3D printed triply periodic minimal surfaces calcium phosphate bone substitute: The effect of porosity design on mechanical properties

Triply periodic minimal surfaces (TPMSs) are relevant structures for building synthetic bone grafts due to their tortuosity and interconnected pores. In order to investigate their porosity and mechanical properties, three different hydroxyapatite-based TPMSs were printed by vat polymerization: the Schwartz diamond, Schwartz primitive, and gyroid scaffolds. Each structure was designed with three levels of porosity, two different pore sizes, and two different wall thicknesses so as to gain an understanding of the effect of pore size and wall thickness on the mechanical properties. The results highlighted the importance of cleaning in the manufacturing process and its impact on the final porosity, especially for small pore sizes. This article intends to be among the first to discuss mechanical testing with macro spherical indentation. Bending and macro spherical indentation resistance tests revealed differences in the mechanical properties between the different structures, with a strong sensitivity of bending strength to the presence of cracks after thermal treatment. Notably, increasing the wall thickness was shown to increase the risk of damage to the solid parts of the scaffolds, therefore lowering the bending strength.