An anisotropic constitutive model for 3D printed hydrogel-fiber composites

Fiber-reinforced soft composites with anisotropic mechanical properties exist widely in nature, forming complex shape-morphing structures during the growth or hydration processes. In this paper, we employ the 3D printing method, termed direct ink writing (DIW), to fabricate the hydrogel-fiber composites. It is found that the fiber distribution is controlled by printing parameters, such as printing direction and nozzle size, which results in complicated anisotropic mechanical and swelling responses. We develop an anisotropic constitutive model for the printed hydrogel-fiber composites. Built upon the isotropic hydrogel model, we add a decoupled freeenergy function for fibers to incorporate the three-dimensional distribution of fibers. The model parameters can be determined from the swelling and uniaxial tension tests. We further implement the model for finite element analysis. The results show that the model can describe the stress response of fiber-reinforced hydrogel composites with different degrees of fiber orientations. More importantly, the model can reproduce the observed anisotropic swelling behaviors in the soft composites. The model is employed to design the shape-morphing structures mimicking a saddle-shaped leaf and a twisted orchid petal. The outcome is verified through comparison with the experimental results. The work provides a mechanics tool for 3D printed hydrogel-fiber composites and can promote the broad applications of soft active materials.