Preparation And Application Study Of Chitosan-based Ionic Liquid Cross-linking Hydrogel

The hydrogel has received widespread interest in the field of flexible stretchable electronics,soft robotics,human-machine interfaces and flexible energy storage devices due to its inherent unique physicochemical property.The weak mechanical property,large swelling rate and poor adhesive toughness of conventional hydrogels limited its application in various fields.Hence,it was particularly important to prepare multifunctional hydrogels to broaden the application of flexible electronic devices.In the first part,a chitosan-based ionic liquid cross-linked anti-swelling hydrogel was prepared.The hydrogel was composed of chitosan（CS）,a copolymer of acrylic acid（AA）,2-hydroxyethyl methacrylate（HEMA）,butyl acrylate（BA）and dimethylaminoethyl methacrylate-maleate（DMAEMA-MA）.The CS improved the internal network structure to reduce swelling effect caused by water molecule penetration.The P（HEMA-BA-AA）-ILs-CS hydrogel exhibited excellent anti-swelling properties driven by balance between hydrophilic segment and hydrophobic segment strategy（equilibrium swelling rate of-3.79%in water for 100 days）.The ionic liquid（DMAEMA-MA）was introduced into P（HEMA-BA-AA）/ILs/CS hydrogel system as cross-linker and electrically conductive substances to endow the hydrogel with outstanding toughness and high ionic conductivity（0.151 S/m）.Meanwhile,it could effectively prevent the loss of hydrogel conductivity after soaking in water.In synergy with chain entanglement,H-bonds and electrostatic interactions endowed the hydrogel with excellent mechanical property（tensile strength 0.321 MPa,elongation at break1164%,modulus 79 k Pa,toughness 1765 k J/m³）.Surprisingly,the tensile strength,elongation at break,modulus and toughness of the hydrogel were significantly boosted after a week of submersion.Based on the excellent swelling resistance,mechanical strength and electrical properties of the hydrogel,the hydrogel has been designed as hydrogel-based strain sensor,which could monitor different strain states and generate real-time feedback signals.Moreover,it implemented underwater motion monitoring and safety alarm.It is expected that the anti-swelling strategy hydrogel driven by the balance of hydrophilic segment and hydrophobic segment would inspire the development of underwater flexible sensors.The second part work,a super-stretchable adhesive hydrogel by adjusted the ratio of acrylic acid（AA）and 2-hydroxyethyl methacrylate（HEMA）was prepared.Chitosan enhanced the internal network structure of the hydrogel and formed a large number of hydrogen bonds with the polymer chain segments within the hydrogel,which endowed the hydrogel with excellent mechanical property.At the same time,the ionic bonding of ionic liquid avoided stress concentration during tension and conferred high toughness to the hydrogel.The hydrogel exhibited fracture stress of 171.14 k Pa,fracture strain of 6050.39%,Young’s modulus of 81.23 k Pa and toughness of 5.92MJ/m³.Furthermore,DMAEMA-MA as a cross-linker,the hydrogel system imparted high ionic conductivity（0.29 S/m）to the hydrogel while it also enhanced the mechanical property of the hydrogel.Additionally,the amino（-NH₂）and hydroxyl（-OH）groups of CS and the carboxyl（-COOH）groups of AA within the system and the electrostatic interactions of the ionic liquids made the hydrogel possess excellent adhesion property.The hydrogel could form strong adhesion to a variety of substrate surfaces.Therefore,a multifunctional hydrogel with excellent adhesion property,mechanical property,high electrical conductivity and excellent sensing property have been designed as flexible wireless strain sensor,which could detect limb movement and physiological signals with real-time feedback electrical signals.We believed that the preparation of chitosan-based ionic liquid cross-linked anti-swelling hydrogel would be expected to facilitate the development of hydrogel in underwater environments.Meanwhile,it would be promoted the application of hydrogel in flexible strain sensor technology.In addition,this chitosan-based ionic liquid cross-linked adhesive hydrogel enhanced the sensitivity and accuracy of the hydrogel-based sensor.These two parts of the work would broaden the application of hydrogel-based strain sensors in complex conditions.