# (Bio)Tribocorrosion in Dental Implants: Principles and Techniques of Investigation
## 1 Introduction  
Currently, tribocorrosion is the process of degradation and irreversibility [1] of various materials due to the combined action of wear (tribo) and the aggressive environment (corrosion). It is capturing the attention of many scientists and engineers because of its dangerous consequences on devices such as valves, pumps, electrochemical instruments and especially medical implants (with the exception of particular fields, such as electrochemical processes, where its impact could be considered positive). Extending the life cycle of these instruments, while ensuring their stability and safety and trying to avoid risky and costly future revisions [2], is a much discussed and studied priority and goal. In the medical field, the term tribocorrosion has been replaced by biotribocorrosion to emphasize the specific environment of this phenomenon, i.e., the human body, animals, plants, or any existing life form. It can involve both natural structures (organs, tissues, cells, etc.) and manufactured ones (hip joints, dental implants, knee implants, etc.) with obvious differences [3] and falls under tribology for its reference to the concepts of friction, contact, wear, etc. One of the first studies on the contact between two metal surfaces with slight movements was conducted by [PERSON] et al. [4], in the early 20 th century, and it is still analyzed in all its fundamental aspects. The tribocorrosion process [5] is the function of many variables, such as the kind of material, the contact geometry, chemical composition and external factors,such as environmental pH and temperature and occupies various industrial fields such as biology, chemistry, physics, statistics and so on. Figure 1 represents the degradation process caused by the mechanical wear due to adhesion, abrasion, fatigue and corrosion (redox reaction) between the sample and the environment. Redox is a process involving two semi-reactions [6]; oxidation (Equation (1)) is generally referred to as the anode and reduction (Equation (2)) is referred to as the following cathode:  
\[M\to M^{n+}+ne \tag{1}\]  
where \(M\) is a general metal, \(n\) is the number of exchanged electrons and \(M^{n+}\) is the ion formed. \[H_{2}O+2e\to 2 OH^{-}+H_{2} \tag{2}\]  
Hence, the electrons lost by the metal are acquired by other species such as water, as described in Equation (2), hydrogen ions and dissolved oxygen. The two formulae are considered the main processes but are not the only ones. For example, another reaction, fundamental for avoiding corrosion, regards the formation of a passive layer according to the following:  
\[M+nH_{2}O\to MO_{n}+2 nH^{+}+2 ne \tag{3}\]  
with \(MO_{n}\), the oxide film formed on the surface of the metal (for example, titanium alloy is TiO\({}_{2}\)) protects the materials from the aggressivity of reagents. The last one is exactly the synergy, i.e., the union, the combination of them, that can be positive (the formation of a passive and hard layer on the surface, capable of protecting the material from wear) or negative (significant loss of material) according to the specific working conditions [7]. The potential disadvantages of tribocorrosion are not only related to the possible loss of function but also to people's health. Indeed, corrosive products could migrate from the implant to the nearby tissues, causing histopathologic problems [8]. Released particles could also cause inflammation (the first sign of discomfort for the patient), followed by the spread of macrophages, which, being cells of the immune system, phagocytize and digest foreign particles [9]. The particles can vary in dimensions and size, extremely complicating their investigation, although the range is about 50-100 nm [10].