*4.2. Chemical and Biological Properties of HA Film Formed by SPT*

Previous studies have revealed that titanium undergoes a time-dependent degradation in biological capability due to the unavoidable contamination of the titanium surfaces by hydrocarbons in air [14]. Contact angle measurements of the water droplets demonstrated that the titanium surface changed from being hydrophilic to hydrophobic with an increase in aging time (Figure 8). This study demonstrated that the contact angle value for a HA-coated titanium disk (Ti-AT-SPT) was significantly lower than that for polished titanium throughout the aging periods. In addition to this, the initial super hydrophilicity of the HA-coated titanium surface by SPT was found to be maintained after 30-day aging in air (Figure 8). Unlike titanium, the HA-coated titanium did not show age-related impaired bioactivity, which is defined as biological aging [15]. The reason for the highly hydrophilic nature of HA-coated titanium can be attributed to the characteristic of HA crystals as HA is an ionic crystal with an abundance of ionic sites that attract water molecules.

The number of MC3T3E1 cells attached to Ti-AT-SPT was approximately 50% larger than that of Polished-Ti at each aging period, suggesting that the HA coating by solution plasma treatment significantly improved the initial cell attachment. This may be one of the reasons why HA-coated titanium implants possess better cytocompatibility and facilitate rapid bone formation due to their excellent osteoconductive property.

These results suggest that an osteoconductive porous-surfaced titanium implant can be developed with a HA coating by solution plasma treatment in a calcium phosphate solution in a short time.
