###### Abstract
However, it was observed that Cr oxides formed naturally on the surface of this alloy prevent the release of Cr ions upon corrosion and that Co and Mo ions are more easily released [15]. Considering the harmful effects of cobalt and the allergic reactions that ensue from nickel [20], which can lead to health risks owing to the release of toxic ions, restrictions have been placed on their use in dentistry. This has led to investigations into finding alternative materials and the need to eliminate CoCr and nickel-chromium (NiCr) alloys from dentistry [21]. Research has estimated that nickel allergy affects more than 28.5% of the general population, which is considered a high potential risk for health [22; 23]. As a result, the toxicological risks of cobalt-chromium alloys in dentistry have become a part of the EU Medical Devices Regulation (MDR) (2017/745), which was scheduled to be fully implemented in May 2021 [24]. According to this EU regulation, cobalt metal has been associated with carcinogenic properties, mutagenic effects and reproductive toxicity [25]. The EU MDR regulation entered into application on 26 May 2021. However, due to global events such as the pandemic, the full implementation of medical device regulation (MDR) has been delayed until December 2022. The new regulation has introduced changes such as the reclassification of existing products and requirements for more clinical trials. In the quest for the perfect equilibrium between biocompatibility, strength, resistance to corrosion and wear, much of the latest research is focused on high entropy alloys (HEAs) [26; 27; 28; 29; 30; 31; 32]. Due to the sensitive nature of the European regulatory changes surrounding the use of CoCr dental alloys [25], the strategy to reduce their toxicological risk has become a challenge for BioHEA investigations. The BioHEAs studied are mainly composed of elements that are common in biocompatible binary and ternary alloys, such as Ti, Zr, Ta, Nb and Mo, which are known for their high cytocompatibility, good strength and increased corrosion resistance [17]. Particularly, the Ti-Zr-based BioHEAs possess high strength and good wear resistance, making them suitable to be used as screws due to their resistance to deformation and fracture [33]. Dental alloy restorations are designed to last for years in the patient's oral cavity, an environment that often contributes to the slow or accelerated degradation of these metal biomaterials. However, it must be taken into account that such biomedical devices must be safe in use so that, in the long term, they do not initiate and/or maintain biocompatibility issues [34]. Most of the time, problems occur as a result of corrosion that leads to metal ions release, which can migrate locally or systemically in the body, leading to undesirable adverse effects [35]. An important factor in achieving long-term clinical success is the nature of the alloy from which the restoration is manufactured. Regarding dental implants, integration problems occur at the interfaces with soft and hard tissues in the form of mucositis (an inflammatory process of the soft tissue surrounding the implant) [36] and peri-implantitis (an inflammatory process of the implant's surrounding connective tissue followed by osseous support loss) [37], which may require complex and costly re-interventions such as tissue regeneration [38], and in severe cases, implant rejection may occur. In the case of HEA implant materials, there are not only obvious drawbacks related to the costs of such materials but also technical, such as poor castability [26] and elemental segregation [39]. Several strategies are being researched and evaluated to address these issues, such as HEA coatings on cheaper substrates and different fabrication methods with highly controlled parameters [40; 41; 42; 43; 44]. Elements such as Ag and Cu can also be used when designing HEA for antimicrobial properties.