Fabrication of calcite foam by inverse ceramic foam method

We have previously reported that calcite foam that had interconnected porous structure could be prepared by ceramic foam method and it transformed to carbonate apatite (CO₃Ap). In the ceramic foam method, polyurethane sponge was used as a template. The polyurethane sponge was immersed in the ceramics slurry, and the strut of the polyurethane foam was covered by ceramic powder. After that it was dried and sintered at high temperature. Calcite foams produced by this approach were comprised of a three-dimensional (3D) interconnected porous structure that facilitated cell penetration. However, all foams have a common limitation: the inherent lack of mechanical strength associated with high porosity. Therefore, in this study, an inverse ceramic foam method was studied; multi polyurethane coating method using polyurethane foam as a template. In this study, the compressive strength was improved by an inverse replication allowed for decreasing porosity while at the same time maintaining the interconnectivity. The burnable synthetic resin coating layer was introduced onto struts of polyurethane foam to make the triangular struts become more round and thick, consequently producing large round capillary within the foam structure fulfilling the requirement for osteoblast colonization. In particular, polyurethane foam was dipped orderly into two monomers, followed by centrifugation to remove excess liquids inside foam. After resin curing, a layer of synthetic resin was coated strut of foam. Calcium hydroxide Ca(OH) ₂ slurry was then infiltrated into resin coated-polyurethane foam. By firing at 600°C in O₂-CO₂ stream, polyurethane template was burnt off and Ca(OH)₂ was converted into calcite. Negative replicated calcite foam was fabricated and characterized micro-structurally with interconnectivity and improved mechanical strength. The results obtained in this study suggested that this method dramatically improved the mechanical strength of the calcite foam without sacrificing the interconnected structure, and this means that the calcite foam obtained in this method could be precursors for the 3D interconnected porous CO₃Ap foam.