Architecture Design And Performance Adjustment For Silver Nanowires Based Flexible Piezoresistive Materials

Flexible piezoresistive materials(FPM)has drawn much attentions and exhibit great potentials in various fields such as medical diagnosises,voice recognitions,wearable electronics and smart robotic systems because they can easily transduce mechanical stresses(compression,bending,twisting,et al.)into electrical signals and conform well to various objects.However,the FPM are at an early stage of development and have a short history,the scarcity of systematic and intensive understanding make many key issues should be further investigated.In addition,it is full of challenge to fabricate FPM with high performance such as high sensitivity,conductivity,stretchability,excellent stability and fast responsibility.This study is focus on the relationships among the inner structures,working mechanisms,sensing performances and applications.Silver nanowires(AgNWs)tend to aggregate in polymer matrix since the intense depletion–induced interactions,which seriously influence the percolation threshold of the composites.In this study,hybrids of AgNWs and layered double hydroxides nanosheets(AgNWs/LDHNs)are prepared by self–assembly of AgNWs and LDHNs and then inkjetable,screen printable and writable conductive ink is fabricated by composed waterbrone polyurethane(WPU)and AgNWs/LDHNs hybrids.The nonconductive 2D LDHNs are embedded into AgNWs network and assist dispersion of AgNWs,which depends on the hydrogen bonding between the two nanostructures.The percolation threshold of the composites decreases from 10.8 vol% to 3.1 vol% due to the dispersion improvement of AgNWs.Finally,a highly stable and sensitive paper based bending sensor is prepared by screen printing WPU/AgNWs/LDHNs composites on office paper,which shows low–cost,excellent conductivity,flexibility(> 3000 bending cycles),sensitivity(0.16 rad–1),fast response(120 ms)and relaxation time(105 ms),making the bending sensor may be meet the needs in numerous applications for robotic systems.A uniform,compact and stable AgNWs conductive network is constructed on cotton fibers through simple “dip–drying” method and following thermal treatment.The prapred bio–based conductive cotton fiber film shows low resistivity as(2.2±0.3)×10–4 Ω·cm.The intrinsically rough AgNWs based cotton fiber films are used to construct FPM based on cotton decorated with AgNWs(AgNWs@Cotton FPM).The fabrication process shows low cost and can be easily scaled up and the bio–based AgNWs@Cotton FPM show very high sensitivity(3.4kPa–1),rapid response and relaxation time(<50 ms)and excellent stability(> 5000 loading/unloading cycles).The working mechanism is theoretically studied and the results demonstrate the contact area between bottom and up cotton fibers increases after applied with external loadings and and the layers of conductive cotton fiber films play a key role in determining the performance due to it is proportional to the contact points.The AgNWs@Cotton FPM can effectively detect dynamic sound–driven vibrations(0.006 dB–1),gentle external forces and muscle motions,illustrating their extensive applications in various fields such as voice recognitions and robotic systems.Reduced graphene oxide(rGO)network with excellent stability is constructed on the surface of cotton fiber by “dip–drying” process.Investigations to the structure of rGO show there exist some defects on the surface or edge of rGO.The topological defects divide rGO into several monocrystal domains(MDs)and grains boundaries(GBs,interfaces between MDs),resulting in high resistivity of the rGO based cotton fiber films(0.26 Ω·cm).In this research,a bio–based flexible piezoresistive sensor based on the “dynamic bridging effect” of highly conductive AgNWs toward rGO(AgNWs–rGO FPM)is presented,in which the highly conductive welded AgNWs networks cross and bridge the neighboring high–resistive grain boundaries and rGO contacts when applied with mechanical stresses.The variation of the charge transport behavior results in drastic decrease in resistance even under subtle loadings.The obtained AgNWs–rGO FPM shows high sensitivity(5.8 k Pa–1),fast response and relaxation properties(29.5 ms and 15.6 ms,respectively),ultralow detection limit(0.125 Pa)and excellent stability(>10000 loading/unloading cycles).The high performance AgNWs–rGO FPM demonstrates great potentials in detection of wrist pulse waves,smart wearable electronics,monitoring or simulation of human body motions and movements.Core–shell conductive fibers with high conductivity((7.30±0.21)×10–5 Ω·cm),stretchability(400 %)and durability(more than 1200 s ultrasonic treatment)are presented and multiscale wrinkled microstructures(about 1.7 μm in height and 2.6 μm in length)are built on the surfaces of fibers via simply writting silver nanowires ink on prestrained commercial PU fibers.The as prepared core–shell elastic fibers are twisted to construct interlocked flexible piezoresistive fibers,which show desirable sensitivity to pressure and bending deformations(0.12 kPa–1 and 0.012 Rad–1),fast response and relaxation time(35 ms and 15 ms),very low detection limit(10 mg)and excellent working stability(> 4000 loading/unloading cycles).The wrinkled microstructures to overcome the viscoelastic delay of polymer composites is observed,making substantial contributions to improve the responsiveness.The investigations to the sensing mechanism indicate that increasing the contact points inner the flexible piezoresistive fibers will significantly improve the sensitivity.Finally,the potential applications of the flexible piezoresistive fibers as wearable devices and smart fabrics are demonstrated.