Tough, mechanochemically responsive double network hydrogels

Abstract

Despite their wide application range owing to the high biocompatibility, conventional single network (SN) hydrogels always suffer from brittleness and weakness. To address this issue, double network (DN) gels consisting of two different polymer networks have been developed to achieve high mechanical performance. Stimulus responsiveness is another potential target for hydrogel bioapplications. Accordingly, the main aim of this thesis is to uncover some fundamental principles for tailoring the properties of tough and mechanochemically active hybrid DN hydrogels via structural control that are suitable for biomedical applications. Poly(ethylene glycol) (PEG) linked by different bonds and ionically linked sodium alginate were selected as covalent and physical networks, respectively. To understand the structure-property relationships of DN hydrogels with strong covalent bonds, PEG (meth)acrylate hydrogels with varying monomer molecular weights (MW) and architectures (linear vs. 4-arm) with and without alginate were used. Compression testing results showed that while PEG SN hydrogels behaved similarly with varied MW and stronger using 4-arm monomers, alginate reinforced DN gels were stronger and tougher when the PEG network was looser with larger MW and/or linear monomers. When using weak dynamic disulfide bonds, alginate reinforced disulfide networks using 4-arm PEG thiol (PEG4SH) with varied MW and mass fractions were investigated with the goal of achieving tough and stretchable DN hydrogels with a capacity for mechanochemical reactions. Tensile testing results demonstrated that the fracture strain and stress of DN gels benefited from looser PEG networks with lower mass concentrations and larger MW of PEG4SH monomers, while stiffness increased with a higher density of disulfide bonds. Considering the mechanochemical response, thiols produced by disulfide bond rupture were sensed by reaction with fluorophores. DN gels showed increased integrated fluorescence intensities upon stretching, demonstrating the activated response of disulfide bond rupture despite alginate reinforcement. Higher mechanochemical reaction rates were obtained from the most stretchable DN gels with looser PEG networks and less alginate reinforcement. In summary, this thesis presents a comprehensive study on how to design tough and mechanochemically active hydrogels using alginate reinforced covalent networks. These results are expected to aid the development of mechanoresponsive DN hydrogels with controllable properties for biomedical applications.