Abstract
For Hydroxyapatite (HA) bone graft substitutes, the choice of fabrication technique is an important aspect of design. Using HA ceases concerns with regards to biocompatibility, however other facets of design need to be addressed, particularly achieving high porosity and high mechanical properties. These two facets are mutually exclusive, however, if porosity can be achieved in a controlled manner, then it is possible for them to coincide. Honeycomb extrusion is a technique capable of achieving the aforementioned feat, with the added advantage of a high degree of pore interconnectivity. Extrusion is also relatively simple, inexpensive, and can achieve large-sized scaffolds. Thus, it was hypothesised that the aforementioned advantages can be translated to bone tissue engineering.
Dynamic mechanical analysis revealed that ceramic pastes formulated using guar gum and Methocel™ (MC) binders, required a shear modulus of 10-1 and 101 MPa, respectively, for extrusion. A higher solids loading was achieved with MC, by 7.6 vol%, and accordingly was used for further studies. The extrusion setup favoured calcined powders for maximising scaffold compressive strength. Initially, a calcined α-tricalcium (α-TCP) and uncalcined HA scaffolds has compressive strengths of 23.6 ± 5.7 and 29.8 ± 8.6 MPa, respectively. When HA was calcined, the strength soared to 105.9 ± 12.2. Calcined powders were less susceptible to both agglomeration and enhanced densification, which resulted in higher solids loading and a higher thermal stability. The calcined powders all possessed a surface area < 10 m2/g, which was regarded as the ideal value to ensure sufficient binder coating of the ceramic primary particles.
HA was also composited with 10 wt% Bioglass® (BG) and 10 wt% canasite glass (CAN), which have been documented to possess improved biological properties over the ceramic. A maximum compressive strength of 30.3 ± 3.9 and 11.4 ± 3.1 MPa, for BG and CAN, respectively, were achieved, and hence revealed that BG was the stronger compositing glass. The calcium oxide (CaO) content in raw BG was then increased by 5 wt%, which resulted in a maximum strength of 56.7 ± 6.9 MPa. The increased CaO was found to lessen the effect of glass-induced HA phase transformation into the mechanically weaker α-TCP, from a α-TCP:HA ratio of 1.01 to 0.84, as determined by XRD; and thus demonstrating that a 5 wt% change was able to have a profound effect on scaffold strength. The compressive strength values obtained herein are a considerable improvement on traditional and commercially-available products, thereby demonstrating the potential of honeycomb extrusion for fabricating porous scaffolds.
