Towards biomimetic, lattice-based, tendon and ligament metamaterial designs

The engineering of tendon and ligament tissue biocompatible restoration materials constitutes a long-standing engineering challenge, from the chemical, biological and mechanical compatibility analysis and design perspective. Their mechanics are inherently anisotropic, exceeding the potential limits of common, non-architected engineering materials. In the current contribution, the design of advanced material or "metamaterial" architectures that can emulate the mechanical properties observed in native tendon and ligament tissues is analytically, experimentally, and numerically investigated. To that scope, anisotropic metamaterial designs that are based on rectangular cuboid architectures with and without inner body-centered strengthening cores are considered. Thereupon, the metamaterial design specifications required for the approximation of the highly anisotropic tissue performance, namely of the characteristic normal, shear and Poisson's ratio attributes are studied. It is shown that certain strengthened, anisotropic body-centered cuboid lattice architectures allow for substantial effective metamaterial stiffness along the primal tissue loading direction, upon a rather low shear loading resistance. The previous mechanical attributes come along with Poisson's ratio values well above unity and moderate relative density values, furnishing a combination of material characteristics that is highly desirable in restoration praxis. The analytically and numerically guided anisotropic metamaterial performance is experimentally reproduced both for the case of uniaxial and shear loads, using a microfabrication stereolithography additive manufacturing technique. The obtained scanning electron microscopy images highlight the fabrication feasibility of the identified metamaterial architectures, in scales that are directly comparable with the ones reported for the natural tissues, having feature sizes in the range of some 10^ths of micrometers and elastic attributes within the range of clinical observation.