Conductive Hydrogel Based Flexible Sensor With Double Network Structure

Conductive hydrogel has become an ideal candidate material for flexible wearable sensing devices due to their inherent high-water content and biocompatibility similar to human tissue.Practical applications for flexible sensor frequently require sensitive and reliable sensing performance and excellent fatigue resistance to repeated loads.Therefore,the development of conductive hydrogels with excellent mechanical and electrical properties is of special significance.In this paper,we prepared two flexible conductive double-network hydrogels based on the design of the hydrogel network structure and conducted a series of studies on the electrical and mechanical properties of the hydrogels.In addition,sensing properties of the hydrogel sensor were specifically explored.The specific content is as follows:（1）To construct a polymer-based flexible strain sensor,the flexible hydrogel network is used as a template to generate conductive polymers in situ and introduce conductive ions.A tough conductive hydrogel,PPy/CMC-Al³⁺/PVA was successfully prepared by generating conductive polymer polypyrrole（PPy）in situ and introducing Al³⁺in the carboxymethyl cellulose/polyvinyl alcohol（CMC/PVA）hydrogel network.Due to the synergistic effect of PPy and Al³⁺coordination,the PPy/CMC-Al³⁺/PVA hydrogel had good mechanical and electrical properties,and its electrical conductivity,tensile strength,and elongation could reach to be 3.5S m^-1.146.21 k Pa,and 478%,respectively.In addition,the interpenetrating electron transport network provided by PPy and the ion conductive path formed by Al³⁺attached to the CMC molecular chain cooperated to construct a continuous conductive network interpenetrating well with the hydrogel network in the entire hydrogel matrix,which realized the simultaneous improvement of the sensitivity and linearity of the hydrogel.Specifically,the hydrogel had a high linear sensitivity in a wide strain range of 0-400%,with GF=2.58.In addition,the hydrogel-based strain sensor has a rapid response to strain（～100 ms）close to the response time of human skin),and its strain sensing performance has excellent stability and durability.The flexible sensor based on PPy/CMC-Al³⁺/PVA hydrogel can monitor many subtle physiological signals,such as bending of finger,wrists,and elbows,and vocal-cord vibration.（2）Constructing a double-network polymer-based flexible sensor doped with nanocomposite conductive polymer.In this paper,by doping nanocomposite conductive polymer and introducing anion(Cit^3-)in the chitosan/polyvinyl alcohol（CS/PVA）hydrogel,we prepared a double network nanocomposite conductive hydrogel with toughness and conductivity.Using the template function of carboxylated cellulose nanocrystals（CNC）to avoid agglomeration of conductive polymers,a nanocomposite conductive polymer（CNC@PPy）with good dispersion is prepared.CNC@PPy is uniformly dispersed in the CS/PVA gel matrix to form a continuous electronic transmission network.Due to the synergistic effect of CNC@PPy conductive network and ionic cross-linking network,the CNC@PPy/CS-Cit^3-/PVA hydrogel had excellent mechanical and electrical properties.In particular,the elastic modulus,tensile strength,elongation and conductivity of the hydrogel can reach to be 364.4k Pa,1.67 MPa,692%,and 0.97 S m^-1,respectively.In addition,the electronic transmission network formed by CNC@PPy and the ion conductive path provided by Cit^3-cooperate to construct a continuous interpenetrating conductive network in the entire hydrogel,which impart the CNC@PPy/CS-Cit^3-/PVA hydrogel with excellent sensing performance.Specifically,the hydrogel had a very high sensitivity of 5.27 at a small strain of 0～2.8%.In the large strain range of 3-650%,the sensitivity is linear（1.24）.Furthermore,the response time of the hydrogel to external strain is close to that of human skin.The flexible sensor based on the CNC@PPy/CS-Cit^3-/PVA hydrogel can monitor the movement of finger,wrist,and elbow,which has broad applications in the field of human movement monitoring.