## 3 Results and Discussion  
The GAXRD and cross-sectional TEM of the PAT film were investigated as shown in Figure 1. The GAXRD pattern and TEM micrograph analysis of the PAT film showed a diffraction peak of (002), which confirmed that the crystalline structure of the PAT film belonged to the hexagonal close-packed (hcp) structure. The cross-sectional TEM image also obviously indicated the formation of nanopolycrystalline grains (\(\sim\)50 nm) in the PAT film. Similar results have been reported by [PERSON] et al. [21]. Figure 2 depicts the high-magnification SEM and cross-sectional TEM images of MG-63 cells cultured for 72 h on the PAT film. Numerous filopodia are visible, as displayed in Figure 2a. Cells on the PAT film were more spread out and had filopodia that were longer and in greater number. Figure 2b shows a TEM micrograph of the PAT film from the cross-sectional direction following cell culturing. The MG-63 cell (as indicated by the arrow and dash-line covered area) is well adhered and obviously a polygon with no prominent nuclei. After 72 h of culture, most of the cells began to differentiate along small irregular regions, and the cells were firmly adhered, flattened, and irregularly networked, demonstrating that the PAT film had great proliferation ability. As mentioned, implants with a surface coating are well known to have a high degree of biocompatibility. After surface modification of the implants, they have better function and biocompatibility. Thus, surface modifications are recommended as coatings for implants that are in contact with blood. The adhesion of blood cells such as platelets, red blood cells, and leukocytes can be improved by surface treatment [22]. Compared with a smooth surface, an increase in the surface roughness from 0.2 to 0.5 mm (arithmetical mean roughness (Ra)) can enhance blood cell adhesion ability [23]. Moreover, the nitrided implants can significantly promote the adsorption of human salivary albumin. This feature proved that human albumin has a high affinity for the nitriding-treated surfaces [24]. Based on the above discussion, it revealed that changes in surface properties caused by surface modifications play a vital role in biocompatibility [13]. Hence, the surface characteristics and biocompatibility of implants could be potentially improved by the nanopolycrystalline PAT coating. Figure 1: Cross-sectional bright-field TEM image and GAXRD pattern of the PAT film. Figure 2: (a) SEM micrograph and (b) cross-sectional TEM image demonstrating cellular morphology on a hcp-PAT film after culturing for 72 h (MG-63 cell). After culturing for 72 h, the PAT film with MG-63 cells exhibited a severe leakage current density at 150 kV/cm in comparison to the PAT film without MG-63 cells, as shown in Figure 3. In particular, the bone cell/PAT specimen showed a high percentage of breakdown specimen (\(\sim\)80%) at 150 kV/cm, revealing that the dielectric layer is passed by most of the currents before the capacitor breakdown. Similar results were also observed in the previous studies [11; 14]. Cells on the PAT film surface were also observed by a cross-sectional microstructure image, as shown in Figure 4. Slit-like microvoids (as indicated by the black arrow) were found across the cell/PAT film. However, no slit-like microvoids could be observed in the PAT film without cells. This feature indicates that the failure is caused by mechanical reasons, and not from electrical reasons. The microvoids act as an electrical path that ultimately breaks down the capacitor with a low applied voltage. This is probably due to the PAT film undergoing some osteogenic reactions such as proliferation and differentiation. The formation of microvoids can be used to realize the stress variation of the PAT biosensing capacitor.