Mechanical and cytotoxic properties of zinc-substituted hydroxyapatite bioceramic derived from eggshells

Eggshells are a rich source of calcium and can be easily converted into biogenic hydroxyapatite for biomedical applications. This research aims to to fabricate hydroxyapatite (HA-Es) and 5 mol% zinc-substituted hydroxyapatite (5ZnHA-Es) dense bioceramic using initial powders synthesized from waste eggshells. The as-prepared powders were presureless sintered at 1250°C and after that their mechanical as well as cytotoxic properties were evaluated. TEM micrographs of the starting powders showed that HA-Es and 5ZnHA-Es particles have nano-rod shape and were highly agglomerated. For densification study, it was revealed that 5ZnHA-Es exhibited higher fracture toughness compared to zinc-free sample with 1.55 ± 0.06 MPa.m^1/2. This enhancement of the fracture toughness was attributed to the presence of minor β-TCP phase when 5 mol% zinc ion was incorporated into HA, combined with enhanced densification at 1250°C. Cytotoxicity analysis through MTT assay analysis indicated that both HA-Es and 5ZnHA-Es were non-toxic. Nevertheless, the 5ZnHA-Es specimen showed more than 90% of cell viability after 24 h exposure in the MTT assay solution, compared to only 81% for HA-Es sample. This finding inferred that the presence of Zn ion may have promoted cells proliferation more effectively than HA-Es. This work had provided an insight on the feasibility of using natural calcium extracted from waste eggshells as starting material in the preparation of Zn-substituted HA, suitable for biomedical applications.