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Non-Invasive Sleep Apnea Sensor
Spring Quarter Final Report
Group 13
Kent Mayzel
Akshay Paul
Edmund Florendo
Kohei Okimura
Avinash Chinchali
Gerard Mendoza

2. 1
Table of Contents
TEAMINFORMATION..........................................................................................................................3
EXECUTIVE SUMMARY ........................................................................................................................4
INTRODUCTION..................................................................................................................................5
INITIAL DESIGN PHASE AND DESIGN CONTROLS REGULATION...............................................................6
Strain Sensor..................................................................................................................................7
Pulse Oximeter...............................................................................................................................8
Central Dongle Unit.........................................................................................................................9
Comparison....................................................................................................................................9
Design Criteria..............................................................................................................................11
FDA Regulation.............................................................................................................................11
Estimated Project Timeline............................................................................................................13
Estimated Budget.........................................................................................................................14
PROJECT TEAM.................................................................................................................................14
Avinash Chinchali..........................................................................................................................14
Edmund Florendo .........................................................................................................................15
Kent Mayzel .................................................................................................................................15
Gerard Mendoza...........................................................................................................................16
Kohei Okimura..............................................................................................................................16
Akshay Paul..................................................................................................................................17
DETAILED DESIGN PHASE..................................................................................................................18
Strain Sensor................................................................................................................................19
Pulse Oximeter.............................................................................................................................20
Data Acquisition Unit ....................................................................................................................21
Apnea Algorithm...........................................................................................................................21
Changes from Initial Design...........................................................................................................22
Cost Breakdown............................................................................................................................24
Project Timeline............................................................................................................................25
MANUFACTURING DOCUMENTATION................................................................................................25
Bill of Materials.............................................................................................................................25
Strain Sensor Manufacturing Process.............................................................................................26
Lithography...............................................................................................................................27

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Miniaturization .........................................................................................................................28
Transfer Step ............................................................................................................................29
Pulse Oximeter and Data Acquisition Unit...................................................................................30
Manufacturing Limitations.........................................................................................................30
MATERIALS SELECTION AND VALIDATION ..........................................................................................30
Foil – Platinum..............................................................................................................................30
Supportive Backing – Ecoflex.........................................................................................................33
Adhesive - KT Tape........................................................................................................................34
DESIGN VALIDATION.........................................................................................................................35
Detailed Validation Testing............................................................................................................36
Validation Results.........................................................................................................................39
FAILURE MODE AND EFFECT ANALYSIS (FMEA)...................................................................................42
LESSONS LEARNED DOCUMENTATION...............................................................................................44
Issue 1: Lead Connection Failure....................................................................................................44
Issue 2: Adhesion Failure...............................................................................................................44
Issue 3: Strain Gauge Overextension..............................................................................................44
USER DOCUMENTATION AND TRAINING............................................................................................44
FUNCTIONAL TRIALS.........................................................................................................................45
FUTURE GOALS.................................................................................................................................46
WORKS CITED...................................................................................................................................47

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TEAM INFORMATION
BME 180 Group 13 Team Information
Non Invasive Sleep Apnea Sensor (nSAS)
ProjectMentor: Dr. Michelle Khine
Department:Biomedical Engineering
Email:mkhine@uci.edu
Phone:(949)-824-4051
Team Leader
Mayzel, Kent (BME)
kmayzel@uci.edu
Okimura, Kohei (BME)
okimurak@uci.edu
Florendo, Edmund (BME)
florende@uci.edu
Chinchali, Avinash (BME)
achincha@uci.edu
Mendoza, Gerard (MSE)
gerardmm@uci.edu
Paul, Akshay (BME)
Apaul1@uci.edu

5. 4
EXECUTIVE SUMMARY
SleepApneaisaverycommonmedical conditionwhereapersonstartsand stopsbreathingrepeatedly
throughoutthe night.Itis usuallyaccompaniedbysnoringandaninabilitytogetagood night’ssleep.
People sufferingfromsleepapneaare notonlyfatiguedbythe lackof sleep,theyare affectedbythe
constanthypoxiacreatedbynotbeingable tobreathe whichputsextrastrainon the cardiovascular
system. There are about22 millionpeople inthe USsufferingfromsleepapneaandof those people
90% remainundiagnosed. CurrentlypatientsmustundergoanOvernightPolysomnograph(PSG) tobe
diagnosedwithsleepapnea. These testsmustbe done inaspecial sleeplabwherethe patientis
hookedupto a plethoraof sensorsthatmonitorrespiration,brainwaves,andECG data. The problem
withPSG testsisthat theyare uncomfortable,expensive,anddifficulttogetan appointmentfor. The
nSASdevice directlytargetsthesemainissuesbybeinglow profile andcomfortable,inexpensive,and
convenienttouse inthe home.
Figure 1: nSAS System
By keepingthe costtomanufacture at a low $40 the nSAScan be marketedtoa large numberof
patients.The conformal strainsensorsare extremelylow profile andperformsimilartoKT AthleticTape
interms of comfortand durabilitywhile maintainingaccurate measurementsof chestexpansionfor
respirationdata. Since the systemiscompletelyportable andthe dataisstoredonto an SD card, itis
convenienttouse inthe home while the patientsleepsintheirownbed.

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INTRODUCTION
Sleepapneaisa commonmedical conditioninwhichapersonstopsbreathingorexperiences
inconsistentbreathingpatternsforseveral secondstominuteswhile asleep.Twoformsof sleepapnea
existwiththe mostcommonbeingobstructive sleepapnea(OSA) andthe lesscommonformbeing
central sleepapnea.OSA iscausedbya collapse of the airwayduringsleepandisusuallyaccompanied
by loudsnoringandgeneral discomfortdue toairsqueezingpastthe blockage [1]. OSA has severe
healthimplicationsif leftuntreatedthatcan drasticallyreduce apatient’squalityof life.The irregular
breathingleadstoa conditionknownaschronicintermittenthypoxia,resultingoxidative stressonthe
sympatheticnervoussystem.TheseconsequencesresultinOSA andhave beenshowntoinduce
hypertension,arrhythmia,stroke,andcoronaryheartdisease [2]. These associatedhealthissuescaused
by OSA are all interconnectedandasfigure 2 shows,eachissue worsensthe others.
Figure 2: The cascading effect of chronic intermittent hypoxia on the cardio metabolic system shows the extensive
consequences of OSA [2].
Withthe discoveryof continuouspositive airwaypressure (CPAP) treatmentnearly30years ago,
clinicianshave increasedpatient care inhopesof reducingthe cardiometabolicproblemsassociated
withOSA and improvingpatientqualityof life [3]. A CPAPdevice appliesconstantairpressure tothe
airwayof the patientkeepingitopenduringsleep[1]. Althoughtreatmentiswidely available,OSA itself
ishard to diagnose inan effective andconvenientmethod.

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It isreportedthat93% of womenand82% of menwithmoderate tosevere OSA goundiagnosed[4] This
isin part due to the primaryform of diagnoses,whichisanovernightpolysomnogram(PSG) inasleep
lab.
A PSG isa device thatmeasuresbrainactivity,eye movements,heartrate,bloodpressure,blood
oxygenation,andchestmovementstoshow aneffortto breathe [1]. The difficultyforpeople toseek
diagnosiscomesfromthe lowavailabilityof beds,the highcost,andthe longwaitinglistsforlab
access. The needformore prevalenthome diagnosisdevicesisshownbythe CentersforMedicare and
MedicaidServicesrecentlyapprovingthe coverage of CPAPtreatmentforpeople diagnosedbyPSGas
well asby inhome devices[4]. The bulkyanduncomfortable nature of PSGtestsaddto the difficultyof
gettingdiagnosed. Patientswhoalreadyhave trouble sleepingare requiredtosleepinabedthat they
are notusedto as well asbe hookeduptoa plethoraof instrumentsasseeninFig. 3.
Figure 3:Patient undergoing a PSG test
The device thatwas createdassistswithdiagnosingsleepapneabyexaminingapatient’sbreathing
patternusingexternal straingaugesadheredtothe patient’schest. Byutilizingconformal strainsensors
placedacross the user'schest,the device isable toachieve new levelsof patientcomfortwhilestill
offeringreliablesignal acquisition.Inaddition,the manufacturingprocessof thissensorsystemoffersan
inexpensive andeasytofabricate method;thiswillallow forthe devicetobe marketable toawide
range of users. The designalsoincorporatesapulse oximeterwornonthe fingerof the patientto
gatherdata on bloodoxygenation. The datafromthe strain sensorandthe pulse oximetercanthenbe
easilyuploadedtoa computerwhere itisanalyzedandthencan be sentto a physician.
INITIAL DESIGN PHASE AND DESIGN CONTROLS REGULATION
The device,nSAS(non-invasivesleepapneasensor),consistsof three maincomponents;awearable
strainsensor,a pulse oximeter,andanArduinoUNO.Usingthese components,the device measures
respiratoryeffort,andbloodoxygenation.Twostrainsensorplacedonthe centerof the chestcomputes
the respiratoryeffortfromchestexpansion.The bloodoxygenationismeasuredusingthe pulse
oximeterplacedonthe finger.Boththe sensorandpulse oximeterisconnectedtothe Arduinobyathin
wire andthe data isacquiredandsavedonto a microSD card forconvenience.The nSASisthe first

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device thatutilizesstraingaugesasthe primarysensortodiagnose sleepapnea.Withthisnovel
implementation,nSASisable tooffernon-invasiveness,comfort,affordability,andhighaccuracyto the
users.
Strain Sensor
The central componentof our device isa wearable sensorthatissensitive tochangesinstrainbetween
normal breathingandan apneaevent.Thisstrainsensorismade froma bondedstraingauge. A strain
gauge is a device inwhichthe electrical resistancechangesinproportiontothe appliedstrain. Figure 4
showsthe currentstrain gauge design.The blackgriddingismade froma thinfilmof platinum, whichis
boundedtoan elasticpolymerbackingknownas Ecoflex. The strain gauge isthenattachedto a piece of
KT AthleticTape sothat itcan adhere tothe body.Dependingonwhetherthe straingauge isstretched
or compressed,the electricresistivityincreasesordecreasesrespectively.Usingthischaracteristicof the
straingauge,we can monitorthe user’schestdisplacementduringrespiration. Rhythmicstrainof the
sensorimpliesnormal breathingwhereasanabsence of strainindicatesthatthe userisexperiencingan
apnea.
Figure 4:Custom Strain Sensor
Because the straincausedby chestexpansionissmall,there isaneedtoaccuratelymeasure small
changesinresistance. A Wheatstone bridge circuitwill be implementedinsidethe dongle unitforthis.A
Wheatstone bridge isacircuitconsistingof 4 resistorsthatenablesustofindunknownresistances.A
schematicof the circuitin the designisshowninFigure 5. Thisconfigurationisknownasthe quarter
bridge straingauge circuit. It consistsof 3 knownresistorsand1 unknownresistorrepresentedbya
straingauge.The outputvoltage ismeasuredacrossthe middle of the twoarmsof the bridge bya
galvanometer. Initially,the valuesof the knownresistorsare adjustedsothatthe ratio 𝑅1 𝑅3⁄ and
𝑅2 𝑅 𝑆𝑡𝑟𝑎𝑖 𝑛 𝐺𝑎𝑢𝑔𝑒⁄ are equal whenthe straingauge isunderno strain.Asa resultof this symmetry,the
voltage differenceacrossthe galvanometerisequal tozero. Anychangesinthe resistance of the strain
gauge (𝑅 𝑆𝑡𝑟𝑎𝑖𝑛 𝐺𝑎𝑢𝑔𝑒) will unbalance the ratiosymmetryand create a voltage difference betweenthe
arms, resultinginanon-zerovoltage output[7]. Thisvoltage outputallowsustocalculate the amount
of strainthe sensorisunder.

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Figure 5: Quarter-bridge strain gauge circuit
However,usingthe Wheatstonebridge circuitintroducescoupleof variability.The firstof these is
temperature dependence.The sensitivityof the straingauge,alsoknownasgauge factor tendsto
change as temperature increases.The severityof change depends largelyonthe material selectedfor
the metal gridding.However,the operatingtemperature mustbe significantlyhighinordertosee large
slopesingauge factorsfor all materials. Fora platinumalloy,datashow thatthe gauge factor changed
only2% per100°F of temperature increase between320°Fto 1500°F [8]. Because the device’s
operatingtemperature isaroundbodytemperature,the variabilityisnegligible.
Pulse Oximeter
Anothercomponentof the device isapulse oximeterthatisusedtomonitorthe patient’sheartrate and
oxygensaturation.A traditional pulse oximeterasshowninFigure 6, containsa pair of small light-
emittingdiodes(LED) thatissituatedinfrontof a photodiode.The LEDshave a wavelengthof 660 nm
(red) and940 nm(infrared) andare flashedabout30 timespersecond.Inthe transmissive
configuration,pulse oximetersare usuallyplacedonathinpart of the body,such as the fingertipor
earlobe.The photodetectormeasuresthe change inabsorbance causedbythe pulsing arterial bloodby
detectingthe twowavelengthsthatare passedthroughthe body.The absorptionof lightatthese
wavelengthswilldifferdependingonthe amountof oxygenpresentonthe hemoglobin. Deoxygenated
hemoglobinabsorbsredlightbetterandallowstopassthrough.Onthe other hand,oxygenated
hemoglobinallowsmore redlighttopassthroughbutabsorbsinfraredlightbetter.

10. 9
Figure 6: Pulse Ox used by the nSAS
Central Dongle Unit
The nSASwill acquire signalsfrom itstwophysiological sensors –the straingauge and pulse oximeter.
Signalsfromthese twosensorsare acquiredandstoredbyan ArduinoUNO.The Wheatstone Circuit,as
discussedabove,isincorporatedintothe CDU.The Wheatstone Circuitoutputsachange in voltage,
basedon the displacementof the straingauge,directlyintoCDUmemory. The currentprototype relies
on an ArduinoUnoto acquire and save data frombothsensorsontoa microSD card. The Arduinoisset
to have a samplingfrequencyof 40Hz, whichmeansthat40 data pointswill be acquiredfromthe strain
sensorandpulse oximeteroverthe course of 1 second.The 40Hz samplingfrequencywaschosen
because itis200 timesgreaterthanthe frequencyof the breathingcycle (0.20Hz).Therefore,the nSAS
samplingfrequencymore thansatisfiesthe Nyquisttheory.
Figure 7: Central Dongle Unit powered by a 9V battery.
Comparison
As statedearlier,the existingdiagnosticsituationaddressesaneedforaprevalent diagnosisdevice for
sleepapnea. The currentstate of the art methodof polysomnographydoesprovidesareliable

11. 10
diagnosis,however.There are manyissuesthatcontribute tothe lackof diagnosedpatients.The device
presentsasolutiontoeachof these problems.
Firstof these issuesiscomfort. Because,PSGmeasuresnumerousdifferentdatainorderto diagnose
sleepapnea,patientsare oftenconnectedanoverabundance of equipmentin. A few of these include a
nasal cannulaprongs insertedintothe nostril,electroencephalography(EEG) electrodesattachedto
multiple areasof the head,anda bulkyelectroniccomponentstrappedontothe user’schest.These
equipmentintroduce discomfortforthe patientandfurtheraffectthe qualityof theirsleep.
The nSASdevice minimizesdiscomfortinmultiple wayswhile still maintainingareliablemeasurement.
Comparingtothe PSG,whichoffersanexcessamountof data, the device acquiresthe minimumsignals
necessarytodiagnose sleepapnea.The strainsensormeasuresthe respiratoryeffortwhile the pulse
oximetermeasuresthe bloodoxygenlevel.Byreducingthe parameterstomeasure,the nSASsystem
can consistof 3 components. Thissimple setupdiminishesthe amountof wiringtotwosimple wires,
lesseningthe possibilityof entanglementduringthe user’ssleep.Anotherwaythe device minimizes
discomfortandmaximizessensitivityisthe straingauge designusedinthe strainsensor.The innovative
straingauge offersa comfortlevel andsensitivitythatisnot providedbya conventionalgauge bought
fromthe market.These ordinarygauges,oftenmade fromsilicon,mayperformsufficientlywhen
appliedtoa stable solidstructure.However,the humanbodyhasmanycurvaturesandassociatedwith
complex biomechanicalmotions. These straingaugesare toostiff andcan neitherconformnormeasure
on softsurfacessuchas skinmakingthemunfitfora wearable sensor.Ourdesignimplementsanecoflex
backingto our straingauge providinganelasticproperty.Thisallowsittocomfortablycoverthe
intricate curvilinearsurfacesof the body. Withthe adhesive keepingthe sensorinplace,the elastic
straingauge has the abilitytostrainwiththe natural motionof the chest,reducinganymechanical
constraintsfeltbythe user.
Secondof these issuesisthe highcost.WithPSG testcostingup to $5,000 dependingonthe clinic,cost
isa majorcontributingfactorto the lowpopulationof diagnosedpatients[9].Evenwithinsurance the
PSG istoo expensive toaffordformanypatients. A majordesigncriteriaof the device wastomake the
device affordable forawidermarketof people.Because the strainsensorisabout3mm longwith1
micronin thickness,the manufacturingcostof the sensorisabout25 cents. The pulse oximeterandthe
data acquisitioncomponentiscommerciallyboughtandmodified.Withthis,the device wasable tobe
manufacturedforunder$40.
Lastly,the lack of facilitiesmakesitisextremelydifficulttogetan appointment.The patientsmustsign
up fora longwaitlistandwork aroundthe convenience of the facility.The conceptof maximizing
accessibilityforuserwasat mainaspectwhenconsideringdesigns.WithnSASthe patientcannowtake
the diagnosticat home ontheirowntime.The simple setupof the strainsensorandpulse oximeter
eliminatesthe needforaspecialisttocarryout the procedure.The datafrom the centralizeddongleunit
can simplybe uploadedtoacomputerand sentto a physician.

12. 11
Design Criteria
Whendesigningthe nSAS, the patientwasatthe forefrontof the designconsiderations.The device
shouldbe comfortable towearandeasyto use by patientsof all body-types.Since sleepapneahasa
hightendencytoaffectoverweightandobese patients,the device neededtobe able toconformto
varyingchestsizes,whilestillprovidingreliable sleepqualitydatanormalizedtoeachspecificbodytype.
Thiswill be discussedinmore detail inthe calibrationsection.
To ensure comfortandwearability,the nSASmustmeet acceptance criteriaforbiocompatibility,
portability,weight,andcostaspredeterminedinthe designprocess.Specifically,all componentsof the
nSASdevice (strainsensor,dongle,andcommercialpulse oximeter) thatwill come intocontactwith
humanskinmustbe made frombiocompatiblematerialsthatwill elicitnoadverse orallergicreaction
fromthe skin.Please refertothe Materialssectionformore information.Since the nSASisdesignedasa
wearable technology,itmustmeetspecificcriteriafor weightandportability.The chestattachedstrain
sensormustnot exceedaweight2.0ozinorder to maintaincomfortduringsleep. The portability
criterionforthe nSASismet if the device canfunctionunderbatterypowerfor12 hours(±0.5 hours),
whichmeetsandexceedsthe AASMrecommendedsleepdurationof 7.5 – 8 hoursfor adults.The last
patientorienteddesigncriterioniscost.Consideringthatthere isaverylarge populationof undiagnosed
Americanssufferingfromsleepapnea,the nSASwasdesignedtobe affordableinordertobe accessible
by a largeruserbase.Therefore,manufacturingcostsforthe device mustbe lessthanor equal to$40.
The nSASmust be designedtoaccuratelymeasure the requiredAASMphysiological signals(as
previouslydiscussed) toadequatelydiagnoseandaccessa patient’ssleepapneacondition.Inorderfor
the device tomeetthe accuracy acceptance criteria,itmust be able to measure lineardisplacement
with95% accuracy. Thisrelativelyhighaccuracyisrequired fromoursystemtobe inaccordance with
the Apnea-HypopneaIndex(asdiscussedbefore),whichregistersa30% decrease inthe Chestexp peak-
to-peakamplitudeasfirststage Hypopnea.Therefore,the allowederrorinstrainmeasurementof 5%is
small enoughtonot interfere withAHIthresholds.The devicemustalsohave ahighspatial resolutionof
lessthanor equal to 20µm to effectivelydifferentiatechangesinchestwall displacement.Since strain
gauge measurementresolutionisdependentuponGauge Factor(GF),whichmustbe greaterthan 3 to
achieve the acceptance criteriaof ≤ 20µm.
FDA Regulation
The Food andDrug Administration(FDA) hasestablisheddifferentcriteriaandcontrolstoguarantee the
safetyandeffectivenessof medical devices.Inaddition,underthe Food,Drugand CosmeticAct,the
UnitedStatesFDA has alsodividedmedical devicesintothree classificationswhichdictatesthe pathof
the regulatoryapproval process. Determiningthe classificationof ourmedical device isthe firststepin
establishingthe pathtocompliance underFDA regulation.Table 1providesanoverview of the three
medical device classifications.

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Class I Class II Class III
No official FDA approval needed.
Must register device and
company on FDA website.
FDA clearance required.
510(k) Pre-market Notification
Submission
Need a predicate device that has been
approved by the FDA
FDA approval required.
Pre-market Approval
(PMA) Process
LOW RISK MEDIUM RISK HIGH RISK
Table 1: FDA medical classification overview
Since the nSASprovidesinformationthataidscliniciansindiagnosingsleepapnea,ithasthe riskof
providingfalse negativeandfalse positivedata.Thiscan leadtoindividual healthconsequencesas
misdiagnosiscanleadto improperpatientmanagement.A false positivetestresultmayresultin
incorrectpatienttreatmentwithpossibleadverseeffectsaswell asunnecessarypatientdistress.Itis
importantto include mitigationmeasuressuchasperformance studiesandproductlabel descriptionsin
orderto protectthe consumerfromthese identifiedrisks.Inaddition,the nSASalsohasthe potential to
cause injuryor harm to the userif it malfunctionsorusedimproperlysince ituseselectricityand
containslightemitting diodesthattransmitradiationatknownwavelengths.Forall of the above
reasons,the nSASwill be classifiedasa ClassII medical deviceunderFDA regulation.
The FDA requiresapre-marketnotificationora510(k) for ClassII medical devicestobe clearedand
made available tothe market.Inthis510(k) application,we willneedtoincludeinformationsuchasthe
scope,performance andsafetyof the device.We willalsoneedtoidentifyapredicate device thatis
similartothe nSASthat has alreadybeenclearedbythe FDA.We foundthat the ApnoescreenII,aClass
II sleepapneadiagnostictool,usessimilartechnologytothe nSAS. The ApnoescreenIIusesanoximeter
to measure the bloodsaturationanda piezoelectricsensorplacedviaabandacross the chest to
monitorthe respiratoryeffortof the patient.We expectthe entire review processtotake 4-9 months
fromthe time of 510(k) submission.

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Estimated Project Timeline

15. 14
Estimated Budget
 Ecoflex 30 for elastomerbacking:$200.00
 ThinPlatinum diskforthe sputtercoater:$1,000.00
 Polystyreneshrinkdinksheetsasasubstrate forthe straingauge:$20.00
 Peelablemaskpaperorcontact paper:$20.00
 Acetone todissolve the polystyrene: $30.00
 Toluene todissolvethe polystyrene:$30.00
 The bluetoothdevice (custom-made):$100.00
Total ProjectCost:$1,400.00
PROJECT TEAM
The team structure consistedof one teamleader,KentMayzel,andfive othermembers.Kentwasin
charge of communicationwithpeople outside of teamsuchasmentors,UROPfunding,etc.Inaddition,
Kentwasin charge of strainsensorfabrication,straindataacquisition,strainsensorvalidation,and
manyother aspectsof the project.AlthoughAkshayandEdmundbothalsoheavilycontributedtothese
previouslymentionedaspects,theirmainfocuswasonthe immense taskof successfullyintegratinga
commercial pulse oximeterintothe designandcomparingitsefficiencytoachest basedpulse oximeter
builtfromscratch. Kohei closelycontributedtostrainsensorfabricationand dataacquisition.In
addition,Kohei wasthe CADspecialistandhelpeddevelopandvalidate improvedtrace patternsforthe
fabricatedstrainsensors. Gerardwasincharge of materialsselectionof the sensorsandbackings.
Avinashparticipatedinstrain sensorfabricationandstraindataacquisition,but hismostimportantrole
was indevelopinganautomatedapneadetection.
Avinash Chinchali
Major Task: ApneaDetectionAlgorithm
In orderto developthisdetectionalgorithm,Ifirsthadto do extensive researchintohow apneaswere
classified.Afterreviewingclinical guidelinesestablishedbythe AmericanAssociationof SleepMedicine
(AASM),Idevelopedamathematical classificationsystemtodetermine the numberanddurationsof
cessationsinbreathingbaseduponchestdisplacementinformationfromthe strainsensor.After
programminga code to analyze strainsensorvoltagesovertime,we wantedtoobtainbreathingdatato
run our algorithmonfordebuggingpurposes.We usedourfabricatedsensorstomeasure chest
displacementsonKohei andfedthe recordeddataintothe detectionalgorithmcreated.Forrelatively
short trialsof breathing,the detectionalgorithmworkedwell incalculatingthe numberof apneasand
theirrespective durations.Howeverwhentrialsbecame longerinduration,mis-classificationof regular
breathingasapneasbeganto be apparent.
Upon extensive debuggingof the algorithm, itwasevidentthatthe risingtrendinvoltage outputsasthe
strainsensorgot repeatedlyrepeatedlystretchedwascausingapneamis-classification.Thishysteresisof

16. 15
the strainsensorwas causingan inflatedapneathresholdvalue,therefore more measurementswere
beingrecordedasbelowregularbreathingamplitudesandmore apneaswere beingrecordedthan
actuallyoccurred.I accountedforthisin an updatedversionof the detectionalgorithmthat
incorporatedupdatingmovingaveragestocalculate the apneathresholdvalue.The debuggingprocess
tooka significantportionof thisproject,bythe endof the school yearI had almosttengenerationsof
the apneadetectionalgorithmeachaddressingthe previousgeneration'sshortcomings.The major
designrequirementsthatwere beingaddressedwitheachimprovedgenerationof the apneadetection
algorithmwasbeingable toaccuratelydetect30 +% breathingamplitude dropsforaminimumduration
of 10 seconds(AASMapneaqualifications) inchestdisplacementdatafromthe strainsensor.Thisapnea
detectionalgorithmiswhatultimatelybridgesthe gapbetweenoursensorsandthe user.Thisalgorithm
essentiallyinterpretsthe patient'sdataduringsleepandpresentsthemwiththe diagnosisof whether
he or she islikelytohave sleepapnea (baseduponnumber&durationsof apneasinsleep).Withoutthe
incorporationof thisdetectionalgorithm, the userwouldstill needaspecialisttoanalyze sleepdataand
make a diagnosis,andourdevice wouldoffernoadvantage overthe currentgoldstandardsleepapnea
detectionmethodof overnightpolysomnographobservation.
Edmund Florendo
Major Task: Pulse OximeterandDataAcquisition
Due to the time constraintof the project,ourteamhad to incorporate a commercial pulse oximeterinto
our design. Iwasresponsibleforchoosingalow-costcommercial pulse oximeterthatwasn’tencrypted
so we couldeasilyincorporate ittooursystem.I thencreateda LabVIEWsoftware totestif we couldget
data fromthe pulse oximeterandeventuallyintegratedit intothe Arduino.Iwasalsoresponsiblefor
creatingdata acquisitionsoftware thatloggedvoltage data,andincorporatedittoourdata acquisition
unit.I programmedourArduinosoit can save voltage andoxygensaturationdataalongwith
timestampsintoanSD card. Lastly,I wasresponsibleformakingthe dataacquisitionunitastand-alone
systembyprogrammingtosave intoan SD card and poweringitbya 9V battery.
As mytasks mainlyinvolvedprogramminginLabVIEWandArduino,Idividedeachtaskto: 1) Coding,2)
Testing,3) Debugging.We didnothave specificdesignrequirementspertainingdirectlytothe
programmingrelatedtaskswhichIwasresponsiblefor,howeverwe wantedourdevice tobe portable,
comfortable,andbe able tolastmultiple uses.Forittobe portable andtolast multiple uses,Ihadto
consideraddinganEthernetshieldtosave dataon an SD card and poweritusinga 9V battery.By doing
this,we were able tomeetourdesignrequirementandsuccessfullymake ouroverall design portable.
Kent Mayzel
Major Task: Team Leader
Since I was the Team Leader my main assignment was to coordinate with all members of the
team to structure the design process and get feedback from the mentors and graduate students
and make sure those suggestions were implemented in the final product. I also took it upon
myself to conduct experiments and validate the designs due to my previous undergraduate lab
assistance experience. This included testing the fabricated strain sensors to make sure they
met the design requirements and were capable of use in the device. In order to meet the

17. 16
requirement of being able to accurately detect respiration the sensors needed to be tested for
gauge factor and durability. This meant incremental strain tests and repeated strain tests.
Also, in order to attach the sensor successfully to the skin, I chose KT athletic tape to adhere to
both the sensor and the skin. This fulfilled the durability and robustness requirement of the
sensors. Due to my position as team leader, I played a role in various other aspects of the
project including data acquisition, specifically designing the wheatstone circuit and amplifier.
This helped ensure the systemwould be fully portable and capable of detecting the small strain
caused by respiration. After working with the graduate students they suggested utilizing a
conformal lead to attach the sensor to the data acquisition device. After this I incorporated flat
ribbon cables from Wurth Electronics that could be placed between the tape and sensor and
stick out slightly for connecting to the data acquisition device.
Gerard Mendoza
Major Task: MaterialsSelection andValidation
As the materialsscientistof the group,Iwas assignedthe taskof materialsspecialist.The majortaskwas
dividedintothree subtasks:materialsselection,materialsvalidation,andfabricationof sensors.Manyof
the designrequirementsinfluencedthe selectionof materials.Forexample,inorderforourdevice to
have a highmeasurementresolution,the strain gauge musthave ahighgauge factor. Thus,platinum
was selectedasthe foil materialbecause platinumhasahighergauge factorthan mostmetalsat low
strains.Furthermore,the designrequirementsof biocompatibility,comfort,andlow costof the device
greatlyinfluencedthe selectionof the straingauge components.Componentslike the supportive
backingand adhesive neededtobe sensitive toskinandrelativelylow cost. Afterthe appropriate
materialswere selected,itwasonto fabricationof the sensorsandsubsequentvalidationtestsof its
componentstosee if the strainsensorsmetour designcriteriaandthe criteriasetforeach of the
materials.Fortunately,the straingauge wasable tomeetall the requirementsformaterialsselection,
but the overall device (includingthe dataacquisitionunitandpulse ox) wasnotable meetall of the
designrequirements.The overalldevice didnotmeetthe weightrequirementandthe straingauge was
unable tomeasure repeateddisplacementswithlessthana5% variance.Effectivelyselecting,validating,
and fabricatingthe appropriate materialsisanimportantfactorfor constructingaworkingproductlest
the device failstomeetdesigncriteria.
Kohei Okimura
Major task: DesignandFabricationof the StrainSensor
My role inthe projectwasthe designingandthe fabricationof the strainsensor.AnessentialtaskthatI
had wasto findan optimal designforthe strainsensorinorderto outputthe necessaryamountof
strain.Utilizingsoftware suchas AutoCADandSolidworks,Imade several straingauge trace designs
basedoff of literature thatI read. AdditionallyIusedthe same software torunfinite elementanalysisof
the designtosimulate possible strains.Bydoingsowe were able topredictperformancesbefore
fabricatingthem.

18. 17
Althoughthere were nottoomanysubtasksinthe fabrication,itcan be brokendowntomaterial
selection,designof sensors,andthe actual fabrication.These subgroupshadresearchestodoon their
ownon howto improve uponthe sensorcomponentof the device.
In termsof influence therewere nosignificantchangesthathadtobe made formaterial selection.
However,due tochangesinsensordesigns,some fabricationmethodshadtobe changed.Aswe
progressedthroughthe yearoursensordesignsbecame more complicatedwithserpentinetracesin
orderto achieve more strain.Inmanystepsinthe fabrication,there are stepsthatrequire handson
fabricationsuchas peelingoff the frisketfilmmasks. Withmore complex designs,these processes have
to be approachedwithmore attentionandhandledcarefullyinordertopreventthe sensorsfrombeing
ruined.Hence,there was lessproduction happeningdue tothisslowedprocess.However, moreover,
the sensordesignisinfluencedbylimitationsbythe fabricationprocess.A majorlimitationisthatthe
sputtercoateravailable foruse has an effectiveradiusof 2 inchesdiameter.Withthisthe bulkof the
sensormustbe designedtofitthisdiameter.
Overall,the adjustmenttothe sensordesignwasfoundtohave a positive impact.Withthe new design
we were able toreach strainresultsthatwere notpossible withpreviousdog-bone designs. From
previousvalidations,we sawthatthe dog-bone designonlysaw a5% strainbefore the resistance value
were immeasurable.Withthe adjusteddesign,we canrepeatedlystretchthe sensorsabout10% of the
original sensorwithconsistentreadings.Thiswasanessential designcriteriathatwasyetto be met with
the initial design.
The designsof the strainsensordidnot necessarilychange the overall designof the device.Althoughthe
designmaybe different,the sensor’ssizeorfabricationprocesshasnotchangeda whole lot.The
sensor’sfunctionalityimprovedandallowedthe device tobe closertothe criteriastandards.
Akshay Paul
Major Task: Develop the electronichardware necessaryforthe Wheatstone amplificationcircuitandthe
central data acquisitionunit.
I dividedthe primarytaskof developingthe electrical hardware intosubtasksbasedonthe logical order
of conventionalelectrical hardware productionandthe specificationsof eachsensorthatthe hardware
wouldinterface with.The firstsubtaskIperformedwastoreview ourdevice’sdesigncriteriaand
determine the necessaryfunctionalitythatthe electrical hardware wouldhave toprovide.The second
subtaskthat I performedwascreatingblockdiagramsof the fundamentallogicof eachof the circuits,
takingintoconsiderationthe functionalitycriteriafromthe firstsubtask.The thirdsubtaskwasthe
actual designingof circuitryusingelectrical designsoftware,suchasLTspice.The fourthsubtaskwas
orderingelectrical componentsbasedonthe predeterminedspecificationsandbuildingbreadboard
prototypesof the circuitdesignsestablishedinsubtaskthree. Subtaskfivewastestingthe functionality
of the prototype boardsandactivelymakingadjustmentsandimprovementsthroughvalidationtesting.
The sixthsubtaskwasintegratingseparate circuitsbysolderingthemintoacompactform factor and
removingredundancies.

19. 18
The designrequirementswerethe central motivationineachsubtasklistedabove.Specifically,the
device requirementscalledforanelectrical circuitcapable of communicatingwithapulse oximeterand
twostrain sensors.The circuitneededtoprovide reliable andaccurate measurementsfromthe sensors
at a highfrequency.Therefore,whendesigningthe circuitinsubtaskthree,the properelectronic
componentswere selectedtoensure the device couldsample multiple sensorsonseparate channels
and provide precisionmeasurements.The designrequirementsforthe device tobe a wearable sensor
motivatedsubtasksfourthroughsix,whichallowedustoproduce compact,lightweightcircuitry.
The major task of developingthe electrical hardware allowedforintegrationof all device components
intoa central unit.Specifically,the central dataacquisitionunitandamplifiedWheatstone circuitis
responsible formeasuringandstoringvoltage signalsfromthe strainsensorsanddigital serial outputs
fromthe pulse oximeter.
The major task thatI performedhasa substantial influence onthe designof the device overall.The
electrical hardware begantoprovide the necessaryfunctionalityearlyoninthe project,whichenabled
the team tofocus on strain sensordesignandvalidation.Despite the betterthanexpectedperformance
of the electrical hardware,the Arduinoandassociatedcircuitryare still responsible forthe majorityof
the size and weightof the device.
DETAILED DESIGN PHASE
Figure 8 showsthe illustrationof the overalldesignof the device.Asmentionedearlier,ourdesign
consistsof a conformal strainsensor, a pulse oximeter,adata acquisitioncomponent,andaLabVIEW
algorithm.
Figure 8: nSAS Aolution Design

20. 19
Strain Sensor
The conformal strainsensorsare placedintwo positions:one inbetweenthe upperribcage,andone on
the abdomen.These strainsensorsare variable resistorsthatchange electrical resistance basedonthe
strainexperienced.Whenthe sensorsare stretchedthe tracesare narrowedresultinginanincrease in
resistance,whereasawidentrace bycompressionresultinadecrease inresistance. Byutilizingthis
characteristic,we can monitorthe patient’smovementduringrespiration.
Figure 9 showsa single strainsensor.A serpentine designwasimplementedtomaximize the possible
strain.More regardingsensordesignwill be discussedindepthlaterinthe report. The sensorisplaced
ontocut outelasticfitness tape.These tapesare oftenusedformuscle supportinthe fieldof
kinesiology. The cottonmeshmaterial allowsthe tape tostretchalongwiththe stretchsensorandalso
adhere tothe skin. The twoleadsof the strainsensorare attachedto an insulatedcopperribboncable.
These cablesallowconnectiontothe sensorfordata acquisition.
Figure 9: Strain Sensor
The varyingresistance of the sensorswill be measuredusingwhatisknownasa Wheatstone bridge.A
Wheatstone bridge isacircuitconsistingof 4 resistorsthatenablesthe detectionof unknown
resistances.ThisWheatstonebridge isembeddedinthe dataacquisitionunitattachedtothe hip.A
schematicof the circuitin the designisshownin Figure 10. Thisconfigurationisknownasthe quarter
bridge straingauge circuit. It consistsof 3 knownresistorsand1 unknownresistorrepresentedbya
straingauge.The outputvoltage will be measureacrossmiddle of the twoarmsof the bridge bya
galvanometer. Initially,the valuesof the knownresistorsare adjustedsothatthe ratioR1/R3 and
R2/R_strain sensorare equal whenthe straingauge isunderno strain.Anychange inresistance of the
strainsensorwill unbalance the ratioandresultina non-zerovoltage output[10].Fromthisvoltage
output, the level of strainorcompressionof the sensorisunder.

21. 20
Figure 10: Wheatstone Bridge Circuit
Pulse Oximeter
The third componentisthe commerciallyboughtpulse oximeter.Inorderto diagnose sleepapnea,
bloodoxygensaturationisanessentialparametertomonitor.A conventional pulse oximeterconsistof
a Light-emitting-diodesanda photodetectortomeasure levelof absorptions.Oxygenatedand
deoxygenatedhemoglobinmoleculespresentinbloodare knowntoabsorbsdifferentwavelengthof
light.While oxygenatedbloodare knowntoabsorbinfraredwavelength,deoxygenatedhemoglobinare
more likelytoabsorbredlightmore efficiently[11].Usingthischaracteristic,pulse oximeterare able to
measure oxygensaturationlevel.
We decidedtopurchase the CMS50D model from Contec.Thismodel waschosenmainlyforthe
unencrypteddata.Withthis,informationcaneasilybe recordedfromthe 8-bitcommunicationprotocol
and acquire data viaLabVIEW.By doingthis,we can integrate boththe strainsensorandpulse oximeter
data intoone algorithm. Figure 11 showsthe device specificationprovidedbythe manufacturer.Forthe
purpose of thisdevice,thispulse oximeterfunctionalitywassufficienttomeetthe need.

22. 21
Figure 11: Pulse Oximeter device specifications
Data Acquisition Unit
The centralizedacquisitionunitattachedonthe hipacquirestwophysiological signals;the straindata
fromrespirationandoxygensaturationfromthe pulse oximeter.The currentprototype consistof an
ArduinoUnomicrocontrolleraswell asa breadboard withresistorsandwires.AnArduinoUnowas
chosenbythe teamdue to itsflexibilityandopensource environment.Mostof the membersof the
teamhave usedit inthe past and are quite familiarwiththe Arduinoenvironment. Itisalsoinexpensive
and backedbya large online community.The componentalsocontainsthe Wheatstonebridge circuit
witha digital potentiometertonull the initialreadings.
Throughthisdata acquisitionunitthe strainsensordatawill be collectedasa voltage whereasthe pulse
oximeterdatawill be collectedasaserial input.The currentdesignhasa stand-alone system.The
acquisitionunitisbatterypoweredbya9-voltbatteryand alsothe data is savedontoan SD card. The
userwill remove the SDcard fromthe acquisitionunitandinsertitintoa computer,where the diagnosis
algorithmwill analyzethe data.
Apnea Algorithm

23. 22
In orderto detectthe total numberof apneaeventsduringapatient'ssleepcycle,straingauge voltage
measurementsaswell asdigital pulse oximeterdatamustsimultaneouslybe processedbyaLabVIEW
detectionalgorithm.The ultimategoal of thisalgorithmistotake data measurementsfromthe data
acquisitionunitandtranslate thisintoaclear,understandablesleepapneadiagnosisathome forthe
userwithoutthe needof a medical specialist.
The firststepof the detectionalgorithmistodevelop anapneathresholdvalue.AccordingtoAmerican
Associationof SleepMedicine(AASM) clinical guidelines,anapneais“scoredwhenthe peaksignal
excursionsdropby≥ 30% of pre-eventbaseline”.Therefore,the detectionalgorithmcalculatesthe
apneathresholdvalue asa 30% decrease fromthe maximumvoltage measurementfromthe strain
gauge data.
Once a thresholdvalue iscalculated,the nextstepof the detectionalgorithmistothresholdall the
straingauge voltage data accordingto thiscalculated thresholdvalue.The outputfromthisstepisa1-D
array of time-stamped0'sand 1's; a zeroindicatingavoltage measurementbelow thresholdvalue and
therefore inanapneastate.
The final stepof the detectionalgorithmistoanalyze the thresholded dataarray.Searchingthis
array for groupsof consecutive zerosallowsthe identificationof apneaevents.AccordingtoAASM
clinical guidelines,the associated30+ %drop inthe respiratorysignal mustlastfora minimumduration
of 10 secondstobe scored as an apneaevent.Thereforethisthresholdedarraymustbe searchedfor
groupsof N or more consecutive zerostodetermine the numberof apneaeventsapatientexperiences
duringsleep(whereN equal the numberof datameasurementstakenin10 seconds).Once the number
of apneaeventsare determined,apatient’sapnea-hypopneaindex(AHI) canbe calculated;thus
allowingforarelevantdiagnosistobe made.The durationandbreathingfrequencyof eachapneaevent
isalso calculatedanddisplayedforfurtherbreathinginformation.
Changes from Initial Design
One of the mainchangesthat wasmade from the initial designisthe designof the strainsensors.Our
initial prototype consistedonasimple dog-bone design (Figure 12).Thisdecisionindesignwasmade
mainlyforthe ease of fabrication.However,fromconductingvariousvalidationtests,thisdesignwas
conclude tobe inefficienttomeetthe designcriteria.Frompreviousvalidationtests,we saw thatthe
initial sensorcanonlyaccuratelymeasure strainof about5%. Withcertaindisplacementof stretch,the
traces of the sensorsbegintobreak,causingthe resistance valuestoincrease toanextremelyhigh
value.
Figure 12: Dog-bone Design
To account for this,thisquarterwe beganfabricatinganew designof serpentinedesignedsensors.This
decisionwasmade basedonresearchdone onliteraturesof studiesbeingdone. Table 2showsdata

24. 23
froma studydone onmicrofabricatedstraingaugestomonitorbone deformation.The numberof turns
data referstothe numberof serpentineturnsthatthe trace hadin the design.A 10mPa strainwas
appliedinthe x-axisof the sensor.Aswe see,asthe numberof serpentine turnsthe straingauge had,
the amountof strainincrease upto a certainvalue.Thiscan isthoughtto be due to the the distributed
force across the turns of the serpentine sensorallow lessstressedtobe allowedbyeachturns.
Table 2: Table of strain sensor design data
Figure 13 showsthe CADfile of the currentprototype forthe strainsensor.It isa serpentine design
consistingof 3 turns witha trace thicknessof 4mm.The complexityof ourstrainsensordesignis
extremelylimitedbythe fabricationprocess.The mainlimitationisdue tothe sputtercoater’s
sputteringdiameter. The currentmodel whichthe teamhasaccessto onlyhas an active diameterof
roughly2 inch.Due to thisthe bulkof our sensormustfitin thisdiameteris,preventing usfrom
designingserpentine designswithmore turns.Althoughthere isthe optionof makingthe actual traces
thinner,athintrace increasesthe chancesof cracks whenstretched.
Figure 13: Current Strain Sensor design
Anotherdesignchange thatwasmade to the sensoristhe widthof the trace loop.These adjustments
were basedoff of literature aswell.Figure 14 showsthe reference foreachparameterforease.

25. 24
Figure 14: Serpentine Design parameters.
Accordingto literature,arisingissue withserpentinedesignisafactor knownas transverse loading.
Whenlongitudinalstrainisappliedtothe sensorasshowninfigure 14, the presence of atransverse
area can degrade the sensitivityaswell asintroduce errors[12].These are causedbythe longitudinal
stresson the endof the loopwhere the mechanical stressisperpendiculartothe directionof the
electrical currentflow[13]. However,suchdegradationcanbe preventedbymakingthe w_loopwidth
thickerthanthe widthw_pr.
Additionally,throughfinite elementanalysisfoundthroughresearch,we saw thatthe moststrain occur
inthe loopareaof the serpentine design.Figure 15showsan image of such FEA diagram inwhichthe
straingauge were stretched10 MPa inthe Y-direction.Fromthese factors,the currentdesign
incorporatesa4 mm thickw_pr withan 8mm thickw_loopdimension(Figure 14)
Figure 15: Finite element analysis (FEA) of serpentine design.
Cost Breakdown
Component Price per Unit
Custom-made PlatinumStrain Gauge (Platinum,Reagents) $0.25
Arduino Uno Microcontroller $15.00
Commercial Pulse Oximeter $20.00
Circuit Components (Breadboard, wires, resistors) $4.00
Total $39.25
Table 3: Cost Break down
The cost of our device hasnotchangedfromthe initial designthatwe anticipated.Althoughthe design
and placementsof sensorsmayhave changed,thisdoesnothave animpacton the cost. Initially,we

26. 25
have hope to reduce the cost inthe final prototype bymakingcustomprintedcircuitboards.However,
do to the lack of time,we couldnotaccomplishthatin time.
Project Timeline
In our initial timelinewe createdwe hada couple of goalssetfor where the prototype shouldbe atthe
endof the springquarter.These goalswere tofabricate a sensorthatcan monitorrespiration,develop
an algorithmthatcan detectsome formsof sleepapneas,andlastly,tohave adata acquisitionunitthat
integratesbothsensorandpulse oximetercomponent.However,we accomplishedabulkof these goals
inthe earlystagesof developmentthisquarter,givingthe teamtime toimprove uponthe device.With
this,a newgoal for the endof the quarterwere setfor the final timeline. Asmentionedearlierinthe
previoussections,the firstof these goalswastofindan optimal designforthe strainsensors.Next,our
goal was to actuallytestthe device ona humansubject.Bydoingso the teamwill be able tofindan
optimal placementforoursensors.Lastly,there wasaneedtoadjustthe algorithmtomeetthe new
needsthatemergedfromthe initial testings.These changesincludedetectingadditionalsignal
parametersotherthanamplitude suchasphase shiftsandfrequencychanges.
Initial timeline forend ofspring quarter Final timeline forend of springquarter
Fabricated a sensor that can measure
displacement in respiration
Find an optimal sensor design to maximize
strain.
Develop an algorithm that can detect forms of
sleepapnea
Finding an optimal placement of sensors to
accurately monitor respiration
Have a data acquisition unit that integrates both
sensor and pulse oximeter data.
Improve algorithm to detect phase shifts and
frequency changes.
Improve dataacquisitionunittoloaddata to an
SD card insteadof viaUSB
MANUFACTURING DOCUMENTATION
Bill of Materials
PART COST VENDOR INFO
CMS 50D+ Blue Finger Pulse Oximeter 39.00 Amazon
Arduino Uno Ultimate Starter Kit
(Board,Resistors, Potentiometer, Jumper Cables,
Breadboard)
54.99 Amazon
Arduino Uno Ethernet Shield 12.99 Amazon
4GB Micro SD Card 8.00 Spark Fun
Digital Potentiometer 3.00 Spark Fun
INA125P Op Amp by TI 6.43 Mouser
Polystyrene 2.97 Home Depot

27. 26
Con-Tact Clear Self Adhesive Film 8.44 Amazon
Ecoflex 30 30.10 Amazon
Grafix Frisket Film 14.93 Amazon
Laser Cutter N/A Borrowed from Lab
KT Tape N/A Borrowed from Lab
Oven N/A Borrowed from Lab
Sputter Coater w/ Platinum N/A Borrowed from Lab
Spin Coater N/A Borrowed from Lab
TOTAL 180.85
Table 4: Bill of Materials
Strain Sensor Manufacturing Process
The fabricationprocedure forthe sensorscanbe brokendownintoathree-stage processof lithography,
miniaturizationandlastlythe transferstep. Figure 16showsan overview of the entireprocess.

28. 27
Figure 16: Fabrication process of the strain sensor
Lithography
The firststepof lithographyisessentiallythe stage wherethe sensordesignismade,andcoatedwith
metal.The fabricationbeginsbycreatingthe maskdesignoutof Con-tactpaper. The initial designis
createdusingcomputer-aideddesign(CAD) software whichisuploadedtoalasercutter.The first
generationstrainsensorswere made usingasimple Dog-bonedesign,inordertogetfamiliarwiththe
fabricationprocess.Usingthe lasercutter,the Con-tactpaperiscut according to the CAD drawing. The
cut out Con-tactpaperis thenplacedcarefullyontopof a Polystyrene(PS) filmthathasbeenwiped
clean.Next,the maskedfilmisplacedintoasputtercoaterinorderto coat the filmwithathinlayerof
metal.The sputtercoaterwill laya thin10nm layerof platinumontothe film.Althoughplatinummaybe

29. 28
expensive,usingnano-scale amountsof itallow ustokeepthe manufacturingcostslow.Lastly,the mask
isremovedfromthe film,resultinginthe platinumcoatingonlyinthe outlinedarea.These Coatedfilms
are storedina designatedareatopreventdustanddirt frompotentiallyaffectingthe final sensors.
Figure 17 showsa picture of the final productof the lithographystage,itisa50x100mm PSfilmwitha
simple straingauge designcoatedwithplatinum.
Figure 17: Final Product of the Lithography stage
Miniaturization
The secondstage of the fabricationprocessisminiaturization. Byutilizingshape memorypolymersuch
as PS,we are able to easilyfabricate small scaledsensors.The PSfilmsare placedina 160 Celsiusoven
where theyare thermallyminiaturizedtodownapproximately67% of theiroriginal size.The films
remaininthe ovenuntil noshrinkingcanbe physicallyseen,thenleftouttocool.Figure 18 showsthe
resultof the miniaturizationstage.The outcome isanapproximately20x 40mm PSslab.The wrinkleson
the platinumcoatsmade as a consequenceof the shrinkingenhance the sensitivityof the sensors.The
overlappingplatinumcausedbywrinkle allowsenhancedconductivityevenwhenstretchedleadingtoa
highergauge factor.

30. 29
Figure 18: Final Product of Miniaturization Step
Transfer Step
In the transferstep,the sensorsare coatedwithEcoflex 30. Ecoflex 30 isa commerciallyavailable
silicone basedbiomaterial thatisbeingusedforwearable technologiesbecauseitislightweightandis
elasticallycompatible withpatientmovement,andalsohypoallergenic.
Afterthe Ecoflex isprepared,the polymersolutionisputina vacuumto remove airbubbles.Usinga
spin-coater,the sensorsare coatedwithanevenlayerof Ecoflex.The coatedsensorsare placedina
vacuumto remove anyair bubblesonce again.The sensorsare thenbakedinan 85 degree ovenfor2
hours.Thisenablesthe Ecoflex tobindtothe platinumcoating.The resultedcoatedsensorsare placed
intoa seriesof solventbathstocompletelyremove the PSportionoff of the sensor.Firstthe sensorsare
placedinan Acetone bathfor30 minuteswhile placed ona55 degree Celsiushotplate inorderto
remove the PSsheetsfromthe sensors,resultinginasensorconsistingonlyof Ecoflex andplatinum. To
remove anyexcessPSresiduals,the sensorsare placedina10 minute toluene bathona 70 degree
Celsiushotplate.Lastly,the sensorsare hungtodry overnightwithinthe fume hood. Figure 19shows
the final productfor the firstgenerationstraingauge.
Figure 19: Final Product of Transfer Step

31. 30
Pulse Oximeterand Data AcquisitionUnit
The current prototype forthe pulse oximeteranddataacquisitionunitinvolvescommercial
components.All of these componentswere purchasedthroughavendorandputtogetherbythe team.
Future workinvolveseliminatingthesecommercial componentstominiaturize the device aswell as
reduce the total cost.
Manufacturing Limitations
The current manufacturingprocessdoesnotallow massproductionof strainsensor.The methodlaid
out onthisreport can onlyproduce about6-8 sensorsperweekaseachsensorrequirescomplex,hands
on fabrication.Toenable large scale productionof the strainsensors,anupgraded,large capacity
equipmentwill be need.Specifically,asputtercoaterwitha largersputteringradiuswillincrease
production ascurrentlythisisthe most time-consumingstepof the manufacturingprocess.The
manufacturingprocessalsoneedstobe fine-tunedandtasksinvolvinghumaninterventionthatcan
replacedbymachine needtobe identifiedtoincrease production.
MATERIALS SELECTION AND VALIDATION
The main componentsof astrain gauge can be dividedupintothree categories:the foil,the supportive
backing,andthe adhesive.Afterfabricationof ourconformal straingauge,the final productconsistsof
three materialsusedforthe above components:platinum, Ecoflex,andKTTape,respectively.Selection
and validationof these materialsisdiscussedfurther.
Foil – Platinum
The material forthe straingauge foil mustmeetthe followingthree criteria:
 Goodconductor ofelectricity- Highelectrical conductivityisanobviousbutmostimportant
propertyfora straingauge foil.Withoutaconductingfoil,there will be nochange involtage
across the Wheatstone Bridge andthusnodisplacementof the patient's chestorabdomencan
be measured. Therefore,the foil shouldbe metallic.
 Sensitivityto strain especiallywhenstrainis small - The metal mustalsobe able to outputa
measurable change of resistance atlow strainsbecause the displacementof the chestcanbe
verysmall duringnormal breathing.Thus,ametal witha highgauge factor at low strainsismost
favorable.
 Low reactivity especiallyat hightemperatures - A low reactive material wouldbe optimal for
betterqualitycontrol duringthe heattreatmentprocessesof fabrication.

32. 31
Many metalswere consideredbutnone wasmore suitable thanplatinum.Whileplatinumsurelyisn't
the metal withthe highestelectrical conductivity(Table 5),certainlyitisgoodenoughforthis
application.
Material Conductivity (S/m)
Silver 6.30 × 107
Aluminum 3.50 × 107
Platinum 9.43 × 106
Glass 10-15
to 10-11
Teflon 10-25
to 10-23
Table 5: Conductivities of different materials [14]
Platinumalsohasa highgauge factor at low strains(Table 6) makingitsuitable formeasuringthe small
displacementof the abdomenwhile breathing.Lineardisplacementtestswere donetovalidate this
claim.The change of resistance asa functionof strainwas measuredusingthe firstgenerationtrace
designand a uniaxial tensiontestingapparatusprovidedbythe Khine Lab(Fig. 20).
Material Low Strain GF High Strain GF Ultimate Elongation
Copper 2.6 2.2 0.5
Nickel -12 2.7 --
Platinum 6.1 2.4 0.4
40% gold/palladium 0.9 1.9 0.8
Table 6: Gauge factors (GF) for various materials at different strains [15]

33. 32
Figure 20: CAD model of tension testing apparatus using first generation strain gauge
The strain gaugeswere clampedtocopperelectrodesonthe stretchingapparatuscoupledtoapower
source and computerprogramthat, whena value isinserted,wouldmechanizethe stretchertoexpand
the specifiedamount.Usingasimple multimeter,the resistance acrossthe straingauge can be
measuredbycontactingthe multimetertestleadstothe straingauge terminals.Gauge factorwasthen
calculatedusingthe equation:
𝐺𝐹 =
𝛥𝑅/𝑅
𝜀
=
𝛥𝑅/𝑅
𝛥𝐿/𝐿0
where εis the strain, ΔL isthe change in length, L0 isthe original length, ΔRisthe change inresistance,
and R isthe unstrainedresistance of the straingauge.Afterincrementallyincreasingthe platinum
gauges(upto 8 mm) and measuringthe change of resistance ateachstep,itwas foundthat platinum
didinfact have a veryhighstraingauge (~50) at low strains.

34. 33
Figure 21: Reactivity series of metals [17]
Platinumisalsothe leastreactive metal inthe reactivityseriesof metals(Fig.21).Ithas remarkable
resistance tocorrosion,evenathightemperatures,andistherefore considereda noble metal [16].
Measuringthe reactivityof platinum--like measuringthe conductivity--wasnotnecessaryforvalidation.
The platinumfoil performedasexpectedandultimatelymetall the acceptance criteria.
Supportive Backing – Ecoflex
The three main criteriathe supportive backingmustmeetare:
 High elasticity- The straingauge ismeantto stretchand be conformable.
 Gentle onskin - Must be biocompatibleandshouldnothave anyadverse reactionwiththe skin.
 Insulating- So that noelectriccurrentwill escape fromthe metallicfoil.Leakageof currentwill
skewresistance dataandpossiblyharmthe patient.
Ecoflex®
00-30 performedtothe manufacturer'sspecificationsandwasable tomeetall the acceptance
criteria.Accordingtothe manufacturer,thissuperstretchysiliconepolymercanbe stretchedupto
900% before breaking(Table 7).Usingthe aforementionedtensiontestingapparatus,max elongation
testingof the straingauge was performed.Itwasfoundthatthe backingwas able to stretchpast the 8
mm mark,whichisthe maximumdisplacementof the chestforthe average personaccordingtothe
EuropeanRespiratoryJournal [19].Furthermore,thiscommerciallyavailable elastomerhaspassed
irritationandskinsensitivitytestsbythe InternationalOrganizationforStandardization [18] andis
commonlyusedasthe material forHalloweenmasks,children'sdolls,andCPRtestdummies.Rubber-
like polymerslike Ecoflex are alsoinherentlyinsulating.Usingamultimeterto measure the voltage
across the supportive backingonanactive straingauge showedthatno currentwas able to leakthrough
the polymer.These material propertiesmake Ecoflex asuitable backingforourwearable straingauge.

35. 34
Table 7: Technical overview of Ecoflex material properties
Adhesive - KT Tape
The acceptance criteriafor the straingauge adhesive are:
 Biocompatibility- The adhesive shouldbe gentleonskinwhen removed(i.e.low-trauma) and
shouldcause noallergicreactionswhenincontactwiththe skin.
 Breathable - Moisture such as a sweatbuildupunderthe straingauge canaffectthe properties
of the adhesive andcause discomforttothe patient.Therefore,itisimportantthatthe material
admitsair to the skin andallowssweattoevaporate.
 Durability - Must be able to adhere properlyfor12 hoursand shouldbe easilyrepositionedif
removed.
 Resistant to heat - Adhesivenessshouldnotdiminishatelevatedtemperaturessuchasbody
temperature.
KT Tape isa commerciallyavailable sportsandfitnesstape designedformuscle,ligament,andtendon
painrelief andsupport.The tape isdesignedtobe wornonthe skinforup to 72 hours,attestingtothe
biocompatibility,durability,andheatresistance of the material[20].The adhesive mayalsobe worn
duringstrenuoustasksthatmay cause you to perspire suchasworkingoutor evenswimming,which
furtheratteststo the durabilityandbreathabilityof the material.Anotheradvantage of usingthe KT
Tape isfor its fabricdesignwhichprovidesuni-directional elasticityallowingthe tape tostretchin length
but preventingthe tape fromstretchinginwidth--anattractive propertyforauni-directional strain
gauge.Duringpatienttestingof ourstraingauges,the KT Tape performedtothese specificationsand
more than metthe acceptance criteriafor an adhesive.
The other componentsof the device suchasthe pulse-ox anddataacquisitionunitare readilyavailable
materialsandnotmanufacturedbythe Khine lab.Therefore,nomaterialsvalidationtestswere
necessaryandall componentsperformedasspecifiedbytheirmanufacturers.

36. 35
DESIGN VALIDATION
The nSASdevice consistsof hardware andsoftware elementsthateachrequiredspecializedvalidation
testingtodetermine whetheracceptance criteriahadbeenachievedforeachdesignrequirement.The
hardware componentsthatrequiredvalidationtestingwere the conformal strainsensors,the pulse
oximeter,andthe central dataacquisitionunit.The software componentof the device thatrequired
validationtestingwasthe SleepApneaDetectionAlgorithm.
Table 8: Design and Acceptance Criteria for Device Validation
Strain Sensors
Validationtestingforthe strainsensorswasdone in-vitrousingaliner stretch inducing testing apparatus
(see figure 20).The firstmajortest administeredonthe strainsensorwasmotivatedbythe High
MeasurementResolutiondesigncriteriaandinvolvedgeneratingadisplacement-dependentgauge
factor profile.The secondmajorstrainsensortestwasmotivatedbythe HighPrecisionMeasurement
designcriteriaandinvolvedrepeatedlydisplacingstrainsensorsata controlledfrequencytogenerate
lifecycle plots.
Pulse Oximeter
The pulse oximeterwasvalidatedusingcomparisonstudiesof oxygenationandheartrate
measurementsfromhealthysubjectsandexpectedliteraturevalues.Since the pulseoximeterusedin
the nSASdesigniscommerciallyavailable,the majorityof validationtestinginvolvedassessmentof the
pulse oximeter’sperformance,comparedtoexpectedmanufacturerspecificationsandliterature values.
Central Data Acquisition Unit
Validationtestingforthe Central DataAcquisitionUnitwasmotivatedbythe stabilityandlifetime
designcriteria.The electricalhardware wastestedduringroutine in-vitrotestingtoevaluatethe
performance of the hardware undernormal,repeatedusage andunderstrenuousconditions.

37. 36
The software componentof thisdevice wastestedforitsabilitytodetectsleepapneaeventsunderin-
vitroand in-vivoconditions.The ApneaDetectionAlgorithmwastestedin-vitrobyinputtingsimulated
breathingmeasurements,withknownapneadurationandseverity,toevaluate the performance of the
algorithm’sdetectioncapability.
Detailed Validation Testing
Hardware Components: Strain Sensors
GaugeFactorProfiling
The reasonstrain sensorswere initiallyselectedforbreathingpatternmeasurements,canbe attributed
to theirunique abilitytoundergochangesinresistance,astheiroverall lengthischanged.This
dependence of resistance ondisplacement,wasthe propellingforce inthe initial developmentof the
nSAS,andleadto the needfordesigncriteria#1: High MeasurementResolution.
Highmeasurementresolutionwasanimportantdesigncriteriatotestbecause the strainsensors,
mountedonthe patients’chests,willmeasure relativelysmalldisplacements.Basedonthe average
10mm size of the sensorsalongthe longaxis,eachwascalculatedtoonlyexperience a10% strainwhen
measuringbreathingfrombaselineexpiration,tofull inspirationina1-2mm range. It wouldhave been
difficulttherefore,todistinguishpathological symptomsfromnormal breathingunlessthe devicehasa
fine scale formeasurement.Accordingly,the acceptance criteriafordesigncriteria#requiredstrain
sensorstohave a Gauge Factor (GF) of greaterthan 3, in orderto achieve aspatial resolutionof less
than or equal to10µm. Therebyensuringthatthe device wouldhave atleast100 incremental data
pointsfroma baseline 0mmtothe minimumobservedfull-inspirationdisplacementof 1mm.
Validationtestingfordesigncriteria#1:High MeasurementResolution,wasaccomplishedbyutilizinga
linearstraininducingapparatus (linearstretcher).Newlyfabricatedstrainsensorwere fixedtothe linear
stretcherto generate adisplacement-dependentgauge factorprofile.The linearstretcherinducedstrain
inthe sensorsbyincrementallyincreasinglineardisplacementin50µm steps.The displacement
incrementstepsize wasselectedbasedonrecommendationof Khine labpersonnel andliterature
testingprotocols.The 50µm stepsize providedthe properbalance betweenfinedisplacementand
exaggeratedresistance changes.The resultingresistance changeswere measureddirectlyfromthe
sensorusinga digital multimeter.The Gauge Factorwas thencalculatedandplottedusingthe measured
changesinresistance andthe knownchangesindisplacement.The followingequationprovidesa
detailedlookatthe Gauge Factor calculation.
Figure 22:This is the Gauge Factor (GF) equation which related changes in resistance and changes in displacement.
𝐺𝐹 =
∆𝑅
𝑅
∆𝑙
𝑙
=
∆R
R
𝜀

38. 37
Generatinggauge factorprofilesforstrainsensorsmanufacturedinlabwasan importantvalidation
step,because itallowedthe teamtoevaluate the sensitivityof eachsensorandprovidedfeedback
aboutthe manufacturingprocess.Please refertothe followingsectiontoview the resultsand
conclusionsof these andthe othervalidationtestsperformed.
Precision Validation
The majorityof physiological signalsproducedbythe humanbodyoccur withinanobservable frequency
range.Normal breathinginhealthypatientsduringsleepaveragesatabout12 breathsperminute,or
0.2Hz. Duringa full nightof sleep(7.5hours),a personwill take about5400 breaths.These statistics
motivatedthe teamtocreate designcriteria2: High PrecisionMeasurements of ChestDisplacement.
HighPrecisionMeasurementwasanimportantdesigncriteriatotestbecause the chest-mountedstrain
sensorswouldbe responsibleformeasuringrepeatedchestdisplacementsthousandsof time every
night.Havingconsideredthe small displacementsthe sensorswouldoperatein,the teamemphasized
the importance forthe strainsensorsto reduce randomerror in repeateddisplacementmeasurements.
Large variationsinrepeatedmeasurementsof the same displacementpresentedthe threatof false
positive andmisdiagnosis.Therefore,acceptance criteriaforstrainsensorprecisionrequiredrepeated
displacementmeasurementstohave astandard deviationof lessthanorequal to20µm.
Validationtestingfordesigncriteria#2:High PrecisionMeasurementswasaccomplishedbyutilizingthe
linearstraininducingapparatus.Strainsensorsthathadpasseddesigncriteria#1were againfixedtothe
linearstretcher.The sensorswere stretchedto10% strainand thenback to 0% strain,at a frequencyof
0.4Hz. The strainvariable of 10% was determinedbasedthe literature valuesforchestdisplacement
duringbreathing,aswell as,the team’sownchestdisplacementmeasurementsfromFall Quarterthat
had found10% to be the average inspirationceiling.The cyclingfrequencyof 0.4Hz wasselectedbased
on a literature search,whichrevealedsubjectsexperiencinghyperventilation tookupto24 breathsper
minute,or0.4Hz. Since normal breathingandslowerfrequencieshadalreadybeentestedandproven
successful inpreviousquarters,thisvalidationtestfocusedonfasterthannormal breathing,whichcan
occur in patientsleading uptoor followinganapneaevent.Asthe strainsensorwasstretched
repeatedly,voltage measurementswere acquiredautomaticallybythe nSASCentral DataAcquisition
Unit.The recordedvoltage measurementswere plottedovertime toproduce lifecycle plotsforeach
sensor.Lifecycle plotswere usedbecause theyprovideddetailsaboutmeasurementprecisionand
revealedhysteresis.
Hardware Components: Pulse Oximeter
CommercialComparison
The pulse oximeterusedinthe nSASdevice isacommerciallyavailable medical device thathasbeen
usedinotherbiomedical studiestomeasure heartrate andbloodoxygenation.The manufacturer
providesspecificperformance guidelinesforthe device,aswell as,database valuesforindexfinger
bloodoxygenationmeasurementsundercommonclinical conditions.These resourcesallowedthe team

39. 38
to determine whetherthe pulse oximeterwasperformingreliablyandprovidingaccurate readings.
Pulse oximetervalidationtestingtherefore,involvedaseriesof simple confirmation of expected
performance tests.
Hardware Components: Central Data Acquisition Unit
Normal-UseEvaluation
The Central Data AcquisitionUnitfoundinthe nSASdevice iscomprisedof anamplifiedWheatstone
circuitand a multichannel Arduinodataloggingunit.The Wheatstone circuitwasinitiallyselectedto
interface withthe strainsensorsbecause of itssimplicityandconsistencyinproducingvoltage
measurementsfromchangesinresistance.Sincethe amplifiedWheatstone circuitusedinthe nSAS
device iswidelyusedforsimilarstrainsensorapplications,the objective forthisvalidationtestwas
similartothat of the pulse oximeter–Doesthe circuitperformas isexpectedbasedonthe literature
and manufacturerguidelines?Asforthe Arduinodataloggingunit,validationtestingwasperformedin
directlyduringeveryexperimentinwhichthe dataacquisitionwasusedtomeasure andrecordvoltages.
The performance of the data acquisitionunitwasassessedbasedonwhetherthe unitwasable to
collectmeasurementsforthe full durationof the experiment,andwhetherthe loggeddatahadany
corruptedentries.
SoftwareComponent: Apnea Detection Algorithm
The ApneaDetectionAlgorithmwastestedforitsabilitytodetectapneaandhypopneaevents.
Accordingthe AmericanAssociationof SleepMedicine (AASM),anapneaeventisdefinedbya 30%
decrease inbreathingamplitude.Accordingly,the ApneaDetectionAlgorithmwasvalidatedto
determine whetheritwascapable of detectingapneaeventsundersimulatedin-vitroandin-vivotests.

40. 39
Validation Results
GaugeFactorProfiling
The High MeasurementResolutionvalidationtestingcorrespondingtocriteria#1 were administeredon
nearlyall 2nd
and 3rd
generationplatinumsensorproduced.The measuredchangesinresistancewere
graphedwithrespecttothe knownchangesindisplacement,producingagauge profile.The slope of the
gauge profile isrepresentative of the Gauge Factor(GF) of eachsensor.
Figure 23:A plot demonstrating the linear relationshipof the 3rd generation platinumstrain sensors observedbetweenchanges
in displacement and changes in resistance.
Thisplotshowsthe relativelylinearrelationshipbetweenchangesinDisplacementandchangesin
Resistance fora single serpentine,3rd
generationplatinumsensor.The slope of thisline,orGauge Factor
(GF),isapproximately20.
Figure 24: The Gauge Factor derived from all the strainsensors reproduced above byaveraging GF across 8 tested sensors with
standard error bars.
y = 20.394x - 0.2516
0
0.5
1
1.5
2
2.5
3
3.5
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14
ChangeinResistance(∆R/R)
Strain(∆𝑙/𝑙 = ε)
Platinum Strain Sensor
0
5
10
15
20
25
GF
Gauge Factor
3rd Generation Pt Sensors

41. 40
The resultsof designcriteria#1 showedthatthe average Gauge Factor of the 3rd
generationserpentine
strainsensorswasapproximately20.SolvingfordeltaL inthe GF equation,allowedthe teamto
calculate the spatial resolutionof the strainsensorstobe 10µm. These strainsensorshave metthe
acceptance criteriasummarizedinthe table below.
Figure 25: The Gauge Factor Equation seen here is resolved for delta L, or spatial resolution.
Design Criteria Acceptance
High Resolution Measurement Gauge Factor (GF) > 3 for spatial resolution of ≤ 10µm
Table 9:DesignandAcceptance Criteria for HighMeasurement Resolution. Note that the acceptance criteria was achieved.
Precision Validation
The resultsfromthe High PrecisionMeasurementvalidationtestshow anacquiredvoltage signalfrom
the displacementcycledstrainsensors.The signal showsarelativelyconstantpeak-to-peakamplitude of
0.82 Volts± 0.01 Volts.These valuessuggestthatthe systemhadverygoodprecisionsince peak-to-peak
amplitudesare conservedovertime however,risingbaseline voltagesbecause of strainsensor
hysteresis,make these lifecyclemeasurementsunreliable. A Fouriertransformanalysisof thissignal
revealedalarge andisolatedpeakat0.4Hz, whichshowsthat the systemiscapable of precisely
measuringinputfrequency.
Figure 26:This is a lifecycle plot ofthe voltage measurements acquiredfrom the strain sensor undergoing cyclic stretching.
𝐺𝐹 =
∆𝑅
𝑅
∆𝑙
𝑙
=
∆R
R
𝜀
 ∆𝑙 =
∆𝑅
𝑅
∗
𝐿
𝐺𝐹

42. 41
The validationtestsfordesigncriteria#2reveal asystemthat has the abilitytoreproduce datawitha
small marginof error. The oscillationtestsshow thatanaverage measuredvoltage of 0.82 Voltswitha
standarddeviationof 0.01 volts.Althoughthe criteriaandacceptance are createdintermsof
displacement,notvoltage,these resultsdonotmeetthe acceptance criteriasummarizedbelow.
Design Criteria Acceptance
High Precision Measurement Measure displacements with standard deviation ≤20µm
Table 9:DesignandAcceptance Criteria for HighPrecisionMeasurements. Note that the acceptance criteria was not met.
CommercialPulse Oximeter
The Commercial Pulse Oximeterperformedasitwasexpectedtoaccordingtothe manufacturer’s
guidelines.The validationtestingrevealedthatthe pulse oximeterwascapable of accuratelymeasuring
the heart rate and pulse oxygenationof teammembers.Basedonthe observableperformance of the
commercial pulse oximeter,the productwas acceptedintothe device design,despitethere beingno
explicitacceptance criteriasetforit.
CentralData Acquisition Unit
The central data acquisitionunitperformedwithoutissueunderthe normal-use conditioninwhichit
was tested.There were noindicationsof measurementfailure ordatacorruption.The unithas
functionedproperlythusfarforover50 hours of combinedusage.The central dataacquisitionunit is
acceptedintothe device design,despite there beingnoexplicitacceptance criteriasetforit.
Apnea Detection Algorithm
The acceptance criteriafor the SleepApneaDetectionAlgorithmstatedthatthe software mustbe
capable of detectingapneaeventswithhighspecificityandhighsensitivity.Althoughspecificityand
sensitivitywere nottestedinthe scope of thisclasswithproperstatistical analysis,the apneadetection
algorithmhadprovenitsabilitytodetecthypopnea andapneaeventswithanapproximately30%
decrease inbreathingamplitude andsubsequentdecreasesinmeasuredbreathingfrequency.Without
properlycarryoutthe necessarystatistical analysis,itcanbe saidthat the algorithmmeetsacceptance.
Figure 27:This screenshot of the labview GUI displays breathingsignals fromthe abdomen inred and the chest inwhite. Notice
the four detected sleep apnea events, demonstrated by diminished amplitude and frequency fluctuations.

43. 42
FAILURE MODE AND EFFECT ANALYSIS (FMEA)
Throughthe variousvalidationtestingmultiplefailure modeswere identifiedalongthe process.
Althougheveryaspectof the device canhave a possible failuremode,the teamconcentratedonthree
mainproblemstoaddress.
The firstof these modesof failure isthe inadequate connectionbetweenthe sensorandthe leadsthat
connectto the data acquisition.A goodsolidcontactisnecessaryinorderto acquire the varying
resistance value thatwill ultimatelybe avoltage reading.The main cause of the mode of failure are the
poor leadsthatwere initiallyused.Ourpreviousprototype usedasimple conductivecoppertape that
was foldedmultiple timesintoshape andtapedontothe sensor.As showninFigure 28, the leadsare
quite bigandalso notconformal.Thismethodof contact wasconsistentlyunreliablesince the lead
wouldcome off contact whenthe subjectmovedtooquicklyorforcefully.Anotherproblemwiththe
previousleadswasthatthe majorityof itcame incontact withnot only the sensorbutalsothe subject’s
skin.Thiscontact can introduce unwantedresistance valuesinthe readings. These problemswiththe
leadscan introduce artifactsinthe displacementdata.The ultimate effectof multiple artifactsis
misdiagnosisof sleepapnea.Inorderto mitigate thisproblem, the currentprototype usesaninsulated
copperribboncable,asshownin Figure 29. The structure of the ribboncable allowsittobendand twist
to the patient'sbody.Inadditionbecause the leadsare insulated,thiseliminatesthe contactbetweenit
and the subjectskin.Withthese newleadsthe voltagedatasaw noticeablylessartifactswhentesting
subjectmovementsduringsleep.
Figure 28: initial prototype with copper tape leads
Figure 29 Final prototype with conformal leads
The secondof these mode of failure isthe overextensionof the strainsensor.Thisoverextensioncauses
the sensortraces tocrack. Thiscracking separatesthe conductingmaterials,resultinginextremelyhigh
and inconsistentresistance readings.Althoughnottothe same extent,the same problemwasseenin
previousprototypesof the sensor.

44. 43
Withthe currentprototype,the sensorisable tostretch30% withoutthe tracesbreaking.Thislevel of
strainwas seentobe sufficientintesting.Howeveranystrainsignificantlyhigherthanthislevel canlead
to unreliablereadingorevenworse,ruinthe sensor.Thisoverextensioncanbe causedbymishandling
and improperplacementof the sensor.If the userwere tocarelesslystretchthe sensorwhile applying
or repositioningthe sensor,he/shemayunintentionallycause the tracestobreak.As mentionedearlier,
the inconsistentresistance causedbyoverextensionwill indicateanunreliable displacementreading.
Withthis, the sensorcannot accuratelymonitorthe respiration.Furthermoreif the sensorwere tobe
extendedfurtherthanthislevelthe sensortracescancompletelybreak,permanentlyruiningthe sensor.
If this were tohappennorespirationcanbe monitored,consequentlyresultinginafaileddiagnosis.To
mitigate thisfailure the teamwill mostlikelycreate aclearand userfriendlyinstructionsforthe patient.
Thisusermanual is writteninthe latterpart of the report.The manual will have clearinstructionsonthe
careful handlingof the sensorandalsoa detailedplacementof the sensortopreventthe needof
repositioning.Withthisoverextensionof the sensorcanbe minimized.
The last of the failure mode thatwasassessedisthe adhesive insufficiency.A critical problemwiththe
sensorisif the sensorwere to fall off inthe middle of the night.Thiscanbe causeddue to movements
or to slippingcausedbysweating.Thisinsufficientadhesive of the sensorcanultimatelyleadstono
diagnosishappening.Inordertopreventthisthe designincorporatedanelasticfitnesstape,alsoknown
as KT tape.Used inthe kinesiologyfield,these adhesive tape are oftenusedformuscle support.
Howeverthere biocompatibility,breathability,andaffordabilitymake the tape anideal adhesive forthe
sensor.
Althoughmanypotential modesof failurewere able tobe addressedtherewere several thatwere not
able to assessdue tothe lackof time.One of the modesof failure thatwasnot exactlyassessedinour
device wasthatthe wiresof the device maybe pulledoff duringsleepmovement.The device contains
twowiresthat connectto the data acquisitionunit;one fromthe sensorandone fromthe pulse
oximeter.The mainmitigationthatthe teamwasworkingtowardswasmakingthe device entirely
wireless.Withthisinmindthe teampurchasedandbeganworkingwithaBluetoothmodule thatcanbe
incorporatedintothe Arduinounit.Howeverdue tothe codingandtroubleshootingrequiredforthe
module,the mitigationwas notable tobe incorporatedintothe final designforthisquarter.
Anothermitigationtoeliminate the needforwireswasthe move the pulseoximeterfromafingerbased
device toa chestbaseddevice.Thisplantoincorporate a chestbasedpulse oximeterwasbasedoff of
literature done earlierinthe quarter.The conventional pulse oximeterusesamethodknownastrans-
illuminationinwhichlightistransmittedthroughthe sampletocalculate absorptionof light. However
as opposedtothis,byutilizingepi-illumination,amethodtoanalyze absorptionof lightthrough
reflection, thispulse oximetercanbe placedon areasof the bodyconventional pulse oximeterscan’t.
The original mitigationwastohave thispulse oximeterplacedonthe chestwiththe strainsensorto
completelyremove wiresfromthe device.However,simultaneouslyworkingwiththe strainsensorand
the chestbasedpulse oximeterwasnotplausibledue tothe lackof time andresources.
A potential mode of failure thatwasunable tobe mitigatedwasthe variabilitybetweensensors.
Betweenindividual sensorswithinthe batchesthere wasaclearvariance ininitial resistance. This
variance can be traced downto the fabricationprocess.Additionally,itisextremelycommontosee

45. 44
sensors thatdo not have anyconductivityfromthe beginning. However, manyof the factorsthat can
contribute tothese changesare out of the team’scontrol.Factors suchas how equallythe platinumwas
coatedonto the filmorthe unevenshrinkingof the sensorscanall affectthe qualityof the sensor. With
thisbeingsaid,the nSASdevice doesnotrelyonthe initial resistance of the sensor.Becausethe
algorithmlooksforchange inresistance,aslongas the gauge factor of the fabricatedsensormeetsour
designcriteriathe variabilityof initial resistance shouldnotbe aproblem.However,withmore time and
resourcesthe teamcan assesswaysto improve the fabricationprocesstoidentifyproblemsandbuild
qualityassurance standard.
LESSONS LEARNED DOCUMENTATION
Issue 1: Lead Connection Failure
Sometimes it was found that the leads connecting the sensor to the data acquisition unit would
fail resulting in erroneous data being recorded. Switching to more conformal flat ribbon cables
fixed the issue and provided for a more robust sensor when worn on the chest and abdomen.
In the future it is recommended that leads be short and as thin as possible for use on the body
when the sensor is as small as the one being used.
Issue 2: Adhesion Failure
The first prototype of the device that included the use of KT Athletic Tape would sometimes fail
where the adhesion points on either side of the sensor would come loose. This was remedied
by increasing the size of the adhesion points for more surface area. The failure would lead to
inaccurate strain being reported and would cause the algorithm to detect false apnea events.
In the future it is recommended to use the larger surface area for adhesion points as well as
incorporating a contact detection system (capacitive sensor) for feedback if the adhesion fails.
Issue 3: Strain Gauge Overextension
It was observed that the strain gauge would get overextended from time to time. This would
cause cracks in the sensor which would sometimes seal up as the sensor contracted or would
permanently cause the sensor to break. This was remedied by using smaller strips of KT Tape to
minimize the strain put on the sensor. The high gauge factor of the sensors allowed for very
minimal strain, around 10%, to be able to distinguish an apnea event from normal breathing. In
the future it is recommended to use a backing material that will not stretch more than the
maximum strain allowed for the sensor.
USER DOCUMENTATION AND TRAINING
The setupof the hardware of our designwill ultimatelybe streamlinedforthe simplicity of the userby
eliminatingoroptimizingvariouscomponentsimplementedinourcurrentprototype.The final design

46. 45
goal is to have a strainsensorattachedto a peel-off adhesive backingsothatthe usercan easilyplace
the sensorinplace on the chest,similartoa bandage.In additioninourfinal design,we wishto
implementachestbasedpulse oximeterthatwouldalsobe attachedina similarfashion.Placementof
the two sensorsonthe chestwill be clearlyoutlinedinauserinstructionmanual providedwithourfinal
device.Thiscohesiveunitonthe chestwouldtransmitmeasurementsviawirelesscommunicationtoa
portable dataacquisitionunitmountedonthe user'sbody.These measurementswouldthenbe saved
by the unitontoa SD card so that data couldbe inputdirectlyintothe detectionalgorithm.For
prototypingpurposes,measurementsfromthe SDcard were sentto a detectionalgorithminLabVIEW.
The onlyparametersthatthe usermustadjustin the currentdetectionalgorithminLabVIEWisthe
locationof the data textfile onthe SD card and the data acquisitionunit'ssamplingfrequency(sampling
frequencyspecificationwill be automatedinthe finaldesign).Afterthe usersetsthese inputsandruns
the program, he or she is presentedwiththe numberof apneasexperiencedduringobservationof the
device andtheircorrespondingdurations,allowingforthe diagnosisof sleepapneatobe made.
FUNCTIONAL TRIALS
Afterperformingvalidationtestsonbothourstrainsensorsand commercial pulse oximeterto
appropriatelycharacterizetheirrespective behaviors,ourgroupwasreadyto begintestingthe
functionalityof ourdesign.Inorderto testwhetherourstrainsensorscouldmeasure human
respiration,strainsensorswere placedonagroupmember'schestand abdomenwhiledatawas
recordedbya data acquisitionunit.Uponprocessingandfilteringthe data,voltagesfromthe strain
sensorwere plottedovertime fortrialsof regularbreathingandtrialscontainingsimulatedapnea
events(temporarilystoppingbreathing).These plotsare seeninFigures30& 31. From these plotswe
were able toobserve a91% decrease inbreathingamplitude duringapneaevents,thushighlightingthat
our sensorsare sensitiveenoughtodetectbothregularandabnormal breathingpatternswhenwornon
the body.Afterobservingthe breathingwaveformandrecordingthe parametersof simulatedapnea
events,the datawasinputintothe apnea detectionalgorithm.Afterbeingprocessedbythe algorithmin
LabVIEW,the numberand durationof simulatedapneaswerecomparedtocalculatedestimatesoutput
by the detectionalgorithm.Inall trials,estimatesfromthe detectionalgorithmmatchedsimulated
apneasinboth numberandduration.Duringourexperimental trials,bloodoxygenationmeasurements
were simultaneouslytakenfromoursubject viaafingerbasedpulse oximeter.The readingsfromthe
pulse oximeterdidnotfluctuate fromhealthyrangesbecause simulatedapneaswerenotsubstantially
long(30 secondsorless) due tofeasibilityandsafetypurposes;thuspreventingadetectable dropin
bloodoxygenationfromevenoccurring.
In orderto assessthe accuracy of our device asa whole indetectingsleepapnea,ourdevice wouldhave
to enterclinical trials.Subjectswithsleepapneawouldhave tovolunteertoundergonightsof
observationwhile havingtheirbreathingrecordedbythe goldstandardpolysomnographandourdevice
simultaneously.The accuracyindetectingnumberof apneas&hypopneas,the lengthsof these
breathingevents,breathingfrequencies,andbloodoxygenationlevels betweenbothdetectionmethods
will have tobe quantitativelycompared.Thissetof previouslymentionedparametersare outlinedby
the AmericanAssociationof SleepMedicine (AASM) clinical guidelinesasthe minimumphysiological
signalsnecessarytodiagnose sleepapnea.

47. 46
Figure 30. Experimental normal breathing data over time.
Figure 31. Simulated apnea event during breathing.
FUTURE GOALS
Givenmore time andmoneyto workon thisproject,there wouldbe numerousmethodstoimprove
uponour design. Firstof all inorderto increase the comfortfor the userof our device,ourdesignaims
to move awayfrom all wiredcomponents.Currently,electrical leadsfromthe strainsensorsonthe
patient'schestandabdomenas well aswiresfromthe pulse oximeteronthe fingerare connectedto
the data acquisitionunit.Implementinga Bluetooth module orWi-Fi communicationbetween
componentswill eliminate the needforwiresrunningalongthe patient’s body.Inaddition,forfuture
generationsof thisdesignwe wishtoimplementamore compactand portable dataacquisitionunit
than the ArduinoUno microcontroller.Anothermajorlimitationwithourdesignisthatthe algorithm
that processespatientdatatodetectsleepapneainthe useriscurrentlyrunningin LabVIEW.LabVIEW,
althoughefficientforthe purpose of developingaprototype algorithm, isnotveryuserfriendlyforthe
general publicoutsideof the engineeringandsoftware communities.Byre-writingthe LabVIEWapnea
detectionalgorithminalanguage suchasjava or C++, thiscan allow forthe developmentof amobile

48. 47
app that hasa simplergraphical userinterface (GUI) andwill be more accessibletoalarger subsetof the
general public.Finally,anotherpossibilitythatourdesigncouldpotentially integrateisachestmounted
pulse oximeter.Recentpublishedliterature indicatesthata standardpulse oximetercanbe modifiedto
measure accurate bloodoxygenationandpulse rate levelsbaseduponluminescence of the chest
insteadof the finger.Byhavingone cohesive unitintegratingbothof ourdesign'ssensors(strainsensor
and pulse ox) onthe chest,the userisable to sleepunhinderedbythe sleepapneamonitor.
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