Influence of MgO, SrO, and ZnO Dopants on Electro-Thermal Polarization Behavior and In Vitro Biological Properties of Hydroxyapatite Ceramics

Abstract

The objective of this work was to investigate the influence of trace element (Mg2+, Zn2+, and Sr2+) doping on polarization behavior of sintered hydroxyapatite [HAp, Ca10(PO4)6(OH)2] pertinent to biomedical applications. For this purpose, commercially procured phase pure HAp powder was doped with MgO, SrO, and ZnO dopants in different single, binary, and ternary compositions. All samples were sintered at 1200°C for 2 h and subsequently electro‐thermally polarized via application of an external dc field (2.0 kV/cm) at 400°C. Combined addition of 1 wt% MgO/1 wt% SrO in HAp was found to be the most beneficial in enhancing the polarizability (stored charge ∼4.19 μC/cm2) of pure HAp (stored charge ∼2.23 μC/cm2) by inhibiting high‐temperature HAp phase decomposition. Furthermore, in vitro bone cell–material interaction has been studied for polarized binary doped (1 wt% MgO+1 wt% SrO in HAp) HAp samples by culturing with human fetal osteoblast cells for a maximum of 7 days to establish the significance of dopants on polarized HAp for bone graft applications. Scanning electron microscope images of cell morphology revealed that favorable surface properties and dopants chemistry led to good cellular adherence and extracellular matrix formation on negatively charged surfaces of binary doped HAp samples in comparison with undoped HAp surfaces. MTT assay results at 7 days showed the highest cell proliferation on negatively charged surfaces of binary doped HAp samples.

Nanosized hydroxyapatite and other calcium phosphates: Chemistry of formation and application as drug and gene delivery agents

“…155 As biological entities are predominantly dispersed on the negative side, it comes as no surprise that positively charged HAP promotes good adhesion of cells thereto, whereas poor cellular adhesion and growth was observed on negatively charged HAP surfaces. 156 Extracted biological HAP crystals were thus also shown to comprise a series of discrete and alternating domains of variously charged (in both magnitude and sign) surfaces (Figure 6a), independently of topography, indicating their intrinsic potential for periodic binding of matrix proteins under physiological conditions. In addition to ions in the double layer surrounding the particle, HAP particles in suspension undergo practically all the mechanisms that contribute to the charged surface (that is, adsorption, ionization and selective dissolution), which additionally increases the complexity of this physical effect.…”

Understanding of dopant-induced osteogenesis and angiogenesis in calcium phosphate ceramics

“…Negatively polarized CaP surfaces have been shown to favor osteoblast proliferation and differentiation [96]. Significantly increased stored charge density was achieved in 1 wt.% SrO and 1 wt.% MgO doped HA compared to undoped HA [97]. In phosphate free calcium silicate scaffolds, strontium and zinc substitution demonstrated enhanced expression of alkaline phosphatase, Runx-2, osteopontin, osteocalcin and bone sialoprotein when compared to pure β-TCP scaffolds [98].…”

Influence of pentavalent dopant addition to polarization and bioactivity of hydroxyapatite

“…Several methods have been proposed to manipulate surface properties of HAp to improve osseointegration that is critical particularly for older patients with bone disorders like osteoporosis. Some of those methods include inducing electrical charge on surface through polarization of HAp [6–10], doping with metal cations such as Mg 2+ , Sr 2+ , Zn 2+ , Bi 3+ , La 3+ [8, 11–13], and mixing of HAp with other calcium phosphate ceramics [14]. …”

“…These studies showed that changes in surface properties of HAp improved their structural stability and enhanced biocompatibility. HAp was doped with Mg, Zn, and Sr in single, binary, and ternary combinations to match that of the bone chemistry [8]. A combined doping of 1% Mg and Sr by weight increased the charge storing ability of doped HAp.…”