Department of Applied Chemistry, Graduate School of Engineering Osaka Prefecture University, Gakuen-cho, Naka-ku, Japan
*Corresponding author: Takeuchi M, Graduate School of Engineering Osaka Prefecture University, Gakuen-cho, Naka-ku, Sakai, Osaka, Japan. Tel: +81-722549287; Fax: +81-722549910; Email: masato-t@chem.osakafu-u.ac.jp
Received Date: 14 February, 2018; Accepted Date: 23 February, 2018; Published Date: 6 March, 2018
1. Abstract
In order to clarify the mechanism of
strong osseointegration by UV light irradiation on titanium surface, we have
discussed the correlation between the surface wettability and cleanliness at
the molecular level. When the Ti disks (machined) were irradiated with UV-A
(mainly 365 nm) or UV-C (mainly 254 nm) light, the contact angles of the water
droplets smoothly decreased but leveled off at ca. 35° or 10°,
respectively. This phenomenon could be explained by the low photo catalytic activity due to the
thin TiO2 passive layer. On the other
hand, the Ti disks (acid-etched) showed high wettability by both UV-A or UV-C
light irradiation. From the results of XPS measurements, UV-A light irradiation
of the Ti disks was found to decompose the C-C bonds in hydrocarbons but hardly
decomposed the O-C=O bonds. In contrast, UV-C light irradiation of the Ti disks
could effectively decompose both the C-C and O-C=O bonds in hydrocarbons by photo catalytic and photochemical
effects. Sufficiently strong osseointegration of the disks
(acid-etched) achieved by UV-C light irradiation could be explained by entirely
decontamination of hydrocarbons from the titanium surfaces.
1.1.
Graphical
Abstract_XPS
High wettability of titanium
surfaces can be achieved by partial decontamination of hydrocarbons. However,
in order to obtain the strong enough osseointegration between bone tissues and
titanium implants, hydrocarbons should be entirely decontaminated by UV-C light
irradiation (mainly 254 nm).
Keywords: Osseointegration (bone to titanium attachment); Surface Wettability; Wavelengths of UV Light, TiO2 Passive Layer on Titanium Metal; XPS Analyses
1. Introduction
Titanium metal is widely used for aircraft bodies, golf clubs, glasses, bicycles, and other structural components or systems because of its lightweight property and durability. Moreover, biochemically inert titanium is also used as biomaterials for fracture treatments or restorative treatments such as dental implants [1,2]. However, if a titanium implant does not attach or fuse well to the bone tissues, it may loosen and fall out in a short period. Thus, sufficiently strong osseointegration is desired for successful implant therapy. Ogawa et al. have reported that UV-C light irradiation (mainly 254 nm) on titanium dental implants, of which the surface was fully oxidized by sulfuric acid, dramatically enhanced osseointegration at least 8-fold as compared to conventional commercial dental implants [3-6]. They have also mentioned that osteoblast cells were efficiently cultivated (a cell-philic property) and thick bone tissues were tightly formed on the titanium implants irradiated with UV-C light.
In this study, the correlation between the wettability and cleanliness levels of titanium surfaces have been discussed from the viewpoint of surface science. It is known that TiO2 shows photo catalytic reactivity to decompose various organic compounds into CO2 and H2O under UV light irradiation. However, we have reported that carboxylic acids interacting with the Ti4+ sites of TiO2 surfaces are very slowly decomposed as compared to alcohols under UV-A light irradiation (mainly 365 nm). Therefore, the effects of the UV light wavelengths on the decontamination of titanium surfaces have been evaluated in detail by XPS analyses.
1.1. Experimental
Sample preparation [3, 4]
Two different types of titanium disks (diameter: 20 mm, thickness: 1.5 mm) were prepared as model surfaces of titanium dental implants. One was mechanically polished by a lathe and denoted as Ti disk (machined), clearly showing a metallic luster. The other was treated with a H2SO4 solution (67 %) at 393 K for 75 sec and denoted as Ti disk (acid-etched). The surface of the Ti disk (acid-etched) was a gray color due to the formation of the TiO2 layers.
1.2. Characterization
The Ti disk samples were
characterized by X-ray diffraction (XRD-6100, Shimadzu, Japan), FE-SEM
observation (SU8010, Hitachi, Japan) and X-ray photoelectron spectroscopy
(ESCA3200, Shimadzu, Japan). Before the SEM observation, the samples were
cleaned by ultrasonic treatment in acetone. XPS measurements were performed
under high vacuum conditions (6 x 10-7
Pa) after fixed periods of UV light irradiation. Surface wettability of the Ti
disks was evaluated using a water contact angle meter (CA-X, Kyowa Interface
Science, Japan) in a clean room of Osaka Prefecture University under controlled
conditions (class 10, 273 K, 46% humidity). The Ti disks were irradiated with
UV light for fixed periods and water of ca. 1 mL was dropped onto the surface with a micro syringe. The contact
angles of water droplets were measured at 4 - 5
points of each sample to obtain the average values.
1.3. UV light Irradiation
UV-A region light (l = 315 - 380 nm) was irradiated onto the Ti disks under ambient conditions with a high-pressure Hg lamp (Toshiba, SHL-100UVQ-2, 100 W). The UV light intensity measured by a UV radiometer (Topcon, UVR-2) was ca. 0.5 mW/cm2 (l = 360±20 nm) and ca. 0.03 mW/cm2 (250±20 nm) at an irradiation distance of 23 cm. UV-C region light (l = 200 - 280 nm) was similarly irradiated onto the Ti disks under ambient conditions using a bacterial lamp (low-pressure Hg lamp) (Toshiba, GL-15, 15 W). The UV light intensity was ca. 0.1 mW/cm2 (l = 360±20 nm) and ca. 5.0 mW/cm2 (250±20 nm) at an irradiation distance of 5 cm.
2. Results and Discussion
(Figure 1) shows
the Ti2p
XPS spectra of the Ti disk (machined) and Ti disk (acid-etched) samples. For
the Ti disk (machined), the Ti2p3/2
peak due to a Ti4+ state was observed
at 458.4 eV but no peaks due to reduced Ti3+ or Ti0
states were observed [2,7,8]. These results
clearly indicate that a stoichiometric TiO2
passive layer exists even on the Ti disk (machined) having a metallic luster.
Similarly, the gray-colored Ti disk (acid-etched) showed a Ti2p3/2
peak due to a Ti4+ state at 458.0 eV, suggesting that the TiO2 layer was relatively thicker of at least 10 - 50 nm than the Ti disk (machined). In
order to discuss the different surface morphologies of the Ti disks, SEM
observation was performed.
4.1. Support Information_XPS
(Figure 2)
shows the SEM images of the surface morphologies of the Ti disks were discussed
by SEM images. Ti disk (machined) and Ti disk (acid-etched) samples. Although
the Ti disk (machined) with a metallic luster showed radial grooves due to a
mechanical polish treatment, the surface was relatively smooth in a micron size
order. In contrast, the Ti disk (acid-etched) with a gray color showed a rough
surface with ca. 20 - 30
mm-sized grains. However, from the
XRD patterns shown in (Figure 1) S1, all
diffraction peaks were assigned to titanium metal and no diffraction peaks due
to anatase or rutile phases were observed. This means the stoichiometric TiO2 passive layer on the Ti disks exists as an
amorphous phase.
(Figure 3) shows the time courses of the contact angles of water droplets on the Ti disks under UV-A light irradiation (l = 315 - 380 nm) with a high-pressure Hg lamp. When the Ti disk (machined) was irradiated with UV-A light, the contact angles of water droplets decreased but leveled off at 30 - 35°. Although the UV-A light intensity was increased up to 4 times (irradiation distance was shortened from 23 cm to 11 cm), the water contact angles on the Ti disk (machined) hardly changed because of a relatively smooth surface morphology. In contrast, when the Ti disk (acid-etched) was irradiated with UV-A light irradiation from a 23cm distance (condition 1 [0 - 48 h]: ca. 0.5 mW/cm2 at l = 360±20 nm and ca. 0.03 mW/cm2 at 250±20 nm), the contact angles of water droplets quickly decreased to 0° in 30 min because of a rough surface morphology. These results suggested that the surface wettability of the Ti disks was closely related to the surface morphologies.
·
Condition
1: distance: 23 cm (0 - 48
hours), l = 360±20
nm: ca. 0.5 mW/cm2, 250±20nm: ca. 0.03 mW/cm2.
· Condition 2: distance: 11 cm (48 - 72 hours), l = 360±20 nm: ca. 2.0 mW/cm2, 250±20nm: ca. 0.07 mW/cm2.
In order to discuss the correlation
between surface wettability and the amounts of hydrocarbons on the Ti disks
under UV-A light irradiation (l
= 315 - 380 nm), XPS measurements were carried
out. (Figure 4) shows the C1s XPS spectra of the
Ti disk surfaces under UV-A light irradiation. The main band at ca. 285 eV and sub and at ca. 289 eV
can be assigned to the C-C and O-C=O bonds of the hydrocarbons, respectively [9-11]. It was clearly shown that the band due to the
C-C bond efficiently decreased but the O-C=O bond hardly decreased by UV-A
light irradiation. As reported in previously, carboxylic acids or aldehydes
were very slowly decomposed by the TiO2
photo catalysts with UV-A light irradiation since carboxylic compounds strongly
interact with the Ti4+ sites of TiO2 surfaces [12,13].
These results clearly indicate that high wettability on the Ti disks with a TiO2 passive layer can be achieved only by
partially decontaminating the hydrocarbons on its surface by UV-A light
irradiation [14,15]. However, Ogawa et al. have
mentioned sufficiently strong interaction between bone tissue and the titanium surface
as well as high cell-philic properties were not obtained by UV-A light
irradiation from a high-pressure Hg lamp [3-6].
These results clearly suggest that the carboxylic compounds and/or carboxylate
ions left on the titanium surfaces inhibit the efficient cultivation of
osteobrast cells as well as the formation of bone tissue.
Next, we have investigated the
changes in surface wettability of the Ti disks under UV-C light irradiation (l = 200 - 280 nm) with a bacterial
lamp (Figure 5).
In order to clarify the effect of
UV-C light irradiation (l
= 200 - 280 nm) on the surface wettability of
the Ti disks, hydrocarbons adsorbed on the Ti disks were evaluated by XPS
measurements (Figure 6).
Shows the C1s XPS spectra of the Ti disks under UV-C light irradiation. When the Ti disks were irradiated with UV-C light from a bacterial lamp, both bands due to C-C (285 eV) and O-C=O (289 eV) effectively decreased. As mentioned in (Figure 4), since carboxylic compounds on the Ti disks are hardly decomposed by UV-A light irradiation, the efficient decontamination of carboxylic compounds by UV-C light irradiation is assumed to be a photochemical effect. In fact, USHIO Co. Ltd. (a developer of light sources in Japan) has reported that quartz surfaces showed high surface wettability by vacuum-UV light irradiation from a Xe2 excimer lamp in the presence of O2 or O3 [16]. They have also mentioned that changes in wettability of quartz surfaces are closely related to a decrease in the intensity of the C1s XPS spectra. Since a quartz surace does not show any photo catalytic reactivity, the high surface wettability obtained by V-UV light irradiation can be explained by the efficient decontamination of hydrocarbons. Ogawa et al. have actually mentioned sufficiently strong interaction between bone tissue and titanium implants could be obtained by UV-C light irradiation from a bacterial lamp [3-6]. These results attest to the strong osseointegration between bone tissue and titanium dental implants which can be achieved by entirely decontaminating hydrocarbons by UV-C light irradiation.
Finally, as shown in (Figure 7), we have confirmed the correlation between the
surface wettability and the intensity of the C1s XPS spectra.
When the Ti disk, which showed high wettability by UV-C light irradiation, was
placed in dark conditions for 3 days, the contact angle of water droplets was
still 0°. At this time, the main band
due to the C-C bond at 285 eV slightly increased
but the sub and due to the O-C=O bond at 289 eV hardly increased. When the Ti
disks were placed in the dark for a long period (1 - 4
weeks), the surface wettability recovered to its initial state and the
intensity of the C1s XPS spectra, especially due to the O-C=O bond, gradually
increased. From such experimental evidence, high surface wettability could, in
fact, be achieved by removing some fraction of the hydrocarbons on the Ti disks
by UV-A light irradiation (l
= 315 - 380 nm). However, the entire
decontamination of hydrocarbons on the TiO2
surface by UV-C light irradiation could sustain high wettability for long periods
even under dark conditions.
3. Conclusions
The effect of UV light irradiation of different wavelengths on titanium surfaces used in dental therapy were investigated at the molecular level from the viewpoint of the correlation between increased surface wettability and cleanliness for improved osseointegration. A stoichiometric TiO2 passive layer with an amorphous phase existed even on the Ti disk (machined) having a metallic luster. On the other hand, a thicker TiO2 layer was confirmed to exist on the surface of the Ti disk (acid-etched) resulting in a gray color.
Although there is a difference in
degree, UV-A light (l
= 315 - 380 nm) irradiation could improve the
surface wettability of the Ti disks. However, although UV-A light irradiation
could efficiently decompose the C-C bonds in hydrocarbons, it hardly decomposed
the O-C=O bonds in carboxylic compounds. That is, the highly wettable titanium
surface with a TiO2 passive layer
can, in fact, be achieved by decontaminating even some fraction of the
hydrocarbons. In contrast, UV-C light irradiation (l = 200 - 280 nm) could more
efficiently improve surface wettability of the Ti disks as
compared to UV-A light irradiation. Moreover, UV-C light irradiation on the Ti
disks effectively decomposed the C-C and O-C=O bonds in hydrocarbons by both photo
catalytic and photochemical effects. The Ti disks (acid-etched) on which most
of the carboxylic compounds were decomposed by UV-C light irradiation showed
high surface wettability for a period as long as one week. Studies are underway
to further elucidate the photo catalytic mechanisms involved in order to
strengthen osseointegration.
Shows the time courses of the
contact angles of water droplets on the Ti disks (A) under UV-C light
irradiation and (B) under dark conditions. When the Ti disk (acid-etched) was
irradiated with UV-C light, the contact angles of water droplets immediately
decreased to 0° in 30 min.
Interestingly, when the Ti disk (acid-etched) was placed in the dark, high
wettability (water contact angle of 0°)
was maintained for 7 days. In contrast, when the Ti disk
(machined) was irradiated with UV-C light in the same manner, the contact
angles of water droplets decreased but leveled off at ca. 10°. As compared to the results in (Figure 3-(a)), UV-C light irradiation (mainly 254 nm)
was clearly more effective in obtaining high surface wettability of the Ti
disks than UV-A light irradiation (mainly 365 nm). When the Ti disk (machined)
was placed in the dark after UV light irradiation, the contact angles of water
droplets immediately increased
Figure 1: Ti2p
XPS spectra of: (a) Ti disk (machined) and (b) Ti disk (acid-etched). Inset: photographs
of the Ti disks.
Figure 1:
S1 XRD patterns of (a) Ti disk (machined) and (b) Ti disk (acid etched)
Figure
2: SEM images of: (a) Ti disk (machined) and (b) Ti disk
(acid-etched).
Figure
3: Time courses for changes in the contact angles of water
droplets on the Ti disks under UV-A light irradiation using a high-pressure Hg
lamp.
Figure
4: C1s XPS spectra of: (a) Ti disk (machined) and (b) Ti disk
(acid-etched) under UV-A light irradiation (distance: 23 cm, l = 360±20
nm: ca. 0.5 mW/cm2, 250±20 nm: ca. 0.03 mW/cm2)
using a high-pressure Hg lamp.
Figure
5: Time courses for changes in the contact angles of water
droplets on the Ti disks (A) under UV-C light irradiation (distance: 5 cm, l = 360±20
nm: ca. 0.1 mW/cm2, 250±20 nm: ca. 5.0 mW/cm2)
using a 15 W bacterial lamp (low-pressure Hg lamp) and (B) under dark
conditions. (a) Ti disks (machined) and (b) Ti disk (acid-etched)
Figure
6: C1s XPS spectra of: (a) Ti disk (machined) and (b) Ti disk
(acid-etched) under UV-C light irradiation (distance: 5 cm, l = 360±20
nm: ca. 0.05 mW/cm2, 250±20 nm: ca. 5.0 mW/cm2)
using a bacterial lamp (low-pressure Hg lamp).
Figure
7: Contact angles of water droplets on the Ti disk
(acid-etched) under dark conditions and the corresponding C1s XPS spectra.
4.
Att W, Hori N, Takeuchi M,
Ouyang J, Yang Y, et al. (2009) Time-dependent degradation of titanium
osteoconductivity: An implication of biological aging of implant materials. Biomaterials 30: 5352-5363.
14.
Takeuchi M, Sakamoto K,
Martra G, Coluccia S, Anpo M (2005) Mechanism of photoinduced
superhydrophilicity on the TiO2
photocatalyst surface. J Phys Chem B 109:
15422-15428.
Citation: Takeuchi M, Anpo M (2018) Effect of UV light irradiation of different wavelengths on the surface wettability of titanium metal for dental implants. J Mater Sci Res: JMSR-109. DOI: 10.29011/ JMSR-109/100009