## 1 Introduction  
Dental implants are now widely used as a prosthodontic treatment for missing teeth, and products with various surface properties are commercially available. Titanium implants developed early had machined mirror polished surfaces, but it has been clarified that blasted and acid-treated moderately rough surfaces increase protein adsorption and cell adhesion, being advantageous for osseointegration, and these became the current mainstream [1; 2].  
However, it has recently been reported that biological aging, representing decreases in hydrophilicity and protein adsorption due to adherence of carbon and nitrogen in the air to the titanium implant surface with time, influences osseointegration [3]. This is caused by a decrease in hydrophilicity and changes in the surface electrical potential due to reduced surface energy by adherence with carbohydrates in the air to the titanium surface. It has been reported that as the surface becomes hydrophobic, adsorption of blood adhesion protein, such as fibronectin, significantly decreases compared with adsorption to the hydrophilic surface [4], and this is considered to influence the bone-implant contact ratio (BIC) at an early stage of the implant placement [5; 6; 7]. This hydrophilicity of the titanium surface is evaluated based on the contact angle, and a surface with a contact angle smaller than 60\({}^{\circ}\) and 5\({}^{\circ}\) or smaller are regarded as hydrophilic and superhydrophilic, respectively [3].  
It has also been reported that the speed of change to hydrophobic increases as the surface roughness increases [4]. Furthermore, electrical polarity decreases on hydrophobic metal surfaces, to which positively charged ions, such as Ca\({}^{2+}\), and negatively charged cells do not readily adhere [1; 8; 9].  
Against this biological aging of the titanium surface, various countermeasures have recently been taken. The photofunctionalization technique, modifying the titanium surface to make it superhydrophilic by ultraviolet (UV) irradiation [3], and a method to prevent exposure to the air by enclosing implants in saline at the time of factory shipment [10] have been investigated, developed, and clinically used. In photofunctionalization, carbohydrates adhered to the titanium surface are degraded by reactive oxygen generated by UV irradiation and the surface property is changed to superhydrophilic. As a result, elevation of the maximum value of BIC to nearly 100% from the normal value, 45% \(\pm\) 16% or 50%-75%, has been reported [1; 5].  
On the other hand, we have developed and reported an original technique in which aged Ti-6 Al-4V disks were immersed in 3% H\({}_{2}\)O\({}_{2}\) and treated with hydrothermal oxidation at 121\({}^{\circ}\)C /0.2 MPa for 20 min using an autoclave to form a new titanium oxide layer with an about 90 nm thickness, changing the titanium surface to hydrophilic [11]. Using this method, a hydrophilic titanium surface was acquired and protein adsorption and cell adhesion increased. However, this method had the disadvantage that it changes the original titanium surface microstructure.  
In this study, to safely and easily improve biological aging, we focused on electrolytic reducing ionic water (S-100\({}^{\text{\textregistered}}\)) [12]. S-100\({}^{\text{\textregistered}}\) is special functional water containing abundant OH\({}^{-}\) ions and it removes/cleans off adhering substances by inducing an electrical repulsion force between the adhering substance and matrix. Therefore, in this study, we investigated whether surface treatment by S-100\({}^{\text{\textregistered}}\) can easily remove pollutants such as carbon, which cause biological aging without changing the surface structure of titanium.