Strengthening poly(2-hydroxyethyl methacrylate) hydrogels using biochars and hydrophobic aggregations

As the first version of synthetic hydrogel, poly(2-hydroxyethyl methacrylate) hydrogels have found broad applications. However, their poor mechanical performances have been long-standing hurdles for practical deployments. Herein, we report on strengthening the poly(2-hydroxyethyl methacrylate) hydrogels with biochar nanoparticles and hydrophobic aggregations, which are induced by solvent exchange and reinforced by freeze-thaw. Both the vast anchoring points on the multifunctional surfaces of the biochar nanoparticles and the aggregates formed between poly(2-hydroxyethyl methacrylate) chains engender strong and dissipative physical crosslinks. The resulting hydrogels exhibit marked mechanical properties, encompassing high stretchability ~7, high fracture toughness ~1360 J m⁻², high elastic modulus ~180 kPa, low friction coefficient ~0.2, self-recovery, and non-swellability. Furthermore, we demonstrate the versatility of the proposed strategy by using water/ionic liquid binary solvent as the solvent system, Laponite as the nano-reinforcement, and dry-anneal as the hydrophobic aggregation enhancer to synthesize mechanically robust hydrogels. Poly(2-hydroxyethyl methacrylate) hydrogels of superior mechanical properties are expected to enable previously inaccessible applications in biomedicine and engineering.