Merging strut-based and minimal surface meta-biomaterials: Decoupling surface area from mechanical properties

The rational design of bone-substituting biomaterials is relatively complex because they should meet a long list of requirements for optimal performance. Meta-biomaterials are micro-architected materials that hold great promise for meeting those requirements as they offer a unique combination of mechanical, mass-transport, and biological properties. There are, however, inherent couplings between the different types of properties of many such materials that make it impossible to simultaneously achieve all the design criteria. An example of such a coupling exists between the mechanical properties and the surface area. Strut-based, metallic meta-biomaterials are known to offer bone-mimicking mechanical properties, but they have limited surface area for cell adherence. Increasing the surface generally results in an undesirable increase in the mechanical properties that could lead to stress shielding. Here, we combine strut-based lattices with minimal surfaces to decouple these two properties. We added minimal surface patches to the designs of both auxetic and non-auxetic meta-biomaterials while minimizing their contribution to the mechanical properties of the resulting meta-biomaterials through the rational application of cuts or "slits". All designs were additively manufactured using selective laser melting and mechanically tested to obtain their quasi-static mechanical properties, including their Poisson's ratio, in two configurations. A finite element-based computational homogenization code was used to compute the elastic moduli and anisotropy of the structures. The results show that the minimal surface patches substantially increase the available surface area without significantly affecting the mechanical properties. Without the slits, the surfaces significantly affected the elastic modulus and deformation behavior of the meta-biomaterials. A similar strategy could be used to tune the biodegradation rate of biodegradable metals and the permeability of metabiomaterials in general.