Preparation And Property Investigation Of Transparent Adhesive Conductive Hydrogel Based On Ion Interaction

Flexible sensors can be widely used in the fields of flexible electronics and intelligent wearable devices due to their excellent flexibility,ductility,and freedom to bend or even fold.Conductive hydrogels stand out among many materials for flexible sensors because of their similar mechanical properties to biological tissues and good biocompatibility.Compared with flexible sensors made of other materials,hydrogel-based flexible sensors have many advantages,including:elastic modulus suitable for human skin,excellent extensibility,and adjustable mechanical and conductive properties,which endow hydrogel sensors with high sensitivity and wear comfort.However,due to the existence of a large amount of water in the hydrogel,the hydrogel will freeze below zero degrees Celsius,and will lose water in the open environment,resulting in poor environmental stability,which seriously affects its application scope.In addition,most of current-reported hydrogel sensors lack adhesion,and need additional adhesives or tape to fix the sensor on the surface of the measured object,which not only affects the beauty and comfort,but also reduces the sensitivity and stability of the sensor.Based on the above analysis,on the basis of previous studies,in this paper,a series of conductive hydrogels with external transparency,excellent mechanical properties,adhesion properties,antifreeze,and dehydration resistance based on the ion interaction between polymer networks were prepared.The relationship between the composition of hydrogels and their performance were investigated,and the application prospect of hydrogels as strain sensors were also demonstrated.Firstly,PAHS-LiCl nanocomposite hydrogel was successfully prepared using Al₂O₃NPs as crosslinking agent,AA,HEMA and zwitterionic SBMA as monomers,and LiCl as conductive carrier.The prepared hydrogels possess excellent optical transparency,and the transmittance at 550 nm higher than 85%.Based on various non covalent interactions between Al₂O₃NPs and polymer matrix,the hydrogels reveal excellent mechanical properties.In addition,due to the various functional groups on the polymer chain,the PAHS-LiCl gels display strong adhesion to different substrates（such as glass,rubber,skin,etc.）.In addition,the introduction of LiCl into PAHS gel not only provides ionic conductivity（conductivity up to 2.25 S/m）,but also improves the freezing resistance of hydrogel,endow them good stablility at-18℃.The flexible sensor based on PAHS-LiCl gel has high sensitivity（GF=2.69）,fast response（response time 275 ms）and signal stability,which can be directly attached to human skin for monitoring various human movements.In order to overcome the dehydration and freezing drawbacks of hydrogel,the introduction of ionic liquids[EMIm][DCA]with low vapor pressure,non-volatile and thermal stability into the PAHS gel system was explored,and PAHS-IL gels with anti-dehydration and antifreeze properties were successfully prepared.It is found that the addition of[EMIm][DCA]not only improved the elastic modulus and adhesive strength of the hydrogel,and but also significantly enhanced the freezing resistance and anti-dehydration capacity.The resules form DSC test showed that the freezing temperature of the gel was below-80℃,and the hydrogel could maintain 88.1%of the residual mass after being placed at 60℃for 24 hours.The excellent environmental stability widens the operating temperature range of PAHS-IL gel sensor,making it suitable for applications under different environments.In order to avoid the high cost and toxicity of ionic liquids,we replaced it with a low eutectic solvent（DES）consisting of choline chloride and urea,and prepared PAHS-DES gels with high transparency,high tensile property,excellent adhesion and excellent environmental stability.The strong solubility of DES enables each component in the gel system to be well dissolved and dispersed,improving the transparency of gel（the transmittance over 92%）.At the same time,the plasticizing effect of DES makes PAHS-DES gel with excellent mechanical flexibility,and the elongation can reach 1005%.The adhesion of gel to various substrates can also be achieved by hydrogen bonding and electrostatic interaction.In addition,the addition of DES makes the gel have good heat resistance and freezing resistance,and the anion and cation in choline chloride provide it with conductivity.Therefore,green,non-toxic and inexpensive DES is expected to become a solvent for green hydrogels.