Effect of Fabrication Methods on the Porosity, Microstructure, Strength and In-Vitro Bioactivity of Porous Hydroxyapatite Scaffolds

Abstract

Porous Hydroxyapatite (HA) scaffold has been prepared from stoichiometric HA powder. The HA powder has been prepared in the laboratory by the wet chemical method. The as-prepared powder was amorphous. On calcination at 850oC, stoichiometric HA powder crystallized. The HA phase was stable till 1250oC and Tricalcium phosphate (TCP) formed at 1300oC.
The HA scaffolds were prepared by three different routes. In the Solid State Fugitive route, porous HA scaffolds were prepared by mixing HA powder and Naphthalene (NA) granules. Five different volume percent (30, 40, 50, 60 and 70) NA was used. At a lower NA content, mostly isolated open pore were observed. Large and interconnected pores were seen at 50 and 60 vol% NA addition. The strength - porosity variation showed an inverse relation and the strength was low at 60 vol% NA. At 70vol%, NA strength could not be measured due to the fragile nature of the sample. The microstructures of Simulated body Fluid (SBF) aged porous HA scaffold show that apatite formation starts from the surface of large grains.
The scaffolds prepared by Gel Casting Route had only 22% porosity. When NA was additionally added, the porosity and interconnectivity increased. At 45vol% NA addition, the pores were mostly open pores. The interconnectivity increases with the increase in NA content. At 45 vol% NA, the compressive strength was 0.96 MPa. In-vitro bioactivity showed that the apatite growth was more in higher porosity samples. With the increase in aging time, the morphology of the deposited apatite changed from granular to flower like. After 21 days aging, petal-like apatite crystals were observed.
Porous scaffolds were also prepared by Protein Coagulation Casting method using Egg white as the foaming and binding agent. The foaming behavior was modified by varying the Egg White: Water (EWH) ratio (1:1, 1:2, 1:3, and 1:4). The foaming was less at higher water contents. The foaming was also less in pure Egg white sample. It was also noted that at a higher Egg White: Water ratio, foaming was more. At any EWH ratio, the minimum solid loading that could be cast without cracking was nearly ten times the foam height. The cracking of the samples at a lower solid loading was related to the fast foam drainage rate. Use of Poly-vinyl alcohol (PVA) and Nitric acid (HNO3) reduced the drainage rate and reduced the cracking tendency. The in-vitro bioactivity tests showed that the apatite crystals were globular shaped. The porosity development was non-uniform in the Protein Coagulation Casting method.
In the summary, it can be said that the three different methods of scaffold preparation produced different microstructures and pore sizes. The different microstructures resulted in varying compressive strength and bioactivity.