## 5 In Vitro Biological Performance of HEA
The SLM process & Cell density: \(>\)8000 cells/cm\({}^{2}\) & [39] \\ BioHEAs & Showed the most promising potential. & Similar biological performance to CP-Ti but superior to SUS316L and Co-Cr-Mo in terms of osteoblast cytomorphology, adhesion and spreading, as well as cytoskeleton organization & Cell density: \(-\)8000 cells/cm\({}^{2}\) & [69] \\ Ti\({}_{2.83}\)Zr\({}_{2.83}\)Hf\({}_{2.83}\)Hf\({}_{2.83}\)Hf\({}_{2.83}\)Hf\({}_{2.83}\)Mo\({}_{0.74}\)Mo\  
It was found that the osteoblasts on the surfaces of this BioHEA, both under as-cast and annealed forms, exhibit an extensive morphology akin to the cellular morphology on CP-Ti as well as a significantly higher number of cells than SUS-316 L, showing promising potential as a new class of metallic bioalloys. On the other hand, [PERSON] et al. developed four compositions of non-equiatomic Ti-Nb-Ta-Zr-Mo BioHEA and showed that the osteoblasts in contact with Ti\({}_{1,4}\)Zr\({}_{1,4}\)Nb\({}_{0.6}\)Ta\({}_{0.6}\)Mo\({}_{0.6}\) exhibited a widespread morphology and a cell density that was resembling that of the cells observed on CP-Ti and equiatomic TiNbTaZrMo but higher than on SUS-316L surface [75]. Moreover, on this BioHEA surface, more mature focal adhesions could be noticed as compared to osteoblasts in contact with the SUS-316L sample. Also, the focal adhesions were significantly longer than in the cells on Ti\({}_{0.6}\)Zr\({}_{0.6}\)Nb\({}_{1.4}\)Ta\({}_{1.4}\)Mo\({}_{0.1.4}\), suggesting that Ti\({}_{1.4}\)Zr\({}_{1.4}\)Nb\({}_{0.6}\)Ta\({}_{0.6}\)Mo\({}_{0.6}\) was the most potent in stimulating the focal adhesions maturation in the osteoblast cells. It is well known that the maturation of focal adhesions is closely associated with tissue integration of the implanted biomaterials [79], and this finding proves that the non-equiatomic Ti\({}_{1.4}\)Zr\({}_{1.4}\)Nb\({}_{0.6}\)Ta\({}_{0.6}\)Mo\({}_{0.6}\) BioHEA possessed a superior in vitro biological performance when compared to that of SUS-316L alloy. This study brings to attention that by manipulating the alloy compositions in the Ti-Nb-Ta-Zr-Mo bioHEAs, implant biocompatibility can be optimized. This study has also demonstrated that the features of the oxide layer from the interface between cells and the metallic substrate play an important role in influencing cell behavior. Thus, as evinced by the XPS analysis, the oxide layer on the SFP specimen is rich in ZrO\({}_{2}\), which is known to be highly biocompatible and supports cell viability and multiplication. The same research group investigated pre-alloyed Ti\({}_{1.4}\)Nb\({}_{0.6}\)Ta\({}_{0.6}\)Zr\({}_{1.4}\) Mo\({}_{0.6}\) BioHEA powders and SLM-built parts with low porosity and demonstrated that the cell density on the SLM-built sample was similar to that on the conventional CP-Ti biomaterial and the cast counterpart but significantly higher when compared to that on SS316L [39]. Likewise, the fluorescent images of the actin cytoskeleton and focal adhesions exhibited by the osteoblasts adhered to the SLM-processed HEA samples revealed a widespread morphology with a dense actin filament network similar to those on CP-Ti. Therefore, the uniform compositional distribution of the SLM-built samples endows them with advantages for cell spreading, which was demonstrated to be a biophysical cue playing a critical role in the osteogenic commitment and differentiation of stem cells [80]. On the contrary, due to elemental segregation in cast counterpart, the distribution of filopodia in osteoblasts was restricted by griping the adhesion spots in Ti- and Zr-enriched inter-dendrite regions, leading to reduced cell spreading. Overall, both as-cast and Ti\({}_{1.4}\)Nb\({}_{0.6}\)Ta\({}_{0.6}\)Zr\({}_{1.4}\)Mo\({}_{0.6}\) proved to show more promising potential for biomedical applications than SS316L. Moreover, based on the above results showing the suitability of the Ti\({}_{1.4}\)Nb\({}_{0.6}\)Ta\({}_{0.6}\)Zr\({}_{1.4}\)Mo\({}_{0.6}\) for bone implants, [PERSON] et al.