Bioinspired hierarchical-gradient designs for tendon-to-bone interfaces: Continuous and discontinuous fatigue behavior

Mimicking the natural tendon-to-bone interface poses a significant challenge due to its complex hierarchical and transitional properties. In this study, novel bioinspired hierarchical-gradient mechanical metastructures were designed and fabricated using 3D printing to address these challenges. By rationally integrating intermediate hybrid unit cells, the designs successfully reproduce a transition in Poisson’s ratio, spanning from negative values in tendon to positive values in bone. Experimental and computational analyses demonstrate that the optimal architecture significantly enhances static mechanical properties. Specifically, it exhibited a 106.1 % increase in Young’s modulus and a 28.4 % improvement in ultimate stress compared to the conventional design. These enhancements are primarily driven by the mitigation of localized stress concentrations, yielding a strain-distribution that more closely resembles the mechanical transition of the native tendon-to-bone interface. Under continuous fatigue loading, the bioinspired regulation of local auxeticity extended the fatigue life by 132.9 %. Furthermore, discontinuous fatigue assessments revealed critical insights into the failure mechanisms of viscoelastic metastructures. While short resting intervals had minimal impact on the number of cycles to failure, prolonged intervals significantly accelerated mechanical degradation. Ultimately, this study establishes a bioinspired mechanical design framework for developing mechanically resilient tendon-to-bone metastructures, providing a basis for future scaffold development and orthopedic applications.