## 4 BioHEAs
### Coatings  
Coated biomaterials, utilized extensively in various medical applications, should ideally exhibit a combination of specific properties to ensure their effectiveness and biocompatibility. Firstly, these coatings must possess superior biocompatibility to prevent adverse reactions when in contact with living tissues or bodily fluids. Moreover, they should encourage cellular adhesion and growth, facilitating tissue integration and regeneration. Additionally, the coatings must exhibit suitable mechanical properties, providing the necessary strength and flexibility to withstand physiological forces without compromising the structural integrity of the biomaterial. They should also be resistant to degradation, ensuring prolonged functionality within the body. Finally, the surface of the coated biomaterial should ideally be designed to minimize bacterial adhesion and biofilm formation, thereby reducing the risk of infections and promoting long-term implant success. These properties are depicted in Figure 3. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Alloy Studied** & **Microstructure** & **Mechanical Properties** & **Resistance to Corrosion** & **Reference** \\ \hline Ti\({}_{20}\)Zr\({}_{20}\)Hf\({}_{20}\)Nb\({}_{20}\)Ta\({}_{20}\) & Single BCC solid solution & HV—320 & Corrosion rate—5.5 \(\times\) 10\({}^{-4}\) & \\  & & E–79 GPa & mm/year (Hank’s solution) & \\ \hline Ti\({}_{25}\)Zr\({}_{25}\)Hf\({}_{25}\)Nb\({}_{12.5}\)Ta\({}_{12.5}\) & & HV—293 & Corrosion rate—8.8 \(\times\) 10\({}^{-4}\)mm/year & \\  & & E–68 GPa & (Hank’s solution) & \\ \hline Ti\({}_{27.78}\)Zr\({}_{27.78}\)Hf\({}_{27.78}\)Nb\({}_{8.33}\)Ta\({}_{8.33}\) & & HV—287 & Corrosion rate—9.3 \(\times\) 10\({}^{-4}\)mm/year & \\  & & E–56 GPa & (Hank’s solution) & \\ \hline  & & & RP—251.9 \(\mathrm{R}\mathrm{C}\mathrm{m}^{2}\) (saliva pH = 3.83) & \\  & & bi-phase of two & RP—51.4 \(\mathrm{R}\mathrm{C}\mathrm{m}^{2}\) (saliva pH = 7.84) & \\ CoCrNbMoZr & hexagonally Co-based & E—107.88 GPa & R—163.3 \(\mathrm{R}\mathrm{C}\mathrm{m}^{2}\) (saliva pH = 9.11) & \\  & & phases & RP—206.5 \(\mathrm{k}\mathrm{\Omega}\mathrm{c}\mathrm{m}^{2}\) (saliva + 0.05 M & \\  & & & & NaF pH = 8.21) & \\ \hline  & & & Ion concentration after 28 days & \\  & & & immersion in fetal bovine serum & \\  & & & (FBS): & \\  & & & Ti—309.32 ppb & \\  & & & Ta—1.14 ppb & \\ TTaHfNb & & n/a & Nb—0.87 ppb & \\  & & & Total ion concentration after 28 days & \\  & & & & of immersion in: \\  & & & & -artificial saliva (AS): approx. 70 ppb & \\  & & & & –in simulated body fluid (SBF): & \\  & & & approx. 20 ppb & \\ \hline  & & & Ion concentration after 28 days of & \\  & & & immersion in FBS: & \\  & & & Ti—347.24 ppb & \\  & & & Ta—9.34 ppb & \\  & & & Nb—29.86 ppb & \\  & & & Zr—10.76 ppb & \\  & & & Total ion concentration after 28 days & \\  & & & of immersion in: & \\  & & & \(-\)AS: approx. 70 ppb & \\  & & & \(-\)SBF: approx. 15 ppb & \\ \hline  & & & Ion concentration after 28 days of & \\  & & & immersion in FBS: & \\  & & & Ti—309.32 ppb & \\  & & & Ta—9.45 ppb & \\  & & & Mo–162.4 ppb & \\  & & & Zr—4.39 ppb & \\  & & & Total ion concentration after 28 days & \\  & & & of immersion in: & \\  & & & \(-\)AS: approx. 20 ppb & \\  & & & \(-\)SBF: approx. 70 ppb & \\ \hline Ti\({}_{1.4}\)Nb\({}_{0.6}\)Ta\({}_{0.6}\)Zr\({}_{1.4}\)Mo\({}_{0.6}\) & BCC solid solution & E—140 GPa & n/a & [39] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._The passive oxide film formed naturally on some alloys helps to increase the resistance to corrosion and acts as a barrier for the release of cytotoxic metal ions. However, due to wear, local areas may appear where the substrate is exposed. The galvanic coupling then formed between the passivated and depassivated zone causes the acceleration of corrosion. This may also happen between a coating and the substrate; therefore, it is highly important to develop coatings with superior adhesion [26]. Research on corrosion is thus conducted using complex environments that would mimic those found in the human body.