## 4 BioHEAs
### Bulk
predicted that introducing Mo in a smaller amount would still lead to solid solution formation and have thus developed a nonequiatomic HEA, Ti\({}_{28.33}\)Zr\({}_{28.33}\)Hf\({}_{28.33}\)Nb\({}_{6.74}\)Ta\({}_{6.74}\)Mo\({}_{1.55}\). The biocompatibility of this HEA assessed in osteoblast culture proved to be higher than in the case of Co-Cr-Mo and slightly superior to that of CP-Ti, which is frequently utilized for orthopedic and dental implants [69]. Other BioHEAs for which the biocompatibility was proven to be comparable to that of CP-Ti were developed using a treelike diagram starting from TiZrHf and the commercially available CoCrMo, obtaining TiZrHfCr\({}_{0.2}\)Mo and TiZrHfCo\({}_{0.07}\)Cr\({}_{0.07}\)Mo [70]. Besides the arc melting preparation method, which appears to be the common choice for BioHEAs, SLM was also proposed and used to fabricate Ti\({}_{1.4}\)Nb\({}_{0.6}\)Ta\({}_{0.6}\)Zr\({}_{1.4}\)Mo\({}_{0.6}\). The authors postulated that by significantly increasing the cooling rate to \(10^{5}\)-\(10^{7}\) K/s, the probability of elemental segregation will be lowered. Moreover, the material's biocompatibility was evaluated by using primary osteoblasts and compared to the same alloy prepared through arc melting, observing that the uniform distribution in the SLM alloy led to an increased cell density [39]. [PERSON] et al. prepared and analyzed several BioHEAs, looking for Ni-free shape memory alloys and identifying the most promising candidate among those studied, the (TiZrHf)\({}_{82}\)Nb\({}_{5}\)Ta\({}_{5}\)Al\({}_{8}\) alloy. The alloy was found to be superelastic, with low magnetic susceptibility, good biocompatibility and corrosion resistance [71]. Iron, which is inexpensive, may also be added to improve the resistance to wear and corrosion, as shown by [PERSON] et al. in the case of the developed TiZrHfNbFe alloy. Despite the fact that the microhardness increases with the Fe content, as well as the resistance to wear, they observed that the smallest amount tested of Fe (0.5 molar ratio) provided the best corrosion resistance in PBS, without traces of pitting corrosion [72]. Several BioHEAs studied as bulk are presented in Table 1, along with their microstructure and properties. \begin{table}
\begin{tabular}{c c c c c} \hline
**Alloy Studied** & **Microstructure** & **Mechanical Properties** & **Resistance to Corrosion** & **Reference** \\ \hline  & & & Corrosion rate–\(5.6\times 10^{-4}\) mm/year & \\  & & & (Hank’s solution) & \\  & & & Ion concentration after 28 days of & \\  & & & immersion in Hank’s solution: & \\ TiZrHfNbTa & BCC structure & n/a & Ti–\(19.8\) ppb & [61] \\  & & & Zr–\(1.4\) ppb & \\  & & & Hf–\(0.76\) ppb & \\  & & & Nb–\(8.4\) ppb & \\  & & & Ta–\(9.8\) ppb & \\ \hline  & & Higher mechanical & & \\ Ti\({}_{28.33}\)Nb\({}_{6.74}\)Ta\({}_{6.74}\)Mo\({}_{1.55}\) & BCC structure & strength than CP-Ti & & \\  & & observed through the & & n/a & [69] \\  & & stress–strain curve & & \\ \hline  & BCC dendritic structure & & & \\ TiZrHfCr\({}_{0.2}\)Mo & & with minor & HV—\(531\) & & [70] \\  & & interdendritic phases & HV–\(532\) & & \\ \hline  & & HV–\(532\) & & \\ Ti\({}_{40}\)Zr\({}_{20}\)Hf\({}_{10}\)Nb\({}_{20}\)Ta\({}_{10}\) & BCC structure & HV—\(294\) & The polarization behavior in 3.5\% NaCl indicates significantly higher & \\  & & E—\(86.4\) GPa & corrosion resistance compared & \\  & & with CP-Ti & & \\ \hline  & & & & \\ \end{tabular}
\end{table} 
Table 1: Microstructure and properties of the bulk BioHEAs.