Anisotropic tough poly(2-hydroxyethyl methacrylate) hydrogels fabricated by directional freezing redox polymerization

This work reports a novel method for fabricating anisotropic hydrogels starting from monomers by combining directional freezing and redox polymerization (DFRP), and poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogels with anisotropic porous structures and mechanical properties are obtained. Scanning electron microscopy (SEM) investigations show that the hydrogels have long and wide (up to several tens of micrometers) aligned channels in the direction parallel to the freezing direction, and pores in the perpendicular direction. The sizes of the channels and pores decrease with increasing freezing rate. Tensile, compressive and tearing tests show that the hydrogels (70 wt% water content) show very good mechanical properties, with tensile strength up to 0.44 MPa, compressive strength more than 20 MPa, and fracture energy up to 1000 J m^-2. More importantly, the hydrogels exhibit significant anisotropy in their mechanical properties, which are better in the parallel direction. The hydrogels also show different swelling behaviour in comparison with conventional synthetic hydrogels. The anisotropic tough PHEMA hydrogels may find further application.