*3.4. Change in the Contact Angle of Water Droplets on Polished-Ti and Ti-AT-SPT with Aging Time*

Figure 8 shows the changes in the contact angle of 20 μL water droplets on Ti, Ti-AT, and Ti-AT-SPT with aging time. The contact angle value for Ti increased as the Ti disk aged, suggesting that the surface property changed from being hydrophilic to hydrophobic. This phenomenon is well known as the time-dependent degradation in biological capability or biological degradation, which arises from the absorption of hydrocarbon contaminants in air. In contrast, the contact angles for Ti-AT and Ti-AT-SPT were much lower than that for Ti throughout the aging period. The initial super hydrophilic surface remained almost unchanged up to 30 days of aging in air, although the tough hydrophilicity significantly decreased.

**Figure 8.** The changes in contact angle of water droplets on Polished-Ti, Ti-AT, and Ti-AT-SPT with aging time are shown. The contact angle for Ti-AT and Ti-AT-SPT was much lower than that for Ti throughout the aging period.

#### *3.5. Bioactivity Analysis*

Figure 9 shows the number of cells attached to the Polished-Ti and Ti-AT-SPT, which were subjected to aging in air for 0, 7, and 30 days, as measured with the hemocytometer. The number of MC3T3E1 cells attached to Ti-AT-SPT was approximately 50% more than the amount attached to Ti at each aging period, which suggested that the HA coating by solution plasma treatment significantly improved the initial cell attachment (*p* < 0.05). The number of cells that attached to the Polished-Ti with

30 days of aging in air was approximately 50% of that for the Polished-Ti without aging. In contrast, the number of attached cells on the Ti-AT-SPT after aging for 30 days remained above 85% of that for the SPT-Ti without aging.

Figure 10 shows the SEM photographs of the cells attached to Porous-Ti-AT-SPT after incubation for 4 h. The attached cells indicated by the arrows were located not only on the outer surface (Figure 10a) but also on the surface of the concavity (Figure 10b), which indicated that the induction of bone tissue on the entire porous surface was going to occur successfully and that bone tissue growth could be expected.

**Figure 9.** The number of attached cells on surfaces after 4 h incubation. The number of MC3T3E1 cells attached to the Polished-Ti and Ti-AT-SPT with 0, 7, and 30 days aging in air is shown in the SEM and graph. Different letters in the graph mean statistical difference at *p* < 0.05.

**Figure 10.** (**a**) Cell attached to porous Ti-AT-SPT after 4h incubation. (**b**) Cell attached at the outer surface of concavity in the porous structure. (**c**) Cell attached at the inner surface of concavity in the porous structure. SEM images of the cells attached to Porous Ti-AT-SPT after 4 h incubation. The attached cells indicated by arrows were located not only on the outer surface (upper yellow square represents c) but also on the surface of concavity (lower square represents b).
