# Antimicrobial Effect of Titanium Hydroxapatite in Denture Base Resin
## 1 Introduction  
With a recent increase of the aging population, the number of people missing teeth is also increasing. Oral function compromised by tooth loss is generally restored by prosthodonic treatments, including the use of dentures [1]. Dentures, which have hard and non-shedding surfaces like teeth, accumulate dental plaque and calculus in a similar way to teeth [2]. The adhesion process is influenced by several factors, such as the structure and composition of the surface of these materials and the chemical/physical properties of microbial cell surfaces [3; 4]. Poor denture cleansing not only leads to denture stomatitis [5], but also results in more serious infectious diseases, such as aspiration pneumonia [6; 7].  
Although cleaning of dentures is effective at ensuring oral hygiene and general health, denture cleanliness and oral hygiene of denture wearers is generally poor, thereby facilitating the formation and accumulation of an oral biofilm [8; 9]. Clinical studies have reported that disrupting the biofilm may be more important than the use of antifungal or antimicrobial agents in the prevention and treatment of denture stomatitis [10; 11]. Mechanical cleansing is an effective measure for routinebiofilm control on dentures [12]. Chemical cleansing also aids denture wearers, especially geriatric patients and those with limited motor capacity [5], because the complicated shapes and rough surfaces of dentures make them difficult to clean [13; 14]. Several materials, including the photocatalyst, have recently been used to manage denture stomatitis by killing the microorganisms attached to dentures. The application of titanium dioxide (TiO\({}_{2}\)), a representative photocatalytic material [1], on prosthodontic materials [15; 16], including dentures [1; 17; 18] and implants [19; 20; 21], has been investigated. This material exerts an antimicrobial effect by photo-oxidizing in response to ultraviolet A (UVA) irradiation [22; 23; 24]. However, the effect might not be necessarily pronounced, since organisms, such as bacteria and proteins, do not adsorb onto TiO\({}_{2}\)[25; 26]. Titanium hydroxyapatite (TiHA; Taihei Chemical Industrial Co., Ltd., Osaka, Japan), in which Ca\({}^{2+}\) is partially replaced by Ti\({}^{4+}\)[25], is a recently developed photocatalytic material. In contrast to TiO\({}_{2}\), TiHA exhibits a high affinity for organic material, thereby effectively killing bacteria [25]. Upon UVA irradiation, methylene blue, an organic compound, is effectively removed from a TiHA-containing denture [27]. Several planktonic bacteria from TiHA-containing tissue-conditioning materials are also killed upon UVA irradiation [28]. However, denture plaque mostly consists of bacterial biofilms composed of multilayers of a wide variety of oral bacteria, not of planktonic bacteria. The antimicrobial effect of TiHA on bacterial biofilms has not yet been reported.  
In the current study, we evaluated the antimicrobial effect of TiHA against bacterial biofilms formed on denture base resin, and the effect against planktonic bacteria. For a clinical perspective, the biofilms used in the evaluation were composed not only by a single bacterial species but also by various species of bacteria from the human saliva. We also examined the effect of irradiation and extensive soaking on the antimicrobial effect of TiHA.