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Pulse Oximeter Calibrator - Worcester Polytechnic Institute

1. ccccceeeeeeeeeeeeeeeeeeeeeeeeeeeeeees 60 Figure 37 ee 61 Figure 38 CMSS0F connected to Human Luna LI Redes 62 Figure 39 CMS50F connected fo Fliman ee 63 Figure 40 Oxygenation Deviation with CMS50F Pulse Oximeter ccceeeeeeeeeeeeeeeeeeeeeeeeseeeeeeeeees 64 Figure 41 Specified oxygenation value vs measured oxygenation ueeessssssnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn 64 Figur 42 Elida EPM ee 65 Fouc ET EU DEA TE 69 tree II II I OR 69 EE 70 rates AA Adapted B ISE o trate eee EEE 70 ELLE AS USO Eege gees 71 Pear AO SPPE ee 71 Pisure Ee 71 Figure A8 Adapted servo attachment to stepper bat 71 Fize 39 Cam for linear acido ad Ge eed eddie 71 Figure A10 Cam walls with guide grove and shaft bhole 71 Piste Alu Einser KE a A E 12 Figure A12 Conical device sectional view and side view eesssseseeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeees T2 Figure A l3 Device with probe attachment Over care a ae a 72 Figure A14 Full device with probe attached and user interface 00000004nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn nn 73 Figure A15 Full device top view with probe attached and circuit connections ueeesssssnnnnnnnnnnnnnnnnnnnnnn nn 73 Table of Tables Table 1 Model design requirement Compartson 24 Table 2 Morpuolo a Chir ee ee ee ee oe ee 25 Table 3 Numeriealevaluat on Matix ann ee 26 Table 4 Pair wise comparison chart for design objectives ccccccccccccnnonononono
2. 2 3 4 Introduction This device has been created for the purpose of imitating the optical properties of a human finger to a pulse oximeter The purpose behind this is to create a device which provides a more efficient and cost effective way of calibrating pulse oximeters This device is capable of simulating levels of blood oxygenation between 70 100 as well as frequencies of heart rates between 30 250BPM as they would be seen by a pulse oximeter Necessary Equipment a Power Supply Alligator Clips c USB Cable Set up a Attach power supply to board i Using alligator clamps connect the power supply to the boards The red wire of the servo header should be connected to 5V The M pin of the stepper driver should be connected to 12V Connect each ground pin of both boards to Ground b Plug USB cable into computer Plug the micro USB end into the USB Bit Whacker Microcontroller Installation a Use the install disk to install the drivers for the device to the computer Run the executable Pulse Oximeter Calibrator to begin running the device 5 Operation a The User Interface UI consists of 5 buttons arranged in a small window 1 Rotate Stepper Adjusts the stepper motor by the defined degree amount ii Set BPM Sets the Beats Per Minute of the settings Help servo motor Pulse Osimeter Calibrator 11 Set Time Sets the run time the for pulsations males SEE iv Start Starts the pulsatio
3. PULSE OXIMETER CALIBRATOR A Major Qualifying Project Report Submitted to the Faculty Of the WORCESTER POLYTECHNIC INSTITUTE In partial fulfillment of the requirements for the Degree of Bachelor of Science by David Reid Heidi Robertson Matthew Sonntag Submitted on April 29 2011 Approved by Professor Yitzhak Mendelson Advisor Biomedical Engineering Table of Contents ger RE 3 POMOC US TN 1S sensies iea aian E iian ea ana N Eida 4 O EEE AE E EEE E A EAE 5 rad A E E Ea 6 Tablero EE 8 DNA a eege 9 PO te era e te 516 E 12 20 Bt ET 13 2 1 OR IE Eege 13 22 A ee re 13 2 3 Current Pulse Oxamicter ali ir atO0s sacate onceacarasiesscoascaneccnaiesade Ea O asai ii 15 253 1 Mock e Uer e EE 15 2 3 2 Basic Pinger ee reset enter beten nee inne eh lesen rise 18 2 3 3 Electtical SimulatOrs tege Eege 20 30 io e PR PET UR A 22 3 1 Tual TE EE 22 sa Revised E ee re 22 3 3 D vice Regue N ee ee ee leitet 22 e Dove UD ECU ES ee ee ee ee 23 3 5 Component ompa ISOM EE 24 40 Device Developme iie EAE E EE 27 4 1 Dn AAA o E 27 4 1 1 MOCK Cito latory Syste 2 una 27 4 1 2 Ni eegene 29 4 2 Evaltation of Alternative Designs iesciccccxnsccwscursauesstseannenessdeossebeccosabsansanstseneeiens ncdbsncavomleasanowene 34 4 2 1 Mock Circulatory Sy Stemi EE 34 4 2 2 Wonne Mode E 35 4 3 TEE 36 O EE 38 5 1 COMIC AIG OR EN N E E E 38 32 Core MAN ACHING ent nea acia 45 5 3 MOOI O EE 45 5 4 Base and Linkages Manutactung 4 3
4. Assemble device PIPE PPP pp PT tT TT tT TT TT TT Post assembly testing PTT tT EEE TE EE TE TT EE TT EET TE ET Test motor control Teste gt hee Ao Test dye light absorption AAA e Start user manual fT tT Drat MQP report Draft MQP presentation A AA tt a alee eee OOO 9 10 12 12 13 14 15 16 17 18 19 20 22 22 23 24 25 26 27 28 29 30 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 1 2 3 4 5 Test core with dye light absorption HH PT TTT TT TT tt tT Ty Test control and accuracy of oxygenation AA SE Test control and accuracy of heart rate Pt tt tt ty January February March Table D3 Gantt Chart D term a ee Task Weeki week Week3 Week4 Week week Test core with dye light absorption O E Test control and accuracy of oxygenation AN y Test control and accuracy of heart rate pr Complete user manual PTT tT TT TT tT TT tt Bro Beet Final Deadline Complete MQP report EC CT tT tT tT tt tt a SSES Pt Tt tT tT Tt Complete MQP presentation Program Motors AAA eee ae ls ee Create GUI RSR SES EIS ES EC E RU EES ER BEHEBEE ARR Complete device assembly III 1111 Bee 7 2 2 ZA 1 Pee ee O 0 O O O O O O O O O O O O O O OO EEE EEE EEE Pt SEER EE RE ee ee ee eT EE March Sl Appendix E User Guide l
5. The team was unable to identify why the simulated signal on the Masimo pulse oximeter was different from that of the CMSSOF but reasoned that the differences could be due to differences in the processing techniques that are employed internally by the pulse oximeters Alternatively the difference could be related to the pulse simulation which used motor wait times instead of varying the motor speed With some improvements in the design the accuracy of the calibrator can be improved This was apparent when testing began as alterations to the device needed to be made upon first assembly to obtain results Other improvements would reduce motion artifacts during operation and resolve a method for repeatable probe placement such that measurements are taken from the desired location Probe placement seemed to be the largest variable during testing and adjustment of the probe would take a considerable amount of time before each test The length of the core was slightly too small for the size of the LED and photodiode windows of the probe The partitions of the core also caused particulates to catch in the corners causing staining and further affecting the oxygenation results Given the large amount of variability of our design the reproducible results obtained only further solidify the theory behind the design of the core Simulated signals were obtained by the pulse oximeter for both heart rate and oxygenation even with the many problems found during test
6. potential Its conical shape provides the volumetric change needed for the pulse oximeter to read an oxygenation and heart rate and the values for blood oxygenation follow a pattern similar to those we had expected to be produced Thus making a few basic alterations to the overall design of our device would improve its performance significantly 66 10 0 Future Improvements It is our feeling that most of the reason we were unable to bring our calibrator model to a state in which it could move into more extensive and regular testing 1s that there wasn t enough time Should we have had time to move on with our testing and adjustments it is possible we could have had the calibrator working in a state in which it could be considered for production In addition some of the smaller design flaws could have been addressed with a second or third iteration of the design Some issues with the device which are immediately obvious and have foreseeable solutions are the length of the core component the method by which the probe is attached to the finger dock the strength of the base materials and the maximum speed of the servo motor The length of the core component has a significant effect on the PPG waveform If the core was pushed beyond its length the pulse oximeter would display odd readings and the results would be corrupt It would seem that much of the differences between the wave form produced by our device and that of a human subject are caused
7. probe pulse oximeter a b Figure 7 Reynolds er al finger dock a mock circulatory system b 8 blood out 2 3 2 Basic Finger Models Basic finger models eliminate the need for a mock circulatory system by focusing just on the finger portion of blood flow and pulsations There is a wide variety of these models as there are a number of different ways to simulate light absorption by blood light absorption by tissue and pulsations within the artery One model by Volgyesi 9 patent 5 166 567 uses a squeeze ball to simulate arterial pulsations and two solutions of known concentrations of red and infrared absorbers such that the oxygen saturation it represents is known The outside of the finger model is rigid while the inside walls containing the liquid are flexible such that air within the tubes as created by pressure on the bulb creates pulsations The device can be seen in Figure 8 18 DOSIS AAA AA DIN N IBA i HARRI H Figure 8 Finger model squeeze ball pulsation mechanism cross sectional view 9 Yount describes a device that includes two hollow wedges on top of each other as seen in Figure 9 10 The wedges are filled with two different solutions of 5 sheep s blood in agar solution The blood of the first solution had 100 oxygen saturation while the second solution had 0 oxygen saturation The choice of 5 blood in agar solution was supported by the fact that between 5 8 blood is seen in fin
8. 167 thickness 0 24 167 sin Eq 14 Router Rinner Due to the change in the thickness of the spiral partition the outer radius thickness would have to increase as well to keep the net gain of the non blood component through the path length at zero The outer radius thickness was then a function described by Equation 15 Rinner 1s constant throughout the cone chosen to be 0 25 cm and represents the inner radius Route 15 the changing outer radius resulting 1n0 534 cm lt Router lt 0 55 cm This allows for a 3 increase in path length which will represent the 3 change in blood due to pulsations Thickness outside radius D 5 Router Rinner sin 24 16 Eq 15 The core was then created using Solidworks using the parameters described above A sketch of the core design is shown in Figure 23 along with the three dimensional CAD model The resulting partitions create four half cresent compartments for the two solutions Figure 23 Sketch of the core design and CAD model isometric view 42 A recent modification of the design eliminates the ramp increase of the spiral partition thickness This increases the range of degrees over which the spiral line extends from the inner radius to the outer radius by 47 63 current range 120 833 new design range 168 463 This will be over 39 more accurate This design has the same principles but eliminates increasing the thickness of the partitions The equat
9. 2 1 2 Disadvantages The system despite 1ts small size and combined parts may still require assembly disassembly cleaning time however it would take less time than current models and the size of the gas tank may be a hindrance if smaller tanks are unavailable The small size may cause issues with the behavior of the flow such that residual pulsations form even with the addition of the reservoir oxygenator Blood also has its own disadvantages such as handling expense and availability We will have to investigate how this blood will be acquired and the proper procedures for being handled 4 2 1 3 Testing There are several points of this design which must be tested before 1t can be set as the final design First 1t would be necessary to test how well the custom motor system would simulate a pulse The motor pump mechanics would have to be tested in combination with the tubing and finger dock to ensure that the device will simulate photoplethysmograms similar to those from human subjects The other potential complication is the size of the set up Ideally the system would be as small as possible however it is necessary to ensure that reducing the size does not adversely affect the accuracy of the readings taken by the pulse oximeter To do this we would need to use a device to measure the pulsations of the blood through the tubing and look for irregularities potentially this could be done with a pulse oximeter The finger model must also be
10. 7 3 Pulse Oximeter Testing Figure 34 shows the measured BPM that was output by the device versus the specified BPM output for the device in blue The red line indicates the identity A linear regression was performed and is displayed in the graph by the black line The standard deviation is also shown for the BPM values the standard deviation from the user specified value was 2 441 BPM This shows that the device is both accurate and predictable in the nature of its heart rate simulation User specified BPM vs Measured BPM Be U dai gt gt 2 Lo OU Q OU o N o co y 1 024x R 0 9958 50 60 70 80 90 100 110 120 130 140 150 160 BPM measured by pulse oximeter Figure 34 Specified heart rate value vs measured BPM Figure 35 displays the deviation from the user specified value versus the average of the measured value and the user specified value This shows that the device has a slightly faster pulsation rate at low BPM and a slightly slow pulsation rate at high BPM 59 Heart Rate Deviation from User Value 30 50 70 90 110 130 150 Average of Reference Heart Rate and Specified Heart Rate BPM Figure 35 Masimo Heart Rate Deviation Figure 36 shows the PPG waveform of the device while attached to the CMS50F Wrist Oximeter It shows a one third rise time and two thirds fall time characteristic to human PPGs In
11. Eine self button pack side RIGHT drsel BPM SSII self BPM int tkSimpleDialog askinteger BPM Enter BPM please derset tine self self max time nt tksimplebialog askintegert Time Enter Time please Section 3 This section is a part of the aforementioned App class it is the function called by the start button in the UL It operates by setting the start time to the current moment based on the epoch and comparing the 74 current moment to the start time and checking to see if the difference between that and the start time is still less than the user defined max time defhalfstep self start time time time remember when we started print max time printseltf m x time while time time start time lt self max time ser serial Serial 3 Opens COM port 4 ser write Fir resers device ser write C 0 0 0 0 r ser write 0 0 0 082 ser write RC B 0 7500 r sleep 125 self BPM ser write RC B 0 5002 sleep 0 1625 self BPM sleep 0 0625 self BPM ser write RC B 0 1 r sleep 0 59375 self BPM ser write ere ser close Section 4 This 1s the section of the code which adjusts the degree of the stepper It works by first asking the user for a degree value converting it to a time value and then running the program for the amount of time 1t would take for the motor to step to that position der adjust degree selt self Degree float tkSim
12. Figure 9 Wedges in square formation a wedges of different sizes in a larger wedge formation fr 9 RA cee e EC O A access uaonse aadennieueteeeee 19 Figure 10 Finger model with in and out motion a enclosed finger model with in and out motion b finger model with rotating disk motion eil 20 Figure 11 SmartSat by Clinical Dynamics 2 onncncn nincnnnnonococononoconoconenenanenenenenonecananenanenenonecanancnanininnas 21 Figure 12 Kent and Mendelson mock circulatory system set up a 5 block diagram of mock ao E 21 Figure 13 Reynolds et al finger model side view top and top view bottom a 6 mock circulatory finger model of similar design side view b ccccccccccccccnnnononononocononnnonnnononnnons 28 Figure 14 Wedges of 0 and 100 oxygenation with pulse oximeter probe attached showing light path position and direction through wedges ccccceeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeees 30 Figure 15 Light path arrow through wedges Right side up view a upside down view b 30 Figure 16 Outer frame and motor shaft a frame configuration attached to wedges b 31 Figure 17 Top view of wedges showing axis of rotation and the direction of wedge turning a light path through wedges before and after wedge turning bi 31 Figure 18 Motor control system of oxygenation content and arterial pulsation simulation 32 Figure 19
13. Gantt charts below visually outline the aforementioned tasks carried out during the four project terms A term e Literature review o Principals of pulse oximetry o Pulse oximeter function o Soft tissue mechanics o Current pulse oximeter calibrators e Project proposal o Revised client statement o Alternative designs o Schedule e Preliminary design of critical components o Determine what methods will be used for different portions of the calibrator o Simulation of pulse blood surrounding tissue potentially motion artifacts and other Hb variants B term e Evaluate alternative designs e Determine final design o Establish specifications for oxygenation Rotation Path Length Wedge Partitions o Establish specifications for pulse Conical Shape angle and size Displacement Movement e Establish method of locomotion o In out o Rotational e Establish Orientation and Fixation e Establish exact specifications for components e Determine materials and manufacturing process o Finger casing o Core component o Base assembly e Determine devices for motion system o Determine pricing and suppliers for components o Order components 79 C term Complete CAD design of major components and full model Develop a complete Bill of Materials BOM Manufacture base and linkages o Acrylic laser cutting Manufacture core component o 3D printing o Polver Fullcure 720 material o Amber translucent material Assemble Device Testing o Servo
14. Pulse oximeter probe attachment to outer semi transparent C3g1ng 32 Figure 20 Welehted OD CCUVES Ces src iia 36 Figure 21 Core filled with oxygenated liquid Blue lower oxygenation Crimson higher Ae E 40 Fieure 22 Example lisht path Throush core ana a e 40 Figure 23 Sketch of the core design and CAD model isometric view 42 Figure 24 Sketch of new core design and CAD model isometric view unsssssssssnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn 44 Figure 25 Conical assembly with cap and bottom piece 45 Figure 26 Pulse Oximeter Calibrator Device ana a a iento 47 Figure 27 GUI for Pulse Oxmeter te 49 Figure 28 Spectrometer results of blue food coloring green food coloring and wild berry EIERE 53 Figure 29 Example photodetector circuit to measure light intensity ccceeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeees 54 Figure 30 Four bar crank slider linkage attached to servo S6 Figure 31 New linkage using cam attached to servo wheel top view a side view bi 57 Figure 32 Center wall screw holes for gtepper 58 Figure 33 Finger dock support a finger dock additional wall bi 58 6 Figure 34 Specified heart rate value vs measured BPM ccccceeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeees 59 Figure 35 Masimo Heart Rate Deviation 0 ccccccceeeeeseeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeees 60 Figure 36 PPG waveform of Device with CMSSOF Pulse Oximeter
15. by the shortness of the core and the limitation of a single speed for the servo motor We therefore propose lengthening the core component to effectively increase the range in which the device could actuate In addition our device would be able to reach heart rates of up to 250 BPM However we were limited to one available speed and the maximum heart rates we could achieve were 150 160 BPM This problem could be rectified by using a different higher speed servo motor in its place Another issue with the servo motor is its use of delay times in the programming rather than actually changing the speed at which it turns This is a problem which can only be rectified by altering the program used to run it and the firmware installed on the microcontroller Another essential change would be improvement is the finger dock This part of the device is one which has caused much trouble during our testing The current model causes a great deal of trouble when trying to properly attach the pulse oximeter probe to the dock The device therefore could benefit greatly from the addition of some mechanism which would allow for consistent and easy attachment to the proper location on the dock Finally improvements to the strength and stability of the current base would improve the accuracy of our results At the moment the base is composed of five interlocking pieces of acrylic which are held together by glue Because of this design structure during operation a slight
16. components and their placement in the path of blood flow a b Figure 12 Kent and Mendelson mock circulatory system set up a 5 block diagram of mock circulatory system b 27 This design variation would focus on the decreasing the size increasing the ease of assembly disassembly and cleaning We will also try to modify 1t to be used as an initial calibrator for pulse oximeters We would attempt to do this latter part by simulating the relevant optical properties that would be measured by both reflectance and transmission oximeters This device would be scaled down to use minimal tubing and hence minimal blood The other components will also be miniaturized and combined where possible Smaller gas tanks may be obtainable to be used with this system in setting the oxygen saturation The device would be designed to easily and securely attach each member and allow cleaning of the components between uses The motor must be capable of pumping between 30 250 times per minute the same operating range as the heart without damaging the blood cells Most pulse oximeters are programmed to measure within this range A control panel would be attached for varying the pulse rate More research is needed in determining any further requirements and the necessary implementation of the motor pump mechanism The finger dock ought to mimic the human finger in optical properties that are relevant to the oximeter and model finger absorption spectra
17. half quarter and eighth steps It has adjustable current control from 150mA phase to 750mA phase and can be controlled by a power supply ranging from 7V to 30V The servo is driven by the USB Bit Whacker PIC 18F2553 Development Board Sparkfun Electronics which comes preprogrammed with boot loader and UBW_D firmware to allow easy programmability 46 The entire system can be seen in Figure 26 On the left 1s the stepper motor connected in the middle to the servo which in turn is connected to the conical device on the right The tube surrounding the conical device and shaft is the finger dock on which the pulse oximeter will attach The red pieces attached to the base are the circuit boards driving both motors the stepper driver next to the stepper and the USB Bit Whacker in the section next to the servo The stepper is raised such that the servo can be suspended and the shafts of the stepper and the conical device are on the same linear axis Figure 26 Pulse Oximeter Calibrator Device 5 4 Base and Linkages Manufacturing In order to gain the precision necessary to keep both motor movements aligned the base components had to be laser cut out of acrylic The two end walls were adhered to the base platform for stability using an acrylic solvent The stepper support was also adhered to the back wall to align the middle wall which can be removed if the device needs to be disassembled The stepper attachment to the servo as well as the li
18. not The wedge model was then pursed and redesigned to meet all the objectives 37 5 0 Current Design The current design came about due to the large problem found with the wedge model The wedge model did not allow easy pulse oximeter attachment This conical model incorporates wedge type elements but the wedges are made into a three dimensional cone like shape This allows the wedges to be the proper shape and size of a finger This cone has inner partitions that are shaped in such a manner that when the cone is rotated the oxygenation is changed Because the device is cone shaped moving it in and out would increase and decrease the amount of volume seen by the pulse oximeter which will create pulsations This cone is attached to a motor system that moves the device in the proper manner to obtain these results The main core of the device and the motor systems to obtain pulsations and oxygenation are further explained in the following sections 5 1 Conical Core The core was designed to provide a pulse oximeter with a specified heart rate and oxygen saturation This requires knowledge on how a pulse oximeter obtains ts heart rate and oxygen saturation measurements from a finger The pulse oximeter measures the intensity of light transmitted through the tissue at each wavelength The amount of light transmitted through a material at a given wavelength can be described using Beer Lambert s law shown in Equation 4 where alpha is the ab
19. ratio measured by the pulse oximeter is related to the actual saturation as measured by the CO oximeter This relationship is called the calibration curve and an example can be seen previously in Figure 3 Calibration procedures using human volunteers are limited to 70 saturation and values below 70 need to be extrapolated from the data and as a result pulse oximeters are less accurate in reading lower saturation values For our device to be equally as accurate at all saturation values specified the design must allow for adjustable increases in oxygen saturation between 70 100 To achieve better accuracy our device must allow for adjustable oxygen saturations below 70 Another consideration that needed to be taken into account is heart rate The pulse oximeter measures the volumetric change through the light absorption mechanism described above Pulsations in vivo represent a 1 3 change in volume from the DC component The design must simulate a similar volumetric increase and decrease by increasing and decreasing the path length through the core This is achieved using a three dimensional cone shape The simulated heart rate is created using a back and forth motions that changes the total path length through which the light had to be transmitted A much more difficult consideration is the change the oxygen saturation This is achieved through rotation of the core The inside of the core will be filled with two solutions that have opposite oxygen s
20. tested to ensure that the materials used will simulate the proper light absorption 34 properties Again this testing could be done using a pulse oximeter in tandem with a CO oximeter to make sure oxygenation levels remain at the desired levels One circulatory model examined had a secondary venous circulatory system running in parallel to the artificial arterial circulatory system This more closely resembles the structure seen in a finger and the affects should be investigated as to its influence in the readings of pulse oximeters 4 2 2 Wedge Model 4 2 2 1 Advantages This model would provide a bloodless calibrator to the market that would require no set up or cleaning between uses and would utilize a user interface that would allow modulation of oxygenation and heart rate and would be able to assess the accuracy of a pulse oximeter or be used to pre calibrate a pulse oximeter Many components are easy to obtain or manufacture and can be purchased at minimal costs below 20 Oxygen saturation levels can be simulated below the normal calibration range of 70 100 improving upon the standard pre calibration techniques The movements although shown exposed to the environment can be enclosed to allow for more durability of the device 4 2 2 2 Disadvantages This device has moving parts and the wedges and frame would have to be sized properly to fit the transmission pulse oximeter probe The movements to simulate the pulse would also hav
21. the oxygenation as seen according to the pulse oximeter changes due to design of the partitions within the conical device The amount of change is dictated by the amount of rotation With saturation variation between 0 100 an angle change of 1 2 1 3 will cause a change of 1 The stepper motor used has a minimum step angle of 1 8 which would cause a 1 3 increase in oxygenation The stepper motor shaft is attached to the servo which in turn is attached through the crank slider to the conical device All components are proportioned such that the linkage attached to the conical device and the stepper motor shaft is on the same linear plane supported by the base walls As long as the shafts are rotating on the same axis there will not be any distortion between the stepper rotation and the rotation of the conical device The servo is suspended such that it can be rotated by the stepper The stepper has the ability to rotate a full 360 but our device will not rotate beyond this point due to the limitations imposed by the cord extending from the servo motor When all of these constraints are taken into account the stepper can rotate the conical device to the proper oxygenation position The stepper motor used in our device is a ROB 09238 Stepper Motor Sparkfun Electronics These motors are driven by two circuits The stepper has its own driver an EasyDriver Stepper Motor Driver ROB 09402 Sparkfun Electronics with a micro stepping resolution to full
22. two things are required The program must be able move based on user defined degree values and reset to the original starting position These two functionalities are achieved in much the same way First the specified degree rotation function stores the user input and inserts it as a time delay between the activation and its deactivation of the stepper The stepper is currently set to receive a square wave at 2Hz At this frequency it moves exactly 3 65 degrees for each second See Appendix B section 4 The second function the reset button works by using the same user defined variable as above again converting it to a time and inserting it as a delay between activation and deactivation Due to the way the board controlling the stepper works to move it backwards a continuous signal must be sent to the direction pin and another continuous signal sent to the stepper control pin This causes the stepper to rotate backwards continuously and stop after traveling the same distance backwards as it did forward a distance defined by the user See Appendix B section 5 A few compromises had to be made with the structure and functionality of the Stepper code For instance before the user can define a new degree value for the device they must first press the reset button to return it to the neutral starting position This is caused by the inability of the program to store any but the very last user defined variables of any one given type These functions were com
23. using a Double Layer Pulsation Flow Cell Engineering in Medicine and Biology Society 2009 8 Reynolds K J et al In Vitro Performance Test System for Pulse Oximeters Medical amp biological engineering amp computing 30 6 629 35 1992 9 Volgyesi George A Method of Testing the Accuracy of Pulse Oximeters and Device Thereof Patent 5166517 November 24 1992 10 Webster J G Design of Pulse Oximeters IOP Publishing Ltd 1997 11 Yount John E Devices and Procedures for In Vitro Calibration of Pulse Oximetry Monitors The State of Oregon Acting by and through the State Board of Higher Education on Behalf of Oregon Health Sciences University assignee Patent 4 834 532 May 30 1989 12 Yount John E Devices and Procedures for In Vitro Testing of Pulse Oximetry Monitors The State of Oregon Acting by and through the State Board of Higher Education on Behalf of Oregon Health Sciences University assignee Patent 4 968 137 November 6 1990 13 Zhou G X et al Pulse Oximeter Calibrator Based on a Liquid Crystal Light Valve Annual International Conference of the IEEE Engineering in Medicine and Biology Society 12 5 2012 13 1990 68 Appendix A CAD Models Figure A2 Adapted Full Device 69 Figure A3 Original Base Adapted Base Figure A4 Figure A5 Servo Motor Figure A7 Original servo attachment Figure A6 Stepper Motor Figure A8 Adapted s
24. where possible Mechanical properties of the dock ought to be similar to the human finger if it causes significant differences in measurement Whether it causes significant differences in measurement remains to be investigated as well The tubing within the finger dock has to be capable of producing photoplethysmographic similarities to the photoplethysmogram of a human patient The feasibility in achieving high accuracy in these properties is still unknown and will have to be researched Our finger dock would be modeled from Reynolds et al which allows flow into and out of a finger shaped protrusion to more closely mimic the blood flow within a finger Figure 13 shows these two finger models blood out in light from pulse oximeter probe Blood flow Finger casing blood Ju directions A Perspex Perspex Blood channel blood in view of cuvette from above b blood out a Figure 13 Reynolds er al finger model side view top and top view bottom a 6 mock circulatory finger model of similar design side view b 28 Further analysis of a device is still required to ensure that this design at least retains all features in the current model if not improving upon them With sufficient improvements in design this model could potentially find use as a tester for clinicians a calibrator for manufacturers and for research in pulse oximetry so long as the device can mimic human finger absorption spectra and
25. while not allowing for a GUI Both designs could be easy to manufacture depending on the components used Device for Pulse Oximeter Calibration 1 0 1 0 Calibrates multiple pulse oximeters 0 25 0 25 Ease of use Manufacturability 0 15 0 15 0 1 0 1 Simulation of signal 0 5 0 5 Heart rate Refelctance 0 35 0 175 0 2 0 05 Easy to assemble GUI 0 50 0 075 0 3 0 03 Oxygenation Transmission 0 50 0 25 0 8 0 2 Set up 0 30 0 045 Durable 0 4 0 04 PPG 0 15 0 075 Cleaning 0 20 0 03 E 0 3 0 03 Figure 20 Weighted objectives tree 36 Table 4 Pair wise comparison chart for design objectives E Oxygenation Cost Portability Dysfunctional Heart Rate Compatibility User Objectives y range hemoglobin Range with probes Interface Oxygenation gt e 1 See ee EEE EC gt lt A lt n O AS lt lt er Portability Heart Rate Range Compatibility with probes User Interface MEN LL A BEN CN o CAS _ pe A e A Sum SGL e NN Dysfunctional 1 hemoglobin Not only this the mock circulatory system that was designed was not a unique enough design and would not significantly improve upon the current model The design would also be too expensive to implement with the budget provided The wedge design surpassed the mock circulatory system in each objective except compatibility This problem could be overcome however while the mock circulatory design problems such as expense could
26. 37C 8 saturation Uncontrolled ar Better than human a au Better than human pulsation variability Drau ul Repeatability fj human accuracy of accuracy of ingers E fingers Oxygenation will fingers motors motors remain the same Extensive Dimensions cleaning e Small size and Small size and Small size and j of 60cm x disassembly and sh weight may weight very 40cm set up time for 8 be fragile mobile experiments Power source Outlet 2 Outlet power Manual labor power tissue Outlet power supply power S Bo Battery Battery component supply PPly PPly hemoglobin Some of the designs appear similar in this assessment and further analysis is required to show any Extensive cleaning disassembly and set up time for experiments Mobility differences in components used A morphological chart was generated based on these models seen in Table 2 which relates each function with possible components to achieve the function The lists of possible components for fulfilling each function were gathered from the aforementioned models 24 Table 2 Morphological Chart Functions Y 32 2 3 Ps 6 1 7 Semi Attenuate Simulate soft tissue Plastic Rubber Silicon transparent Quartz light signal a plastic film 5 animal Human Ania blood in Light Real blood blood cow blood agar absorption sheep pig solution of blood SC Semi Liquid component Simulation Dyed plastic transparent crystal LEDs of blood mat
27. Di 3 oO E Ces oO 800 Wavelength nm Figure 1 Light absorption spectra of hemoglobin variants 6 2 2 Pulse Oximetry While traditional oximetry has the ability to determine oxygen saturation in blood there was a major limitation preventing its use in real time patient monitoring systems The light scattering through the tissue could not be distinguished from that of the blood The development of pulse oximeter was after the 1970 s when Takuo Aoyagi invented photoplethysmography which is the use of optically generated measurements of the volume of an organ to identify a time dependent volumetric change 1 In this case the time dependent volumetric change can be associated with that of arterial blood This can be seen in Figure 2 where the amount of light absorbed can be seen to vary in time with the arterial pulse state 13 Arterial pulse Residual arterial blood 23056 4 UU Venous blood See Oe LA Bloodless tissue RATA AA A AAA we OO OOF 9 9 0 0 9 0 0 E 4 Oe Gesin neen wi Time Figure 2 Material within a finger showing arterial blood volume changing over time photoplethysmogram 6 The pulse oximeter 1s able to disregard portions of the signal that remain constant by taking a ratio of the changing AC and unchanging DC signal per wavelength as defined in Equation 1 This ratio is called the R ratio Eq 1 AC Dad ON ACinfrar
28. EEGEN Ee 48 A press Bra AA OAAR 51 A SEENEN 32 6 1 Dye selec sense 52 02 licht EnS EE 54 6 3 Bicie e T EAE E A EE E E AEN EAS AE E E E AEE 55 TO EE 56 7 1 Device e ONS ae ze ee ao 56 72 MOr FUN ere 58 3 Par ORTE AA ee 59 O ISOS 6 1 pe A A S 65 9 0 Conclusion co 66 10 0 TUU O ls 67 A RR 68 Appendix A CAD Models ai lis 69 Appendix Be Code dt td ii ii ii ehe 74 JO ee 74 SECLON A O 74 AS ee ee A A A cect 74 PP O A 75 IL ON Re Nene Re ERE eS es ERNE er enge 76 Appendix Es Circuit DAA EE 78 Appendix D Schedule and Timeline of Project Tasks nnennosssssseeseeeennssssssssssssssecceeeersssssssssssssseseceereee 79 Appendix E Uer Gde aei ed 82 Authorship Page David Reid Heidi Robertson and Matthew Sonntag all contributed to the research and writing of this report The following is a breakdown of how the report was written for this project David Reid contributed to Component Comparison Conical Core Experimentation Pulse Oximeter Testing and Discussion as well as provided editing and formatting support Heidi Robertson contributed to Current Pulse Oximeter Calibrators Wedge Model Evaluation of Alternative Designs Current Design Bill of Materials Device Alterations Motor Functionality Appendix A and Appendix C as well as provided editing and formatting support Matthew Sonntag contributed to the Introduction Mock Circ
29. a piece of foam to act as a spring The plate is attached to the core shaft such that when the plate is compressed and released the shaft moves inward and outward within the finger dock The plate has a grove in which the cam rotates This allows the rotation of the servo to also rotate the shaft since the plate will rotate when the cam rotates The servo was lifted slightly place the cam directly centered on the plate This type of linkage allowed movement without dependence on each component being level It also allowed easier attachment and component variation and alteration The base had originally been designed to allow easy assembly and disassembly The center wall was not adhered to the base plate or stepper support The stepper also was placed on the stepper support with the shaft through the center wall without any other way of anchoring the motor These design choices caused large motion artifacts in the PPG when servo rotation occurred The servo had a large amount of torque to move the linkages and caused the back wall to jump with each turn The stepper would also move when the wall jumped and when pressure was applied to the servo attachment On the other end the outer wall with the finger dock would also bow out during pulsations This caused large movement of the finger dock often hitting the core The finger dock could not properly support the pulse oximeter probe when attached Certain probes were also too large to easily fit onto the fin
30. addition a dicrotic notch can be observed Figure 36 PPG waveform of Device with CMS50F Pulse Oximeter Figure 37 shows readings from the CMS50F when connected to the device The data displayed shows two trials the first trial had a heart rate set to 72 BPM and the second trial had a heart rate set to 75 BPM The graph indicates a high level of accuracy in the control of the heart rate However 1t can be seen that the accuracy of the oxygen level differs between the two trials The first trial was able to stabilize the oxygenation value after approximately 30 seconds and then remained at the same value The second trial shows a stabilization of the oxygenation value after approximately 30 seconds as well but continued to vary by 3 for the rest of the trial length 60 Pulse bpm 255 255 Figure 37 CMS50F connected to device Figure 38 shows the CMS50F readings for a human volunteer The data shows a variance in the oxygenation value of 7 over the ten minute testing length In addition the heart rate is seen to vary by about 10 BPM during the testing interval The subject was sitting prior and during the test and was told to breathe normally 61 Pulse bpm 255 25 Figure 38 CMS50F connected to Human 1 Figure 39 shows CMSSOF readings for a second human volunteer The data showed a variance in oxygenation values of 5 over the ten minute testing length The heart rate was initially measured as 90 BPM but two m
31. as low as 70 This method of calibration is expensive and time consuming for pulse oximeter manufacturers Eliminating the human element of calibration would reduce overall cost and save time An artificial system was developed to simulate blood oxygenation levels between 70 100 and heart rate readings between 30 150 BPM A programmed motorized dual axis system was designed to provide certain calibration values as specified by the user through a computer interface Tests conducted with the CMSS50F Wrist Oximeter showed that values obtained from the calibrator fall within 4 oxygenation and 3 BPM heart rate from that of the user defined values Table of Figures Figure 1 Light absorption spectra of hemoglobin variants Toi 13 Figure 2 Material within a finger showing arterial blood volume changing over time Photopleihysmosram Oasen deeg ana 14 Figure 3 Normalized R ratio for oxygen saturation levels Toi 15 Figure 4 Kent and Mendelson Finger model a mock circulatory system b 5 16 Figure 5 Diagram of double flow cell 7 0 0 0 0 ccccecceeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeees 17 Figure 6 Complete setup of the mock circulatory system by Oura et al II 17 Figure 7 Reynolds et al finger dock a mock circulatory system b 8 eseeeeseeesseesesesssesssseesessese 18 Figure 8 Finger model squeeze ball pulsation mechanism cross sectional view 9 19
32. aturation levels The inner design of the core creates four partitions two that are filled with the highly oxygenated solution and two that are filled with the lower oxygenated solution Rotation of the core will provide a change in absorbance of the two wavelengths of light through alterations in the ratio of solution high and solution low To increase oxygenation the path length through solution high will increase while the path length through solution low will decrease Overall the total path length through the cone will remain constant The center of the core is hollow such that the center will not play a role in the ratio of the two solutions A model of the core is shown in Figure 21 The blue represents the lower oxygenated liquid while the crimson represents the higher oxygenated liquid 39 Figure 21 Core filled with oxygenated liquid Blue lower oxygenation Crimson higher oxygenation The light path extends through the entire core such that a ratio of the two liquids can be taken by simply rotating the core This can be seen in Figure 22 The red arrow represents the light path through the core enlarged for easier viewing The outer material will be translucent such that the light could penetrate the core Figure 22 Example light path through core At a given positive angle of rotation from Oj the design would result in a ratio of material depths within the LED path length of solution 2 to solution 1 solution 2 tha
33. be inexpensive to manufacture Currently we have little knowledge of how the oxygenator works or how exactly the motor pump mechanism interacts with the blood to cause the pulsatile flow 4 1 1 1 Bill of Materials e 1 5 meters rubber tubing e Small electric motor e Circuit control board e 10cm cube of Delrin e Artificial finger dock e Blood 1L It is important to note that many of the components listed here are estimations of what will be need and not exact values While we currently believe that we have a general understanding of what will need to go into this device at the moment it is impossible us to say with absolute certainty what materials the final design will require All materials need to be biocompatible if they are to come into contact with blood before or during the use of this circulatory system 4 1 2 Wedge Model The wedge model designed comes from the patent by Yount which includes wedges filled with blood of known oxygenation content articulated within the oximeter probe to simulate pulsating The patent also uses a pair of polarizing disks to simulate pulsations Here we use the wedge portion of this patent with slight modifications but with a different means of wedge movements to simulate pulsations In this case there are two wedge shaped pieces These wedges are aligned to form a rectangle Both wedges are made of solid material one has optical properties that are similar to reduced hemoglobin and anothe
34. bowing effect can be observed This bowing can lead to motion artifacts in the pulse oximeter signal and even misalignment of the pulse oximeter probe If the base of the device were to be made from a more solid material such as aluminum or even if the current base were to be reinforced with some type of support structure such as a roof or thicker walling 1t would make great strides in reducing this bowing effect and therefore the motion artifacts in the signal 67 References 1 Aoyagi Takuo et al Apparatus for Calibrating Pulse Oximeter Patent 5278627 January 11 1994 2 Clinical Dynamics SmartSat Information SmartSat Pulse Oximetry Analyzer 2009 http www clinicaldynamics com smartsat_specs htm 3 Hornberger Ch et al A Prototype Device for Standardized Calibration of Pulse Oximeters Journal of Clinical Monitoring and Computing 16 3 161 9 2000 4 Hornberger Ch et al A Prototype Device for Standardized Calibration of Pulse Oximeters II Journal of clinical monitoring and computing 17 3 4 203 9 2002 5 Kent Joel C and Mendelson Yitzhak An In Vitro Tissue Model for Evaluating the Effect of Carboxyhemoglobin Concentration on Pulse Oximetry IEEE Transactions on Biomedical Engineering 1989 6 Mendelson Yitzhak Pulse Oximetry Wiley Encyclopedia of Biomedical Engineering John Wiley amp Sons Inc 2006 7 Oura M et al Calibration System for Pulse Spectrophotometry
35. d laminated object manufacturing at Intelitek Manchester NH This process was performed twice with walls of various thicknesses The walls in the first process were 0 5 mm thick which caused the walls to collapse after device printing The walls in the second process were 1 5 mm thick which did not allow for the manual inner material extraction necessary for this process This required us to find a company that specialized in rapid prototyping such that our device could be processed using a different type of 3D printing The third attempt at core printing was done by Cideas Cary IL using a process called Polyjet ideal for detailed high resolution pieces The resolution of this process was 0 005 which was necessary in our device in order to obtain the proper 1 3 change in the walls of the core Fullcure 720 was the material used as it produced a semi transparent amber material the closest to skin color achieved in this type of 3D printing The piece was printed with the base attached and the top piece was printed separately in order to allow septum closure attachment between the top piece and the conical portion Two copies of the core were printed 5 3 Motor Systems The design uses two motors in order to obtain the correct pulsations and oxygenation The pulsations come about by reciprocating the conical device within the light path of the pulse oximeter This 1s achieved using a servo motor in combination with a four bar crank slider linka
36. d the establishing functionality of the motors Each motor was tested for accurate control through a GUI The device was then connected to a pulse oximeter probe to test the accuracy of the heart rate and oxygenation values displayed by the pulse oximeter The servo motor was tested by inputting a user value into the interface and recording the beats outputted during each test The test ran for two minute intervals and the number of simulated beats was counted by two team members These BPM results were then compared to the user defined value The stepper motor rotation tests were conducted by inputting step values of 5 10 30 90 and 180 increments These step angles were then measured with a protractor at both the stepper shaft and the core shaft These values were compared to the defined rotation angle Measurements were taken at both locations to determine if step degrees were lost through the linkage system connected at the servo wheel Pulse oximeter results were compared to the user defined value for oxygenation and heart rate The pulse oximeter probe was attached to the finger dock such that the LED and photodiode were aligned vertically over the smaller end of the core Once the pulse oximeter probe was aligned the pulse oximeter display was monitored for two minute intervals for heart rate tests and four minute intervals for oxygenation tests The larger time interval length for oxygenation values was used to allow the pulse oximeter r
37. e tested required almost only food coloring to be used to obtain the necessary absorbance for calculating the extinction coefficient This high concentration was not practical for the device The concentrations used for testing can be seen in Table 5 52 Table 5 Spectrometer results and calculated extinction coefficient Test Substance 660 absorbance Concentration Path Extinction coefficient flavor average length EN mae ve 0 2848 0 02634 g mL 10 81348 mL cm g Eessen Fruit Fruit Punch 0 1987 0 02638 g mL SEN 7 533649 mL cm g 0 0111 0 02655 g mL tomo 0 418079 mL cm g CEA 0 0 1613 0 02623 g mL E 6 15062 mL cm g ee EE cas REENEN reso ooo aise tom ETC aos oo omas num EE EE The results from the spectrometer are shown in Figure 28 The vertical axis shows the absorbance values while the horizontal axis is the wavelength The three dye solutions that had the highest absorption in the red light spectrum are shown in the table and in the graph by a color that corresponds to the solution Absorbance 640 650 Wavelength nm Figure 28 Spectrometer results of blue food coloring green food coloring and wild berry pomegranate mix Once the extinction coefficient was known the two dye concentrations could be calculated for 100 oxygenation and 70 oxygenation to match that of a finger A finger can range in size from one person to the next and thus it is known that the absorbance of one fing
38. e time consuming and expensive Thus it would be of benefit to the manufacturer to have a device that provides an efficient universal and cost effective calibration of pulse oximeters Our device seeks to meet this need for initial calibration that reduces cost time and risk to volunteers 2 3 Current Pulse Oximeter Calibrators A pulse oximeter calibrator must fulfill certain criteria According to Hornberger et al it is important that the R ratio signals that the pulse oximeter receives are within the calibration range of the device The pulse oximeter must identify the device as a patient It must also be able to assess the accuracy and correctness of data compared to an accepted standard Finally it must provide an absorbance that would be the same as a finger with known arterial blood saturation 3 To be detected as a patient the calibrator would have to have a non changing absorbing component that absorbs proportional to the tissue and venous blood of a patient a pulsatile absorbing component that absorbs proportional to pulsating arterial blood Current pulse oximeter calibrators do not meet the need of the commercial market There are many designs ideas and patents of pulse oximeter calibrators and simulators testers and use various methods to function Most of them fall into one of three categories mock circulatory finger models and electronic simulators 2 3 1 Mock Circulatory Systems A variety of mock circulatory syst
39. e to be very small to properly simulate a human subject Potential difficulty would come about from simulating high BPMs using the servo There is still skepticism as to whether or not pulsations can be created using the motion devised for this purpose Further testing would need to be done in order to determine most parts necessary for device operation Using this design the calibrator would not fit within most commercially available pulse oximeters 4 2 2 3 Testing Methods First the proper material needs to be chosen to simulate fully oxygenated and fully reduced hemoglobin This will be done through research and testing of the accuracy in blood light absorption replication through these materials A basic circuit will be set up in order to determine the amount of red light and infrared light absorbed by the oxygenated blood and deoxygenated blood The optimal size and angle of inclination for each block will be tested by using an LED and photodiode so that the internal components will fit into a finger sized casing and that the accuracy is maintained to less than 1 2 Through equations and light absorption testing it can be further shown that when the path length is changed through equal displacement of the wedges the ratio can be maintained and a pulse can be simulated Other ways of 35 moving the two pieces to simulate the pulse and oxygen saturation while maintaining adjustability will be considered One such alternative is moving both
40. eadings time to stabilize The values displayed on the pulse oximeter were recorded and a weight was assigned based on the amount of time each value was displayed in an interval The weighted average was found and compared to the user defined value The PPG displayed on the pulse oximeter was also compared to the compared to a sample human PPG waveform to determine signal accuracy This however was not necessary for calibration purposes Each pulse oximeter test was conducted with a CMSSOF Wrist Oximeter and the Masimo SET Radical pulse oximeter 55 7 0 Results 7 1 Device Alterations When testing began several problems with the device design became apparent and several alterations needed to be made to allow device function The largest problems were linkage movement and stability The original design used a four bar crank slider linkage to translate the rotational movement of the servo to linear actuation shown in Figure 30 Figure 30 Four bar crank slider linkage attached to servo While sound in principle and in CAD modeling this approach did not create smooth movement over the intended degree range Each portion of the linkage system needed to be supported to keep it level and aligned Only level movement would create the linear actuation necessary for pulsation simulation The shaft diameter was too small to allow a hole to be drilled through it Without this hole there was no easy way to secure the shaft to the linkage to still a
41. ed R Empirical calibrations allow for conversion from the R ratio to SpO Equation 2 shows an example of how an empirical calibration could look Some pulse oximeter models use more calibration coefficients In Equation 2 both A and B represent calibration coefficients Eq 2 The AC DC ratio at each wavelength normalizes the signal such that differences in individuals tested AC Red DCRed Ree Infrared SpO are ignored Equation 3 shows that the normalization of the signal yields the changing component of the signal Eg 3 n l Igxe T 0101 a9C Al Ipxe 41Cc1 a2c2 l0 In Today pulse oximetry is recognized worldwide as the standard of care in anesthesiology and is widely used in intensive care operating rooms and during patient transport However all pulse oximeters require initial empirical calibration Companies wishing to bring new pulse oximeters to the market need to the FDA s standards in providing proper oxygenation initially calibrate their device such that it meets readings A graph showing a typical R ratio is shown in Figure 3 14 E 2 ba _ be 5 D gt x lt O o o 2 0 3 0 Normalized R ratio Figure 3 Normalized R ratio for oxygen saturation levels 6 The R ratio is displayed by the pulse oximeter and matched to the corresponding oxygenation value as the intravenous blood measured by a CO oximeter The calibration procedures ar
42. ed oxygen saturation values within 5 and heart rate values within 3 BPM from values defined by the user The team investigated previous pulse oximeter calibrators that were presented in journal articles and patents These models were categorized into three different types finger models mock circulatory systems and electrical simulators Whole blood was generally used in the mock circulatory systems the use of blood is expensive and inefficient as it requires proper handling and preparation Electrical simulators were found incapable of performing the initial calibration of a pulse oximeter because they depend on previously calibrated look up tables for oxygenation values specific to pulse oximeter models A finger model design was found to be the most efficient and cost effective design that can be used to calibrate a pulse oximeter Simulation of heart rate was achieved by mimicking the 1 3 volumetric change of arterial blood within a finger Pulse oximeters measure the pulsatile volumetric change within the finger To simulate pulsations a conical shape is actuated within the light path of the probe to increase and decrease the volume of the simulated blood compartment Pulse oximeters determine SpO by measuring light intensity transmitted through tissue in the red and infrared spectrum The difference in absorbance of oxygenated hemoglobin HbO and reduced hemoglobin Hb in these spectra can be used to find the ratio of HbO and Hb calcula
43. eed for the device While we currently believe that we can obtain the necessary items to go into this device at the moment we cannot say with absolute certainty what materials the final design will require We are also unsure at this point what materials will best suit our purposes This will require further testing and research to determine the best material suited for our purposes 33 4 2 Evaluation of Alternative Designs After the alternative designs were established and described they were assessed and evaluated Below are the advantages and disadvantages of both alternative designs along with the testing that would need to be completed before the design could be chosen as the final design 4 2 1 Mock Circulatory System 4 2 1 1 Advantages This device will be made small in size compared to current devices and will attempt to reduce the number of components A possible combination of oxygenator and reservoir could eliminate one component and the use of an artificial finger dock eliminates the need for a mounting unit for the oximeter The finger model also allows for the mounting of any type of pulse oximeter since it will be sized and shaped similarly to an actual human finger The device would also provide adjustable oxygen saturation by means of adjusting the oxygen partial pressure and would allow for adjustment of pulse rate through a circuit board The device meets all of the requirements established in the requirements list 4
44. ems exist which seek to provide an identical photoplethysmogram as measured in vivo This is done by reproducing the pressure wave of blood as it pulses through the finger Oxygenation of the blood is controlled within the system by an oxygenator and gas supply Some 15 oxygenators may be more efficient in the time it takes to oxygenate the blood but this wasn t taken into consideration when comparing the models Some of these models have special points of attachment for the pulse oximeter while others better reproduce certain physiological conditions The following are mock circulatory system models that were looked at in the literature review A tissue model by Kent and Mendelson was developed to have similar optical properties as tissue recreate photoplethsomgrams that are similar to those natural to the finger and have a whole blood tissue volume comparable to that in the finger 2 5 A 35 systole and 65 diastole at a rate of 70 stroke min was created by the pump Experiments in this paper were conducted using physiological conditions of 41 hematocrit and 1 5 HbCO however the temperature of the blood was at 37 Celsius The tissue model uses a block of Dow Corning silicone elastomer with 15 symmetrically distributed cylindrical conduits shown in Figure 4 to simulate capillaries and relevant optical properties of tissue The finger model is placed within the circulatory system that creates pulsations oxygenates the blood and regu
45. er is different from the next This means that 53 the core absorbance does not need to match the absorbance of a single finger with the dye solution but rather have the dye solution concentrated enough to produce an absorbance that is in the dynamic range of finger absorbance 6 2 Light Intensity A pulse oximeter can only read light through a finger at certain intensities Excess amounts of light or not enough light going through to the photodetector will cause errors in the readings of the pulse oximeter Outside of this error range the pulse oximeter will not be able to read the light signal Light intensity through the core of the device needs to be in the proper range in order for the pulse oximeter to obtain a correct reading The material for the core was chosen such that 1t would approximately match the finger tone and would not be overly transparent While visually this material may look like it would give the proper oximeter reading it needed to be tested to determine if it would give the proper numerical results similar to that of a finger Not only did we need to test the material for the core but also the material with the dyes in the selected concentration These materials and solutions were tested without the pulse oximeter using a simple photodetector circuit Using two LEDs at the proper red and infrared wavelength and a photodetector we measured the light intensity through a finger the core and the core with dyes An exam
46. erial Gg plastic film light valve Multiple Multiple wae Simulate oxygen wedges SE be with known Database saturations position er oxygen variation content Artificial Disk Oxygenate blood lung oxygenator Means of supplyin Surroundin oxygen environment BEE Pressure Plunger Rotating Rotating generated Modulating Peristaltic Simulate pulse pump pump core disk alternating distance pump light signal Means of powering the Power Manual H 5 Batteries Wall outlet device supply labor The morphological chart provided insight to possible designs through the mixing of components between the previous designs Several different types of systems were developed based on this combination of functions The two main systems were the mock circulatory models and finger models The mock circulatory model usually consisted of a series of tubes with a pump attached to represent the flow of blood through the body with the pulse oximeter attached at a location to represent the finger The finger models used a device to represent change in blood volume within a finger only Three separate finger models were considered the liquid crystal light valve the pump model and the wedge model The electrical simulator was also considered in the numerical evaluation matrix An evaluation of the systems developed was completed in order to determine which could best fulfill the constraints and objectives The evaluation assigned numerical values between 0 100 to t
47. ervo attachment to stepper shaft wu A SS Figure A10 Cam walls with guide grove and shaft hole d Figure A9 Cam for linear actuation 71 Figure A12 Conical device sectional view and side view Figure A13 Device with probe attachment over core 12 sy TES E cp Ca D SE SC KE Figure A15 Full device top view with probe attached and circuit connections 73 Appendix B Code Section 1 Python is a language that has a very limited set of core functions and features However the language has an extensive library of modules which can be called to give Python an impressively robust set of tools The below section of the code is what calls all the additional modules which are necessary to run the motor fromTkinter import importtkMessageBox import serial import time importtkSimpleDialog from time import sleep Section 2 This section is the heart of the program This App class creates the user interface UI window the buttons within it and the functions which are called when the user clicks on any of the buttons class App BPM O max time 0 def n e Self master frame Frame master frame pack self button Button frame text Start command self halfstep self button pack side LEFT self button Button trame text Set BPM Comma o selt set BPM self button pack side RIGHT e button Button franc L xt set time Ccommand seLf set
48. ge This configuration 45 turns rotational motion into reciprocating motion The servo provides the rotating crank while the shaft connected to the conical model is the reciprocating slider This type of motor was chosen because it varies easily between two extremes rotating a total of 60 This is necessary to obtain pulsations within our system When the servo rotates between its extremes the conical device is pushed and pulled forward and backward This creates a pulse according to the pulse oximeter Since the device is cone shaped moving it backward and forward will decrease and increase the amount of liquid as seen by the pulse oximeter This is similar to the increase and decrease in volume within a finger during pulsations The servo can be programmed to allow pulses to be created at a speed between 30 250 BPM It also is possible to alter the rotations of the servo such that the subsequent movement of the conical device will create a waveform similar to that of a normal arterial blood pressure waveform The servo used in our device is an HS 77BB HiTec Low Profile Servo The servo has a running speed of 60 rotation in 0 17 sec at 5 0V The oxygenation as seen by the pulse oximeter is altered by rotating the conical device This is accomplished using a stepper motor The stepper motor can accommodate 360 of rotation with controllable step angles This will allow precise control over the change in oxygenation As the conical device rotates
49. ger tissue This orientation of wedges yields a continuous change in oxygen saturation values to the pulse oximeter Pulsations are created in this device by a rotating device that has openings spaced to allow the light to travel This does not allow for photoplethysmographic waveforms to be obtained as the light signal is temporarily blocked at some points by the rotating disk In other words the pulsation is not characteristic to in vivo pulsations 76 99 70 9 108 em EEE u ee oe ID UN eee EA ae UN wate VOI UI IO II UA IA UA IA we Oe UI Oe EE N OS Se A N N a b Figure 9 Wedges h d i formation a wedges of different sizes in a larger al b 10 Patent 5 278 627 by Aoyagi et al 1 describes that the device needs to provide highly repeatable and highly reliable signals to the pulse oximeter but notes that calibrations involving human fingers have poor repeatability making it difficult to judge whether the value measured is reliable or not He proposes a tissue model enclosing a blood model which is moved into or out of view of the pulse oximeter The blood model is moved rotationally on one axis and linearly on another axis to present pulsations and oxygen saturations respectively The motors are powered via a power supply that is stored within the 19 device The blood model in this patent uses a solid material compound that consists of an acrylic base with different fluorescent materials in varying ra
50. ger dock The center wall was redesigned to allow stepper attachment and was glued to the base plate and the stepper support to keep it from moving during Screw holes were added to the middle wall to secure the stepper during movement Figure 32 57 Figure 32 Center wall screw holes for stepper While the stepper has four screw holes only the top holes were used bottom screws would hinder servo rotation Additional supports were added on the outer finger dock wall A shaft support and new finger dock wall were designed as seen in Figure 33 a b Figure 33 Finger dock support a finger dock additional wall b All additional parts were constructed out of acrylic and manufactured using the laser cutter With these improvements the overall functionality of the device increased and results could be obtained The results in the following sections were obtained after the adjustments had been made 7 2 Motor Functionality Each motor was tested individually to determine if the output matched that of the user defined value The servo motor was run for two minutes and two team members counted out the beats This showed that the servo was within 4 BPM from the user defined value with higher deviations in the upper range with larger variations near the higher BPM range While our intended range was 30 250 BPM the servo did not have the capacity to produce over 150 BPM The motor itself had the ability to move 60 in 0 17
51. ges of the wedges This frame is attached to a motor shaft at one short side of the wedges This shaft is connected to a motor configuration that will move the wedges in order to simulate a pulse This configuration can be seen in Figure 16 Motor Shaft a 30 Pulse Oximeter P Motor Shaft Outer Frame b Figure 16 Outer frame and motor shaft a frame configuration attached to wedges b Two different types of motors would be used to move these two pieces of material One motor a servo motor will be used to simulate the arterial pulsations while another motor a stepper motor 1s used to move the wedges into the proper ratio of thicknesses to simulate an oxygenation level as seen by the pulse oximeter The servo is used to turn the device side to side on the bottom longitudinal axis of the wedges This motion would cause an increase in the path length while maintaining an equal ratio of thicknesses of the wedges relative to the light source An increased path length means more light will be absorbed and thus an ac component is being simulated to the pulse oximeter This is shown in the two diagrams in Figure 17 Figure 17 a shows the axis along which the wedge will turn and the direction of turning It also shows where the light source would be located with respect to the wedges Figure 17 b shows how the light path of the pulse oximeter light source is affected by the wedge turning Top View of Wedges P
52. he constraint objective and each was multiplied by a priority factor based on relative importance Each team member completed the evaluation independently and individual responses were averaged to give a final score for each category The results are displayed in Table 3 25 Table 3 Numerical evaluation matrix Design Contraints C and Objectives O Priority Mock Circulatory System Can perform initial calibrations for pulse oximeters Electrical Simulator Liquid Crystal Light Valve Wedge Model Pump Model A O O i AAA A A C Simulate pulse rate between 30 2508PM J C Simulate blood oxygen saturation between 70 100 C Complete within MQP time constraints Co EE AA ARA AS O Highlevelofaccuray OOOO zw Le Tom O Allows attachment of commercialy available pulse oximeters 000 20 2301 250 i AAA E A S s e 32 OIO O Ease of Maintenance clean assemble disassemble O Simulate blood oxygen saturations below 70 Totais 613 Kl XxX 2 mm The resulting score led the team to the conclusion that a model similar to the wedge model proposed by Yount would be the most viable calibrator A mock circulatory system design was also investigated The electrical simulator was not considered because the device is dependent on pre calibrated oxygenation values which are only specific to pulse oximeter models The liquid crystal light valve was eliminated due to complexi
53. he path length of the plastic will increase until only plastic is seen at 6 43 Plastic will then be the only absorber for the next 2 3 in the negative direction Solution low then begins to show an increase in path length and the plastic will show a decreasing path length for the next 2 81 in the negative direction Our design will only need to rotate 180 which is accounted for 168 46 6 43 2 3 2 81 180 The negative degrees represent rotation of the model in the clock wise direction A full 180 of the design will be identical to the remaining 180 from the design so that 1 looks the same as 179 The zero degree point is considered to be along the positive x direction and 180 is along the negative x direction with reference to the center point of the device 43 The above describes one half of the cone and the other half is the duplicate of this simply rotated 180 The light from the pulse oximeter goes through both sides to reach the photodiode This means that the ratio calculated from one side needs to be doubled when determining oxygenation The final design can be seen in Figure 24 A sketch of the smaller end is shown as well as an isometric view of the cone The length of the cone is 1 25 cm Figure 24 Sketch of new core design and CAD model isometric view In order for the core to perform the necessary functions it needs to hold liquid This functionality required that the device is completely enclosed Als
54. hosen such that the combined absorbance of the core and dyes were comparable to a human finger The motors and a graphic user interface GUI were programmed in the scripting language Python The GUI was created to allow user controlled variation in heart rate and SpO Device accuracy was measured against a portable pulse oximeter to determine how closely user entered values followed pulse oximeter readings Each heart rate and oxygenation test was conducted for four minutes Blood substitute dye solution tests involved the three most red absorbing dyes that were tested When the core was attached to the pulse oximeter the powdered drink solution Wild Berry Pomegranate proved most effective in simulating signals This powder based solution also allowed easy concentration adjustment Heart rate tests evaluated the motor s ability to produce a user defined heart rate in beats per minute as well as the pulse oximeter s ability to read these heart rate values correctly Test results showed that the servo motor was able to produce heart rate values within the range of 30 150 BPM within 4 BPM Motor specifications would not allow higher speeds and thus 150 160 BPM was the maximum range achievable Pulse oximeter readings of the heart rate were accurate to within 3 BPM although the device needed approximately10 seconds to stabilize to the correct value Oxygenation testing produced a trial with SpO values between 70 100 which settled near the i
55. ing 65 9 0 Conclusions Based on the experimental results derived several conclusions can be drawn Through extensive testing of the core with the two different pulse oximeters we saw a great deal of evidence supporting our theory that conical shape accurately reproducing the volumetric change of a human subject to a pulse oximeter and with a reasonable level of error less than 2 the pulse oximeter read the same heart rate values input by the user However the waveform created by the pulsations does still need some improvement Largely due to inadequacies in the program and issues with the probe attachment the wave form is only similar and not identical to that produced by a normal human subject The blood oxygenation functionality of the core also seems to work well but there were issues with maintaining the reproducible probe attachment and so the testing results would vary Results with the CMSSOF pulse oximeter typically followed a pattern in line with the increase or decrease we expected however the values were not exactly where they should be suggesting that the pulse oximeter might not be vertically aligned This indicates that the idea behind the core design seems to work as expected however due to design issues of the probe attachment site and other design flaws the oxygenation values were not reproducible or consistently accurate Overall we can conclude from our results that the theory governing the design of the core has great
56. intended oxygenation level can be compared to the oxygenation level read by the pulse oximeter The same can be done for pulse rate The SmartSat device can be seen in Figure 11 Figure 11 SmartSat by Clinical Dynamics 2 21 3 0 Design Approach The team identified the problem with current methods for calibration Then the team looked at what was achieved by the current methods and what should be achieved for a device that could replace these methods Finally several designs and their components were compared 3 1 Initial Client Statement Current pulse oximeter calibration techniques are expensive time consuming and limited risky for human volunteers The purpose of this project is to develop a pulse oximeter calibrator which reproduces clinical situations that can provide users with a simple inexpensive and effective tool to test the basic functional capabilities of a pulse oximeter and perform initial calibrations 3 2 Revised Client Statement Pulse oximeter calibration is expensive time consuming and risky for human volunteers The purpose of this project is to develop a simple efficient and cost effective way to initially calibrate and later test pulse oximeters without human volunteers The device created will simulate heart rate between 30 250 BPM and blood oxygenation between 70 100 in order to properly calibrate and test the basic functional capabilities of a pulse oximeter for medical use This calibration device
57. inutes after the trial began the pulse oximeter measured approximately 60 BPM which continued for the rest of the interval The volunteer was at rest prior and during testing and was told to breathe normally 62 255 Pulse bpm 25 Figure 39 CMS50F connected to Human 2 Several test trials were conducted for oxygenation however results varied a great deal between different trials and pulse oximeters The Masimo pulse oximeter could detect the heart rate but rarely displayed the correct oxygenation value If probe placement was exact certain values could be achieved for small intervals of time When the device was connected to the CMS50F oxygenation values were detected however the oxygenation values were inconsistent at different speeds Oxygenation values would rise and fall appropriately with the given direction when turning the device however the degree of the rise and fall was different such that 1t was difficult to return to a specific oxygenation value Closer inspection revealed that the linear actuation of the core would cause the core to shift left or right during the diastole end of pulsation This meant that the light path was not going through the core in the center during all points in the pulsation but rather off to one side at different points With the given time and resources this mechanical problem could not be resolved Given these problems reproducible results could not be obtained Only one testing run showed p
58. ions below were used in defining this design The first spiral is defined by Equations 16 18 and the second spiral is defined by Equations 19 21 t x Rinner cos t ar ThicknesSpartition Rivne cos t Carers Eq 16 i R t y Rinner sin t T Roues 7 ThicknesSpartition gt Rinner S sin t T raras Eq 17 0 lt t lt 168 463 Eq 18 x Rinner T ThickneSSpartition E cos t T Routes ThickneSSpartition z Rima X cos t Ea Eq 19 t x Rinner ES ThicknesSSpartition sin t T Router E ThicknesSpartition e Rimer S sin t E Cae Eq 20 0 lt t lt 168 463 Eq 21 This gives a fixed thickness of the spiral partition throughout the cone which was set at 0 05 cm Rinner Was chosen to remain at 0 25 cm and Route variation remained the same 0 534 cm lt Router S 0 55 cm With a fixed partition thickness the outside radius thickness also remained fixed at 0 05 cm This new design is less complex to design making it easier to change the dimensions The straight partitions no longer need to flair outward from the inner to outer radius and now can stay parallel with each other and 0 05 cm apart offset by 180 Rotating the core the 168 463 of the spiral will give a linear change in the saturation percentage from that of solution high to solution low as a function of the incident light angle through the center From 0 to 6 43 solution high will have a decreasing path length and t
59. kness 0 635 cm Length 6 5 cm Diameter 0 317 cm Plexiglas 5 715cm tube Outer diameter of 0 75 in and Wall thickness of 0 0625 in Polyjet process Fullcure 720 material 2 copies 13 mm in diameter SV logic levels 12V stepper supply Programming power for Microcontroller 31 94 Servo City 18 89 Sparkfun Electronics 14 99 Sparkfun Electronics 28 89 Sparkfun Electronics 5 00 Plastics Unlimited Plastics Unlimited N A N A 1 04 5 94 60 00 2 08 N A Z Fi gt N A 51 6 0 Experimentation 6 1 Dye selection The blood component is represented in our design by two solutions one which represents 100 oxygenated blood and one which represents 70 oxygenated blood The saturation as seen by the pulse oximeter is varied by rotating the devices such that the light from the pulse oximeter s LEDs will pass through each section with a specified path length and is thus a function of the core s angular position Fully oxygenated blood absorbs less than fully deoxygenated blood in the red light spectrum and absorbs slightly more in the infrared spectrum The normalized ratio of change in red absorbance to change in infrared absorbance is measured by the pulse oximeter and compared to the pre calibrated oxygenation values associated with this ratio value Thus the oxygenation can be most easily controlled through changing the amount of red light that is absorbed This change i
60. lates flow through the finger model The pulse oximeter then attaches around this finger model Results of experiments showed that the presence of elevated HbCO caused an overestimation in Sp The model was shown to allow controlled and reproducible SpO measurements Light Emitting f Diodes Disk Oxygenator w k kl Hollow Channels hi Compliance Blood In Device Finger Model Photodetector Blood Sampling Port a b Figure 4 Kent and Mendelson Finger model a mock circulatory system b 5 Oura et al created a mock circulatory system that included arterial and venous circulation in parallel 7 This model was developed to produce an in vitro pulsatile optical density ratio that would agree with the in vivo pulsatile optical density ratio They reported that single layer pulsation flow cells show a discrepancy in pulsatile optical density ratios when at the same oxygen saturation This model was designed to be compact and has dimensions of 60 cm x 40 cm The arterial circulation part of the system includes a compliance chamber reservoir chamber clamp pumping unit and artificial lung This circuit is able to produce pulsatile waveforms that are similar to aortic pressure 120 80 mmHg and flow waveforms and the heart rate was set to 60 BPM The venous system contained a centrifugal pump reservoir chamber and artificial lung Flow in this circuit was steady and the pressure was set to 0 mmHg 16 The f
61. lfill necessary specifications Bane Ve a Ekel PERHEEN Determine pricing and suppliers for components PEREN AE a Wel PERHEEN Decide on what final components will be used I m iii SEEREN Complete preliminary testing of oxygenation and pulse rate simulation Complete CAD design of major components and full model EEEN BEHBEEBBBBERHEER PERHEEN Order and receive components PEHEN PERHEEN Machine components III il Determine testing procedures Program the motor systems PEREN III PERHEEN Assemble attachment system bearing block finger PEREN a PERHEEN Assemble core component and linkages Assemble motor system and electrical components Begin testing device P ft tt ft dt td Fix problems found in testing WESSEN DE ARES SER ERR sim riwirlelsIsimlriwirlels simfr wlalelsis mir wirlels simfr wlalels s mlrfwirlels simfriwlelelsis mirlwirlels 24 25 26 27 26 29 30 33 12 1314 15 15 7 6 5 30 a 12 13 14 15 6 27 18 19 20 24 22 23 24 25 26 27 26 29 20 1 2 3 4 5 6 7 5 0 November E Table D2 Gantt Chart C term C Term Week0 Weeki Week2 Week3 Week4 Week5 Week6 week Task Program Motors Create GUI Select proper dye combination Order and receive dye Pt tT tet tT tT TT TT TT O EN Pt tT ttt tT yt tre EET TT TT Manufacture base components Pt tT tT tt A ads Pt tT E ty tt T Manufacture and receive core component HH HH HH HH EE EEE
62. llow rotation To achieve the small amount of movement by the core the linkage between the shaft and the servo needed to be 2 cm in length This size was too small to easily attach 1t to both the servo and the shaft The original CAD model also had the servo wheel on the one side of the motor but the servo can only move within a certain degree range and needed to be on the other side to allow this rotation Overall the linkage system created had many problems and could only achieve pulsations within 30 of servo rotation This was not acceptable and needed to be improved A new linkage system was designed to overcome this problem Instead of using a crank slider a cam was designed to allow 0 6 cm of shaft movement for 90 of servo rotation The amount of space within the walls and the necessary volumetric change determined the amount of linear movement while the servo specifications determined the degree value This design can be seen in Figure 31 56 b Figure 31 New linkage using cam attached to servo wheel top view a side view b In this set up the cam is attached to the servo wheel in order to rotate between two positions The start position and end positions are indicated by the lines etched into the cam the smaller line is the start while the larger line is the end This forms a 90 angle the allowable rotation for the servo motor When the cam rotates 1t compresses and releases a plate Between the plate and the wall is
63. low from these two systems meets in the middle within a double layered pulsation flow cell This flow cell consists of two elastic diaphragms that move in response to the pulsations created in the arterial side of the system Figure 5 shows a diagram for the double layer flow cell and Figure 6 shows the complete circulatory system with arrows indicating the direction of the flow The pulse oximeter would attach around the flow cell Photo diode Arterial Diaphragm War Arterial blood Venous blood Venous Diaphragm LED Figure 5 Diagram of double flow cell 7 Centrifugal Blood Pump KK E ie A Venous 0 E KS 2o Circulation ZEN WW td d 2 S BT Reserve Chamber Blood Pump Compliance Chamber Arte Circulation Figure 6 Complete setup of the mock circulatory system by Oura et al 7 An alternative iteration of the mock circulatory model was developed by Reynolds et al 8 which included a slightly altered flow path to allow for easier pulse oximeter probe attachment around a finger shaped silicone rubber membrane Figure 7 shows the finger model inner flow system and Figure 9 shows a diagram of the circulatory system 17 gas supply reservoir oximeter probe direction of personal computer bypass line le port i blood e samp licone rf Perspex Perspex a computer _ model finger blood in view of cuvette contralled from above pump head pulse oximeter
64. mming our motors Firstly the actual functional code for the motor drivers involves simply printing text commands to a serial port Python can do this in one line of code after importing the serial module a task requiring only one more line of code However to accomplish the same task in LabView one would need several blocks connected correctly Another issue is the amount of times the code needs to be edited revised or almost completely redone With python this is a simple matter of changing around a few lines deleting a section of code or moving a line to a new spot to change its functionality This simple editing is more complex and time consuming in LabView Complete revision of the code would require the 48 deletion of some of the blocks used previously and their replacement by new blocks with different functionality To give more specific examples of Python s functional diversity the current build of the program uses three modules They are Tkinter Serial and Time Tkinter is a module for python made specifically for the purpose of creating a Ul or User Interface also referred to as a GUI or Graphic User Interface It adds functions that allow python to create a window and place things within that window such as message bars menu bars buttons and dialogue boxes It also allows the program to make pop up dialogue boxes such as error messages and make requests for user input Serial 1s a module which allo
65. motor movement and programming Obtain proper speed and position o Dyes acquisition and testing Produce desired light absorption o Light intensity Neither too low nor too high to obtain results Light intensity o LEDs and photodetector circuit o Match dye and core absorbance with finger absorbance Stepper motor o Program and test proper speed and position Servo motor o Program and test proper speed Pulse Oximeter Testing o Shows appropriate oxygen saturation o Shows proper BPM Final MQP Report Final MQP Presentation User Manual 80 Table D1 Gantt Chart B term B Term Task Week3 Week4 Week5 Week6 week Weeks Evaluate alternative designs PE te IN fe Establish specifications for oxygenation rotation path length wedge partitions HEN Dave Establish specifications for pulse concial shape displacement movement HEHE MOMO OOOO BB EBENE Heidi Establish method of locomotion in out rotational EA Dave Heidi Establish component orientation and fixation Matt Heidi Develop a complete bill of materials Matt Establish exact specifications for components RER Pt tt OA IMatt Dave Determine material for finger casing EEREN MOMO TITT al ars tm EC GE CDI Determine devices for motion system Find components that fu
66. n absorbance is caused by pulsations which effectively increase the path length of the blood component in the finger and is mimicked by the stepper motor in our device The team looked at a variety of commercially available dyes that could be mixed with a solvent for representing each component in the device However these dyes were available only in increments of one milligram and one gram would cost anywhere from 50 190 depending on the supplier This cost was more than the team had left in available funds so an alternative was sought Everyday products such as food coloring beverages and powdered drink mix were purchased and tested with a spectrometer to determine absorbance Once the absorbance of the solution was known the extinction coefficient could be determined using the path length of the spectrometer cuvette and the concentration of the solution The purchased dyes were mixed with water to obtain a concentration within the spectrometer scale The spectrometer can only measure the absorbance of a dye solution accurately when the absorbance is between 0 2 and 1 0 absorption units The volume of food coloring was measured and the powdered drink mix was weighed before each was added to individual samples of a known volume of water After all the testing was complete the team found that there were three dye solutions that could be used Powdered drink mix concentrations were measured to avoid saturation The other food color solutions that wer
67. n content and arterial pulsation simulation Finally in order to allow pulse oximeter attachment while also moving the wedges the wedge portion of the device will be enclosed within a casing This case would be made out of Delrin rubber silicone or a similar material that has optical properties which mimic human tissue 1 The case and inner wedges would be small and thin enough to allow it to fit within a transmission oximeter probe If a rubber silicone material is used the casing would be slightly flexible as well to allow better shape confirmation The material would be semi transparent to allow the light to pass through the device This can be seen in Figure 19 In this figure the casing has been made fully transparent such that you can see the wedges inside Figure 19 Pulse oximeter probe attachment to outer semi transparent casing 32 All of the positions and motion of the device would potentially be controlled with a programmable intuitive user interface The motors would be controlled by control circuit which would include a microcontroller that could be programmed using C or another device interface programs Lab View is one program often used for biomedical applications that could be utilized for this device The position of the wedges within the pulse oximeter probe and the speed of turning to simulate pulsations would be set by the user before each use of the device An alternative variation of the design would use hollo
68. n the ratio of blood substitutes within the light emitted and received by the pulse oximeter Much of our project was spent in developing the concept design and testing of a prototype for this idea It is our hope that this device and project will aid in future investigations regarding the production of an alternative solution to the current methods of pulse oximeter calibration 12 2 0 Literature Review The team performed a thorough literature involving the science and techniques employed by pulse oximeters Then the team reviewed previous designs for the calibration of pulse oximeters 2 1 Oximetry Oximetry is the optical measurement of HbO content in the blood The field of oximetry takes into account the difference in optical absorption of HbO and Hb at specific light wavelengths in order to measure the oxygen saturation in blood The maximum difference in light absorption occurs at red light at 660nm as seen in Figure The ratio of absorbance at 660 nm to the absorbance at a known wavelength in the infrared spectrum between 880 940nm can be used to determine the saturation of HbO in the blood Often oximetry uses a number of wavelengths to determine the oxygen content in blood which is more accurate because other hemoglobin variants can then be detected such as MetHb and HbCO The absorbance spectrum for MetHb and HbCO is displayed along with the absorbance spectrum for Hb and HbO in Figure 1 MetHb y A AR TT d SS
69. nkage from the servo to the core shaft was laser cut as well It was essential to obtain the correct height of the servo motor to maintain alignment of the shafts The resolution in the laser cuter was specified to be 0 005 Each piece was designed to slip fit into the other base pieces however due to the laser beam width and the resolution not all the pieces were as tight as originally designed and needed to be adhered to be level and maintain stability The notches within the finger were cut using a vertical mill The finger 47 can then slip fit into the outer wall to allow pulse oximeter probe attachment and guide movement of the core during pulsations The shaft connected to the core was originally a threaded rod and the threads were removed using a lathe This was done in order to maintain a standard thread size and 1t was simpler than threading a bare rod 5 5 Programming Due to the nature of our device and the ways in which it will be used it is necessary to have a way to easily facilitate repeated and consistently reproducible movements For this reason it is important to have some sort of simple user interface which would allow users to consistently produce the same functional results with minimal effort and technical knowledge of the motor systems The program used to control the device was tasked with performing three basic functions First it had to control two motors a servo motor and a stepper motor Second it had to all
70. nnnnnnnnnnnnnnnnnnnononononnnanannnnnnnnss 31 Table 5 Spectrometer results and calculated extinction coefficient neneessssssseeeeeeeeeresssssssssssssseeeereee 53 Table DE Gantt Chart tC sustento cidcid caigas 81 Table De CHE CHI LEE ii aca A AAA a ee ee 81 Table DI Gantt Chant DIT tiesa ee 81 Executive Summary Pulse oximeters are standard medical devices used for continuous patient monitoring of arterial oxygen saturation SpO and heart rate HR Initial calibration of pulse oximeters is conducted by inducing hypoxemia in healthy human volunteers to obtain blood oxygenation levels as low as 70 and comparing pulse oximeter measurements to that of a reference CO oximeter These data are used to determine calibration coefficients and create a look up table that is programmed into the pulse oximeter This method of calibration is expensive and time consuming for pulse oximeter manufacturers and it is required by the FDA in order to bring a new pulse oximeter to market Eliminating the human element of calibration would reduce overall cost and increase efficiency To eliminate the need for human volunteers a calibration system was developed to simulate blood oxygenation levels between 70 100 and heart rate readings between 30 150 BPM This programmable motorized dual axis system was designed to provide certain calibration values as specified by the user through a computer interface Tests conducted with a portable pulse oximeter display
71. ns with the user defined parameters set time Set BPM v Reset Moves the stepper motor back to original location before Rotate Stepper command 82 b Warnings 1 11 After each usage you MUST reset the stepper motor using the reset button before inputting a new degree amount If the stepper 1s not reset 1t will not be able to return to the O degrees Extended use of the System can cause the stepper motor and stepper motor driver to become very hot It is recommended that continuous use not exceed 30 minutes This can also be avoided by turning off the power between uses 83
72. ntended value but varied within 4 over time The oxygenation displayed was observed to fluctuate anywhere between 0 100 given alterations in finger dock to probe attachment probe alignment and wall stabilization The generated photoplethysmogram PPG waveform by the calibrator is similar to a physiological waveform The PPG waveform was created by programming in wait times as opposed to adjusting motor speeds This caused slight differences between the programmed and physiological waveforms and made higher BPMs unachievable 10 Oxygenation tests revealed that the calibrator was sensitive to movement Oxygenation values also depend greatly on maintaining proper probe placement on the finger dock The position of the probe relative to the core is critical to obtaining correct values and the PPG waveform Overall accuracy increased when a wait time of 30 seconds was added between consecutive tests The current design did not meet original design specifications However the calibration device helped to verify the theoretical concept behind the core design Testing revealed possible improvements to the design to increase accuracy and reproducibility 11 1 0 Introduction Pulse oximeters are standard for continuous patient monitoring of arterial oxygen saturation and heart rate in a clinical setting The current method for the calibration of this device involves inducing hypoxemia upon human subjects and comparing pulse oximeter measurement
73. o there needs to be a way to fill the partitions with the proper oxygenated liquid Due to the size of the device it was determined that the only way to fill the device would be using a needle and syringe It would be very difficult to have a cap that screws on or snaps on given the size Due to these constraints a septum closure will be used over holes in the cap The proper liquid can be injected into the core through the septum The device also needed a method of attachment to the rest of the device The easiest mechanism for attachment in order to keep the device secure would be a screwing mechanism The base includes a tapped hole that will screw onto the main device The CAD model of this conical device with the cap and bottom piece is shown in Figure 25 The cap is shown with the holes for fluid injection The bottom portion is attached and the screw hole is shown without the tapered edges 44 Figure 25 Conical assembly with cap and bottom piece 5 2 Core Manufacturing The core needed to be 3D printed due to the conical shape It would be extremely difficult to use a standard machine as 1t would need three degrees of freedom to produce the tapered internal features The small size of the device also limited the type of 3D processes that could be used Machines at WPI were not equipped to deal with our small cone and because of this found an outside company to produce the core In the first attempt to print the device the team use
74. ow for user defined values to be inputted for the Beats per Minute Running time and Stepper degree orientation Finally the third function called for both of the above to be accessible through a clearly defined and intuitive user interface The aforementioned program is written using Python The reasons for this are twofold Python is a very minimalistic language By itself it has only a very limited number of core functionalities Its strength lies in its ability to utilize a vast library of plug in modules which give it increased functionality This means that a program written in python will tend to be both smaller and faster than one written in a more robust language that has a great number of functionalities constantly present The second reason for choosing Python is simply that it was a language which was already familiar to the team This meant that the time spent learning the language would be minimal and work could be conducted more efficiently When programming medical devices t has become somewhat of an industry standard to use at least in part the block language LabView The team decided against using LabView for multiple reasons While LabView boasts a simplistic and understandable editor for non programmers it has a base level of complexity which cannot be reduced Python by nature is a language that was built to be very bare bones with infinite potential for features to be added onto it It is this nature that made it idea for progra
75. piled into an executable file that can be opened by the user This file can be used after the install drivers have been downloaded to the computer The program would need to be altered depending on the USB port connected to the Microcontroller An additional feature would allow the program to function no matter which port the USB is plugged into Overall the function of the GUI is simple and intuitive Several people outside of the project group have been able to use the interface without any problems 50 5 6 Bill of Materials Suppliers HS 77BB Low Profile Servo Stepper Motor with Cable ROB 09238 EasyDriver Stepper Motor Driver ROB 09402 USB Bit Whacker PIC 18F2553 Development Board Base Stepper attachment Cam components Core Shaft Finger dock Conical device Septum Closure Power supply Program Computer Speed 0 14 sec 60 running at 6 0V Size 1 73 x 0 90 x 0 98 in Step Angle 1 8 Rated voltage 12V Rated current 0 33A Size 1 67x1 67x1 34 in Shaft size 0 945 in 5 mm diameter Microstepping resolution to full half quarter and eighth steps Adjustable current control from 150mA phase to 750mA phase Power supply range from 7V to 30V Size 2 x 4 8 cm 16 General I O Preprogrammed with Boot loader and UBW_D firmware Size 2 54 x 4 064 cm Acrylic Thickness 0 5334cm Base Size 16 523 x 14 355 cm Acrylic Size 2 96 x 2 20 cm Thickness 0 5334cm Acrylic Thic
76. ple of this circuit is shown in the Figure 29 R1 100 Q Vg FUNCTION INFRARED PHOTODIODE GENERATOR LED LTE 302 LTR 516AD Figure 29 Example photodetector circuit to measure light intensity The material being tested was placed between the LED and the photodiode similar to that of a pulse oximeter A guide tube was placed between the LED and the photodiode to keep the finger in the same place during each test The input and output of the circuit was measured by an oscilloscope to determine the amount of light is going to the photodetector This experiment was conducting with multiple fingers of different skin tones to get an average result Then the finger result was compared to that of the core with and without the dyes chosen for the device Since only a small amount of light will be going to the 54 photodiode through a finger the gain at the output stage was increased to detect the light This is why the circuit above is only an example the gain stage was adjusted by altering the IMQ resistor to a 2MQ resistor The results of this test showed that using high concentrations of dyes would provide absorbance that was similar to a finger However the team found that the concentration was limited by the amount of powder mix that could be dissolved The team chose to use the highest concentration of the wild berry pomegranate solution that would be allowed without saturation 6 3 Device Testing Testing of the device first involve
77. pleDialog askfloat Degree Please enter degree print Step 1 Complete ser serial Serial 3 Opens COM port 4 Ser write reir Ffresets device ser write C 0 0 0 0 2 ser write 0 0 0 0 2 Ser writel T 2 2 1 2 ser write F 0 r ser write r r print step 2 complete sleep float self Degree 3 65 somehow because of the timing of the motor it ser write C 0 0 0 0 Ser write O 0 0 0 15 ser Write E 0 ser write r r ser write ee 75 ser close Section 5 This is the reset function It recalls the last user defined degree of rotation for the stepper and move the motor back that exact amount Unfortunately this code does not remember how far the motor has turned in total therefore 1t must be used after every rotation of the stepper to reset 1t to the original position def reset self print reset ser serial Serial 3 sleep 1 Sor writes UE 00 DAD sleep 1 Ser write 0 0 0 0 7 sleep 1 ser write RC A 1 100 ser write RC A 1 100 ser write RC A 1 100 sleep 1 Ser wrice RC A 1 0 sleep 1 Pee RC A 0 10 BESA Oy LO RCA Oy LOT ser write ser write ser write ser write serverite 1 2 ser write RC 1 0 ser close sleep float self Degree 000000000001 ser serial Serial 3 Opens COM port 4 ser write r r resets device ser write 0 000 052 ser write 0 0 0 0 z2 Se
78. r write RC A 1 0 ser write r r ser close ser serial Serial 3 sleep 1 ser write 0 0 0 0 01 2 sleep 1 ser write 0 0 0 0 ne sleep 1 ser write RC A 1 100 ser weite RC A 1 100 ser write RC A 1 100 sleep 1 ser write RC Ay 1 0 sleep 1 ser write ser write ser write ser write SODA WETLTS MEAT serra ORE Aa 120 ser close sleep float self Degree 00000000001 110 04 ser serial Serial 3 Opens COM port 4 ser write r r resets device Ee Ee ME DO ON Serner ite 0 00 207 Sero wiiee URC Ae 0 VENEN RCA MRE Ay Oy LO CRC AGO E Sser write re ser close 71 Appendix C Circuit Diagram Logic Levels Ground Rail 5V Rail Pin BO Servo Variable current approx 330mA rd 7 D e x LI o e A A E Au A ae a p EN Tag Stepper Step u t gt ur Stepper Direction Stepper Power Ground Rail 12V Rail Ban Se 4 p yo KA SN u D x i gt y E gt gt ew rid A A MATA A e D We 4 A Lo I lt Pin Al Stepper Step Pin AO Stepper Direction Stepper Wires A Red Green B Yellow Blue 78 Appendix D Schedule and Timeline of Project Tasks The team outlined the tasks for each term over which the project extends and estimated time frames within which these tasks were completed The
79. r with optical properties similar to fully oxygenated hemoglobin These two values for oxygenation are chosen because only two wedges can produce any oxygenation value between 0 100 The wedges are placed on top of each other and the pulse oximeter probe is placed such that light emitted goes through the top flat portion of the top wedge and the sensor is located on the bottom flat portion of the bottom wedge The oxygen saturation is then equal to the ratio of thickness of the 100 oxygenated material and the 0 oxygenated materials This can be seen in Figure 14 the light path of the pulse 29 oximeter probe goes through the middle of both wedges such that the oxygenation 1s a ratio of the width of the two wedges Figure 14 Wedges of 0 and 100 oxygenation with pulse oximeter probe attached showing light path position and direction through wedges Figure 15 more explicitly demonstrates the light path through the wedges It would go through the middle of the two wedges such that the edges and sides are not within the light path 0 O 100 O 100 O a b Figure 15 Light path arrow through wedges Right side up view a upside down view b The wedges simulate the oxygen saturation and movement of the wedges simulates the arterial pulsations The wedges are placed in a frame that surrounds the outer edge of the bottom and short sides of the wedges This frame would be unseen by the pulse oximeter as 1t holds only the ed
80. racy and possible improvements The overall assessment was that heart rate could be reproducible but depended on the pulse oximeter used and could be improved upon with alterations in servo speed Oxygenation could also be achieved 1f probe placement was verified and mechanical stability could be achieved Testing results verified the theory behind the core design since simulated signals could be achieved 64 8 0 Discussion From the results 1t can be seen that our device did meet most original specifications The heart rate could be simulated between 30 160 BPM and the oxygenation could be simulated between 70 100 These values could be easily varied by changing the programming or concentrations of the dyes in the core Both the CMS50F and the Masimo pulse oximeter could simulate a steady heart rate over two minute intervals The results of the CMSSOF pulse oximeter indicate that this method of calibration can controllably simulate different oxygenation levels The PPG waveform of the device as measured by the CMSSOF shows many similarities to the human PPG waveform seen in Figure 42 Figure 42 Human PPG waveform Both show a one third rise time and two thirds fall time along with a dichotic notch While this 1s not necessary for the calibration of a pulse oximeter it does show the accuracy of the simulated signal The PPG waveform of the device when connected with the Masimo pulse oximeter showed a signal that appeared to be inverted
81. romising results as seen in Figure 40 This run was done after taking a large amount of time to adjust the pulse oximeter probe Based on the position of the core the user specified value was visually estimated to verify oxygenation The team created a reference paper that could aid the visual verification of the oxygenation value Furthermore the team found it impossible to make correlations between the tests due to the high sensitivity of probe placement 63 Deviation from Oxygenation CMS50F ei O a c 2 Fe E gt 0 O 75 80 85 90 95 Average of Reference Oxygenation and Specified Oxygenation SpO Figure 40 Oxygenation Deviation with CMS50F Pulse Oximeter Given the lack of reproducible oxygenation results statistical analysis could not be properly conducted Analysis was done using the data above Figure 41 shows the measured oxygenation that was output by the device versus the specified oxygenation in blue The red line indicates the identity A linear regression was performed and is displayed in the graph by the black line The standard deviation is also shown for the oxygenation values the standard deviation from the user specified value was 1 632 Specified vs Measured Oxygenation Specified Oxygenation y 0 9467x 3 7626 R 0 9571 80 85 95 Oxygenation Figure 41 Specified oxygenation value vs measured oxygenation Qualitative assessments were made to determine approximate accu
82. s to that of a CO oximeter This data 1s used to determine calibration coefficients and create a look up table that is programmed into the pulse oximeter FDA regulations require this calibration method for pulse oximeter approval However calibration with human subjects can only be done between 70 100 Inducing hypoxemia at the lower end of this range is potentially dangerous to the test subjects while oxygenation levels below 70 are considered too dangerous to be conducted This makes such a calibration process both costly to the company producing the device and inaccurate below 70 since the oxygenation data can only be determined through extrapolation It was the goal of this project to create a device which would increase the accuracy of the calibration process and potentially set the stage for the elimination of the need for human volunteers Such a device would also significantly reduce cost and time for companies attempting to bring a new model of pulse oximeter to market A device such as this even if it is not used as a substitute for testing on humans would allow the company the opportunity to test their device before making the expensive commitments involved with FDA testing We propose to do this by creating a device which uses a motorized dual axis system to adjust the positioning of a core component and thereby altering oxygenation and creating a pulsing signal The core consists of two partitions that respond to rotation with a change i
83. sec at SV however this specification refers to no load on the servo and does not account for the speed of commands from the program The stepper motor was tested at 5 10 30 90 and 180 increments This rotational motion needed to translate through the mechanisms of the device all the way to the core to change the oxygenation Because of this rotation was also measured at the shaft end The stepper motor shaft moved the proper increment amount during each test The core shaft however lost rotation during the middle 90 of the 58 necessary165 rotation The shaft successfully turned the proper amount for the 90 and 180 increment tests Smaller increments had errors once the rotation had reached approximately 35 When this point was reached approximately 1 4 of rotation would be lost with each increment Larger increments would have a smaller deviation while smaller increments would have a larger deviation During these tests the shaft would usually fall to 4 behind stepper shaft rotation The reset button would return the stepper to the previous position after a rotation increment Identical errors were seen when the stepper moved in the opposite direction These errors can be accounted for due to the cam plate connection To have ideal rotation the grove needed to be a tight fit over the cam which was not the case The grove for cam movement could only be a certain size to allow both cam rotation and shaft rotation
84. sorption coefficient is the path length and c is the concentration This can be related to the optical density of a material by Equation 5 eralc Eq 4 Io A aLC In T Eq 5 If the tissue 1s composed of multiple absorbers the total absorbance of the material 1s the concentration times the absorption coefficient all multiplied by the total path length of the material Equation 6 describes the total absorbance of a medium with multiple absorbers with ly indicated the total path length through both material one and material two Al Cy lC 04 Cy AC lr Eq 6 Within a finger the measured path length through the material is not constant the total path length changes over time due to pulsations but keeps a constant baseline amount For this reason a normalization of the signal is required This is represented in Equation 7 where l indicates the smallest path length associated with the unvarying DC component 38 se air n Ipxe 4161 2C2 lo D e 41C1 42C2 Al Eq 7 The normalized function for the light path is independent of the light intensity from the pulse oximeter s LEDs and the non linearities associated with the photodiode emitting the light To determine the saturation of the LEDs the pulse oximeter relates the normalized absorbance of the two wavelengths used in what is called an R ratio described in Equation 8 R Bed Eq 8 Alnfrared When the pulse oximeter is calibrated the R
85. t 145 t x Rinner cos t Router Rinner cos t GE Eq 10 f t y Rinner Sin t Router Rinner sin t Eech Eq 11 This first set of equations created a line but since the partitions needed to have width a second set of equations was created This second spiral is defined by Equations 12 and 13 which when combined with the first spiral line gives created the partition This curve started at 24 167 and ended at 169 167 such that 24 167 lt t lt 169 167 t x 5 cos t Rinner cos t Router Rinner cos t e Eq 12 y t y 5 sin t Rinner Sin t Router Rinner Sin t Gre Eq 13 The equations were defined using the Rinner and Router Which made the equation easy to change to fit the small end of the cone as well as the large end of the cone The thickness of the spiral partition had to vary so that the saturation remained the same during pulsations With a fixed thickness the angle at which the second spiral would begin was not the same for the two different outer radii the smaller bottom in comparison to the larger top The thickness of the spiral partition was defined by Equation 14 and was based on the angle at which the second spiral starts 24 167 The remaining 169 23 not taken up by the 41 straight partitions were made up of two 24 167 ramp increases of partition thickness and the actual range of changing saturations 120 833 145 24
86. t 1s a function of the angle of rotation over the range of possible rotation where solution two is in the circular wedges that has no externally faced sides and solution 1 is in the circular wedge that has one externally faced side 40 The first design developed for the inner partitions had a range of rotation of 120 833 145 24 167 will be shown later The partition width was 0 05 cm which resulted from the requirement of small size while retaining mechanical stability The reason for choosing a small thickness was to allow for a greater angle over which the path length ratio of solution high to solution low could change A greater angle would make the device more accurate The outer limit was approximately the size of a finger 1 2 cm in diameter The inner hollow circle was 0 5 cm in diameter Two partitions extend from the hollow inner radius to the outer radius laying 180 degrees apart Each was 0 0467 cm thick at the junction with the inner radius This partition thickness reduced the angle over which we could change the saturation from 180 to 169 23 because the wall thickness takes up 10 77 Equation 9 shows the angle taken up by the thickness 1 467 0 sin 2 5 10 77 Eq 9 A partition would also extend from the inner radius to the outer radius as the device was rotated This would give a spiral shape that is defined by Equations 10 and 11 which started the curve at 0 and ended it at 145 such that 0 lt t l
87. ta can be simulated 2 Usable for both transmission and reflectance oximeters 3 Accounts for dysfunctional hemoglobin 4 Allows alteration of calibration standards 25 3 5 Component Comparison A detailed description of many current calibration and simulation devices found during the team s literature review was provided in an earlier section This section of the paper compares the models and the components that have been utilized for each function Table 1 determines the success of each design with respect to our developed requirements and objectives Table 1 Model design requirement comparison Kent and Mendelson Oura et al Criteria in vitro tissue dual flow Reynolds et al oe ou Sal model cell ball pump wedges finger model Control ea iome madal Odie EISE Ce S Gas supply Gas supply Gas supply simulate different saturation remains same wedges oxygenations Saturation Aia o 0 100 0 100 0 100 ie can be simulated min max continuous not continuous measurements Pulsation Shows natural May not mimic Yes Yes No No waveform photoplethysmogram natural waveform Pulsation rate Determined Determined by Can be varied but is 8 Determined by motor min max by motor motor not accurate Kate pulsation Decided by e Changeable 35 Standard of systole 7OBPM 2 Can be Canbechansed Decided by fluid fluid filled material used to data blood per volume changed filled model dais represent oxygen 41 hematocrit
88. ted as SpO in the blood by comparing a normalized ratio of absorption at each of the two wavelengths In this device the ratio of light absorption was controlled by using two different concentrations of a red absorbing dye solution The two solutions were separated by rounded partitions such that the volumetric ratio of the different concentrate solutions would change linearly as the device is rotated within the sensor light path Heart rate values are simulated through linear actuation of the device A servo motor rotates a cam which compresses a plate attached to the core This produces a change in volume within the light path of the pulse oximeter that resembles a blood pressure waveform Oxygenation values are changed by rotating the core using a stepper motor A 5 5 rotation of the core corresponds to a 1 change in oxygenation A finger dock allows for the attachment of a pulse oximeter probe while the core moves internally The core creates a 1 volume change when moved 0 6 cm The absorbencies of various dye solutions for use in the core were analyzed using a spectrophotometer Concentrations of these solutions were measured and a portion of the solution was placed in a standard cuvette The extinction coefficient was then determined using Beer Lambert s Law Using a custom designed light intensity circuit the absorbance of the human finger was compared to the absorbance of the core filled with the dye solutions Final concentrations were c
89. tio to create an absorbance and reflectance that 1s characteristic to differing oxygen saturation levels However the device 1s in the number of different oxygen saturations that 1t can represent The device includes a finger shaped dock for the pulse oximeter probe to attach to and can be seen in Figure 10 In his patent he notes that calibrations have been done electronically without the use of a measuring probe DEE LLAR Figure 10 Finger model with in and out motion a enclosed finger model with in and out motion b finger model with rotating disk motion c 1 2 3 3 Electrical Simulators Electric signal simulators were lastly considered for the very attractive quality of minimal parts and no liquid or material to simulate blood light absorption Electric simulators calibrators essentially perform like a reverse pulse oximeters A sensor takes in the light emitted by the pulse oximeter and transmits a properly altered signal to the pulse oximeter sensor using its own LEDs The altered signal would correspond to a set value for light absorption by a specific oxygen saturation and pulse rate as found in a database of a pulse oximeter Certain models have the ability to test specific pulse oximeter models as 20 they contain the databases from these specific devices The SmartSat made by Clinical Dynamics contains over 10 different device databases to look up the proper oxygenation level 2 Once inputted into the system the
90. tion it is a simple task to identify the source of the problem since the break will only happen when that function is called by its corresponding button The servo motor is responsible for creating the rapid pulsations which are read by the pulse oximeter as a heartbeat The program accomplishes these motions by sending the motor a series of text commands through the virtual serial port The board which we are using interprets text commands which are defined 49 by the board s firmware The servo is commanded using the format RC port letter Pin Number Position number The program we are using works by printing these text commands to the serial port in a specifically timed and ordered pattern meant to simulate the pulsations seen in an actual human specimen See Appendix B section 3 The BPM defined by the user actually works by modifying the delay placed in between each arches of the beats within this artificial pulsation The time duration set by the user works by measuring the time between when the program was started with relation to the Epoch and the current time also with relation to the Epoch If this measured time is equal to or exceeds the time defined by the user in seconds then the program terminates 1f not 1t continues See Appendix B section 3 The stepper motor is responsible for rotating our finger model into the correct orientation which in turn gives the desired oxygen saturation To accomplish this function
91. ty and time constraints to complete the design 26 4 0 Device Development The development of the device began with the evaluation of the possible design alternatives as well as the advantages and disadvantages associated with each design 4 1 Design Alternatives When developing a pulse oximeter calibrator two design alternatives were considered the mock circulatory system and the wedge model Each design accomplishes the outlined requirements through distinct simulation mechanisms These designs were evaluated in order to determine the most accurate efficient and cost effective means of calibration 4 1 1 Mock Circulatory System This artificial circulatory system will work similarly to previous circulatory models specifically the model as developed by Kent and Mendelson 5 Blood vessels are represented with tubing that takes the blood from one system component to another Blood will come from an oxygenator and will be pumped by a motor to an artificial finger to which the oximeter will be attached After the finger blood moves to a reservoir that will remove or at least dampen any residual pulsations from the system and return the blood to the oxygenator Oxygen will be provided by an oxygen tank and can be combined with other gases with varying partial pressures thus changing the saturation A diagram of the full system as used by Kent and Mendelson is shown in Figure 12 alongside a block diagram of our device showing the major
92. ulatory System Programming Conclusions Future Improvements Appendix B and Appendix E as well as provided editing and formatting support In addition to writing individual sections of the report David Reid Heidi Robertson and Matthew Sonntag as a group established the Design Approach Evaluation of Alterative Designs Design Selection and Appendix D Acknowledgements The design team would like to thank the following individuals and institution for their support throughout the course of our Major Qualifying Project e Professor Yitzhak Mendelson for his continual guidance and advice in all aspects of our project e Lisa Wall for her assistance in obtaining supplies for our design e Professor Destin Heilman for his assistance in testing our dye substitutes e Professor John McNeill for his help during light intensity testing e Our peer consultants Andrew Nehring Jeffrey O Rourke Joe Amato Daniel Jones and Michael Fagan for their advice regarding the design development and manufacturing of this project as well as our other peer consultants Huan Lai Jesse Basset and Kevin McManus for their advice in the design of the python program e Worcester Polytechnic Institute for providing funding and work space for our project Abstract Initial calibration of pulse oximeters 1s conducted by inducing hypoxemia in healthy human volunteers to obtain blood oxygenation levels
93. ulse Oximeter LED Location of pulse oximeter light sourc Axis of Rotati N KR Oximeter Photodiode a a b Figure 17 Top view of wedges showing axis of rotation and the direction of wedge turning a light path through wedges before and after wedge turning b The second motor the stepper motor would be used to initially position the wedges within the device such that the photodiode would read the intended oxygenation level This motor would be attached to a gear pulley system that would move two gears simultaneously in one direction or the other A bearing 31 block is on this gear pulley system upon which the stepper motor is attached and from this via the motor shaft the wedges are also attached The wedges would have a set position before any measurements begin This can be seen in Figure 18 where a servo is connected to a set of gearing that in turn 1s connected to a shaft that goes through the bearing block to the wedge frame The bearing block 1s set atop a platform guide block which 1s attached to the pulley gear system which moves the platform with a stepper motor to place the wedges at the correct position within the pulse oximeter probe A diagram of this system 1s shown in Figure 18 Wedges with pulse oximeter probe attached Gearing BEB EEE SE Shaft through bearing block Platform for bearing block Pulley system attached to stepper motor Figure 18 Motor control system of oxygenatio
94. w internal wedges that could be filled with 100 oxygenated blood and 0 oxygenated blood or dyes that would have a similar optical properties These types of wedges can be used 1f solid wedges cannot be found or depending on the FDA premarket approval processes This method would be used only as a backup plan and potentially during initial pulse oximeter calibration instead of human testing because using blood is undesirable as it is expensive requires proper handling and is sometimes difficult to obtain This method also requires extra cleaning between uses and extra preparation before use To use the device for reflection oximeters an additional component would be added to reflect the light back to the photo detector This involves simply adding a metal plate The metal plate would sit within the metal frame at the bottom of the wedge system and reflect the light back up into the sensor 4 1 2 1 Bill of Materials Material to simulate 100 oxygenated HbO Material to simulate 0 oxygenated Hb Metal plate Delrin or silicone rubber casing Stepper motor Servo motor Guide block potentially 6723K9 McMasters Belts Gearing Metal shaft Internal metal framing Pulse width modulation circuit Microprocessor Program software It is important to note that many of the components listed here are estimations of what will be needed and not exact materials or values that will be used This bill of materials 1s an estimation of what we will n
95. wedges inward and outward simultaneously to increase the path length and produce a pulse Other testing will determine 1f the location size and type of motors 1s optimal for the device function The pulsation mechanism and positioning mechanism along with the program used to control these mechanisms must be tested to insure proper simulated oxygen content and pulse rate All of these functions will undergo final testing with a functional pulse oximeter 4 3 Design Selection While both designs have positive attributes that make them desirable designs there are specific disadvantages that eliminated both designs The objectives are stated in the objectives tree below The weights in the sub objectives indicate relevance to the stated objectives of the device including simulation of signal compatibility ease of use and manufacturability The pair wise comparison chart Table 4 also shows the importance of each objective in a slightly different format Simulation of signal was most important while calibration of multiple pulse oximeters was second Both the wedge design and the mock circulatory system could simulate the signal however the wedge design was not compatible with multiple models of pulse oximeter probes Further research into current pulse oximeter models showed that there are several different types of probes that would not fit onto our square wedge design The mock circulatory system would require a great deal of set up and clean up
96. will serve as an alternative method to allow manufacturers to gain FDA acceptance of their pulse oximeters 3 3 Device Requirements 1 Calibrate and Test the Accuracy of Pulse Oximeters 1 1 Must calibrate and test how well pulse oximeter measures saturation and heart rate 1 1 1 Simulate oxygen saturation 70 100 1 1 2 Simulate heart rate between 30 250 BPM 1 2 Must be compatible with multiple models of commercially available pulse oximeters 1 2 1 Allows pulse oximeter probe attachment around the section of the calibrator simulating oxygenation and heart rate 1 3 Calibrator should be insensitive to the surrounding environment 1 3 1 Readings are not be significantly affected by ambient temperature 1 3 2 Readings are not be significantly affected by ambient light 2 Ease of Use 2 1 User interface 22 3 4 2 1 1 Intuitive design 2 1 2 Easily adjustable to desired oxygenation and pulsation levels 2 2 Attachment to Pulse Oximeter 2 2 1 Fits in transmission pulse oximeters small enough 2 2 2 Blocks ambient light 2 2 3 Works with devices that do not adjust the LED drive current under low transmittance 2 3 Portability 2 3 1 Easy to carry lt 20 pounds 2 3 2 Easy to assemble disassemble if required 2 3 3 Durable Manufacturability 3 1 Accuracy not effected by manufacturing tolerances 3 2 Inexpensive 3 3 Reproducible Device Objectives 1 Larger range of oxygen saturation a 0 100 oxygen saturation da
97. ws Python to communicate with the computer s serial ports This is the most essential plug in for this program as all interfacing with the motors is done via a virtual serial port this topic will be discussed in more detail later Finally Time is a module which provides a simple yet essential function It allows the program to recognize the exact time and date since the epoch January 1 1970 and thereby recognize the passage of time It also allows for the use of a delay function The UI for the device is fairly minimalistic It consists of five buttons arranged in a small window From right to left are the set BPM set time Rotate Stepper reset and Start buttons Figure 27 settings Help Pulse Osimeter Calibrator Aotate Stepper set time Set BPM Figure 27 GUI for Pulse Oximeter Calibrator For the most part the names of these buttons are fairly self explanatory though their exact functionality will be discussed later In relation to the code itself the buttons are packaged in a very efficient fashion Each button is essentially its own compartment containing a description and a reference to the function which it activates The functions are completely separate entities from the buttons which call them This division and compartmentalization means that making changes to the actual program is as simple as swapping out one component for another Also should there be an issue with any one func

Pulse Oximeter Calibrator - Worcester Polytechnic Institute

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