ElectrochemicalCarbonNanotubeArtificialMuscleswithHighStroke,Work,andEfficiency

Abstract

Artificialmusclesthatmimicnaturalmusclescanreversiblyrotate,contract,orexpandduetoanexternalstimulussuchaselectricity,temperature,pressure,orchemicals.Inparticular,electrochemicallydrivenartificialmuscleshavelowoperatingvoltages,negligiblethermaleffects,andhighcontrollability.Inaddition,theelectrochemicaldoublelayerofmusclesprovidesanaturallatchingstateandhighenergyconversionefficiency.Especially,carbonnanotube(CNT)yarnisausefulmaterialforelectrochemicalartificialmuscleduetoitshighmechanicalstrength,electricalconductivity,andcapacitance.Basedontheseproperties,previouslyreportedCNTmusclesshowedhighperformance,inparticularaworkcapacity30timeshigherthanthatofhumanskeletalmuscles.However,muscleperformanceneedstobeimprovedforvariousapplications.Hence,thisstudydemonstratesthreewaystoimprovetheperformancesofelectrochemicalCNTmuscles,includingtensilestroke,workcapacity,andenergyefficiency. First,grapheneparticleswereaddedtoCNTyarntoimprovethetensilestrokeoftheyarn.Thegrapheneparticleincreasesthecapacitanceoftheyarn.Asaresult,morechargesareinjectedintotheCNTyarn,resultinginalargervolumechange.Therefore,theCNTmuscleswithgrapheneprovideahighertensilestrokethanneatCNTmuscles. Second,astructuralapproachwasusedtoimprovetheworkcapacityofCNTmuscle.WhenfabricatingacoiledCNTyarn,ahighandlowtensileloadwereappliedforyarntwistandcoiltwist,respectively.Thehightensileloadprovidesahightensilemodulus,andthelowloadprovidesahighspringindexofthecoil.Asaresult,itprovideshigherworkcapacityandalsoimprovesenergyefficiency. Third,anelectrolytewhichhavehigherelectrochemicalstabilitywasappliedtoincreaseenergyefficiency.PreviouslyreportedCNTmuscleswereusedelectrolyteswhichhaveawidevoltagerangetomaximizetensilestrokeandworkcapacity.However,awidevoltagerangecauseselectrochemicalenergyloss,leadingtodecreaseenergyefficiency.Hence,thisstudysuggestsanelectrolyteforthemusclesthathaveastablevoltagerangecanincreasemuscleefficiencyashighasnaturalmuscles.|인공근육은생체근육을모방하여전기,온도,압력및화학물질과같은자극에의해가역적인회전,수축,이완운동을할수있다.특히전기화학적인자극으로구동하는인공근육은필요전압이낮고,발열이적으며,조절이쉽다는장점이있다.또한근육의전기화학적이중층은추가적인에너지소모없이근육이수축상태를유지할수있게하고,높은에너지효율을가능하게한다.특히탄소나노튜브섬유는강한물성과높은전기전도도및정전용량을가지고있어전기화학적인공근육의재료로유용하다.이러한특성을바탕으로,기존에보고된탄소나노튜브인공근육은높은성능을제시하였고,특히사람의근육보다30배높은일용량(workcapacity)을보여주었다.그러나다양한응용을위해추가적인성능의개선이필요하다.본연구에서는전기화학적탄소나노튜브인공근육의수축률(tensilestroke),일용량,에너지효율과같은성능을높이는세가지방법을제시한다. 첫째로,근육의수축률을높이기위해탄소나노튜브섬유에그래핀을첨가하였다.그래핀은섬유의정전용량을높이고,그결과더많은이온이인공근육의팽창에관여하게한다.따라서그래핀이첨가된탄소나노튜브인공근육은기존의탄소나노튜브인공근육보다상승된수축률을보인다. 둘째로,탄소나노튜브인공근육의구조를개선하여일용량을상승시켰다.코일형태의탄소나노튜브인공근육을제조하기위해꼬임을인가할때,코일구조가형성되기전후로각각높은부하와낮은부하를가하였다.초기높은부하는높은인장탄성률(tensilemodulus)을제공하고,코일형성과정에서낮은부하는높은스프링지수(springindex)를제공한다.이러한과정으로제조된코일구조의탄소나노튜브인공근육은높은일용량을제공하고,그에따라에너지효율도개선되었다. 셋째로,안정적인전압범위를가지는전해질을통해인공근육의효율을높였다.기존탄소나노튜브인공근육은높은전압범위를통해근육의수축률과일용량을극대화시켰다.하지만높은전압은전기에너지의손실을수반하고,그에따라에너지효율이감소한다.따라서본연구에서는안정적인전압범위를가지는전해질을통해탄소나노튜브인공근육의효율을생체근육과비슷한성능으로향상시켰다.