Anisotropic contraction of fiber- reinforced hydrogels

Hydrogels reinforced by fibers can undergo remarkable anisotropic contraction triggered by external stimuli, which has a broad appeal for various applications. However, little is known about how to optimize the contraction anisotropy by tuning the microstructures of fiber-reinforced hydrogels. Here, we investigate the underlying mechanisms controlling the anisotropic contraction of fiber-reinforced hydrogels. Using a simplified model incorporating the directional constraint of the fibers, we show that the contraction anisotropy can be substantially enhanced if the hydrogel is prestretched along the fiber direction. We further explicitly model fibers as periodically distributed cylindrical rods in the finite element simulations, and find that the contraction anisotropy can be maximized by varying the transverse fiber-fiber distance; this maximum anisotropy can be improved by reducing the longitudinal fiber-fiber distance and increasing the fiber length. Our study provides insights into designing novel fiber-reinforced hydrogels, suggesting possible applications in soft robotics, tissue engineering and beyond.