Strain-stiffening in photo-clickable bottlebrush hydrogels via crosslink clustering

Biological tissues, extracellular matrices, and biopolymers rely on nonlinear elasticity such as strain-stiffening to maintain integrity under large deformation. By contrast, synthetic polymer hydrogels with flexible linear chains remain linearly elastic under strains exerted by cells. Here, we report photo-clickable bottlebrush hydrogels that recapitulate strain-stiffening behavior characteristic of natural semiflexible biopolymers. These hydrogels are formed by thiol-ene crosslinking of norbornene-functionalized bottlebrush polyethylene glycol (PEG) polymers. We discover a power-law relation between shear modulus (G₀) and onset shear strain of strain-stiffening (γ_c ~ G₀^-1/3) in networks crosslinked by point-like difunctional linkers. Increasing the size and functionality of crosslinkers yields stiffer hydrogels with reduced critical strains. We rationalize this behavior by modeling each network strand as a composite of semiflexible bottlebrush and flexible coil polymers. Experiments show that larger multifunctional crosslinkers generate more elastically effective network strands but also promote clustering of crosslinks within the same molecule, which effectively reduces the coil size and thus lowers critical strain. These results advance fundamental understanding of nonlinear elasticity in synthetic networks and establish bottlebrush hydrogels as a versatile platform for designing tissue-mimicking biomaterials in mechanobiology and tissue engineering.