**1. Introduction**

The use of dental implants has revolutionized the current treatment of partially and fully edentulous patients given their high level of predictability and wide variety of treatment options. In 2006, the estimated number of dental titanium implants placed in the United States was over five million [1]. A major consideration in designing dental implants is the production of surfaces that promote desirable responses in the cells and tissues.

Hydroxyapatite (HA) coatings on titanium implants facilitate rapid bone formation due to their excellent osteoconductive property. HA-coated implants have been reported to stimulate bone healing, which enhances improvement in the rate and strength of the initial implant integration. For enhanced implant stability and bio-integration to bone tissue, various methods have been

applied to coat the titanium implant with HA, for example, plasma spraying [2], electrophoretic co-deposition [3], ion-beam-sputter deposition [4], dip coating in a simulated body fluid [5], electrochemical deposition [6], blast coating [7], and thermally induced liquid-phase deposition [8,9]. Among these methods, titanium implant bodies coated with HA in dry processes, such as the plasma spraying and ion-beam-sputter deposition methods, have already been used clinically. The HA coating methods in dry processes, however, cannot be applied to titanium implant bodies with porous surface structures. To coat titanium implants with a three-dimensional porous surface structure with HA film, Kuroda et al. and Tamura et al. developed the titanium substrate heating method in a liquid [8,9]. Since the solubility product of HA decreases with an increase in temperature above 16 ◦C, they electrically heated the titanium substrate by applying a large current with an AC or DC power source to the substrate in calcium phosphate solutions to precipitate HA crystals on the titanium surface. These two studies demonstrated that the precipitated HA crystals were needle-like or plate-like in shape and that the formation of a uniform and dense HA film consisting of fine and spherical HA crystals could not be achieved. Another shortcoming of the titanium substrate heating method is that it requires a large electric power source exceeding 1000 V for the deposition of HA crystals on the surface of the dental implant body by Joule heating.

The solution plasma technique is an entirely new technology to coat an implant surface with HA spherical particles. To date, this solution plasma technique has been used in the fields of chemistry and applied physics [10]. When a pulsed voltage is applied between the two electrodes, a glowing discharge takes place in the liquid, which generates plasma in the gas phase. This plasma produces thermal energy and light, along with reactive chemical species such as hydrogen and hydroxide (OH) ions. With this novel surface modification technique, the temperature of the solution in contact with the titanium surface can easily be increased with the thermal energy generated by the solution plasma. In this case, a thin and uniform HA layer is expected to be formed on a porous-surfaced titanium implant, which will induce rapid bone growth into the pore space of the implant body, and also enhance the biological anchorage of the implant with bone.

The present study aimed to achieve a thin and uniform HA layer covering titanium with a porous surface structure by using solution plasma technology. The mechanism of HA precipitation on the titanium during solution plasma treatment was elucidated.
