Eggshell waste derived nano-hydroxyapatite/metakaolin composites for bone scaffold applications

This study presents a novel, zero-waste wet chemical precipitation method for synthesizing eggshell-derived nano-hydroxyapatite (HA) reinforced with metakaolin (MK) for potential biomedical applications. Calcium oxide obtained from calcined eggshells served as the calcium source, and MK was incorporated at varying weight ratios (25-100% of HA mass) prior to cold pressing. The composites were characterized using XRD, FTIR, SEM-EDX, and BET analyses to assess phase composition, functional groups, microstructure, and surface area. Results confirmed successful HA formation with rod-like nanocrystals, while increasing MK content reduced crystallinity and crystallite size (37.3-21.1 nm) due to aluminosilicate incorporation. Cold pressing decreased porosity (63-55%), increased bulk density (1.57-1.66 g cm^-3), and improved mechanical performance, with compressive strength and diametral tensile strength reaching 16.4 MPa and 11.8 MPa, respectively, within the range of cancellous bone. In vitro bioactivity tests revealed progressive apatite layer formation over 21 days, while antimicrobial assays showed broad-spectrum inhibition against Gram-positive, Gram-negative, and fungal strains. MTT cytotoxicity assays using MC3T3-E1 pre-osteoblasts demonstrated high cell viability (>95%) at ≤12.5 µg mL^-1, indicating good biocompatibility. The developed HA/MK composites exhibited promising mechanical, bioactive, and antimicrobial properties, supporting their potential use in bone tissue engineering scaffolds.