Characterizing porosity for 3d printed hydroxyapatite scaffolds of varying density and composition

Abstract

Recent advances in materials science and tissue engineering give the ability to create three-dimensional biocompatible ceramic scaffolds for bone repair. Applications include a variety of bone defects such as non-unions, replacing bone loss from trauma or disease, and maxillofacial reconstruction. An important, yet understudied, aspect of creating artificial bone requires knowledge of porosity. In this study, hydroxyapatite (HA) synthetic bone scaffolds were fabricated through an extrusion-based 3D printing technique called robocasting to characterize three modes of porosity distinguished by a range of pore diameter size: (i) ceramic pores (100nm – 2µm), (ii) intermediate pores (2µm – 70 µm), and (iii) structural pores (>70µm). Highly porous scaffolds were designed with a CAD software and printed with an ‘ink’ composed of a stable suspension of colloidal HA and some contained a fugitive pore former of poly(methyl methacrylate) (PMMA) particles between 20 and 65 microns. Porosity was characterized in two ways: (i) pure HA scaffolds were printed and sintered at 1200°C (highest sintering temperature), 1175°C, and 1150°C and, (ii) composite scaffolds were printed in HA/PMMA volumetric ratios of 95/5, 85/15, 75/25, 60/40, and 50/50 and were each sintered at 1200°C. As intuition would assume, the HA scaffolds showed a decrease in porosity as sintering temperature was increased and the HA/PMMA scaffolds showed an increase in porosity as the fugitive pore former volume fraction was increased. From this work, a method is established for creating a range of ceramic porosities due to sintering temperature, a range of intermediate porosities due to the volume fraction of fugitive pore former, and the ability to control structural porosity through CAD (this was only shown through the two scaffold models used and was not the focus of this study).