High-strength and self-recoverable silk fibroin cryogels with anisotropic swelling and mechanical properties.

Creating mechanically strong macroporous hydrogels with anisotropic properties as observed in many biological tissues is a major challenge in the gel science. Here we describe a directional freezing/cryogelation method of producing high-strength and rapid self-recoverable silk fibroin scaffolds with a high degree of mechanical anisotropy similar to that of tendon. By adjusting the synthesis parameters, we were able to create fibroin scaffolds exhibiting the highest modulus anisotropy so far reported, 21 ± 5, with moduli E = 2.3 ± 0.5 and 0.11 ± 0.03 MPa measured along parallel and perpendicular to the freezing direction, respectively. The cryogels are squeezable under load whereas, upon unloading, the squeezed-out water is taken back immediately. It was shown that the squeezability of the cryogels results in significant viscous stresses and energy dissipation. Cyclic mechanical tests reveal that the friction between the fibroin pore walls is the primary factor responsible for the energy dissipation. Independent on the fibroin concentration or direction of the measurements, 60% of the mechanical energy given to the cryogels are dissipated due to the friction between the pore walls, which is responsible for their almost complete squeezability and self-recoverability.