Biomimetic anisotropic poly(vinyl alcohol) hydrogels with significantly enhanced mechanical properties by freezing–thawing under drawing

Biological hydrogels usually possess anisotropic structures to achieve maximum functionalization at a low material cost. Unfortunately, most conventional synthetic hydrogels are intrinsically isotropic in microstructures and hence in properties. Here, we report a freezing–thawing under drawing (FTD) method to prepare poly(vinyl alcohol) (PVA) hydrogels with an anisotropic microstructure and significantly enhanced mechanical properties. Drawing of an as-prepared PVA hydrogel leads to the orientation of PVA crystallites and PVA chains, which enables the formation of more and even cooperative hydrogen bonding via freezing under drawing. The PVA hydrogels prepared with the FTD method are much stronger than the corresponding PVA hydrogels prepared with a common freezing–thawing (FT) method. For instance, after only one cycle of FTD at a drawing ratio of 100%, the tensile strength of the PVA hydrogel is dramatically increased, about 3.6 times that of the original hydrogel. Both the tensile strength and elastic modulus of the PVA hydrogels increase significantly with the increase of FTD cycles, drawing ratio and PVA content, and they are higher in the direction parallel to the drawing direction than the perpendicular direction, showing significant mechanical anisotropy. For the PVA hydrogel prepared with a PVA content of 16 wt% and via freezing at a drawing ratio of 200%, its tensile strength and elastic modulus in the parallel direction reach 5.00 MPa and 0.50 MPa, respectively. This work provides a simple and convenient strategy to design anisotropic hydrogels with significantly enhanced mechanical properties for potential applications in biomedical materials and soft machinery.