A specific area of emphasis in biomaterial research is on bone regeneration because of the prevalence of bone injuries and diseases.^1,2 Traditionally, the treatment includes the use of autografts, but the donor site morbidity,³ besides the limitations due to a scarcity of usable tissue, severely restricts this mode of therapy. Therefore, synthetic alternatives that mimic the characteristics of natural bone have attracted considerable research interest. Among these, hydroxyapatite (HA), which is a natural mineral form of calcium apatite, has emerged as prominent material due to its biocompatibility and bioactivity.^1,3 HA closely resembles the mineral component of bone, hence it can be considered as an ideal candidate for bone grafting and tissue engineering applications.^4,5

However, pure HA lacks mechanical strength and cannot avoid bacterial colonization⁶ that leads to infections associated with implantation.⁴ In light of such challenges, researchers have examined the doping of HA with various metal ions, among which magnesium is very promising.⁷ Magnesium is one of the most important elements involved in bone metabolism; hence, it takes a vital role in osteogenesis and maintains healthy bones.⁸ Incorporation of Mg in the HA structure enhances not only its mechanical properties but also its biological performance, such as increased osteoblastic activity and enhanced biocompatibility.²

Recent studies have demonstrated that magnesium-doped hydroxyapatite (Mg-HA) nanofibers possess superior properties compared to their undoped counterparts.^4,9 Mg-HA nanofibers can be prepared by different methods, e.g., microwave-assisted hydrothermal treatments, where shape and crystallinity of the resulting nanofibers can be well controlled.⁴ Different characterization methods such as X-ray diffraction (XRD), SEM, and Fourier transform infrared spectroscopy (FTIR) have been employed to analyze successful doping of magnesium into Mg-HA nanofibers and structural features of Mg-HA nanofibers.^1,2,10

Mg-HA nanofibers also possess excellent antimicrobial activity. It has been reported that the addition of magnesium ions enhances not only the antibacterial activity of HA but also reduce the opportunistic pathogens, such as Staphylococcus aureus and Escherichia coli.¹¹ These antimicrobial actions can help inhibit infections that may develop with bone implants, thereby avoiding more severe complications and healthcare costs.⁶ The mechanisms underlying the antimicrobial activity of Mg-HA are believed to involve the release of Mg ions, which lyse bacterial cell membranes and induce oxidative stress.^10,12

Besides their antimicrobial action, Mg-HA nanofibers were found to be highly biocompatible In-vitro.^8,9 Cell viability assays analysis showed that Mg-HA nanofibers enhance the proliferation of fibroblasts and osteoblasts, which makes them promising for bone regeneration.^7,13 Most importantly, cytotoxicity of Mg-HA has been found to be low, which further confirms its potential for application in the biomedical field.^5,14

This review aims to present a full picture of the current research situation in the case of Mg-HA nanofibers,³ such as their synthesis, characterization, biocompatibility, antimicrobial activity, and uses in bone regeneration. With the focus put on the developments in this field, it directs attention towards the significance of Mg-HA composite, as a promising biomaterial for future clinical uses.^15,16

There are several routes to synthesize Mg-HA, and each of these has merits in scalability as well as in material properties. Four of the most commonly used techniques are the Microwave-Assisted Wet Chemical Method, Sol-Gel Method, Hydrothermal Synthesis, and Precipitation Method.

Microwave-Assisted Wet Chemical Method: Among the various reported methods, the microwave-assisted wet chemical approach is frequently cited for its accuracy and efficiency in synthesizing Mg-HA. The process starts with dissolving calcium hydroxide (Ca(OH)₂) in distilled water to produce a homogeneous suspension. Ammonium dihydrogen phosphate (NH₄H₂PO₄) is added gradually at a pH of 10, adjusted by adding ammonium hydroxide. Magnesium is added to the HA structure by adding magnesium oxide (MgO), followed by microwave irradiation that increases nucleation and crystallization processes, reducing reaction times by a significant margin compared to conventional processes. During the synthesis, the precipitate is washed, filtered, dried, and calcinated at 900°C to enhance crystallinity (Figure 1). The major advantages of this method are a rapid reaction time, be energy efficient, have uniform magnesium distribution, and reduce particle agglomeration.¹

Sol-Gel Process: The Sol-Gel process (Figure 2) offers a route to good microstructure and compositional control. Phosphoric acid, calcium nitrate, and magnesium nitrate are dissolved in ethanol to create the precursor solution. This is stirred to create a gel-like structure, which is aged for 24–48 hours. After gelation, the material is low-temperature dried (~100°C) and calcined at 600–900°C to enhance crystallinity and burn out organic residues. This route offers high purity homogeneity, and the ability to control particle size and morphology, and hence is extremely suitable to the production of high-quality Mg-HA.

Hydrothermal Synthesis: Hydrothermal synthesis is the method of synthesis of crystalline nanocomposites at high pressure and temperature. Calcium, magnesium, and phosphate aqueous precursors are combined and charged into a sealed autoclave (Figure 3). The solution is heated at 180–200°C for a few hours, and it is possible to synthesize high-quality material. The product is filtered, washed, and dried after the reaction is over. Hydrothermal synthesis is also renowned for the synthesis of highly crystalline, phase-pure material with few impurities and reproducibility.

Precipitation Method: This method is one of the easiest and cheapest approaches for large-scale production. Calcium and magnesium salts, for example, are calcium nitrate (Ca(NO₃)₂) and magnesium nitrate (Mg(NO₃)₂), which are dissolved in distilled water. Precursors of phosphate are added dropwise while continuously stirring to allow for precipitation. Ammonium hydroxide is used to maintain a pH of 9–10, and the resulting precipitate is washed, dried, and calcined. This method is scalable and relatively straightforward, thus being suitable for mass production of Mg-HA.¹ The method referred in this study is Microwave-Assisted Wet Chemical Method due to its accuracy and efficiency in synthesizing Mg-HA.²⁰

This review was conducted following the PRISMA 2020 (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines to ensure transparency and reproducibility.

This review compiles data from studies that analyzed antibacterial performance of Mg-HA specimens prepared through various synthesis routes. The methodology outlines (Figure 5) the preparation of HA particles for various nanocomposite samples, which are to be applied in the human body. Since the composites are designed for implantation in the human body, antibacterial studies are vital to assess their interactions during the degradation process.^1,4

Reported studies have examined various weight percentages of Mg incorporation to identify optimal compositions.

Although there is substantial evidence to support the antibacterial and osteogenic potential of Mg-HA, there are several factors that cause heterogeneity in the available results. The crystallinity, ion release, and surface area are greatly affected by the differences in the particle size, the level of Mg²⁺ substitution, and the synthesis pathways, thereby influencing the antibacterial and biocompatibility results. Besides this, assay conditions (bacterial strain, culture medium, inoculum density and incubation period) among studies are rather diverse which limits direct comparison of inhibition-zone and MIC data.

Other methodological biases are the usage of small samples, non-standardized antibacterial procedures, or inadequate reporting of statistics. The only reports that use such reference standards as CLSI or ISO antimicrobial testing methods may influence reproducibility.

At the translational level, the Mg-HA systems have to match the antibacterial effectiveness with corrosion control and the release of Mg-ion because too much corrosion might increase the local alkalinity and surpass the cytotoxicity levels. It is exploring using surface coating, polymer encasement or alloying as a method of controlling the kinetics of corrosion. Moreover, the majority of the existing data are based on the in-vitro assessments; there is little in-vivo data available on the long-term osseointegration, inflammatory reactions, and the controlled Mg-ion dissolution. Future research must thus incorporate standardized testing protocol, quantitative corrosion evaluation as well as preclinical in-vivo validation to enhance translational relevance.

Mg-HA nanofibers, which are formed by a variety of procedures like microwave-assisted hydrothermal processing, are an important improvement of biomaterials. The presence of magnesium also improves mechanical and biological characteristics, giving them effective antimicrobial efficiency against most implant-associated pathogens and demonstrated ability to induce bone regeneration. This bi-functionality along with proven biocompatibility and low cytotoxicity, makes Mg-HA nanofibers a very promising alternative to existing orthopedic and dental bone graft material. The next wave of research should be conducted on the clarification of the underlying mechanism of its biological property and its performance in living models to further confirm its clinical potential in bone regeneration and infection prevention.