Strain-Stiffening and Tough Hydrogels via Hydrophobic Folding and Hydrophilic Expansion: A Topological Design with Amphiphilic Comb-Like Networks

Tough and strain-stiffening hydrogels are highly desirable for applications such as tissue engineering, artificial skin, and soft robotics. Here, we develop a class of such hydrogels through the topological design of amphiphilic comb-like polymer networks. Using living ring-opening metathesis polymerization, we synthesize networks by copolymerizing a hydrophilic macromonomer with a hydrophobic comonomer, enabling independent control over the hydrophilic side-chain length and the grafting spacing of hydrophobic units. In aqueous media, the competition between hydrophobic association (driving backbone folding) and hydrophilic side-chain expansion leads to microphase separation, yielding a lamellar morphology, where hydrophilic layers are confined between hydrophobic layers. The resulting hydrogels exhibit high toughness and strain-stiffening behavior with mechanical properties comparable to those of soft biological tissues. By accounting for the “double dilution” effect arising from side-chain dilution and network swelling, we demonstrate that the mechanical response of the grafting-spacing series is governed primarily by network swelling, whereas the side-chain length series is influenced by both dilution mechanisms, leading to master stress–strain curves. Furthermore, in situ small-angle X-ray scattering (SAXS) reveals that the strain-stiffening behavior originates from the rapid decay of long-range order within the phase-separated lamellar structure, an energy-dissipating process driven by the unfolding of the backbone. This work establishes clear molecular design principles for tailoring hydrogel mechanics via a macromolecular architecture, offering a versatile platform for the development of biomimetic soft materials.