Tuning structural and mechanical anisotropy of PVA hydrogels

Hydrogel materials are widely applicable due to their high water content, biocompatibility, and broad tunability. While the overall mechanical performance of hydrogels can be tuned by methods such as altering the recipe, introducing a secondary network, and adding a post-treatment step, anisotropic mechanical properties with specific anisotropic ratios are not readily achievable. Moreover, the relation between anisotropic structures and the resulting difference in mechanical performance along various directions is not yet clearly understood. In this paper, we fabricated poly (vinyl alcohol) hydrogels with tunable anisotropic ratios of Young's moduli by a bidirectional ice-templating method. Then we characterized the morphologies and mechanical properties in different directions to study the correlation between structure and property anisotropy. An analytical model based on Eshelby's equivalence principle and Mori-Tanaka's mean-field theory was established to predict the anisotropic ratios of the effective moduli for the porous hydrogels, which agrees well with the experimental results. This model reveals the key parameters that govern the degree of anisotropy and has the potential of providing guidance for designing micro-structured hydrogels with specified mechanical properties.