Design and evaluation of triple-doped nano-hydroxyapatite (Mg/Si/Zn-HAP) as multi-step intelligent scaffold for bone healing

Introduction/Objective Severe bone defects remain a significant clinical challenge due to the limitations of traditional biomaterials. The aim of this study is to design and synthesize a novel triple-doped nano-hydroxyapatite (Mg/Si/Zn-HAP) with a specific formula to act as a multi-stage intelligent scaffold, mimicking the complex time-dependent sequence of natural bone healing. Methods Mg/Si/Zn-HAp (target formula Ca 9 . 70 Mg 0 . 20 Zn 0 . 10 (SiO4) 0 . 20 (PO 4 ) 5 .7 0 (OH) 0 ) was synthesized and precipitated via the control method. The material was characterized using XRD, FT-IR and AFM. Ion release kinetics (Ca 2+ , Mg 2+ , Zn 2+ , Si 4+ ) were studied in simulated body fluid (SBF) over 28 days including the effect of pH (5.5-8.0). Biocompatibility was evaluated via MTT assay on MG-63 osteoblastic cells, and S. Antimicrobial efficacy was tested against E. aureus and E. coli. A prototype 3D porous scaffold was prepared and its mechanical properties were evaluated. Results XRD and FT-IR confirmed a pure, nano-sized (≈17 nm) hydroxyapatite phase with successful multi-ion replacement. The material showed a sustained, biphasic ion release profile in SBF. A pH-responsive "smart release" was observed, where zinc release increased by 37.5% in an acidic environment (pH 5.5) compared to physiological pH. The material showed excellent biocompatibility (cell viability >95% at ≤1 mg/ml) with 35% higher IC50 (3.8 ± 0.2 mg/ml) than pure HAp. This S. showed significant antimicrobial activity against E. aureus and E. coli with MIC values of 1.5 mg/mL and 2.0 mg/mL, respectively. The fabricated 3D scaffold had an interconnected porous structure (65 ± 3% porosity) and a compressive strength of 18 ± 2 MPa. Discussion The triple-doping strategy (Mg, Si, Zn) provides a synergistic effect, which leads to non-linear improvement of the material properties. Controlled, pH-sensitive ion release, particularly enhanced Zn 2+ release under acidic conditions, provides the potential for intrinsic antimicrobial protection during the inflammatory phase, followed by sustained release of osteogenic ions (Mg, Si) to support subsequent healing phases. Conclusion The designed Mg/Si/Zn-HAp nanomaterial successfully integrates multi-step osteogenic stimulation, infection resistance, and manufacturability into a smart, responsive scaffold. This integrated approach presents a promising strategy for developing next-generation biomaterials to treat severe and infected bone defects more effectively and safely.