Anisotropic Swelling-Induced Deformation of Responsive Hydrogels Incorporating Non-Uniform Fibers

The functionality of anisotropic natural materials has motivated researchers to develop anisotropic biomimetic materials, such as fiber-reinforced responsive hydrogels. Thus, several models have been proposed to investigate the anisotropic swelling of fiber-reinforced hydrogels. There are many studies that have investigated the non-uniform deformation of hydrogels resulting from the non-uniform distribution of uniform fibers, while shape-morphing structures can also be obtained by incorporating non-uniform fibers. To investigate the swelling-induced deformation of responsive hydrogels caused by incorporating non-uniform fibers in the hydrogels, a different approach, rather than equivalent free energy formulations, is used in this paper. For this purpose, the fiber is modeled explicitly, featuring stiff and soft parts, inspired by the structure of cellulose nanofibrils. A representative volume element (RVE) is used as a suitable approach for demonstrating the local effects of the fiber on swelling, and periodic boundary conditions are applied to ensure that the fibers are long enough and arranged periodically within the hydrogel. It is shown that the non-uniform fiber results in a swelling gradient; thus, swelling-induced deformation is not uniform along the fiber as well as perpendicular to it. Consequently, the sections perpendicular to the fiber are curved, particularly those close to the borders of the stiff and soft parts. Non-uniform deformation of the hydrogel caused by the swelling gradient is essential in developing complex geometries. The influence of different properties of the non-uniform fiber on the deformation of the RVE and the RVE’s surfaces, a significant feature in thin-layered structures, is also studied. Furthermore, the potential application of non-uniform fibers for fabricating integrated hydrogel-based shape-morphing structures is demonstrated, showing that when the stiffness ratio of the hard part to the soft part of the fiber is [Formula: see text], controllable shape-morphing devices for a vast range of actuators are provided.