# The Modification of Titanium Surface by Decomposition of Tannic Acid Coating
## 4 Discussion  
The surface functionalization of various classes of biomaterials, including metals, ceramics and polymers, are required to improve their performance for potential medical applications. Titanium and its alloys play a crucial role in orthopedic and dental procedures, mostly due to their improved mechanical properties and biocompatibility [15]. Despite being biocompatible, the antibacterial and antioxidant characteristics of titanium are inferior and can thereby lead to implant-related infections. One of the causes of implant failures can be attributed to allergic reactions associated with titanium ion release. There have been reports of hypersensitive reactions, such as erythema, urticaria, eczema, swelling, pain, necrosis and bone loss, due to titanium dental implants. In order to circumvent these limitations, surface functionalization various techniques have been extensively studied. The present work outlines the use of a facile, cost effective and sustainable technique to develop tannic acid-based surfaces on Ti metallic surfaces via a vacuum deposition technique. Tannic acid is a natural phenolic compound that has antibacterial and long-term antioxidant properties, a fact that has been widely documented [16; 17; 18]. It has been studied as a dental implant coating and considered to be a bio-safe additive [19]. The presence of phenolic hydroxyl groups allows tannic acid to eliminate free-radicals and display good \begin{table}
\begin{tabular}{c c} \hline
**Specimen—Titanium Plate** & **Rate of Hemolysis [\%]** \\ \hline without TA & \(-0.3054\pm 0.1884\) \\ \hline with 5\% TA & \(-0.4223\pm 0.0715\) \\ \hline with 10\% TA & \(-0.4808\pm 0.0260\) \\ \hline \end{tabular}
\end{table} 
Table 2: Hemolysis rate for each type of titanium plate contacted with blood smears. Figure 2: Total polyphenols released from titanium plates. antioxidant properties. Tannic acid is able to adhere to the titanium surfaces of various metal oxides to form a thin coating through metal-ligand coordination [20; 21; 22]. As a result, a tannic acid-metal complex is formed. Tannic acid is capable of combining with the metal ions present in the defects and the dangling bonds present on the native oxide layer of titanium by means of a coordinate bond. It is also evident from morphological studies that the increase in tannic acid concentration has led to a reduced homogenization of the developed tannic acid on the titanium surface. This could be due to the fact that the abovementioned defects and dangling bonds available on the native oxide layer surface will be reduced during direct contact after the initial layer of tannic acid is deposited onto the surface. Surface free energy is one of the most important surface properties that dictates cell attachment, cellular proliferation and differentiation; moreover, cells generally prefer to grow on high energy surfaces. As a result of the deposition of tannic acid, the polar component of the surface energy was increased, which can be attributed to the presence of numerous hydroxyl groups in the acid structure. The increased polar components can effectively interact with the surface groups present on the cell walls and can generate chemical bonds, whereas non-polar groups are capable of developing non-specific short range interactions, such as in case of van der Waals interactions. In addition, the presence of a large number of phenolic hydroxyl groups allows them to play the role of donor in the process of forming hydrogen bonds. This is advantageous, as the presence of these large numbers of hydrogen bond donors from phenolic groups present in tannic acid attract individual water molecules. Hydrophilic surfaces have a positive effect on the adhesion of cells, e.g., osteoblasts, which means an increase in the compatibility of the material [23]. It was reported by [PERSON] et al. [24] that tannic acid can be utilized to immobilize glucose oxidase on the surface of biomaterials.