Strontium‑Doped Biogenic Hydroxyapatite/Glass‑Ceramic Composites: Synthesis, Structural Characterization, and Enhanced Bioactivity

Background

Biogenic hydroxyapatite (BHA) mirrors the nano‑ to micro‑scale architecture of natural bone mineral and, when combined with bioactive glasses, forms composites that support bone regeneration. Earlier studies have explored various ion substitutions—Fe²⁺/Fe³⁺, Cu²⁺, Ce²⁺, Si⁴⁺, La³⁺—to enhance osteogenic potential [1–14]. Strontium, in particular, has demonstrated dual actions: stimulating osteoblast proliferation while inhibiting osteoclast activity, thus favoring bone formation [15–18]. Clinical interest in Sr‑containing materials is heightened by the prevalence of osteoporosis, a condition marked by decreased bone density [19–21]. This work aims to develop a Sr‑doped BHA/glass composite that balances mechanical integrity with bioactivity for potential clinical application.

Methods

Sample Preparation

Nanostructured BHA was blended with sodium borosilicate glass (46 % SiO₂, 28 % B₂O₃, 26 % Na₂O) in a 50/50 wt ratio. The blend was first sintered at 1100 °C to form a precursor, then milled. SrO (1 wt %) was incorporated, followed by pelletizing (2.5 g, 11 mm diameter) and sintering at 780 °C for 1 h.

Characterization

Phase analysis: XRD (DRON‑3M, Cu Kα, λ = 1.54178 Å). IR spectra: FSM 1202, 4000–400 cm⁻¹. Microstructure: SEM (REM‑106I). Porosity: apparent density and open/closed porosity measured per ASTM C1701 using ethylene saturation. In‑vitro solubility: 0.9 % NaCl (solid/liquid = 1:30) at 36.5 °C for 2, 5, 7 days; mass loss recorded with an OHAUS Pioneer PA214C balance (0.0001 g).

Results and Discussion

Phase Composition

Initial BHA matched the JCPDS 72‑1243 pattern with lattice parameters a = 9.432 Å, c = 6.881 Å (V ≈ 530 ų). The 50/50 wt composite retained the HA phase while introducing Na₂Ca₃Si₃O₁₀, Ca₂SiO₄, Na₄SiO₄, and Na₂BO₂ phases, confirming heterogeneous solidification. SrO addition did not alter the phase assemblage but shifted diffraction peaks toward higher 2θ, indicating lattice contraction due to Sr²⁺ substitution for Ca²⁺ (Sr²⁺ has a larger scattering factor).

XRD of pristine BHA

XRD of BHA/glass composite

XRD of BHA/glass‑Sr composite

Microstructure and Porosity

SEM revealed a matrix‑type structure: glass formed a continuous framework embedding HA crystals. Porosity analysis showed total porosity of the Sr‑doped composite reaching ~61 %, nearly double that of the undoped material (~30 %). Open porosity increased from 6 % to 10 %, while closed porosity decreased accordingly (Figure 6). The high porosity aligns with natural cancellous bone (55–70 %) and is expected to enhance cell infiltration and vascularization.

Porosity distribution for BHA/glass and BHA/glass‑Sr composites

In‑vitro Dissolution

After 2 days, the Sr‑doped composite exhibited a dissolution rate of 0.19 % mass/day, 2–4× higher than the undoped counterpart. The enhanced solubility correlates with increased open porosity and a slightly contracted HA lattice, facilitating ion release (Figure 8).

Mass loss in saline for BHA/glass vs. BHA/glass‑Sr (2, 5, 7 days)

IR Spectroscopy

The composite spectra displayed the characteristic HA bands at ~1040 and ~570 cm⁻¹, with additional absorption due to the glass network. Sr addition broadened and shifted bands in the 700–1050 cm⁻¹ region, indicative of lattice perturbation.

IR spectra of BHA/glass and BHA/glass‑Sr composites

Conclusions

We successfully fabricated Sr‑doped BHA/sodium borosilicate glass composites (50/50 wt, 1 wt % SrO). XRD confirmed the persistence of the HA phase with lattice contraction upon Sr incorporation. Sr addition markedly increased porosity and in‑vitro dissolution, suggesting superior osteoconductivity. These materials hold promise for clinical bone repair, particularly in osteoporotic contexts where enhanced resorption and new bone formation are desirable.