Inducing Hidden Length in Bottlebrush Hydrogels for Improved Resilience

Biological soft tissues exhibit remarkable resilience by distributing deformation across hierarchically organized collagen scaffolds. Inspired by this mechanism, we investigate the emergence of “hidden length” in single-stranded bottlebrush networks and its role in enhancing extensibility at extreme swelling. The networks consist of poly(2-hydroxyethyl methacrylate) (PHEMA) backbones grafted with super hydrophilic poly(2-methyl-2-oxazoline) (PMOx) side chains, forming microphase-separated domains that act as reversible deformation reservoirs. By systematically varying crosslink density and backbone-to-side-chain volume ratios, we establish quantitative correlations between molecular architecture, swelling behavior, and mechanical response. Selective swelling enhances microphase separation of the PHEMA backbone, promoting strain-induced release of hidden length and uniform stress redistribution throughout the network. The resulting hydrogels combine extreme water content (Q =4-128) with high extensibility (λ = 2-10), approaching the toughness of jellyfish tissue. This study establishes molecular design rules for resilient, tissue-mimetic hydrogels where hierarchical structure governs both swelling and mechanical behavior.