Performance Evaluation of Knitted and Stitched Textile Strain Sensors

sensors

Review Performance Evaluation of Knitted and Stitched Textile Strain Sensors

Kaspar M.B. Jansen

Emerging Materials Group, Department Industrial Design Engineering, Delft University of Technology, 2628 DE Delft, The Netherlands; [email protected]

Received: 22 October 2020; Accepted: 15 December 2020; Published: 17 December 2020

Abstract: By embedding conductive yarns in, or onto, knitted textile fabrics, simple but robust stretch sensor garments can be manufactured. In that way resistance based sensors can be fully integrated in textiles without compromising wearing comfort, stretchiness, washability, and ease of use in daily life. The many studies on such textile strain sensors that have been published in recent years show that these sensors work in principle, but closer inspection reveals that many of them still have severe practical limitations like a too narrow working range, lack of sensitivity, and undesired time-dependent and hysteresis eﬀects. For those that intend to use this technology it is diﬃcult to determine which manufacturing parameters, shape, stitch type, and materials to apply to realize a functional sensor for a given application. This paper therefore aims to serve as a guideline for the fashion designers, electronic engineers, textile researchers, movement scientists, and human–computer interaction specialists planning to create stretch sensor garments. The paper is limited to textile based sensors that can be constructed using commercially available conductive yarns and existing knitting and embroidery equipment. Within this subtopic, relevant literature is discussed, and a detailed quantitative comparison is provided focusing on sensor characteristics like the gauge factor, working range, and hysteresis.

Keywords: textile strain sensors; conductive yarns; knitted sensor; stitched sensor; performance evaluation

1. Introduction Garments are intimate, close to the body and have a natural potential for collecting and monitoring body-related signals. Whereas in the early research of electronic textiles (or e-textiles), off-the-shelf sensors, electronic parts, and connecting wires were simply added to the textile, recent developments now allow for an almost complete integration of sensor and actuator functions in a textile [1,2]. Garments with well integrated electronics can be personalized, are comfortable, unobtrusive, and do not show visible connecting wires and sensor electrodes. Such functional clothing therefore has a much higher chance of being accepted to be worn in everyday life situations. In the near future stretch sensors will be embedded into everyday objects like pillows and car seats and we will use them during our fitness workouts, to monitor and correct our posture or during our virtual reality gaming. Apart from that, strain sensor garments are particularly beneficial for rehabilitation purposes where there is an urgent need for monitoring the recuperation of impaired body kinematics in situations of daily life activities [3]. Although textile strain sensors are now being evaluated in clinical rehabilitation settings, they still seem to lack sufficient resolution [4]. The era of smart textiles started with the introduction of conductive yarns and fabric at the end of last century which enabled it to integrate sensors and interconnections in garments in an unobtrusive way [2,5]. Conductive yarns are usually blending of traditional textile fibers and thin metal filaments like stainless steel, copper, or metal coated fibers consisting of a polymer core and a thin

Sensors 2020, 20, 7236; doi:10.3390/s20247236 www.mdpi.com/journal/sensors Sensors 2020, 20, 7236 2 of 28 metal cladding (usually silver). Because of the blending with traditional textile fibers these conductive yarns still have their textile like properties and appearance. Current yarn manufacturing processes enable the production of yarns with non-conductive filaments wrapped around conductive core fibers. Such composite yarns have a fully textile appearance and are electrically insulated at the outside, and thus may prove useful as conductor lines in future e-textile applications [6,7]. An alternative way to achieve insulation is by coating the conductive filaments with micrometer thin polyurethane (enamel) layers. These types of insulated conductive yarns are however still not widely commercially available at present. Reviews on wearable and flexible sensors in general are given by [1,2,8–13], whereas [14–16] more particularly focus on textile strain sensors. Textile compatible strain sensors can be produced in three ways: (1) by integrating prefabricated stretchable sensor yarns in a garment; (2) by coating an existing fabric surface with a conductive substance, and (3) by integrating loop structures of conductive (non-stretchable) yarns in a textile fabric. In the latter case the resistance changes result from changes in contact resistance between loops of the conductive yarns during stretching. Sensor yarns or fibers can be manufactured by coating or dyeing yarns with a conductive layer or by embedding conductive particles in a stretchable polymer matrix [14–16]. In both cases the yarn resistance changes when stretched and sensor garments can be constructed by integrating them into the textile. Examples are the coating of stretchable yarns with carbon nanotubes [17], polypyrrole [18] or PEDOT [19], wrapping them with graphene film [20,21] or dyeing with silver nano crystal precursor solution [22]. Composite yarns were fabricated by adding e.g., carbon black [3], carbon nanotubes [23] or by blending with PEDOT solution [24]. Most of these sensor yarns show a high sensitivity, stretchability and good cyclic behavior. Although these yarns thus show high potential for future use as textile sensors, they are still in a relatively early research stage and require dedicated and well controlled processing, with the recently published kilometer scale conductive yarn sensor as the notable exception [24]. Coated fabrics (as studied in e.g., [25–28]) on the other hand have a lower sensitivity and tend to wear during use [14]. The advantage of the third category, the embedding of conductive loop structures in fabrics, is that the technology is directly accessible since it can be manufactured in mass production with standard equipment and uses commercially available types of yarns. The purpose of this review is therefore to present an overview of textile strain sensors that can be constructed using commercially available conductive yarns with standard textile manufacturing methods like knitting and stitching or embroidery. The paper is subdivided in 6 sections. In Sections2 and3 the basics and terminology used in the fields of textile engineering and sensors are discussed. Next, in Section4, the studies on knitted and stitched sensors are reviewed with a focus on the types of conductive yarns, stitches and substrate materials that were used. A detailed performance evaluation of the discussed sensors is treated in Section5, while the main conclusions are summarized in Section6.

2. Textile Basics and Terminology Yarns can be combined to fabric structures by technologies like weaving and knitting. In weaving horizontal and vertical yarns are interlaced to form a fabric layer with a strong and deformation resistant structure. During knitting, on the other hand, consecutive rows of yarns are looped together to form a fabric structure which in principle consists of a single yarn. Knitted fabrics allow for a much higher comfort level because of its breathability and inherent ability to adapt conform to the human body shape without introducing pressure points. The fact that knitted garments stretch and are tight ﬁtting makes them the ideal candidate as a platform to embed textile strain sensors.

2.1. Knitting Basics The most common knitting technique is weft knitting in which horizontal rows of loops (called courses) are interconnected. With the consecutive addition of courses the knitted fabric grows in vertical direction. With the less common warp knitting method, however, loops are added to a vertical column of stitches which makes the fabric grow in horizontal direction. With modern Sensors 2020, 20, x FOR PEER REVIEW 3 of 30

Sensors 2020, 20, 7236 3 of 28 vertical column of stitches which makes the fabric grow in horizontal direction. With modern flatbed knitting machines it is possible to integrate warp knitted vertical structures of (functionalized) yarns ﬂatbedinto a knittingweft knitted machines base structure it is possible using to integratespecial intarsia warp knitted needles. vertical A wale structures is a vertical of (functionalized) column of loops yarnsproduced into a by weft the knitted same baseneedle structure in a weft using knitted special structure, intarsia knitted needles. at A successive wale is a vertical cycles column(Figure 1a). of loopsCourses produced are rows by theof loops same needleacross inthe a width weft knitted of a fabric structure, and are knitted produced at successive in the same cycles knitting (Figure 1cyclea). Courses(Figure are1b). rows of loops across the width of a fabric and are produced in the same knitting cycle (Figure1b).

(a) (b)

FigureFigure 1. 1.Single Single jersey jersey knit knit stitch stitch showing showing (a) ( coursea) course and and (b) wale(b) wale direction direction and theirand their density density (number (number per unitper length). unit length).

ComputerizedComputerized knitting knitting machines machines have have the possibilitythe possibility to make to make a wide a varietywide variety of stitches. of stitches. The single The jerseysingle stitch jersey (or stitch plain (or stitch) plain is stitch) the most is the standard most standard one (Figure one1 ).(Figure A less 1). dense A less fabric dense can fabric be created can be bycreated using onlyby using half only of the half needles. of the needles. Other typical Other stitch typical types stitch include types include the interlock the interlock stitch, Milan stitch, stitch Milan andstitch rib and stitches. rib stitches. More details More candetails be found can be in found textbooks in textbooks on knitting on technologyknitting technology [29,30]. Each [29,30]. type Each of stitchtype hasof stitch its own has electromechanical its own electromechanical behaviour behaviour and the selectionand the selection of stitch of type stitch is therefore type is therefore one of the one importantof the important design parametersdesign parameters in the development in the development of knitted of strainknitted sensors. strain sensors. DuringDuring the the knitting knitting process process conductive conductive yarns yarns can can be be introduced introduced as as single single yarns yarns or or co-knitted co-knitted withwith a a non-conductive non-conductive support support yarn yarn in in either either the the standard standard way way or or with with the the so-called so-called plated plated knitting knitting (vanis(vanisè)è) technology. technology. In In the the former former technique technique the conductivethe conductive yarn yarn is wrapped is wrapped around around the support the support yarn suchyarn that such both that theboth top the and top bottomand bottom face face of the of fabricthe fabric are are conductive, conductive, whereas whereas with with plated plated knitting knitting thethe upper upper and and lower lower yarnsyarns areare kept separate, separate, resultin resultingg in in a afabric fabric element element with with both both a conductive a conductive and anda non-conductive a non-conductive face. face. Such Such a separa a separationtion of functional of functional layerslayers can also can be also obtained be obtained by double by doubleknitting. knitting.In that Inway that pocket way pocket structures structures consisting consisting of two of two distinct distinct layers layers can can be be made [[29].29]. AnAn important important considerationconsideration for for the the knitting knitting with with functional functional yarns yarns is is the the matching matching of of the the yarn yarn thickness thickness with with the the machinemachine gauge gauge or or the the needle needle spacing spacing of of the the knitting knitting machine machine (usually (usually given given as as the the number number of of needles needles perper inch). inch). The The higher higher the the machine machine gauge, gauge, the the thinner thinner the the yarns yarns should should be. be. The The yarn yarn thickness thickness is is expressedexpressed in in terms terms of of its its linear linear density: density: the the mass mass per per unit unit length. length. Several Several systems systems and and units units are are used. used. TheThe commonly commonly used used tex, tex, decitex decitex (dtex) (dtex) and and denier denier (den) (den) are are deﬁned defined as: as: tex tex= =g /g/km,km, dtex dtex= =g /g/1010 km km andand den den= =g /g/99 km. km. In In textile textile engineering engineering these these linear linear yarn yarn densities densities are are often often referred referred to to as as the the yarn yarn countcount numbers. numbers. Other Other deﬁnitions definitions and and conversion conversion factors factors can can be be found found in in [29 [29],], p. p. 5. 5. TheThe dimensions dimensions (as (as well well as as the the electrical electrical characteristics) characteristics) of knittedof knitted fabrics fabrics depend depend on howon how tight tight it isit knitted. is knitted. This This is characterized is characterized by the by stitchthe stitch density, density,S, which S, which is the is total the numbertotal number of needle of needle loops loops in a givenin a given area and area is and the productis the product of the courseof the course count andcount the and wale the count wale (the count number (the number of courses of courses and wales and perwales unit per length, unit see length, also Figuresee also1). Figure The wale 1). countThe wale is often count expressed is often as expressed wales per as inch wales ( wpi per), the inch course (wpi), countthe course as courses count per as inch courses (cpi). per A fabric inch ( withcpi). A 12 fabric wales with per inch 12 wales and 15 per courses inch and per inch15 courses is said per to have inch a is fabric count of 12 15. The stitch length is the length of yarn in a knitted loop. The longer the stitch said to have a fabric× count of 12 × 15. The stitch length is the length of yarn in a knitted loop. The lengthlonger the the more stitch open length and the lighter more the open fabric. and In lighter weft knitted the fabric. fabrics In weft the courseknitted and fabrics wale the densities course areand inverselywale densities proportional are inversely to this stitchproportional length, tol,[ this29]: stitch length, l, [29]: k k cpi = c , wpi = w (1) l l

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Sensors 2020, 20, 7236 = , = 4 of(1) 28

in which kc and kw are constants referring to courses and loops, respectively. With this the stitch indensity which Sk cbecomesand kw are constants referring to courses and loops, respectively. With this the stitch density S becomes =∗=kckw , = cpi = kc , (2) S = cpi wpi = , R = = , (2) ∗ l2 wpi kw TheThe ratioratio betweenbetween thethe coursescourses and wales per unit length is called the loop shape factor, R. AccordingAccording toto Spencer Spencer the thek ckcvalue value varies varies between between 5.0 5.0 and and 5.5 5.5 whereas whereas the the loop loop shape shape factor factor tends tends to beto alwaysbe always close close to 1.3.to 1.3. TheThe planar size of of knitted knitted fabrics fabrics depends depends on on the the knitting knitting compactness. compactness. The The tightness tightness Factor, Factor, TF, TFis ,defined is defined as the as thearea area covered covered by the by yarn the yarn in one in oneloop loop relative relative to the to total the total area areaoccupied occupied by that by loop that loop[30], [p.30 219:], p. 219: √√tex TF= =S l ∗ d ∗ ~ (3)(3) ∗ ∗ ∼ l For the scaling use is made of Equation (2) and the relation m =106 = π 2. Note that in the For the scaling use is made of Equation (2) and the relation L = 10 tex = 4ρd . Note that in the SpencerSpencer bookbook thethe proposedproposed scalingscaling isis givengiven asas √(tex/(tex/l) which is inconsistent inconsistent with the derivation derivation above. above. InIn mostmost plainplain fabricsfabrics thethe tightnesstightness factorfactor rangesranges betweenbetween 1.41.4 andand 1.51.5 butbut notenote thatthat for conductive yarnsyarns containingcontaining metalmetal filamentsfilaments withwith higherhigher densitydensity thisthis approximationapproximation maymay nono longerlonger hold.hold. It is toto bebe expectedexpected thatthat thisthis tightnesstightness factorfactor willwill alsoalso aaffectffect thethe electricalelectrical contactcontact properties.properties. TheThe tightnesstightness isis ofof coursecourse alsoalso directlydirectly relatedrelated toto thethe porosityporosity andand breathabilitybreathability ofof aa fabric.fabric. ItIt cancan bebe variedvaried byby changingchanging thethe NPNP settingsetting ofof thethe knittingknitting machinemachine whichwhich determinesdetermines thethe looploop size.size. InIn practice, justjust knitted fabricsfabrics alwaysalways havehave tensiontension betweenbetween thethe yarnsyarns whichwhich relaxesrelaxes duringduring movementmovement andand whenwhen wet.wet. Therefore, thethe tightnesstightness ofof a a fabric fabric is is always always measured measured in in the the fully fully relaxed relaxed state, state, i.e., i.e., after after 24 24 h washingh washing in waterin water at 40at 40◦C °C followed followed by by 1 h 1 drying h drying at 70at 70◦C[ °C29 [29],], p. 281.p. 281. OtherOther knittingknitting parametersparameters toto considerconsider areare tension,tension, take-downtake-down speedspeed and cam speed [[30],30], p.p. 217. AccordingAccording toto [[31]31] tensiontension isis thethe mostmost importantimportant factorfactor forfor knittingknitting conductiveconductive yarn.yarn. ItIt isis thethe amountamount ofof distancedistance thatthat eacheach needleneedle pullspulls downdown afterafter aa knittingknitting movementmovement andand controlscontrols thethe tightnesstightness ofof thethe stitches.stitches. Higher tension meansmeans looserlooser stitches.stitches. WithWith inlayinlay knittingknitting (Figure(Figure2 2)) an an external external (functional) (functional) yarn yarn can can be be embedded embedded in in the the knit knit structure structure whichwhich allowsallows conductiveconductive externalexternal interconnectinterconnect wires to bebe integratedintegrated in thethe garmentgarment asas wellwell asas thethe constructionconstruction ofof strainstrain gaugesgauges consistingconsisting ofof aa singlesingle meanderingmeandering strainstrain sensitivesensitive wire.wire. In orderorder toto preventprevent discomfortdiscomfort andand wrinkleswrinkles duringduring stretching,stretching, it isis importantimportant to ensureensure thatthat thethe stistiffnessffness ofof thethe inlayinlay yarnsyarns isis comparablecomparable toto thatthat ofof thethe basebase fabric.fabric.

Figure 2. InlaidInlaid yarns yarns (black) (gray) embeddedembedded inin aa doubledouble jerseyjersey structurestructure [[32]32]. . 2.2. Stitching and Embroidery 2.2. Stitching and Embroidery In a knitted fabric the functional yarn is part of the structure. However, with embroidery and In a knitted fabric the functional yarn is part of the structure. However, with embroidery and stitching the (functional) yarns are attached to the surface of an existing fabric. Embroidered conductive stitching the (functional) yarns are attached to the surface of an existing fabric. Embroidered yarn structures can also act as strain sensors and will therefore also included in this review. They offer conductive yarn structures can also act as strain sensors and will therefore also included in this a simple way to add sensors to an existing garment. review. They offer a simple way to add sensors to an existing garment. Figure3a shows the structure of the basic double lockstitch in which and upper thread is fixated in the fabric by a lower thread supplied by the bobbin spool. Since both threads are confined to a single Sensors 2020, 20, x FOR PEER REVIEW 5 of 30

Sensors Figure2020, 20, 3a 7236 shows the structure of the basic double lockstitch in which and upper thread is fixated5 of 28 in the fabric by a lower thread supplied by the bobbin spool. Since both threads are confined to a single face of the fabric, the use of threads with different properties allows it to design patterns which faceare ofconductive the fabric, at the one use face of and threads insulate with at di thefferent other properties face. A more allows complete it to design overview patterns of embroidery which are conductivestitches can at be one found face andin [33]. insulate Here atwe the only other mention face. A the more tailored complete fiber overview placement of (TFP) embroidery stitch (Figure stitches can3b) be with found which in [33 it]. is Here possible we only to mentionfixate an theexternal tailored element fiber placement like a hollow (TFP) tube, stitch optical (Figure fiber,3b) with or whichconductive it is possible filament to to fixate a fabric an external surface, element similar liketo the a hollowyarn inlay tube, technique optical fiber, used or in conductive knitting. filament to a fabric surface, similar to the yarn inlay technique used in knitting.

(a) (b)

FigureFigure 3. 3.( a(a)) Double Double lockstitchlockstitch embroidery.embroidery. The upper thread can can be be conductive; conductive; ( (bb)) Tailored Tailored fiber fiber placementplacement (TFP) (TFP) embroidery; embroidery; useful useful for forfixating fixating ofof functionalfunctional fibersfibers [[34]34]..

3.3. Strain Strain Sensors Sensors 3.1. Sensor Types 3.1. Sensor Types Sensors are materials of which we can use the change of one of their properties (like electrical Sensors are materials of which we can use the change of one of their properties (like electrical resistance)resistance) to to detect detect andand monitormonitor thethe evolutionevolution of an external action action (a (a force, force, deformation deformation or, or, e.g., e.g., changechange in in humidity). humidity). TheThe strainstrain sensorssensors commonlycommonly used in mechanical engineering, engineering, for for example, example, consistconsist of of aa metalmetal filmfilm pattern on on a a thin thin polymer polymer su substrate.bstrate. When When the thesubstrate substrate material material deforms, deforms, the thestrain strain is transferred is transferred to the to the metal metal grid grid and and the theresistance resistance change change in the in metal the metal film filmis used is used to sense to sense the thedeformation. deformation. Metal Metal film film strain strain gaug gaugeses are therefore are therefore an example an example of strain-resistive of strain-resistive type of type sensors of sensors (i.e., (i.e.,a resistance a resistance change change due to due a deformation to a deformation input). input). In literature In literature also the alsoterm the piezo-resistive term piezo-resistive is used for is usedthese for type these of type sensors. of sensors. In this In work, this work, however, however, we wereserve reserve the theword word piezo piezo for for the the out-of-plane out-of-plane (squeezing)(squeezing) type type ofof deformationdeformation as as detected detected by by pressure pressure sensors sensors and and use use the the term term strain-resistive strain-resistive to torefer refer to to in-plane in-plane deformations. deformations. The The difference difference is important is important because because sensors sensors preferably preferably should should be beselective: selective: the the strain-responsive strain-responsive sensors sensors that that we we wish wish to to design design should should be be sensitive sensitive to to in-plane in-plane deformationdeformation changes changes and and as as inert inert toto pressurepressure changeschanges asas possible.possible. TheThe strain-resistive strain-resistive sensorssensors discusseddiscussed so far usually operate under under constant constant current current condition. condition. AsAs an an alternative, alternative, it it is is also also possiblepossible toto observeobserve thethe changes in the electrical impedance impedance if if an an alternating alternating currentcurrent (AC) (AC) is is applied. applied. ImpedanceImpedance measurementsmeasurements cancan be done using the same sensor sensor layouts layouts as as the the commoncommon DC DC sensors sensors [ 35[35]] but but also also allowsallows forfor newnew layouts.layouts. The distance between parallel parallel conducting conducting lineslines can can for for example example be be easily easily measuredmeasured ifif oneone conductorconductor line carries a a carrier AC AC current current and and the the inducedinduced current current is is picked picked up up by by the the second second conductorconductor lineline [[36].36]. Based on this this a a network network of of cm cm spaced spaced carriercarrier and and sensor sensor lines lines could could bebe constructed,constructed, allowingallowing for higher resolution strain strain mapping mapping than than is is currentlycurrently possible possible with with DC DC based based sensors.sensors. AnAn additionaladditional advantage is that with impedance impedance sensing sensing datadata over over a a range range of of frequencies frequencies is isobtained obtained whichwhich cancan bebe usedused toto filterfilter outout spurious behaviour.

3.2.3.2. Sensor Sensor Response Response CharacterizationCharacterization IdeallyIdeally a a sensor sensor signal signal should should bebe stablestable overover time,time, reproduciblereproducible and linear with with the the input input signal signal (a(a 10% 10% input input increaseincrease willwill then result result in in a a 10% 10% increase increase of of the the output output signal). signal). Metal Metal film filmgauges gauges are aregood good examples examples of reliable, of reliable, robust, robust, and stable and sensors stable sensorswhich can which be considered can be considered as a mature as technology a mature technology now. The textile based sensors that we wish to explore here are not. These sensors show hysteresis, are seldom linear, suffer from baseline drift and degrade over time. Moreover, Sensors 2020, 20, x FOR PEER REVIEW 6 of 30 Sensors 2020, 20, x FOR PEER REVIEW 6 of 30 Sensorsnow. The2020 ,textile20, 7236 based sensors that we wish to explore here are not. These sensors show hysteresis,6 of 28 arenow. seldom The textile linear, based suffer sensors from baseline that we drift wish and to deexploregrade here over are time. not. Moreover, These sensors details show about hysteresis, this non- idealare seldom behaviour linear, are suffer seldom from reportedbaseline driftin literature and degrade which over complicates time. Moreover, direct details comparison. about this Before non- details about this non-ideal behaviour are seldom reported in literature which complicates direct discussingideal behaviour reported are sensorseldom characteristics, reported in literaturewe first outline which thecomplicates types of no directn-ideal comparison. sensor behaviour Before comparison. Before discussing reported sensor characteristics, we first outline the types of non-ideal whichdiscussing may reportedappear in sensor experiments. characteristics, A typical we signal first outlineplot of athe non-ideal types of sensor non-ideal is shown sensor in behaviour Figure 4. sensor behaviour which may appear in experiments. A typical signal plot of a non-ideal sensor is Thiswhich figure may showsappear how in experiments. the resistance A changes typical signalover ti meplot after of a applicationnon-ideal sensor of two is distinct shown strainin Figure steps, 4. shownThis figure in Figure shows4. how This the figure resistance shows howchanges the over resistance time after changes application over time of aftertwo distinct application strain of steps, two εapp (shown as the dashed lines). distinctεapp (shown strain as steps,the dashedεapp (shown lines). as the dashed lines).

Figure 4. Schematic response of sensor after applied strain steps (black pulses). Figure 4. Schematic response of sensor after applied strain steps (black pulses). After applicationapplication of of the the strain strain the the sensor sensor response response∆R does ΔR not does remain not constantremain constant but slowly but decreases slowly withdecreasesAfter time (relaxation).withapplication time (relaxation).of In textilethe strain based Inthe sensorstextile sensor base thisresponsed is sensors usually ΔR associateddoesthis isnot usuall remain withy the associatedconstant readjustment but with slowly andthe readjustmentslidingdecreases of yarnwith and segments. time sliding (relaxation). Theof yarn offset segments. isIn thetextile di ffTheerence base offsetd in sensors signalis the difference before this andis usuallin after signaly the associated before pulse. and The withafter baseline the pulse.isreadjustment the measuredThe baseline and response sliding is the measured whenof yarn the segments. sensorresponse is The notwhen offset loaded. the is sensor the It often difference is not slowly loaded. in changes signal It often before over slowly time, and whichafterchanges the is overdenotedpulse. time, The as whichbaseline drift. Thisis isdenoted driftthe measured can as be drift. either response This due drift to when artefacts can bethe either insensor the due electronics is notto artefacts loaded. or to Itin structural often the electronics slowly relaxation changes or to of structuraltheover yarn time, if relaxation thewhich garment is denoted of isthe under yarn as drift. tension. if the This garment Baseline drift can is changes underbe either tension. may due also to Baseline occurartefacts during changes in the physical electronicsmay also activity occur or orto duringchangesstructural physical in relaxation the environment activity of theor changes (temperature,yarn if thein the garment sweat,enviro nment oris under humidity). (temperature, tension. Dynamic Baseline sweat, baseline changes or humidity drift may describes). also Dynamic occur how baselinemuchduring the physical drift sensor describes activity signal athow or zero changes much extension the in thesensor drifts enviro signal overnment consecutive at zero (temperature, extension cycles; driftssweat, static over baselineor humidity consecutive drift). shows Dynamic cycles; the baseline drift describes how much the sensor signal at zero extension drifts over consecutive cycles; staticchange baseline in sensor drift resistance shows the during change static in sensor conditions. resistance during static conditions. static baseline drift shows the change in sensor resistance during static conditions. A directdirect consequenceconsequence of of the the relaxation relaxation is thatis that the signalthe signal during during stretching stretc dihingffers differs from that from during that A direct consequence of the relaxation is that the signal during stretching differs from that duringthe subsequent the subsequent retraction. retraction. This is called This is hysteresis called hysteresis and is shown and is in shown Figure5 in below. Figure Non-linearity 5 below. Non- in during the subsequent retraction. This is called hysteresis and is shown in Figure 5 below. Non- linearityitself is not in itself a real is problem not a real regarding problem the regarding signal interpretation the signal interpretation since each measuredsince each∆ measuredR/R0 value Δ thenR/R0 linearity in itself is not a real problem regarding the signal interpretation since each measured ΔR/R0 valuestill corresponds then still corresponds to a single strain to a single value (casestrain 3 value in Figure (case5). 3 The in Figure only di 5).fference The only is then difference that the is gauge then value then still corresponds to a single strain value (case 3 in Figure 5). The only difference is then thatfactor the (linearity gauge factor constant) (linearity has to constant) be replaced has by to abe calibration replaced curveby a calibratio or fit function.n curve When, or fit however,function. that the gauge factor (linearity constant) has to be replaced by a calibration curve or fit function. When,a maximum however, occurs a (casemaximum 4) a measured occurs (case∆R/ R4)0 valuea measured can correspond ΔR/R0 value to two can strain correspond values. to The two practical strain When, however, a maximum occurs (case 4) a measured ΔR/R0 value can correspond to two strain values.range of The the practical sensor (the range working of the range)sensor is(the then workin limitedg range) to the is strain then valuelimited at whichto the strain the maximum value at values. The practical range of the sensor (the working range) is then limited to the strain value at whichoccurs. the In knittedmaximum and occurs. embroidered In knitted structures and embroidere this typed of structures response curvethis type is often of response observed curve and is which the maximum occurs. In knitted and embroidered structures this type of response curve is oftenrelated observed to the stitch and structureis related andto the the stitch knit density.structure and the knit density. often observed and is related to the stitch structure and the knit density.

Figure 5. Non-linearity and hysteresis examples. Figure 5. Non-linearity and hysteresis examples.

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The sensitivity of a sensor is expressed asas thethe gaugegauge factorfactor GFGF:: ∆/ =∆R/R (4) GF = 0 (4) ε The higher the response to a certain strain, the higher the gage factor (or gauge factor). A gauge factorThe of 1 higher means the that response every 10% to a elongation certain strain, results the higherin a 10% the resistance gage factor increase. (or gauge Note factor). that the A gauge factor ofis 1usually means thatnot everydefined 10% as elongation the initial results slope inbut a 10%as the resistance average increase. value over Note a that certain the gauge range. factor The istransverse usually not sensitivity, defined as TS the, is initial a number slope butwhich as the indicates average how value sensitive over a certain the sensor range. is The to transverse transverse sensitivity,deformationTS with, is a numberrespect to which length indicates changes. how It sensitiveis defined the as sensor the ratio is to transversebetween the deformation corresponding with respectgauge factors: to length changes. It is defined as the ratio between the corresponding gauge factors:

GF(transverse)(transverse) TS= = (5)(5) GF(axial)(axial) Ideally the transverse sensitivity should be close to zero. The working range can be defined as Ideally the transverse sensitivity should be close to zero. The working range can be deﬁned as the the strain range with a monotonic increase or decrease in the resistance-strain curve which strain range with a monotonic increase or decrease in the resistance-strain curve which unambiguously unambiguously relates a strain value to a given resistance change. Figure 6 shows an example with a relates a strain value to a given resistance change. Figure6 shows an example with a working range working range of 5–25%. of 5–25%.

(a) (b)

Figure 6. DefinitionsDeﬁnitions of: ( (aa)) Working Working range; ( b) Horizontal and vertical hysteresis.

Hysteresis is the didifferencefference in signal measured during a forward and a backward movement and can be defineddefined based on the relative deviation on the horizontal (strain) axis, or that on the vertical (resistance) axisaxis (Figure(Figure6 6bb and and Equation Equation (6)) (6)) ∆ ∆ ∆εhys ∆Rhys == , == (6) Hε − , HR − (6) εmax ε Rmax R − min − min In the few studies that report the hysteresis error sometimes the deviation in resistance (vertical In the few studies that report the hysteresis error sometimes the deviation in resistance hysteresis) is chosen [37], whereas other times the horizontal hysteresis is used [38]. In practice, (vertical hysteresis) is chosen [37], whereas other times the horizontal hysteresis is used [38]. In practice, however, we are interested in how accurate we can obtain strain values from our measurements so it however, we are interested in how accurate we can obtain strain values from our measurements so it would make more sense to use the strain based definition, Hε to quantify hysteresis errors. Further, would make more sense to use the strain based definition, H to quantify hysteresis errors. Further, note that the amount of hysteresis strongly depends on theε starting point. A sample which is note that the amount of hysteresis strongly depends on the starting point. A sample which is deformed deformed up to, say 40% will have much more yarn slippage and thus show a much larger hysteresis up to, say 40% will have much more yarn slippage and thus show a much larger hysteresis than a than a sample that is only 5% strained (see Figure 6). Part of this effect is already taken into account sample that is only 5% strained (see Figure6). Part of this e ffect is already taken into account by scaling by scaling the hysteresis to the range maximum (Equation (6)), however, since yarn slippage (or the hysteresis to the range maximum (Equation (6)), however, since yarn slippage (or relaxation) may relaxation) may be disproportionate at larger strains, it is a good habit to also mention the maximum be disproportionate at larger strains, it is a good habit to also mention the maximum strain when strain when reporting hysteresis errors. reporting hysteresis errors. There are several types of experiments which are used to characterize a strain sensor. First of all, There are several types of experiments which are used to characterize a strain sensor. First of all, we have the step-up step-down or block pulse strain experiments as shown in Figure 4. Typically in we have the step-up step-down or block pulse strain experiments as shown in Figure4. Typically in these experiments the applied strain is increased stepwise in groups of three (e.g., three times 10% these experiments the applied strain is increased stepwise in groups of three (e.g., three times 10% strain, followed by three 20% pulses, etc.). An alternative way is to increase the strain in an incremental, stepwise way (staircase profile). In both methods the gauge factor is evaluated by

Sensors 2020, 20, 7236 8 of 28 strain, followed by three 20% pulses, etc.). An alternative way is to increase the strain in an incremental, stepwise way (staircase profile). In both methods the gauge factor is evaluated by plotting the relative resistance change after a fixed time interval versus the applied strain. The results depend on the (arbitrary) choice of the time interval and the choice to take R0 as the resistance at the start of the first measurement or as the value just before the next strain step. In the third type of experiment the strain is applied at a constant speed and the resistance change versus strain curve is obtained as a direct result. If the upward curve is followed by a strain decrease step a hysteresis curve as shown in Figure5 (case 2 and 5) is obtained. What is often not realized is that during such constant speed experiments also relaxation occurs and that there is more relaxation in a slow experiment than in a fast one. All results are thus, in principle, speed dependent, although [39] showed that for Electrolycra based sensors the velocity dependency was low. If a newly fabricated sensor is loaded to a certain strain for the first time, its behaviour often differs from subsequent loading cycles. Therefore, the first of the three block pulses in experiment type 1 described above is often discarded. The explanation is that during the first stretch microcracks occur (in metallic coated fibers) and that the fabric structure changes irreversibly. Experimentalists therefore often precondition their sensor by stretching it to the highest intended strain level before doing the actual experiment. In some studies hysteresis and relaxation are compensated with dedicated data processing algorithms [40] or by comparison with extra sensors [41]. This is possible but goes at the expense of simplicity. A probable better way would be to redesign the sensors such that hysteresis is minimized and the maximum is shifted towards the end of the required sensor range (for skin deformation this is typically near 40% [3]).

3.3. Sensor Resistance The resistance of an object increases with increasing length and decreases with increasing width and thickness. Equation (7) shows the formula used in literature for a rectangular body, a wire and a plane L L R = ρ = ρ L = ρ (7) V h W L A W · 2 in which ρV is the resistivity in Ω/m, ρL = ρV/h (Ω/m ) and ρA = ρV/A(Ω/sq) denote the length and area resistivity, respectively. The ρL is the property used to characterize conductive wires and yarns, whereas ρA is the resistivity property of a conductive fabric. Its units are in ohm but are usually expressed as Ω/sq (ohm per square) to diﬀerentiate it from the resistance itself (also in Ω). The ratio L/W is usually referred to as the aspect ratio.

4. Knitted and Stitched Strain Sensors In this section we will discuss the manufacturing details (types of yarn and knitting parameters) and electromechanical behaviour of knitted and stitched structures, which are able to detect and monitor deformation. The focus will be on sensors which can be knitted with a computer controlled knitting machine and commercially available conductive yarns. Knitted fabric sensors can easily be stretched to over 100%, and such behaviour is indeed often reported. However, the applications that we have in mind are for on the body worn garments where strains are not larger than 35–45% [3]. For respiration monitoring the sensing range of interest is even smaller (up to 5%). While discussing the sensitivity and other properties we will therefore in the following limit ourselves to the ﬁrst 40%. The ideal sensor properties for these type of applications can be formulated as [42]:

- Wide working range (up to 45% for measuring stretch on the human body); - High enough gauge factor (sensitivity); - Low hysteresis; - No signal drift; - Good repeatability (i.e., after multiple cycles and after washing). Sensors 2020,, 20,, x 7236 FOR PEER REVIEW 99 of of 30 28

4.1. Knitted Sensors 4.1. Knitted Sensors 4.1.1. Strain Sensitive Knitted Structures (1999–2009) 4.1.1. Strain Sensitive Knitted Structures (1999–2009) The number of published studies on knitted strain sensors in which details are given about the stretchThe strain number behaviour of published and electrical studies onresponses knitted strainare limited. sensors One in whichof the detailsfirst studies are given showing about the usefulnessstretch strain of textile behaviour based and sensors electrical is that responses of Farringdon are limited. [43]. They One knitted of the 10 first mm studies wide sensor showing strips the fromusefulness conductive of textile fibers based with sensors a nominal is that resistance of Farringdon 1 MΩ/m. [43]. These They sensors knitted 10were mm sewn wide on sensor a jacket strips to monitorfrom conductive limb and fibersupper with body a motion nominal and resistance had a quite 1 M Ωhigh/m. gage These factor sensors (close were to sewn17 for onthe a first jacket 20% to elongation).monitor limb Unfortunately and upper body no details motion about and hadthe cond a quiteuctive high yarn gage and factor knitting (close parameters to 17 for thewere first given. 20% Laterelongation). on Vogl Unfortunately et al. [44] used no the details same aboutidea to the manufacture conductive simple yarn and textile knitting user parameters interfaces by were stitching given. conductiveLater on Vogl yarns et al. on [ 44elastic] used fabric the same strip. idea to manufacture simple textile user interfaces by stitching conductiveAnother yarns early on study elastic was fabric that strip.by Bickerton [45] who used conductive carbon fibers to construct a knittedAnother fabric early sensor study with was an that initial by resistance Bickerton [of45 about] who 90 used kΩ conductive which increased carbon up fibers to 460 to constructkΩ at 30% a stretchknitted ( fabricGF = 14) sensor and withhad a an usable initial working resistance range of about up to 90about kΩ which50%. He increased interpreted up tothe 460 shape kΩ atof 30%the resistancestretch (GF curve= 14) in and terms had of a fiber usable slipping working during range extension up to about and 50%.the associated He interpreted increase the in shape conducting of the pathresistance between curve contact in terms points. of fiber slipping during extension and the associated increase in conducting pathThe between Advanced contact Textiles points. group of Tilak Dias (Nottingham Trent University) did much of the preliminaryThe Advanced work on Textilesknitted textile group sensors of Tilak and Dias claimed (Nottingham a first patent Trent in University) this area [28]. did Some much details of the arepreliminary given in [35] work who on knitteddescribe textile relatively sensors wide and knitted claimed sensor a first mesh patent structures in this area of 45 [28 mm]. Some (course) details by 25are mm given (wale) in [35 directions] who describe (Figure relatively 7a). Their wide measurements knitted sensor show mesh that structures these sensors of 45 mesh mm (course) are much by more25 mm sensitive (wale) directions in course (Figuredirection7a). (GF Their = 2.4 measurements over the first show10% stretch) that these compared sensors to mesh wale are direction much more (GF =sensitive 0.42, over in the course first direction 10%). The (GF sensitivity= 2.4 over quickly the first leve 10%ls of stretch) after 10% compared stretch. to No wale details direction about (GF the= yarn0.42, typesover thearefirst given 10%). however. The sensitivity In his 2006 quickly study, levels Wijesiriwardana of after 10% stretch.[46] also No considered details about a different the yarn way types of strainare given sensing however. by measuring In his 2006 the study, change Wijesiriwardana in inductance between [46] also two considered knitted aconductive different way coils of after strain a linearsensing or byangular measuring displacement. the change This in technique inductance could between not only two be knitted used for conductive respiration coils monitoring after a linear but alsoor angular for angular displacement. movement This monitoring technique on couldfingers not and only arms be (Figure used for 7b). respiration They claimed monitoring that these but types also offor sensors angular are movement less sensitive monitoring to temperature on fingers drifts and as arms well (Figure as to 7agingb). They due claimedto washes that as these compared types ofto resistivesensors are transducers. less sensitive These to temperature types of sensors drifts ascan well in asprinciple to aging be due used to washes both as as linear compared and as to resistiveangular sensor.transducers. These types of sensors can in principle be used both as linear and as angular sensor.

(a) (b)

FigureFigure 7. 7. (a(a) )Knitted Knitted sensor sensor [35] [35];; ( (bb) Knitted coil sensors in arm sleeve [[46]46]..

TheThe possibility possibility to to continuously continuously monitor monitor the the body body kinematics kinematics of of patients patients during during daily daily life life would would bebe a a breakthrough breakthrough in the rehabilita rehabilitationtion field field [47]. [47]. The The Pisa Pisa group group in in a a collaboration collaboration with with Smartex Smartex have have beenbeen working on this since the early 2000s. Their Their early early work considered sensing fabrics constructed byby impregnating impregnating fabrics with conductive polymer or elastomer coatings [48,49]. [48,49]. Because Because of of relatively relatively lowlow sensor sensor accuracy, accuracy, long response response time, time, and and hysteres hysteresisis effects, effects, the group then developed a series of of knittedknitted sensorssensors basedbased on on a carbona carbon filled filled nylon nylon fiber fiber (Belltron, (Belltron, [47,50 [47,50]] and referencesand references therein). therein). The sensor The sensordiscussed discussed in [47] hadin [47] a gauge had a factor gauge of factor about of 6.7, about a working 6.7, a rangeworking of atrange least of 10% at andleast an 10% acceptable and an acceptablehysteresis. hysteresis. In subsequent In subsequent work they buildwork and they investigated build and investigated several health several care systems, health care as summarized systems, as summarized in [51,52]. Sensors were fabricated by applying a conductive carbon filled silicone rubber

Sensors 2020, 20, 7236 10 of 28

Sensors 2020, 20, x FOR PEER REVIEW 10 of 30 Sensors 2020, 20, x FOR PEER REVIEW 10 of 30 in [51,52]. Sensors were fabricated by applying a conductive carbon filled silicone rubber coating to coating to the fabric or by constructing knitted strain sensors. In [51] they presented a knitted sensor coatingthe fabric to orthe by fabric constructing or by constructing knitted strain knitted sensors. strain In sensors. [51] they In presented [51] they a presented knitted sensor a knitted with sensor gauge with gauge factor 0.75, high linearity (up to 18% strain) and low hysteresis. Their findings suggest withfactor gauge 0.75, highfactor linearity 0.75, high (up linearity to 18% strain) (up to and 18% low strain) hysteresis. and low Their hysteresis. findings Their suggest findings that hysteresis suggest that hysteresis is suppressed by applying the conductive yarn in an ordered structure. Although they thatis suppressed hysteresis by is suppressed applying the by conductive applying the yarn conduc in antive ordered yarn structure. in an ordered Although structure. they Although were not clearthey were not clear how this is done, the most likely interpretation is that they achieved this by using the werehow thisnot isclear done, how the this most is done, likely the interpretation most likely isinterpretation that they achieved is that thisthey by achieved using the this plated by using knitting the plated knitting technique (see Section 2.1). platedtechnique knitting (see Sectiontechnique 2.1 ).(see Section 2.1). Zhang [53] knitted heat resistant structures consisting of pure steel and carbon yarn bundles Zhang [53] [53] knitted heat resistant structures consisting of pure steel and carbon yarn bundles (Figure 8). They argued that the change in contact resistance between conductive wire loops is the (Figure8 8).). They They argued argued that that the the change change in in contact contact resistance resistance between between conductive conductive wire wire loops loops is the is keythe key factor for the electromechanical response of knitted strain sensors. For the experiments, they used keyfactor factor for thefor the electromechanical electromechanical response response of of knitted knitted strain strain sensors. sensors. For For the the experiments, experiments, theythey used conductive yarns consisting of 120 stainless steel fibers of 30 μm diameter, knitted into a plain knitted conductive yarns consisting of 120 stainless steel fibers fibers of 30 μµm diameter, knitted into a plain knitted fabric with wale and course densities of 26 and 40 units per 50 mm. Their results indicated a gauge fabric with wale and course densities of 26 and 40 units per 5050 mm.mm. Their results indicated a gauge factor of about −10 over the first 10% stretch. After 20% stretch the resistance almost dropped to zero, factor of about −1010 over over the the first first 10% 10% stretch. stretch. After After 20% 20% stre stretchtch the the resistance resistance almost almost dropped dropped to to zero, zero, most probably− since they used only pure stainless steel fiber yarns without support yarns. The main most probably since they used only pure stainless steel fiberfiber yarns without support yarns. The main focus of their 2005 work [54] was to model the electromechanical behaviour of the knitted conductive focus of their 2005 work [54] [54] was was to to model model the the el electromechanicalectromechanical behaviour of the knitted conductive networks shown in Figure 8. Their experimental data showed that such networks have a linear networks shown in Figure8 8.. TheirTheir experimentalexperimental datadata showedshowed thatthat suchsuch networksnetworks havehave aa linearlinear responseresponse upup toto 10%10% andand aa gaugegauge factorfactor ofof aboutabout −10.10. response up to 10% and a gauge factor of about −10.

(a) (b) (c) (a) (b) (c) Figure 8. (a) Knitted single warp steel and carbon structures; (b,c) tubular structures [53]. FigureFigure 8. 8. (a(a)) Knitted Knitted single single warp warp steel steel and and carbon carbon structures; structures; ( (b,c) tubulartubular structuresstructures [[53]53].. Yang [55] modelled the electrical performance of knitted 1 × 1 rib structures. These structures Yang [[55]55] modelledmodelled thethe electricalelectrical performanceperformance ofof knittedknitted 11 × 1 ribrib structures.structures. These structures showed a resistance decrease while being stretched, an effect ×which was well predicted by their showed aa resistanceresistance decrease decrease while while being being stretched, stretched, an eanﬀect effect which which was wellwas predictedwell predicted by their by model their model (see Figure 9). The gauge factor for the first 20% stretching turned out to be close to −1.1. (see Figure9). The gauge factor for the ﬁrst 20% stretching turned out to be close to 1.1. model (see Figure 9). The gauge factor for the first 20% stretching turned out to be close− to −1.1.

(a) (b) (a) (b) FigureFigure 9.9. ((aa)) 11 × 1 rib knitted structure; and (b) itsits elongationelongation behaviourbehaviour [[55]55].. Figure 9. (a) 1 ×× 1 rib knitted structure; and (b) its elongation behaviour [55]. LiLi andand coworkerscoworkers [[56]56] measuredmeasured thethe confiningconfining pressurepressure andand electricalelectrical resistanceresistance ofof fabricsfabrics Li and coworkers [56] measured the confining pressure and electrical resistance of fabrics manufacturedmanufactured withwith eighteight didifferentfferent knittingknitting stitches.stitches. SinceSince theirtheir focusfocus waswas onon thethe confiningconfining pressurepressure manufactured with eight different knitting stitches. Since their focus was on the confining pressure theythey diddid notnot mentionmention thethe resistanceresistance changeschanges withwith strain.strain. Similarly,Similarly, in in a a laterlater workwork [[57]57] theythey usedused silversilver they did not mention the resistance changes with strain. Similarly, in a later work [57] they used silver coatedcoated polyamidepolyamide yarns yarns (Statex) (Statex) of 0.295 of 0.295 mm diameter mm diameter and 100–200 and Ω100–200/m conductivity Ω/m conductivity and determined and coated polyamide yarns (Statex) of 0.295 mm diameter and 100–200 Ω/m conductivity and thedetermined electromechanical the electromechanical behaviour of behaviour (i) single yarns, of (i) single (ii) looped yarns, yarns, (ii) looped and (iii) yarns, an intarsia and (iii) knitted an intarsia yarn determined the electromechanical behaviour of (i) single yarns, (ii) looped yarns, and (iii) an intarsia knitted yarn structure (together with a 58 tex cotton yarn). In that paper, however, they only reported knitted yarn structure (together with a 58 tex cotton yarn). In that paper, however, they only reported resistance changes versus applied load (and not strain) so gauge factors could again not be calculated. resistance changes versus applied load (and not strain) so gauge factors could again not be calculated.

Sensors 2020, 20, 7236 11 of 28

structureSensors 2020 (together, 20, x FOR PEER with REVIEW a 58 tex cotton yarn). In that paper, however, they only reported resistance11 of 30 changes versus applied load (and not strain) so gauge factors could again not be calculated. 4.1.2. Strain Sensitive Knitted Structures (2010–2020) 4.1.2. Strain Sensitive Knitted Structures (2010–2020) Zieba [58] constructed a knitted respiration sensor consisting of a cotton yarn base fabric a 20 Zieba [58] constructed a knitted respiration sensor consisting of a cotton yarn base fabric a 20 mm mm band of knitted silver plated polyester yarns (Xsilver, China). No details about knitting and band of knitted silver plated polyester yarns (Xsilver, China). No details about knitting and gauge gauge factor were reported but the respiration measurements results appeared reasonable. factor were reported but the respiration measurements results appeared reasonable. In a series of publications the group of Ozgur Atalay (University of Manchester) studied the In a series of publications the group of Ozgur Atalay (University of Manchester) studied the behaviour of knitted strain gages in more detail. In their 2013 study they used a silver coated nylon behaviour of knitted strain gages in more detail. In their 2013 study they used a silver coated nylon yarn (235 dtex, 200 Ω/m) plus several elastomeric yarns and varied the input tension of the yarn (235 dtex, 200 Ω/m) plus several elastomeric yarns and varied the input tension of the elastomeric elastomeric yarn to produce fabrics with different compactness [38]. The conductive yarn was yarn to produce fabrics with different compactness [38]. The conductive yarn was embedded in the embedded in the interlock base structure in a series of single jersey loops located only on the technical interlock base structure in a series of single jersey loops located only on the technical face of the fabric face of the fabric (Figure 10). The run-in tension was varied between 0.062 and 0.125 cN/tex. For each (Figureof the three 10). fabrics The run-in they tensiondetermined was the varied wale between and course 0.062 density, and 0.125 stitch cN length/tex. Forand each Tightness of the Factor three fabrics(Equation they (3)). determined They observed the wale a andbilinear course electromec density, stitchhanical length response and Tightnesswith a gauge Factor factor (Equation 3.7 below (3)). They19% strain observed and a2.2 bilinear above, electromechanical as well as hysteresis response effects. with The a gaugemost tightly factor 3.7knitted below structure 19% strain had and the 2.2highest above, linearity as well (GF as = hysteresis 0.75). The e fabricsffects. Thecould most be extended tightly knitted up to 350% structure before had breaking the highest and were linearity seen GF (to be= stable0.75). during The fabrics cyclic could testing. be With extended the increase up to 350% of fabric before tightness breaking the andrelaxation were seen effects to increased be stable duringslightly cyclic(from testing.4 to 16%) With [38]. the increase of fabric tightness the relaxation effects increased slightly (from 4 to 16%) [38].

(a) (b)

FigureFigure 10.10.(a ) Loop-wise(a) Loop-wise embedded embedded conductive conductive yarn in knitted yarn interlockin knitted structure; interlock (b) electromechanical structure; (b) responseelectromechanical [38]. response [38].

InIn aa follow-upfollow-up studystudy [[59]59] theythey usedused aa silversilver coatedcoated nylonnylon yarnyarn (Swicoﬁl,(Swicofil, 22 ohmohm/cm)/cm) andand threethree typestypes ofof LycraLycra basebase yarnyarn coveredcovered withwith wrappedwrapped nylonnylon ﬁlamentfilament (800,(800, 570570 andand 156156 dtex,dtex, 88 cNcN feedingfeeding tension).tension). TheThe 800800 dtex Lycra yarnyarn waswas studiedstudied inin moremore detaildetail andand thethe conductive yarnyarn feedingfeeding tensiontension waswas variedvaried fromfrom 5,5, 10,10, toto 2020 cN.cN. ItIt showedshowed thatthat atat lower yarn feeding tension the loop structures werewere looserlooser and the the conductive conductive contact contact area area was was increase increased.d. The The 20 20cN cNfabric fabric showed showed lowest lowest hysteresis hysteresis and anda bi-linear a bi-linear electromechanical electromechanical response response (GF ( GF= 1.86= 1.86 up upto to18% 18% and and 0.68 0.68 up up to to 40%, 40%, Figure Figure 1010b).b). IncreasingIncreasing thethe numbernumber ofof conductiveconductive coursescourses (separated(separated byby non-conductivenon-conductive material)material) diddid notnot increaseincrease thethe performanceperformance duedue toto increasedincreased tendencytendency ofof thethe fabricfabric toto buckle.buckle. AsAs before,before, thethe sensorssensors werewere stablestable andand showedshowed a lowa low drift. drift. In theirIn their 2015 paper2015 paper [60] they [60] optimized they optimized the knitting the ofknitting multiple of line multiple conductive line samplesconductive and samples produced and a sensor produced for respiration a sensor for monitoring respiration which monitoring was linear which of to was 8% linear with GF of to= 3.448% with and whichGF = 3.44 did and not showwhich hysteresis did not show (800 dtexhysteresis Lycra base(800 yarndtex withLycra 235 base dtex yarn silver with plated 235 dtex nylon, silverTF = plated1.39). nylon,In TF their = 1.39). last study [61] they compared two types of conductive yarns (Bekinox polyester blended stainlessIn their steel last and study silver [61] plated they compared nylon), plain two types versus of conductive interlock base yarns structure (Bekinox and polyester elastic blended versus non-elasticstainless steel base and structure silver plated yarn. nylon), During plain knitting, versus interlock conductive base and structure elastic and yarn elastic are fed versus to same non- needleelastic (platingbase structure technique) yarn. to During be able knitting, to shield cond the backuctive of and the sensorelastic fromyarn skinare contact.fed to same From needle their electro-mechanical(plating technique) tests, to be it turnedable to outshield that the sensors back withof the a non-elastic sensor from plain skin knitted contact. base From structure their showedelectro- amechanical negative gauge tests, factorit turned whereas out that elastic sensors structures with a resultednon-elastic in aplain positive knitt gaugeed base factor. structure They showed attribute a negative gauge factor whereas elastic structures resulted in a positive gauge factor. They attribute this difference to the elastic tension which initially compressed the conductive loops in the elastic structure but which is absent in the non-elastic case. All of their results on these types of sensors, however, showed a large scatter and reproducibility, which is probably due to the lack of sufficient

Sensors 2020, 20, 7236 12 of 28

Sensorsthis di 2020fference, 20, x FOR to the PEER elastic REVIEW tension which initially compressed the conductive loops in the elastic12 of 30 structure but which is absent in the non-elastic case. All of their results on these types of sensors, contractionhowever, showed in the asensor large scatterarea compared and reproducibility, to the base whichstructure is probably which may due lead to the to lackbuckling of su ffiofcient the sensorcontraction part. inThey the sensorexplained area the compared unreliable to the response base structure of their which stainless may steel lead tosensors buckling to the of theirregular sensor orientationpart. They explainedof the steel the fibers unreliable within responsethe structure. of their Reported stainless gauge steel sensorsfactors in to their the irregular paper ranged orientation from −of0.7 the to steel1.05. fibers within the structure. Reported gauge factors in their paper ranged from 0.7 to 1.05. − A systematic study on knitted sensor performance was was conducted conducted by by the the textile textile group group of the Niederrhein university [62]. [62]. This This group group studied studied the the effect effect of knitting structure on the strain sensing capability and considered the following parameters: - Stitch type: double face, single face, Milano rib and full cardigan; - Stitch type: double face, single face, Milano rib and full cardigan; - Stitch cam settings NP = 9.5 (small), 10.5 (medium) and 11.5 (large); - Stitch cam settings NP = 9.5 (small), 10.5 (medium) and 11.5 (large); - Conductive yarn type: four types of S-Shield PES and cotton blended stainless steel yarns (single - thread,Conductive two thread; yarn type: 50/2 with four 20% types steel of fibers S-Shield or 15/1 PES Nm and with cotton 50% blended steel); stainless steel yarns - Fabric(single orientation: thread, two 0, thread; 30, 45, 50 60,/2 and with 90°. 20% steel fibers or 15/1 Nm with 50% steel); - Fabric orientation: 0, 30, 45, 60, and 90◦. They calculated wale and course densities as well as the loop lengths, which would allow them to relateThey the calculated observed wale electromechanically and course densities behaviou as wellr to as textile the loop parameters lengths, whichlike the would tightness allow factor. them Regardingto relate the the observed stitch types electromechanically they observed that behaviour the single to textileface fabrics parameters often showed like the oscillatory tightness factor. time dependenceRegarding the and stitch that the types Milano they rib observed structure that had the a singlerather facesmall fabrics sensitivity. often The showed full cardigan oscillatory and time the doubledependence face structures and that the gave Milano the best rib structureresults (Figure had a 11). rather small sensitivity. The full cardigan and the double face structures gave the best results (Figure 11).

Figure 11. Effect of stitch size on electromechanical behaviour of full cardigan fabrics. (a) With respect Figure 11. Effect of stitch size on electromechanical behaviour of full cardigan fabrics. (a) With to strain and (b) with respect to time [62]. respect to strain and (b) with respect to time [62]. Figure 11 also shows that for these knitted structures the resistance decreases when stretched (negativeFigure gauge 11 also factors) shows and that that for after these the knitted first 10% stru stretchctures thethe sensorsresistance rapidly decreases became when less stretched sensitive. (negativeFurthermore, gauge the factors) figure shows and that that after the smallerthe firststitched 10% stretch structures the sensors had a considerablerapidly became lower less gage sensitive. factor Furthermore,than the medium the andfigure larger shows ones that ( 2.2 the versus smaller5.8, stitched as obtained structures over thehad first a considerable 10%). On the lower other hand,gage − − factorthe signal than decrease the medium over and time larger of the ones smaller (−2.2 stitched versus − structures5.8, as obtained was considerable over the first better 10%). than On the the other more hand,dense the structures signal (Figuredecrease 11 overb). time of the smaller stitched structures was considerable better than the moreThe studydense ofstructures Ehrmann (Figure et al. [11b).62], showed that the difference between the 50/2 conductive yarn containingThe study 20% of steel Ehrmann filaments et al. and [62], that showed of the 15that/1 yarnthe difference with 50% between steel was the not 50/2 large, conductive as depicted yarn in containingFigure 12. The20% 50 steel/2 yarn filaments had a smallerand that working of the 15/1 range yarn (up with to 15 50% to 20%) steel but was also not a large, considerable as depicted smaller in Figurerelaxation. 12. The More 50/2 interesting yarn had a to smaller note is working that the range blending (up into of15 cottonto 20%) threads but also resulted a considerable in undesirable smaller relaxation.(first increase, More then interesting decrease to in note resistance) is that asthe well blending ambiguous in of cotton results threads (peaks andresulted scatter in inundesirable relaxation (firstexperiments). increase, Thisthen confirmeddecrease in the resistance) conclusions as ofwell Atalay ambiguous et al. [61 results] that mixed (peaks filament and scatter conducting in relaxation yarns experiments).were unsuited This for constructing confirmed knittedthe conclusions strain sensors. of Atal Theay authorset al. [61] also that showed mixed that filament when elongatedconducting in yarns were unsuited for constructing knitted strain sensors. The authors also showed that when wale direction (90◦) the sensitivity increased at the costs of a decreased working range (limited to 20%) elongated in wale direction (90°) the sensitivity increased at the costs of a decreased working range (limited to 20%) and relaxation behaviour. Interestingly enough, all tests at 30, 45, and 60° closely followed this wale direction behaviour. Wang et al. [63] proposed an electrical resistive mathematical model using an empirical relation for the inter-loop resistance increase during stretching and validated their model with measurements

Sensors 2020, 20, x FOR PEER REVIEW 13 of 30 SensorsSensors 20202020,, 2020,, 7236x FOR PEER REVIEW 13 of 2830 atat 2,2, 5,5, andand 10%10% elongation.elongation. TheirTheir resultsresults showedshowed linearitylinearity ofof thethe knittedknitted sensorssensors upup toto 5%5% andand gaugegauge factors of 5 to 10. Unfortunately, most experimental conditions were not reported and a good andfactors relaxation of 5 tobehaviour. 10. Unfortunately, Interestingly most enough, experime allntal tests conditions at 30, 45, and were 60◦ nocloselyt reported followed and this a good wale directioncomparisoncomparison behaviour. betweenbetween experimentsexperiments andand modelmodel waswas missing.missing.

Figure 12. Eﬀect of yarn materials [62]. FigureFigure 12.12. EffectEffect ofof yarnyarn materialsmaterials [62].[62]. Wang et al. [63] proposed an electrical resistive mathematical model using an empirical relation for Oks [64] knitted plain stich sensors with alternating conductive and non-conductive courses the inter-loopOks [64] resistanceknitted plain increase stich during sensors stretching with alte andrnating validated conductive their model and non-conductive with measurements courses at 2, (Figure 13). Due to the compressive effect of the elastomeric yarns the conductive courses made 5,(Figure and 10% 13). elongation. Due to the Their compressive results showed effect linearityof the elastomeric of the knitted yarns sensors the upconductive to 5% and courses gauge factors made resistive contact when unloaded but separated gradually when loaded (in wale direction). For the ofresistive 5 to 10. contact Unfortunately, when unloaded most experimental but separated conditions gradually were when not loaded reported (in andwale a direction). good comparison For the experiments they used betweenexperiments experiments they used and model was missing. • • OksBaseBase [ yarn:64yarn:] knitted Elastane/polyesterElastane/polyester plain stich sensors 22/7822/78 yarnyarn with alternating conductive and non-conductive courses • (Figure• FunctionFunction 13). Due non-conductivenon-conductive to the compressive yarn:yarn: 2525 e TexﬀTexect cottoncotton of the elastomeric yarns the conductive courses made • resistive• FunctionalFunctional contact conductive:conductive: when unloaded ShieldexShieldex but dTex110 separateddTex110 andand gradually dTex110*2dTex110*2 when loaded (in wale direction). For the experiments they used

((aa)) ((bb)) ((cc)) Figure 13. (a,b) Alternating courses of conductive and non-conductive yarns; (c) resistance change Figure 13.13. ((aa,,bb)) AlternatingAlternating courses courses of of conductive conductive and and non-conductive non-conductive yarns; yarns; (c) resistance (c) resistance change change [64]. [64][64].. Base yarn: Elastane/polyester 22/78 yarn; • These sensors work well for strains up to 5 or 10% and can be designed such that the resistance FunctionThese sensors non-conductive work well for yarn: strains 25 Tex up cotton; to 5 or 10% and can be designed such that the resistance •increases to infinity if the strain exceeds a certain value (if the conductive courses are separated too increasesFunctional to infinity conductive: if the strain Shieldex exceeds dTex110 a certai andn value dTex110*2. (if the conductive courses are separated too •much,much, seesee FigureFigure 13).13). ThisThis typetype ofof designdesign howehoweverver seemsseems lessless suitablesuitable forfor workwork rangesranges aboveabove 20%.20%. TheThe sensorsThesesensors sensors showedshowed work hysteresishysteresis well for whenwhen strains thethe up knittingknitting to 5 or wawa 10%ss donedone and can atat highhigh be designed coursecourse densitydensity such that (16.8/cm).(16.8/cm). the resistance WhenWhen increasesknittedknitted withwith to inﬁnitylowerlower coursecourse if the densitydensity strain exceeds (15.2/cm)(15.2/cm) a the certainthe hysthyst valueeresiseresis (ifwaswas the almostalmost conductive absent.absent. courses TheThe resistanceresistance are separated changechange toowithwith much, appliedapplied see strainstrain Figure curvecurve 13). waswas This non-linearnon-linear type of design (Figure(Figure however 13c)13c),, butbut seems whenwhen lessevaluatedevaluated suitable overover for thethe work firstfirst ranges 5%5% strainstrain above thethe 20%.gaugegauge The factorsfactors sensors variedvaried showed betweenbetween hysteresis 2020 andand when42.42. IfIf thethe 110*2110*2 knitting doubledouble was yarn doneyarn waswas at high usedused course thethe linearitylinearity density waswas (16.8 betterbetter/cm). Whenbutbut thethe knitted gaugegauge withfactorsfactors lower werewere course aa bitbit lowerlower density (10(10 (15.2 toto 30).30)./cm) TheThe the sensorssensors hysteresis werewere was seenseen almost toto bebe stable absent.stable duringduring The resistance 1010 cyclescycles ofof loading.loading. TheyThey presentedpresented knittedknitted sensingsensing glovesgloves andand sockssocks butbut diddid notnot discussdiscuss theirtheir properties.properties.

Sensors 2020, 20, 7236 14 of 28 change with applied strain curve was non-linear (Figure 13c), but when evaluated over the first 5% strain the gauge factors varied between 20 and 42. If the 110*2 double yarn was used the linearity was better but the gauge factors were a bit lower (10 to 30). The sensors were seen to be stable duringSensors 2020 10, cycles20, x FOR of PEER loading. REVIEW They presented knitted sensing gloves and socks but did not discuss14 of 30 their properties. XieXie [[65]65] fabricated aa yarn consisting of 50% cotton and 50% 8 µμm thin stainless steel fibersfibers which lookedlooked andand feltfelt likelike cottoncotton andand whichwhich werewere robustrobust andand washable.washable. JerseyJersey knittedknitted sensorssensors showedshowed aa high,high, negativenegative gagegage factorfactor of of −2020 whenwhen loaded loaded in in wale wale direction direction but but after after 5% 5% strain strain this this diminished diminished to − to1.52 −1.52 (Figure (Figure 14 14c).c). Due Due to to this this large large change change this this type type of of sensor sensor is is in in its its currentcurrent statestate ofof lessless practicalpractical − useuse butbut maymay showshow toto havehave aa largelarge potentialpotential whenwhen improved.improved. In course direction the gaugegauge factorfactor changedchanged fromfrom −3.7 to −1. As As a a comparison comparison they they also also tested tested a sensor knitted from silversilver platedplated − − multifilamentmultifilament nylonnylon (110(110 dtexdtex/40f)./40f). These sensors were linearlinear upup toto 30% strain and had gauge factors ofof 0.360.36 (course(course direction) direction) and and 0.05 0.05 (wale (wale direction, direction, Figure Figure 14d). 14d). This This once once again again shows shows that silver that platedsilver nylonplated yarns nylon are yarns much are more much suitable more for suitable sensor constructionfor sensor construction than stainless than steel stainless filaments steel blended filaments with non-conductiveblended with non-conductive material. material.

FigureFigure 14.14. Course-wise response of ( a) Cotton-steel knitted sensor and (b) nylon plated material; Wale-wiseWale-wise (transverse)(transverse) responsesresponses ofof (c(c)) cotton-steel cotton-steel and and ( d(d)) nylon nylon plated plated material material [ 65[65].].

RibRib structures havehave thethe tendency to contract laterally, resultingresulting inin aa patternpattern inin which knit stitches comecome forwardforward andand purl purl stitches stitches recede. recede. They They provide provide highly highly elasticity elasticity and and are are therefore therefore often often used used for cuforff scuffs and and edges edges that requirethat require form-fitting. form-fitting. During During stretching stretching the vertical the vertical rib structures rib structures unfold unfold which makeswhich themmakes an them interesting an interesting candidate candidate for (large for deformation) (large deformation) stretch stretch sensors. sensors. A detailed A detailed study on study the eonffect the of effect the rib of structure the rib structure on the electrical on the electrical behaviour behaviour was performed was performed by Raji [66 ]by for Raji a future [66] for application a future asapplication respiration as sensorrespiration in a Smartsensor Bra.in a TheySmart used Bra. aThey silver used coated a silver nylon coated yarn nylon (77 ohm yarn/cm (77 resistivity) ohm/cm asresistivity) a conductor as a and conductor both bare and strand both (BS)bare andstrand nylon (BS) covered and nylon yarn covered (CY) spandex yarn (CY) as the spandex elastic yarns.as the Theelastic four yarns. types The of mockfour types rib structures of mock rib studied structures are shown studied in are Figure shown 15. in They Figure reported 15. They the reported main textile the parametersmain textile (wale parameters and course (wale density and course and fabric density dimensions) and fabric as dimensions) well as the resistance as well as changes the resistance during cyclicchanges loading. during Their cyclic main loading. findings Their were: main Gauge findings factors were: of samples Gauge withfactors the of bare samples strand with elastane the bare (BS) hadstrand systematically elastane (BS) higher had syst valuesematically (GF = higher2.4, 2.8, values 2.3 and (GF 2.2 = 2.4, for the2.8, 12.3 1,and 1 2.23, for 1 the2, and1 × 1, 2 1 ×2 3, ribs) 1 × × × × × than2, and those 2 × 2 of ribs) the coveredthan those yarn of elastane the covered samples yarn (GF elastane= 1.25, samples 1.5, 1.6, and(GF 1.4).= 1.25, The 1.5, sensor 1.6, and performance 1.4). The ofsensor the CY performance 1 1 and 1 of 2the as CY well 1 as× 1 the and BS 1 1 × 23 as samples well as were the seenBS 1 to× 3 decrease samples somewhat were seen after to decrease 50 days × × × somewhat after 50 days of testing. Unfortunately, no direct resistance versus strain plots were reported so no conclusion about linearity can be given.

Sensors 2020, 20, 7236 15 of 28 of testing. Unfortunately, no direct resistance versus strain plots were reported so no conclusion about Sensors 2020, 20, x FOR PEER REVIEW 15 of 30 linearitySensors 2020 can, 20, be x FOR given. PEER REVIEW 15 of 30

Figure 15. Images of knitted rib structures. (a) 1 × 1, (b) 1 × 3, (c) 1 × 2 and (d) 2 × 2 structure [66]. FigureFigure 15. 15.Images Images of of knitted knitted rib rib structures. structures. ( a(a)) 1 1 × 1,1, ((bb)) 11 × 3, (c) 1 × 22 and and ( (d)) 2 2 × 22 structure structure [66]. [66]. × × × × In a subsequent paper they selected the CY 1 × 1 structures and varied the knitted sensor InIn aa subsequent subsequent paper paper they they selected selected the CYthe 1CY1 structures1 × 1 structures and varied and thevaried knitted the sensor knitted geometry sensor geometry for aesthetic reasons [67]. They conclu×ded that complex shaped samples delivered poor forgeometryrepeatability aesthetic for reasons andaesthetic noisy [67 ].reasons results, They concluded [67].probably They thatbecauseconclu complexded of thethat shaped truncating complex samples shaped of conductive delivered samples poor yarns delivered repeatability inside poor the andrepeatabilitynon-conductive noisy results, and gauge probablynoisy areas.results, because An probably extended of the because truncating test se ofries ofthe with conductive truncating 14 rectangular yarns of conductive inside samples the yarns non-conductive with inside different the gaugenon-conductiveaspect areas.ratio showed An extendedgauge first areas. of test all An series that extended the with gauge 14 test rectangular resistance series with samplesincreased 14 rectangular with linearly different withsamples aspect the aspectwith ratio different showedratio, as firstaspectexpected of ratio all from that showed the resistance gauge first of resistance all expression that the increased gauge Equation resistance linearly (7). Not with increased shown the aspect by linearly the ratio, authors with as expectedthebut aspect based from ratio,on their the as resistanceexpecteddata, the fromsurface expression the resistivity resistance Equation could expression (7). be Not determined shownEquation by as(7). the 1.83 Not authors Ω shown/sq (see but by Figure based the authors on16). their Combining but data, based the this on surface theirwith resistivitydata,Equation the surface could(7) and be resistivitythe determined given couldlinear as 1.83 beresistivity determinedΩ/sq (see of Figure77 as Ω 1.83/cm 16 ).thisΩ Combining/sq implies (see Figure a this fabric 16). with thickness Combining Equation of (7) 0.24this and withmm, the givenEquationwhich linear could (7) resistivity beand realistic. the given of Secondly, 77 Ωlinear/cm thisresistivitytheir implies results of a show 77 fabric Ωed/cm thickness that this the implies gauge of 0.24 factora mm,fabric of which thicknessthe rib could knitted of be 0.24 realistic. sensors mm, Secondly,whichhave a could maximum their be results realistic. between showed Secondly, 40 thatto 120 thetheir Ω gauge. resultsThey factor reasoned show ofed the thatthat rib knitted thethe gauge decrease sensors factor in have gaugeof the a maximum ribfactor knitted below between sensors 40 Ω 40havewas to due 120a maximum toΩ. an They increased between reasoned resistive 40 that to 120 the heating Ω decrease. They resulting reasoned in gauge in thata factor less the efficient below decrease 40sensing Ωin wasgauge and due factor that to an forbelow increased too 40high Ω resistivewasresistances due heating to thean increasedcurrent resulting becomes inresistive a less too e ffiheating cientlow to sensing resultinggive reliab and in thatle a results. less for tooefficient Although high resistancessensing the latterand the that currentinterpretation for becomestoo high is tooresistancesdoubtful low to (other give the current reliablestudies becomes results.worked Although toowith low sensors to the give latterof reliabseveral interpretationle results.kΩ [50]), Although the is doubtful idea thethat (otherlatter gauge interpretation studies factors worked show is a withdoubtfulmaximum sensors (other depending of severalstudies on kworked Ωtheir[50 ]),geometry with the ideasensors is thatinteresting of gauge several factorsto kfurtherΩ [50]), show explore the a maximumidea for that other gauge depending knitting factors structures. on show their a geometrymaximum is depending interesting on to their further geometry explore is for interesting other knitting to further structures. explore for other knitting structures.

(a) (b) (a) (b) FigureFigure 16.16.( a(a)) Resistance Resistance and and (b )( gaugeb) gauge factor factor rectangular rectangular knitted knitted sensors sensors of diff erentof different length andlength 6width; and Figure 16. (a) Resistance and (b) gauge factor rectangular knitted sensors of different length and Based6width; on Based [67]. on [67]. 6width; Based on [67]. OuOu etet al. al. [31 [31]] used used a 1000 a 1000Ω/m Ω 450/m denier 450 denier (500 dtex) (500 silver dtex) plated silver nylon plated blended nylon with blended non-conductive with non- yarnsconductiveOu and et an al.yarns interlock [31] and used an knit a interlock 1000 structure. Ω/m knit 450 They structure. denier first knitted(500 They dtex) first a series silver knitted of plated rectangular a series nylon of sensors rectangularblended with with sensors aspect non- ratiosconductivewith aspect varying yarns ratios between and varying an 0.31 interlock between and 320 knit and0.31 structure. confirmedand 320 They and the firstconfirmed linear knitted scaling the a series withlinear Lof /Wscaling rectangular, as expected with Lsensors/W from, as Equationwithexpected aspect (7)from andratios Equation also varying found (7) andbetween in Figure also found 0.3116. Thean ind surfaceFigure 320 and 16. resistivity confirmedThe surface was the resistivity 2.63 linearΩ/sq, scalingwas which 2.63 iswith Ω of/sq, the L /whichW same, as orderexpectedis of the as thesame from value order Equation for as the the (7) study value and discussed also for thefound study above in Figurediscussed [67]. The16. Theabove sensitivity surface [67]. to Theresistivity stretching sensitivity was was to2.63 onlystretching Ω/sq, shown which was for aisonlysingle of theshown sensorsame for order geometry a single as the sensor (with value 60geometry forΩ initial the study (with resistance). discussed 60 Ω initial The above sensor resistance). [67]. response The The sensitivity wassensor more response to or stretching less linearwas more was but theonlyor less strain shown linear range for but a and single the sensitivity strain sensor range geometry could and not sensitivity (with be determined 60 Ω could initial sincenot resistance). be the determined sample The length sensor since was response the not sample reported. was length more orwas less not linear reported. but the strain range and sensitivity could not be determined since the sample length was not reported. 4.1.3. Modelling of Knitted Sensor Electromechanical Behaviour 4.1.3. Modelling of Knitted Sensor Electromechanical Behaviour About the physical interpretation for the observed decreasing and increasing resistance-strain curvesAbout there the seems physical to be interpretation a general consensus for the inobserv the literature,ed decreasing although and theyincreasing have neverresistance-strain been listed curves there seems to be a general consensus in the literature, although they have never been listed

Sensors 2020, 20, 7236 16 of 28

4.1.3. Modelling of Knitted Sensor Electromechanical Behaviour About the physical interpretation for the observed decreasing and increasing resistance-strain Sensorscurves 2020 there, 20, x seems FOR PEER to be REVIEW a general consensus in the literature, although they have never been16 listed of 30 together. Summarizing the literature, we can discern the following four distinct stage during the together.stretching Summarizing of a knitted conductor: the literature, we can discern the following four distinct stage during the stretching of a knitted conductor: Stage 1: Reduction of contact points: the opening of the structure (gradually) disconnects •• Stage 1: Reduction of contact points: the opening of the structure (gradually) disconnects contacting loops which were connected in the initial state due to elastic contraction of the base contacting loops which were connected in the initial state due to elastic contraction of the base structure. This is typical for the first 10–20% stretching and causes a resistance increase; structure. This is typical for the first 10–20% stretching and causes a resistance increase; Stage 2: Yarn slippage and shifting of contact points due to a rearrangement of the knitted structure. •• Stage 2: Yarn slippage and shifting of contact points due to a rearrangement of the knitted This results in a larger yarn length (and resistance) between a fixed number of contact points and structure. This results in a larger yarn length (and resistance) between a fixed number of contact can be a dominating effect for high resistance yarns. Between 10 and 40% strain; points and can be a dominating effect for high resistance yarns. Between 10 and 40% strain; Stage 3: Increased contact pressure between hooked conductor loops. This effect starts to play a •• Stage 3: Increased contact pressure between hooked conductor loops. This effect starts to play a role for strains above, say, 20% and causes the resistance to decrease. In bare metal conductive role for strains above, say, 20% and causes the resistance to decrease. In bare metal conductive yarns this may start earlier and is the dominating effect; yarns this may start earlier and is the dominating effect; Stage 4: The stretching of the conductor yarn segments itself. This occurs when all slack is out of •• Stage 4: The stretching of the conductor yarn segments itself. This occurs when all slack is out of thethe system andand looploop segments segments have have straightened. straightened. Depending Depending on theon structurethe structure this maythis startmay abovestart above50 to 80%. 50 to For80%. metal For coatedmetal coated and carbon and carbon filled yarns filled this yarns results this inresults an increase in an increase in resistance in resistance whereas whereasfor yarns for consisting yarns consisting of metal filaments of metal blended filament withs blended non-conductors with non-conductors the resistance the decreases resistance due decreasesto the squeezing due to the effect. squeezing effect. TheThe modelling modelling isis donedone by by first first calculating calculating the the resistance resistance and and then then separately separately addressing addressing the above the abovefour stages. four stages. We will We not will discuss not discuss the literature the literature on resistance on resistance modelling modelling in detail but in indetail general but this in general is based thison firstis based considering on first the considering resistances the of loopresistances segments of loop and contactssegments in and a unit contacts cell (Figure in a unit17a), cell followed (Figure by 17a),calculating followed the by equivalent calculating resistance the equivalent of a network resistan ofce such of a unit network cells (Figureof such 17unitb). cells This (Figure always led17b). to Thisunnecessary always led large to andunnecessary complex expressionslarge and complex which are expressions difficult to which handle are and difficult did not to give handle much and insight. did notRefreshing give much is therefore insight. Refreshing to follow the is waytherefore of reasoning to follow of the Li et way al.[ 68of ].reasoning These authors of Li et take al. into[68]. account These authorsthat most take segment into account resistances that most are of segment equal value resistances and that are adjacent of equal unit value cells and therefore that adjacent act as unit balanced cells thereforeWheatstone act bridgesas balanced through Wheatstone which no br currentidges through flows. which no current flows.

(a) (b) (c)

FigureFigure 17. 17. (a(a,b,b)) Unit Unit cell cell of of knit knit [68]; [68]; ( (cc)) Resistor Resistor network network approach approach [53]. [53].

TheThe current current then then in in fact fact appears appears to to flow flow only only along along the the courses courses without without any any interaction interaction between between adjacentadjacent courses, whichwhich considerablyconsiderably simplifies simplifies the the network network model model (see (see Figure Figure 18). Note18). Note that thethat upper the upperand lower and lower courses courses of the of conductive the conductive area have area ahave resistance a resistance which which differs differs from those from inthose between in between due to duethe factto the that fact the that outer the part outer of part the loopsof the there loops are th notere are in in not contact in in withcontact other with conductors other conductors and miss and the misstwo contactthe two resistancecontact resistance points (points pointsR (pointsc1 and Rcc12 andFigure Rc2 17Figureb). 17b).

Sensors 2020, 20, 7236 17 of 28 Sensors 2020, 20, x FOR PEER REVIEW 17 of 30

FigureFigure 18.18. SimpliﬁedSimplified equivalentequivalent networknetwork modelmodel [[68].68].

ForFor structuresstructures withwith moremore thanthan threethree coursescourses thethe equivalentequivalentresistance resistance then then becomes becomes [ 68[68]]

2 R++R ∥ =2nwR1 ; = 1 1 3 (8) Req= +; K = −3k (8) nc + K R1 3R1∥3 − k in which both R1 and K depend on the contact resistance. It is clear that for large enough number of in which both R1 and K depend on the contact resistance. It is clear that for large enough number of courses the resistance scales with the ratio of the number of wales nw and courses nc, or as the length courses the resistance scales with the ratio of the number of wales n and courses n , or as the length over width ratio L/W, as seen in Equation (7). w c over width ratio L/W, as seen in Equation (7). Next the question is how to calculate the resistance increase due to stretching. Although some Next the question is how to calculate the resistance increase due to stretching. Although some studies on the topic are reported, a generally accepted theory is not available yet. Stage 1 (the gradual studies on the topic are reported, a generally accepted theory is not available yet. Stage 1 (the gradual decrease of contact points) is discussed by e.g., [60] who take into account Holm’s contact pressure decrease of contact points) is discussed by e.g., [60] who take into account Holm’s contact pressure theory. Xie et al. [69] address stage 2 and assume that during stretching the yarns slide, resulting in theory. Xie et al. [69] address stage 2 and assume that during stretching the yarns slide, resulting in longer loop segments in tensile direction (Rl3 in Figure 17b). The loop segment length changes during longer loop segments in tensile direction (R in Figure 17b). The loop segment length changes during stretching were empirically determined andl3 with this and their (complicated) network model they stretching were empirically determined and with this and their (complicated) network model they could could correctly predict the overall resistance change due to stretching. In other works often the correctly predict the overall resistance change due to stretching. In other works often the empirically empirically determined contact resistance variation with pressure is used to describe stage 3 [53]. The determined contact resistance variation with pressure is used to describe stage 3 [53]. The fourth stage fourth stage can be taken into account by first estimating how the yarn stretching relates to the can be taken into account by first estimating how the yarn stretching relates to the macroscopic strain macroscopic strain in the structure and then using the (measured) resistance increase of the in the structure and then using the (measured) resistance increase of the conductive yarns. conductive yarns. 4.2. Stitched Sensors 4.2. Stitched Sensors With knitting the sensor placement and orientation is limited in flexibility because of the fact that manyWith hidden knitting conductive the sensor tuck yarnsplacement are needed and orientation to realize more is limited complex in shapesflexibility as wellbecause as orientations of the fact dithatfferent many from hidden wale andconductive course directions.tuck yarns Althoughare needed these to hiddenrealize yarnsmore arecomplex not visible, shapes they as tendwell toas corruptorientations proper different functioning from ofwale the and sensor course [67]. direct Stitching,ions. Although on the other these hand, hidden can beyarns directly are not applied visible, to athey finished tend to garment corrupt and proper is not function limiteding on oforientation the sensor [67]. direction. Stitching, Moreover, on the other stitched hand, sensors can be have directly no perceptibleapplied to a compromise finished garment for user and comfort is not and limited a minimal on orientation impact on direction. garment productionMoreover, stitched [70]. The sensors sensor responsehave no perceptible however relies compromise heavily fo onr user the physicalcomfort and characteristics a minimal impact of the textileon garment substrate production to which [70]. it isThe stitched. sensor response however relies heavily on the physical characteristics of the textile substrate to whichMuch it is ofstitched. the work on stitched strain sensors has been done by the group of Dunne (University of Minnesota).Much of In the their work first on work stitched on the strain topic sensors Gioberto has and been Dunne done [ 70by] presentthe group a top-thread of Dunne cover(University stitch stretchof Minnesota). sensor using In their a silver first platedwork on nylon the yarntopic (LamGiobertoé Lifesaver, and Dunne 0.81 ohm[70]/ cm)present on aa 98% top-thread polyester cover/2% Lycrastitch jerseystretch knit sensor fabric. using The a sensorsilver plated had a worknylon range yarn of(Lamé about Lifesaver, 12% and 0.81 a gauge ohm/cm) factor on of a about 98% 0.54.polyester/2% In their Lycra 2013 paperjersey [knit71], fabric. they tested The sensor overlock-stitched had a work range strain of sensor about using 12% and a Shieldex a gauge 235 factor/34 silverof about coated 0.54. nylonIn their yarn 2013 (50 paperΩ/m) [71], and they five tested types ofoverlock-stitched jersey knit fabrics strain with sensor different using elastomeric a Shieldex content.235/34 silver Their sensorscoated turnednylon yarn out to (50 have Ω a/m) work and range five betweentypes of 18 jersey and 29% knit and fabrics relatively with small different drift andelastomeric hysteresis. content. Gauge Their factors sensors were turned not reported out to ha butve a can work be range estimated between to vary 18 and between 29% and 0.48 relatively and 2.2. Thesmall sensor drift and with hysteresis. the highest Gauge work factors range were was the not one reported stitched but on can fabric be estimated consisting to ofvary 82% between nylon and0.48 18%and spandex.2.2. The sensor In another with paperthe highest [72] they work compared range wa overlocks the one and stitched bottom on thread fabric cover consisting stitches of using 82% nylon and 18% spandex. In another paper [72] they compared overlock and bottom thread cover two, four, and five-ply silver plated nylon yarns (Shieldex 177/17, Shieldex 235/34 and a custom made stitches using two, four, and five-ply silver plated nylon yarns (Shieldex 177/17, Shieldex 235/34 and thread). Their results showed that sensors fabricated with the two-ply thread were less repeatable and a custom made thread). Their results showed that sensors fabricated with the two-ply thread were less repeatable and showed significantly more noise that the other sensors. The working range of the

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top-thread cover stich and overlock sensors are limited to 5–10% and 20–25%, respectively. The showed significantly more noise that the other sensors. The working range of the top-thread cover bottom thread cover stitch sensor, on the other hand, showed decreasing resistance upon stretching stich and overlock sensors are limited to 5–10% and 20–25%, respectively. The bottom thread cover and had a working range to above 40% strain. They tested their sensors on a leotard and two types stitch sensor, on the other hand, showed decreasing resistance upon stretching and had a working of shirts for a spinal posture sensing application. The sensors were stitched to the dorsal side along range to above 40% strain. They tested their sensors on a leotard and two types of shirts for a spinal the spine and insulated with 3M7012 stitchless bonding film. Those tests showed that the T-shirts posture sensing application. The sensors were stitched to the dorsal side along the spine and insulated were less suitable for posture sensing since during multiple bending they tended to ride up on de with 3M7012 stitchless bonding film. Those tests showed that the T-shirts were less suitable for posture body and do not slide back. sensing since during multiple bending they tended to ride up on de body and do not slide back. The response of stitched sensors depends on the stitch structure and the anisotropy of the base The response of stitched sensors depends on the stitch structure and the anisotropy of the base or or substrate material whereas for the suppression of hysteresis it is important how the (elastic) substrate material whereas for the suppression of hysteresis it is important how the (elastic) response response of the substrate or surrounding yarns are. In [37] the effect of the substrate materials on of the substrate or surrounding yarns are. In [37] the effect of the substrate materials on strain gauges strain gauges embroidered at different stitch angles was investigated. The authors used a two-way embroidered at different stitch angles was investigated. The authors used a two-way and four-way and four-way stretch weft knit as well as a double knit structure and Shieldex 235/34 four-ply silver stretch weft knit as well as a double knit structure and Shieldex 235/34 four-ply silver plated nylon plated nylon as the conductive yarn. The stitches considered were the ISO401 chain stitch and the as the conductive yarn. The stitches considered were the ISO401 chain stitch and the ISO406 bottom ISO406 bottom cover stitch. They showed that the chain stitch had a considerable effect on local fabric cover stitch. They showed that the chain stitch had a considerable effect on local fabric stiffness stiffness (stiffness increase of 12% at 0° orientation and 200% at 60°). The sensors were linear up to at (stiffness increase of 12% at 0 orientation and 200% at 60 ). The sensors were linear up to at least 30%. least 30%. The highest gauge◦ factor was obtained for the◦ two-way knitted fabric with chain stitch (GF The highest gauge factor was obtained for the two-way knitted fabric with chain stitch (GF = 2.24) = −2.24) and the lowest was for the two-way knit fabric with coverstitch (−1.01). Remarkably the −gauge and the lowest was for the two-way knit fabric with coverstitch ( 1.01). Remarkably the gauge factors factors did not depend much on the sensor orientation for all− combinations tested. If on the other did not depend much on the sensor orientation for all combinations tested. If on the other hand the hand the force direction was varied the effects were larger. The transverse sensitivities of the two- force direction was varied the effects were larger. The transverse sensitivities of the two-way knit fabric way knit fabric with coverstitch sensor were unacceptable high (65% up to 98%), the two-way chain with coverstitch sensor were unacceptable high (65% up to 98%), the two-way chain stitch and the stitch and the four-way chain stitch combinations showed much lower values (4 to 12%). The chain four-way chain stitch combinations showed much lower values (4 to 12%). The chain stitch is thus the stitch is thus the preferred stitch geometry. A typical example is shown in Figure 19. preferred stitch geometry. A typical example is shown in Figure 19.

(a) (b)

FigureFigure 19.19. ((aa)) CoverstitchCoverstitch structurestructure andand ((bb)) resistanceresistance versusversus displacementdisplacement curvecurve [[37].37].

Ruppert-StroescuRuppert-Stroescu andand coworkerscoworkers [[73]73] investigatedinvestigated thethe sensorsensor propertiesproperties ofof threethree conductiveconductive yarnyarn typestypes ShieldexShieldex 117 117/7/7 (a (a silver silver coated coated two-ply two-ply nylon, nylon, 985 Ω985/m), Ω Shieldex/m), Shieldex 234/34 234/34 (a four-ply (a four-ply silver coated silver yarn,coated 80 yarn,Ω/m) 80 and Ω/m) Lame and LifesaverLame Lifesaver yarn (a yarn three-ply (a three-ply silver silver coated coated yarn yarn with with 65 Ω 65/m Ω resistance)/m resistance) on plainon plain woven woven cotton cotton fabric fabric (muslin). (muslin). Furthermore, Furthermore, they they considered considered ﬁve five stitch stitch types types (310,(310, 304,304, 315,315, andand 401,401, andand adaptedadapted 304)304) withwith ﬁxedfixed stitchstitch densities.densities. ItIt turnedturned outout thatthat forfor somesome ofof thethe stitchstitch/wire/wire combinationcombination the resistance resistance after after stitching stitching was was considerab considerablyly less less than than that that of the of thebare bare wire wire (all per (all unit per unitwidth) width) whereas whereas for formost most other other combinations combinations this this was was not not the the case. case. Elongation Elongation tests tests were onlyonly performedperformed withwith thethe barebare threadsthreads soso nono gaugegauge factors factors of of the the stitch stitch types types could could be be determined. determined. VoglVogl [[44]44] stitchedstitched conductiveconductive silversilver platedplated yarnsyarns onon aa smallsmall stretchablestretchable bandband andand thusthus createdcreated stretchstretch bandband sensorssensors whichwhich cancan bebe usedused asas futurefuture useruser interfaces.interfaces. ToTo constructconstruct this,this, theythey usedused silversilver platedplated nylonnylon (Inventables,(Inventables, tripletriple twine,twine, 3030 ΩΩ//cm),cm), aa highlyhighly stretchablestretchable 70%70% polyesterpolyester/30%/30% elastodieneelastodiene fabricfabric andand a a zig-zag zig-zag stitch stitch with with 3.3 3.3 mm mm width. width. The The response response during during stretching stretching however however appeared appeared to be to quitebe quite non-linear. non-linear. Another stitched sensor study was performed by Greenspan [74] with the aim to apply it for unobtrusive human movement monitoring. They selected a silver plated 22/1 dtex nylon yarn, since preliminary experiments showed that it had the highest strain sensitivity. They further considered

Sensors 2020, 20, 7236 19 of 28

Sensors 2020, 20, x FOR PEER REVIEW 19 of 30 Another stitched sensor study was performed by Greenspan [74] with the aim to apply it for unobtrusivethree fabric substrates human movement (nylon/10% monitoring. spandex, polyester/10% They selected spandex a silver and plated cotton/5% 22/1 dtex spandex). nylon yarn, The sincestitch preliminarygeometries experimentsstudied were showed 304, that402, it406, had an thed highest 514. The strain 514 sensitivity. stitch (overedge They further stitch) considered showed threenegligible fabric response substrates to (nylonstretching/10% ( spandex,GF close polyesterto 0) and /was10% not spandex further and analysed. cotton/5% Note spandex). that such The stiches stitch geometrieshowever can studied be useful were as 304, interconnects 402, 406, and in which 514. The resistance 514 stitch changes (overedge are often stitch) undesired. showed negligible The best responsesensors candidates to stretching were (GF theclose Zig- tozag 0) and(304) was and not the further chainstitch analysed. (402) Noteon a thatcotton/spandex such stiches substrate. however canThe beZig-zag useful on as polyester/spandex interconnects in which combination resistance ho changeswever showed are often better undesired. repeatability The best after sensors cyclic candidatestesting. The were gauge the factor Zig-zag as derived (304) and from the chainstitchtheir graphs (402) was on near a cotton 1.0. /spandex substrate. The Zig-zag on polyesterAs mentioned/spandex above, combination the response however depends showed on better the substrate repeatability fabric, after the cyclic yarn, testing. as well The as on gauge the factorstitch asstructure. derived If from the theirsubstrate graphs shows was nearhysteresis 1.0. after stretching, the fabricated sensor will also. TangsirinaruenartAs mentioned [42] above, ther theefore response suggested depends to first on select the substrate the substrate fabric, fabric the yarn,and then as well consider as on the stitchyarn and structure. stitch geometry. If the substrate They showsevaluated hysteresis six fabrics, after two stretching, conductive the fabricatedthreads (four-ply sensor willShieldex also. Tangsirinaruenart234/34 and two-ply [42 ]117/17) therefore and suggested four stitch to ﬁrst type selects (numbers the substrate 304, 406, fabric 506, and and then 605). consider Preliminary the yarn andexperiments stitch geometry. showed that They the evaluated fabric with six the fabrics, highes twot elastic conductive recovery threads (93%) (four-plywas a single Shieldex jersey234 nylon/34 and(two-ply, two-ply 4.44 117 tex)/17) with and 25% four spandex stitch types (7.78 (numbers tex) combination. 304, 406, 506,Using and that 605). fabric Preliminary they showed experiments that the showed304 stitch that (the the Zig-zag fabric lock with stitch) the highest in combination elastic recovery with the (93%) two-ply was wire a single gave the jersey best nylon results: (two-ply, gauge 4.44factor tex) 1.61, with working 25% spandex range 50%, (7.78 good tex) combination.linearity, low Usinghysteresis, that fabricand good they repeatability showed that (Figure the 304 20a,b). stitch (theA more Zig-zag typical lock response stitch) in cure combination is shown with in Figu the two-plyre 20d showing wire gave a resistan the bestce results: maximum gauge and factor limited 1.61, working rangerange. 50%,Note goodthat their linearity, experience low hysteresis, with the andtwo-ply good conductive repeatability yarn (Figure is different 20a,b). from A more that typicalreported response by Gioberto cure isand shown Dunne in Figure[72] who 20d conclude showingd athat resistance these yarns maximum resulted and in limited noisy workingand less range.repeatable Note sensors. that their experience with the two-ply conductive yarn is diﬀerent from that reported by Gioberto and Dunne [72] who concluded that these yarns resulted in noisy and less repeatable sensors.

(a) (b)

Figure 20.20.( a)(a Zigzag) Zigzag stitch stitch and (band) corresponding (b) corresponding resistance resistance plot; (c) Coverstitch plot; (c) andCoverstitch (d) corresponding and (d) resistancecorresponding plot [resistance42]. plot [42].

5. Discussion In the above sections we have discussed all studiesstudies that involve the development and testing of knitted andand stitchedstitched textile textile sensors. sensors. In In the the following following two two sections sections we summarizewe summarize the performancethe performance data obtaineddata obtained from from those those studies studies with thewith aim the to aim be ableto be to able facilitate to facilitate the selection the selection of the mostof the suitable most suitable sensor designsensor design for a speciﬁc for a specific project. project. As performance As performanc indicatorse indicators we selected we selected the initial the resistance,initial resistance, the gauge the gauge factor, working range and the strain related hysteresis (as defined by Equation (6)). If the chosen indicators were not calculated by the authors themselves, they were extracted from the

Sensors 2020, 20, 7236 20 of 28 factor, working range and the strain related hysteresis (as defined by Equation (6)). If the chosen indicators were not calculated by the authors themselves, they were extracted from the resistance versus strain graphs in the papers. In this way we hope to provide a clearer and more uniform comparison of the sensor performance. While selecting a sensor design the working range is the first indicator to pay attention to. Sensors needed for respiratory sensing need to detect chest expansions with strain of only 5%. Taking into account that the garment needs an additional 5% pre-strain when fitting around the chest, a working range of 10% seems sufficient for these purposes. The gauge factor however should be high enough such that the 5% strain variation is still measurable. Since for this application only the correct detections of peak-to-peak distances are relevant, the linearity of the response curve, hysteresis, and signal drift are less important. For movement and activity classification purposes the limb and upper torso deformations need to be detectable with sufficient accuracy, resulting in a desired working range of 30 to 35% and a preferably low hysteresis value. Applications like the monitoring of sports kinematics, rehabilitation, and movement tracking for gaming or VR require much higher demands on hysteresis and drift stability. Demands which probably cannot be met with the sensors presented in this review. Similarly, long term monitoring of small strain changes, as, e.g., needed for edema sensing, are currently still challenging since they require low drift and hysteresis levels. Applications in the human–computer interaction field, on the other hand, are much less demanding since in that case the sensing thát a surface is being stretched is already sufficient.

5.1. Knitted Sensors In Section 4.1 we discussed 13 studies in the ﬁrst 10 years of this century and another 17 papers in the decade that followed. As could have been expected, the early papers were more about demonstrating that knitted conductive yarn structures are able to sense strains whereas in later years the focus was more on the improvement of the sensor performance and therefore contained much more details about aspects like initial resistance, gauge factor, working range and hysteresis. Table1 summarizes these performance indicators. In the table we ranked the studies as much as possible in chronological order but kept publications from the same author groups together. Based on Table1 we can make the following observations.

5.1.1. Initial Resistance The initial resistances of knitted sensors are seen to decrease from kOhms in the early works to around 10–50 ohms in later years (Table1). This decrease in resistance is probably directly related to the introduction of yarns with better conductivity in the early 2010s. The question we want to pose here is how beneficial this trend of lower resistances is for the performance of the sensors. It can be argued that the gauge performance is likely to decrease if the gauge resistance drops below a level of about 10 ohm because of the increased influence of contact resistances. If we assume that the lead wires which connect the strain gauge to the data processing and power units have a resistance of a few ohms (depending on the length and conductivity), the resistance of the gauge should be much larger (e.g., a factor 10) to avoid that resistance changes in these lead wires and contact points during body movements contribute too much to the signal. This agrees with the observation of [67] that sensors with lower resistances showed a noisier behaviour. Moreover, low resistance gauges consume more power. Measurements are usually done by supplying a constant voltage and measuring the changes in current. For a sensor with 10 ohm resistance and 5 Volt applied, the power consumption amounts to P = V2/R = 2.5 W (and I = V/R = 500 mA), which is quite high. A sensor with a resistance of, e.g., 1000 Ω, is much less sensitive to lead wire effects and consumes 100 times less energy. From this we conclude that in order to suppress noise from fluctuating contact resistances and in view of energy efficiency future strain gauges should aim at initial resistances in the range of 100–1000 Ohm rather than the current practice of 10–50 Ohm. Sensors 2020, 20, 7236 21 of 28

Table 1. Performance comparison of knitted sensors. Sensors with working range above 30% and low hysteresis are highlighted.

Working Reference Knit Type Conductive Yarn 1 Non-Conductive Yarn R (Ω) GF (Range) 2 Hysteresis H 4 Remarks Application 0 Range (%) 3 ε Farringdon 1999 [43] - - - 50.000 17 40% Sensor strips Activity sensing Bickerton 2003 [45] plain Carbon ﬁbre Lycra 90.000 14 40% Body kinematics Wije 2003 [35] - Carbon rubber elastic 400.000 2.4 10% Body kinematics 1-course loopwise Atalay 2013 [38] interlock 235 dtex Sil-Ny, 200 Ω/m 800 dtex elastic 166 3.75 10–40% 0.10 Body kinematics embedded 1-course loopwise Atalay 2014 [59] interlock Sil-Ny, 200 Ω/m 800 dtex elastic - 1.86 6 to >40% 0.07 embedded Atalay 2017 [61] plain Sil-Ny, 200 Ω/m 800 dtex elastic 10.4 1.05 80% noisy plain Sil-Ny, 200 Ω/m 167 dtex polyester 5.3 0.38 20% noisy − interlock Sil-Ny, 200 Ω/m 800 dtex elastic 2.3 0.97 40% noisy interlock Sil-Ny, 200 Ω/m 167 dtex polyester 4.6 0.70 55% noisy − 1-course loopwise Respiration Atalay 2015 [60] interlock Sil-Ny, 200 Ω/m 800 dtex elastic 261 3.44 (8%) >40% 0.08 embedded monitoring Zhang 2005 [54] plain Bare SS - 12 8.6 (10%) 22% − Zhang 2006 [53] Single warp carbon - 15.000 13 (2%) 8% 0.08 − Yang 2009 [55] 1 1 rib Bare SS - 11.5 1.1 (40%) 50% × − Resistance drops to Ehrmann 2014 [62] full cardigan PES/SS, 525 k Ω/m cotton - 6.2 (10%) 20% − zero Respiration Double face PES/SS, 525 k Ω/m - 2.2 (20%) 55% Large relaxation − monitoring Rehabilitation, Pacelli 2013 [50] intarsia Cf-Ny (Belltron) Lycra 44.300 0.75 (16%) >16% 0.14 Body kinematics Alternating Oks 2014 [64] plain Sil-Ny Elastane, PES, cotton 12 >10 (5%) 5–10% 0.10 Sensing glove, sock conductive rows Rehabilitation, Tognetti 2014 [75] intarsia Cf-Ny (Belltron) Lycra 90.000 6.7 >10% 0.14 Single layer strip Body kinematics Human Computer Ou 2019 [31] interlock Sil-Ny Spandex, PES 60 2.3 L estimated 0 Interaction Custom blended Xie 2016 [65] plain Cotton/SS, 500 Ω/m cotton 12 3.7 (10%) 40% Body kinematics − yarn plain Sil-Ny, 500 Ω/m 2.8 0.36 (>30%) >30% Respiration, Raji 2018 [66] 1 1 rib Sil-Ny, 7700 Ω/m Nylon covered spandex 32.8 1.25 >10% × smart bra 1 2 rib 25.3 1.5 × 1 3 rib 25.9 1.6 × 2 2 rib 31.7 1.4 × 1 1 rib Bare spandex 14.9 2.45 × 1 2 rib 12.3 2.85 × 1 3 rib 11.1 2.3 × 2 2 rib 15.1 2.2 × Respiration, Raji 2019 [76] 1 1 rib Sil-Ny, 7700 Ω/m Nylon covered spandex 47.1 1.31 >10% 12 other sizes tested × smart bra 1 Sil=silver coated; Ny = nylon; SS = stainless steel; PES = polyester; Cf = carbon ﬁlled; Cc = carbon coated; 2 range for which the GF approximately applies; 3 notation ‘>x%’ means tested until x% but possibly larger; 4 hysteresis in length relative to maximum length (mm/mm) or as ∆ε/εmax (%/%), see Equation (6). Sensors 2020, 20, 7236 22 of 28

5.1.2. Conductive Yarns The best results seem to be obtained with silver plated nylon yarns (i.e., Shieldex or Lamé Lifesaver yarns). Most often used is the Shieldex 234/34 four-ply yarn from Statex. Bare metal fiber yarns and metal fiber yarns blended with non-conductive cotton or polyester (Bekinox) have a negative gauge factor. The blended yarn combinations however are much less suitable for application as sensor since the irregular arrangement of the non-conductive thread and stainless steel filaments result in noise and less reproducible results. Greenspan [74] argues that the yarns should be of 140–700 dtex and have linear conductivities above 1 Ω/m.

5.1.3. Performance Indicators The gauge factors reported in literature range from 0.3 to 17 and can be both positive and negative. The value of the gauge factor itself is in fact not that important as long as it allows to collect data with low enough noise levels, which is typically true for gauge factors above one. Note that the development of sensors with extremely high gauge factors is useless for on-body applications as we are discussing here, since we do not need to be able to detect ultra-low strain levels, as opposed to typical mechanical engineering applications. In Table1 we highlighted those strain sensors which had a working range of at least 30% and had hysteresis values below 0.10. The three best performing sensors are all variations to the method developed by Atalay et al. [38] which utilizes a single course, loop-wise embedded conductor yarn, as discussed in Section 4.1. The reported working ranges vary from 8 up to 80%. Typical values are around 30 to 40% which is just enough to detect body movements like elbow and knee deﬂection. Hysteresis values were more diﬃcult to obtain but all values were in the range from 0.07 to 0.14.

5.2. Stitched Sensors The work on stitched textile strain sensors started much more recently with the work of Gioberto and Dunne [70]. In all studies silver-plated nylon yarns are used as conductors with the Shieldex 117/17 and 234/34 types as the preferred candidates. For stitched sensors the substrate material is an additional parameter with effect on the sensor performance. It is important to realize that for optimum sensor performance care should be taken in selecting a suitable substrate fabric with low mechanical hysteresis. The best performing sensors have jersey knitted substrates containing 10–25% elastomeric (Spandex) yarn. In case the sensors have to be integrated in a different, less elastic, fabric it may be useful to integrate patches of a more elastic substrate with stitched-on sensor in the garment. An important conclusion from the work of Dupler [37] is that the sensitivity did not vary much with the sensor orientation and is always largest in sensor direction. This in fact simplifies the positioning of sensor elements on the textile. The 406 bottom cover stitch and the 304 Zig-zag stitch seem to give the best results, as shown in Table2. The gauge factors are typically in the range of 1 to 1.6 and can be either positive or negative, depending on whether the resistance changes due to the gradual breaking of loop–loop contacts during stretching (positive gauge factor) or due to the increased squeezing contact between adjacent conductive lines (negative gauge factor). For the functioning of the sensor both variants can be expected to work well. Compared to the previous evaluation of knitted sensors we can see that the working range for stitched sensors is usually higher (often exceeding the 50%), whereas hysteresis levels are similar. The best performing sensor can be considered as the bottom coverstich sensor of Dupler [37] which has an extremely low hysteresis value of 0.02 and thus outperform the knitted sensors in this respect. Sensors 2020, 20, 7236 23 of 28

Table 2. Performance comparison of stitched sensors. Sensors with working range above 30% and low hysteresis are highlighted.

Working Author Stitch Type Conductive Yarn 1 Substrate Fabric R (Ω) GF (Range) 2 Hysteresis H 4 Remarks Application 0 Range (%) 3 ε Sil-Ny, Lamé, Non-linear, Gioberto 2012 [70] 602, top coverstitch - 120 0.54 (12%) 10% - 81 Ω/m low hysteresis

Gioberto 2013 [71] 514, Overlock Sil-Ny, 50 Ω/m 100% PES jersey 43 0.94 (10%) 19 - HR = 0.19

60 cotton/40 PES 55 0.48 (10%) 21 - HR = 0.35

90 PES/10 spandex 58 2.22 (10%) 17 - HR = 3.04

94 cotton/6 spandex 43 1.41 (10%) 22 0.17 HR = 1.15

82 Ny/18 spandex 43 1.43 (10%) 29 - HR = 0.82

602, bottom 2-ply Sil-Ny Gioberto 2016 [72] Jersey knit elastomeric blend 2370 1.22 (50%) >50% - Noisy signal coverstitch Shieldex −

4-ply Sil-Ny Jersey knit elastomeric blend 61 0.89 (50%) >50% - Good signal Spinal posture sensing Shieldex −

406, Bottom Dupler 2019 [37] Sil-Ny Shieldex 4-way PES/Spandex 10.9 1.15 (30%) >30% 0.02 H = 0.23 coverstitch − R

401, Chain stitch 7.7 1.76 (30%) >30% - H = 0.34 − R 406, Bottom 2-way PES 10.9 1.13 (30%) >30% - H = 0.08 coverstitch − R

401, Chain stitch 7.7 2.20 (30%) >30% - H = 0.34 − R Sil-Ny/PET, 30 Greenspan 2018 [74] 304, Zigzag 90% PES/10% elastane 7000 1.0 (5–28%) >30% 0.10 Body kinematics kΩ/m

Tangsiri 2019 [42] 304, Zigzag 2-ply Sil-Ny 117/17 75% Ny/25% Spandex 125 1.61 (50%) 50% 0.07 HR = 0.09

406, chain stitch 2-ply Sil-Ny 117/17 71.5 3.71 (25%) 25% 0.20 HR = 2.51

4-ply Sil-Ny 234/34 55.6 2.71 (8%) 8% 0.55 HR = 0.71

506, overlock 2-ply Sil-Ny 117/17 649 0.099 (16%) 16% 0.36 HR = 0.05

4-ply Sil-Ny 234/34 46.7 5.16 (12%) 12% 0.51 HR = 0.54

605, coverstitch 2-ply Sil-Ny 117/17 240 0.21 (18%) 18% 0.04 HR = 0.59

4-ply Sil-Ny 234/34 39.3 1.65 (18%) 18% 0.14 HR = 0.09 1 Sil = silver coated; Ny = nylon; SS = stainless steel; PES = polyester; Cf = carbon ﬁlled; Cc = carbon coated; 2 range for which the GF approximately applies; 3 notation ‘>x%’ means tested until x% but possibly larger; 4 hysteresis in length relative to maximum length (mm/mm) or as ∆ε/εmax (%/%), see Equation (6). Sensors 2020, 20, 7236 24 of 28

Note that there is an inconsistency in the naming of the stitch geometries in literature. The 406 type is according to the ASTM-D6193 classiﬁcation the two-needle bottom cover stitch but is referred to by [42] as the two-needle rear side chain stitch. In their recommendations [42] suggest to increase the stitch density in order to improve the performance of the strain sensors. This recommendation is based on the fact that the initial gauge factor of stitched sensors is mainly determined by the gradual opening of loop-to-loop contacts [70], a process which can be expected to proceed more uniformly for larger stitch densities.

6. Conclusions

6.1. Sensor Selection (“Do’s and Don’ts”) From the above study we can conclude that

Knitted strain gauges are best produced with silver plated nylon yarn as the conductor thread. • Blended yarn combinations like stainless steel/polyester should be avoided since this results in less reproducible sensor performance. The loop-wise embedding configuration of the conductive yarn as used by Atalay [38] results in • the best overall sensor performance. Knitted gauges of non-rectangular shapes suffer from sensor noise and poor reproducibility due • to the need to truncate conductive yarns outside the sensor area [67]. In order to reduce hysteresis and relaxation effects in stitched strain sensors it is important to use • a highly elastic substrate fabric with minimum mechanical hysteresis. The bottom coverstich sensor of Dupler [37] has hardly any hysteresis, and is thus the preferred • configuration. As alternative the zig-zag stitch also performs well. The sensitivity of stitched sensors can probably be improved by increasing the stitch density. • Researchers or manufacturers aiming to fabricate garments with embedded strain sensors using currently available materials and technology, have the choice between knitting and stitching of conductive yarns in their fabrics. Both of them do not compromise comfortability and washability and can result in robust and easy to manufacture sensors. Stitched sensors are simple to apply and are most likely the preferable choice for prototyping sensor garments. As mentioned above, when constructing stitched sensors, care should be taken to select a substrate with good elastic properties. The bottom coverstitch and the zig-zag stitch are the best candidates for sensors with low hysteresis values. Note that for zig-zag stitches to perform well it is required to have a high enough stitch density such that consecutive loops can contact in the unloaded state. Knitted sensors can be completely embedded in the fabric structure and can be fabricated in a single process step. They are therefore the preferred option when the sensor garments have to be mass produced. The best options here are to use silver plated nylon yarns, in combination with the loop-wise embedding configuration proposed in [38].

6.2. Topics for Future Research In most of the studies discussed above textile sensors were created by constructing rectangular strips of which the resistance changes uniformly upon stretching. There were however two alternative ideas proposed with diﬀerent sensing mechanisms. Oks and co-workers [64] for example made structures consisting of separated conductive lines (courses) which touch when unloaded but separate when loaded. This resulted in an a-typical sensor curve in which the resistance rapidly rises above a threshold value which depends on the knitting structure. Such constructions can serve as a starting point for designing knitted structures in which the resistance increase, e.g., occurs in a stepwise way by introducing conductive bridges of diﬀerent size in the structure. This type of parallel line structures can also be helpful for developing direction sensitive sensors. Sensors 2020, 20, 7236 25 of 28

Another interesting idea for strain sensing in textiles is the inductance idea of Wijesiriwardana [46]. They propose to knit a structure of parallel lines or coils and use the change in inductance as a measure of deformation. This technology could be interesting since it has the option of adjusting the sensing frequency to suppress noise (see also the study of Dhawan [36]). The time-dependent sensor responses like the dependency on the loading velocity, the signal relaxation, drift, and hysteresis largely complicate the data analysis process. One should therefore first try to reduce these effects as much as possible by selecting an appropriate substrate and sensor structure. If one then still wants to correct for these effects it may be useful to use the modelling and characterization techniques developed to describe polymer viscoelasticity since this provides a consistent framework for describing time dependent effects with a limited number of parameters. In that way it should be possible to have a simple unified model which both predicts relaxation, hysteresis and drift with a single set of parameters. More information can be found in standard viscoelasticity textbooks. An idea for further study is to repeat the aspect ratio of knitted sensors for different stitch types and observe if also a maximum in the gauge factor is obtained as reported by Raji [67]. In addition, our hypothesis that gauges with low initial resistances are more susceptible for contact point related noise can be verified with a study in which the yarn conductivity is systematically varied. Finally, the determination of gauge factors at temperatures different from room temperature can be used to find out if the gauge factor is affected by heating or not. Such studies are expected to contribute to a more fundamental understanding of the working principles of knitted sensors than we presently have. Future research should focus on reliability and robustness of the sensors. In current studies the reliability is often studied by cyclic loading in laboratory test setups. This of course gives a good initial indication of the sensor stability but not about the sensor behavior during actual wearing conditions. In such situations sweating, mechanical friction and washing may lead to unexpected sensor degradation. In addition, extra research is needed regarding the interconnection of the sensors with data processing and power units since these should be unobtrusive, robust and washable as well. In particular, it can be expected that the connection points between flexible interconnect wiring and rigid power or data processing units easily fail during extensive bending such as can be expected to occur during machine washing. The sensor types we have discussed here are good candidates to be used in future smart garments in which the sensors are combined with local energy harvesting elements and are part of a body area network, which processes the data and transmits it wirelessly to the cloud.

Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Cherenack, K.; van Pieterson, L. Smart textiles: Challenges and opportunities. J. Appl. Phys. 2012, 112.[CrossRef] 2. Castano, L.M.; Flatau, A.B. Smart fabric sensors and e-textile technologies: A review. Smart Mater. Struct. 2014, 23.[CrossRef] 3. Gibbs, P.T.; Asada, H. Wearable Conductive Fiber Sensors for Multi-Axis Human Joint Angle Measurements. J. Neuroeng. Rehabil. 2005, 2, 7. [CrossRef][PubMed] 4. McLaren, R.; Joseph, F.; Baguley, C.; Taylor, D. A review of e-textiles in neurological rehabilitation: How close are we? J. Neuroeng. Rehabil. 2016, 13, 59. [CrossRef] 5. Post, E.R.; Orth, M.; Russo, P.R.; Gershenfeld, N. E-broidery: Design and fabrication of textile-based computing. IBM Syst. J. 2000, 39, 840–860. [CrossRef] 6. Poupyrev, I.; Gong, N.-W.; Fukuhara, S.; Karagozler, M.E.; Schwesig, C.; Robinson, K.E. Project Jacquard. In Proceedings of the 2016 CHI Conference on Human Factors in Computing Systems, San Jose, CA, USA, 7–12 May 2016; pp. 4216–4227. 7. Roh, J.-S. Conductive Yarn Embroidered Circuits for System on Textiles. Wearable Technol. 2018.[CrossRef] Sensors 2020, 20, 7236 26 of 28

8. Stoppa, M.; Chiolerio, A. Wearable electronics and smart textiles: A critical review. Sensors 2014, 14, 11957–11992. [CrossRef] 9. Patel, S.; Park, H.; Bonato, P.; Chan, L.; Rodgers, M. A review of wearable sensors and systems with application in rehabilitation. J. Neuroeng. Rehabil. 2012, 9, 21. [CrossRef] 10. Rodgers, M.M.; Pai, V.M.; Conroy, R.S. Recent Advances in Wearable Sensors for Health Monitoring. IEEE Sens. J. 2015, 15, 3119–3126. [CrossRef] 11. Nag, A.; Mukhopadhyay, S.C.; Kosel, J. Wearable Flexible Sensors: A Review. IEEE Sens. J. 2017, 17, 3949–3960. [CrossRef] 12. Bosowski, P.; Hoerr, M.; Me Fika, V.; Gries, T.; Jockenhoevel, S. Design and Manufacture of Textile-Based Sensors. In Electronic Textiles; Woodhead Publishing: Cambridge, UK, 2015. 13. Wilson, S.; Laing, R. Fabrics and Garments as Sensors: A Research Update. Sensors 2019, 19, 3570. [CrossRef] 14. Seyedin, S.; Zhang, P.; Naebe, M.; Qin, S.; Chen, J.; Wang, X.; Razal, J.M. Textile strain sensors: A review of the fabrication technologies, performance evaluation and applications. Mater. Horiz. 2019, 6, 219–249. [CrossRef] 15. Lee, J.; Llerena Zambrano, B.; Woo, J.; Yoon, K.; Lee, T. Recent Advances in 1D Stretchable Electrodes and Devices for Textile and Wearable Electronics: Materials, Fabrications, and Applications. Adv. Mater. 2020, 32, 1902532. [CrossRef][PubMed] 16. Wang, J.; Lu, C.; Zhang, K. Textile-Based Strain Sensor for Human Motion Detection. Energy Environ. Mater. 2020, 3, 80–100. [CrossRef] 17. Zhang, R.; Deng, H.; Valenca, R.; Jin, J.H.; Fu, Q.; Bilotti, E.; Peijs, T. Carbon nanotube polymer coatings for textile yarns with good strain sensing capability. Sens. Actuator A Phys. 2012, 179, 83–91. [CrossRef] 18. Xue, P.; Wang, J.; Tao, X. Flexible textile strain sensors from polypyrrole-coated XLA™ elastic fibers. High Perform. Polym. 2014, 26, 364–370. [CrossRef] 19. Ding, Y.; Invernale, M.A.; Sotzing, G.A. Conductivity Trends of PEDOT-PSS Impregnated Fabric and the Effect of Conductivity on Electrochromic Textile. ACS Appl. Mater. Interfaces 2010, 2, 1588–1593. [CrossRef] [PubMed] 20. Foroughi, J.; Spinks, G.M.; Aziz, S.; Mirabedini, A.; Jeiranikhameneh, A.; Wallace, G.G.; Kozlov, M.E.; Baughman, R.H. Knitted Carbon-Nanotube-Sheath/Spandex-Core Elastomeric Yarns for Artificial Muscles and Strain Sensing. ACS Nano 2016, 10, 9129–9135. [CrossRef][PubMed] 21. Wang, H.; Liu, Z.; Ding, J.; Lepró, X.; Fang, S.; Jiang, N.; Yuan, N.; Wang, R.; Yin, Q.; Lv, W.; et al. Downsized Sheath–Core Conducting Fibers for Weavable Superelastic Wires, Biosensors, Supercapacitors, and Strain Sensors. Adv. Mater. 2016, 28, 4998–5007. [CrossRef][PubMed] 22. Li, Y.; Li, Y.; Su, M.; Li, W.; Li, Y.; Li, H.; Qian, X.; Zhang, X.; Li, F.; Song, Y. Electronic Textile by Dyeing Method for Multiresolution Physical Kineses Monitoring. Adv. Electron. Mater. 2017, 3, 1700253. [CrossRef] 23. Behabtu, N.; Young, C.C.; Tsentalovich, D.E.; Kleinerman, O.; Wang, X.; Ma, A.W.K.; Bengio, E.A.; ter Waarbeek, R.F.; de Jong, J.J.; Hoogerwerf, R.E.; et al. Strong, Light, Multifunctional Fibers of Carbon Nanotubes with Ultrahigh Conductivity. Science 2013, 339, 182. [CrossRef] 24. Seyedin, S.; Moradi, S.; Singh, C.; Razal, J.M. Continuous production of stretchable conductive multifilaments in kilometer scale enables facile knitting of wearable strain sensing textiles. Appl. Mater. Today 2018, 11, 255–263. [CrossRef] 25. Wang, J.; Xue, P.; Tao, X.; Yu, T. Strain Sensing Behavior and Its Mechanisms of Electrically Conductive PPy-Coated Fabric. Adv. Eng. Mater. 2014, 16, 565–570. [CrossRef] 26. Oh, K.W.; Park, H.J.; Kim, S.H. Stretchable conductive fabric for electrotherapy. J. Appl. Polym. Sci. 2003, 88, 1225–1229. [CrossRef] 27. Tadesse, M.G.; Loghin, C.; Chen, Y.; Wang, L.; Catalin, D.; Nierstrasz, V. Effect of liquid immersion of PEDOT: PSS-coated polyester fabric on surface resistance and wettability. Smart Mater. Struct. 2017, 26, 065016. [CrossRef] 28. Dias, T.; Beatty, P.; Cooke, W.; Wijesiriwardana, R.; Mukhopadhyay, S.; Hurley, W. Knitted Transducer Devices. Europe Patent EP1624800, 15 January 2009. 29. Spencer, J.D. Knitting Techonology; Woodhead Publishing Limited: Cambridge, UK, 2001. 30. Au, K.F. Advances in Knitting Technology; Woodhead Publishing Limited: Cambridge, UK, 2011. 31. Ou, J.F.; Oran, D.; Haddad, D.D.; Paradiso, J.; Ishii, H. SensorKnit: Architecting Textile Sensors with Machine Knitting. 3D Print. Addit. Manuf. 2019, 6, 1–11. [CrossRef] 32. Guignier, C.; Camillieri, B.; Schmid, M.; Rossi, R.; Bueno, M.A. E-Knitted Textile with Polymer Optical Fibers for Friction and Pressure Monitoring in Socks. Sensors 2019, 19, 3011. [CrossRef][PubMed] Sensors 2020, 20, 7236 27 of 28

33. Nayak, R.P. Rajiv Garment Manufacturing Technology; Woodhead Publishing Limited: Cambridge, UK, 2015. [CrossRef] 34. Mecnika, V.; Hoerr, M.; Krievins, I.; Jockenhoevel, S.; Gries, T. Technical Embroidery for Smart Textiles: Review. Mater. Sci. Text. Cloth. Technol. 2015, 9.[CrossRef] 35. Wijesiriwardana, R.; Dias, T.; Mukhopadhyay, S. Resistive ﬁbre-meshed transducers. In Proceedings of the Seventh IEEE International Symposium on Wearable Computers, White Plains, NY, USA, 21–23 October 2003; pp. 200–209. 36. Dhawan, A.; Ghosh, T.K.; Seyam, A.M.; Muth, J.F. Woven Fabric-Based Electrical Circuits: Part II: Yarn and Fabric Structures to Reduce Crosstalk Noise in Woven Fabric-Based Circuits. Text. Res. J. 2004, 74, 955–960. [CrossRef] 37. Dupler, E.; Dunne, L.E. Eﬀects of the textile-sensor interface on stitched strain sensor performance.

In Proceedings of the 23rd International Symposium on Wearable Computers—ISWC019, London, UK, 9–13 September 2019; pp. 45–53. 38. Atalay, O.; Kennon, W.R.; Husain, M.D. Textile-based weft knitted strain sensors: Effect of fabric parameters on sensor properties. Sensors 2013, 13, 11114–11127. [CrossRef] 39. Grassi, A.; Cecchi, F.; Maselli, M.; Roling, M.; Laschi, C.; Cianchetti, M. Warp-Knitted Textile as a Strain Sensor: Characterization Procedure and Application in a Comfortable Wearable Goniometer. IEEE Sens. J. 2017, 17, 5927–5936. [CrossRef] 40. Oliveri, A.; Maselli, M.; Lodi, M.; Storace, M.; Cianchetti, M. Model-Based Compensation of Rate-Dependent Hysteresis in a Piezoresistive Strain Sensor. IEEE Trans. Ind. Electron. 2019, 66, 8205–8213. [CrossRef] 41. Liu, R.; Shao, Q.; Wang, S.; Ru, C.; Balkcom, D.; Zhou, X. Reconstructing Human Joint Motion with Computational Fabrics. Proc. ACM Interact. Mob. Wearable Ubiquitous Technol. 2019, 3, 1–26. [CrossRef] 42. Tangsirinaruenart, O.; Stylios, G. A Novel Textile Stitch-Based Strain Sensor for Wearable End Users. Materials 2019, 12, 1469. [CrossRef][PubMed] 43. Farringdon, J. Wearable sensor badge & jacket for context awareness. In Proceedings of the Third International Symposium on Wearable Computers, San Francisco, CA, USA, 18–19 October 1999. 44. Vogl, A.; Parzer, P.; Babic, T.; Leong, J.; Olwal, A.; Haller, M. StretchEBand. In Proceedings of the 2017 CHI Conference on Human Factors in Computing Systems, Denver, CO, USA, 6–11 May 2017; pp. 2617–2627. 45. Bickerton, M. Effects of fibre interactions on conductivity, within a knitted fabric stretch sensor. In Proceedings of the 2003 IEE Eurowearable, Birmingham, UK, 4–5 September 2003; pp. 67–72. 46. Wijesiriwardana, R. Inductive fiber-meshed strain and displacement transducers for respiratory measuring systems and motion capturing systems. IEEE Sens. J. 2006, 6, 571–579. [CrossRef] 47. Lorussi, F.; Carbonaro, N.; De Rossi, D.; Tognetti, A. Strain- and Angular-Sensing Fabrics for Human Motion Analysis in Daily Life. In Smart Textiles: Fundamentals, Design, and Interaction; Schneegass, S., Amft, O., Eds.; Springer: Cham, Switzerland, 2017; pp. 49–70. [CrossRef] 48. Scilingo, E.P. Strain-sensing fabric for wearable kinaesthetic-like systems. Sens. J. 2003, 3, 460–467. [CrossRef] 49. Tognetti, A.; Lorussi, F.; Bartalesi, R.; Quaglini, S.; Tesconi, M.; Zupone, G.; De Rossi, D. Wearable kinesthetic system for capturing and classifying upper limb gesture in post-stroke rehabilitation. J. Neuroeng. Rehabil. 2005, 2, 8. [CrossRef] 50. Pacelli, M.; Caldani, L.; Paradiso, R.; IEEE. Performances evaluation of piezoresistive fabric sensors as function of yarn structure. In Proceedings of the 35th Annual International Conference of the IEEE Engineering in Medicine and Biology Society, Osaka, Japan, 3–7 July 2013; pp. 6502–6505. 51. Paradiso, R.; Caldani, L.; Pacelli, M. Chapter 3.1—Knitted Electronic Textiles. In Wearable Sensors; Academic Press: New York, NY, USA, 2014. [CrossRef] 52. Paradiso, R.; Loriga, G.; Taccini, N. A wearable health care system based on knitted integrated sensors. IEEE Trans. Inf. Technol. Biomed. 2005, 9, 337–344. [CrossRef] 53. Zhang, H.; Tao, X.; Yu, T.; Wang, S. Conductive knitted fabric as large-strain gauge under high temperature. Sens. Actuators A Phys. 2006, 126, 129–140. [CrossRef] 54. Zhang, H.; Tao, X.; Wang, S.; Yu, T. Electro-Mechanical Properties of Knitted Fabric Made From Conductive Multi-Filament Yarn Under Unidirectional Extension. Text. Res. J. 2005, 75, 598–606. [CrossRef] 55. Yang, K.; Song, G.-L.; Zhang, L.; Li, L.-W. Modelling the Electrical Property of 1x1 Rib Knitted Fabrics Made from Conductive Yarns. In Proceedings of the 2009 Second International Conference on Information and Computing Science, England, UK, 21–22 May 2009; pp. 382–385. Sensors 2020, 20, 7236 28 of 28

56. Li, L.; Au, W.M.; Li, Y.; Wan, K.M.; Chung, W.Y.; Wong, K.S. A Novel Design Method for an Intelligent Clothing Based on Garment Design and Knitting Technology. Text. Res. J. 2009, 79, 1670–1679. [CrossRef] 57. Li, L.; Liu, S.; Ding, F.; Hua, T.; Au, W.M.; Wong, K.-S. Electromechanical analysis of length-related resistance and contact resistance of conductive knitted fabrics. Text. Res. J. 2012, 82, 2062–2070. [CrossRef] 58. Zi˛eba,J.; Frydrysiak, M.; Błaszczyk, J. Textronic clothing with resistance textile sensor to monitoring frequency of human breathing. In Proceedings of the 2012 IEEE International Symposium on Medical Measurements and Applications, Budapest, Hungary, 18–19 May 2012; pp. 1–6. 59. Atalay, O.; Kennon, W.R. Knitted strain sensors: Impact of design parameters on sensing properties. Sensors 2014, 14, 4712–4730. [CrossRef] 60. Atalay, O.; Kennon, W.R.; Demirok, E. Weft-Knitted Strain Sensor for Monitoring Respiratory Rate and Its Electro-Mechanical Modeling. IEEE Sens. J. 2015, 15, 110–122. [CrossRef] 61. Atalay, O.; Tuncay, A.; Husain, M.D.; Kennon, W.R. Comparative study of the weft-knitted strain sensors. J. Ind. Text. 2017, 46, 1212–1240. [CrossRef] 62. Ehrmann, A.; Heimlich, F.; Brücken, A.; Weber, M.O.; Haug, R. Suitability of knitted fabrics as elongation sensors subject to structure, stitch dimension and elongation direction. Text. Res. J. 2014, 84, 2006–2012. [CrossRef] 63. Wang, J.; Long, H.; Soltanian, S.; Servati, P.; Ko, F. Electro-mechanical properties of knitted wearable sensors: Part 2—Parametric study and experimental verification. Text. Res. J. 2013, 84, 200–213. [CrossRef] 64. Oks, A.; Katashev, A.; Litvak, J. Knitted Resistive Fabric: Properties and Applications. Mater. Sci. Text. Cloth. Technol. 2015, 9.[CrossRef] 65. Xie, J.; Long, H.R.; Miao, M.H. High sensitivity knitted fabric strain sensors. Smart Mater. Struct. 2016, 25, 105008. [CrossRef] 66. Raji, R.K.; Miao, X.; Zhang, S.; Li, Y.; Wan, A. Influence of Rib Structure and Elastic Yarn Type Variations on Textile Piezoresistive Strain Sensor Characteristics. Fibres Text. East. Eur. 2018, 26, 24–31. [CrossRef] 67. Raji, R.K.; Miao, X.; Zhang, S.; Li, Y.; Wan, A.; Boakye, A. Knitted piezoresistive strain sensor performance, impact of conductive area and profile design. J. Ind. Text. 2019.[CrossRef] 68. Li, L.; Au, W.; Wan, K.; Wan, S.; Chung, W.; Wong, K. A Resistive Network Model for Conductive Knitting Stitches. Text. Res. J. 2010, 80, 935–947. [CrossRef] 69. Xie, J.; Long, H. Equivalent resistance calculation of knitting sensor under strip biaxial elongation. Sens. Actuators A Phys. 2014, 220, 118–125. [CrossRef] 70. Gioberto, G.; Dunne, L. Theory and Characterization of a Top-Thread Coverstitched Stretch Sensor. In Proceedings of the 2012 IEEE International Conference on Systems, Man, and Cybernetics (SMC), Seoul, Korea, 14–17 October 2012; pp. 3275–3280. 71. Gioberto, G.D.; Dunne, L.E. Overlock-stitched stretch sensors. J. Text. Appar. Technol. Manag. 2013, 8, 1–14. 72. Gioberto, G.; Compton, C.; Dunne, L. Machine-stitched E-textile stretch sensors. Sens. Transducers 2016, 202, 25–37. 73. Ruppert-Stroescu, M.; Balasubramanian, M. Effects of stitch classes on the electrical properties of conductive threads. Text. Res. J. 2017, 88, 2454–2463. [CrossRef] 74. Greenspan, B.; Hall, M.L.; Cao, H.T.; Lobo, M.A. Development and testing of a stitched stretch sensor with the potential to measure human movement. J. Text. Inst. 2018, 109, 1493–1500. [CrossRef] 75. Tognetti, A.; Lorussi, F.; Mura, G.D.; Carbonaro, N.; Pacelli, M.; Paradiso, R.; De Rossi, D. New generation of wearable goniometers for motion capture systems. J. Neuroeng. Rehabil. 2014, 11.[CrossRef] 76. Raji Rafiu, K.; Miao, X.; Zhang, S.; Li, Y.; Wan, A.; Frimpong, C. A comparative study of knitted strain sensors fabricated with conductive composite and coated yarns. Int. J. Cloth. Sci. Technol. 2019, 31, 181–194. [CrossRef]

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