Anisotropic tough poly(vinyl alcohol) hydrogels

Anisotropic tough hydrogels are of great importance in biomedical fields. Tough poly(vinyl alcohol) (PVA) hydrogels with anisotropic porous structure and mechanical properties are obtained with a facile directional freezing–thawing (DFT) technique. The PVA gels have an aligned porous structure, with long aligned channels in the direction parallel to the freezing direction and pores with similar sizes in the perpendicular direction. The degree of crystallinity of the freeze-dried PVA hydrogels increases with number of DFT cycles, and it can reach 55.8%. The PVA hydrogels have excellent mechanical properties, as exhibited by the high tensile strengths (0.3–1.2 MPa), medium moduli (0.03–0.10 MPa) and high fracture energies (160–420 J m−2) of the gels with solid contents of 10–12%. More importantly, the gels exhibit significant anisotropy in the mechanical properties, and their tensile strengths, moduli and fracture energies are higher in the perpendicular direction than those in the parallel direction. Anisotropic mechanical behaviors can also be found in the cyclic tensile tests of the PVA hydrogels. The anisotropic mechanical properties of the DFT PVA hydrogels could be attributed to the oriented arrangement of crystalline regions along the direction perpendicular to the direction of freezing.