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The use of dental implants has been increasing exponentially over the last few decades, making it essential to understand their degradation behaviour in the oral environment. Dental implants are generally made out of titanium (Ti) because of its outstanding biocompability and high corrosion resistance. Even though the degradation of Ti, in vivo, is minimal due to its ability to develop a highly stable passive oxide layer (TiO2); Ti is not inert to corrosive attack and presents poor wear resistance. Several studies have already been done on the corrosion and wear behaviour of titanium in artificial saliva and fluoride solutions. However, the microbial effect on the corrosion and wear of metallic biomaterials still remains unknown. The major novelty of this work relies on the first ever evaluation of the combined behaviour of wear and corrosion, process known as tribocorrosion, of Ti dental implants covered by biofilms in artificial saliva with different fluoride solutions. Therefore, the main aims of this work are: - to assess the effect of surface topography on the microbial adhesion and biofilm formation; - to evaluate the biotribological behaviour of Ti samples covered with mixed biofilms of Candida albicans and Streptococcus mutans, in artificial saliva solutions in the absence or presence of different fluoride concentrations; - to study the influence of mixed biofilms on the biotribocorrosion behaviour of Ti under an artificial saliva solution with fluoride; - to analyze the biocorrosion behaviour of Ti samples covered with mixed biofilms in artificial saliva solutions in the absence or presence of different fluoride concentrations.

Even though titanium (Ti) has the ability to develop a highly stable passive oxide layer (TiO2) that provides an excellent corrosion resistance; it’s not inert to corrosive attack. Under in vivo conditions, such as in orthopedic and dental implants, a tribological contact may be formed in association with a chemical aggressive environment. Consequently, wear debris and corrosion products or ions can lead to inflammatory reactions, such as osteolysis (bone resorption), provoking peri-prosthetic bone loss and subsequent loosening of the implant/prosthesis [1]. The microbial effect on the corrosion of metallic biomaterials remains unknown. Chang et al. [2] reported an increase on the corrosion behaviour of dental metallic materials in the presence of Streptococcus mutans and its growth byproducts. Moreover, brushing can lead to the production of wear debris, which together with the attachment of microbes can disturb the passivity of the metal oxide. In addition, the organic acids produced by bacteria may reduce the pH, favoring the corrosion and tribocorrosion of implants. The aim of this work was to study the influence of mixed biofilms on the biotribocorrosion behaviour of commercially pure (cp) Ti for dental implants, and the consequent effect of fluoride.

The use of dental implants has been increasing even though failures do occur. The presence of wear debris and oral microorganisms can contribute to infections and jeopardize implant integration. The aim of this work was to study the influence of mixed biofilms in the tribological behaviour of commercially pure titanium for dental implants under different concentrations of fluoride. Samples of titanium with two different surface topographies were used. Mixed biofilms of Candida albicans and Streptococcus mutans were formed on both surfaces at 37 °C in a tryptic soy broth containing mucin, peptone, yeast extract and sucrose. After 8 days, biofilm biomass was analysed by crystal violet staining method. Biofilm biomass was significantly higher for the samples with higher roughness. Some samples with biofilms were analysed under friction (using a force of 100 mN) in an artificial saliva solution (Fusayama) without or with different concentrations of fluoride (30 and 227 ppm). It was verified that the coefficient of friction (COF) decreased in the presence of biofilms. Moreover, samples with more biomass (0.4 μm of roughness) presented the lowest values of COF. Concerning the effect of the presence of fluoride, although there were no significant differences on the COF for 30 ppm, for 227 ppm a transition regimen was observed. These results were confirmed by sample observation under scanning electron microscopy. In conclusion, it can be highlighted that biofilm formation on dental implants can significantly affect the tribological behaviour of titanium, namely, the presence of biofilms reduces the release of wear debris.

Tribocorrosion covers the material degradation process resulting from the combined interaction of wear and corrosion phenomena on surfaces subjected to a relative contact movement. Bio-tribocorrosion is the designation used to describe the tribocorrosion behavior of materials in contact with biological environments, as it happens with instruments used in the medical field and devices inserted in the human body. Under in vivo conditions, a complex tribological contact may be formed in orthopedic and dental implants in association with a chemical and biological aggressive environment. Consequently, wear debris and corrosion products or ions can lead to inflammatory reactions, such as osteolysis (bone resorption), provoking bone loss and subsequent loosening of the implant/prosthesis. An overview of the aspects intervening on the tribocorrosion and biotribocorrosion of biomaterials is given in this chapter, with a particular focus on the topics related with the oral environment and dental implants, especially those built in titanium. These implants are under a complex congregation of mechanical solicitations and chemical aggressive substances, which can change considerably in magnitude or nature over the day and between individuals. Moreover, the interaction with cells, particularly with microorganisms, will result in local modification of the surrounding environment, both from the chemical and mechanical points of view. Biotribocorrosion response will depend on the multifaceted combination of all those factors. This review gathers knowledge from distinct fields such as materials sciences, microbiology, and dentistry, aiming at contributing to a better understanding of bio-tribocorrosion processes in the oral environment.