Fabricating High-strength Shape Memory Hydrogel Based On Hydrophobic Interaction Enhanced Hydrogen Bonding

As a kind of wet and soft materials containing large amounts of water,hydrogels with good biocompatibility and excellent stimuli responsiveness have been widely used in many fields.The traditional synthesized hydrogels are usually brittle and weak,limiting their application.High strength hydrogels with good mechanical properties overcome this shortcoming and show a better practical application prospect consequently.At the same time,shape memory hydrogels which change shapes upon the triggering of external stimuli,have shown the potential as implant materials,micro-robots,etc.In this paper,based on the hydrogen bonding enhanced by hydrophobic interactions,we developed a kind of high-strength,thermally-activated hydrogen-bonded hydrogels using several readily available monomers.The main research contents of this paper are as follows:(1)We prepared a kind of high-strength shape memory hydrogels by copolymerizing methacrylic acid(MAAc),N-vinyl pyrrolidone(NVP)and poly(ethylene glycol)methacrylate(PEGMA)in aqueous solution.The hydrophobic groups of MAAc and NVP effectively enhance the hydrogen bonds.PEGMA was introduced to the system as a temporarily hydrogen bonding shielding monomer to prevent phase separation.As the monomers were polymerized into polymer chains,the shielding effect of PEGMA is weakened,and multi-strength hydrogen-bonding domains enhanced by hydrophobic interactions formed in situ.These strong and large amounts of hydrogen-bonding crosslinks endowed the hydrogels with excellent mechanical properties,the tensile stress of the hydrogels reached up to 3.9 MPa.Besides,based on the temperature sensitivity of hydrogen bonding,the hydrogels also demonstrated a rapid thermally-activated shape memory performance.(2)To further improve the mechanical properties of hydrogen-bonded hydrogels,we synthesized hydrogels by copolymerizing 2-acrylamidopyridine(NP2AM)and methacrylic acid(MAAc),of which the monomers are more hydrophobic and capable of forming multiple intermolecular hydrogen bonds.Since the monomer NP2AM is slightly soluble in water,we firstly polymerized the monomers in DMSO and then dialyzed the gel in water to prepare hydrogels.The obtained hydrogels which were swelling equilibrium(after dialysis)showed excellent mechanical properties,its tensile fracture strength and elastic modulus can reach up to 6.4 MPa and 66 MPa,respectively,and self-recovery and fatigue-resistance abilities are excellent.At the same time,the thermo-sensitivity of the hydrogen bonds also brought the hydrogels a very fast and repeatable thermally-activated shape memory function.