The influence of contact force on forensic trace collection efficiency when sampling textiles with adhesive tape

Delft University of Technology

The influence of contact force on forensic trace collection efficiency when sampling

textiles with adhesive tape

Damsteeg-van Berkel, Selma; Beemster, Fleur; Dankelman, Jenny; Loeve, Arjo J.

DOI

10.1016/j.forsciint.2019.03.017

Publication date

2019

Document Version

Final published version

Published in

Forensic Science International

Citation (APA)

Damsteeg-van Berkel, S., Beemster, F., Dankelman, J., & Loeve, A. J. (2019). The influence of contact

force on forensic trace collection efficiency when sampling textiles with adhesive tape. Forensic Science

International, 298, 278-283. https://doi.org/10.1016/j.forsciint.2019.03.017

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The

in

fluence

of

contact

force

on

forensic

trace

collection

ef

ficiency

when

sampling

textiles

with

adhesive

tape

Selma

Damsteeg-van

Berkel

a

,

Fleur

Beemster

b

,

Jenny

Dankelman

a

,

Arjo

J.

Loeve

a,c,

*

a

DepartmentofBioMechanicalEngineering,FacultyofMechanical,MaritimeandMaterialsEngineering,DelftUniversityofTechnology,Mekelweg2,2628 CD,Delft,TheNetherlands

b

DepartmentofHumanBiologicalTraces,NetherlandsForensicInstitute,TheHague,TheNetherlands c_{CovanLedden-HulseboschCenterforForensicScienceandMedicine,Amsterdam,TheNetherlands} A R T I C L E I N F O

Articlehistory: Received7January2019

Receivedinrevisedform12February2019 Accepted10March2019

Availableonline15March2019

Keywords: Stubbing Adhesivetape Contactforce Micro-traces textile DNA A B S T R A C T

Purpose:DNAisahighlyvaluableleadtoidentifypeoplewhowerepossiblyinvolvedinacrime.Evenby smallcontactevents,minuteamountsofDNA(‘traceDNA’)canbetransferredfromaDNAsourcetoan evidentiaryitem,whichcanbeenoughforasuccessfulDNAanalysis.Thefocusofthisresearchistoget moreinsightinthecollectionoftraceDNAfromtextilesby‘stubbing’,whichisatape-liftingmethod usingdouble-sidedtapeplacedonastub.Therelationbetweenthe‘stubbingforce’(thenormalforcethat isappliedduringstubbing)andthecollectionefficiencyofmicrospheresisinvestigated.

Methods:Microspheres(Ø25mm)wereusedasmocktracestomimicDNA-containingmicro-traces.The particleswereappliedtotextilesubstratesinasuspensionofethanolthatwaslefttoevaporatebefore sampling.Experimentswereperformedonthreedifferentpolyestersubstrates.Traceswerecollectedby stubbingwhileusing5differentstubbingforces.Thenumberofmicrospheresplacedoneachsubstrate wascountedbeforesamplingandallstub-tapeswereanalysedaftersamplingtocounthowmanyofthe microsphereswerepickedup,bothbyusingstitchedimagesfromadigitallightmicroscope. Custom-madeimagerecognitionsoftwarewasusedtoautomaticallycountthemicrospheres.

Results:Onalltestedpolyestersubstrates,themeanefficiencyofthecollectionofmicrospheresincreased with increasing stubbing forcein a concave down increasing function. The increaseof collection efficiencystagnatedaround3–12N,dependingonthesubstratematerial.Thetheoreticalmaximum collectionefficienciesvariedbetween38%and78%,dependingonsubstratematerialaswell. Conclusions:Stubbingwithaforcehigherthan12Ndoesnotnotablyinfluencethecollectionefficiency fromthevarietyoftextilesthatweretested.However,becausethetheoreticalmaximaofthecollection efficiencieswerefarfrom100%,itishighlylikelythatstubbingmultipletimesonthesamespotofa substrateincreasesthetotalcollectionefficiency.Thegainedknowledgewillhelptostandardizeand improvetheeffectivenessofstubbing.

©2019ElsevierB.V.Allrightsreserved.

1.Introduction

DNA tracesarefrequently recovered fromevidentiaryitems,

becauseitisahighlyvaluableleadtoidentifypeoplewhowere possiblyinvolvedinacrime.EvenfromminuteamountsofDNAa samplecanbeobtainedfromwhichinterpretableDNAprofilescan bederived.

Whenanevidentiaryitemissampled,theidealsamplecontains asmuchaspossibletargetedDNAandaslittleaspossiblenoise

(i.e.:othercontaminatingmaterial)astomaximizethechanceof obtaininga usefulDNAprofile.TheratiobetweentargetedDNA

and noise inthesample is determinedbyallstepsin theDNA

transferprocess(seeFig.1).TheamountofDNAthathumans(the DNAsources)transfertotheirsurroundings(knownas‘shedding’) variesbetweenpersons,butalsowithinpersonsovertime[1].This isinfluencedby,forexample,handwashing[2]orprevioustouch events[3].TheactivitybywhichDNAistransferredindicatesthe contactevent.IntransferofDNAbetweendifferentsubstratesit

hasbeenshown thatfrictioncontactstransfermuch moreDNA

than passive or pressured contacts [4–6]. Ageing indicatesthe degradationofDNA,whichdependsonvariousfactors,including time,temperature,humidity,ultra-violetlight,exposuretovarious chemical substances andotherenvironmental factors[7,8]. The

substrate of the evidentiary item affects the deposition and

*Correspondingauthorat:DelftUniversityofTechnology,FacultyofMechanical, MaritimeandMaterialsEngineering,DepartmentofBioMechanicalEngineering, RoomF-0-200,Mekelweg2,2628CD,Delft,TheNetherlands.

E-mailaddress:a.j.loeve@tudelft.nl(A.J. Loeve).

https://doi.org/10.1016/j.forsciint.2019.03.017

0379-0738/©2019ElsevierB.V.Allrightsreserved.

ForensicScienceInternational298(2019)278–283

ContentslistsavailableatScienceDirect

Forensic

Science

International

collection efficiencies of micro-traces on and from it. Besides, specificsubstratematerialsmayinteractwithand degradeDNA [9].ForsamplingaDNA-containingtrace,differentmethodsexist, suchascutting,swabbingandtape-lifting. Tape-liftinghasbeen comparedtootherrecoverymethodsandwasshowntobean

easy-to-useand reliableDNAcollectionmethodfor sampling

micro-tracesfromtextiles[10–12].

Ofall5mentionedfactorsintheDNAtransferprocessfromthe sourcetothesecuredDNAsample,samplingistheonlystepthatis controllable bythe forensic investigator at the crime scene. Therefore itisquintessentialtounderstandtheworkingprincipleofasampling methodwellinordertoachieveoptimalDNAcollection.

Thisstudyfocusseson‘stubbing’;atape-liftingmethodusinga stubcoveredwithdouble-sidedadhesivetapetoliftmicro-traces fromasubstrate(Fig.2).Thisisaneasytouseandreliablemethod [10]thatismostlyusedforthecollectionofmicro-particlesfrom textile.Samplingfromtextileevidentiaryitems,suchasclothing, involvesextra challenges as comparedto solid objects, because textileisporousandhighlydeformable.Duetotheshapeandsizeof thestubs,stubbingoffersmuchpotentialforfurtherstandardisation ofsamplingmicro-traces. Furthermore,thestubholdercreatesa

distancebetweenthesampledsurfaceandthehandinorderto

reduce contamination risks. It should benoted that,justas with other adhesivetapebasedmethods,stubbingislessornotsuitablefor textilesthatarewet(noadhesivebonding),thathavesoextremely beenexposedtotheelementsthattheyfallapart(textileandtraces bothgettinglifted),or arecoveredindirt(samplewillcontainmostly dirt,whichcouldofcoursebeatraceitself).

Eventhoughvariouscollectionmethodshavebeencompared

witheachother[10–14],onlyafewstudiesconsideredtheeffectof variationswithinthestubbingtechniqueonthe_finalsample:the effectoftapebrand[9,15]andthenumberoftape-liftings[9]have beenstudied.Inthestubbingprocedurethatiscurrentlyusedat ourforensicinstitute,the‘stubbingforce’(i.e.:maximumnormal

force applied on the stub during trace collection) is applied

manually,whichmakesithardtocontrol.However,thestubbing forceissuspectedtobeoneofthemostinfluentialvariablesduring stubbing,basedonthefollowingworkingprinciple.Underahigher stubbingforce,textilefibresandtapearecompressedmore,which increasestheactualcontactareabetweenthetapeandthetextile.

Therefore, it was hypothesised that stubbing identical samples withahigherstubbingforceresultsinalargeramountofcollected micro-traces,asillustratedinFig.3.Thecollectionislimitedbya

maximumcontactareaandtheamountofmicro-particlesonthe

substrate.Toourknowledge,thisisthefirsttimethattheeffectof contact force on the collection efficiency is studied for trace collectionmethodsingeneral.

2.Materialsandmethods

Fluorescent microspheres (Ø25

m

m,12%Coefficientof Varia-tion,Fluoro-MaxTM,Thermoscientific)wereusedtomock micro-tracesthatmaycontaintraceDNA.Thesespheresaresizedinthe sameorderofmagnitudeasskincells(25

m

m)[16],whichareone ofthecomponentsoftraceDNA,nexttothemuchlargerskinflakes andthemuchsmaller_‘freeDNA_’.Furthermore,thesemicrospheres werevisuallyquantifiableunderamicroscope.

Thisstudywasfocusedonpolyestertextilesubstrates,because polyesterismostoftenpresentinthetextilessampledinourown casework.Furthermore,becausethisstudywasthefirstofitskind, thetextilesampleswerekeptrelativelysimpletotest thisnew method.Tolimitthevariationbetweensamplesandtoexcludethe effectoftextileweavingsandcompositions,itwaschosentouse samplesofparalleloriented100%polyesterthreads.The experi-mentswereconductedonflatsteelspoolswrappedinoneofthree differenttypesofthreads:sewingpolyester(SP),Ø0.22mm;extra strongsewingpolyester(ESSP),Ø0.35mm;andcrochetpolyester (CP),Ø1.13mm(Fig.4).

Fig.1.DNAtransferfromaDNAsourcetoasamplefromwhichDNAcanbe extractedtocreateaDNAprofile.

Fig.2. Tapestubbingisusedmainlytocollectmicro-tracesfromtextiles.This methodoriginatesfromtheGunShotResiduescollectionmethod,forwhicha carbon-layerisattachedtothestubinsteadofadhesivetape.

Fig.3.Schematicrepresentationofthehypothesizedeffectoftheforceappliedon thestub:(top)withalowforce,onlyafewparticlesareincontactwiththetape. (Bottom)Whencontactforceincreases,thecontactareabetweenthetapeandthe fibresincreases,enablingcollectingmoremicro-particles.

Fig. 4. Microscope images of a sewingpolyester (‘SP’),extra strong sewing polyester(‘ESSP’)andcrochetpolyester(‘CP’)spool.

The microspheres were deposited on the substrates in a suspensionofethanol.Inordertoputthemicrospheresasevenly aspossibleonthesubstrates,anasuniformaspossiblesuspension wascreatedbyputtingitinanultrasonicbathfor2min(atroom

temperature) and frequently shaken thereafter to keep the

suspensionwell-mixed.Next,twentydropletsofthesuspension, of0.025mLeach,withaconcentrationof9.2E4microspheres/mg, wereevenlydistributedoveronesideofeachspoolusingapipette. Thisamountwaschosen arbitrarilytodepositsufficient micro-tracesonthesamplestoallowmeasuringsamplingdifferences,but avoidexcessivecoverageofthetextile.Afterdryingforatleast12h atroomtemperature,thespoolswereoverlainbyamaskingplate with5circularholestocreate5separatesamplelocations(Fig.5).

Aluminiumstub-pinswere,usedasshownin Fig.6,without

stub holders, directly clamped into the setup. Double sided

adhesivetape(Scapa4405,Ø10mm)wasstucktothesurfaceofthe

stub-pin (Ø12.7mm, see Fig. 6). The surface underneath the

transparenttape was paintedblack toimprovethevisibilityof microspheresonthetape.

Forforensicpurposes,adhesivetapeonstubsisUVirradiated beforeusetomakeitDNA-free.UVlightcandegradepolymersand therebyinfluencetheadhesivebonding.Therefore,theeffectofthe

standardUVirradiationprocedureontheadhesivebondingwas

investigatedbeforethetests,whichshowednochangeofthetape’s adhesiveproperties.Consequently,tosavetime,thetapewasnot UV-irradiatedinthisstudy.

The microspheres were collected from the substrates by

stubbing with controlled force. The stubs were fixed onto the

headofatensiletester(Instron,modelno.:4505,serialno.:H2164) witha
100Nstaticloadcell(Instron,serialno.:65883),seeFig.6. Thetensiletesterwasusedtocontrolthestubbingforceapplied duringsampling.Fivedifferentstubbingforces(maximumnormal forcesimposedbythetensiletester)weretested:0.1,0.2,0.5,1and 7N.Foreachcombinationofstubbingforceandsubstratematerial 3trialswereperformed.Forpracticalreasonsthematerialswere tested in the order CP, SP, ESSP with thethree repetitions per stubbingforcegroupedperforce.

A Keyence VHX-5000 Digital Microscope (Keyence, Osaka,

Japan)wasused(usingimagestitchingof200magnifiedimages) tocapture thesubstrate surfacebeforestubbing and countthe numberofmicrospheresplacedonthesubstrate,andtocapture

the stub-tape surface after stubbing and count the number of

microspherescollectedonthetape.

Themicroscopepictureswereallidenticallypost-processedin

Adobe Photoshop CS6 (Adobe Systems, San Jose, CA, USA) for

contrastenhancementinordertodiscriminatethemicrospheres

fromthebackground.Acustom-madeMATLABimagerecognition

script(versionR2013b,Mathworks,NatickMA,USA)wasusedto

Fig.5.(Top)Spoolwoundwiththread(oneofthetypesshowninFig.4).(Bottom) Bycoveringthethreadwithanaluminiummaskingplate,5separate,roundsample locations(Ø14mm)werecreated.

Fig.6.(Top)Stubpinsthatwereusedinthisexperiment.(Bottom)Thespool containingthesubstratewasclampedtothecrossheadofthetensiletester.The stub-pinwasclampedtotheloadcell.

Fig.7.(Top)Microscopephotoofastub-tapewithcollectedmicrospheresand fibresonit.(Middle)ImageeditedinPhotoshop.(Bottom)Resultofautomated particledetectioninMATLABwasverifiedbyindicatingthelocationofdetected spheresintheoriginalmicroscopephoto,seeaddedthicklinedcircles. 280 S.Damsteeg-vanBerkeletal./ForensicScienceInternational298(2019)278–283

automaticallycountthemicro-particlesintheprocessedpictures (Fig. 7).Allend-results werevisuallychecked forfalse positive identifications of microspheres, sometimesoccurringwhen, for example,atextilefibrewaspickedupbythetapeandshimmering edgeson thefibrewere falselyidentifiedas clustersof

micro-spheres. Such clusters of falsely detected microspheres were

subtractedfromthetotalnumberofdetectedparticles.

Part of the end-results, 7 microscope pictures of tape, 2

microscope pictures of both CP and ESSP, and 4 microscope

picturesofSP,werevisuallycheckedindetail.Theabsolutenumber offalselydetectedmicrospheresandthepercentageofundetected microsphereswerecalculatedtoestimatethemethod_’saccuracy. The collection efficiencies,

h

collection, were calculated using

Eq.(1).

h

collection¼

ntape

Atape

r

substrate

100% ð1Þ

Becausetheexactplacementlocationofthetapeonthesample

circle was unknown, the number of particles present in the

sampledsubstrateareawascalculatedbymultiplyingtheparticle densityonthesubstratebeforestubbing,

r

substrate(averagenr.of

particles/mm2),bythesurfaceareaofthetape,Atape(mm2).The

numberofdetected particlesonthestub-tapeisrepresentedby ntape.

Theambienttemperatureandrelativeair humiditynearthe

tensiletesterweremeasured,becausechangesintheseconditions mightinfluencetheadhesivecapacityofthetape.

3.Results

ThetestresultsaredisplayedinTable1.InFig.8theresulting collectionefficiencies(n=3perforcelevel)forthethreesubstrates andfivestubbingforcesareshown.Curveswerefittedthroughthe meancollectionefficienciesaccordingtotheexponential behav-iour y=a  ebx+c,withx the stubbingforce, ythestubbing efficiency,anda,b,andcthefittingparameters.Thistrendmatched thehypothesisandresultedinalowresidualerror(Table2).

Thetheoreticalmaximallyachievablecollectionefficiencyfor eachsubstratematerialwastakenasthevaluethatitsfittedcurve

approaches asymptotically. The stubbing forces at which the

collectionefficiencystagnatedweredefinedastheforceatwhich thecollectionefficiencyreached95%ofthetheoreticalmaximum

efficiency.Thetheoreticalmaximaandthestagnationforcesare giveninTable3.

Duringstubbing,theambienttemperaturevariedbetween21

and 24C (mean 22.9C). The relative humidityof ambient air

variedbetween36and 38%(mean36.6%,accuracy
5%).These

limitedchangesinenvironmentalconditionsarenotexpectedto haveanysignificantimpactontheadhesivebondingofthetape. 4.Discussion

The results matched the hypothesis that the collection

efficiencyincreasesinasimilarmannerastheactualcontactarea betweenthetapeand thesubstrate.Thecollectionefficiencyof

microspheresincreasedmostquicklyamonglowstubbingforces

uptoabout1N.Theriseofcollectionefficiencywithincreasing

stubbing force stagnated between 3 and 12N, depending on

substratematerial (Table 3).These resultsindicatethat usinga stubbingforcehigherthan12Nwouldnotbereallyusefulwhen

requiring a maximum collection efficiency on the substrate

materials tested. Based on the fitted curves, the theoretical

maximaofthecollectionefficiencies arefarbelow100%.These

results suggest that when aiming at collecting as much trace

Table1

Thenumberofdetectedmicrospheresonthesubstratebeforestubbing(‘substr’) andthenumberofdetectedmicrospheresonthestubtapeafterstubbing(‘tape’) for each combination of used stubbing force (‘Fstub’) and substrate material (‘CP’=crochetpolyester,‘SP’=sewingpolyesterand‘ESSP’=extrastrongsewing polyester).

Fstub(N) Trialnr. Nr.ofmicrospheresdetected

CP SP ESSP

substr tape substr tape substr tape 0.1 1 549 18 537 8 349 11 2 695 25 496 15 479 17 3 630 21 436 1 430 19 0.2 1 797 56 509 15 453 6 2 717 43 420 11 304 11 3 830 46 456 9 314 7 0.5 1 671 69 405 28 310 32 2 573 40 350 34 328 11 3 737 74 371 14 337 50 1 1 941 146 383 58 302 46 2 667 125 314 14 329 22 3 524 104 381 13 323 39 7 1 617 306 272 72 307 70 2 674 323 348 115 321 73 3 618 235 354 135 299 68 Table2

Parametersofthecurvesfittedthroughthecollectionefficiency(y)andstubbing force (x) relation,and the coefficients of determinationfor the three tested substratematerials. Substrate material Equation Coefficientof determination(R2 ) SP y=68.12e0.21x_{+69.32 0.9934} ESSP y=36.30e0.70x_{+37.62 0.9734} CP y=76.63e0.45x_{+78.00 0.9972} Table3

Theoreticalmaximallyachievabletracecollectionefficiencies,andstubbingforces atwhichtheincreaseofcollectionefficienciesstagnated(i.e.:reached95%ofthe theoreticalmaximumefficiency).

Substratematerial Theoreticalmax. collectionefficiency Stubbingforceat stagnationofcollection SP 69% 12N ESSP 38% 3N CP 78% 6N

Fig.8. Meancollectionefficiencies(representedbythedots,witheachdotbeing the meanof3trials thatare eachrepresentedby an‘X’ sign)forall tested combinationsofstubbingforceandsubstratematerials.Foreachsubstratematerial anexponentialcurveisfittedthroughthedatapoints.

materialaspossible,itmightbemoreusefultostubmultipletimes withlowforceonthesamelocationofasubstratethantofurther increasethestubbingforce.

Prior to the conducted measurements, the compressive

stiffness of the substrate materials were measured using the

sametensiletester as in the testsand a blankstub-pin. This

showed that stiffer, and thus less deformable, substrate

materials had lower limit collection efficiencies. This might

be explained by the fact that more compliant threads allow

morecompressionanddeformationandhenceenablestubbing

deeper into and between the threads. Consequently, it is

expectedthatusingmoredeformablestubpinsorthicker,soft

stubtapesmay compensate partly for the reducedcollection

efficiencyfromstiffthreads. 4.1.Limitations

Themicrospheresusedinthisexperimentdifferedfromskin

cellsinshapeandothermechanicalproperties,whichmayhave

affected the adhesive bonding. However, the demonstrated

relationbetweenstubbingforceandtracecollectionefficiencyis expected to be a valid indication for skin cells, based on the similarityofsize(andthusofcontactareawiththestubbingtape) withtheseoftenoccurring DNA-containingtraces. Additionally, DNA-containingtracesdonotonlyexistofskincells,butalsoof

muchlargerskinflakesandmuchsmallerfreeDNA.However,by

usingtheapproximatelyskin-cellsizedmicrospheres thenewly

developed method for measuring stubbing efficiency could be

testedforanintermediateparticlesizerange.Furthermore,usinga specificandactualDNA-containingtracewouldjustaswellhave

been a limited representation of the vast range of sizes and

materialpropertiesofDNA-containingtraces,butatamuchhigher economiccost.

Microspheresmighthavepenetratedinthesubstratefibresand thenstillmighthavebeencollectedbythetape,despitenothaving beenvisibleatthesurfaceatthetimeofanalysingthesubstrate.If so,theactualdensityofmicrospheresonthesubstratewouldhave beenhigherthanvisuallyquantifiable,sothecalculatedcollection efficiencies would be lower than determined. Additionally,the exactdistributionofthemicrospheresonthesubstrateswas,for practicalreasons,notconfirmedvisuallyforallsamples.However,

the suspension holding the particles was constantly kept as

homogeneous as possible and applied to the samples in a

standardisedmannertoensurethatallsamplescouldbeassumed tohavesimilarmicrospheredistributions.Randomvisualchecks duringpilottestsdidsuggestthatthis wasthecase, butfuture

studies may improveon this by making sample-wide stitched

microscopeimagesofeachsample.

Becausesomemicrosphereswerehardlyvisibleandbecause

sometimesglareonshinyedgesoftextilefibreswasdetectedas amicrospherebytheusedsoftware,bothfalsepositiveandfalse

negative counts existed in the data. Manually verifying the

outcomesof the detectionsoftwarein some randomlypicked

datasetsshowedthatthe numberof undetectedparticleswas

proportionaltothenumberofparticles present(about2–15%

depending on substrate material). The number of falsely

detectedparticles (about28–32perentire sample) depended

onlyonthesubstratematerial.Thereforethenumberoffalsely

detected particles was similar for all samples of the same

substrate, regardless of the number of present particles.

Withoutthesedeviations,therelationbetweenstubbingforce

and collection efficiency would show to start at a lower

collectionefficiencyand endat a higher collection efficiency,

but the general trends and the conclusions drawn from the

results would not have differed, as these are only marginal

deviations.

4.2.Futureresearch

One couldimaginethat onrough substrates suchastextile,

trace DNA can be present on specific depths of the substrate

structure only, due to DNA sources that transfer DNA to the

substrateduringdifferentactionsandatdifferenttimes.Toobtain

DNA samples that represent a single DNA source instead of a

mixtureofmultipleDNAsources,itwouldbevaluabletogetmore insight in thedistributionof traceDNAalong thedepthof the substrate structure. In addition, it should be investigated if selectively collecting traces from specific substrate structure depths is possible. Trace collection from only the top layer of thesubstratestructurecouldbedonewhenusingaverylowand controlledstubbingforcethatbarelyimpressesthesubstrate.From therelationbetweenstubbingforceandtracecollectionefficiency, itcanbededucedthattosampleonlythetoplayer,forcesinthe orderof0.1N(~10g)wouldhavetobeapplied.Anunpublishedpilot studybyWendt[17]showedthatforensicinvestigatorsgenerally usehighlyvaryingstubbingforcesbetween1and10N.Therefore, itisexpectedthattherequiredforcesintheorderof0.1Ncannotbe consistentlyappliedmanually,butwouldrequireanaidingdevice tocontrolthestubbingforce.Suchadevicehasbeenproposedby Van Eck et al. [18] and enabled controlling the stubbing force between2.1and 30Nwitha standarddeviationofabout0.5N. Furtherdevelopmentofthatdevicetoachieveconsistentstubbing forces around0.1N would benecessarytoallow depth-specific stubbinginthefuture.

Asthemanualcheckofthemicrospheresdetectionalgorithm showed,thealgorithmcouldbeimprovedintermsofspecificity

andsensitivity. Someimprovementcouldpossiblybegained by

furtherimprovingthemanuallyestablishedimageprocessingthat

wasnextautomated.However,therearemanyautomatedimage

recognition systems, e.g. for medical image recognition and

diagnosis,thatoutperformhumansintermsofspeed,specificity andsensitivitythankstotheapplicationofmachinelearningand

deep learning algorithms. Similar developments may greatly

enhancetheperformanceofthemethodsdescribedinthecurrent study.

In order to gain further knowledge about the efficiency of

stubbing of different trace materials on other substrates, the currentstudyshouldbefollowedupbytwotypesofstudies:(1) validationtestsinvestigatingwhethertheresultsobtainedwith theusedmicrospheresaretransferabletoactualskincells.(2)Tests onothersubstratesandwithtracematerialsthatalsomimicskin flakes,freeDNAandothermaterialsofinterest.Thiswouldalso showwhetherthelowcollectionefficienciesandhighvariations arecharacteristicforpolyestertextilesorforthestubbingmethod ingeneral.Furthermore,thesefuturestudieswilleventuallyhelp establishingquantitativestatementsaboutactivitiesrelatedtothe transferoftrace-DNA.

5.Conclusion

Thepresentedresultsstronglysupportthehypothesisthatthe

collection efficiency of microspheres from textile substrates

increases with increasing stubbing force in a concave down

increasingfunction.Accordingtoafitthroughthemeasuredtrace collectionefficiencies(measuredforstubbingforcesof0.1,0.2,0.5, 1and7N),thecollectionefficienciesseemedtostagnateat3,6and

12N for extra strong sewing polyester, sewing polyester and

crochetpolyester,respectively.Stubbingwithaforcehigherthan 3–12N (depending onthesubstrate material)doesnotnotably influencethecollectionefficiency.However,becausethe theoreti-calmaximaofthecollectionefficiencieswerefarfrom100%,itis

expected that stubbing multiple times on the same spot of a

substrateincreasesthetotalcollectionefficiency.

Declarationofinterests

Theauthorsdeclarethattheyhavenocompetinginterests.

CRediTauthorshipcontributionstatement

SelmaDamsteeg-vanBerkel:Conceptualization,

Methodolo-gy,Software,Validation,Formalanalysis,Investigation,Writing -originaldraft,Writing - review &editing,Visualization, Project administration.FleurBeemster:Conceptualization,Methodology,

Resources, Writing - review & editing. Jenny Dankelman:

Conceptualization, Methodology, Validation, Resources, Writing -review&editing,Supervision.ArjoJ.Loeve:Conceptualization, Methodology,Software,Formalanalysis,Writing-originaldraft, Writing-review&editing,Visualization,Supervision.

Acknowledgments

The authors would like to thank Martine Verhoeff of the

NetherlandsForensic Institutefor providing test materials, and

SanderLeeflangand SandervanAsperenofthedepartmentof

Materials Science at the Delft University of Technology for

providinganalyticalequipmentandpracticalsupportduringthe experiments.

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