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.

近年来，各向异性水凝胶的发展迅猛。特别地，通过将​​自然界现有的各向异性软质材料转变为水凝胶，出现了一种新型的基于微纤维的各向异性水凝胶。例如，木质水凝胶的特征是交联网络充当基质，而较硬的微尺寸纤维素束则作为增强材料。这些各向异性水凝胶类似于生物体的各向异性微观结构，具有广阔的应用前景。尽管其前景令人鼓舞，但对于微纤维增强的各向异性水凝胶而言，仍然无法获得结构良好的力学模型。现有的本构模型仅限于简化的光纤配置，使其仅适用于具有大尺寸纤维的各向异性水凝胶，但不足以捕获复杂的微纤维分布。此外，与纤维素微纤维在木质水凝胶中的非线性行为形成鲜明对比的是，大多数现有模型中的纤维通常是线性弹性的。为了解决这一不足，我们建立了适用于微纤维增强的各向异性水凝胶的微机械本构模型。纤维分布包括在建议的本构模型中，这使得研究各种纤维增强结构成为可能。我们探索了微纤维增强水凝胶的几个重要的各向异性力学行为，包括各向异性溶胀和单轴拉伸载荷中的各向异性应力-应变关系。更重要的是，我们将本模型应用于基于木材水凝胶和聚酰亚胺的双层结构的对湿度敏感的执行器的性能分析。所提出的本构模型可以促进对各向异性水凝胶和基于各向异性水凝胶的软机械的机械性能的理论理解。