## 5 In Vitro Biological Performance of HEA  
Thus far, most of the tests to evaluate the biological performance of HEAs rely on cell culture experiments by direct cell contact methods, as it is further described. However, although the majority of these tests envisaged the demonstration of the HEA suitability for orthopedic implants, these studies can be expanded to applying these alloys for dental implants since in vitro responses of bone-derived cells have been mainly investigated (Table 3). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Coating Studied** & **Substrate** & **Microstructure** & **Mechanical Properties** & **Resistance to Corrosion** & **Reference** \\ \hline  & & & & & Rp \\  & & & & (Tafel)—78.2 \(\times\) 104 \(\Omega\)cm\({}^{2}\) & \\  & & & H—11.43 GPa & (PBS) & (PBS) \\  & & Amorphous structure & E—180 GPa & Rfilm & \\  & & & Critical & (EIS)—5.10 \(\times\) 104 \(\Omega\)cm\({}^{2}\) & \\  & & Roughness—2.05 nm & load—190 mN & (PBS) & \\  & & & & Rct (EIS)—4.25 \(\times\) 106 \(\Omega\)cm\({}^{2}\) & \\  & & & & (Tafel)—81.8 \(\times\) 104 \(\Omega\)cm\({}^{2}\) & \\  & & & & (Tafel)—83.0 \(\times\) 104 \(\Omega\)cm\({}^{2}\) & \\  & & Amorphous structure & E—185 GPa & Rfilm & \\  & & Roughness—2.11 nm & Critical & (EIS)—6.10 \(\times\) 104 \(\Omega\)cm\({}^{2}\) & \\  & & & load—280 mN & (PBS) & \\  & & & & Rct (EIS)—6.95 \(\times\) 106 \(\Omega\)cm\({}^{2}\) & \\  & & & & (Tafel)—83.0 \(\times\) 104 \(\Omega\)cm\({}^{2}\) & \\  & & & & (Tafel)—83.0 \(\times\) 104 \(\Omega\)cm\({}^{2}\) & \\  & & Amorphous structure & H—11.49 GPa & (PBS) & \\  & & & & E—183 GPa & Rfilm \\  & & Roughness—2.27 nm & Critical & (EIS)—8.43 \(\times\) 104 \(\Omega\)cm\({}^{2}\) & \\  & & & load—400 mN & (PBS) & \\  & & & & Rct (EIS)—1.54 \(\times\) 107 \(\Omega\)cm\({}^{2}\) & \\  & & & & (PBS) & \\ \hline TiTaHfNbZr & Ti6 Al4V & Amorphous structure & H—12.51 GPa & & \\  & & Roughness—2.78 nm & E—181.3 GPa & & n/a & [74] \\ \hline  & & & & Lower Ni ion concentration & \\  & & & & after 28 days of immersion & \\  & & & & both in artificial saliva & \\ TiTaHfNbZr & NiTi & structure & E—183.2 GPa & (pH = 2.3) and gastric fluid & \\  & & & & (pH = 2) for the coated & \\  & & & & samples & \\ \hline TiTaHfNbZr & NiTi & 750 nm coating & H—12.44 GPa & 1.88 ppb Ni after 28 days & \\  & & thickness & E—182.8 GPa & immersion in SBF compared & \\  & & Roughness—4.35 nm & Critical & to 265.55 ppb Ni for & \\  & & & load—158 mN & uncoated alloy & \\ \hline  & & & 1500 nm coating & H—11.82 GPa & \\  & & thickness & E—175.1 GPa & & \\  & & Roughness—4.35 nm & Critical & & n/a & \\  & & & load—204 mN & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Microstructure and properties of the BioHEAs coatings. The cell behavior was investigated especially at the early stages of cell-surface interactions like cell attachment and spreading as well as cell proliferation (Figure 4, created with BioRender.com (accessed on 22 April 2023) as indicators of the materials' biological performance. For instance, [PERSON] et al. developed a novel equiatomic TiNbTaZrMo HEA and investigated its cytocompatibility with human osteoblasts in terms of cellular morphology, spreading and density in comparison to 316 L stainless steel (SUS-316L) and CP-Ti as reference biomaterials [78]. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**BioHEA Studied** & **Investigated Cell Type** & **In Vitro Biological Performance** & **Quantitative Assessments** & **Reference** \\ \hline Non-equiatomic BioHEAs (Ti\({}_{1.4}\)Zr\({}_{1.4}\)Nb\({}_{0.6}\)Ta\({}_{0.6}\)Mo\({}_{0.6}\)) & Human osteoblasts & Cell morphology and density are similar to CP-Ti and equiatomic TiNbTaZrMo but higher than on SUS-316L surface. More mature local adhesions than on SUS-316L. & Size regulation of fibrillar adhesions: 810/mm\({}^{2}\) & [75] \\ As-cast and SLM-built & Mouse primary but superior to SS316L in terms of cell density, morphology and spreading as well as osteogenic differentiation.