What are smart and electronictextiles?According to the CEN defenitionseverallevels of smart functionscanbedefined. In this paper we consider a textile to be smart whenit has the capability to measure and/or to react. The reactioncanbeanintrinsicmaterialpropertyquch as a colour change, oritcanbeactivelysteeredbyelectronics. The latterinvolvesactive data processing.
A Intelligent textiel can have 5 basic functions, namely:SensorActuatorData processingCommunicationEnergyNot all functions are foreseen in all appications of Intelligent textiel.Thispresentationaddressesexamples of eachfunction.
Beforechosingtextile as a basic tool forembeddingor presenting intelligenceone must consider the addedvcalue of a textile product for the envisagedapplication.A first issue is thattextiles are all around. We wearclothes in severallayers, ourhouses are decoratedwithcarpets, textilewall paper, curtains, upholsteryonfurniture etc.. This offers manypossibilitiesforintegration in a distreteway.Textiles are versatile: they are composed of oneor more polymerfibreswhichcanbecoarseor fine (up to the nanoscale), arranged in 1, 2 or 3 dimensionalstructures.A major advantage is theirlarge contact areawith the body without negativelyaffectingthermal comfort oraethetics.Last butnotleasteveryone is familiarwithtextilessono user guidelines have to begiven. Moreover the active areas are always at the right place.
This picture shows anexample of the scaleeffectsthatcanbeachievedwithtextilestructures. A nanofibre web is depositedon a (polyester) wovenfabric. The picture left shows the nanofibres. The yarnscomposing the wovenfabriccanbeseen at the background. The right picture shows the fabriccoveredby a nanowebwhichnowlookslike a thinmembrane.
The objective of smart textile is to absorb a series of activecomponentsessentially without changingitscharacter of flexible and comfortable product. Sensors, devices, wiresshoulddisappearinto the textile. In a firstphaseconventionalcomponents are beingreshaped in order to fit in the textile, ultimately the fibres and textilestructuresthemselveswillbecome the activeelements.
A smart textilecanbeactive in manyfields. Theycanabsorborgenerate, reflect, shield, measureor affect a series of parameters such as:Temperature,Heat flux,Electrostatic and electromagneticfields,Humidity,Chemicals in liquidorgaseousphase,Radiation,Movements,Forces,Odour,Biologicalactivity,…
Smart textilescanbeusedformeasuring body parameters as well as inforamtionfrom the environment. A relativelyunexplored field is measuringitown status and properties. Self test functionscanbe of vitalimportance in applcaitionslikehealthorprotection.Itheaciton of the textilecanbepassive, activeorwith intelligent control.
The basis of Intelligent textiel are electroconductivematerials. Severalmaterials are in use. Metal fibressuch as stainless steel and copper are readilyavailable. The have a fair to excellent conductivity level. Theirpropertiesbeingverymuch different frompolymerfibrestheymaybe hard to process and show someparticularproblems of long term stability.Alternativelypolylmerfibresmaybecoatedwith a conductivelayersuch as polypyrol, copperor gold. The conductivitywillbemaintained as long as the layer is intact and stilladhering to the fibre.
One of the firstapplicationswhere a conductivetextilestructure was used as a sensor are electrocardiogramrecordings. Suchstructures are calledtextrodes. They are truetextilecomponents, as theyconsist of 100% textilefibres. Simplyreplacingconventionalelectrodesbysuchtextrodesleads to a working smart textile system. Additional filtering considerablyimproves the quality of the signals. A secondapplication is the monitoring of the respiration. Two major principlescanbeused to this end. Both are basedonconductiveyarnsthat are integrated in a belt (calledrespibelt) that is wornaround the chest. When a staplefibreyarn is used the current has to pass fromoneconductivefibre to another. In addition to the resistance of the fibres as such the overall resistance is determinedby the contact resistance at the fibre-to-fibrecontacts. When the yarn is subject to strain (forinstancebystretching the fabric) itbecomes more compact, causing the number of contact points to increase and the contact resistance to decrease. Thisleads to a reduction of the overall resistance of the yarnstructure. Breathinginduces a cylcicexpansion and shrinkage of the chest and thisleads to anincrease/decrease of the length of the respibelt.The secondmechanismconsiders the conductiveyarns in the belt as anelectromagneticcoil of which the inductancechangesdue to breathing.
The textrodesessentiallymeasure the biopotentialthat is needed to make the heartmuscle contract. Anymucscle in the body is steeredby a biopotential. Sobasically the textrodescanalsobeusedformeasuringmuscleactivity. However the skin conductivity is low and due to the compressible nature of textilematerials and due to movements the contact resistance is subject to largevariations. These phenomenacause a lot of noise. Within the European project Context a contactless sensor has been developed. Theyuseembroidery and laminationtechnology to produce EMG sensors formonitoring stress in professional situations.
Pressure sensors canconsist of a 3 dimensionaltextilestructure, composed of an upper and lowerconductivelayerseparatedby a non conductivelayer. Whenpressure is applied the conductivelayersmake contact and as a resultanelectricalconnection is made between upper and lowerlayer. This is anonoff sensor thatcandetectwhetherornot the pressureexceeds a preset level. A secondapproach is basedon the quantumtunneling effect. the conductivity of suchmaterialschangescontinuouslywithpressure as illustrated in fig. such a sensor allowsquantitativemeasurements.
The conductivity of materials is oftenaffectedbyseveral parameters whichmaybeexploitablemechanismsforuse as a sensor. Pouillet’slaw is a firstexample. Thislawdescribeshowconductivitydecreasesdue to materialextension. Butalsoheating, wetting and absorption of chemicalcompounds in generalmayincreaseordecreaseconductivity. Carbon nanotubesincorporated in fibresmake the fibresconductive. Anymechanismthatleads to swellingorshrinkage of the fibresalters the distancebetween the nanoparticles, causing the conductivity to change. Thermalexpansion in this respect is the base of thermoresistivefibres. Polymersthatswell in the presence of water orchemicalcompounds in generalcanbeused as sensors when carbon nanotubes are added. This has been the topic of the Inteltex project.
As previouslydescribed, conductivematerialscanbeapplied in textile sensors in variousways. Textrodesdesignedformonitoringbiopotentials are alsosuited as electricalactuators. Electrostimulation has shown to beeffectiveformanypurposes. Dependingon the application the impulsecanbeveryweak up to strong. A tactile stimulus forinstance does notneed a high currentintensity. A higher level of stimulation is requiredfor skin stimulation, forinstanceforreinforcing the tissue in order to prevent bed sores. Anotherapplication is the increase of skin permeability in order to enhance drug delivery. Anadditionalelecrical field canenhance drugs to pass trough the skin.Realactivation of the musclesrequires the highest level of stimulation. Targetedeffects are warming up orexercising the musles.
Actualstimulationcanonlybeachievedbyhighercurrentintensity. The current distribution in conductiveelectrodesbeinginhomogeneous, localpeakscanoccur. Heat production is proportional to the square of currentintensity, socurrentdensitypeakscorrespond to local hot spots. Thispossiblyleads to burninginjuries. Currentconcentrationoccurs at the edges of anelectrode and parricluarly in the corners. A more homogeneouscurrentdensityprofilecanbeachievedbychanging the localconductivity of the electrodes. Models have been establishedforcalculating the currentdensity distribution forelectrodeswithvariousconductivity profiles. As shown in fig. usinganelectrodewithconcentric zones of conductivity (high in the middle, low at the outside) instead of a homogeneouselectrode the peakcurrentdensitycanbereducedby more than half.
Several prototypes have been manufactured. Theyuseseveralconductivefibres (stainless steel, coppercoated, gold coated) and productionmethods (full insertion of conductiveyarns, embroidery, lamination). In the framework of a European project ….
As discussed in previousparagraph, conductivematerials heat up as a result of current. Polar has presented a heatingfleece. Itusesstainless steel fibrespoweredbyregularbatteries. AlsoSefarcommercialiseswovenheatingelements.
Cooling is far more challengingthanheating. The Eu project Prospie consideresseveralapproachesforcooling. Ventilation is the first. Secondlyphasechangematerials are used. Theywillbediscussedfurtheron. For somematerialswetting is anendothermalprocess. Suchsaltswillbeembedded in textilestructures in order to generate a cooling effect initiatedbywetting (due to sweating).The Italiancompanygrado Zero has embeddedultrathin tubes in textilestrcuturesthroughwhich a coolingliquidcanbecirculated. A F1 pilot racer suit has been mafuncatered. The liquid is cooledby a smallPeltier element that is fixed at the back side of the suit.
As conditionsmaycange (temperature, intensity of activity, …) the needforinsulationmayvary as well. Adjustableinsulationcanbeachievedbymultilayerfabricswhere the disctancebewteen the layerscanbechanged. Anexample of such a product is aninflatable jacket where air has to beblown in to separate the layers. Another product consists of a comfortablecotton inner layer and a heat resistantaramidouterlayer. Shapememorysprings separate the layersby opening up when a transitiontemperature is exceeded. This product is thin and comfortable at low temperatures, werehas at elevatedtemperaturesitautomatically provides thermalprotection. At this moment it is stilloneway.
Adaptableventilation has been basedon the structure of pineapples. Theyconsist of multiple layerswithlargedifference in hygralexpansioncoefficient. In a wet state the leaves of the pineapple are closed. When the humiditydecreases (in spring time) the leavesbendawayfromeachother and the pineapple opens allowing the seeds to get out. Similary a multilayercoating has been coatedon a fabric. U-shapedperforations are punched in the fabric and coating. When the material is wetted (simulatingsweating) the blades of the perforationsbendcreating a largeventilation effect. The system is fullyreversible.
Phasechangematerials have been developedby NASA in the 90’s for coping withlarge and fasttemperaturevariations in spacecabins. Theyconsist of microcapsules with a hard shellfilledwithvariousparaffinwaxes. Theyexploitmelting heat which is muchhigherthan the caloricvalue. As a result the material has a very high thermal buffer capacity. The meltingtemperaturedetermines the temperature range of use.
Humiditycontrol is very important in thermal comfort. Sweating is a naturalmechanism of thermocontrol. Evaporation of sweat is anendothermalprocess. Butsweatalsogivesanunpleasant feeling of wetness and cold. In firefightersuits the sweatcannot escape and migratesinside the suit. Localcondensationmaydrasticallyreduceinsulation and evaporationmaylead to formation of steam. Both effectscangiverise to burninginjuries. Byanappropriate design consisting of selection and combination of proper materials and a correct shapeit is possible to evacuatehumidityadequately. Moisturecanbetransportedforisntance to a specificareawherethey are absorbedbyhydrogels. On the other hand sweat is reflecting the health state of the body. In the Biotex project devices have been developedformeasuringseveral parameters of sweatsuch as pH, conductivity, sweatingrate, concentration of K, Cl and Na ions. The lattergive a goodindication of dehydration of the body, which is very relevant forfirefighters and sportspeople. Apart fromsweat, pH is also a good indicator of the healingprocess of a wound.
Thermal comfort is very important. Thermal stress leads to increasedfatigue. In extreme conditionsitmaycause a person to faint. Full thermoregulatingtextileshowever are notyetwithinreach. It is a goodexample of whatneeds to bedone in order to fullyexploit the potential of Intelligent textiel.The firstquestion is to measure the level of thermal comfort. A number of body parameters canbemeasured (skin temperature, humidity, conductivity, …), buttruetextile sensors are stillunderdevelopment. Thenit is notyetclearhow to interprete these data, that is how to quantify the level of thermal comfort. Secondly adequate actuators are neededthatcan heat, cool, insulate, ventilate, regulatemoisture. Apart fromheating, noeffectivetextileactuators are notyetavailable.Thirdlythermal comfort is veryindividual and so is the body response. Advancedselflearningmodelling and controlstrategies are needed.So a lot of workstillneeds to bedonebeforetextilethermoregulation is a fact.
Adequate mechanicalactuators are expected to reactfast, to consume a minimum of energy, to befullyreversible and to produce big deformations and forces. Severalmechanismscanbeused to achieve a mechanicalaction. Diffusionbasedactuatorsconsist of gel basedmaterialsthatswellwhen water oranyothercompound (likeions) are absorbed and shrinkwhenthey are released. Temperatureoranelectrical field canbe the trigger. They are fullyreversible and do notrequire a lot of energy, but the diffusionmakes the process to berather slow. The speed canbeenhancedbyapplyingverythinlayers. Shapememorymaterialscanbetemperaturecontrolled. They are prettyfastbutcan act onlyoneway; they and canbecontrolledbytemperature. Temperature in its turn canbecontrolledbyanelectricalcurrent. Electrostrictivesystems are veryfast and fullyreversible, buttheyrequire high voltage. Energy consumption is low. So the perfect mechanicalactuator is notyetwithinreach.
Electroactivefibreswillplay a major role in the futuredevelopment of Intelligent textiel. A genericstructure is given in fig. Anelecroactivefibretypicallyconsists of a conductive core, the electroactivelayer and anouterconductivecoating. The inner and outerconductivelayer act as the positive and negativeelectrodes. The electroactivelayerdetermines the functioning of the fibre.Someexamples are givenhereafter.The firstexample is a mechanicalfibre actuator basedonanelectrochemicalprocess. A polymer is chargedwithanions and cations. Applyinganelectrical field causesone of the ions to beexpelledorabsorbed. Thisleads to shrinkageorswelling of the fibre. The system is fullyreversible and does notconsume a lot of energy.
Opticalfibrescanalsobeused as sensors and actuators. Threemechanismscanbeapplied, namely colour change, Snell’slaworBragg’slaw.
The colour change of materialsthat are coatedontoanopticalfibrecanberead out by a device. A maforchallenge is to fixsuchdyes in a polymericmaterial. Indeed the dyemay bind on the polymer in a groupthatinterfereswith the colour changingmechanism and this bond maycause the colour change to disappear. Colour change is not a newphenomenon. Alexander the Greatalreadyused a dyestuffextractedfromsnailsthatchanges colour under UV light (TyrianPurple).At UGent research is carried out onpH sensitive dyes. The goal is to usethem in wound dressings in order to follow up the healingprocess of the wound. Heretoo a major challenge is to bind the dyeon a textilesubstrate without loosing the colour change.
Opticalfibreactuatorscanbeachievedeasilybydamaging the claddinglayer of the fibres as thiscauses the light to protrude. France Telecom has developed a shirt withembeddedopticalfibres in an 8*8 matrix as shown in the followingfigure. Eachopticalfibre is litby a smallLED.The shirt is meant as a visualcommunucation tool when the noise of the environment does notallowcommunicationby sound.
MiniaturisedLED’s are built in the textiledirectly in the Lumalive project from Philips. Within the Stella project they have been embedded in a stretchable print.
Furtherprogress is made byprinting the LED directlyon the textilesubstrate. Challenges at this moment are that the efficiency of organicactivelayers are notveryefficientyetso the yield is still low, and that the material is very sensitive to oxidation.
Anotherclass of actuatorsabsorbor release chemicalcompounds in a controlledway. Microcapsules, active gels orcyclodextrinscanbeused to this end. Microcapsules release their content when the shellallowsso. Varyingpermeability, dissolvingorbreakage of the shelltriggeredbyheate, friction, water orchemicals are exploitablemechanisms.
Communication is a very important aspect. It does notmakesense to carry out anymonitoringwhennoaction is taken in specificcirconstances. First of all allactivecomponents in a smart textilesuch as sensors, electronics and actuatorsneed to beconnected. Conductivefibres and opticalfibres are used to this end. Communciationwith the wearer is handled by a textile keyboard or a textile display. A wirelessconnectioncanbe made with the wide environment via aninductive link and an ISM band textile antenna. In the Proetex project a GPS system embedded in a rescuesuitallows to determine the location of a person.Important criteria forselecting the otimalsolutionhighlydependon the conditions of use of the system such as whetherornot the wearer is moving, whether the user is located in a field or in a building etc.
Energy forsure is one of the keychallenges in a smart textile system. Energy canbestored in a form of battery and/orharvestedfrom the environment. As forenergystorage, twoconcepts are availablenamelyelectrochemicalbatteries and capacitybasedbatteries. Electrochemcialbatteries are widelyused. Todaytheyexist in the form of thinbendablefoils. Unfortunatelytheircapacitymainlydependsontheir volume, sothat a reduction of the thickness must becompensatedbyanexpansion of the surfacearea. The batterybeingimpermeable to moisture, thiscauseslocal discomfort due to sweataccumulation.As forcapacitybasedbatteries, carbon nanotubes have been reported to have supercapacitiveproperties.Energy is available in the environment under the form of heat, light, motion. Severalmechanisms are known to harvestthem.
Infineon has developed a devicesthatharvestselectricityfrom body heat. It is basedon the Seebeck effect. Itconsists of a repetitivesequence of conductive, n semiconductive, conductive and p semiconductivematerials. Electroncswillrather move from n semiconductive to p semiconducivematerialthan the otherwayaround. Thismeanstheywill speed up in the conductive areas between n and p semiconductive and slow down in the conductive areas between p and n semiconductivematerials. Speeding up requiresenergy, slowing down releases energy, in this case in the form of heat. Sosome areas willcool down, otherwill heat up. Whenanappropriateconfiguration is chosen, forisntance as illustrated in fig. a flat device is obtainedthat has a cooling and a warming surface. As long as both surfaces are at different temperatureanelectricalcurrentwillbegenerated.The demonstratordevelopedbyInfineon has the dimensions of a eurocoin and producesenoughenergyfor a small sensor. Thismeansthat a stand alone sensor canbeachievedbyintegrating a localthermogeneratortogetherwith the sensor.
Smart textilesmayrequiresome data processing devices. Soelectronics must be made compatible withtextiles. Electronics shouldbestretchableinstead of rigidorbendable, and the textileshouldbeused as motherboard. A grid of opticalfibres (wearablemother board developedbyGeorgia Tech) orelectroconductivefibres (ETH Zurich; Zefar) has been developed to this end.
Electronic componentsneed to beintegratedseamlessly in the textilestructure. The WearableMotherboardbyGeorgia Tech is the firstdevelopment in this respect. Itconsists of a grid of opticalfibresembedded in a wovenorknittedstructure; at severalconnectorsactivecomponentssuch as sensorscan beattached.Basedon the work of ETH Zurich, the Swiss companySefar has designed a woventextilestructurewith a grid of electroconductiveyarnsontowhichelectroniccomponentscanbeattachedsothat the appropriateconnections are achievedwith a good level of durability.
The next step in thisevolution is to make the electroniccomponents in the form of fibres. Within the EU project Proetex, a fibre transistor has been designed. The firstgenerationstartedfrom a ribbon shapewhichcanbewoveninto a fabric. A truefibre transistor requiresfurhterwork in coatingorotherforms of deposition of subsequentactivelayerson a conductivefibre
Unfortunately the characteristics of such transistors are notverystable. Theyvarydue to deformationcausedby bending, stretching etc.. On the other hand thismakesthemsuitable as a sensor.As alreadymentionedbefore the organic semiconductors that are currently in usealsosufferfromoxidation.
Onceelectroactivefibres have been achieved the nextchallenge is to integratethem in a fabricstructure in anindustrialproductionprocess. Itrequires design of the appropriatefabricstructure in order to ensure the contacts are right and stable and thatnofalsocontactsarise.
Today the most advanced smart textile product has beendeveloped in the framework of the proetex project. The Proetex smart suitprotectsrescueworkers and allowsfastfollow up of victims.The suititselfconsists of an inner and outergarment and boots for the resueworker and a victimpatchforvictims.
The inner garmentcontainstextile sensors formeasuringheart and respirationrate, temperature and sweatcomposition. These signals are used to tracewhether the rescueworker is at risk.
The outergarment is equippedwithtextile compatible componentssuch as accelerometers, temperature sensor, GPS module, visual alarm. Anelectronic box processes the data and controls the system. A flexiblebatterysuppliesenergy. The results are communciated to a base station via a textile antenna.
The victimpatchcontains the sametechnology as the inner garment. Itcanbeappliedeasilyonvictims. Itmeasuresheart and respirationrate and body temperatureenablingfastevaluation of urgent needfor care.
Key issues for the design of the system are:ComfortWorking conditions – relevant parameters: only relevant information should be provided in order to avoid additional work load; this includes indication of danger and need for help.Effective alarm generation: the rescue worker or a responsible person should be informed adequately on what needs to be done.System maintenance: it must be possible to treat the suit according usual mainatenance procedures.Ease of use: the use of the smart system should not requireanyadditinal effortWeight : the additionalloadshould not reduceoperation time of the rescueworker.Cost must be justifiedRobustnessEnergy constraints: energy requirements must be optimisedLong range transmission: transmission range must be adjusted to the situation of use. Fighting a fire in a building is different from in an open field.
A smart textile is a powerfull tool thatcancontribute to health and safety. Itcan monitor man, the environment and itself. Itcandetectunusualconditionsthatmightindicatesomthingmight go out of hand; thisallowspreventionorearlyintervention. Incident protectionmightbeprovided without hindering comfort and ease of use. The smart textile system can monitor the impact aneventmight have had. Itcanidentifymeasures to be taken liketreatmentorcallingfor help.Later onitcan support and follow up rehabilitation.Ultimately the potential is endless. At this moment developments are mainly at the level of embeddingtextile compatible components. But research is onitsway to developtruefibrebasedtextileelementsthatcan act as actoivecomponents and thatcanbeintegratedseamlessly in a garment.
Informationonprojects, people and productsregarding smart textilescanbefound at www.systex.org.
Intelligent textiel: science fiction in dekleerkastProf. Lieva Van LangenhoveVakgroep Textielkunde
Ieder mechanisme dat geleidbaarheidverandert, kan basis zijn voor sensor• CNT koolstof nanobuisjes• uitzetting van het materiaal verandertgeleidbaarheid: • Uitrekking • Thermsiche expansie • Zwelling oiv vocht, chemicaliën
Intelligent textiel is in staat om…Een persoon, zijn omgeving en zichzelf te monitorenAfwijkende condities te detecterenRisico’s te detecterenOngevallen te voorkomenTe beschermen bij ongevalDe impact van een ongeval te beoordelenEerste hulp te biedenHerstel op te volgen en te ondersteunen
Coordination action for enhancing thebreakthrough of intelligent textile systems(e-textiles and wearable Microsystems) www. .orgCOLAE: Commercialisation Clusters of OLAE www. .eu