## 3 Results and Discussion  
### Characteristics of the Ar Plasma-Modified Ti Samples  
Figure 1 illustrates the SEM micrographs of the polished Ti sample and Ar plasma-modified Ti sample (100 W, 12 min, and 190 mTorr). Obviously, the surface of the polished Ti sample showed some remaining polishing tracks (Figure 1a). After the Ar plasma modification, the sample revealed a flat surface with an ion bombarded feature as shown in Figure 1b. From a higher magnification observation by AFM (Figure 2), there were pores on the surface (as indicated by arrows), which indicated that Ar plasma modification is an effective method to generate porous topography on the Ti surface. Similar topography characteristics could also be found in other Ar plasma-modified Ti surfaces with different parameters (Table 1). The topography became slightly rougher as the plasma modification duration and power increased. This result was also similar to a previous study that demonstrated that under an Ar atmosphere with plasma spray, graded porous Ti coatings were deposited [21]. The SIMS spectra obtained from the Ti surface modified with Ar plasma (100 W, 12 min, and 190 mTorr) and without Ar plasma are shown in Figure 3. The native Ti oxide layer of \(\sim\)13 nm was formed on the Ti surface without Ar plasma modification (Figure 3a). After Ar plasma modification, it was found that Ar plasma modification can remove residues possibly left from other procedures (Figure 3b), thus effectively producing purer and expected surfaces compared to materials prepared by other traditional methods [22]. Under different designed plasma parameters, similar SIMS spectrum features could also be detected on the modified Ti surfaces. The formation of a clean and porous surface can be attributed to when all the Ar atoms are ionized, most of the Ar ions react with the Ti material through ion bombardment by Ar plasma.  
Figure 1: SEM micrographs of (**a**) the polished Ti sample and (**b**) Ar plasma-modified Ti sample (100 W, 12 min, and 190 mTorr).  
Figure 2: AFM image of the Ar plasma-modified Ti sample (100 W, 12 min, and 190 mTorr).  
Figure 4 presents the RSM analysis result (contact angle) of the Ar plasma-modified Ti sample. As the Ti surface was modified at power 100 W, time 12 min, and chamber pressure 190 mTorr, the minimum contact angle of 14.50\({}^{\circ}\) was achieved, and the surface had the maximum hydrophilic property. However, it was found that the contact angle of the Ti surfaces is not proportional to the setting factors of power, time, and pressure in Ar plasma cleaning modification, according to the RSM analysis results. The Ar plasma cleaning process can obtain surfaces with a lower contact angle. The feature reveals that the wettability property of biomaterial surfaces can be improved by a controlled degree of surface cleaning. In a previous study, the contribution of polar interactions to the solid-liquid interface interaction energy increased with the increase in the glassy phase content of the oxide, resulting in a decrease in the measured contact angle [23]. Wettability should be considered when choosing dental implant biomaterials. Cell and tissue adhesion can be affected by the wettability performance of implants after implantation [24,25].  
Figure 4: The RSM analysis result (contact angle) of the Ar plasma-modified Ti sample (100 W, 12 min, and 190 mTorr).  
Figure 3: SIMS spectra obtained from the Ti surface modified with (**a**) Ar plasma (100 W, 12 min, and 190 mTorr) and (**b**) without Ar plasma.