Anisotropic Hydrogels with a Multiscale Hierarchical Structure Exhibiting High Strength and Toughness for Mimicking Tendons

Owing to their anisotropic and hierarchical structure, tendons exhibit an outstanding mechanical performance despite the low polymer concentration and softness of the constituent materials. Here, we propose a tendon-mimicking, strong, and tough hydrogel with a multiscale hierarchical and anisotropic structure. An isotropic, precursor double-network hydrogel is transformed into an anisotropic hydrogel by stretching, solvent exchange, and subsequent fixation via ionic crosslinking. Solvent exchange induces densification of the stretched polymer network, enhancement of linear alignment of polymer chains, and microphase separation, leading to anisotropic toughening of the hydrogel. The resulting anisotropic hydrogels show high strength and toughness, which vary over a wide range (1.2-3.3 MPa of strength and 4.9-8.8 MJ/m of toughness, respectively), controlled by the degree of pre-stretching. Furthermore, a hierarchical architecture is constructed by braiding the anisotropic hydrogel strands into a rope, resulting in an improved mechanical performance (4.7 MPa of strength in a four-strand hydrogel rope) compared to separated unbraided strands of a hydrogel (2.3 MPa of strength). The higher hierarchical hydrogel cable, prepared by braiding four hydrogel ropes, can withstand a heavy load even up to 13 kg. These results represent that a hierarchical assembly of anisotropic hydrogels exhibits high mechanical performance and a hierarchically anisotropic structure, which are reminiscent of tendons.