Engineering Robust Anisotropic Hydrogels via Fracture-Mechanics-Inspired Dissipation Design for Tendon Repair

Abstract: Tendon injuries create a pressing demand for biomimetic materials capable of replacing damaged tissues. Anisotropic hydrogels are promising candidates, yet their mechanical performance and structural fidelity often fall short of native tendons. Here, we present a theory-inspired design strategy for robust anisotropic hydrogels by distilling essential principles from classical fracture mechanics that emphasize the concurrent enhancement of the energy-dissipation length L and the energy-dissipation density W. We show that enlarging L requires a modulus contrast within the network, whereas increasing W relies on chain densification. Guided by this framework, we employed an orientation/salting-out (O/S) approach to construct anisotropic hydrogels from an alginate (Alg)/polyvinyl alcohol (PVA) system. The selection of this pair is key: semi-rigid, metal-cation-responsive Alg chains retain orientation and form fibrillar domains during O/S treatment, while flexible, anion-sensitive PVA chains undergo salting-out-induced densification into a disordered matrix. This complementary arrangement yields a tendon-like architecture featuring a pronounced modulus contrast that extends L and a dense flexible matrix that enhances W. The resulting hydrogels exhibit exceptional mechanical properties, rivalling state-of-the-art tough hydrogels and biological tissues. Their biocompatible anisotropic structure supports directional cell alignment, and in vivo evaluation in a rabbit model confirms their promise as artificial tendon substitutes.