A Novel High-Energy Vacuum Ultraviolet Light Photofunctionalization Approach for Decomposing Organic Molecules around Titanium

Titanium undergoes biological aging, represented by increased hydrophobicity and surface accumulation of organic molecules over time, which compromises the osseointegration of dental and orthopedic implants. Here, we evaluated the efficacy of a novel UV light source, 172 nm wavelength vacuum UV (VUV), in decomposing organic molecules around titanium. Methylene blue solution used as a model organic molecule placed in a quartz ampoule with and without titanium specimens was treated with four different UV light sources: (i) ultraviolet C (UVC), (ii) high-energy UVC (HUVC), (iii) proprietary UV (PUV), and (iv) VUV. After one minute of treatment, VUV decomposed over 90% of methylene blue, while there was 3-, 3-, and 8-fold more methylene blue after the HUVC, PUV, and UVC treatments, respectively. In dose-dependency experiments, maximal methylene blue decomposition occurred after one minute of VUV treatment and after 20–30 min of UVC treatment. Rapid and effective VUV-mediated organic decomposition was not influenced by the surface topography of titanium or its alloy and even occurred in the absence of titanium, indicating only a minimal photocatalytic contribution of titanium dioxide to organic decomposition. VUV-mediated but not other light source-mediated methylene blue decomposition was proportional to its concentration. Plastic tubes significantly reduced methylene blue decomposition for all light sources. These results suggest that VUV, in synergy with quartz ampoules, mediates rapid and effective organic decomposition compared with other UV sources. This proof-of-concept study paves the way for rapid and effective VUV-powered photofunctionalization of titanium to overcome biological aging.

UV photofunctionalization or UV activation was developed to overcome the biological aging of titanium [20,23,34,35,. The treatment of titanium with UV light decomposes and removes carbon-containing molecules, reducing the atomic percentage of carbon from 40-75% to 15-20% [20,[48][49][50]76]. Treated titanium surfaces consequently regain hydrophilicity. UV-treated, decarbonized titanium surfaces then recruit osteogenic cells, facilitate cell attachment and adhesion, promote osteoblastic proliferation and differentiation, and accelerate and increase bone formation around titanium [21,23,24,48,49,57,62,77,78]. However, there remain three major challenges to the practical and effective use of UV photofunctionalization in clinical practice [50,51,53,79,80]. First, UV treatment has not been optimized; there is still uncertainty about the optimal UV dose or if there even is an optimal dose. Second, the treatment time remains long; despite significant advances in photo-technology, the proposed UV treatment time is still 48 h [48,49,81], 20 min [38,44,[51][52][53]63,80,[82][83][84], or 12 min [33,37,42] depending on the light source; these intervals which may be acceptable for research but are all too long for routine clinical use. Third, current UV treatment can only be achieved when titanium specimens have been removed from their sterile containers due to only very limited penetration of UV light into these containers [53,60,63,85,86]. Implantable medical devices, including dental and orthopedic implants, pins, plates, and screws, are packed in sterilized plastic or metal containers and are expected to be immediately delivered to patients directly from these containers. Exposing the devices to the atmosphere or surroundings during UV treatment carries a risk of contamination. Unfortunately, the higher the UV photon energy is, the lower the penetration through current container materials is. UV photofunctionalization was originally described using a bactericidal UV lamp for 48 h [49]. The lamp emitted UV-C (200-280 nm wavelength), with a peak around 254 nm. The UV-mediated decomposition of carbon-containing molecules is thought to occur via two mechanisms: direct decomposition or through photocatalytic decomposition via titanium dioxide [49,50,[87][88][89][90]. In the direct UV decomposition mechanism, UV light breaks atomic bonds according to the strength of photon energy; for instance, a relatively weak, single oxygen-carbon (C-O) bond can be broken by low-energy UV-A (374 nm wavelength), while double oxygen bonds (O=O) can be dissociated by high-energy UV-B (243 nm wavelength). Another mechanism of direct UV-mediated organic decomposition is the generation of oxygen radicals. For example, UV-C produces radicals through ozone, which attacks organic molecules, with dissociated molecules such as CO 2 , H 2 O, and O 2 being released into the atmosphere. In the second mechanism, photocatalytic organic decomposition is induced by two reactive oxygen species produced on titanium dioxide [89][90][91][92]. UV energy releases free electrons (e − ) and forms a positive hole (h + ) on titanium dioxide as a result. Free electrons (e − ) react with atmospheric oxygen (O 2 ) to produce superoxide anions (O 2 .− ), which in turn dissolve organic molecules through oxidation. Positive holes (h + ) react with atmospheric water (H 2 O) to generate hydroxy radicals ( . OH), which act in a manner similar to that of superoxide anions.
Recent technical advances have resulted in a high-energy, low-pressure mercury vapor lamp that reduced UV treatment to 20 min to generate a similar effect to 48 h bactericidal UV treatment, and the latest UV device uses a proprietary wavelength for 12 min treatment. Here, we tested the potential of a novel, xenon excimer lamp emitting 172 nm wavelength vacuum UV (VUV; defined as UV < 200 nm) to decompose organic molecules around titanium. Specifically, we determined whether the combined use of 172 nm VUV and a quartz ampoule [93,94] as a specimen container overcame the abovementioned three challenges. Methylene blue (C 16 H 18 ClN 3 S), a benzene ring, hydrocarboncontaining molecule, was used as a model organic molecule. We tested the device on microroughened commercially pure titanium surfaces (most commonly used in dental and orthopedic implants), machined titanium surfaces, and machined titanium alloy surfaces. The efficacy of organic decomposition was assessed according to (i) direct decomposition induced by UV light and (ii) photocatalytic decomposition induced by UV-excited titanium dioxide. Finally, we attempted to optimize the UV dose by identifying a plateau in efficacy.

Surface Characteristics of Titanium Materials
The surface morphology and hydrophilic/hydrophobic state of three titanium test materials were examined. The surface of acid-etched commercially pure grade 4 titanium was significantly rougher than that of machined grade 4 titanium and machined grade 5 titanium alloy according to low-magnification SEM (Figure 1). The high-magnification images showed that the acid-etched commercially pure titanium was evenly roughened, with a combination of scattered supra-micron concavities (arrowheads in lower-left panel) and micro-pits over the surface. The machined surfaces, regardless of titanium type, had no defined topography except for linear traces from machine milling.

Surface Characteristics of Titanium Materials
The surface morphology and hydrophilic/hydrophobic state of three titanium test materials were examined. The surface of acid-etched commercially pure grade 4 titanium was significantly rougher than that of machined grade 4 titanium and machined grade 5 titanium alloy according to low-magnification SEM (Figure 1). The high-magnification images showed that the acid-etched commercially pure titanium was evenly roughened, with a combination of scattered supra-micron concavities (arrowheads in lower-left panel) and micro-pits over the surface. The machined surfaces, regardless of titanium type, had no defined topography except for linear traces from machine milling. The hydrophilicity of acid-etched microroughened commercially pure titanium was examined before and after UV treatment for 1 min using four different light sources: (i) ultraviolet C (UVC), (ii) high-energy UVC (HUVC), (iii) proprietary UV (PUV), and (iv) VUV ( Figure 2). Original titanium surfaces were hydrophobic, with a contact angle > 90°. All UV treatments converted the surfaces to hydrophilic (defined as a contact angle < 30°), the degree varied according to the UV source, with the HUVC, PUV, and VUV treatments all resulting in a contact angle of 0° and UVC in a contact angle of ~30%. Hydrophilic/hydrophobic state of acid-etched microroughened titanium surfaces before and after UV treatment exposed to four different UV light sources for one minute: UVC from a lowpressure mercury lamp (UVC); high-energy UVC (HUVC); a UV device with a proprietary protocol The hydrophilicity of acid-etched microroughened commercially pure titanium was examined before and after UV treatment for 1 min using four different light sources: (i) ultraviolet C (UVC), (ii) high-energy UVC (HUVC), (iii) proprietary UV (PUV), and (iv) VUV ( Figure 2). Original titanium surfaces were hydrophobic, with a contact angle > 90 • . All UV treatments converted the surfaces to hydrophilic (defined as a contact angle < 30 • ), the degree varied according to the UV source, with the HUVC, PUV, and VUV treatments all resulting in a contact angle of 0 • and UVC in a contact angle of~30%.

Organic Molecule Decomposition Induced by Different UV Light Sources
We next compared the ability of different UV sources to decompose organic molecules. Microroughened commercially pure titanium was used as a representative test material and methylene blue as a model organic molecule. Methylene blue placed in a quartz ampoule was treated with UV light for 1 min with or without a titanium specimen. All UV treatments significantly decomposed methylene blue regardless of the light source; however, the amount of remaining methylene blue varied considerably (p < 0.001, two-way ANOVA), with UVC being the highest and VUV the lowest ( Figure 3A) (p < 0.001, Bonferronicorrected). With a titanium specimen, one minute of VUV treatment decomposed over 90% of methylene blue, versus~25% for UVC and~75% for HUVC and PUV. UV treatment with a titanium specimen decomposed more methylene blue than without a titanium specimen for all light sources (p < 0.05, Bonferroni-corrected). The difference in methylene blue decomposition with and without a titanium specimen was represented as photocatalytic contribution (%), which was significantly lower for HUVC, PUV, and VUV (<5%) than for UVC (~45%) (p < 0.001; Figure 3B).

Figure 1.
Surface morphology of titanium specimens used in this study. Low (upper panels)-and high (lower panels)-magnification SEM images of three different specimens.
The hydrophilicity of acid-etched microroughened commercially pure titanium was examined before and after UV treatment for 1 min using four different light sources: (i) ultraviolet C (UVC), (ii) high-energy UVC (HUVC), (iii) proprietary UV (PUV), and (iv) VUV ( Figure 2). Original titanium surfaces were hydrophobic, with a contact angle > 90°. All UV treatments converted the surfaces to hydrophilic (defined as a contact angle < 30°), the degree varied according to the UV source, with the HUVC, PUV, and VUV treatments all resulting in a contact angle of 0° and UVC in a contact angle of ~30%. Figure 2. Hydrophilic/hydrophobic state of acid-etched microroughened titanium surfaces before and after UV treatment exposed to four different UV light sources for one minute: UVC from a lowpressure mercury lamp (UVC); high-energy UVC (HUVC); a UV device with a proprietary protocol

Organic Molecule Decomposition Induced by Different UV Light Sources
We next compared the ability of different UV sources to decompose organic molecules. Microroughened commercially pure titanium was used as a representative test material and methylene blue as a model organic molecule. Methylene blue placed in a quartz ampoule was treated with UV light for 1 min with or without a titanium specimen. All UV treatments significantly decomposed methylene blue regardless of the light source; however, the amount of remaining methylene blue varied considerably (p < 0.001, twoway ANOVA), with UVC being the highest and VUV the lowest ( Figure 3A) (p < 0.001, Bonferroni-corrected). With a titanium specimen, one minute of VUV treatment decomposed over 90% of methylene blue, versus ~25% for UVC and ~75% for HUVC and PUV. UV treatment with a titanium specimen decomposed more methylene blue than without a titanium specimen for all light sources (p < 0.05, Bonferroni-corrected). The difference in methylene blue decomposition with and without a titanium specimen was represented as photocatalytic contribution (%), which was significantly lower for HUVC, PUV, and VUV (<5%) than for UVC (~45%) (p < 0.001; Figure 3B).

Dose Dependency of Organic Decomposition Induced by VUV and UVC
After establishing that VUV had the greatest methylene blue decomposition efficacy, we tested for dose dependency by varying the treatment time. With a titanium specimen in a quartz ampoule, the amount of remnant methylene blue significantly decreased with the increase in treatment time up to 60 s to then plateau at about 10%, fitting a negative exponential curve with a remarkably high coefficient of determination ( Figure 4A). VUV treatment without a titanium specimen showed a similar dose dependency, but with a lower efficacy of decomposition. The photocatalytic contribution decreased with longer treatment time ( Figure 4B). Thus, the maximum efficacy of organic decomposition induced by VUV was achieved with an optimized treatment time of 1 min.

Dose Dependency of Organic Decomposition Induced by VUV and UVC
After establishing that VUV had the greatest methylene blue decomposition efficacy, we tested for dose dependency by varying the treatment time. With a titanium specimen in a quartz ampoule, the amount of remnant methylene blue significantly decreased with the increase in treatment time up to 60 s to then plateau at about 10%, fitting a negative exponential curve with a remarkably high coefficient of determination ( Figure 4A). VUV treatment without a titanium specimen showed a similar dose dependency, but with a lower efficacy of decomposition. The photocatalytic contribution decreased with longer treatment time ( Figure 4B). Thus, the maximum efficacy of organic decomposition induced by VUV was achieved with an optimized treatment time of 1 min. We further tested for the presence of UV dose dependency using UVC. Again, the length of UVC treatment was proportional to the degree of methylene blue decomposition ( Figure 5A), plateauing at 20-30 min along a negative exponential curve. With both UV sources, UV treatment was unable to completely decompose methylene blue within the treatment time tested, suggesting a general limit to UV-mediated organic decomposition. Photocatalysis induced by a titanium specimen contributed less as treatment time increased ( Figure 5B).  We further tested for the presence of UV dose dependency using UVC. Again, the length of UVC treatment was proportional to the degree of methylene blue decomposition ( Figure 5A), plateauing at 20-30 min along a negative exponential curve. With both UV sources, UV treatment was unable to completely decompose methylene blue within the treatment time tested, suggesting a general limit to UV-mediated organic decomposition. Photocatalysis induced by a titanium specimen contributed less as treatment time increased ( Figure 5B).

Effect of Quartz Ampoules on UV-Mediated Organic Decomposition
We next compared the impact of quartz ampoules and laboratory-grade plastic tubes on organic decomposition efficacy by assessing methylene blue decomposition using one minute of UV treatment with or without a titanium specimen. Methylene blue decomposition was greater in quartz ampoules than plastic tubes for all UV treatments ( Figures 3A and 6A); for instance, there was >90% residual methylene blue in plastic tubes after UVC treatment compared with 75% in quartz ampoules. VUV showed the highest efficacy of decomposition among the UV sources, with approximately 55% remnant methylene blue, although significantly more than the 9% seen in the quartz ampoule ( Figures 3A and 6A). The photocatalytic contribution was significantly higher in plastic tubes than in quartz ampoules ( Figures 3B and 6B), with the majority of methylene blue decomposition in plastic tubes being due to titanium photocatalysis during UVC decomposition.

Effects of Different Titanium Materials and Surfaces
We next determined if the organic decomposition efficacy differed between titanium specimens using UV treatment under four representative conditions (1 min UVC in quartz ampoules, 1 min UVC in plastic tubes, 1 min VUV in quartz ampoules, and 1 min VUV in plastic tubes). There was no significant difference in efficacy between microroughened commercially pure titanium, machine-surfaced commercially pure titanium, and machine-surfaced titanium alloy (Figure 7), indicating that the photocatalytic activity was not significantly affected by different titanium materials or surfaces.
quantified and plotted against UV treatment time; it showed a highly fitted, negative exponential curve. (B) Photocatalytic contribution (%) calculated as the difference in the amount of decomposed methylene blue with and without a microroughened titanium specimen.
We further tested for the presence of UV dose dependency using UVC. Again, the length of UVC treatment was proportional to the degree of methylene blue decomposition ( Figure 5A), plateauing at 20-30 min along a negative exponential curve. With both UV sources, UV treatment was unable to completely decompose methylene blue within the treatment time tested, suggesting a general limit to UV-mediated organic decomposition. Photocatalysis induced by a titanium specimen contributed less as treatment time increased ( Figure 5B).

Effect of Quartz Ampoules on UV-Mediated Organic Decomposition
We next compared the impact of quartz ampoules and laboratory-grade plastic tubes on organic decomposition efficacy by assessing methylene blue decomposition using one minute of UV treatment with or without a titanium specimen. Methylene blue decomposition was greater in quartz ampoules than plastic tubes for all UV treatments ( Figures 6A  and 3A); for instance, there was >90% residual methylene blue in plastic tubes after UVC treatment compared with 75% in quartz ampoules. VUV showed the highest efficacy of decomposition among the UV sources, with approximately 55% remnant methylene blue, although significantly more than the 9% seen in the quartz ampoule ( Figures 6A and 3A). The photocatalytic contribution was significantly higher in plastic tubes than in quartz ampoules ( Figures 6B and 3B), with the majority of methylene blue decomposition in plastic tubes being due to titanium photocatalysis during UVC decomposition.

Effects of Different Titanium Materials and Surfaces
We next determined if the organic decomposition efficacy differed between titanium specimens using UV treatment under four representative conditions (1 min UVC in quartz ampoules, 1 min UVC in plastic tubes, 1 min VUV in quartz ampoules, and 1 min VUV in plastic tubes). There was no significant difference in efficacy between microroughened commercially pure titanium, machine-surfaced commercially pure titanium, and machine-surfaced titanium alloy (Figure 7), indicating that the photocatalytic activity was not significantly affected by different titanium materials or surfaces.

Load Testing of UV-Mediated Organic Decomposition
We next load-tested the four different UV sources to determine their tolerance to or capacity of organic decomposition. Using a one-minute treatment protocol, various concentrations of methylene blue were decomposed in a quartz ampoule. The UVC treatment did not increase the amount of decomposed methylene blue, rather declining it to near zero as the methylene blue concentration increased (Figure 8). The HUVC and PUV treatments showed a load-dependent increase up to x4 load, followed by a plateau or decline. The regression analysis fitted a polynomial equation for UVC, HUVC, and PUV treatments, with very high coefficients of correlation. In contrast, methylene blue decomposition linearly increased with the increase in VUV treatment load, with greater concentrations of methylene blue resulting in greater removal. These results indicate unlimited tolerance of VUV-mediated organic decomposition compared with a maximal capacity for other UV sources within the range of loading tested in this study.

Load Testing of UV-Mediated Organic Decomposition
We next load-tested the four different UV sources to determine their tolerance to or capacity of organic decomposition. Using a one-minute treatment protocol, various concentrations of methylene blue were decomposed in a quartz ampoule. The UVC treatment did not increase the amount of decomposed methylene blue, rather declining it to near zero as the methylene blue concentration increased (Figure 8). The HUVC and PUV treatments showed a load-dependent increase up to x4 load, followed by a plateau or decline. The regression analysis fitted a polynomial equation for UVC, HUVC, and PUV treatments, with very high coefficients of correlation. In contrast, methylene blue decomposition linearly increased with the increase in VUV treatment load, with greater concentrations of methylene blue resulting in greater removal. These results indicate unlimited tolerance of VUV-mediated organic decomposition compared with a maximal capacity for other UV sources within the range of loading tested in this study.

Discussion
In pursuit of advancing UV photofunctionalization for routine application to dental and orthopedic implants, here, we conducted a series of experiments to optimize the speed and efficacy of organic molecule decomposition induced by UV light sources, especially a novel, xenon excimer lamp emitting 172 nm wavelength VUV. VUV showed particularly high efficacy, which here we tested for the first time on titanium implants [95][96][97][98][99]. Only 9% of methylene blue remained after one minute of VUV treatment, while 25%

Discussion
In pursuit of advancing UV photofunctionalization for routine application to dental and orthopedic implants, here, we conducted a series of experiments to optimize the speed and efficacy of organic molecule decomposition induced by UV light sources, especially a novel, xenon excimer lamp emitting 172 nm wavelength VUV. VUV showed particularly high efficacy, which here we tested for the first time on titanium implants [95][96][97][98][99]. Only 9% of methylene blue remained after one minute of VUV treatment, while 25% remained after HUVC and PUV treatments, a 2.8-fold difference in efficacy. UVC was the least effective light source tested, with 74% of methylene blue remaining after treatment. HUVC and PUV have been commercialized for photofunctionalizing dental implants using 20 and 12 min treatment protocols, respectively [33,52,100,101]. The substantial improvement offered by VUV demonstrated here strongly suggests that this approach requires further clinical validation. Furthermore, our dose-dependency experiments revealed that VUV achieved maximum organic decomposition in one minute, considerably faster than existing protocols. This optimization (1) provides empirical evidence on which to further refine the UV photofunctionalization protocol for bone integration and (2) drastically expedites the processing time for clinical application.
Robust VUV-mediated organic decomposition was only possible with the synergy afforded by the use of quartz ampoules. VUV decays proportionally with the increase in the distance from the source due to progressive absorption by the atmosphere or any intervening materials [102][103][104][105][106]. Unfortunately, higher-energy UV light with shorter wavelength decays even more when passing through materials. Furthermore, UV light decays by 60 to 100% through plastic, depending on its thickness and molecular structure, and even glass nearly completely absorbs UV light <300 nm due to impurities such as iron oxide. Indeed, we found that methylene blue decomposition was significantly compromised by plastic, but despite this, VUV retained high performance. As mentioned in the Introduction section, implantable medical devices, including dental and orthopedic implants, are packed in sterilized plastic or metal containers. The present result, revealing excellent UV permeability of quartz ampoules, provides a novel strategy and solution for packaging medical implants, compatible with photofunctionalization. Synthetic quartz can be shaped freely as shown in the present study and is widely used in the industrial field. Now that the clinically viable one-minute protocol has been introduced for effective and rapid photofunctionalization, re-designing implant packages is justified. Thus, exploiting a synergy of VUV and quartz ampoules could open the door to a new standard of implant therapy.
Three mechanisms of UV-mediated organic decomposition are outlined in Figure 9, in which contribution to titanium-mediated photocatalysis was remarkably smaller than that of direct decomposition induced by UV light (photochemical and photophysical decomposition in Figure 9) for all light sources tested. Titanium is a semiconductor, so its photocatalytic activity is induced by UVA at wavelengths of~375 nm, whose energy corresponds to the band gap of 3.2 eV on TiO 2 surfaces to release e − to the conduction band. The longer wavelength of UVA affords it higher permeability than UVB and UVC. Therefore, the photocatalytic contribution of UVC might be due to the relatively low intensity of UVC used in this study absorbed by the atmosphere and water, while the majority of UVA reached the titanium surfaces. As a result, UVA-triggered photocatalysis was pronounced. By contrast, the other UV sources of HUVC, PUV, and VUV, emitting high-intensity, short-wavelength UV rays, allowed the partial permeation of shorter UV rays through the atmosphere and water and less UVA-driven photocatalysis to be obtained. More importantly, our results suggest that photocatalytic decomposition is generally slow and has low efficiency and that direct decomposition induced by UV light is the most important parameter to consider when meeting requirements for clinical use (Figure 9). Previous studies have shown that 48 h of treatment is required to functionalize titanium using low-power bactericidal UV light [31,48,49].
1 Figure 9. Schematic of three different mechanisms of UV-mediated organic decomposition. During photochemical and photocatalytic decomposition, the generated reactive oxygens (highlighted in green) attack organic molecules. Note that 172 nm VUV light has both photochemical and photophysical advantages, enabling faster and more efficient decomposition.
We postulate that our finding that different materials and surfaces contributed little to the overall decomposition is due to the small contribution of photocatalysis to organic molecule decomposition. We conducted the experiment under the selected conditions using UVC and VUV light sources because information on light intensity was only available for these two sources. We selected the conditions in order to examine the effect of treatment time and type of container. There were no significant effects of different materials or surfaces under all conditions. In theory, the enlarged surface area of acid-etched microroughened titanium may increase photocatalysis. However, this must also be considered in the context of the different crystal properties of titanium surfaces processed in different ways. Heatmanufactured titanium dioxide particles form two crystal types, anatase and rutile; in general, anatase is stable at relatively low temperatures and more reactive to photocatalytic stimuli due to the favorable structure of the conduction band, while rutile is formed at high temperatures and is less photocatalytic [107][108][109][110][111][112][113]. Regular bulk titanium such as the machined surface used in this study is unlikely to have a crystal structure, but acidetched titanium may have an altered crystal structure due to high-temperature acid etching. Future studies should further explore these material structures, including the effect of grade 5 titanium alloy; nevertheless, the photocatalytic contribution appears to be a relatively small component of the rapid organic decomposition required for clinical applications.
High-energy 172 nm VUV should be able to break strong carbon bonds. As shown in the photophysical decomposition illustrated in Figure 9, for instance, oxygen double bonds (O=O) and carbon double bonds (C=C) can be dissociated by 243 nm and 204 nm UV. However, UVC, with a peak wavelength of 254 nm, cannot break them. Thus, shorter wavelengths should decompose more molecules. Methylene blue is a synthetic, organic chloride salt (C 16 H 18 ClN 3 S) containing many carbon double bonds in their benzene rings, making degradation very difficult [97,114]. In light of this, the remarkable capability of VUV to decompose methylene blue and its linear load-testing capacity are of particular note. In photochemical decomposition (Figure 9), reactive oxygen species (ROS) generated by UV play an essential role. Although a low-pressure mercury lamp can generate ROSexcited atomic oxygen, O( 1 D), through the production of ozone O 3 , VUV excimer lamps can also generate O( 1 D) directly from O 2 with a higher efficiency in addition to ozonemediated generation, providing another explanation for the rapid and efficient VUVmediated decomposition seen here.
One of the objectives of this study was to find a peak of decarbonization for VUV treatment. In light of this, as mentioned earlier, the one-minute treatment optimized in the dose-dependency experiment was more satisfactory than we anticipated, while the UVC light source took 20-30 min to accomplish the same. We conducted a dose-dependency test for HUVC and PUV sources but had to discontinue because the methylene blue solution started to evaporate after 1.5-3 min. Future studies using a dry technique instead of the wet technique used in this study need to be planned. More importantly, VUV is known to generate less heat than UVC, which has been demonstrated in this study and can be another advantage of VUV in clinical applications over other UV sources.
In this study, hydrophilicity/hydrophobicity and organic decomposition outcomes were not closely related. The efficacy of methylene blue removal varied considerably between the different UV light sources tested, while hydrophilic conversion was not significantly different. In fact, all light sources except for UVC converted titanium surfaces from hydrophobic to very hydrophilic or superhydrophilic (defined as a contact angle of 0 • ). This confirmed the importance of surface carbon rather than hydrophilicity as a marker for osseointegration, as reported previously [20,23,47,49]. Indeed, residual methylene blue varied between 75% and 9% on hydrophilic surfaces. We propose that surface hydrophilicity is an early marker of titanium photofunctionalization, i.e., hydrophilicity is a necessary but not sufficient factor for improved titanium bioactivity. More significantly, VUV treatment resulted in maximal hydrophilicity and organic decomposition within a minute.

Titanium Specimens and Surface Characterization
Titanium test specimens in rectangular plate form (14 mm × 6 mm, 2 mm in thickness) were machined from commercially pure grade 4 titanium and grade 5 Ti-6Al-4V alloy ( Figure 10A). To modify the surface from machine-smooth to microroughened, grade 4 commercially pure titanium plates were sandblasted and acid-etched. All test specimens were prepared and provided by DIO (Busan, Korea). Surface morphology was examined using scanning electron microscopy (SEM; Nova 230 Nano SEM; FEI, Hillsboro, Oregon). The hydrophilicity/hydrophobicity of titanium surfaces with and without UV treatment was evaluated by measuring the contact angle of 3 mL of ddH 2 O.

Methylene Blue as a Model Organic Molecule and Containers
Methylene blue was used as a model organic molecule to decompose using UV light. A 0.002% stock solution was prepared. Quartz ampoules in hollow cylinder form (10 mm in diameter, 25 mm in height, 1 mm thick) made from synthetic quartz glass ( Figure 10B) and laboratory-grade 1.5 mL plastic tubes (Fisher Scientific, Pittsburgh, PA, USA) ( Figure 10C) were prepared as containers for methylene blue solution during UV treatment. Quartz ampoules or plastic tubes were filled with 750 mL of methylene blue stock solution for UV treatment. UV treatment was administered with and without a titanium specimen in each container.

UV-Mediated Decomposition of Methylene Blue
Methylene blue in quartz and plastic containers was treated with four different UV light sources ( Figure 11): (i) UVC from a commercially available low-pressure mercury lamp (1.2 mW/cm 2 ; Iwasaki Electric, Tokyo, Japan); (ii) high-energy UVC (HUVC) from a commercially available UV device for dental implants (TheraBeam Affiny, Ushio, Tokyo, Japan); (iii) a commercially available UV device for dental implants with a proprietary protocol (PUV) (SuperOsseo, Ushio, Tokyo, Japan); and (iv) 172 nm vacuum UV (VUV; 60 mW/cm 2 ) (DIO, Busan, Korea). UVC and VUV were irradiated at 6 mm and HUVC and PUV following the manufacturers' instructions. The treatment time was 1 min for most experiments and was varied for dose-dependency experiments. The methylene blue concentrations in the original solution and after UV treatment were measured using a microplate reader at 650 nm (Synergy H1; BioTek Instruments, Winooski, VT, USA), and remnant methylene blue was calculated as a percentage relative to the original solution.

Titanium Specimens and Surface Characterization
Titanium test specimens in rectangular plate form (14 mm × 6 mm, 2 mm in thickness) were machined from commercially pure grade 4 titanium and grade 5 Ti-6Al-4V alloy ( Figure 10A). To modify the surface from machine-smooth to microroughened, grade 4 commercially pure titanium plates were sandblasted and acid-etched. All test specimens were prepared and provided by DIO (Busan, Korea). Surface morphology was examined using scanning electron microscopy (SEM; Nova 230 Nano SEM; FEI, Hillsboro, Oregon). The hydrophilicity/hydrophobicity of titanium surfaces with and without UV treatment was evaluated by measuring the contact angle of 3 mL of ddH2O.

Methylene Blue as a Model Organic Molecule and Containers
Methylene blue was used as a model organic molecule to decompose using UV light. A 0.002% stock solution was prepared. Quartz ampoules in hollow cylinder form (10 mm in diameter, 25 mm in height, 1 mm thick) made from synthetic quartz glass ( Figure 10B) and laboratory-grade 1.5 mL plastic tubes (Fisher Scientific, Pittsburgh, PA, USA) ( Figure  10C) were prepared as containers for methylene blue solution during UV treatment. Quartz ampoules or plastic tubes were filled with 750 mL of methylene blue stock solution for UV treatment. UV treatment was administered with and without a titanium specimen in each container.

Statistical Analyses
Three test specimens were used for all methylene blue decomposition experiments (n = 3). One-or two-way ANOVA was performed to examine the effect of different UV light sources, the presence or absence of a titanium specimen, and treatment time. The Bonferroni test was used as a post hoc multiple comparison test where appropriate. p-values < 0.05 were considered to be statistically significant. Regression analysis was applied to determine the associations between methylene blue decomposition parameters and UV treatment factors. protocol (PUV) (SuperOsseo, Ushio, Tokyo, Japan); and (iv) 172 nm vacuum UV (VUV; ~60 mW/cm 2 ) (DIO, Busan, Korea). UVC and VUV were irradiated at 6 mm and HUVC and PUV following the manufacturers' instructions. The treatment time was 1 min for most experiments and was varied for dose-dependency experiments. The methylene blue concentrations in the original solution and after UV treatment were measured using a microplate reader at 650 nm (Synergy H1; BioTek Instruments, Winooski, VT, USA), and remnant methylene blue was calculated as a percentage relative to the original solution.

Statistical Analyses
Three test specimens were used for all methylene blue decomposition experiments (n = 3). One-or two-way ANOVA was performed to examine the effect of different UV light sources, the presence or absence of a titanium specimen, and treatment time. The Bonferroni test was used as a post hoc multiple comparison test where appropriate. pvalues < 0.05 were considered to be statistically significant. Regression analysis was applied to determine the associations between methylene blue decomposition parameters and UV treatment factors.

Conclusions
Here, we determined the capability of a novel xenon excimer-generated VUV light source to decompose organic molecules around titanium, with a view to optimizing the photofunctionalization or activation of titanium implants for clinical use. VUV decomposed methylene blue molecules in a quartz ampoule significantly faster and with greater

Conclusions
Here, we determined the capability of a novel xenon excimer-generated VUV light source to decompose organic molecules around titanium, with a view to optimizing the photofunctionalization or activation of titanium implants for clinical use. VUV decomposed methylene blue molecules in a quartz ampoule significantly faster and with greater efficacy than other UV light sources, including low-pressure mercury-generated UVC and commercially available UV devices for dental implants. We achieved >90% VUV-mediated organic decomposition in one minute. Load testing revealed that VUV light decomposed more methylene blue as it was more concentrated, whereas there was a limit to the decomposing capacity of other UV light sources. All UV light sources tested here generated hydrophilic titanium surfaces after one minute of treatment. Thus, the ability and capacity of different UV light sources to induce organic decomposition varied considerably, with the fastest and most effective decomposition being achieved with VUV, paving the way for VUV-mediated photofunctionalization protocols and devices in clinical practice.
Author Contributions: T.S. performed experiments and data analysis and drafted the manuscript; T.M. contributed to the interpretation of data and reviewed the manuscript; K.K. was involved in experimental design and performed experiments and data analysis; T.O. was involved in experimental design, data analysis, interpretation, and the drafting of the manuscript; all authors discussed the results and commented on the manuscript. All authors have approved the final version of the manuscript.
Funding: This work was partially supported by a research gift from DIO IMPLAMT Co., Ltd.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.