A constitutive model of microfiber reinforced anisotropic hydrogels: With applications to wood-based hydrogels

Recent years have witnessed a surging growth in developing anisotropic hydrogels. Particularly, a new type of microfiber-based anisotropic hydrogel has emerged by transforming nature's existing anisotropic soft materials into hydrogels. For example, wood-based hydrogels feature crosslinked networks serving as the matrix with stiffer micro-sized cellulose bundles as the reinforcement. These anisotropic hydrogels resemble the anisotropic microstructure of living organisms and hold promise for broad applications. Despite its promising outlook, well-formulated mechanical models remain unavailable for microfiber-reinforced anisotropic hydrogels. The existing constitutive models are limited to simplified fiber configurations, making them only suitable for anisotropic hydrogels with macro-sized fibers but inadequate to capture complex microfiber distribution. Moreover, in sharp contrast to the nonlinear behavior of cellulose microfibers in wood-based hydrogels, fibers in most existing models are usually linear-elastic. Aiming to address this deficiency, we have established a micromechanical constitutive model suitable for microfiber-reinforced anisotropic hydrogels. Fiber distributions are included in the proposed constitutive model, which makes possible the investigation of various fiber reinforcement configurations. We explore several important anisotropic mechanical behaviors of the microfiber-reinforced hydrogel, including the anisotropic swelling, and anisotropic stress-strain relation in uniaxial tensile loading. More importantly, we apply the present model to analyze the performance of a humidity-sensitive actuator based on a bilayer of wood-based hydrogel and polyimide. The proposed constitutive model may promote theoretical understandings on the mechanical properties of anisotropic hydrogels and anisotropic-hydrogels-based soft machines. (C) 2020 Elsevier Ltd. All rights reserved.