## 3 Results
### Doping of HA Nanoparticles  
Doping of hydroxyapatite (HA) nanoparticles is a critical strategy to overcome its inherent limitations, enhance its properties, and improve its integration with polymer matrices and other biomaterials. This approach increases dispersion stability in various media and optimizes the interface between nanoparticles and surrounding materials, particularly in composite systems [42]. Recent advancements in HA composite research have focused on surface modification with polymers to leverage HA's bioactivity while addressing its mechanical and functional shortcomings [43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75].  
Various organic molecules and polymers have been grafted onto HA nanoparticles to enhance their performance. Examples include silane coupling agents, stearic acid, titanate coupling agents, chitosan, RGD-containing peptides, poly(N-isopropylacrylamide), and poly(\(\gamma\)-benzyl-l-glutamic acid) [43]. These surface modifications not only improve the chemical compatibility of HA in composite systems but also enhance its biological stimulation properties. Furthermore, the incorporation of trace amounts of ions, typically found in natural bone minerals, into HA has gained considerable attention as an effective strategy to address its limitations and improve its biofunctionality [44].  
Metal ion doping, in particular, has emerged as a transformative approach to enhance HA's properties. Ions such as silver (Ag\({}^{+}\)), zinc (Zn\({}^{2+}\)), magnesium (Mg\({}^{2+}\)), and strontium (Sr\({}^{2+}\)) significantly influence bone regeneration by supporting bone formation and contributing to metabolic processes during healing. Additionally, anions like chloride (Cl\({}^{-}\)), fluoride (F\({}^{-}\)), and carbonate (CO\({}_{3}\)\({}^{2-}\)) play essential roles in the physiological function of hard tissues, affecting HA's crystallinity, mechanical strength, degradation, and bioactivity [45]. For example, Sr\({}^{2+}\) enhances osteoblast differentiation and bone mineralization, while Ag\({}^{+}\) offers antimicrobial properties to reduce the risk of infection around implants. These modifications allow HA to mimic natural bone more closely, improving its integration and functionality within the physiological environment [11].  
Notable effects of doping of the chosen metals and non-metals on hydroxyapatite are shown in Table 2.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Method**} & **Process** & \multirow{2}{*}{**Cost**} & \multirow{2}{*}{**Size (nm)**} & \multirow{2}{*}{**Purity**} & **Doping** & \multirow{2}{*}{**References**} \\  & **Complexity** & & & & & \\ \hline Microemulsion & & & & & & [PERSON] et al. [30] \\ processing & Medium & Medium & 24–70 & High & High & [PERSON] et al. (2011) \\  & & & & & & [31] \\ \hline Sol-Gel & Low & Low & 28–60 & High & High & [PERSON] (2014) [35] \\  & & & & & & [PERSON] (2013) [36] \\  & & & & & [PERSON] (2019) [37] \\ \hline Microwave & & & & & & [PERSON] (2017) [40] \\ Assisted & Low & Low & 11–32 & High & High & [PERSON] (2014) [41] \\ \hline \hline \end{tabular}
\end{table}
Table 1: _Cont._