Characterization and protein-adsorption behavior of deposited organic thin film onto titanium by plasma polymerization with hexamethyldisiloxane

Tohru Hayakawa^{, , a}, Masao Yoshinari^b and Kimiya Nemoto<sup>a

a</sup> Department of Dental Materials, Research Institute of Oral Science, Nihon University School of Dentistry at Matsudo, 2-870-1, Sakaecho-nishi, Matsudo, Chiba 271-8587, Japan
^b Department of Dental Materials Science and Oral Health Science Center, Tokyo Dental College, 1-2-2 Masago, Mihama-ku, Chiba 261-8502, Japan

Received 2 December 2002;  accepted 15 June 2003. ; Available online 22 August 2003.
Biomaterials
Volume 25, Issue 1 , January 2004, Pages 119-127

1. Abstract

Plasma polymerized hexamethyldisiloxane (HMDSO) thin film was deposited onto titanium using a radio-frequency apparatus for the surface modification of titanium. A titanium disk was first polished using colloidal silica at pH=9.8. Plasma-polymerized HMDSO films were firmly attached to the titanium by heating the titanium to a temperature of approximately 250°C. The thickness of the deposited film was 0.07-0.35 
m after 10-60 min of plasma polymerization. The contact angle with respect to double distilled water significantly increased after HMDSO coating. X-ray photoelectron spectroscopy revealed that the deposited thin film consisted of Si, C, and O atoms. No Ti peaks were observed on the deposited surface. The deposited HMDSO film was stable during 2-weeks immersion in phosphate buffer saline solution. Fourier transform reflection-absorption spectroscopy showed the formation of Si-H, Si-C, C-H, and C=O bonds in addition to Si-O-Si bonds. Quartz crystal microbalance-dissipation measurement demonstrated that the deposition of HMDSO thin films on titanium has a benefit for fibronectin adsorption at the early stage. In conclusion, plasma polymerization is a promising technique for the surface modification of titanium. HMDSO-coated titanium has potential application as a dental implant material.

Author Keywords: Hexamethyldisiloxane; Plasma polymerization; Titanium; Thin film coating; Protein adsorption; Quartz crystal microbalance-dissipation

2. Article Outline

1. Introduction

2. Materials and methods

2.1. Titanium substrate

2.2. Plasma polymerization

2.3. Surface characterization

2.4. Protein adsorption

2.4.1. XPS analysis

2.4.2. QCM-D measurement

3. Results

4. Discussion

Acknowledgements

References

3. 1. Introduction

During the past decade, physical vapor deposition (PVD) techniques such as ion plating [1], magnetron sputtering [2], and ion beam dynamic mixing [3] have been introduced for the deposition of a thin calcium phosphate coating on oral and medical implants of titanium or titanium alloys as a means of improving tissue responses. The advantage of PVD is that this method avoids some intrinsic shortcomings of plasma-sprayed Ca-P coatings [4, 5 and 6]. For example, PVD deposited Ca-P coatings have a stronger adherence to the underlying titanium surface and are less prone to form cracks [2 and 3].

However, little attention has been given to the use of chemical vapor deposition techniques other than PVD, such as plasma polymerization for depositing thin films onto titanium for modifying the surfaces of titanium substrates. Films formed by plasma polymerization are generally free of pinholes, have a strong degree of adherence to a wide variety of materials, and have a greater degree of resistance to chemical and physical treatment because of their cross-linked structure [7 and 8]. Monomers for plasma polymerization need not contain the traditional functional groups, and polymerization of organic compounds can be initiated that will not form polymers under normal polymerization conditions. The polymer films obtained by plasma polymerization are quite different from those derived from polymerized films formed by conventional radical or ion initiation methods. Plasma polymerization of a number of compounds such as hydrocarbons, aliphatic hydrocarbons, organosilicones, and methacrylates has been reported [9, 10, 11, 12 and 13].

Among the wide variety of possible monomers for plasma polymerization, considerable attention has been given to the plasma polymerization of organosilicon monomers, particularly hexamethyldisiloxane (HMDSO) [14, 15, 16, 17, 18 and 19]. These monomers are of interest because of their high deposition rates and the ability to control their structure and properties by varying the deposition conditions. In addition, HMDSO is an easy and safe monomer to handle, especially compared to silane compounds.

In contrast to the extensive literature on the chemistry of HMDSO plasma-deposited films, relatively little attention has been given to the application of these films in the medical or dental fields in respect of their biological properties. A few studies reported the application of plasma polymerization for the immobilization of bioactive molecules on the surfaces of bioinert materials. Miyachi et al. [20] reported the application of plasma-polymerized hexamethyldisiloxane film to immobilize streptavidin on a glass substrate. Puleo et al. [21] found that surface modification of titanium by plasma polymerization of allyl amine is useful for the immobilization of bioactive molecules such as BMP-4.

In this paper, in order to learn more about the efficacy of plasma polymerization as a technique for modifying the surface of titanium, we used plasma polymerization to deposit HMDSO thin films onto titanium and then characterized the chemical properties of the deposited thin films. The chemical properties of the plasma-polymerized thin films were analyzed by measuring the contact angle with respect to distilled water, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared reflection-absorption spectroscopy (FT-IR-RAS). Recent studies have proven that the quartz crystal microbalance-dissipation (QCM-D) technique is useful for evaluation of surface-related processes in liquids, including protein adsorption [22, 23 and 24]. The adsorption behavior of protein towards the deposited thin films was also investigated by XPS measurement and by the QCM-D technique.

4. 2. Materials and methods

2.1. Titanium substrate

Commercially pure wrought titanium disks (JIS, Japan industrial Specification H 4600, 99.9 mass% Ti, Furuuchi Chemical Corp., Tokyo) with a diameter of 6 or 15 mm were used. The 6-mm diameter titanium disk was used for the XPS analysis and the 15-mm diameter titanium disk was used for the FT-IR-RAS analysis. The disks were ground progressively finer down to 1200 grit, finely polished using colloidal silica at pH=9.8, and then ultrasonically cleaned with acetone only in order to avoid affecting the surface conditions of the titanium after mirror polishing.

2.2. Plasma polymerization

Plasma-polymerized thin films were prepared using a commercially available plasma deposition system (High Vapor Deposition System, VEP-1000, ULVAC Inc., Kanagawa, Japan). The titanium substrate was rotated in the stainless steel chamber (300 mm diameter×500 mm height) at a rotational velocity of 30 rpm and was then heated to a temperature of approximately 250°C. The reservoir (50 mm diameter×150 mm height) of HMDSO was heated to 40°C and the pipe (15 mm diameter) for introducing the HMDSO was heated to 80°C. The HMDSO gas was then introduced into the equipment chamber through the heated pipe at the HMDSO gas flow rate of 35 sccm. The plasma was generated using a radio-frequency generator operating at 13.56 MHz at a power of 50 W. The pressure of the chamber was maintained at approximately 0.17 MPa throughout the plasma polymerization process. Plasma polymerization was performed for periods of 10, 30, and 60 min. After the plasma was turned off, nitrogen gas was used to purge the chamber, which was then allowed to return to atmospheric pressure. The process resulted in the deposition of plasma-polymerized film on the titanium surface.

2.3. Surface characterization

The thickness of the deposited organic films was determined using a Dektak Stylus Profiler System (Veeco Instrument Inc., Woodbury, NY, USA). The stylus force was 25 mg, and the scanning speed was 200 
m/s. Three measurement runs were performed for each surface.

The contact angle with respect to the double-distilled water was measured using a Contact Angle Meter (CA-D, Kyowa Interface Science Co. Ltd., Tokyo, Japan). Five measurements of 15 s each were made for each surface type, and all analyses were performed at the same temperature and humidity. The data were analyzed by an analysis of variance (ANOVA) and Sheffe's test for the multiple comparisons among the means at p=0.05.

Surface analyses of the plasma-polymerized titanium samples were performed using XPS (ESCA-750, Shimadzu, Kyoto, Japan) by evaluating the intensity of Si2p, C1s, O1s, and Ti2p. Argon-ion sputtering was performed at 2 kV and 15 
A/cm². The binding energies of each of the spectra were calibrated with C1s at 284.6 eV.

The stability of deposited HMDSO film was evaluated by the immersion of HMDSO-coated specimen in phosphate-buffered saline (PBS) solution. After a 10-min deposition, the HMDSO plasma-polymerized titanium disk was immersed in PBS solution with pH=7.4 for 2 weeks. Afterwards, the specimen was rinsed with double-distilled water and the surface was then characterized by XPS as described above. The PBS solution was exchanged every day.

The organic groups in the deposited films were analyzed by Fourier transform infrared reflection-absorption spectroscopy (FT-IR-RAS, FT-IR-430, Jasco Corp., Tokyo, Japan) at 4 cm⁻¹ resolution. Two measurements for each surface type were made. The incident angle of the infrared ray to the specimen was 75°. Mirror-polished uncoated titanium was used for the background while calculating the spectrum of the specimen to obtain a spectrum due only to substances formed on the titanium by plasma polymerization.

2.4. Protein adsorption

2.4.1. XPS analysis

Human cell-binding fibronectin (Upsatate Biotechnology, Lake Placid, NY, USA) was dissolved in PBS solution with pH=7.4 at a concentration of 0.1 mg/ml. The mirror-polished titanium disk and the HMDSO plasma-polymerized titanium disk (30-min coating) were immersed into the fibronectin/PBS solution for 24 h at 37°C, and then rinsed with double-distilled water. Finally, the disks were dried in a gentle stream of dry air and stored in a desiccator. The adsorption of fibronectin onto titanium and HMDSO-polymer coated titanium was confirmed by the N1s peaks of amide bonds of protein. Argon-ion sputtering was performed at 2 kV and 15 
A/cm². The binding energies of each of the spectra were calibrated with C1s of 284.6 eV.

2.4.2. QCM-D measurement

The QCM-D technique was used for the evaluation of the adsorption behavior of fibronectin in the presence and absence of HMDSO-polymer coating. The QCM-D instrument (QCM-D, Q-Sense, AB, Göteborg, Sweden) was operated with AT-cut single crystal quartz sensors of 5 MHz resonant frequency. As a sensor, titanium coated QCM-D sensor crystal (14 mm diameter) was used. The HMDSO-coated sensor (30-min HMDSO coating) was prepared according to the method described above. The titanium and HMDSO-coated titanium sensor crystals were cleaned prior to each adsorption measurement in an UV/ozone chamber for 15 min. The crystal resonant frequency (Δf) and the dissipation factor (ΔD) of the oscillator were measured simultaneously at a fundamental resonant frequency (5 MHz) and at a number of overtones, including 35 MHz.

Monitoring the resonance behavior of piezoelectric oscillation allows measurements of the mass being adsorbed at the surface of the oscillation in real time, usually as a function of the decrease in the resonance frequency (f). The frequency shift (Δf) is related to the adsorbed mass (Δm) according to the Sauerbrey relation [25]. At 5 MHz, a frequency shift of 1 Hz corresponds to a mass change of approximately 18 ng/cm². This relation is applicable strictly to sufficiently thin, rigid films [26]. A second measurement parameter, dissipation (D), gives qualitative information about the viscoelastic properties of the adsorbed layer.

Fibronectin/PBS solution (20 ppm) was introduced into an axial flow chamber comprising a T-loop in order to thermally equilibrate the sample at 37±0.1°C. The sequence of injections into the QCM cell for an experimental run was as follows: 0.5 ml of distilled water, 0.5 ml of PBS solution, 2 ml of fibronectin/PBS solution.

5. 3. Results

The thickness of the deposited HMDSO films and the contact angles with respect to double-distilled water are listed in Table 1. The film thickness was nearly proportional to the coating time. After a 60-min coating process, the thickness of the deposited film was approximately 0.35 
m. The contact angle with respect to double-distilled water increased after the HMDSO coating, and there were significant differences in the contact angle between un-coated titanium and the HMDSO-coated titanium. After a 30-min coating, the HMDSO-coated surface showed contact angles with respect to double-distilled water of approximately 83°, which were significantly the highest.

Table 1. Film thickness and contact angles of plasma-polymerized HMDSO film

( ) Standard deviation.

Mean values with the same superscripts are not significantly different at P>0.05.

The elements detected by the XPS analysis of the mirror polished titanium were Ti, C, and O, and those of the HMDSO-coated titanium were Si, C, and O. No Ti peaks were observed on the surface of the HMDSO-coated titanium. XPS analyses revealed no titanium peaks on an HMDSO-coated surface before argon-ion sputtering, and only Si, O, and C atoms were identified as components of the deposited films.

Fig. 1 shows the Si2p spectra of the HMDSO-coated titanium specimens analyzed by XPS measurement. In the spectra of 10-min coating, the Si2p peak of the HMDSO layer was observed at 102.2 eV before argon-ion sputtering, as indicated by the dotted line in Fig. 1, which shifted approximately 2 eV lower in comparison with the reported energy value [15]. These peaks almost disappeared after 60 min of argon-ion sputtering. In the spectra of the 30-min coating, the Si2p peak of the outermost layer of the HMDSO film appeared at 101.3 eV, as indicated by the dotted line. After 60 min of argon-ion sputtering, the Si2p peak shifted to 102.2 eV, and almost disappeared after 90 min of argon-ion sputtering. The Si2p spectra of the 60-min coating are similar to those of the 30-min coating. The Si2p peak of the outermost layer of HMDSO film was also observed at 101.3 eV, as indicated by the dotted line, and also shifted to 102.2 eV after 90 min of sputtering. The Si2p peaks of 60-min HMDSO deposition almost disappeared after 150 min of argon-ion sputtering.

(16K)

Fig. 1. Si2p spectra of HMDSO film deposited onto titanium by XPS analysis: (a) 10-min coating, (b) 30-min coating, (c) 60-min coating.

Fig. 2 shows the O1s spectra of the HMDSO-coated titanium specimens analyzed by XPS measurement. The O1s peaks of the three types of outermost layers of HMDSO films appeared at 532.6 eV, as indicated by the dotted line, which corresponded with the reported energy value [15]. These peaks also disappeared after long-term argon-ion sputtering. Afterwards, new O1s peaks appeared at 531.4 eV, which corresponds to the bulk oxygen in the titanium dioxide. [27]

(17K)

Fig. 2. O1s spectra of HMDSO film deposited onto titanium by XPS analysis: (a) 10-min coating, (b) 30-min coating, (c) 60-min coating.

Fig. 3 shows the Ti2p spectra of HMDSO-coated titanium specimens analyzed by XPS measurements after argon-ion sputtering. Two Ti2p peaks were observed at 459.8 and 453.9 eV after long-term argon-ion sputtering, as shown in Fig. 3 (dotted line). The Ti2p_1/2 peak at 459.8 eV and the Ti2p_3/2 peak at 453.9 eV represented the Ti of metallic titanium [28].

(10K)

Fig. 3. Ti2p spectra of HMDSO film deposited onto titanium after argon-ion sputtering by XPS analysis: (a) 10-min coating, (b) 30-min coating, (c) 60-min coating.

Fig. 4 shows the Si2p and O1s spectra analyzed by XPS measurement of HMDSO-coated specimen after immersion in PBS for 2 weeks. The Si2p and O1s peaks were still observed at 102.2 and at 532.6 eV, respectively.

(8K)

Fig. 4. Si2p and O1s spectra of HMDSO film deposited onto titanium by XSP analysis after immersion for 2 weeks in PBS solution.

The FT-IR-RAS spectra of HMDSO-coated specimens are shown in Fig. 5. The peak attributed to Si-O-Si bonds appeared at around 1130 cm⁻¹ for the 10-min coated specimen, and at around 1080 cm⁻¹ for the 30- and 60-min coated specimens. The peaks around 800 cm⁻¹ are attributed to Si-H and Si-C bonds. The two clusters appearing at 1500 and 1700 cm⁻¹ in the 30- and 60-min coated specimens are attributed to C-H and C=O bonds, respectively [15, 16 and 17]. In the 10-min coated specimens, these two peaks are not clearly distinguished.

(8K)

Fig. 5. FT-IR-RAS spectra of HMDSO film deposited titanium: (a) 10-min coating, (b) 30-min coating, (c) 60-min coating.

Fig. 6 shows the XPS spectrum of HMDSO film deposited titanium after the adsorption of fibronectin. N1s peaks were detected at 399.9 eV. These peaks were derived from the amide groups of fibronectin [29]. This derivation was confirmed by XPS observation of the original fibronectin. In addition, Si2p peaks were detected at 101.3 eV.

(4K)

Fig. 6. XPS analysis of fibronectin attached HMDSO coated titanium. The HMDSO plasma polymerization was performed over 30 min.

Fig. 7 compares the N1s spectra produced by the XPS analysis of (a) the fibronectin-attached titanium and (b) the fibronectin-attached HMDSO-coated titanium. Both specimens were rinsed with double-distilled water. N1s peaks of the fibronectin-attached titanium peaks almost disappeared after 10 s of argon-ion etching. However, the N1s peaks of the fibronectin-attached HMDSO-coated titanium remained after argon-ion sputtering for 60 s.

(9K)

Fig. 7. N1s spectra of fibronectin-attached titanium (a) and fibronectin attached HMDSO coated titanium (b). The HMDSO plasma polymerization was performed over 30 min. The specimens were rinsed with double distilled water after the fibronectin adsorption.

Fig. 8 and Fig. 9 show the frequency and dissipation shift obtained via QCM-D measurement. In Fig. 8, the HMDSO-coated specimen revealed a more rapid frequency decrease very quickly and a greater frequency decrease compared to the non-coated titanium surface during 120-min adsorption, which indicates a greater amount of fibronectin adsorbed onto the HMDSO-coated titanium. The dissipation shift differed during 120-min adsorption between the titanium and HMDSO-coated titanium specimens, as shown in Fig. 9. Fibronectin attached to titanium surface showed a greater shift of dissipation than HMDSO-coated titanium.

(5K)

Fig. 8. Frequency shift with respect to time for exposure of a titanium QCM-sensor (a) and HMDSO-coated titanium QCM-sensor (b) to fobronectin.

(4K)

Fig. 9. Dissipation shift with respect to time for exposure of a titanium QCM-sensor (a) and HMDSO-coated titanium QCM-sensor (b) to fobronectin.

6. 4. Discussion

The purpose of the present study was to characterize the HMDSO-deposited film on titanium and to examine protein adsorption on the HMDSO plasma-polymerized film.

Compared with PVD techniques such as RF magnetron sputtering, plasma polymerization has the advantages of a high deposition rate and a wide variation of source monomers. Generally, the films deposited by plasma polymerization are free of pinholes and show a strong degree of adhesion to the substrate [7 and 8]. However, there have been few studies examining plasma polymerization to deposit organic thin films for the surface modification of dental titanium implants. We devised a technique of applying plasma polymerization for modifying the surface of titanium implant materials.

Investigation via XPS revealed that the titanium surface was completely covered by plasma polymerized HMDSO film. The deposited film was insoluble with organic solvents such as acetone, due to the cross-linking of the deposited film, and firmly adhered to the titanium substrate. According to the procedures given by the Japanese Industrial Standard Committee [30], adhesive tape was applied to deposited film in order to determine the degree of adherence of the film. No organic film deposited under the present procedures was removed from the titanium substrate. XPS measurement after immersion for 2 weeks of HMDSO-coated specimens also confirmed the stability of the deposit film on titanium. No distinct dissolution of deposited films and no swelling were observed during the immersion for 2 weeks.

Two types of Si2p peaks were detected at 102.2 and at 101.3 eV by XPS measurement. Endo et al. [31] analyzed the titanium surface by XPS after the coupling reaction between the silanol end of the silane coupling agent and the surface hydroxyl groups of titanium. They reported that the Si2p peaks of the T-O-Si bond were detected at 102.7 eV. On the other hand, Kalman et al. [32] reported the XPS analysis of silicone rubber in which they detected Si2p peaks of dimethyl silicone rubber near 100 eV. Thus, the Si2p peaks at 102.2 eV could be assigned to Si of the Ti-O-Si bond and the Si2p peaks at 101.3 eV could be assigned to the Si of the siloxane network.

Endo et al. [31] also reported two O1s peaks at 532.0 and 533.1 eV after the same coupling reaction of the silane coupling agent described above. They assigned the O1s peaks at 532.0 eV to the oxygen in the Ti-O-Si bond, and the O1s peaks at 533.1 eV to the oxygen of the cross-linked siloxane network (Si-O-Si). In the present study, O1s peaks other than those for the oxygen derived from TiO₂ were detected at 532.6 eV, which were almost in the middle of 532.0-533.1 eV. It is presumed that the O1s peaks at 532.6 eV could be assigned as the combination of two oxygens derived from the oxygen of Ti-O-Si bonds and Si-O-Si bonds.

FT-IR-RAS measurement showed the difference of Si-O-Si bonds of the siloxane network in the HMDMO deposited film between 10-min coating, and 30 or 60-min coating. The peaks derived from Si-O-Si bonds of deposited HMDSO film in the 30- and 60-min coated specimens appeared around 1080 cm⁻¹, which corresponded to the peaks derived from the Si-O-Si bonds in polydimethyl siloxane [33]. The peaks derived from the Si-O-Si bonds in the 10-min coated specimen appeared at higher wavelengths than those from 30- and 60-min coated specimens. It is presumed that the difference in wavelength of the Si-O-Si bond in 10-min coating is due to the influence of the formation of Ti-O-Si bonds.

It is postulated that the initial step of HMDSO plasma polymerization is the fragmentation of methylsilyl groups and their cleavage off methyl and silyl radicals after the ionization of an HMDSO molecule [17 and 34]. We suggested that silyl radicals first attach the hydroxyl group of titanium and then formed a metallosiloxiane bond (Ti-O-Si) during the present HMDSO plasma polymerization.

FT-IR-RAS measurement of HMDSO-deposited films showed the presence of new bonds that were absent in the monomer structure, namely, Si-H and C=O bonds in addition to the corresponding monomer bonds: Si-O-Si and Si-C. This resulted from the fragmentation of the Si-C and C-H bonds of the HMDSO monomers and a slight oxidation reaction of the deposited films during the plasma polymerization [18].

The titanium deposited via HMDSO plasma polymerization is more hydrophobic than is mirror-polished titanium, resulting in the high contact angles. It is generally understood that a surface having a greater degree of hydrophobicity results in a greater degree of protein adsorption [35 and 36]. McDonald et al. [37] reported that hydrophobic preparation of the titanium surface enhanced the fibronectin adsorption as compared to hydrophilic surface preparation.

For a biological examination of HMDSO-deposited films, the adsorption behavior of a protein, fibronectin, onto the HMDSO deposited surface was investigated as a next step of our experiment, including an investigation into the adsorption mechanism of fibronectin. Among the HMDSO plasma-polymerized specimens, the 30-min coating specimen was used because of the significantly higher contact angles, i.e. higher hydrophobicity. XPS analysis revealed that adsorbed fibronectin did not completely cover the surface of the HMDSO-coated specimen. Comparison of N1s spectra between fibronectin-attached titanium and fibronectin-attached HMDSO-coated specimen suggested a difference in the adsorption behavior of fibronectin, for example, a greater amount of the adsorbed fibronectin or tighter binding of adsorbed fibronectin on the HMDSO-coated surface.

QCM-D measurement was performed in order to obtain more detailed information about the protein adsorption. Höök et al. [23] reported the effectiveness of the QCM-D technique for the analysis of adsorption kinetics of three model proteins on titanium oxide surfaces, compared with ellipsometry and optical waveguide lightmode spectroscopy. They also pointed out that the mass calculated from the resonance frequency shift included both protein mass and water that bound or hydrodynamically coupled to the protein adlayer.

In the present study, the amount of fibronectin adsorbed to HMDSO-coated titanium by frequency shift was greater than that adsorbed to titanium at an early stage of adsorption, i.e. 120 min adsorption. Moreover, there was a difference in dissipation shift between titanium and HMDSO-coated titanium specimens during 120-min adsorption. The difference in dissipation shift suggests that fibronectin adsorbed onto HMDSO-coated titanium has a more rigid structure than that on titanium. The hydrophobic surface of HMDSO-coated titanium will allow a stronger binding with fibronectin, resulting in a more rigid conformation. Further detailed study of the fibronectin adsorption using QCM-D to examine, for example, the influence of different concentrations of fibronectin or the influence of longer adsorption periods or adsorption kinetics, should be conducted in the future.

Fibronectin is a well-known cell-adhesive protein and is believed to play an important role in governing the interactions of biomaterials with their surrounding matrices [38]. It was reported that the precoating of titanium with fibronectin enhanced the adhesive characteristics of cells to titanium [39 and 40]. The results obtained in this study suggest that an HMDSO-coated titanium surface will easily be covered with protein after implantation into living tissue at an early stage, and will improve the degree of cell activity at the implant-tissue interface.

In conclusion, coating of titanium with organic thin films was shown to be possible via plasma polymerization of HMDSO, and the deposition of HMDSO thin films onto titanium will be beneficial for protein adsorption at the early stage of implantation. It is expected that the application of HMDSO plasma polymerization will become a useful technique for the surface modification of titanium and that HMDSO-coated titanium has potential for application as a dental implant material. Biological activity such as cultured cell growth or tissue response in relation to HMDSO-coated titanium will be the subject of future investigation.

7. Acknowledgements

This study was supported in part by a Grant from the Ministry of Education, Culture, Sports, Science and Technology of Japan to promote 2001-Multidisciplinary Research Projects (in 2001-2005), and by a Grant-in-Aid for Scientific Research (C)(2)(15592073) from the Japan Society for the Promotion of Science, and by an Oral Health Science Center Grant 5A10 from Tokyo Dental College.

8. References

1. M. Yoshinari, K. Ozeki and T. Sumii, Properties of hydroxyapatite-coated Ti-6Al-4V alloy produced by the ion-plating method. Bull Tokyo Dent Coll 32 (1991), pp. 147-156. Abstract-MEDLINE  

2. J.A. Jansen, J.G.C. Wolke, S. Swann, J.P.C.M. van der Waerden and K. de Groot, Application of magnetron sputtering for producing ceramic coatings on implant materials. Clin Oral Impl Res 4 (1993), pp. 28-34. Abstract-MEDLINE   | Full Text via CrossRef

3. T. Hayakawa, M. Yoshinari, H. Kiba, H. Yamamoto, K. Nemoto and J.A. Jansen, Trabecular bone response to surface roughened and calcium phosphate (Ca-P) coated titanium implants. Biomaterials 23 (2002), pp. 1025-1031. SummaryPlus | Full Text + Links | PDF (365 K)

4. C.P.A.T. Klein, J.G.C. Wolke, J.M.A. de Blieck-Hogervorst and K.A. de Groot, Calcium phosphate plasma-sprayed coatings and their stability: an in vivo study. J Biomed Mater Res 28 (1994), pp. 909-917. Abstract-MEDLINE | Abstract-EMBASE | Abstract-Compendex  

5. P. Cheang and K.A. Khor, Addressing processing problems associated with plasma spraying of hydroxyapatite coatings. Biomaterials 17 (1996), pp. 537-544. SummaryPlus | Full Text + Links | PDF (1749 K)

6. M. Ogiso, Y. Yamashita and T. Matsumoto, Microstructural changes in bone of HA-coated implants. J Biomed Mater Res 39 (1998), pp. 23-31. Abstract-MEDLINE | Abstract-Compendex   | Full Text via CrossRef

7. S. Kurosawa, N. Kamo, D. Matsui and Y. Kobatake, Gas sorption to plasma-polymerized copper phthalocyanine film formed on a piezo-electric crystal. Anal Chem 62 (1990), pp. 353-359. Abstract-EMBASE | Abstract-Beilstein Abstracts  

8. F.F. Shi, Recent advances in polymer thin film prepared by plasma polymerization: synthesis, structural characterization, properties and applications. Surf Coat Technol 82 (1996), pp. 1-15. Abstract | PDF (1382 K)

9. F-.O. Fong, H.C. Kuo, J.C. Wolfe and J.N. Randall, Plasma polymerized styrene: a negative resist. J Vac Sci Technol B6 (1988), pp. 375-378. Abstract-INSPEC   | Full Text via CrossRef

10. N. Inagaki, K. Nishino and K. Katsuura, Some optical properties of polymer films prepared by glow discharge polymerization from methane, tetrramethylsilane, and tetramethylin. J Poly Sci Polym Ed 18 (1980), pp. 765-770. Abstract-INSPEC   | Full Text via CrossRef

11. M.W. Horn, S.W. Pang and M. Rothschild, Plasma-deposited organosilicon thin films as dry resists for deep ultraviolet lithography. J Vac Sci Technol B8 (1990), pp. 1493-1496. Abstract-INSPEC   | Full Text via CrossRef

12. A.M. Wrobel, J. Kowalski, J. Grebowicz and M. Kryszewski, Thermal decomposition of plasma-polymerized organosilicon thin film. J Macromol Sci Chem A17 (1982), pp. 433-452. Abstract-Compendex  

13. A.J. Ward and R.D. Short, A time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy investigation of the structure of plasma polymers prepared from the methacrylate series of monomers. Polymer 34 (1993), pp. 4179-4185. Abstract

14. G. Akovali, Plasma polymerization of hexamethyldisiloxane in the gas. Phase I. Polymerization studies. Polym Eng Sci 21 (1981), pp. 658-661. Abstract-Compendex  

15. K.G. Sachdev, Characterization of plasma-deposited organosilicon thin films. Thin Solid films 107 (1983), pp. 245-250. Abstract

16. K. Kashiwagi, Y. Yoshida and Y. Murayama, Hybrid films formed from hexamethyldisiloxane and SiO by plasma process. Jpn J Appl Phys 30 (1991), pp. 1803-1807. Abstract-INSPEC | Abstract-Compendex  

17. C. Rau, Mechanisms of plasma polymerization of various silico-organic monomers. Thin Solid Films 249 (1994), pp. 28-37. Abstract

18. S.H. Lee and D.C. Lee, Preparation and characterization of thin films by plasma polymerization of hexamethyldisiloxane. Thin Solid Films 325 (1998), pp. 83-86. SummaryPlus | Full Text + Links | PDF (141 K)

19. J. Schwarz, M. Schmidt and A. Ohl, Synthesis of plasma-polymerized hexamethyldisiloxane (HMDSO) films by microwave discharge. Surf Coat Technol 98 (1998), pp. 859-864. Abstract | PDF (430 K)

20. H. Miyachi, A. Hiratsuka, K. Ikebukuro, K. Yano, H. Muguruma and I. Karube, Application of polymer-embedded proteins to fabrication of DNA array. Biotechnol Bioeng 69 (2000), pp. 323-329. Abstract-Elsevier BIOBASE | Abstract-MEDLINE | Abstract-EMBASE | Abstract-Compendex   | Full Text via CrossRef

21. D.A. Puleo, R.A. Kissling and M-.S. Sheu, A technique to immobilize bioactive proteins, including bone morphogenetic protein-4 (BMP-4), on titanium alloy. Biomaterials 23 (2002), pp. 2079-2087. SummaryPlus | Full Text + Links | PDF (188 K)

22. M. Rodahl, F. Höök and B. Kasemo, QCM operation in liquids: an explanation of measured variations in frequency and Q factor with liquid conductivity. Anal Chem 68 (1996), pp. 2219-2227. Abstract-Compendex   | Full Text via CrossRef

23. F. Höök, J. Vörös, M. Rodahl, R. Kurrat, P. Böni, J.J. Ramsden, M. Textor, N.D. Spencer, P. Tengvall, J. Gold and B. Kasemo, A comparative study of protein adsorption on titanium oxide surface using in situ ellipsometry, optical waveguide lightmode spectroscopy, and quartz crystal micreobalance/dissipation. Colloids Surf B Biointerfaces 24 (2002), pp. 155-170. SummaryPlus | Full Text + Links | PDF (284 K)

24. Andersson A-S, Glasmästar K, Sutherland D, Lidberg Ulf, Kasemo B. Cell adhesion on supported lipid bilayers. J Biomed Mater Res 2003;64A:622-9.

25. G. Sauerbrey, Verwendung non Schwingquarzen zur Wägung dünner Schichten und zur Mikrowägung. Z Phys 155 (1959), pp. 206-222.

26. M. Rodahl, F. Höök, C. Fredriksson, C.A. Keller, A. Krozer, P. Brzezinski, M. Voinova and B. Kasemo, Simultaneous frequency and dissipation factor QCM measurements of biomolecular adsorption and cell adhesion. Faraday Discuss 107 (1997), pp. 229-246. Abstract-MEDLINE | Abstract-INSPEC   | Full Text via CrossRef

27. T.K. Sham and M.S. Lazarus, X-ray photoelectron spectroscopy (XPS) studies of clean and hydrated TiO₂ (rutile) surfaces. Chem Phys Lett 68 (1979), pp. 426-432. Abstract

28. C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. Muilenberg. In: G.E. Muilenberg, Editor, Handbook of X-ray photoelectron spectroscopy, Parkin-Elmer Corporation, Minnesota (1979), p. 68.

29. D.E. MacDonald, B. Markovic, M. Allen, P. Somasundaran and A.L. Boskey, Surface analysis of human plasma fibronectin adsorbed to commercially pure titanium materials. J Biomed Mater Res 41 (1998), pp. 120-130. Abstract-MEDLINE | Abstract-Compendex | Abstract-EMBASE   | Full Text via CrossRef

30. Methods of adhesion test for metallic coatings. H8504-1996, Japanese Industrial Standard Committee.

31. K. Endo, Chemical modification of metallic implant surfaces with biofunctional proteins (Part 1) Molecular structure and biological activity of a modified NiTi Alloy surface. Dent Mater J 14 (1995), pp. 185-198. Abstract-MEDLINE  

32. P.G. Kalman, C.A. Ward, N.B. McKeown, D. McCullough and A.D. Romaschin, Improved biocompatibility of silicone rubber by removal of surface entrapped air nuclei. J Biomed Mater Res 25 (1991), p. 1990211.

33. J.H. Park, K.D. Park and Y.H. Bae, PMDS-based polyurethanes with MPEG graft: synthesis, characterization and platelet adhesion study. Biomaterials 20 (1999), pp. 943-953. Abstract | PDF (304 K)

34. A.M. Wrobel, J. Kowalski, J. Grebowicz and M. Kryszewski, Thermal decomposition of plasma-polymerized organosilicone thin films. J Macromol Sci Chem A17 (1982), pp. 433-452. Abstract-Compendex  

35. C.A. Haynes and W. Norde, Globular properties at solid/liquid interfaces. Coll Surfaces B: Biointerfaces 2 (1994), pp. 517-566. Abstract

36. D.R. Absolom, W. Zingg and A.W. Neumann, Protein adsorption to polymer particles: role of surface properties. J Biomed Mater Res 21 (1987), pp. 161-171. Abstract-Compendex | Abstract-EMBASE | Abstract-MEDLINE  

37. D.E. McDonald, N. Deo, B. Markovic, M. Stranick and P. Somasundaran, Adsorption and dissolution behavior of human plasma fibronectrin on thermally and chemically modified titanium dioxide particles. Biomaterials 23 (2002), pp. 1269-1279.

38. T.L. Seitz, K.D. Noonan, L.L. Hench and N.E. Noonan, Effect of fibronectin on the adhesion of an established cell line to a surface reactive biomaterial. J Biomed Mater Res 16 (1982), pp. 195-207. Abstract-Compendex | Abstract-EMBASE | Abstract-MEDLINE  

39. M. Cannas, F. Denicolai, L.X. Webb and A.G. Gristina, Bioimplant surfaces: binding of fibronectin and fibroblast adhesion. J Orthop Res 6 (1998), pp. 58-62.

40. J.W. Dean, K.C. Culbertson and A.M. D'Angelo, Fibronectin and laminin enhance gingival cell attachment to dental implant surfaces in vitro. J Oral Maxillofac Implants 10 (1995), pp. 721-728. Abstract-MEDLINE  

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