Preparation And Properties Of Conductive Hydrogels Crosslinked With Modified Polyaspartic Acid And Their Application In Flexible Strain Sensors

The electroconductive hydrogels that combine the superior properties of conductive materials and hydrogels have good mechanical properties,and excellent electron transport and ion transport capabilities.It has attracted wide attention as a new type of functional material and has become an ideal material for fabricating flexible sensors.Poly?aspartic acid??PASP?has abundant carboxyl groups and amide groups and shows good biocompatibility and biodegradability even with side chain modifications.It is a very promising biomaterial owing to its protein-like chain structure.Therefore,we mainly developed a simple synthesis method for preparing two kinds of flexible strain sensors based on biodegradable conductive hydrogel combining with the structure and properties of polyaspartic acid and conductive hydrogel.In this paper,polysuccinimide?PSI?was synthesized by acid-catalyzed thermal condensation polymerization using L-aspartic acid as raw material.Then,the PSI was modified by partial ring opening reaction of ethanolamine?EA?and acrylylation of acryloyl chloride?AC?,subsequently hydrolyzing of the remaining succinimidyl units to abtain a biodegradable macromolecular crosslinker with an acryloyl groups side chain?MPASP?.The biodegradable MPASP-PAA/Fe³⁺conductive hydrogels were prepared by free-radical polymerization in an aqueous solution using acrylic acid?AA?as a monomer,biodegradable macromolecular MPASP as covalent cross-linking agent,FeCl₃ as physical cross-linking agent and ammonium persulfate?APS?as initiator.The results showed that the amide bonds and ester bonds introduced by MPASP made the hydrogels have good biocompatibility and biodegradability.The dynamic ionic interaction between the carboxyl group and Fe³⁺ion,and the hydrogen bond between the molecular chains can act as reversible sacrificial bonds to improve the energy dissipation efficiency of the hydrogels by breaking-recombination.The chemical cross-linking network formed by the covalent bond between MPASP and PAA can serve as an elastic support network.Furthermore,the large amount of conductive ions provided by FeCl₃,MPASP and PAA made the hydrogels have good electrical conductivity.These properties achieved the synergistic mechanism of multiple components in the hydrogel material,thus the hydrogels exhibited excellent properties,such as high transparency?96.1%?,good swelling properties?85.84 g/g?,high tensile/compressive strength?150 kPa/1043.5 kPa?,high stretchability?339%?and conductivity(0.1173 S·m^-1).In addition,the hydrogels showed good linear responsiveness and strain sensitivity?4.93 at 250%strain?.It also exhibited high repeatability and good stability in repeated tensile-unloading experiments.We revealed that the MPASP-PAA/Fe³⁺hydrogels can be employed as flexible strain sensors which can instantly monitor large movements of the human body such as fingers,wrists,fists and elbows,and can recognize subtle motions such as vocalization and swallowing.The strain sensors showed rapid response,good repeatability and stability.The biodegradable MPASP-P?AA-co-AM?/GO conductive hydrogels were successfully prepared by free-radical copolymerization of AA and acrylamide?AM?and addition of graphene oxide?GO?aqueous solution.The biodegradable macromolecular MPASP and APS were acted as covalent cross-linking agent and initiator,respectively.The results showed that the addition of GO has a significant effect on the swelling properties and mechanical properties of hydrogels.When the MPASP contents were 2.5 wt%/?AA+AM?,the GO contents were 0.3 wt%/?AA+AM?,the molar ratio of AM and AA were 16:4,and the APS contents were 0.8 mol%/?AA+AM?,the hydrogel showed great swelling properties and mechanical properties.The swelling ratio of hydrogel was 84.96 g/g,the compressive strength was 775.45 kPa,the tensile strength was 293 kPa,and the elongation at break was up to 523%.In addition,MPASP-P?AA-co-AM?/GO hydrogels also exhibited good electro-mechanical properties.The resistance change rate and gauge factor of hydrogels increased linearly with the increase of tensile strain,which showed high sensitivity?3.55at 130%strain?.The hydrogels also exhibited good repeatability and stability,enabling the sensing and detection of corresponding variables.When the hydrogels were employed as flexible strain sensors,it can monitor subtle vocal cord vibrations and detect large human movements such as fingers,wrists,fists and elbows joints bending.