To address this limitation, researchers have explored strategies to impart nonlinear elasticity to hydrogels by incorporating stiff components. Hydroxyapatite, a naturally rigid biomineral, is the primary component of human bones and is responsible for its exceptional mechanical strength including an elastic modulus of approximately 20 GPa. Although homogeneous biomineralization of hydroxyapatite into hydrogels enhances their stiffness, the resulting composites generally remain confined to the regime of linear elasticity.
In this study, we present a hydrogel composite that exhibits nonlinear elastic behavior through the formation of anisotropic hydroxyapatite patterns within the hydrogel matrix. The resulting pattern consists of hydroxyapatite-hydrogel-hydroxyapatite-hydrogel sequence extending inward from the outer surface. Supersaturation-mediated biomineralization enables localized nucleation within the hydrogel interior, spatially separated from the continuous mineral deposition occurring at the surface. The independent nucleation layer functions as an ion sink through the Ostwald ripening effect, facilitating anisotropic pattern development.
The anisotropic pattern significantly alters the mechanical response of hydrogels. Homogeneously biomineralized hydrogels exhibit linear elastic behavior, with the storage modulus increasing proportionally to the degree of mineralization. In contrast, the anisotropically patterned hydrogel displays nonlinear elasticity, characterized by a linear and moderate storage modulus increase at low deformation, and nonlinear and rapid increase at higher deformation levels.
The nonlinear elasticity enabled by the anisotropic pattern can be interpreted using a spring–dashpot viscoelastic model, where the dashpot resists loading and the spring provides elastic recovery. In the context of the anisotropically patterned hydrogel, the stiff hydroxyapatite domains function as the dashpot, while the soft hydrogel matrix behaves like the spring. Under increasing deformation, the hydrogel domains experience minimal stress, as the hydroxyapatite patterns effectively buffer concentrated mechanical energy without structural failure. As a result, the soft hydrogel regions can consistently maintain their elastic behavior even at high strain levels.
Furthermore, as this nonlinear elasticity arises from the internal anisotropic pattern, all hydrogel configurations, including disc and fiber geometries, exhibit consistent nonlinear elastic behavior. In conclusion, this work represents a significant step toward hydrogel-based materials for advanced biomedical applications.