The Fabrication And Characterization Of Shape Memory Hydrogel Assisted By Hofmeister Effect

Shape memory hydrogels have been extensively studied in the past decades because their exceptionally promising potential in a wide range of applications.The morphology and structure of polymer chains could be regulated and assisted by Hofineister effect.Many studies have been performed on Hofmeister effects on the ductility,strength,and toughness of hydrogels.However,shape memory effect through reversible physical interactions assisted by Hofineister effect has not been reported.Here,we present a shape memory hydrogel assisted by Hofineister effect.The main research of this paper are as follows:(1)We present a gelatin/polyacrylamide double network with thermal-activated and salt-activated shape memory effect.The chemically cross-linked PAM network was formed through the radical polymerization in the presence of gelatin.The thermal-activated is attributed to the reversible triple helices transformation of gelatin under temperature and the salt-activated is attributed to form chain bundling,hydrophobic interaction domains under Hofineister effect.Hydrogels soaking with(NH4)2SO4 solution show an obvious enhancement in terms of the failure strain and G’.The hydrogel can memorize a temporary shape successfully through soaking with(NH4)2SO4 solution or decreasing temperature,and recovers its permanent shape by extracting ions with deionized water or increasing temperature.In particular,the hydrogel exhibits excellent shape fixity ratio(94.4%)and shape recover ratio(89%).The reversible hydrophobic interaction domains endow the hydrogel with a good reproducible.The outstanding shape-fixing capacities of hydrogels make it possible to deform complicated temporary shapes.Moreover,according to Hofineister series implying various ions could induce SME.The presented strategy could enrich the construction as well as application of biopolymers based shape memory hydrogels.To the best of our knowledge,our system is the first investigation on SMHs assisted by Hofmeister effect.(2)In order to further improve the mechanical properties of hydrogels,gelatin/P(acrylamide-co-acrylic acid)(gelatin/P(AM-co-AAc))shape memory hydrogels were prepared by replacing part of acrylamide in gelatin/polyacrylamide hydrogels with acrylic acid.Gelatin/P(AM-co-AAc)hydrogels was prepared with adjustable mechanical properties while still have thermal/salt-activated shape memory properties.The carboxyl group in gelatin/(AM-co-AAc)hydrogels can be complexed with Fe3+ as a temporary cross-links.After complexed with Fe3+,it could recover to initial state by soaking with EDTA or reducing agent solution.In addition,the carboxyl groups in gelatin/P(AM-co-AAc)hydrogels can be cross-linked by Fe3+ complexation to obtain the cross-linked interpenetrating network hydrogels.The maximum tensile stress of the after cross-linked interpenetrating network hydrogels could reach to 1MPa and did not affect the chain entanglement or hydrophobic interaction assisted by the Hofmeister effect,also,it still exhibited salt-activated shape memory behaviors.We successfully prepared multiresponsive hydrogels with shape memory and adjustable mechanical properties by simply replacing part of acrylamide monomer.