- Email: [email protected]
Ion surface treatments on organic materials
Nuclear Instruments and Methods in Physics Research B 175±177 (2001) 368±374
www.elsevier.nl/locate/nimb
Ion surface treatments on organic materials Masaya Iwaki
*
Surface Characterization Division, RIKEN, Hirosawa 2-1, Wako, Saitama 351 0198, Japan
Abstract A study has been made of surface modi®cation of various organic materials by ion bombardment or implantation to make the surface properties of high and multiple functions in RIKEN. Substrates used were polyimide (PI), polyacetylene, polytetra¯uoroethylene (PTFE), polystyrene (PS), silicone rubber, various kinds of proteins and so on. Bombarded or implanted ions were inert gas elements, chemically active gaseous elements and metallic elements. Surface properties such as electrical conductivity, wettability and cell adhesion of implanted layers have been investigated. Surface characterization of implanted materials has been carried out by means of transmission electron microscopy, laser Raman spectroscopy, X-ray photoelectron spectroscopy and Rutherford backscattering spectroscopy. In this paper, studies in RIKEN are reviewed of electrical conductivity, optical absorbance, wettability and cell adhesion depending on current densities and doping elements. Applications of ion bombardment to biomedical materials are introduced using cell adhesion control. It is concluded that ion bombardment or implantation is useful to change and control surface properties of various organic materials. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 52.75.Rx; 61.41.+e; 61.82.Pv; 81.05.Lg Keywords: Ion implantation; Polymer; Surface analysis; Cell adhesion
1. Introduction Ion implantation as impurity doping has been succeeded for semiconductor device fabrication, in which the dose is relatively low, generally. In the case of ion beam modi®cation of metals to improve the wear and corrosion resistance, the dose is very high. Therefore, the ion implantation in metals has been restricted in industrial applications because of long time processing [1]. The re-
*
Tel.: +81-48-467-9570; fax: +81-48-462-4701. E-mail address: [email protected] (M. Iwaki).
search study of ion beam modi®cation of organic materials has been gradually progressed. The use of electron beam irradiation has been succeeded in industry for modi®cation of organic materials for curing and hardening [2]. On the other hand, using ion beam bombardment, it is thought that organic materials are easily destroyed and consequently their favorite functions will disappear. Really, this situation has blocked the progress of ion beam modi®cation of polymers. However, we have considered that the destruction of chemical bonds via ion bombardment means synthesis of surfaces with new functions. Ion beam modi®cation of organic materials is classi®ed into two categories. One is surface
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 5 4 3 - 7
M. Iwaki / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 368±374
modi®cation inclined as a function of depth and the other is uniform modi®cation as a function of depth. The former is ion implantation modi®cation near surface layers and the latter is ion beam bombardment through thin ®lms [3,4]. Ion bombardment in the latter case includes the surface modi®cation of ®lms and ion beam mixing at the interface of organic ®lms and substrates due to radiation eects. In this review, at ®rst, electrical conductivity, optical absorbance and wettability are introduced from the standpoints of current density dependence and impurity doping. Secondly, cell adhesion of modi®ed surfaces is described from the standpoints of biomedical applications. 2. Electrical conductivity and wettability 2.1. Current density dependence When ion bombardment and ion implantation are carried out in polymers, most of polymer surfaces change in color from brown to metallic black and have electrical conductive layers. Fig. 1 shows the relationship between sheet resistivity and ¯uence in the case of Ar-bombarded polyimide (PI) ®lms [5]. Ar-ion bombardment in PI ®lms has been carried out with doses ranging from 1 1015 to 2 1017 ions/cm2 using various current densities. In general, the sheet resistivity of the bombarded layers becomes low as the ¯uence increases. Using a low current density, the sheet resistivity is saturated at 1 105 X=cm2 . At a high current density, it monotonously decreases as the dose increases. At the intermediate current density of 1.5 lA=cm2 , the sheet resistivity is saturated. The saturated value is in between those with low and high current densities. The results show that a current density gives a great in¯uence on electrical properties even at the same dose. The electrical conductivity is induced by surface carbonization. Rutherford backscattering spectroscopy (RBS) of 1.5±2.0 MeV He ions was used to measure the change in surface composition of Ar-bombarded PI ®lms [5]. RBS spectra measured showed the reduction of nitrogen and oxygen contents as compared with those for a pristine
369
specimen. The amount of remained Ar atoms obtained from RBS yields is lower than Ar ¯uence, that means that Ar atoms move out during and/or after Ar bombardment. RBS spectra measured after 30 days of Ar bombardment show that the Ar amount reduced from 40% to 26% with 5 1016 Ar/cm2 . No change in Ar amount by aging was found for Ar bombardment with 1015 ±1016 Ar/cm2 . Anyway, Ar bombardment in PI ®lms causes the reduction of nitrogen, oxygen and argon to form carbonized layers. The laser Raman spectra show the formation of amorphous carbon for Ar-bombarded PI ®lms [5,6]. The Raman spectra change depending on beam current densities. At a low current density, the spectra exhibit an asymmetric triangular broad band with a peak at about 1550 1 cm, which is similar to amorphous carbon, as well known. Using a high current density, Raman spectra exhibit a trapezoidal one with ¯at region extending from 1300 cm 1 to 1600 cm 1 , which is considered to contain graphitic structure. The results correspond to those of electrical properties.
Fig. 1. Relationship between sheet resistivity and ¯uence for Arbombarded PI ®lms as a function of beam current density. Implant energy is 150 keV.
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The structure of Ar-bombarded layers was investigated by means of cross-sectional transmission electron microscope (TEM). TEM micrographs show that the bombarded layers and bulk are distinctly separated by the image contrast [5]. It suggests that the formation of conductive layers induced by ion bombardment is not due to thermal eects such as pyrolysis of hydrocarbons at high temperatures. Using a low current density, ion bombarded layers consist of two layers with dierent contrasts in which one is darker near the surface and a little dark in the deeper region. The thickness is a little shallower than the predicted range. Using a high current density, a TEM micrograph shows the appearance of three layers. Two deeper layers at the bulk side have similar contrast to that for a low current density. The top layer has porous structures. As compared with electrical properties, the porous layer may cause the strong reduction of sheet resistivity [5]. The color of PI ®lms is originally brownish yellow and turned dark brown after Ar bombardment. Optical absorbance spectra, obtained by subtracting the absorbance of the pristine specimen from Ar-bombarded specimens, monotonously increase toward shorter wavelength. The absorbance increases as the ¯uence and current density increase. The increase in optical absorbance at the high current density even at the same dose seems to be similar to the tendency of sheet resistivity, as already shown in Fig. 1. Fig. 2 shows the relationship between electrical conductivity and absorbance of 800 nm for Ar-bombarded PI ®lms, polyethyleneterephthalate (PET) ®lms and polyphenylenesul®de (PPS) ®lms [6]. It is quite noteworthy that the conductivity has evident correlation with the optical absorbance. The same correlation has been con®rmed to be held at other wavelengths such as 600, 1000, 1500 and 2000 nm. Moreover, the dependence of the conductivity on the absorbance was found to be almost the same for all the polymers studied in our experiments. The clear dependence of wettability on current density was found recently by Hitachi group [7]. Ar-ion bombardment in polytetra¯uoroethylene (PTFE) has been carried out to investigate the contact angle of water and surface morphology as
Fig. 2. Relationship between conductance and absorbance for Ar-bombarded PI, PET and PPS ®lms.
a function of energy, ¯uence and current density. The surface is roughened by Ar bombardment and the protuberance can be found at special conditions. The contact angle of water of 170.1° was found. It is the highest at the energy of 30 kV and the beam current density of 0.25 mA/cm2 in the experiment. These results show that a current density must be controlled to obtain desired properties. 2.2. Doping eects The sheet resistivity of metal ion-implanted PI ®lms depends on ion species. Fig. 3 shows the sheet resistivity of Ag-, W- and Pd-implanted PI ®lms as a function of dose [8]. The sheet resistivity decreases for all specimens as the dose increases and is saturated at the high dose. The dierence among these saturated sheet resistivities is over one order of the magnitude. Since the dierence of modi®ed thickness is estimated below twice, the saturated sheet resistivity depends on the ion species and the carbon structure in¯uenced by the species.
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Fig. 4. TEM micrographs of (a) Cu- and (b) Zn-implanted PI ®lms. Ion implantation was carried out at 150 keV with doses of 1 1017 Cu/cm2 and 5 1017 Zn/cm2 . The solid bars indicate 100 nm.
Fig. 3. Relationship between sheet resistivity and dose of Ag-, W- and Pd-implanted PI ®lms. Ag and W ions were implanted at average energies of 130 and 190 keV, respectively. Pd-ions were implanted at 100 keV.
RBS spectra for all specimens show the reduction of amounts of nitrogen and oxygen and the enrichment of carbon near the surface layers [9]. This means surface carbonization. Implanted metals are remained in the implanted layers to form a Gaussian-like distribution. Raman spectra observed for Ag- and W-implanted specimens exhibit the formation of amorphous carbon. On the other hand, the spectra for Pd-implanted specimens are not found [8]. XPS spectra show that Ag is metallic, W is a carbide state and Pd is unknown states [9]. The structures of metal-implanted layers are investigated by means of TEM. Cross-sectional TEM micrographs of Cu- and Zn-implanted PI ®lms show small spherical particles of a few nm in diameter in the implanted layers, as shown in Fig. 4 [5]. Such spherical and crystalline particles can be also found in Ag-implanted PI ®lms [10]. Wettability of organic materials was changed by ion implantation. Fig. 5 shows the contact angle of water of Ne-bombarded and Na-implanted polyethylene (PE) [11]. The contact angle of
Fig. 5. Relationship between contact angle of water and dose for Ne-bombarded and Na-implanted PE ®lms. Implant energy is 150 keV.
modi®ed specimens below the dose of 1 1016 ions/cm2 is almost the same as a pristine specimen. Over the dose, it is kept at the same angle for Nebombarded specimen, but that for Na-implanted specimens decreases steeply. The reduction becomes larger at the lower energy implantation.
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The masses of Ne- and Na-atoms are almost the same so that the change in contact angle is due to doping eects of Na. XPS measurements were carried out to investigate the change in composition and structure at the modi®ed surfaces [11]. The results showed that the amorphous carbon is formed for both samples and the high concentration of Na is found at the surface for Na-implanted specimens with higher doses. Therefore, the reduction of contact angles is caused by high concentration of Na atoms. Doping eects can be obtained on electrical conductivity and wettability, induced by high dose metal implantation. Metal ion implantation in polymers form metal-doped amorphous carbon, that can be called new diamond-like carbon.
3. Cell adhesion and its applications 3.1. Cell adhesion Cell adhesion study has been made for applying ion-implanted or bombarded polymers to biomedical materials. Fig. 6 shows micrographs for endothelial cell adhesion on polystyrene (PS), segmented polyurethane (SPU) and collagencoated polystyrene (CC-PS) [12,13]. Na ions were implanted in PS and SPU, and Ar ions were bombarded to CC-PS. The modi®ed area is within the circle of 0.1 mm in diameter. Implant conditions are shown in the ®gure.
Endothelial cells adhere on pristine surfaces of PS and ion implantation more improves cell adhesion. In the case of SPU, the cells hardly adhere on unimplanted area, and they adhere on implanted layers. Implanted layers were characterized by means of Raman and XPS measurements. Raman and XPS spectra of implanted layers show the formation of amorphous carbon. It is considered that cells adhere easily on amorphous carbon for polymers without suitable amino acids. Fig. 6(c) shows that HeLa cells adhere on CCPS very good, but they do not adhere on bombarded regions. In contrast, endothelial cells adhere on both unbombarded and bombarded regions, not shown in the ®gure. High ¯uence bombardment to organic materials forms the amorphous carbon on which HeLa cells do not adhere on amorphous carbon. The cause is considered as HeLa cells adhere selectively on organic materials with amino acids rather than on amorphous carbon. The results show that the cells select adhesive area by themselves. However, the results suggest that ion bombardment has a possibility to control cell adhesion. This technique can be applied to biomedical materials. If it is applied to make small vascular grafts, important characteristics needed on ionmodi®ed regions are the ability of selective adhesion of cells, that is the formation of surfaces with adhesion of endothelial cells and no adhesion of platelets because the adhesion of platelets causes the formation of thrombus to occlude the grafts. The ion-modi®cation condition has been searched
Fig. 6. Micrographs of endothelial cell adhesion on Na-implanted PS (a) and SPU (b) with a dose of 1 1017 ions/cm2 , and HeLa cell adhesion on Ar-bombarded CC-PS (c) with a ¯uence of 1 1015 ions/cm2 . The energy is 150 keV.
M. Iwaki / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 368±374
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to satisfy the above status, using ion bombardment of inert gases such as He, Ne, Ar and Xe. From the experimental results until now, He ion bombardment of 1 1014 ions/cm2 at 150 keV is more effective for good endothelial cell adhesion and no adhesion of platelets. In such a way, the suitable selection of modi®cation condition is able to perform the control of cell adhesion. 3.2. Biomedical applications The use of ion bombardment has been reported for production of small vascular grafts and modi®cation of Guglielmi detachable coils (GDCs) [14,15]. In the case of small vascular grafts, ion bombardment must be performed to the modi®cation of inner walls of small tubes. For GDCs, relatively uniform bombardment must be performed to small, long springs. We have produced the new target system to perform such bombardments. Small vascular grafts were produced as the following process [14]. At ®rst, Ne ions were bombarded in inner walls of expanded PTFE (ePTFE) with 2±3 mm in inner diameter to get good adhesion of collagen. Secondly, collagen is coated on Ne-bombarded surfaces. Thirdly, He-ion bombardment was carried out to collagen-coated surfaces according to the results of fundamental studies of cell adhesion. Such surfaces have properties of endothelial cell adhesion and no adhesion of platelets. Furthermore, He-ions go through collagen to e-PTFE so that ion beam mixing occurs at the interface between collagen and e-PTFE. This mixing causes the tight adhesion of collagen to substrates. This sample was implanted to femoral artery of dogs. Fig. 7 shows the photographs of inner walls of small grafts after in vivo evaluation. In the case of a control specimen, severe growth thrombus is found after 3 days, and we cannot ®nd thrombus on the modi®ed tubes using ion bombardment even after 180 days. 4. Summary In this paper, ion beam modi®cation of polymers studied in RIKEN is reviewed. A current
Fig. 7. Micrographs of protein hybrid type small diameter arti®cial grafts after in vivo tests.
density is an important factor for getting desired properties even at the same dose. Inert gaseous ion bombardment can be used for controlling surface structure and metal implantation can give the doping eect. Especially, the small metal particle formation is hopeful as new functional carbon materials. A typical application of ion beam modi®ed organic materials is applicable to biomedical ®elds because ion beam bombardment can be used for selective adhesion of cells. Acknowledgements The author would like to thank Dr. A. Nakao, Dr. T. Kobayashi and Dr. K. Yoshida for performing surface characterization, and Dr. Y. Suzuki and Dr. H. Nakajima for performing animal study. References [1] M. Iwaki, in: Proceedings of the 1998 International Conference on Ion Implantation Technology, IEEE, 1999, p. 824.
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[2] R. Holman (Ed.), UV and EB Curing Formulation for Printing Inks, Coatings and Paints, SITA-Technology, London, 1984. [3] J. Davenas, P. Thevenard, Nucl. Instr. and Meth. B 80±81 (1993) 1021. [4] B.E. Fischer, Nucl. Instr. and Meth. B 30 (1990) 284. [5] K. Yoshida, M. Iwaki, Nucl. Instr. and Meth. B 19±20 (1987) 878. [6] K. Sakamoto, M. Iwaki, K. Takahashi, J. Mater. Res. 11 (1996) 1656. [7] Y. Inoue, Y. Yoshimura, Y. Ikeda, A. Kohno, Colloids and Surfaces B: Biointerface 19 (2000) 263. [8] H. Watanabe, K. Takahashi, M. Iwaki, Mater. Res. Soc. Symp. Proc. 354 (1995) 369. [9] M. Iwaki, K. Yabe, A. Fukuda, H. Watanabe, A. Itoh, M. Takeda, Nucl. Instr. and Meth. B 80±81 (1993) 1080.
[10] T. Kobayashi, T. Iwata, Y. Doi, M. Iwaki, Nucl. Instr. and Meth. B 175±177 (2001) 548. [11] A. Nakao, Y. Suzuki, M. Iwaki, J. Colloid, Interf. Sci. 197 (1998) 257. [12] Y. Suzuki, M. Kusakabe, J.-S. Lee, M. Kaibara, M. Iwaki, H. Sasabe, Nucl. Instr. and Meth. B 65 (1992) 142. [13] Y. Suzuki, M. Kusakabe, M. Kaibara, M. Iwaki, H. Sasabe, T. Nishisaka, Nucl. Instr. and Meth. B 91 (1994) 588. [14] Y. Suzuki, H. Iwata, A. Nakao, M. Iwaki, M. Kaibara, H. Sasabe, S. Kanko, H. Nakajima, M. Kusakabe, Nucl. Instr. and Meth. B 127±128 (1997) 1019. [15] Y. Murayama, F. Vinuela, Y. Suzuki, Y. Akiba, A. Ulihoa, G. Duckwiler, P. Gobin, H. Vinters, M. Iwaki, S. Abe, Am. J. Neuroradiol. 20 (1999) 1992.
www.elsevier.nl/locate/nimb
Ion surface treatments on organic materials Masaya Iwaki
*
Surface Characterization Division, RIKEN, Hirosawa 2-1, Wako, Saitama 351 0198, Japan
Abstract A study has been made of surface modi®cation of various organic materials by ion bombardment or implantation to make the surface properties of high and multiple functions in RIKEN. Substrates used were polyimide (PI), polyacetylene, polytetra¯uoroethylene (PTFE), polystyrene (PS), silicone rubber, various kinds of proteins and so on. Bombarded or implanted ions were inert gas elements, chemically active gaseous elements and metallic elements. Surface properties such as electrical conductivity, wettability and cell adhesion of implanted layers have been investigated. Surface characterization of implanted materials has been carried out by means of transmission electron microscopy, laser Raman spectroscopy, X-ray photoelectron spectroscopy and Rutherford backscattering spectroscopy. In this paper, studies in RIKEN are reviewed of electrical conductivity, optical absorbance, wettability and cell adhesion depending on current densities and doping elements. Applications of ion bombardment to biomedical materials are introduced using cell adhesion control. It is concluded that ion bombardment or implantation is useful to change and control surface properties of various organic materials. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 52.75.Rx; 61.41.+e; 61.82.Pv; 81.05.Lg Keywords: Ion implantation; Polymer; Surface analysis; Cell adhesion
1. Introduction Ion implantation as impurity doping has been succeeded for semiconductor device fabrication, in which the dose is relatively low, generally. In the case of ion beam modi®cation of metals to improve the wear and corrosion resistance, the dose is very high. Therefore, the ion implantation in metals has been restricted in industrial applications because of long time processing [1]. The re-
*
Tel.: +81-48-467-9570; fax: +81-48-462-4701. E-mail address: [email protected] (M. Iwaki).
search study of ion beam modi®cation of organic materials has been gradually progressed. The use of electron beam irradiation has been succeeded in industry for modi®cation of organic materials for curing and hardening [2]. On the other hand, using ion beam bombardment, it is thought that organic materials are easily destroyed and consequently their favorite functions will disappear. Really, this situation has blocked the progress of ion beam modi®cation of polymers. However, we have considered that the destruction of chemical bonds via ion bombardment means synthesis of surfaces with new functions. Ion beam modi®cation of organic materials is classi®ed into two categories. One is surface
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 5 4 3 - 7
M. Iwaki / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 368±374
modi®cation inclined as a function of depth and the other is uniform modi®cation as a function of depth. The former is ion implantation modi®cation near surface layers and the latter is ion beam bombardment through thin ®lms [3,4]. Ion bombardment in the latter case includes the surface modi®cation of ®lms and ion beam mixing at the interface of organic ®lms and substrates due to radiation eects. In this review, at ®rst, electrical conductivity, optical absorbance and wettability are introduced from the standpoints of current density dependence and impurity doping. Secondly, cell adhesion of modi®ed surfaces is described from the standpoints of biomedical applications. 2. Electrical conductivity and wettability 2.1. Current density dependence When ion bombardment and ion implantation are carried out in polymers, most of polymer surfaces change in color from brown to metallic black and have electrical conductive layers. Fig. 1 shows the relationship between sheet resistivity and ¯uence in the case of Ar-bombarded polyimide (PI) ®lms [5]. Ar-ion bombardment in PI ®lms has been carried out with doses ranging from 1 1015 to 2 1017 ions/cm2 using various current densities. In general, the sheet resistivity of the bombarded layers becomes low as the ¯uence increases. Using a low current density, the sheet resistivity is saturated at 1 105 X=cm2 . At a high current density, it monotonously decreases as the dose increases. At the intermediate current density of 1.5 lA=cm2 , the sheet resistivity is saturated. The saturated value is in between those with low and high current densities. The results show that a current density gives a great in¯uence on electrical properties even at the same dose. The electrical conductivity is induced by surface carbonization. Rutherford backscattering spectroscopy (RBS) of 1.5±2.0 MeV He ions was used to measure the change in surface composition of Ar-bombarded PI ®lms [5]. RBS spectra measured showed the reduction of nitrogen and oxygen contents as compared with those for a pristine
369
specimen. The amount of remained Ar atoms obtained from RBS yields is lower than Ar ¯uence, that means that Ar atoms move out during and/or after Ar bombardment. RBS spectra measured after 30 days of Ar bombardment show that the Ar amount reduced from 40% to 26% with 5 1016 Ar/cm2 . No change in Ar amount by aging was found for Ar bombardment with 1015 ±1016 Ar/cm2 . Anyway, Ar bombardment in PI ®lms causes the reduction of nitrogen, oxygen and argon to form carbonized layers. The laser Raman spectra show the formation of amorphous carbon for Ar-bombarded PI ®lms [5,6]. The Raman spectra change depending on beam current densities. At a low current density, the spectra exhibit an asymmetric triangular broad band with a peak at about 1550 1 cm, which is similar to amorphous carbon, as well known. Using a high current density, Raman spectra exhibit a trapezoidal one with ¯at region extending from 1300 cm 1 to 1600 cm 1 , which is considered to contain graphitic structure. The results correspond to those of electrical properties.
Fig. 1. Relationship between sheet resistivity and ¯uence for Arbombarded PI ®lms as a function of beam current density. Implant energy is 150 keV.
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M. Iwaki / Nucl. Instr. and Meth. in Phys. Res. B 175±177 (2001) 368±374
The structure of Ar-bombarded layers was investigated by means of cross-sectional transmission electron microscope (TEM). TEM micrographs show that the bombarded layers and bulk are distinctly separated by the image contrast [5]. It suggests that the formation of conductive layers induced by ion bombardment is not due to thermal eects such as pyrolysis of hydrocarbons at high temperatures. Using a low current density, ion bombarded layers consist of two layers with dierent contrasts in which one is darker near the surface and a little dark in the deeper region. The thickness is a little shallower than the predicted range. Using a high current density, a TEM micrograph shows the appearance of three layers. Two deeper layers at the bulk side have similar contrast to that for a low current density. The top layer has porous structures. As compared with electrical properties, the porous layer may cause the strong reduction of sheet resistivity [5]. The color of PI ®lms is originally brownish yellow and turned dark brown after Ar bombardment. Optical absorbance spectra, obtained by subtracting the absorbance of the pristine specimen from Ar-bombarded specimens, monotonously increase toward shorter wavelength. The absorbance increases as the ¯uence and current density increase. The increase in optical absorbance at the high current density even at the same dose seems to be similar to the tendency of sheet resistivity, as already shown in Fig. 1. Fig. 2 shows the relationship between electrical conductivity and absorbance of 800 nm for Ar-bombarded PI ®lms, polyethyleneterephthalate (PET) ®lms and polyphenylenesul®de (PPS) ®lms [6]. It is quite noteworthy that the conductivity has evident correlation with the optical absorbance. The same correlation has been con®rmed to be held at other wavelengths such as 600, 1000, 1500 and 2000 nm. Moreover, the dependence of the conductivity on the absorbance was found to be almost the same for all the polymers studied in our experiments. The clear dependence of wettability on current density was found recently by Hitachi group [7]. Ar-ion bombardment in polytetra¯uoroethylene (PTFE) has been carried out to investigate the contact angle of water and surface morphology as
Fig. 2. Relationship between conductance and absorbance for Ar-bombarded PI, PET and PPS ®lms.
a function of energy, ¯uence and current density. The surface is roughened by Ar bombardment and the protuberance can be found at special conditions. The contact angle of water of 170.1° was found. It is the highest at the energy of 30 kV and the beam current density of 0.25 mA/cm2 in the experiment. These results show that a current density must be controlled to obtain desired properties. 2.2. Doping eects The sheet resistivity of metal ion-implanted PI ®lms depends on ion species. Fig. 3 shows the sheet resistivity of Ag-, W- and Pd-implanted PI ®lms as a function of dose [8]. The sheet resistivity decreases for all specimens as the dose increases and is saturated at the high dose. The dierence among these saturated sheet resistivities is over one order of the magnitude. Since the dierence of modi®ed thickness is estimated below twice, the saturated sheet resistivity depends on the ion species and the carbon structure in¯uenced by the species.
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Fig. 4. TEM micrographs of (a) Cu- and (b) Zn-implanted PI ®lms. Ion implantation was carried out at 150 keV with doses of 1 1017 Cu/cm2 and 5 1017 Zn/cm2 . The solid bars indicate 100 nm.
Fig. 3. Relationship between sheet resistivity and dose of Ag-, W- and Pd-implanted PI ®lms. Ag and W ions were implanted at average energies of 130 and 190 keV, respectively. Pd-ions were implanted at 100 keV.
RBS spectra for all specimens show the reduction of amounts of nitrogen and oxygen and the enrichment of carbon near the surface layers [9]. This means surface carbonization. Implanted metals are remained in the implanted layers to form a Gaussian-like distribution. Raman spectra observed for Ag- and W-implanted specimens exhibit the formation of amorphous carbon. On the other hand, the spectra for Pd-implanted specimens are not found [8]. XPS spectra show that Ag is metallic, W is a carbide state and Pd is unknown states [9]. The structures of metal-implanted layers are investigated by means of TEM. Cross-sectional TEM micrographs of Cu- and Zn-implanted PI ®lms show small spherical particles of a few nm in diameter in the implanted layers, as shown in Fig. 4 [5]. Such spherical and crystalline particles can be also found in Ag-implanted PI ®lms [10]. Wettability of organic materials was changed by ion implantation. Fig. 5 shows the contact angle of water of Ne-bombarded and Na-implanted polyethylene (PE) [11]. The contact angle of
Fig. 5. Relationship between contact angle of water and dose for Ne-bombarded and Na-implanted PE ®lms. Implant energy is 150 keV.
modi®ed specimens below the dose of 1 1016 ions/cm2 is almost the same as a pristine specimen. Over the dose, it is kept at the same angle for Nebombarded specimen, but that for Na-implanted specimens decreases steeply. The reduction becomes larger at the lower energy implantation.
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The masses of Ne- and Na-atoms are almost the same so that the change in contact angle is due to doping eects of Na. XPS measurements were carried out to investigate the change in composition and structure at the modi®ed surfaces [11]. The results showed that the amorphous carbon is formed for both samples and the high concentration of Na is found at the surface for Na-implanted specimens with higher doses. Therefore, the reduction of contact angles is caused by high concentration of Na atoms. Doping eects can be obtained on electrical conductivity and wettability, induced by high dose metal implantation. Metal ion implantation in polymers form metal-doped amorphous carbon, that can be called new diamond-like carbon.
3. Cell adhesion and its applications 3.1. Cell adhesion Cell adhesion study has been made for applying ion-implanted or bombarded polymers to biomedical materials. Fig. 6 shows micrographs for endothelial cell adhesion on polystyrene (PS), segmented polyurethane (SPU) and collagencoated polystyrene (CC-PS) [12,13]. Na ions were implanted in PS and SPU, and Ar ions were bombarded to CC-PS. The modi®ed area is within the circle of 0.1 mm in diameter. Implant conditions are shown in the ®gure.
Endothelial cells adhere on pristine surfaces of PS and ion implantation more improves cell adhesion. In the case of SPU, the cells hardly adhere on unimplanted area, and they adhere on implanted layers. Implanted layers were characterized by means of Raman and XPS measurements. Raman and XPS spectra of implanted layers show the formation of amorphous carbon. It is considered that cells adhere easily on amorphous carbon for polymers without suitable amino acids. Fig. 6(c) shows that HeLa cells adhere on CCPS very good, but they do not adhere on bombarded regions. In contrast, endothelial cells adhere on both unbombarded and bombarded regions, not shown in the ®gure. High ¯uence bombardment to organic materials forms the amorphous carbon on which HeLa cells do not adhere on amorphous carbon. The cause is considered as HeLa cells adhere selectively on organic materials with amino acids rather than on amorphous carbon. The results show that the cells select adhesive area by themselves. However, the results suggest that ion bombardment has a possibility to control cell adhesion. This technique can be applied to biomedical materials. If it is applied to make small vascular grafts, important characteristics needed on ionmodi®ed regions are the ability of selective adhesion of cells, that is the formation of surfaces with adhesion of endothelial cells and no adhesion of platelets because the adhesion of platelets causes the formation of thrombus to occlude the grafts. The ion-modi®cation condition has been searched
Fig. 6. Micrographs of endothelial cell adhesion on Na-implanted PS (a) and SPU (b) with a dose of 1 1017 ions/cm2 , and HeLa cell adhesion on Ar-bombarded CC-PS (c) with a ¯uence of 1 1015 ions/cm2 . The energy is 150 keV.
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to satisfy the above status, using ion bombardment of inert gases such as He, Ne, Ar and Xe. From the experimental results until now, He ion bombardment of 1 1014 ions/cm2 at 150 keV is more effective for good endothelial cell adhesion and no adhesion of platelets. In such a way, the suitable selection of modi®cation condition is able to perform the control of cell adhesion. 3.2. Biomedical applications The use of ion bombardment has been reported for production of small vascular grafts and modi®cation of Guglielmi detachable coils (GDCs) [14,15]. In the case of small vascular grafts, ion bombardment must be performed to the modi®cation of inner walls of small tubes. For GDCs, relatively uniform bombardment must be performed to small, long springs. We have produced the new target system to perform such bombardments. Small vascular grafts were produced as the following process [14]. At ®rst, Ne ions were bombarded in inner walls of expanded PTFE (ePTFE) with 2±3 mm in inner diameter to get good adhesion of collagen. Secondly, collagen is coated on Ne-bombarded surfaces. Thirdly, He-ion bombardment was carried out to collagen-coated surfaces according to the results of fundamental studies of cell adhesion. Such surfaces have properties of endothelial cell adhesion and no adhesion of platelets. Furthermore, He-ions go through collagen to e-PTFE so that ion beam mixing occurs at the interface between collagen and e-PTFE. This mixing causes the tight adhesion of collagen to substrates. This sample was implanted to femoral artery of dogs. Fig. 7 shows the photographs of inner walls of small grafts after in vivo evaluation. In the case of a control specimen, severe growth thrombus is found after 3 days, and we cannot ®nd thrombus on the modi®ed tubes using ion bombardment even after 180 days. 4. Summary In this paper, ion beam modi®cation of polymers studied in RIKEN is reviewed. A current
Fig. 7. Micrographs of protein hybrid type small diameter arti®cial grafts after in vivo tests.
density is an important factor for getting desired properties even at the same dose. Inert gaseous ion bombardment can be used for controlling surface structure and metal implantation can give the doping eect. Especially, the small metal particle formation is hopeful as new functional carbon materials. A typical application of ion beam modi®ed organic materials is applicable to biomedical ®elds because ion beam bombardment can be used for selective adhesion of cells. Acknowledgements The author would like to thank Dr. A. Nakao, Dr. T. Kobayashi and Dr. K. Yoshida for performing surface characterization, and Dr. Y. Suzuki and Dr. H. Nakajima for performing animal study. References [1] M. Iwaki, in: Proceedings of the 1998 International Conference on Ion Implantation Technology, IEEE, 1999, p. 824.
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