Preparation And Strain Sensing Properties Of Anisotropic Carbon Fiber Reinforced Hydrogels

The growing overlap between materials and sensing systems has brought many challenges to sensor research.In recent years,high-performance soft pressure sensors have had a significant impact on wearable electronics,human-machine interaction,electronic skin,and environmental monitoring.Soft conductive hydrogels,due to the synergistic advantages of the conductivity of conductive fillers and the flexibility of hydrogels,have become candidate materials for biosensors.In order to improve the mechanical strength and sensing performance of hydrogels,different conductive fillers are often introduced.It has also been shown that anisotropic hydrogels have the advantage of improving the mechanical and sensing properties of hydrogels.For the orientation method,researchers have done a lot of research and exploration.Carbon fiber has excellent mechanical properties,electrical conductivity,anisotropy and other properties,and long carbon fiber has high strength and modulus along the fiber axis due to its graphite microcrystal orientation along the fiber axis,which is usually fused with resin,ceramics and other rigid matrix to form composite materials.Inspired by the anisotropy of long carbon fibers,the combination of short carbon fibers with hydrogels and the endowment of anisotropy can improve the mechanical and sensing performance of hydrogels.However,in recent years,due to the influence of carbon fiber size on dispersion performance,there have been few reports on carbon fibers as hydrogel reinforcement fillers,and carbon fibers have characteristics such as larger size and weaker inherent magnetism,making the orientation method more complex,and its effectiveness has not been well studied.Therefore,this paper used a simple stretching method to orient carbon fibers and prepared anisotropic carbon fiber-reinforced conductive hydrogels.Fiber orientation enhanced the mechanical and sensing properties of the hydrogel composite materials,and the application of anisotropic conductive hydrogels in flexible sensing was investigated.The specific work is divided into the following two parts:(1)Using carbon fiber(CF)and polyvinyl alcohol(PVA)as experimental materials,anisotropic carbon fiber-polyvinyl alcohol conductive hydrogels were prepared by a simple stretching and oriented freezing method without introducing initiators and crosslinking agents.During the preparation process,stretching-induced PVA chains and carbon fiber orientation arrangement,followed by oriented freezing,were used to fix the orientation structure formed inside the hydrogel.The orientation structure increased the distance between fibers,formed more hydrogen bonds inside the hydrogel,and promoted the internal stress transfer of the PVA matrix to the CF.The mechanical strength of the prepared hydrogel was significantly increased,and it showed anisotropic mechanica properties.The tensile strength of the hydrogel composite material in the orientation direction and the vertical orientation direction were 759.24 k Pa and 454.7 k Pa,respectively,and the tensile strength in the orientation direction was 1.7 times higher than that of the hydrogel without orientation.Meanwhile,the hydrogel’s anisotropic microstructure formed new conductive paths inside,and the sensitivity coefficient(GF)of the hydrogel increased from 0.076 to 0.133 in the range of 0%-100% strain,showing anisotropic sensing properties.This hydrogel can be used as a wearable sensor to detect various human movements,such as joint and throat sound before and after exercise.(2)This experiment used acrylic acid(AA)and chitosan(CS)as the hydrogel matrix,and utilized the silver mirror reaction to deposit elemental silver on the surface of carbon fiber to enhance its conductivity.The cross-linking agent N,N-di(acryloyl)cystamine(BACA)and the initiator ammonium persulfate(APS)were used to polymerize the hydrogel into a conductive hydrogel.The hydrogel and conductive filler were oriented using the stretching method from the previous experiment,and then immersed in a solution of 3-acrylamidophenylboronic acid.This increased the network density of the hydrogel and further fixed the internal orientation structure of the gel.The prepared ACCP hydrogel exhibits anisotropic mechanical and sensing properties,with electrical signal response at a small strain of 0.5%,a wide detection range,and no significant change in electrical signal response after 100 cycles of stretching at 30% strain,demonstrating excellent cyclic stability.