Cork-derived hierarchically porous hydroxyapatite with different stoichiometries for biomedical and environmental applications

Abstract

Hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂) is the major mineral component of human bone, but has a wide range of interesting and useful properties, and many applications beyond biomedicine. Here we produce HA-based materials from a naturally templated precursor which can have a variety of tailored microstructures and hierarchical porosities, and we investigate their use as designed functional materials for several applications. HA-based materials with different stoichiometries of Ca/P = 1.67 (HA) and 1.5 (Ca-deficient HA) were successfully synthesised from a previously prepared and reported cork-derived CaCO₃. The CaCO₃ was reacted with a phosphorus containing precursor solution at 60 °C, and also successively recalcined at 700 °C. The samples prepared at 60 °C maintained the 3D honeycomb structure of ∼20 μm hexagonal cells typical of cork, and showed high specific surface areas (SSA > 85 m² g⁻¹) and a high mesoporosity (average mesopores ∼10–15 nm). These features made them suitable for environmental remediation applications such as heavy metals (i.e. lead) removal from water, with calcium-deficient HA (higher degree of mesoporosity) adsorbing >99% of Pb(II) ions from solutions containing up to 1.5 g of lead per L. Recalcined samples (SSA = 29 m² g⁻¹), on the other hand, had very low cytotoxicity towards MG-63 osteoblasts, showing them to be biocompatible for biomaterials. All had cell viabilities of over 70% against a control, and the calcium-deficient HA proved to be even less cytotoxic than the control (>100% viability after 24 and 48 h). Therefore, cork was shown to be an excellent template agent for the design of HA materials with varied properties, tailored for different applications in the environmental and biomedical fields.