The Unusual Mechanical Evolution Of Biodegradable Double Hydrogen Bonding Strengthened Hydrogels In Response To PH Change

In this work, pH-responsive and biodegradable high strength hydrogels were prepared by photopolymerization of two hydrogen bonding-containing monomers, 2-vinyl-4,6-diamino-1,3,5-triazine(alkali active H-bonding monomer), 3-acrylamidophenyl boronic acid(acid active H-bonding monomer), oligo(ethylene glycol)methacrylate(OEGMA) and disulfide-containing crosslinker, N,N'-bis(acryloyl)cystamin(BAC). For the conventional biodegradable hydrogels, they end up with quick disintegration and consequently fail to bear external loading after degradation. Although the double hydrogen bonding hydrogels constructed in this study were degraded in the chemical crosslinkers in the presence of reducing agent, such as 1,4-dithio-DL-threitol(DTT) or L-glutathione reduced(GSH), the degraded hydrogel evolved into supramolecular network reinforced by diaminotriazine-diaminotriazine and phenylboronic acid-phenylboronic acid dual H-bonded physical crosslinkings. The degraded hydrogels were able to retain macroscopic integrity and exhibit high tensile and compressive strengths up to MPa levels over a broad pH range from acid to base due to the unique ability to maintain at least one type of hydrogen bonding crosslinkings without chemical crosslinkage. The tensile, compressive strength and Young's moduli of original gels were significantly enhanced with increasing VDT and AAPBA content because of the increased strengthening effect of the double H-bondings of DAT-DAT and PBA-PBA. We envision that this biodegradable double hydrogen bonding strengthened hydrogel may provide a simple and robust approach to fabricate advanced functional high strength soft and wet materials.