Preparation And Performance Of Hydrogels Based On Double Network Toughening Mechanism

As soft and wet materials, hydrogels are substances that are reminiscent of soft biological tissue and have been investigated over the past 30 years as candidate materials for soft tissue engineering scaffolds. Conventional hydrogels are usually weak, brittle and not very stretchable. However, many applications such as soft robotics and cartilage tissue scaffolds require hydrogels to endure significant mechanical loads in aggressive environments. Despite recent progress, developing hydrogels that are both mechanically robust and chemically stable is still a challenge.Traditional double network(DN) hydrogels with two strong asymmetric networks being chemically linked possess extraordinary mechanical strength and toughness.However, classical method still encounters some limitations: the candidate materials for the first network are limited to polyelectrolyte gels; the whole multi-step polymerization process which often involves swelling, diffusion is tedious and time consuming. And the research and development of new DN hydrogels are still at a very early stage.On the basis of traditional double network, we had conducted the three parts of exploration work. In the first investigation, we successfully prepared extremely stretchable,transparent and tough PVA-SA/PAM DN hydrogels by using neutral synthetic polymer-poly(vinyl alcohol)(PVA) as the first network, polyacrylamide(PAM) as the second network and sodium alginate(SA) as a molecular stent. The first semi-interpenetrating polymer network(semi-IPN) was synthesized via a polycondensation of neutral synthetic polymer PVA. It was found that the introduction of SA significantly influenced the swelling property of PVA semi-IPN hydrogel in acrylamide aqueous media and corresponding mechanical properties of DN hydrogels. The hydrogels with nearly 90wt% water had achieved fracture strain of about 2400% and a maximum compression stress of 35.2 MPa, which is 39 times higher than that of PVA-SA semi-IPN hydrogels(0.9MPa). Molecular stent method helps extend the DN gel concept to various functional polymers. In the second investigation, we reported the one-pot synthesis of mechanically tough Agar/PAM DN hydrogels by using the physical-linked agar gel as the first network.The whole progress was achieved via a heating-cooling process that eliminates the uncontrollable swelling process and unnecessary diffusion process. In the third investigation, we reported the preparation of extremely stretchable and tough pH-sensitive hydrogels by introducing a linear polysaccharide-agar into poly(acrylamide-co-acrylic acid)(P(AM-co-AA)) network. Here, we chose hydrophilicity PEG diacrylate(PEGDA) as a chemical crosslinker instead of small molecular crosslinker MBA for the radical polymerization of acrylic monomers. PEGDA crosslinked hydrogels showed an obviousincrease of swelling ratio and stretchability. Also, Agar/P(AM-co-AA) DN hydrogels exhibited excellent reversible swelling and recoverable deformation. Agar/P(AM-co-AA)hydrogels could achieve a tensile strength of 182 kPa and dissipated energy of 1.22 MJ m-3,which was superior to that of P(AM-co-AA) hydrogels(tensile strength 51 kPa, dissipated energy 0.27 MJ m-3) under 800% strain. The compression strength and dissipated energy could reach 4.56 MPa and 0.56 MJ m-3under the deformation of 85%, respectively. The improved mechanical property enables the pH-sensitive hydrogels to be potential candidates in the field of load-bearing soft tissues.