Degradable magnesium-hydroxyapatite interpenetrating phase composites processed by current assisted metal infiltration in additive-manufactured porous preforms

Abstract

• Introduction of novel hydroxyapatite-magnesium interpenetrating phase composites. • Produced by ceramic additive manufacturing and liquid metal infiltration. • Magnesium provides mechanical strength in the range of cortical bone. • An osteoconductive hydroxyapatite scaffold is generated after magnesium degradation. • The structural and biological properties are controlled through the digital design. This work explores ceramic additive manufacturing in combination with liquid metal infiltration for the production of degradable interpenetrating phase magnesium/hydroxyapatite (Mg/HA) composites. Material extrusion additive manufacturing was used to produce stoichiometric, and calcium deficient HA preforms with a well-controlled open pore network, allowing the customization of the topological relationship of the composite. Pure Mg and two different Mg alloys were used to infiltrate the preforms by means of an advanced liquid infiltration method inspired by spark plasma sintering, using a novel die design to avoid the structural collapse of the preform. Complete infiltration was achieved in 8 min, including the time for the Mg melting. The short processing time enabled to restrict the decomposition of HA due to the reducing capacity of liquid Mg. The pure Mg-base composites showed compressive yield strength above pure Mg in cast state. Mg alloy-based composites did not show higher strength than the bare alloys due to grain coarsening, but showed similar mechanical properties than other Mg/HA composites that have significantly higher fraction of metallic phase. The composites showed faster degradation rate under simulated body conditions than the bare metallic component due to the formation of galvanic pairs at microstructural level. Mg dissolved preferentially over HA leaving behind a scaffold after a prolonged degradation period. In turn, the fast production of soluble degradation products caused cell metabolic changes after 24 h of culture with not-diluted material extracts. The topological optimization and reduction of the degradation rate are the topics for future research.