Langmuir 1998, 14, 1887-1891
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Protein-Silicone Interactions: How Compatible Are the Two Species? Vasiliki Bartzoka,† Michael A. Brook,*,† and Mark R. McDermott‡ Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4M1, and Department of Pathology, McMaster University, 1200 Main Street West, Hamilton, Ontario, Canada L8N 3Z5 Received October 10, 1997. In Final Form: January 5, 1998 Protein-on-silicone and silicone-on-protein films were made by the sequential coating of the human serum albumin (HSA) onto silicone films on glass or vice versa. The silicones used were either trimethylsilylterminated poly(dimethylsiloxane) (unfunctionalized PDMS) or (triethoxysilyl)propyl-terminated poly(dimethylsiloxane) (functionalized TES-PDMS). Angular-dependent X-ray photoelectron spectroscopy (AD-XPS) and contact angle measurements (CA) were used to characterize the modified surfaces. Irrespective of the order of building the films, protein-on-silicone or silicone-on-protein both showed essentially identical surface compositions, suggesting a significant degree of mixing between the protein and silicone. The TES-PDMS was found to have a greater affinity for HSA: thicker and more homogeneous silicone films were found with TES-PDMS/HSA than with PDMS/HSA films.
Introduction The exposure of a protein solution to a solid surface results in spontaneous adsorption at the solid/liquid interface.1 This tendency of proteins affects many natural and synthetic processes and therefore has attracted attention in various biological, medical, and technological fields (i.e., biofouling, thrombus development, emulsions).2 Protein adsorption is the net result of the interactions between the protein molecules, the solvent, and the sorbent surface. In addition to these intermolecular interactions, intramolecular forces within the protein macromolecule are important. When a protein interacts with a surface, structural rearrangements in the protein macromolecule and dehydration of the protein may occur, leading to denaturation and subsequent modification of the protein bioproperties. In particular, when the surface is hydrophobic, such as a silicone surface, for example, there is evidence that proteins undergo structural rearrangements at the solid silicone/water interface.3 The adsorption of silicone polymers (poly(dimethylsiloxane)s) from dilute solutions on solid surfaces has also received considerable attention.4,5 Poly(dimethylsiloxane)s (PDMS) can interact with substrates both through dispersion forces from the induced dipoles in the methyl groups and through permanent dipoles in the partially polar siloxane backbone. Thus, PDMS will primarily interact with polar substrates through the siloxane backbone and with nonpolar substrates through the methyl groups. The backbone flexibility of silicones allows them to adjust to the availability of reactive sites on * Corresponding author. E-mail:
[email protected]. Phone: (905) 525-9140, ext. 23483. Fax: (905) 522-2509. † Department of Chemistry. ‡ Department of Pathology. (1) Lipatov, Y. S.; Sergeeva, L. M. Adsorption of Polymers; Wiley: New York, 1974. (2) Kondo, A.; Oku, S.; Higashitani, K. J. Colloid Interface Sci. 1991, 143, 214. (3) Haynes, C. A.; Norde, W. Colloids Surf. B: Biointerfaces 1994, 2, 517. (4) Ashmead, B. V.; Owen M. J. J. Polym. Sci. A-2 1971, 9, 331. (5) Britcher, L. G.; Kehoe, D. C.; Matisons, J. G.; Smart, R. St. C.; Swincer, A. G. Langmuir 1993, 9, 1609.
surfaces.6,7 End-functionalized siloxanes (e.g., NH2, COOH, OH, alkoxy, epoxy) are additionally able to interact through the end groups. These siloxanes have received attention as macromolecular coupling agents since strong end group interactions with the substrate can result in greater control and reproducibility of surface modifications, while taking advantage of the hydrophobic properties of the siloxane polymers.5-9 Silicones have been extensively used as materials for medical applications. Numerous biomaterials used for prostheses (e.g., breast implants, finger joints, etc.) and devices for the controlled release of drugs are siliconebased compounds.10 This is mainly due to their good biocompatibility.11 However, there have been many discussions concerning the risks that such devices may pose, such as harmful immune reactions. For instance, it has been suggested that silicones (gel, not oil12) introduced into the body (e.g., via a prosthetic device) have the ability to enhance the immune response of proteins (adjuvant activity), suggesting that a protein-silicone interaction yields an immunogenic moiety,13 although the proposition has come into question.14 A better understanding of the interaction of silicones with substances present in the body is, thus, an important focus of research.15 In a recent study, starch/protein microparticles that were surface-modified with silicone polymers were shown, (6) Valignat, M. P.; Fraysse, N.; Cazabat, A. M.; Heslot, F. Langmuir 1993, 9, 601. (7) Villette, S.; Valignat, M. P.; Cazabat, A. M.; Jullien, L.; Tiberg, F. Langmuir 1996, 12, 825. (8) Lenk, T. J.; Lee, D. H.; Koberstein, J. T. Langmuir 1994, 10, 1857. (9) Pluddemann, E. P. Silane Coupling Agents; Plenum: New York, 1982. (10) Williams, J. M., Nichols, F. M., Zingg, W., Eds. Biomedical Materials (Mater. Res. Soc. Pro.); Material Research Society: Pittsburgh, PA, 1986; Vol. 55. (11) Noll, W. Chemistry and Technology of Silicones; Academic: New York, 1968. (12) Robinson, O. G.; Bradley, E. L.; Wilson, D. S. Ann. Plas. Surg. 1995, 34, 1. (13) Kossovsky, N.; Heggers, J. P.; Robson, M. C. J. Biomed. Mater. Res. 1987, 21, 1125. (14) (a) Segal, M. FDA Consumer 1995, 29, 11. (b) Gott, D. M.; Tinkler, J. J. B. Med. Dev. Agency 1994, 1. (15) Peters, W. Ann. Plast. Surg. 1995, 34, 103.
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in mice, to elicit antibodies upon oral administration, unlike the unmodified protein (human serum albumin, HSA).16 In the study, it was shown that only the use of (triethoxysilyl)propyl-terminated silicone (TES-PDMS, (EtO)3Si(CH2)3SiMe2(OSiMe2)nOSiMe2(CH2)3Si(OEt)3) resulted in the generation of antibodies: the use of unfunctionalized PDMS (Me3Si(OSiMe2)nOSiMe3) resulted in no immunological effect. The role of the silicone and the nature of its interaction with the protein were not established during the course of these studies. Although physicochemical analysis suggested similar protein:silicone interactions for both PDMS and TES-PDMS, no specific effect could be found to account for the enhanced antigenicity observed only for the functionally terminated silicone.17 In view of these observations, the objective of this research was to study the nature of the interaction between both types of silicone and HSA. Covalent bonding or physical adhesion between the silicone and protein could both serve to provide a hydrophobic barrier that might protect the protein on its passage through the gut. We have investigated these possibilities by studying the behavior of protein-silicone composites under a variety of hydrolytic conditions similar to those found in the body. The results of these experiments are presented in the following paper in this issue. Herein we report the preparation and characterization of model siliconeprotein films prepared with both unfunctionalized and functionalized silicone polymers. Experimental Section Materials. Microscope slides (1 mm, 25 × 75 mm, precleaned) and cover glass slides (18 mm2; No. 11/2) were obtained from Corning or Fisher. Human serum albumin (HSA, MW ∼ 67 500; Sigma) was obtained as a powder and was dissolved in phosphatebuffered saline solution (0.1 M NaCl, 0.086 M, KH2PO4, pH ) 7.2). γ-(Aminopropyl)trimethoxysilane (APTS; Aldrich)18 was distilled prior to use. 3-(Triethoxysilyl)propyl-terminated poly(dimethylsiloxane) (1000 cs, MW ∼ 28 000) was prepared as previously described.16 Me3Si-terminated poly(dimethylsiloxane) (1000 cs, Dow Corning), glutaraldehyde (25 wt % aqueous solution, BDH), anhydrous ethanol or methanol (Aldrich), triethylamine (Fisher), and diethyl ether (Caledon) were used as provided. Instrumentation. A Picotron radio-frequency plasma cleaner was used for cleaning the glass surfaces.19 Static (advanced) contact angle, θ, of a sessile drop of distilled water was measured by using a NRLCA goniometer (Rame´-Hart Inc.). Low-resolution XPS spectra were obtained on a Leybold MAX 200 XPT system. Methods. Preparation of Protein-Silicone Films. Protein-silicone surfaces were prepared by stepwise modification of glass surfaces (microscope slides). Before modification, the slides were cleaned of any adsorbed contaminants in a Picotron radio-frequency plasma cleaner under an argon atmosphere for 10 min. After this treatment, the contact angle of the surface was
