Surface Modification of Stainless Steel by Plasma-Based Fluorine and Silver Dual Ion Implantation and Deposition Yukari SHINONAGA and Kenji ARITA

Dental Materials Journal 2009; 28(6): 735–742

Surface modification of stainless steel by plasma-based fluorine and silver dual ion implantation and deposition Yukari SHINONAGA and Kenji ARITA

Department of Pediatric Dentistry, Social and Environmental Medicine, Integrated Sciences of Translational Research, Institute of Health Biosciences, The University of Tokushima Graduate School, Tokushima, Japan Corresponding author, Yukari SHINONAGA; E-mail: [email protected]

The aims of this study were to modify dental device surface with fluorine and silver and to examine the effectiveness of this new surface modification method. Stainless steel plates were modified by plasma-based fluorine and silver ion implantation-deposition method. The surface characteristics and brushing abrasion resistance were evaluated by XPS, contact angle and brushing abrasion test. XPS spectra of modified specimens showed the peaks of fluoride and silver. These peaks were detected even after brushing abrasion test. Water contact angle significantly increased due to implantation-deposition of both fluorine and silver ions. Moreover, the contact angle of the modified specimen was significantly higher than that of fluorine only deposited specimen with the same number of brushing strokes. This study indicates that this new surface modification method of fluorine and silver ion implantation- deposition improved the brushing abrasion resistance and hydrophobic property making it a potential antimicrobial device.

Keywords: Plasma-based ion implantation-deposition, Fluorine, Silver

Received May 7, 2009: Accepted Aug 17, 2009

a high negative potential relative to the chamber wall. INTRODUCTION Ions generated in the overlying plasma accelerate Bacterial colonization and subsequent device infection across the ion sheath formed around the samples and are common complications of medical and dental are implanted into their surfaces24). For example, a devices1). Metallic biomaterials such as stainless steel, sample possessing a complicated shape can be treated titanium and titanium alloys are widely used in the with good conformity and uniformity without beam fields of pedodontics, orthodontics, prosthodontics, oral scanning and special target manipulation. In addition, surgery and cardiovascular surgery. Medical and dental the use of multiple processes, such as simultaneous devices have complicated shapes and are susceptible to and consecutive ion implantation-deposition, and adhesion by bacteria resulting to dental caries2-6), etching, are possible by varying the instrumental periodontal disease7-9), or infection of soft oral tissue10). parameters without breaking vacuum. In particular, Moreover, oral bacteria can be released from dental the ion deposition with simultaneous ion implantation plaque into salivary secretions and then aspirated into (plasma-based ion implantation and deposition: PBII- the lower respiratory tract causing pneumonia11,12). D) is desirable for efficient processing and has an There is also evidence that periodontal infection with advantage over conventional methods25). circulating antibodies against some oral pathogens Recently, several researchers have carried out increase the risk for stroke and cardiovascular surface modification using fluorine (F) ion and found it disease13,14). Also, biomedical-related infections often to be a useful means of inhibiting bacterial require device removal and replacement and immediate adhesion17,18,20,22,26). In our previous study, stainless steel aggressive antibiotic therapy to avoid the development implanted with F ion by PBII could provide of severe complications. Consequently, additional antibacterial activity and inhibit bacterial adhesion20). treatment greatly increases patient’s discomfort and It was suggested that the effects could be attributed to treatment costs15,16). Considering all these, it is the increased contact angle and existence of metal- therefore important to inhibit the adhesion of oral fluoride complexes on the surface of the F implanted bacteria on exposed device surfaces in the oral cavity to stainless steel. In the PBII-D, since both processes of keep them plaque-free. ion implantation and deposition are done Device surface modifications are generally simultaneously, it was anticipated that required ions classified into physical and chemical modifications. In existed more on the surface of the materials. In the case of metallic materials, physical modifications addition, it is well known that silver (Ag) possesses are mainly chosen. For instance, several studies have antibacterial property without any toxic effects in indicated that surface modifications, such as ion comparison to other heavy metal ions. Therefore, Ag is implantation17-21) and film coating17,22), are effective in the first candidate used in antibacterial research. reducing bacterial infection. Plasma-based ion Several reports have indicated an effective treatment implantation (PBII) in particular, is a promising to reduce bacterial infection based on the Ag zone near method for the surface modification of three- the surface of biomaterials17,19,27-29). In the dental field, dimensional materials23). In the PBII, the samples are agents and materials containing both F and Ag, such surrounded by high-density plasma and pulse-biased to as diammine silver fluoride solution30) for inhibition of 736 Dent Mater J 2009; 28(6): 735–742 caries progress and silver glass ionomer cement31) for depth profile analysis for the F+Ag implanted-deposited filling or lining of cavities, are widely used. However, stainless steel was performed using Ar etching at 20 there have been no known reports on device surface mA current and 2 kV voltage under the pressure of modification using both F and Ag. 5×10–4 Pa. The Ar etching rate was approximately 2

We then developed the new technique to nm/min on Al2O3. The binding energies obtained by simultaneously implant and deposit both F and Ag ions XPS were corrected against that of C1s, an internal into stainless steel using PBII-D for the first time. The standard with binding energy at 285.0 eV for the C-C aim of this study was to examine the effectiveness of F and C-H (saturated hydrocarbons)32). and Ag implanted-deposited into stainless steel by evaluating the surface characteristics and brushing Contact angle measurements abrasion resistance. Samples were washed in ultrasonic bath (J.M. Ultrasonic Cleaner SUW-50D, J. Morita Co., Tokyo, Japan) containing distilled water for 10 minutes, and MATERIALS AND METHODS then dried at room temperature. Static contact angle Plasma-based F and Ag ion implantation-deposition measurements were conducted by the sessile drop Stainless steel 316L plates (composition in wt%: technique using a contact angle meter (CA-DT, Kyowa C=0.12, Si=0.48, Mn=0.84, P=0.27, S=0.02, Ni=12.02, Kaimenkagaku Co. Ltd., Saitama, Japan) with distilled Cr=17.42, Mo=2.03, Fe=balance; Nippon Steel & water. Ten points per group were measured. Sumikin Stainless Steel Co., Tokyo, Japan) with measurements of 10 mm × 10 mm × 1 mm were used. Brushing abrasion test The stainless steel plates were polished mechanically The brushing abrasion test machine (MANA-63S, to a mirror finish and washed in an ultrasonic bath MASUDA Co., Osaka, Japan) was used. A commercial with 98% ethanol. The stainless steel plates were then toothbrush (Dr. Bee Young II soft, Bee Brand Medico modified by plasma-based ion implantation-deposition Dental Co., Ltd., Osaka, Japan) was attached to the equipment at Plasma Ion Assist Co., Ltd., Kyoto, toothbrush holder in contact with the F deposited and Japan. A schematic diagram of both F and Ag ion F+Ag implanted-deposited stainless steel set on the implantation-deposition method by PBII-D is illustrated sample holder. Distilled water without dentifrice was in Fig. 1. Fluoride gas used for F ion implantation- deposition was octafluoropropane (C3F8). For the Ag ion implantation-deposition, a 99.8% Ag mesh was set 10 mm above the stainless steel plates and sputtered by C3F8 gas. The conditions of plasma-based F and Ag ion implantation-deposition are shown in Table 1. The stainless steel plates were grouped into: control group, those that were not treated with any ion; F deposited group, those that were deposited with F ion; and F+Ag implanted-deposited group, those that were implanted- deposited with both F and Ag ions.

Surface analysis by XPS The surfaces of the control, F deposited and F+Ag implanted-deposited stainless steel were characterized by X-ray photoelectron spectroscopy (XPS). XPS spectra were obtained using an X-ray photoelectron spectrometer (ESCA-1000AX, Shimadzu Co., Kyoto, Japan) with Mg-Kα radiation operated at 30 mA Fig. 1 Schematic presentation of Plasma-based F and Ag current and 10 kV accelerating voltage. Specifically, ion implantation-deposition technique.

Table 1 Experimental groups and specific condition for fluorine and silver ion implantation-deposition into the surfaces of stainless steel plates Group Fluorine treatment Silver mesh Control None None

C3F8 F deposited None –0.5 kV / 60 min

C3F8 C3F8 F+Ag implanted-deposited Used –5 kV / 30 min + –0.5 kV / 60 min Dent Mater J 2009; 28(6): 735–742 737 then poured into the vessel, and the machine was run steel surface (Fig. 2c). In the F deposited and F+Ag at 80 rpm with a 200 g load. The contact angle implanted-deposited stainless steel, F1s spectra were measurements of the F deposited and F+Ag implanted- divided into the two regions (Figs. 2b and 2c). deposited stainless steel were done every 10,000 1.2. Narrow -scan XPS analysis strokes up to 60,000 strokes. In the control stainless Fe2p3/2, Cr2p3/2 and F1s XPS narrow-scan spectra of the steel, the contact angle was measured without control (a), F deposited (b) and F+Ag implanted- brushing. Five points per group were measured. deposited (c) stainless steel are shown in Fig. 3. In

Furthermore, XPS analysis of the F+Ag implanted- Fe2p3/2 spectra for the control stainless steel surface, deposited stainless steel was done every 10,000 strokes several peaks at 710-711.5 eV and 707.5 eV were up to 60,000 strokes. detected. These peaks energies were approximated to 33) iron oxides . Chemical shifts of Fe2p3/2 spectra were Statistical analysis observed in the higher binding energy region (711.8 eV) The results of the contact angle measurements before in the F deposited and F+Ag implanted-deposited and after the brushing abrasion test were expressed as stainless steel. It was suggested that the shifted peak the mean ± standard deviation. The data were analyzed energy approximated to iron fluoride (FeF2) (711.4 34) using one-way ANOVA and Tukey’s multiple eV ). Cr2p3/2 peak position of the control stainless steel 35) comparison tests (α = 0.05). was detected at chromium oxide (Cr2O3) (576.8 eV ).

Chemical shifts of Cr2p3/2 spectra were observed in the higher binding energy region in the F deposited and RESULTS F+Ag implanted-deposited stainless steel. This XPS analysis suggested that the shifted peak energy was 1. XPS analysis before brushing approximated to chromium fluoride binding energy 35) 1.1 Wide-scan XPS analysis (578.1 eV ). Ag3p3/2 signal appeared at 573.2 eV on the XPS wide-scan spectra of the surfaces of the control (a), surface of the F+Ag implanted-deposited stainless steel. F deposited (b) and F+Ag implanted-deposited (c) On the surfaces of the F deposited and F+Ag stainless steel are shown in Fig. 2. The elements C, O, implanted-deposited stainless steel, F1s signal was Cr, Fe, Ni and Mo were detected in the wide-scan divided into two peak positions. The peak at 685.0 eV spectrum of the control stainless steel (Fig. 2a). F1s was approximated to the metal-fluoride bonding state35). peak appeared on the F deposited stainless steel Another peak energy was approximated to the carbon- surface (Fig. 2b), and both F1s and Ag3d peaks fluoride binding energy, such as p-(CF2=CF2) at appeared on the F+Ag implanted-deposited stainless 689 eV35). The F-Ag bonding state was not detected

Fig. 2 XPS wide-scan spectra of the control (a), F deposited (b) and F+Ag implanted-deposited (c) stainless steel. Dashed lines indicate the peak positions of stainless steel’s components. Black triangles indicate the peak positions of implanted F and Ag. 738 Dent Mater J 2009; 28(6): 735–742 in Fig. 3. metal-fluoride bonding state; shown in Fig. 3) was observed maximally. Furthermore, the metal-fluoride 1.3. Depth profile of the F+Ag deposited stainless steel peak decreased as the Ar etching increased, and was The F1s and Ag3d XPS depth profiles of the F+Ag not detected after 2.0 minutes of Ar etching. In implanted-deposited stainless steel are shown in Fig. 4. addition, the peak at 690-691 eV in F1s region was also After 0.5 minutes of etching by Ar gas, the peak at detected in the depth profile of the control stainless 689.0 eV (the carbon-fluoride bonding state; shown in steel. Thus, the peak is not dependent on the presence Fig. 3) disappeared and the peak at 685.0 eV (the of fluoride. Ag3d signal decreased with depth. However,

Fig. 3 Fe2p3/2, Cr2p3/2 and F1s XPS narrow-scan spectra of the control (a), F deposited (b) and F+Ag implanted-

deposited (c) stainless steel. Dashed lines indicate the peak positions of Fe (707.5 eV) and FeF2 (711.8 eV)

in the Fe2p3/2 region, Ag3p3/2 (573.2 eV), Cr2O3 (576.8 eV) and CrF2 (578.1 eV) in the Cr2p3/2 region, F (685.0

eV) and p-(CF2=CF2) (689.0 eV) in F1s region. The horizontal bar indicates the peak position of iron-oxide.

Fig. 4 F1s and Ag3d XPS depth profiles of the F+Ag implanted-deposited stainless steel. Dashed lines indicate

the peak positions of metal-fluoride (685.0 eV) and 2p-(CF =CF2) (689.0 eV) in the F1s region, and Ag (368.1 eV) in the Ag3d region. Dent Mater J 2009; 28(6): 735–742 739

Fig. 5 Cr2p3/2 and F1s XPS profiles of the F+Ag implanted-deposited stainless steel before/after the

brushing abrasion test. Dashed lines indicate the peak positions of Ag3p3/2 (573.2 eV), Cr2O3

(576.8 eV) and CrF2 (578.1 eV) in the Cr2p3/2 region, and F (685.0 eV) and p-(CF2=CF2) (689.0 eV) in the F1s region.

Ag was detected after Ar etching for 4.5 minutes in Ag3d depth profile.

2. XPS analysis of the F+Ag deposited stainless steel after the brushing abrasion test

Cr2p3/2 and F1s XPS narrow-scan spectra of the F+Ag implanted-deposited stainless steel surface before and after the brushing abrasion test are shown in Fig.5.

Ag3p3/2 peak area at 573.2 eV in Cr2p3/2 region decreased with the increase in brushing strokes.

However, Ag3p3/2 peak area was still detected after 60,000 strokes brushing. Similarly, the decreased carbon-fluoride bonding state in 688-689 eV after brushing was visible in F1s region spectra. The peak of the carbon-fluoride bonding state was not detected after 60,000 brushing strokes. Nevertheless, the peak of metal-fluorides was detected even after 60,000 brushing strokes.

Contact angle measurements The typical water drop pictures on the control, F Fig. 6 (A) Contact angles and typical water drops on the deposited and F+Ag implanted-deposited stainless steel surface of the control (a), F deposited (b) and plates before the brushing abrasion test, and contact F+Ag implanted-deposited (c) stainless steel. angle values of the F deposited and F+Ag implanted- (B) Contact angles of the F deposited and F+Ag deposited stainless steel before and after the brushing implanted-deposited stainless steel before and abrasion test are shown and compared to that of the after the brushing abrasion test. Horizontal lines control without the brushing abrasion test in Fig. 6. indicate no significant differences (ANOVA, The contact angles of the F deposited and F+Ag Tukey, α = 0.05). implanted-deposited stainless steel were significantly higher than that of the control stainless steel (p<0.001). Moreover, the contact angle of the F+Ag implanted- 740 Dent Mater J 2009; 28(6): 735–742 deposited stainless steel was higher than that of the F Several related researches have indicated effective deposited stainless steel (p<0.001). In the F deposited treatments to reduce bacterial infection based on the stainless steel, the contact angles after 10,000, 30,000 Ag zone near the surface of biomaterials19,27-29). In this and 60,000 brushing strokes were significantly lower study, the F+Ag implanted-deposited stainless steel than before brushing (10,000 and 30,000 strokes had the metal-fluoride complexes and Ag near the p<0.05, 60,000 strokes p<0.001). Particularly, there surface. Therefore, this F+Ag implanted-deposited was no significant difference in the contact angle stainless steel was expected to be effective in promoting between the F deposited stainless steel after 30,000 surface antibacterial activity. This is best to be and 60,000 brushing strokes and the control stainless confirmed in future studies. steel. The F deposited and F+Ag implanted-deposited Oka et al.39) discussed the microstructure in nano- stainless steel exhibited a similar pattern in the contact interface between an aluminum alloy substrate and a angle after the brushing abrasion test; that is, the diamond-like carbon (DLC) film prepared by PBII-D, contact angles of the F+Ag implanted-deposited showing the formation of the mixing layer which was stainless steel after 10,000, 30,000 and 60,000 brushing constructed by the implanted ions and the substrate strokes were significantly lower than before brushing elements. In the F+Ag implanted-deposited stainless (10,000 strokes p<0.01, 30,000 and 60,000 strokes steel, the results of XPS spectra suggested that the p<0.001). However, the F+Ag implanted-deposited deposited layer had the carbon-fluoride bonding state stainless steel after the brushing abrasion test resulted from the C3F8 gas plasma, metal-fluorides and Ag, and in a significantly higher contact angle compared to the the mixing layer formed by metal-fluorides and Ag. In control stainless steel (10,000, 30,000 and 60,000 Figure 4, it was shown that the carbon-fluoride bonding strokes p<0.001). Moreover, the contact angle of the state disappeared after 0.5 minutes of Ar etching

F+Ag implanted-deposited stainless steel was (etching speed; approximately 2 nm/min on Al2O3) in significantly higher than the F deposited stainless steel the F+Ag implanted-deposited stainless steel which for the same number of brushing strokes (10,000 may indicate that the carbon-fluoride deposited layer strokes p<0.01, 30,000 strokes p<0.05, 60,000 strokes was less than 1 nm in depth. After 60,000 brushing p<0.001). strokes, the carbon-fluoride almost disappeared on the surface of the F+Ag implanted-deposited stainless steel (Fig. 5), suggesting that the deposited layer could last DISCUSSION until 60,000 brushing strokes. The Ag3p (573.2 eV) In the present study, both F and Ag ions were peak and the F1s peak (685.0 eV) from metal-fluoride simultaneously implanted-deposited into stainless steel complexes were still visible on the surface of the F+Ag (SUS 316L) using the PBII-D with silver mesh and implanted-deposited stainless steel even after 60,000 were examined for effectiveness by evaluating the XPS brushing strokes (Fig. 5), suggesting that the mixing spectra and the contact angles. One of the austenitic layer could resist more than 60,000 brushing strokes. stainless steels, SUS 316L, has long been used in Kanter et al.40) estimated that 20,000 brushing strokes orthopedic implants for fracture fixation and joint were the equivalent to approximately 5 years of replacement. Because this type of stainless steel has brushing. The present study confirmed that the mixing also been used in dentistry for orthodontic appliances layer of the F+Ag implanted-deposited stainless steel and preformed crowns for primary teeth restoration, it could withstand brushing for more than 15 years, and was chosen in the present study. Ag could withstand 60,000 brushing strokes or the Based on the results of the XPS analyses, it was equivalent of 15 years of brushing with a toothbrush. obvious that there was indeed F and Ag on the surface The contact angle is characteristic of the surface of the F+Ag implanted-deposited stainless steel (Fig. energy of a solid surface, and has been used for 2). Further, the results suggested that F combined with determining the wettability and hydrophobic property carbon from the C3F8 gas and with metallic elements of various solid materials. Several reports showed that from the stainless steel substrate (Fig. 3). The carbon- bacterial adhesion decreased with the increasing fluoride bonding state on the surface disappeared after contact angle of the solid surfaces and the decreasing Ar etching for 0.5 minutes to a depth of about 1nm. surface energy of the substrate surfaces18,20,41,42). Some While the metal-fluoride complexes were dominant research found that bacteria adhered to the inside the F+Ag implanted-deposited stainless steel hydrophobic materials could be more easily removed by after Ar etching for 2.0 minutes, to a depth of about an air-bubble jet43,44). It was also reported that there 4 nm (Fig. 4). The metal-fluoride complexes are also exists direct the relationship between the contact angle responsible for fluoride inhibition of proton- and the antibacterial property of various fluoride- translocating F-ATPases and are thought to mimic implanted stainless steel18,20). In this study, the contact phosphate to form complexes with ADP at the reaction angle of the F+Ag implanted-deposited stainless steel center of enzymes. Indeed, ATPase plays an important (101.9±1.4°) was higher than those of the modified role in the maintenance of intracellular pH by pumping stainless steel in previous studies (Chao et al.18): out protons and in the inhibition of the enzyme that 33.3±0.1° and Nurhaerani et al.20): approximately 90°). disrupts the bacterial metabolism and aciduric In addition, the contact angle of the F+Ag implanted- capability of Streptococcus mutans17,36-38). deposited stainless steel after the brushing abrasion Dent Mater J 2009; 28(6): 735–742 741 test was significantly higher than that of the control 323-331. stainless steel (Fig. 6). Furthermore, contact angle of 5) Øgaard B, Rølla G, Arends J. Orthodontic appliances and the F+Ag implanted-deposited stainless steel was enamel demineralization Part 1. Lesion development. 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