Preparation And Properties Of Ionic Hydrogel Strain Sensors And Their Application In Human Motion Detection

Flexible strain sensors have attracted extensive attention in recent years by virtue of their potential applications in many fields,such as human motion detection,soft robots and implantable devices.Ionic hydrogels have exhibited great potential as another way to develop strain sensors by virtue of their intrinsic conductivity,stretchability and biocompatibility.Nevertheless,ionic hydrogel sensors typically suffered from poor mechanical strength and low strain sensitivity.Therefore,two kinds of high-performance ionic hydrogel strain sensors were fabricated which not only retained their intrinsic stretchability,conductivity and biocompatibility,but also improve their mechanical strength and strain-sensitive performance.Thus,we have successfully accomplished critical performance combination of flexible strain sensors.（1）Preparation of high-performance PVA/glycerol/Na Cl ionic hydrogel strain sensors and their application in human motion detectionBiocompatible polyvinyl alcohol（PVA）and nontoxic glycerol was mixed evenly in water,the mixture was subjected to the freezing treatment and subsequently soaked in the saturated sodium chloride（Na Cl）solution to obtain the desired PVA/glycerol/Na Cl（PGN）hydrogel sensors.The resulting PVA/glycerol/Na Cl hydrogels exhibited good transparency（>88%）,favorable stretchability（570.7%）,outstanding mechanical strength（3.1MPa）,and high toughness（8.65 MJ/m³）.Meantime,the PGN ionic hydrogel sensors displayed excellent stretching sensitivity with a gauge factor of 4.01,and it could repeatedly and stably detect accurate electrical signals under both large strains and subtle strains.Furthermore,our hydrogel sensors were endowed with anti-freezing feature on account of the presence of both glycerol and Na Cl,the sensors could still maintain good mechanical and strain-sensitive performance at low temperatures（-20°C）.Based on the above characteristics,the biocompatible PGN hydrogel sensors could be employed as wearable devices to detect various human motions（eg.finger,wrist,elbow,and knee joints）at room temperatures and low temperatures.（2）Preparation of high-performance chitosan/poly（acrylamide-acrylic acid）-Al³⁺ionic hydrogel strain sensors and their application in human motion detectionHerein,AlCl₃·6H₂O aqueous solution,as a sort of new solvent,was utilized to facilely manufacture high performance hydrogel ionic conductors with novel one-pot dual ionic cross-linking approach.We first took advantage of Al³⁺solution to attain one-step dissolution and cross-linking of chitosan（CS）at high temperatures,which could erect one ionic cross-linking network by forming Al³⁺-CS complex.Subsequently,another ionic cross-linking network was further constructed by ionic bonds formed between the carboxyl groups of the copolymer chains and Al³⁺through introducing the Al³⁺-CS complex into the copolymer system to obtain the desired hydrogel sensors.The resulting model hydrogel sensors manifested favorable transparency（>84%）,excellent stretchability（1400%）,good mechanical strength（421.2 k Pa）,outstanding rapid self-recovery（nearly full recover）and fine anti-fatigue.Furthermore,our ionic hydrogel sensors demonstrated brilliant stretching sensitivity（gauge factor as high as 9.6）,and it displayed stable sensing performance for repeatedly real-time monitoring of both large and subtle deformations.Hence,the hydrogel sensors successfully achieved critical features combination and could serve as wearable devices to sense various human motions（eg.finger,wrist,elbow,knee）and even the vibrations of the larynx.