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Ti and Ti alloys are currently the most commercially used materials. In particular, since it has good corrosion resistance and biocompatibility, many studies are being conducted to apply it to medical devices for dental and orthopedic implants (1, 2). However, since Ti is a bioinert metal, direct binding to living bones can lead to inflammation and allergy symptoms (3). The biological activity of Ti and its alloys mainly depends on the structure, shape and chemistry of the surface layer (4). Ti has a much higher modulus of elasticity than pure bone, but the lowest modulus of elasticity compared to other metals (5). Ti-mesh is used in the implant procedure in the dental area and it is used to extract teeth or to transplant bone to a patient with low bone density. At this time, the mesh requires two requirements. It must have excellent biocompatibility on the surface, and it must be removed well when a certain amount of time has elapsed after surgery. Because it has to be well attached to the bone, it requires an excellent biocompatibility on the surface.

Therefore, in order to improve the bioactivity of Ti surface, surface modification of Ti mesh has been researched to increase the bonding strength between bone and mesh surface and prevent various side effects such as inflammation and allergies (6, 7). A surface modifications for improving bioactivity of Ti-mesh are physical vapor deposition (PVD), chemical etching, titanium plasma spraying (TPS), titanium sintered pore coating, RF-sputtering, anodizing, resorbable blasting media (RBM), and sandblasting (5). Among these methods, the plasma electrolytic oxidation (PEO) method can produce thick, hard, and strong oxides by the electrochemical oxidation process by variously controlling the applied voltage, current, time, and temperature with a relatively simple experimental method and low cost (8). PEO film can be formed on light metals and alloys in aqueous electrolyte solutions under plasma discharge conditions that exceed the threshold of polarization potential. Due to this reaction, during the PEO process, Ca²⁺ and PO₄^3- ions react through the micro-discharge potential generated at high temperature through Ca and P ions on the Ti surface to form a hydroxyapatite (HA) phase as a coating (9, 10). In addition, Mg ions play an important role in bone metabolism as they affect osteoblast activity. Previous studies have found that Mg ions are one of the substances that enhance bone formation in Ti alloys. It is predicted that there will be a difference in bioactivation when PEO is performed in a solution containing Ca and P ions and in a solution containing Ca, P and Mg ions, and this difference can be predicted through changes in physical and biological properties. However, a few studies on this have been reported, so we intend to investigate it in this study.

Therefore, in this study, surface characteristics of plasma electrolytic oxidized Ti-mesh for dental use were studied using various experimental instruments.

Figure 1 shows the PEO coating process from 0 to 3.5 seconds. In this process, it was seen that the PEO film was rapidly formed at the beginning of the coating under the PEO condition of 280 V for 180 seconds due to a change in plasma discharge and electrolyte polarity, and then stabilized thereafter. In the Figure 1, it can be seen that the case of doping with Ca and P (a) and the case of doping with Ca, P, and Mg (b) show similar spark discharge patterns, but when Mg is added, the reaction is stronger and more active compared to CaP sample. This can be judged to be due to a change in the composition of the electrolyte, and it is thought that Mg is formed on the surface by contributing to the passivation film formation process on the surface by adding Mg.

Figure 1. 
The PEO process of Ti-mesh with different electrolytes from 0 to 3.5 seconds : (a) CaP (b) CaMgP.

In the PEO process, the pore formation process is affected by various conditions, and the mechanism is explained as follows (11) ; the PEO steps were to form and grow a passivation film on the substrate as a result of applying an extremely high voltage to the electrolyte. As the thickness of the oxide layer increased, the voltage between the substrate and the electrolyte increased. The weak part of the oxide layer will suffer dielectric breakdown due to the high voltage. The oxide layer melted by the high temperature generated in the micro-discharge region tends to be ejected from the substrate coating interface to the coating surface, leading to rapid solidification and recrystallization by the cooling electrolyte. After all, it is possible to obtain coatings composed of complex compounds in various electrolytes. In the case of PEO-treated surface in solution containing Mg ions, MgO can be formed in the oxide film by sparking of anodic oxidation. At high temperatures, MgO is present in a fused state, but during the spark of anodic oxidation and spacing of micro-arcs, due to the cooling effect of the electrolyte (12). From the research on PEO-treated Mg alloy (16), the alkaline treatment of the PEO coatings of pure Mg could lead to the formation of Mg(OH)₂ on the coating surface. The formation of Mg(OH)₂ did not generate any detrimental effect on the cytocompatibility in cell culture medium. Mg is generally known as a bioactive element, so it is expected to play a role in increasing biocompatibility even if it forms an oxide.

Figure 2 shows the FESEM images of PEO-treated Ti-mesh surface in electrolyte containing Ca and P ions at 280 V for 3 min. Figure 2 (a) shows the entire surface at low magnification, (b) enlarges it and observes the distribution of pores, and (c) and (d) shows the observation of round, oval, and irregular shapes of pores. Although the shape of the pores is mainly circular, the reason for such an irregular shape is considered to be various factors related to the PEO process. It has been reported that the size and shape of the pores formed in the oxide layer depend on the composition and concentration of the electrolyte and depend on the elements of the alloy (14, 15). In the PEO process, the pore size is determined by the formation and discharge of the oxide film in several stages. For example, there are cases in which secondary pores are formed again in the pores formed in first pores, so that the pores are enlarged and small pores are present. By repeating the process of forming other pores within the first pore as described above, micropores having different sizes and shapes can be obtained. The surface of the PEO (CaP) specimen showed many pores, and no sediment was observed on the surface in the enlarged figures, and pores could be formed in a short time. As shown in Figure 2, the size of the pores is uniform and the size of the nano-particles grows evenly, indicating that it is consistent with the previous study (14).

Figure 2. 
FESEM micrographs showing PEO(CaP) surface morphologies formed on Ti mesh : (a) PEO(CaP) and (b-d) high magnification of (a).

Figure 3 shows the FESEM images of PEO-treated Ti-mesh in electrolyte containing Ca, P, and Mg ions at 280 V for 3 min. Figure 3 (a) shows the entire surface at low magnification, (b) enlarges it and observes the distribution of pores, and (c) and (d) shows the observation of round, oval, and irregular shapes of pores. In Figure 3 (a), compared to Figure 2 (a), the surface is much roughened. In order to confirm this, it can be seen from the Figure 2 (b) that the surface was formed in the same manner as a defect on the surface due to the active PEO reaction. In Figure 3 (c) and (d), it can be seen that a lot of things such as precipitates are covered on the surface. It is considered that Mg was added and not only doped inside during the PEO process, but also formed precipitates such as MgO on the surface. What is unusual is that the size of the pores formed on the surface is greatly increased. This is because it is a case of treatment in an electrolyte solution to which Mg active element is added. It is expected that it will be good for osteoblast activity, which is important for bone metabolism, due to the pores that appear to be many Mg precipitates on the surface (16). Therefore, the microstructure of the generated PEO layer depends on the type of micro-discharge in the PEO process, and the composition of the electrolyte such as Mg affects the morphology of PEO and the change in the size of the pores (11).

Figure 3. 
FESEM micrographs showing PEO(CaMgP) surface morphologies formed on Ti mesh : (a) PEO(CaMgP) and (b-d) high magnification of (a).

Figure 4 and 5 show the EDS peaks and mapping images of PEO-treated Ti-mesh in electrolytes containing Ca, P, and Mg ions. In Figure 4, (a) shows a CaP sample and (b) shows a CaMgP sample, respectively. Overall, the distribution of the desired element is detected homogeneously, and in particular, in the case of CaMgP specimen, Mg is detected and the coating seems to be good. In addition, it can be seen that the pore size is significantly increased compared to that of the CaP specimen. In Figure 5, (a) shows a CaP sample and (b) shows a CaMgP sample. As shown in Figure 5, in the case of CaP specimen, Ca and P elements were uniformly distributed in the pores and surface, whereas Mg was mainly detected at the surface, which is in good agreement with the precipitates shown in Figure 3.

Figure 4. 
FESEM images and EDS peaks for coating surface of PEO-treated Ti mesh : (a, a-1) CaP and (b, b-1)

Figure 5. 
EDS mapping images for coating surface of PEO-treated Ti mesh: (a) CaP and (b) CaMgP.

Figure 6 shows the X-ray diffraction peaks of PEO-treated Ti-mesh of (a) bulk, (b) CaP, and (c) CaMgP, respectively. All patterns of XRD were confirmed by comparing with JCPDS (Joint Committed on Powder Diffraction standards, PCPDFWIN), HA(ICSD #083352, #026261), Ti(#7440-32), TiO₂-rutile (ICSD #085494, #085495, #082656), TiO₂-anatase (ICSD #202243, #202242, #200392), Mg(#35-0821). The PEO-treated specimen was mainly composed of anatase and HA, and it was determined that the oxide film formed during the PEO process had an anatase structure to improve biocompatibility. In CaP and CaMgP samples, anatase peaks were observed at 2θ= 25.00 and 25.55°, and HA peaks were observed at 2θ= 29.28 and 29.84°, respectively. And Mg was observed at 2θ = 36.62, 67.31 and 68.63°, respectively, and it can be seen that it is in good agreement with the Figure 3 in which Mg is covered on the surface. In particular, lattice matching of anatase structure and HA structure in the PEO process is known to induce early bone adhesion and growth, so it is expected that the initial biocompatibility will be improved. In addition, as mentioned at the beginning, there is an Mg peak, which is expected to improve biocompatibility (11).

Figure 6. 
X-ray diffraction peaks of PEO-treated Ti-mesh: (a) bulk, (b) CaP, and (c) CaMgP.

Figure 7 shows the surface images AFM(atomic force microscopy) of PEO-treated Ti-mesh (a) CaP and (b) CaMgP. The respective average roughness values were 0.247 μm for CaP and 0.226 μm for CaMgP. The surface roughness of CaMgP measured by AFM showed slightly low, but in reality, the roughness of the defected surface is high as shown in Figure 3. This is thought to be small because the size of the pores is large and the small pores are reduced, if the pores are large, measuring the roughness in nano-units scans the surface rather than the pores.

Figure 7. 
Atomic force microscopy images of PEO-treated Ti-mesh: (a) CaP and (b) CaMgP.

It can be seen that the roughness increases when PEO(CaP and CaMgP) is treated. This is considered to be due to the pores formed on the surface. In the case of coating with Mg, the surface roughness is increased. Surface roughness greatly affects biocompatibility, and an average roughness value within 5 micrometers is recommended for biocompatibility.

Figure 8 shows the results of nanoindentation test to determine the elastic modulus and indentation hardness. The load-displacement graphs can be known as stiffness, contact area, modulus, and hardness etc. The stiffness and elastic modulus are determined according to the slope, and the larger the value of the slope, the stronger the stiffness and the weaker the elastic modulus (7). Figure 8 (a) shows that the stiffness weakens and the modulus of elasticity increases from the bulk to the PEO-treated CaP and CaMgP. This shows that the HA deposition on the surface is good and that HA is more ductile than the conventional Ti-mesh. Therefore, it is seen that CaMgP containing Mg ions is stronger than CaP containing without Mg ions. In general, the modulus of elasticity increases as strength and hardness increase. However, Mg increases the size of pores due to strong electrical stimulation caused by anodization, resulting in larger pores than CaP. When measuring the hardness of the surface, it is also affected by the presence or absence of pores, but when Mg is added, it is thought that the surface is doped with Mg element, resulting in an increase in hardness or an increase in elastic modulus due to solid solution hardening. The inorganic material layer manufactured through PEO technology is expected to provide excellent hardness and adhesion due to properties similar to ceramics having a relatively thick and dense structure. In the case of Mg alloy, it is reported that MgO and Mg₃(PO₄)₂, contribute to hardness (17). Hardness and adhesion values can vary with substrate condition, electrolyte composition, current density, and processing time, which determine the major phase components (18). Therefore, when PEO treatment is performed in an electrolytic solution containing Mg, the formation of Mg oxide is increased, hardness is increased, and as a result, it is considered that the elastic modulus is increased.

Figure 8. 
The results of nanoindentation test on the PEO-treated Ti-mesh in electrolyte containing Ca, P, and Mg ions : (a) load-displacement graphs and (b) indentation hardness and elastic modulus.

CaMgP sample showed the active spark discharge reaction compared to CaP sample. In CaP and CaMgP samples, the PEO surface of Ti-mesh was mainly circular, irregular, and oval shapes of pores. In the case of CaMgP, the defect and precipitates such as MgO was formed on the surface. On the surface of sample, the distribution of the desired element was detected homogeneously, and in particular, in the case of CaMgP specimen, Mg was detected and the coating seems to be good. In the case of CaP specimen, Ca and P elements were uniformly distributed in the pores and surface, whereas Mg was mainly detected at the surface.

The PEO-treated specimen was mainly composed of anatase and HA, in CaP and CaMgP, anatase peaks were observed at 2θ= 25.00 and 25.55°, and HA peaks were observed at 2θ= 29.28 and 29.84°, respectively. And Mg was observed at 2θ = 36.62, 67.31 and 68.63°, respectively. From the AFM result, the average surface roughness was 0.247 μm for CaP and 0.226 μm for CaMgP. CaMgP containing Mg ions was harder than CaP containing without Mg ions, also, elastic modulus of CaMgP sample was higher than that of CaP sample.