Preparation And Strain Response Of Acrylic-based Hydrogel With Antifreezing And Moisture Retention

In recent years,flexible wearable sensors can monitor human physiological activities and reflect health problems in real time,which has attracted much attention.Hydrogel,a soft material with a tissue-like structure and a modulus comparable to that of human skin,is one of the most promising materials for the preparation of flexible wearable devices.However,the practical application of hydrogel-based sensors is greatly limited because of the poor mechanical properties of conventional hydrogels,the easy freezing or evaporation of water in network,the lack of durability and electrical conductivity etc.In this paper,two kinds of hydrogels with stretchable,flexible,fatigue resistance,antifreezing,moisturizing,and electroconductive properties were developed.The mechanical,antifreezing and moisturizing properties,the electrical conductivity and the strain responses of hydrogels were systematically studied and used in flexible strain sensors to monitor human motion.The main research contents and results are as follows:1.PAC-Zn organohydrogels were prepared by thermal initiating free radical copolymerization with acrylic acid（AA）as hydrophilic monomer,cardanol（CA）as hydrophobic monomer,zinc chloride（ZnCl₂）as conductive material and H₂O/1,3-butanediol as dispersing medium.Due to the presence of rigid aromatic rings and flexible alkane chains,CA could simultaneously improve the mechanical strength and toughness of hydrogel materials.The results showed that PCA₁ organohydrogel（CA content of 1%）had a dense pore structure that was conducive to external force transmission,and exhibits good tensile strength（～4 times higher than PAA）,elongation at break and toughness（～3 times higher than PAA）.1,3-butanediol endowed PAC-Zn with excellent tolerance for freezing（-45°C）and drying（over 85%moisture retention after 15 days of storage in a 37°C oven）.Compared with commonly used ethylene glycol and glycerol antifreeze,the hydrogels containing 1,3-butanediol exhibited similar freezing resistance and better mechanical properties.The dynamic effects ofπ-πconjugation,hydrophobic association,chelation,and hydrogen bonding in PAC-Zn network could rapidly dissociate-recover under external forces,which making PAC-Zn hydrogels have excellent resilience（almost no hysteresis loop）and fatigue resistance.In addition,ZnCl₂ imparted good electrical conductivity（1.3 m S/cm）to PCA-Zn.The PCA-Zn organohydrogel-based sensor had a fast response-recovery capability（only 0.20s for 1%strain）,which allowing a real-time monitoring of human motion with a fast,stable and reproducible signal response.2.PAA-CS-G/Gly hydrogels with dual physical networks were prepared rapidly（<0.5 h）without external energy and chemical cross-linking agents.During preparation,silver nitrate accelerated the activation process of ammonium persulfate and the coordination of silver ions with chitosan promoted CS gelation.The first physical network of the PAA-CS-G/Gly hydrogel was CS with a double helix structure,and the PAA obtained by free radical polymerization passed through CS network to form the second physical network.Due to hydrogen bonds formed by networks-networks and glycerol（Gly）-networks,the synergistic effect between networks and well-dispersed graphene（G）in certain viscosity CS,and ion coordination,PAA-CS-G/Gly hydrogel showed ultra-long tensile properties（～2500%）,good toughness（3.11 MJ/m³）,fracture strength（0.27 MPa）,and fatigue resistance.PAA-CS-G/Gly showed good adhesion properties to different substrate surfaces,which reducing response failure due to interface debonding when used as a wearable sensor.A series of PAA-CS-G/Gly hydrogels with good antifreezing and moisturizing properties could be obtained by adjusting the concentration and soaking time of Gly.In particular,when Gly concentration exceeding80%and soaking time exceeding 2 h,hydrogel didn’t solidify at-80℃low temperature.Even if it was placed in a drying oven（RH was 25%）at 37°C for 15 days,the mass retention rate still reached more than 80%.At the same time,the hydrogel after freezing and high-temperature drying still maintained mechanical flexibility and electroconduction,and could monitor human movement normally.More importantly,PAA-CS-G/Gly strain sensor had excellent sensitivity（GF,up to 9.449）at 300～900%strain.It could sensitively detect human joint movements and even subtle activities such as talking and swallowing.The two organohydrogels of PAC-Zn and PAA-CS-G/Gly exhibited excellent comprehensive properties such as antifreeze,moisture retention,stretchability,electroconduction,and fatigue resistance.In particular,the PAC-Zn sensor with excellent resilience had a fast response-recovery ability,and its strained signal was stable and repeatable.The excellent sensitivity of PAA-CS-G/Gly hydrogel provided basis for flexible monitoring of large and small human movements.Moreover,the good adhesion provide PAA-CS-G/Gly hydrogel with direct fit on skin to ensure the stability of signal detection.PAC-Zn and PAA-CS-G/Gly hydrogels have good application prospects for monitoring human motion in extreme environments.