Development of a small- to medium-sized resorbable, bone-regenerating scaffold using additive manufacturing

Abstract

Lithography-based ceramic is a recent additive manufacturing technology for scaffolds with complex geometry and internal structure. This technology opens the possibility of manufacturing custom-made biodegradable bone regeneration scaffolds. All the specimens used in this investigation were made of hydroxyapatite LithaBone HA480 from Lithoz GmbH. This material is an improvement from the previous material HA400 from Lithoz. The manufacturing of HA480 specimens was done on a CeraFab S65 machine with a layer thickness of 25 μm at Lithoz in Vienna, Austria. The other specimens and implant demonstrator were manufactured at the Centre for Rapid Prototyping and Manufacturing (CRPM), South Africa, using the same material and process parameters. The aim of the study was to create a customisable, resorbable three-dimensional porous bone scaffold with controllable degradation and bone regeneration rates by additive manufacturing. Selection of a suitable lattice structure for the study was done through comparing Maxwell’s criteria, porosity and mechanical properties between diamond, rhombic and diagonal lattice structures. The diamond lattice structure was selected because of its Maxwell number indicating a stretch-dominated structure, interconnected pores, and good mechanical properties. The calcium to phosphorus ratio of the HA480 was found to be 1.64±0.21 which was comparable to that of standard hydroxyapatite. Microhardness testing was performed on the solid HA480. Solid HA480 exhibited similar micro-hardness on surfaces parallel (556±25 HV) and perpendicular (559±27 HV) to the build direction. To determine the degradation ability of the material, HA480 diamond lattice structure specimens were immersed in simulated body fluid (SBF) for 1, 7, and 28 days. Biodegradation weight loss results indicated that after 28 days the scaffold degraded by 8.11%. Further investigation regarding biodegradation was done by using scanning electron microscopy (SEM) which revealed evidence of material loss for Day 7 with decreased size of the struts. Surface analysis in the SEM on specimens at Day 28 revealed bone-like apatite, which confirmed that HA480 possessed enhanced bioactivity. Compression tests were performed according to the standard ISO 13175-3:2012. A set of ten HA480 diamond lattice structure specimens were compressed until failure to determine the compressive strength for dry, as well as hydrated specimens immersed in SBF for 1 and 28 days. The dry condition compressive strength was 10.7 MPa, while for the hydrated specimens, the compressive strength was 7.59 MPa after 1 day and 3.04 MPa after 28 days. The modulus of elasticity for HA480 was 0.5±0.4 GPa for the dry specimens which was higher than for cancellous bone. For the hydrated specimens after 1 day and 28 days of biodegradation the modulus of elasticity was 0.4±0.1 GPa and 0.6±0.02 GPa, respectively. Specimens of HA480 were sintered and after 28 days were analysed by Fourier transform infrared spectroscopy. The results were comparable to those of standard hydroxyapatite. The surface roughness of HA480 was measured as Ra = 0.60±0.05 μm which was appropriate for cell attachment. Solid HA480 specimens were seeded with human osteoblast cell line hFOB 1.19 for determining the material’s biocompatibility and cell attachment properties. The fluorescence level results were lower on Day 1, but cell proliferation increased on Day 3, while fluorescence decreased on Days 5 and 7. Optimal cell attachment was observed from Day 1 to Day 7. An implant demonstrator with a diamond lattice structure was designed and printed in HA480 material. The accuracy of the LCM and the biomedical and mechanical properties of the implant demonstrated that the LCM technology can be used to manufacture custom-designed scaffolds for bone regeneration.