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Surface modification of silicone medical materials by plasma-based ion implantation
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 257 (2007) 128–131 www.elsevier.com/locate/nimb
Surface modification of silicone medical materials by plasma-based ion implantation Tomohiro Kobayashi a,*, Toshihiko Yokota a,b, Rui Kato b, Yoshiaki Suzuki a, Masaya Iwaki a, Takayuki Terai b, Noriyoshi Takahashi c, Tomonori Miyasato d, Hiroshi Ujiie e a
The Institute of Physical and Chemical Research (RIKEN), Saitama 351-0198, Japan b The University of Tokyo, Tokyo 113-0033, Japan c Tokyo University of Science, Tokyo 162-8601, Japan d Yaesekai Dojin Hospital, Okinawa 901-2133, Japan e Tokyo Women’s Medical University, Tokyo 162-8666, Japan Available online 8 January 2007
Abstract Silicone (polydimethylsiloxane) sheets and tubes for medical use were irradiated with inert gas ions using plasma-based ion implantation (PBII). The affinity of the surface with tissue examined by an animal test was improved by the irradiation at optimal conditions. The cell attachment percentage increased at an applied voltage of less than 7.5 kV; however, it decreased at higher voltage. The specimens irradiated at higher voltages were more hydrophobic than unirradiated specimens. The surface became rough with increasing voltage and textures, and small domains appeared. This effect was caused by different etching speeds in the amorphous and crystalline areas. 2007 Elsevier B.V. All rights reserved. PACS: 68.47.Mn; 52.77.Dq; 87.68.+z Keywords: Plasma-based ion implantation; PBII; Silicone; Carbonization; Biocompatibility; Cell attachment
1. Introduction Silicone tubes are widely used for medical catheters. The biological inertness of this material sometimes causes problems when it is used for a long periods in situations such as continuous ventricular drainage [1] and continuous ambulatory peritoneal dialysis [2]. Owing to poor tissue adhesion, the interface of the catheter with tissue tends to be a path for bacterial entry and growth. If the tissue adheres well to the catheter by surface modification, infections are suppressed. Ion irradiation is one of the most promising methods for improving the biocompatibility of polymers [3,4]. Plasma-based ion implantation (PBII) is perhaps the best of these methods for modifying curved surfaces *
Corresponding author. Tel.: +81 48 467 9358; fax: +81 48 462 4623. E-mail address: [email protected] (T. Kobayashi).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.12.158
because it attracts ions with an electrical field applied directly onto the specimen [5]. The tube surfaces can be modified by PBII without rotating the sample by using a columnar sample holder inserted into the tubes. We irradiated silicone sheets and tubes with inert gas ions, and we investigated the improvement of their biocompatibility. The optimal irradiation conditions were determined by cell attachment tests. The mechanisms of the improvement in biocompatibility were considered with respect to surface properties such as the composition, chemical bonds and morphology. 2. Experimental Silicone sheets (THE740-8U; 1 mm thick, GE Toshiba Silicones, Japan) and tubes (catheters for peritoneal dialysis; 1 mm thick and 4 mm in diameter; Medionics
T. Kobayashi et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 128–131
Fig. 1. Cross-sections of catheters implanted in a rabbit: (a) unirradiated, (b) irradiated with Ar at
International Inc., Canada) were irradiated with He, Ar and Kr ions using PBII for 30 min at 1.33 Pa. The sheet samples were fixed with a conductive adhesive tape on a flat sample holder. Columnar holders were inserted into the tubular samples, and only the outer surfaces of the samples were 6000
100
Cell adhesion rate[%]
4000
He 60
Ar 3000
Kr 40
2000 20
0 Unirradiated
1000
Cells on substrate [cells/mm2]
5000
80
0 0
-2.5
-5
-7.5
-10
Voltage[kV] Fig. 2. The relationship between the irradiation voltage and the cell adhesion rate (left axis); the density of the cells (right axis).
129
5 kV.
irradiated. PBII was performed by applying pulsed RF (13.56 MHz, 500 W, 30 ls) and pulsed fields (from 0 kV to 10 kV, 10 ls) alternately to the electrically floated sample holder at a cycle of 2 kHz. During the irradiation process, the temperature of the rear side of the specimens did not exceed 95 C. The samples were cooled after irradiation in a vacuum for 1 h, and then exposed to air. The sample irradiated with 5 keV Ar ions was implanted in a rabbit with an unirradiated sample for three weeks. The samples with surrounding tissues were sliced with a microtome and the interfaces were examined with an optical microscope. Fibroblast (L929) cells were seeded at 1.0 · 105 cells/ dish on the irradiated specimens and cultured in medium (RPMI1640) supplemented with 10% fetal bovine serum (FBS), 0.057% penicillin and 0.01% streptomycin. The cultures were maintained for three days at 36.5 C in a 5% CO2 normal oxygen atmosphere. The extent of cell attachment and spreading was observed under an optical microscope. The contact angle of water was observed using Kyowa CA-X system using deionized water. The structure and the functional groups were determined using Raman spectrometry using an Ar laser operating at 514.5 nm at 20 mW and Fourier transform–infrared spectroscopy with attenuated total reflectance (FT–IR–ATR). The changes
Fig. 3. The peak intensities of functional groups in FT–IR–ATR spectra of the Ar-irradiated silicone surface.
130
T. Kobayashi et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 128–131
in surface morphology were observed under an atomic force microscope (AFM). Except for the animal tests, all observations and tests were also performed using sheet samples.
Water contact angle [deg]
112
110
3. Results 108
Unirradiated
106
104
102 0
-2.5
-5
-7.5
-10
Voltage [kV] Fig. 4. Contact angle of water at the silicone surface irradiated with Ar ions.
Fig. 1(a) shows a cross-sectional view of an unirradiated tube implanted beneath the skin of a rabbit. The separation between the tube and the tissue is evident. By contrast, the sample irradiated with 5 keV Ar ions has maintained good contact, as shown in Fig. 1(b). These observations demonstrated that PBII is effective. In the following sections, the surface properties are examined to determine the optimal conditions for irradiation. The cell adhesion rate increased for voltages between 0 and 7.5 kV, as shown in Fig. 2. It decreased for higher voltages, with the saturation voltage inversely related to the mass of ions. The optimum irradiation voltage was approximately from 2.5 kV to 5 kV, considering the number of active cells in an area representing the division rate of the cells.
Fig. 5. AFM images of the Kr–PBII-treated silicone surface: (a) unirradiated, (b) irradiated at 10 kV.
5 kV, (c) irradiated at
7.5 kV and (d) irradiated at
T. Kobayashi et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 128–131
131
4. Discussion
1000
Kr 800
Ar
Ra[nm]
600
He 400
200
0 Unirradiated
0
-2.5
-5
-7.5
-10
Voltage[kV]
Cell attachment was enhanced by the formation of new functional groups that increased the adsorption of those proteins involved in cell adhesion [7]. We noted that the adhesion rate decreased in the sample irradiated at 10 kV, in spite of the increase in hydrophilic functional groups. At the higher voltages, the increase in surface roughness disturbed the cell adhesion because the contact area between cells and the silicone surface decreases [8]. The formation of a carbonaceous layer at high voltage will not depress the cell adhesion rate, as indicated by the adhesion rate of 75–85% on the diamond-like carbon (DLC) surface [9]. The energy dependence of the cell attachment (Fig. 2) and roughness (Fig. 6) showed that the heavier ions require less energy to obtain a same level of modification. By using SRIM [10] code, the averaged energy transfers of ions accelerated at 5 keV are calculated to be 72 eV/nm (He), 532 eV/nm (Ar) and 602 eV/nm (Kr), respectively. We presume that using heavier ions, the optimal processing time can be shortened.
Fig. 6. The average roughness (Ra) of the irradiated silicone surface.
5. Conclusions Fig. 3(a) and (b) shows the normalized peak intensities in FT–IR spectra attributed to the CH3 (1260 cm 2), SiO (1020 cm 2), CO (1720 cm 2) and OH (3400 cm 2) chemical bonds. The destruction of the original silicone structures (CH3 and SiO) and the formation of new functional groups (OH and CO) occurred as the applied voltage was increased. The new bonds were formed by O2 and H2O molecules in atmosphere that were adsorbed by the unstable carbon bonds formed by the irradiation. Raman spectra showed broad two peaks around 1550 cm 1 and 1350 cm 1 indicating formation of amorphous carbon structure at the surfaces irradiated with Ar and Kr at 7.5 kV and 10 kV. Fig. 4 shows the contact angle of water on the surface of the samples irradiated with Ar ions. The contact angle at silicone surface decreased by the destruction of hydrophobic CH3 and formation of hydrophilic OH and CO groups caused by the plasma immersion, but these effects decreased with increasing voltage. At 10 kV the contact angle became greater than that for the unirradiated sample. Figs. 5 and 6 show the AFM images of Kr irradiated silicone and the average roughness (Ra) of the surface irradiated at He, Ar and Kr, respectively. The original surface had small scratches due to rolling, but the scratches were less than 20 nm deep. Using PBII, the roughness increased exponentially with increasing DC pulse voltage. Aligned domains were observed in irradiated samples at 10 kV. These unique surfaces were formed because of the different etching speed of the amorphous parts and the crystalline domains [6].
The cell adhesion rate at the surface of a silicone sheet was enhanced using PBII with an inert gas plasma. Destruction of the original silicone structures and formation of new functional groups increased with increasing irradiation voltage. The surface irradiated at 10 kV developed unique shapes and a high carbon concentration, but the cell adhesion rate decreased to that of an unirradiated surface. The optimum voltage for the modification was determined to be from 2.5 kV to 5 kV. A pathway for infection can be eliminated by using an irradiated silicone catheter. References [1] W. Pfisterer, M. Mu¨hlbauer, T. Czech, A. Reinprecht, J. Neurol. Neurosur. Ps. 74 (2003) 929. [2] W.F. Kean, S.R. Alexander, G.R. Balite, Periton. Dialysis Int. 16 (1996) 557. [3] Y. Suzuki, M. Kusakabe, J.S. Lee, M. Kaibara, M. Iwaki, H. Sasabe, Nucl. Instr. and Meth. B 65 (1–4) (1992) 142. [4] Y. Suzuki, M. Kusakabe, M. Kaibara, M. Iwaki, H. Sasabe, T. Nishisaka, Nucl. Instr. and Meth. B 91 (1–4) (1994) 588. [5] J.R. Conrad, R.A. Dodd, F.J. Worzala, X. Qiu, Surf. Coat. Technol. 36 (3–4) (1988) 927. [6] M.M. Silvan, A. Valsesia, D. Gilliland, G. Ceccone, F. Rossi, Appl. Surf. Sci. 235 (2004) 119. [7] G.K. Toworfe, R.J. Composto, C.S. Adams, I.M. Shapio, J. Biomed. Mater. Res. 71A (2004) 449. [8] S. Nomura, H. Kojima, Y. Ohyabu, K. Kuwabara, A. Miyauchi, T. Uemura, Jpn. J. Appl. Phys. 44 (37) (2005) L1184. [9] T. Yokota, T. Terai, T. Kobayashi, M. Iwaki, Nucl. Instr. and Meth. B 242 (1–2) (2006) 48. [10] J.F. Ziegler, Nucl. Instr. and Meth. B 219–220 (2004) 1027.
Nuclear Instruments and Methods in Physics Research B 257 (2007) 128–131 www.elsevier.com/locate/nimb
Surface modification of silicone medical materials by plasma-based ion implantation Tomohiro Kobayashi a,*, Toshihiko Yokota a,b, Rui Kato b, Yoshiaki Suzuki a, Masaya Iwaki a, Takayuki Terai b, Noriyoshi Takahashi c, Tomonori Miyasato d, Hiroshi Ujiie e a
The Institute of Physical and Chemical Research (RIKEN), Saitama 351-0198, Japan b The University of Tokyo, Tokyo 113-0033, Japan c Tokyo University of Science, Tokyo 162-8601, Japan d Yaesekai Dojin Hospital, Okinawa 901-2133, Japan e Tokyo Women’s Medical University, Tokyo 162-8666, Japan Available online 8 January 2007
Abstract Silicone (polydimethylsiloxane) sheets and tubes for medical use were irradiated with inert gas ions using plasma-based ion implantation (PBII). The affinity of the surface with tissue examined by an animal test was improved by the irradiation at optimal conditions. The cell attachment percentage increased at an applied voltage of less than 7.5 kV; however, it decreased at higher voltage. The specimens irradiated at higher voltages were more hydrophobic than unirradiated specimens. The surface became rough with increasing voltage and textures, and small domains appeared. This effect was caused by different etching speeds in the amorphous and crystalline areas. 2007 Elsevier B.V. All rights reserved. PACS: 68.47.Mn; 52.77.Dq; 87.68.+z Keywords: Plasma-based ion implantation; PBII; Silicone; Carbonization; Biocompatibility; Cell attachment
1. Introduction Silicone tubes are widely used for medical catheters. The biological inertness of this material sometimes causes problems when it is used for a long periods in situations such as continuous ventricular drainage [1] and continuous ambulatory peritoneal dialysis [2]. Owing to poor tissue adhesion, the interface of the catheter with tissue tends to be a path for bacterial entry and growth. If the tissue adheres well to the catheter by surface modification, infections are suppressed. Ion irradiation is one of the most promising methods for improving the biocompatibility of polymers [3,4]. Plasma-based ion implantation (PBII) is perhaps the best of these methods for modifying curved surfaces *
Corresponding author. Tel.: +81 48 467 9358; fax: +81 48 462 4623. E-mail address: [email protected] (T. Kobayashi).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.12.158
because it attracts ions with an electrical field applied directly onto the specimen [5]. The tube surfaces can be modified by PBII without rotating the sample by using a columnar sample holder inserted into the tubes. We irradiated silicone sheets and tubes with inert gas ions, and we investigated the improvement of their biocompatibility. The optimal irradiation conditions were determined by cell attachment tests. The mechanisms of the improvement in biocompatibility were considered with respect to surface properties such as the composition, chemical bonds and morphology. 2. Experimental Silicone sheets (THE740-8U; 1 mm thick, GE Toshiba Silicones, Japan) and tubes (catheters for peritoneal dialysis; 1 mm thick and 4 mm in diameter; Medionics
T. Kobayashi et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 128–131
Fig. 1. Cross-sections of catheters implanted in a rabbit: (a) unirradiated, (b) irradiated with Ar at
International Inc., Canada) were irradiated with He, Ar and Kr ions using PBII for 30 min at 1.33 Pa. The sheet samples were fixed with a conductive adhesive tape on a flat sample holder. Columnar holders were inserted into the tubular samples, and only the outer surfaces of the samples were 6000
100
Cell adhesion rate[%]
4000
He 60
Ar 3000
Kr 40
2000 20
0 Unirradiated
1000
Cells on substrate [cells/mm2]
5000
80
0 0
-2.5
-5
-7.5
-10
Voltage[kV] Fig. 2. The relationship between the irradiation voltage and the cell adhesion rate (left axis); the density of the cells (right axis).
129
5 kV.
irradiated. PBII was performed by applying pulsed RF (13.56 MHz, 500 W, 30 ls) and pulsed fields (from 0 kV to 10 kV, 10 ls) alternately to the electrically floated sample holder at a cycle of 2 kHz. During the irradiation process, the temperature of the rear side of the specimens did not exceed 95 C. The samples were cooled after irradiation in a vacuum for 1 h, and then exposed to air. The sample irradiated with 5 keV Ar ions was implanted in a rabbit with an unirradiated sample for three weeks. The samples with surrounding tissues were sliced with a microtome and the interfaces were examined with an optical microscope. Fibroblast (L929) cells were seeded at 1.0 · 105 cells/ dish on the irradiated specimens and cultured in medium (RPMI1640) supplemented with 10% fetal bovine serum (FBS), 0.057% penicillin and 0.01% streptomycin. The cultures were maintained for three days at 36.5 C in a 5% CO2 normal oxygen atmosphere. The extent of cell attachment and spreading was observed under an optical microscope. The contact angle of water was observed using Kyowa CA-X system using deionized water. The structure and the functional groups were determined using Raman spectrometry using an Ar laser operating at 514.5 nm at 20 mW and Fourier transform–infrared spectroscopy with attenuated total reflectance (FT–IR–ATR). The changes
Fig. 3. The peak intensities of functional groups in FT–IR–ATR spectra of the Ar-irradiated silicone surface.
130
T. Kobayashi et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 128–131
in surface morphology were observed under an atomic force microscope (AFM). Except for the animal tests, all observations and tests were also performed using sheet samples.
Water contact angle [deg]
112
110
3. Results 108
Unirradiated
106
104
102 0
-2.5
-5
-7.5
-10
Voltage [kV] Fig. 4. Contact angle of water at the silicone surface irradiated with Ar ions.
Fig. 1(a) shows a cross-sectional view of an unirradiated tube implanted beneath the skin of a rabbit. The separation between the tube and the tissue is evident. By contrast, the sample irradiated with 5 keV Ar ions has maintained good contact, as shown in Fig. 1(b). These observations demonstrated that PBII is effective. In the following sections, the surface properties are examined to determine the optimal conditions for irradiation. The cell adhesion rate increased for voltages between 0 and 7.5 kV, as shown in Fig. 2. It decreased for higher voltages, with the saturation voltage inversely related to the mass of ions. The optimum irradiation voltage was approximately from 2.5 kV to 5 kV, considering the number of active cells in an area representing the division rate of the cells.
Fig. 5. AFM images of the Kr–PBII-treated silicone surface: (a) unirradiated, (b) irradiated at 10 kV.
5 kV, (c) irradiated at
7.5 kV and (d) irradiated at
T. Kobayashi et al. / Nucl. Instr. and Meth. in Phys. Res. B 257 (2007) 128–131
131
4. Discussion
1000
Kr 800
Ar
Ra[nm]
600
He 400
200
0 Unirradiated
0
-2.5
-5
-7.5
-10
Voltage[kV]
Cell attachment was enhanced by the formation of new functional groups that increased the adsorption of those proteins involved in cell adhesion [7]. We noted that the adhesion rate decreased in the sample irradiated at 10 kV, in spite of the increase in hydrophilic functional groups. At the higher voltages, the increase in surface roughness disturbed the cell adhesion because the contact area between cells and the silicone surface decreases [8]. The formation of a carbonaceous layer at high voltage will not depress the cell adhesion rate, as indicated by the adhesion rate of 75–85% on the diamond-like carbon (DLC) surface [9]. The energy dependence of the cell attachment (Fig. 2) and roughness (Fig. 6) showed that the heavier ions require less energy to obtain a same level of modification. By using SRIM [10] code, the averaged energy transfers of ions accelerated at 5 keV are calculated to be 72 eV/nm (He), 532 eV/nm (Ar) and 602 eV/nm (Kr), respectively. We presume that using heavier ions, the optimal processing time can be shortened.
Fig. 6. The average roughness (Ra) of the irradiated silicone surface.
5. Conclusions Fig. 3(a) and (b) shows the normalized peak intensities in FT–IR spectra attributed to the CH3 (1260 cm 2), SiO (1020 cm 2), CO (1720 cm 2) and OH (3400 cm 2) chemical bonds. The destruction of the original silicone structures (CH3 and SiO) and the formation of new functional groups (OH and CO) occurred as the applied voltage was increased. The new bonds were formed by O2 and H2O molecules in atmosphere that were adsorbed by the unstable carbon bonds formed by the irradiation. Raman spectra showed broad two peaks around 1550 cm 1 and 1350 cm 1 indicating formation of amorphous carbon structure at the surfaces irradiated with Ar and Kr at 7.5 kV and 10 kV. Fig. 4 shows the contact angle of water on the surface of the samples irradiated with Ar ions. The contact angle at silicone surface decreased by the destruction of hydrophobic CH3 and formation of hydrophilic OH and CO groups caused by the plasma immersion, but these effects decreased with increasing voltage. At 10 kV the contact angle became greater than that for the unirradiated sample. Figs. 5 and 6 show the AFM images of Kr irradiated silicone and the average roughness (Ra) of the surface irradiated at He, Ar and Kr, respectively. The original surface had small scratches due to rolling, but the scratches were less than 20 nm deep. Using PBII, the roughness increased exponentially with increasing DC pulse voltage. Aligned domains were observed in irradiated samples at 10 kV. These unique surfaces were formed because of the different etching speed of the amorphous parts and the crystalline domains [6].
The cell adhesion rate at the surface of a silicone sheet was enhanced using PBII with an inert gas plasma. Destruction of the original silicone structures and formation of new functional groups increased with increasing irradiation voltage. The surface irradiated at 10 kV developed unique shapes and a high carbon concentration, but the cell adhesion rate decreased to that of an unirradiated surface. The optimum voltage for the modification was determined to be from 2.5 kV to 5 kV. A pathway for infection can be eliminated by using an irradiated silicone catheter. References [1] W. Pfisterer, M. Mu¨hlbauer, T. Czech, A. Reinprecht, J. Neurol. Neurosur. Ps. 74 (2003) 929. [2] W.F. Kean, S.R. Alexander, G.R. Balite, Periton. Dialysis Int. 16 (1996) 557. [3] Y. Suzuki, M. Kusakabe, J.S. Lee, M. Kaibara, M. Iwaki, H. Sasabe, Nucl. Instr. and Meth. B 65 (1–4) (1992) 142. [4] Y. Suzuki, M. Kusakabe, M. Kaibara, M. Iwaki, H. Sasabe, T. Nishisaka, Nucl. Instr. and Meth. B 91 (1–4) (1994) 588. [5] J.R. Conrad, R.A. Dodd, F.J. Worzala, X. Qiu, Surf. Coat. Technol. 36 (3–4) (1988) 927. [6] M.M. Silvan, A. Valsesia, D. Gilliland, G. Ceccone, F. Rossi, Appl. Surf. Sci. 235 (2004) 119. [7] G.K. Toworfe, R.J. Composto, C.S. Adams, I.M. Shapio, J. Biomed. Mater. Res. 71A (2004) 449. [8] S. Nomura, H. Kojima, Y. Ohyabu, K. Kuwabara, A. Miyauchi, T. Uemura, Jpn. J. Appl. Phys. 44 (37) (2005) L1184. [9] T. Yokota, T. Terai, T. Kobayashi, M. Iwaki, Nucl. Instr. and Meth. B 242 (1–2) (2006) 48. [10] J.F. Ziegler, Nucl. Instr. and Meth. B 219–220 (2004) 1027.