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**Figure 2.** Variation of solution temperature in the vicinity of the sample with time, during solution plasma treatment (SPT). The temperature increased rapidly and exceeded 60 ◦C within 3 min after starting SPT, and then increased slowly and reached an almost constant value of 80 ◦C at 30 min.

#### *3.2. Characterization of Titanium Surfaces Subjected to SPT and IT in the Calcium Phosphate Solution*

#### 3.2.1. SEM Observation

Figure 3 shows the SEM photographs of the polished titanium surface after SPT (Ti-SPT) (Figure 3a) and after 5 M NaOH treatment and subsequent SPT (Ti-AT-SPT) (Figure 3b). Crystals formed on both the Ti-SPT and the Ti-AT-SPT. Crystals with diameters between 5 and 20 μm precipitated sparsely and

did not cover the whole surface of the Ti-SPT, while fine spherical crystals with a diameter of 5 μm precipitated and covered the whole surface of the Ti-AT-SPT.

**Figure 3.** SEM images of HA crystals precipitated on Ti-SPT (**a**) and Ti-AT-SPT (**b**). The HA spherical particles formed on both the Ti-SPT and the Ti-AT-SPT. The spherical particles precipitated sparsely and did not cover the whole surface of the Ti-SPT, while the fine spherical crystals precipitated and covered the whole surface of the Ti-AT-SPT.

Figure 4 shows the SEM photographs of the porous titanium surface after 5 M NaOH treatment and subsequent SPT (Porous-Ti-AT-SPT). Fine spherical HA particles were uniformly precipitated over the entire surface (Figure 4a) including the areas recessed in the shape of the porous structure (Figure 4b) as well as the inner surface of the pores (Figure 4c).

**Figure 4.** Scanning electron microscopy (SEM) images of Porous-Ti-AT-SPT. Fine spherical hydroxyapatite (HA) particles were uniformly precipitated over the entire surface (**a**), the areas recessed in the shape of the porous structure (**b**), and inner surface of the pores (**c**).

3.2.2. Analysis of Precipitated Particles on Ti-AT-SPT and Ti-AT-IT60◦ by XRD and EDX

Figure 5 shows the X-ray diffractogram obtained from Polished-Ti (Figure 5a), Ti-AT-SPT (Figure 5b), and synthetic HA powder (Figure 5c). The synthetic HA powder was obtained from the dried-out mineralizing solution which was subjected to SPT.

All of the diffraction peaks were assigned to HA or the titanium substrate under the deposited film. For Ti-AT-SPT (Figure 5b), the diffraction peak of the deposited HA crystals at 25.9◦ (2θ) that corresponded to the (002) lattice plane was relatively higher than the other diffraction peaks at around 32◦, unlike that observed for the synthetic HA powder. This indicated that the spherical HA crystals that precipitated on Ti-AT-SPT were slightly oriented with the c-axis perpendicular to the titanium substrate. These results clearly demonstrated that 5 M NaOH treatment and subsequent SPT for 30 min in the calcium phosphate solution were effective in coating the entire surface of titanium with an HA film composed of fine crystals in a relatively short treatment time.

**Figure 5.** X-ray diffractogram obtained from (**a**) Polished-Ti, (**b**) Ti-AT-SPT, and (**c**) synthetic HA powder. All the diffraction peaks obtained from Ti-SPT were assigned to HA or the titanium substrate under the deposited film.
