High Toughness Composite Conductive Hydrogel Based On Polyacrylamide And Application To Flexible Strain Sensor

Hydrogel-based flexible strain sensors can transform different human motions（including various joints and even muscle micro-movements）and vital signs（including heart rate,pulse rate and respiration rate）into electrical signals,have exhibited potential applications in health monitoring.However,the conventional hydrogel-based flexible strain sensors exhibited limited mechanical properties（including toughness,modulus,and uniaxial tensile capacity）and low electrical conductivity,which is difficult to durable to complex mechanical shape change of natural skin.It is also very difficult to monitor the large movements of the body as well as subtle muscle deformations simultaneously by using conventional hydrogel-based flexible strain sensors.So,this thesis synthesized a composite conductive hydrogel with a high toughness,low modulus and ultra-long stretching capability by using polyacrylamide as the main material,and fabricated it into flexible strain sensor to achieve the goal of health monitoring.The details are as follows:1.Introduced liquid metallic gallium（Ga）into a double network（DN）hydrogel,which was synthesized by using polyacrylamide（PAm）and sodium alginate（SA）(chelated with Ca²⁺).The unique electronic configuration of Ga was utilized to trigger the polymerization of acrylamide radicals to form Ga-centered crosslinking sites,which improved the DN hydrogel network.In addition,the hydrogen released from Ga in alkaline environment could also induce the microporous structures inside the hydrogel,which reduced the hydrogel modulus.Finally,the dynamic chelation-dissociation of SA with Ca²⁺and the elastic deformation of Ga droplets provided the dissipation energy to improve the toughness of the hydrogel.Results showed that the optimized Ga/SA/PAm composite hydrogel had high toughness of（2.25MJ/m³）,and the uniaxial tensile strain and stress at break were 998%and 351k Pa,respectively.The hydrogel also had good electrical conductivity of（1.9S/m）because of the addition of Ca²⁺.It was worth noting that the elastic modulus is only 133 k Pa.The Ga/SA/PAm composite hydrogel-based flexible strain sensor had high sensitivity（GF=4.08）and extremely fast response time（110 ms）,which also had high adaptability to human skin.As a result,the flexible strain sensor could easily monitor not only the different human motions（including finger,elbow and knee bending）and subtle muscle changes（including smiling and swallowing）but it could also monitor and recognize the voice.Furthermore,it is expected to be applied in the training and treatment of people with speech impairment when combined with big data technology.2.For the problem of the flexible strain sensors in chapter 1 that sometimes the flexible strain sensors showed fluctuating signals during multiple cycles of tensile testing and could not effectively monitor finer muscle movements（such as pulse）.So,this chapter synthesized a hydrophobic association with hybrid crosslinking composite hydrogel by using the polyacrylamide（PAm）,surfactant sodium dodecyl sulfate（SDS）and lauryl methacrylate（LMA）.Meanwhile,the compatibility of the conducting polymer polypyrrole（PPy）and SDS/LMPAm hydrogel was improved by the hydrophobic association of SDS,which impart electronic conductivity to the hydrogel.Moreover,the dynamic dissociation-conjugation between LMA and SDS and PPy could dissipate energy to improve the toughness of hydrogels.A maximum toughness of 1.44 MJ/m³,an uniaxial tensile fracture stress of 345k Pa,an uniaxial tensile strain of 1021%,and an electrical conductivity of 0.57S/m could be obtained for the SDS/PPy/LMPAm composite hydrogel.In addition,inspired by the interdigital electrode,the interdigital electrode flexible pressure sensor was designed to replace the bipolar electrodes flexible pressure sensor,which improved the resolution of the flexible pressure sensor.The SDS/PPy/LMPAm composite hydrogel-based interdigital electrode flexible pressure sensor exhibited outstanding stability,it could not only identify different hand restores,but also monitor the pulse signal of human.Moreover,the characteristic systolic peak（P₁）and characteristic diastolic peak（P₂）could be clearly observed in the pulse signal diagram,and the pulse frequency（65 times/min）and the radial artery augmentation index（0.57）could be calculated,which might be important in evaluating the arterial vessel wall and function of the human artery.