Plasma-Based Fluorine Ion Implantation Into Dental Materials for Inhibition of Bacterial Adhesion

Dental Materials Journal 25（4）：684－692, 2006

Plasma-based Fluorine Ion Implantation into Dental Materials for Inhibition of Bacterial Adhesion

NURHAERANI, Kenji ARITA, Yukari SHINONAGA and Mizuho NISHINO Department of Pediatric Dentistry, Institute of Health Biosciences, The University of Tokushima Graduate School, 3-18-15, Kuramoto, Tokushima 770-8504, Japan Corresponding author, Kenji ARITA E-mail :[email protected]

Received July 28, 2006/Accepted September 12, 2006

―――――――――――――――――――――――――――――――――――――――――――――――――――――― The aims of this study were to evaluate the fluorine depth profiles of pure titanium （Ti）, stainless steel （SUS）, and polymethyl methacrylate（PMMA）modified by plasma-based fluorine ion implantation and the effects of fluorine ion implantation on contact angle, fluoride ion release, and S. mutans adhesion. Fluorine-based gases used were Ar＋F2 and CF4. By means of SIMS, it was found that the peak count of PMMA was the lowest while that of Ti was the highest. Then, up to one minute after Ar sputtering, the presence of fluorine and chromic fluoride could be detected by XPS in the surface and subsurface layer. As for the effects of using CF4 gas for fluorine ion implantation into SUS substrate, the results were: contact angle was significantly increased; no fluoride ion release was detected; antibacterial activity was significantly increased while initial adhesion was decreased. These findings thus indicated that plasma-based fluorine ion implantation into

SUS with CF4 gas provided surface antibacterial activity which was useful in inhibiting bacterial adhesion.

Key words : Ion implantation, Fluorine, Bacterial adhesion ――――――――――――――――――――――――――――――――――――――――――――――――――――――

analysis, no beam-transport optics, and no focusing INTRODUCTION or scanning of the beam14）. Therefore, compared to In a dental clinic, various kinds of appliances － such conventional ion implantation, the plasma-based ion as space maintainer and space regainer for occlusal implantation facility is smaller, less expensive, and guidance, and also as orthodontic appliances and den- simpler to maintain and operate13－17）. tures － are used frequently for children and adults. To date, some researchers have carried out sur- These appliances are susceptible to adhesion by oral face modification by fluorine ion implantation18－20） bacteria1－6）, and thus cause dental caries7,8）, periodon- and found it to be a useful means in inhibiting bacte- tal disease, or inflammation of oral soft tissue9,10）. rial adhesion. In this study, therefore, the surfaces Plasma surface modification, as an economical of Ti, SUS, and PMMA were modified by plasma- and effective material processing technique, is gain- based fluorine ion implantation to evaluate this ing popularity in the biomedical field. It is able to technique’s inhibitory effect against oral bacterial ad- change the surface chemical composition of the proc- hesion. It should be highlighted that bacterial adhe- essed material as well as improve the latter’s proper- sion to biomaterial surface is an important step dur- ties such as wettability, adhesion, dyeability, refrac- ing the initial stages of pathogenic infection. Two tive index, hardness, chemical inertness, lubricity, methods have been identified to inhibit the formation and biocompatibility11）. As such, in recent years, of microbial plaque. The first method is to inhibit there is an increasing trend to employ plasma-based the initial adhesion of oral bacteria, while the second ion implantation to modify the surface of method involves surface antibacterial activity to in- biomaterials12,13）. With plasma-based ion implanta- hibit the colonization of oral bacteria21,22）. tion, specimens are immersed in high-density plasma In view of the beneficial effects of fluorine ion and pulse-biased to a high negative potential relative implantation, the aims of this study were to evaluate to the chamber wall. Ions generated in the overlying the depth profiles and XPS spectra of fluorine con- plasma are accelerated across the sheath formed centration in the surface of dental materials, as well around the sample and implanted into the sample as evaluate the effects of fluorine ion implantation surface. However, in the event of ditch contours and on contact angle, fluoride ion release, and bacterial concave corners, it becomes difficult for the sheath to adhesion. follow the surface contour of the complex shape. Moreover, in the event of non-uniform plasma den- MATERIALS AND METHODS sity distribution around the matrix, it also becomes difficult to achieve uniform ion implantation on the Plasma-based fluorine ion implantation whole surface. Nonetheless, despite the aforemen- The principle of plasma-based ion implantation is tioned drawbacks, this technique features non-mass shown in Fig. 1 （modified from Okui et al.23））. NURHAERANI et al. 685

Plates of Ti（TP340C: 99.98 wt％ Ti, 0.003 wt％ H, X-ray photoelectron spectroscopy（XPS）

0.09 wt％ O, 0.01 wt％ N, 0.07 wt％ Fe; Kobe Steel After fluorine ion implantation using CF4 gas, the Ltd., Kobe, Japan）, SUS （SUS316L: 65.67 wt％ Fe, surface of SUS was analyzed by means of X-ray pho- 17.00 wt％ Cr, 13.00 wt％ Ni, 2.40 wt％ Mo, 1.40 wt％ toelectron spectroscopy （XPS）（ESCA-1000AX, Mn, 0.03 wt％ C, 0.50 wt％ Si; Daido Steel Co. Ltd., Shimadzu Co., Kyoto, Japan）. Measurements were Nagoya, Japan）, and PMMA （ACRYPET VH＃001, done using Mg Kα X-rays at 30 mA and 10 keV. Mitsubishi Rayon Co. Ltd., Tokyo, Japan）with size Depth profile analysis was performed using Ar gas of 10×10×1 mm were used. Before ion implantation, sputter method under these conditions: 20 mA cur- Ti and SUS plates were polished to a mirror-like sur- rent, 2 keV voltage, and gas pressure of 5×10－4 Pa. face and washed in an ultrasonic bath with 98％ alco- Sputtering speed was 20Å（SiO2）/min. Binding en- hol. PMMA plates were wiped with alcohol-soaked ergies were corrected against that of C 1s, an inter- cotton. Ti, SUS, and PMMA plates were then modi- nal standard with binding energy at 285.0 eV. fied by plasma-based ion implantation appliance at Ion Engineering Research Institute Co., Osaka, Contact angle

Japan. Fluorine gas for plasma-based ion implanta- After fluorine ion implantation by CF4 gas, the sur- tion was 95％ argon and 5％ F2（Ar＋F2）or CF4 gas. face of SUS was washed in an ultrasonic bath（J.M. Conditions of plasma-based fluorine ion implantation Ultrasonic Cleaner SUW-50D, J. Morita Co., Tokyo, are shown in Table 1. For controls, non-fluorine-ion- Japan） containing distilled water for 10 minutes. implanted Ti, SUS, and PMMA were used. After the washing procedure, the sample was dried at room temperature. Secondary ion mass spectrometry（SIMS） A drop of distilled water was carefully placed on The peak count（count per second, CPS）and depth each slightly dried, prepared surface, and contact profile until 10 counts（nm）of fluorine in Ti, SUS, angle was measured using the sessile drop technique and PMMA samples were measured with a secondary with a contact angle meter （CA-DT, Kyowa ion mass spectrometer （SIMS）（ADEPT 1010, Kaimenkagaku Co. Ltd., Saitama, Japan）. Five ULVAC Inc., Kanagawa, Japan）. Measurements points per sample were measured. A total number of were carried out using 3.0 keV Cs＋. For each mate- 10 samples were prepared for fluorine ion implanta- rial, a total of eight samples were prepared for fluo- tion（i.e., N＝5）and to serve as controls with no ion rine ion implantation by Ar＋F2 gas（i.e., N＝4）and implantation（i.e., N＝5）.

CF4 gas（i.e., N＝4）. Fluoride ion release Five SUS samples which were fluorine ion-implanted

by CF4 gas were immersed in 10 ml of deionized water in a petri dish and shaken with a speed of 110 rpm （Double Shaker NR-3, Taitec Co., Saitama, Japan）at 37℃. The deionized water was changed every 24 hours and evaluated for fluoride ion concentration in ppm daily for a period of seven days. The immersion solution was kept at room temperature before fluoride ion concentration was measured. To provide a constant background ionic strength, 0.8 ml of total ionic strength adjustment buffer solution （TISABШ, Thermo Electron Co., Beverly, MA, USA） was added to 8 ml of immersion solution. Fluoride ion concentration（ppm）was then measured with an ion specific electrode （ionplus Sure-Flow Fluoride, 9609 BN, Orion Research Inc., Beverly, MA, USA） Fig. 1 Principle of plasma-based ion implantation（modi- connected to an expandable ion analyzer （model 23） fied from Okui et al. ）. 702A, Orion Research Inc., Beverly, MA, USA）.

Table 1 Conditions of fluorine ion implantation into surface of dental materials

Gas Ar＋F2 （95％ argon and 5％ fluorine）

CF4 （100％ carbon tetrafluoride） Pressure of gas 1.0 Pa Negative electric charge 1000 Hz, 10μs Time 60 minutes Voltage 5 keV（Ti, SUS）, 3 keV（PMMA） 686 FLUORINE ION IMPLANTATION IN DENTAL MATERIALS

Calibration of the analyzer was performed before the PMMA（p＜0.05 and p＜0.001, respectively）, and SUS test with standard solutions containing 0.1, 0.5, and to be higher than PMMA（p＜0.001）. Similarly, with

1.0 ppm of fluoride. CF4, the peak count of Ti was higher than that of SUS and PMMA （p＜0.001）, and SUS was higher Microbiological examination than PMMA（p＜0.001）. S. mutans（ATCC25175 type c, Summit Pharmaceuti- Fig. 3 shows the depth profiles until 10 counts of cals International Co., Tokyo, Japan）was used for fluorine after ion implantation（N＝4）. As shown, it

the evaluation of bacterial adhesion and antibacterial was only in PMMA that CF4 led to a significantly

activity. Substrate material was SUS which was higher depth profile than Ar＋F2 （p＜0.001）. With

fluorine ion-implanted by CF4 gas. Ar＋F2 as a fluorine-based gas for ion implantation, Ti had a significantly higher depth profile than Antibacterial activity PMMA and SUS（p＜0.001 and p＜0.05, respectively）.

SUS was incubated at 37℃ in 2 ml of BHI broth con- But with CF4 , PMMA was higher than SUS taining S. mutans with a cell density of 2×107 CFU/ （p＜0.05）. ml. After 48-hour incubation, 0.5 ml of solution was immediately added to 4.5 ml of PBS（－）and diluted. X-ray photoelectron spectrometry（XPS）

Following which, 100μl of diluted solution was plated Figs. 4, 5, and 6 show the F 1s, Cr 2p3/2, and Fe 2p3/2 on BHI agar. After another 48 hours of incubation XPS spectra of SUS respectively. On the left side in at 37℃, the number of colonies was counted. A total each figure is the non-fluorine-ion-implanted control, number of 20 samples were prepared for fluorine ion while the right side shows the fluorine ion-implanted implantation （i.e., N＝10） and to serve as controls SUS substrate material. Table 2 lists the binding en- with no ion implantation（i.e., N＝10）.

Initial adhesion Into a petri dish were poured 20 ml of BHI broth and 200μl of S. mutans with a cell density of 4×108 CFU/ml, and SUS material was placed with the fluorine ion-implanted surface upward. After 4-hour incubation at 37℃, the sample was removed from the petri dish and washed three times with phosphate buffered saline without calcium and magnesium （PBS（－））. The material was then placed in a tube containing 2 ml of PBS（－）, and the tube was sonicated（Dentcraft Ultrasonic 3800N, Yoshida Co., Tokyo, Japan）for five minutes. The sample was removed from the tube, and 10 ml of BHI broth was added into the tube. After 24 hours of incubation at 37℃, 0.5 ml of solution was immediately added to 4.5 ml of PBS（－）and diluted. Following which, 100μl Fig. 2 Peak counts of fluorine after ion implantation. of diluted solution was plated on BHI agar. After 48 hours of incubation at 37℃, the number of colonies was counted. A total number of 20 samples were prepared for fluorine ion implantation （i.e., N＝10） and to serve as controls with no ion implantation（i.e., N＝10）.

RESULTS Secondary ion mass spectrometry（SIMS） Fig. 2 shows the peak counts of fluorine after ion implantation（N＝4）. In SUS and PMMA, ion implan-

tation using CF4 yielded significantly higher peak

counts than Ar＋F2 （p＜0.05 and p＜0.001, respectively）. But in Ti, there were no differences between

Ar＋F2 and CF4.

Concerning the role of Ar＋F2 as a fluorine-based gas for plasma-based ion implantation, it caused Ti Fig. 3 Depth profiles until 10 counts of fluorine after ion substrate to yield a higher peak count than SUS and implantation. NURHAERANI et al. 687

Fig. 4 F 1s XPS spectra of SUS. Dashed lines show the peak position of F（685.5 eV）.

Fig. 5 Cr 2p3/2 XPS spectra of SUS. Dashed lines show the peak positions of metal Cr

（574.3 eV）, CrF2（578.2 eV）, and Cr2O3（576.6 eV）.

Fig. 6 Fe 2p3/2 XPS spectra of SUS. Dashed lines show the peak positions of metal

Fe（706.95 eV）, FeF2（711.4 eV）, and Fe2O（3 710.9 eV）. 688 FLUORINE ION IMPLANTATION IN DENTAL MATERIALS ergies given in other published works. F 1s peak po- Contact angle sition was observed on the surface layer even after Fig. 7 shows the contact angles of SUS, where fluo- one minute of Ar gas sputtering （Fig. 4）. Chemi- rine ion-implanted SUS surface showed a signifi- cally shifted peaks of Cr 2p3/2 were observed in the cantly higher contact angle than the non-fluorine- higher binding energy region on the surface and sub- ion-implanted control（p＜0.001）. surface at one minute after sputtering（Fig. 5）. As for the chemically shifted peak of Fe 2p3/2, it was ob- Fluoride ion release served in the higher binding energy region in the Fig. 8 shows the amount of fluoride ion release from surface layer（Fig. 6）. the surface of fluorine ion-implanted SUS. A small amount of fluoride ions was released until the second day. After the third day, fluoride ion release ceased Table 2 Binding energies （eV） given in other published works to be detected.

Compound Binding energy（eV） Reference Antibacterial activity F 685.5 Shimadzu Co.＊） Fig. 9 shows the S. mutans colonies on BHI agar for metal Cr 574.3 D. Briggs et al.37） antibacterial activity evaluation, while Fig. 10 shows 37） Cr2O3 576.6 D. Briggs et al. the CFU/ml values. It could be seen that fluorine 38） CrF2 578.2 K. Hanamoto et al. ion-implanted SUS showed a lower CFU/ml value 37） metal Fe 706.95 D. Briggs et al. than the non-fluorine-ion-implanted control（p＜0.001）. 37） Fe2O3 710.9 D. Briggs et al. 37） FeF2 711.4 D. Briggs et al. Initial adhesion ＊Shimadzu Co.: XPS Spectra Sequential Peak File Fig. 11 shows the adhesion of S. mutans on the surface of SUS after four hours of incubation. Fluorine ion-implanted SUS showed less bacterial adhesion than the non-fluorine-ion-implanted control（p＜0.001）.

Fig. 8 Fluoride ion release from the surface of fluorine Fig. 7 Contact angles of SUS. ion-implanted SUS.

Fig. 9 S. mutans colonies on BHI agar. NURHAERANI et al. 689

ions25）. The peak count of fluorine ions implanted into Ti was the highest. However, the surface of Ti plate became discolored and corroded after plasma-based fluorine ion implantation. Oral prophylactic fluoride agents were reported to cause discoloration26） and corrosion27－32）, as well as increase the fracture sus- ceptibility of Ti33）. The corrosion resistance of Ti de- pends on the balance between the formation and dissolution of the passive oxide film. In general, Ti and its alloys are covered with a thin passive film（surface oxide film） consisting of low-crystalline or amorphous structure, and these films play a critical role in corrosion resistance34,35）. In particular, strong passive films are formed on the surface of Ti, which provide high corrosion resistance when subjected to Fig. 10 Antibacterial activity of the surface of SUS hostile acidic environments and against various kinds against S. mutans. of chemical agents36）. However, the presence of fluoride results in a low dissolved oxygen concentration, and thus hinders the formation of the passive film. It has been suggested that the corrosion of Ti occurs because the balance shifts to the dissolution reaction of the passive film35）. In the present study, the discoloration and corrosion further suggested that plasma-based fluorine ion implantation shifted the balance to dissolution of the passive film. XPS analysis revealed that the binding energies

of chromic oxide（Cr2O3）and chromic fluoride（CrF2） were 576.6 eV37） and 578.2 eV38） respectively. Hence, the shifted peaks in the present study（Fig. 5）were

due to Cr2O3 and CrF2. The binding energies of iron

fluoride（FeF2）and iron oxide（Fe2O3）were reported to be 711.4 eV and 710.9 eV respectively37）, which

meant that iron oxide（Fe2O3）was detected on the surface of fluorine ion-implanted SUS. However, on Fig. 11 Initial adhesion of S. mutans on the surface of the surface and after one minute of sputtering, the SUS. peak was close to that of metal Fe. Compared with the non-fluorine-ion-implanted control, the spectra shifts were not changed. This finding thus sug- DISCUSSION gested that fluorine did not react with Fe. Dental appliances have complex shapes and surfaces. With regard to the effect on contact angle, that With plasma-based ion implantation, fluorine ions of fluorine ion-implanted SUS was significantly in- can be implanted not only into a plane surface, but creased. As the contact angle increases, the also into a complex surface. However, fluorine gas is wettability of the substrate decreases or that the lat- very dangerous. Therefore, fluorine-based gases such ter becomes hydrophobic39,40）. It is known that or-

as CF4,C2F6, SF6 or mixed gas of argon and F2 have ganic polymers are generally hydrophobic as com- been used for plasma-based ion implantation24）. In pared with inorganic materials（such as glass）and

this study, a mixed gas of 95％ argon and 5％ F2 metals － which are hydrophilic. On the other hand,

（Ar＋F2）and CF4 gas were used. As for substrate it has been found that adhesion of bacteria to mate- materials, they were Ti, SUS, and PMMA. rials is determined by the degrees of hydrophobicity In the present study, the peak count of fluorine of both bacteria and material surface21,41,42）. Dankert ions implanted into PMMA was significantly lower et al.43） showed that a hydrophobic bacterium adhere than Ti and SUS. It might be due to the insulation to a hydrophobic surface more easily than its hydro- of PMMA. In general, a polymer surface is charged philic counterpart. According to interfacial thermo- to a very high voltage owing to the positive charge dynamics, high surface free energy strains, such as accumulation of implanted ions without any charge S. mutans, should adhere preferentially to hydro- compensation. The charge-up problem is expected to philic substrata. Riding on this hypothesis, it could affect the dose amount and implantation energy of thus be said that the decreased initial adhesion of S. 690 FLUORINE ION IMPLANTATION IN DENTAL MATERIALS mutans on the surface of fluorine ion-implanted SUS the possible antibacterial mechanism of fluorine ion- was due to the latter’s significantly increased contact implanted SUS was caused by the formation of metal angle. fluoride complexes. With regard to fluoride release behavior, Verbeeck et al.44） reported that fluoride release from CONCLUSIONS glass ionomer cements and composite resins occurred as two different processes. The first process was Ti, SUS, and PMMA were materials implanted with characterized by an initial burst of fluoride release, fluorine ions in the surface and subsurface layers. after which the release rate was markedly reduced For plasma-based fluorine ion implantation, CF4 gas and the release of fluoride continued for a long pe- was found to be more suitable than Ar＋F2. First, riod of time. In the present study, a small amount XPS analysis of fluorine ion-implanted SUS showed of fluorine ions was released from the surface of the peaks of fluorine and chromic fluoride. Moreo- fluorine ion-implanted SUS within 24 hours and an ver, while the contact angle and antibacterial activity extremely small amount of fluorine ions was detected of fluorine ion-implanted SUS were significantly in- up to the second day. However, fluorine ions ceased creased, the initial adhesion of S. mutans on the sur- to be detected after three days. According to our re- face was significantly decreased. It was suggested sults, the maximum amount of fluoride ions released that the decrease in S. mutans adhesion could be as- was about 0.03 ppm, and the detection limit of fluo- cribed to the increase of both contact angle and anti- ride electrode in this study was 0.02 ppm. This find- S. mutans activity. As for the antibacterial activity ing thus suggested that there was no fluoride ion re- of fluorine ion-implanted SUS in this study, it most lease from the surface of fluorine ion-implanted SUS. probably arose from the formation of metal-fluoride With regard to the effect on bacterial adhesion, complexes, such as chromic fluoride. In summary, the fluorine ion-implanted SUS showed increased an- plasma-based fluorine ion implantation into dental tibacterial activity. As a result, the initial adhesion materials could provide antibacterial activity and in- of S. mutans on fluorine ion-implanted SUS de- hibit bacterial adhesion on the surface of dental ma- creased significantly compared with the non-fluorine- terials. ion-implanted control. The decrease in initial adhesion in this study might be due to the increase of ACKNOWLEDGEMENTS both contact angle and antibacterial activity of fluorine ion-implanted SUS. In an in vitro study18）, it We would like to thank Mr. Yutaka Hibino of was found that surface modification through a dry Plasma Ion Assist. Co. Ltd., Kyoto, Japan for his process － such as ion implantation － was useful in collaboration in this study. This study was sup- controlling the initial adhesion of oral bacteria, and ported by a Grant-in-aid for Scientific Research from that fluorine ion-implanted Ti surface exhibited anti- the Japan Society for the Promotion of Science, bacterial activity effectively against both P. KAKENHI（No.14370697）. gingivalis and A. actinomycetemcomitans18）. Moreo- ver, Li et al.19） also reported that cell attachment on REFERENCES PMMA surface could be controlled by fluorine ion implantation. 1） Satou J, Fukunaga A, Satou N, Shintani H, Okuda K. Two mechanisms of antibacterial action have Streptococcal adherence on various restorative materi- been identified for fluorine ion-implanted materials. als. J Dent Res 1988; 67: 588-591. 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