Langmuir 1997, 13, 5697-5701
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Cytomimetic Biomaterials. 2. In-Situ Polymerization of Phospholipids on a Polymer Surface Kacey Gribbin Marra,† Derrick D. A. Kidani,†,‡ and Elliot L. Chaikof*,†,‡ Laboratory for Biomolecular Materials Research, Department of Surgery, Emory University School of Medicine, Atlanta, Georgia 30322, and School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30320 Received May 6, 1997. In Final Form: July 31, 1997X A stabilized, phosphatidylcholine-containing polymeric surface was produced by in-situ polymerization on an amphiphilic polymer surface. The phospholipid monomer, 1-palmitoyl-2-[12-(acryloyloxy)dodecanoyl]sn-gycero-3-phosphorylcholine, was prepared as unilamellar vesicles and fused onto a terpolymer consisting of the monomers 2-(methylthio)ethyl methacrylate, 2-hydroxyethyl acrylate, and 3-acryloyl-3-oxapropyl3-(N,N-dioctadecylcarbamoyl)propionate. Free-radical polymerization was carried out in aqueous solution at 70 °C using the water-soluble initiator, 2,2′-azobis(2-methylpropionamidine) dihydrochloride. The supported monolayer displayed advancing and receding water contact angles of 58° and 31°, respectively. Angle-dependent electron spectroscopy for chemical analysis (ESCA) results confirmed the presence of phosphorus and nitrogen and was consistent with theoretical predictions for a close-packed monolayer. Ellipsometry measurements indicated a film thickness of 134 Å. In the absence of network formation, polymeric films demonstrated acceptable stability under static conditions in water. For comparative analysis, the phospholipid was also fused and polymerized onto a self-assembled monolayer of octadecyl mercaptan (ODT) bound to gold. ESCA data were consistent with a close packed monolayer and ellipsometry revealed a film thickness of 66 Å. Nonetheless, contact angle measurements demonstrated that the polymerized lipid monolayer when supported on an ODT surface exhibited a higher level of instability over time than that supported by the amphiphilic terpolymer.
Introduction One of the most intriguing developments in the past decade has been the recognition that membrane-mimetic systems based on the phosphorylcholine head group limit the induction of surface-associated blood clot formation. While not fully understood, this biological property has been attributed to surface bound water associated with phosphorylcholine zwitterion. It has also been suggested that specific plasma proteins which inhibit the blood clotting process are selectively adsorbed to this head group.1 We have previously described the in-situ polymerization of phospholipids on a self-assembled monolayer of octadecyltrichlorosilane (OTS) on glass.2 We now report the polymerization of phospholipids on an amphiphilic dialkyl containing terpolymer bound to a gold-coated silicon wafer. In addition, we have investigated the polymerization of a lipid monolayer supported on a SAM of octadecyl mercaptan (ODT) on gold. Surface physiochemical properties of lipid monolayers polymerized on different alkylated supports were compared. Biomimetics systems and, in particular, membranebased structures continue to be the topic of wide ranging research efforts. Phospholipids differing in chemical composition, saturation, and size have been utilized as building blocks in the design of structures of complex geometry. Indeed, surface-coupled bilayers have been produced by assembling a layer of closely packed hydrocarbon chains followed by exposure to either a dilute solution of emulsified lipids or unilamellar lipid vesicles.3-5 * Address correspondence to: Professor Elliot L. Chaikof, Department of Surgery, Emory University, 1364 Clifton Road, N.E., Box M-11, Atlanta, GA 30322. Phone: (404) 727-8413. Fax: (404) 727-3660. E-mail:
[email protected]. † Emory University School of Medicine. ‡ Georgia Institute of Technology. X Abstract published in Advance ACS Abstracts, September 15, 1997. (1) Chapman, D. Langmuir 1993, 9, 39. (2) (a) Marra, K. G.; Winger, T. M.; Kidani D. D. A.; Chaikof, E. L. Polym. Prepr. 1997, 38, 682. (b) Marra, K. B.; Winger, T. M.; Hanson, S. R.; Chaikof, E. L. Macromolecules, in press.
S0743-7463(97)00473-3 CCC: $14.00
The dialkyl moiety facilitates the assembly of lipids with dissimilar head groups into surface structures of diverse biomolecular functionality and activity. However, limited stability remains a major practical limitation of substratesupported membranes in which the constituent members are associated solely by noncovalent interactions. In order to create robust surface structures, most membrane-mimetic systems for blood-contacting applications have been designed as copolymers containing the phosphatidylcholine functional group in either side chains or, less frequently, the polymer backbone.6-11 Extensive literature exists on the two-dimensional polymerization of lipid monomers self-assembled in the form of vesicles.12-18 However, far fewer studies have evaluated in-situ polymerization of dialkyl amphiphiles at a solid(3) Spinke, J.; Yang, J.; Wolf, H.; Liley, M.; Ringsdorf, H.; Knoll, W. Biophys. J. 1992, 63, 1667. (4) Florin, E. L.; Gaub, H. E. Biophys. J. 1993, 64, 375. (5) Seifert, K.; Fendler, K.; Bamberg, E. Biophys. J. 1993, 64, 384. (6) Kojima, M.; Ishihara, K.; Watanabe, A.; Nakabayashi, N. Biomaterials 1991, 12, 121. (7) Ueda, T.; Oshida, H.; Kurita, K.; Ishihara, K.; Nakabayashi, N. Polym. J. 1992, 24, 1259. (8) Ishihara, K.; Hanyuda, H.; Nakabayashi, N. Biomaterials 1995, 16, 873. (9) Campbell, E. J.; O’Byrne, V.; Stratford, P. W.; Quirk, I.; Vick, T. A.; Wiles, M. C.; Yianni, Y. P. ASAIO J. 1994, 40 (3), M853. (10) Chen, T. M.; Wang, Y. F.; Li, Y. J.; Nakaya, T.; Sakurai, I. J. Appl. Polym. Sci. 1996, 60, 455. (11) Yamada, M.; Li, Y.; Nakaya, T. JMS Pure Appl. Chem. 1995, A32, 1723. (12) O’Brien, D. F. TRIP 1994, 2, 183. (13) Armitage, B.; Klekotka, P. A.; Oblinger, E.; O’Brien, D. F. J. Am. Chem. Soc. 1993, 115, 7920. (14) Batchelder, D. N.; Evans, S. D.; Freeman, T. L.; Haussling, L.; Ringsdorf, H.; Wolf, H. J. Am. Chem. Soc. 1994, 116, 1050. (15) Hub, H.-H.; Hupfer, B.; Koch, H.; Ringsdorf, H. Angew. Chem., Int. Ed. Engl. 1980, 19 (11), 938. (16) Tsiboulkis, J.; Feast, W. J. TRIP 1993, 1, 16. (17) Singh, A.; Markowitz, M. A. In Novel Techniques in Synthesis of Processing of Advanced Materials; Singh, J., Copley, S. M., Eds.; The Minerals, Metals, and Materials Society: Warrendale, PA, 1994; pp 177-186. (18) O’Brien, D. F.; Whitesides, T. H.; Klingbiel, R. T. J. Polym. Sci., Polym. Lett. Ed. 1981, 19, 95. (19) McLean, L. R.; Durrani, A. A.; Whittam, M. A.; Johnston, D. S.; Chapman, D. Thin Solid Films 1983, 99, 127.
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5698 Langmuir, Vol. 13, No. 21, 1997
liquid interface.19-21 We report the synthesis of stabilized, phosphatidylcholine-containing (PC) polymeric surfaces by in-situ polymerization of a self-assembled lipid monolayer onto various alkylated substrates.
Marra et al. Scheme 1. Polymerization of Phospholipid
Experimental Section Materials. AAPD (2,2′-azobis(2-methylpropionamidine) dihydrochloride), DTBC (2,6-di-tert-butyl-p-cresol), 1,12-dodecanediol, pyridine, DMAP (4-(N,N-dimethylamino)pyridine), DCC (dicyclohexylcarbodiimide), succinic anhydride, 2-(methylthio)ethyl methacrylate (MTEM), and PDC (pyridinium dichromate) were obtained from Aldrich and used as received. 2-Hydroxyethyl acrylate (Aldrich) was vacuum distilled. Dioctadecylamine (Fluka) was used as received. 1-Palmitoyl-2-hydroxy-sn-glycero3-phosphocholine was obtained from Avanti Polar Lipids and used as received. THF, toluene, dichloromethane, and pyridine were obtained from Fisher and dried over 4 Å molecular sieves. Acryloyl chloride was obtained from Aldrich and vacuum distilled prior to use. Chloroform (Aldrich) was washed with water, dried over CaCl2, distilled, and stored over 3 Å molecular sieves. AIBN (Aldrich) was recrystallized from methanol. The resin AG 501X8 was obtained from Bio-Rad and used as received. 1-Palymitoyl-2-[12-(acryloyloxy)dodecanoyl]-sn-glycero3-phosphorylcholine (1) (Scheme 1). This monomer was prepared and polymerized as previously described.2,22 3-Acryloyl-3-oxapropyl-3-(N,N-dioctadecylcarbamoyl)propionate (AOD) (3) (Scheme 2).23 Dioctadecylamine (2.08 g, 4.0 mmol), succinic anhydride (0.80 g, 8.0 mmol), and pyridine (0.36 mL, 8.0 mmol) were heated to reflux in CH2Cl2 for 46 h. The reaction mixture was washed twice each with 2 N H2SO4, NaHCO3, and then H2O. After drying with Na2SO4, the solvent was removed in vacuo to give a residue that was recrystallized with acetone yielding 1.02 g (41%) of white solid (2). To 0.88 g (1.42 mmol) of 2 was added DMAP (6.0 mg, 0.05 mmol) and 2-hydroxyethyl acrylate (0.3 mL, 2.62 mmol) in 20 mL of CH2Cl2 at 0 °C. DCC (0.32 g, 1.55 mmol) in 9 mL of CH2Cl2 was added dropwise. After 1 h at 0 °C, the reaction was stirred overnight at room temperature under Ar. The reaction mixture was suction filtered to remove dicyclohexylurea. The filtrate was washed with water then dried over Na2SO4. The solvent was removed in vacuo to give a residue that was purified by flash chromatography on silica gel (hexanes/ethyl acetate/CH2Cl2, 5:1:1). The product was a clear oil that slowly solidified (3): yield 0.63 g (27%); mp 31.0-32.0 °C; 1H NMR (CDCl3) δ 6.35 (d, vinyl, 1H), 6.09 (q, vinyl, 1H), 5.80 (d, vinyl, 1H), 4.28 (s, CH2OOC, 4H), 3.17-3.23 (m, CH2O, CH2N, 4H), 2.50-2.60 (m, CH2COO, 4H), 1.20 (br, CH2, 60H), 0.82 (t, CH3, 6H) ppm; high-resolution mass spectrometry calculated (FAB) 726.6588, observed 726.6599 (+Li). Polymerization of HEA + AOD + MTEM (4) (Scheme 3). 2-Hydroxyethyl acrylate (0.08 g, 0.70 mmol), 2-(methylthio)ethyl methacrylate (0.02 g, 0.12 mmol), and acrylate 3 (AOD) (0.25 g, 0.35 mmol) were dissolved in 1.5 mL of toluene. AIBN (2.8 mg, 0.01 mmol) was added. The solution was purged with Ar, sealed, and placed in a 70 °C oil bath for 18 h. The solution was cooled to room temperature then was slowly added to 50 mL of MeOH. The white precipitate was recovered and dried to give polymer 4: yield 0.06 g (17%); molecular weight determined by gel permeation chromatography (GPC), Mw ) 19248 g/mol; PDI ) 2.16; 1H NMR (CDCl3) δ 4.12-4.26 (m, CH2OOCO), 3.79 (t, CH2OH), 3.23 (m, CH2O, CH2N), 2.62-2.73 (m, CH2S, CHCOO, CH2COO), 2.16 (s, CH3S), 1.70 (s, CH3C), 1.45-1.65 (m, CH2CHCOO) 1.30 (br, CH2’s), 0.88 (t, CH3) ppm. Instrumentation. Contact angles were measured on a Rame´Hart goniometer, Model 100-00. The values reported are an average of at least five readings. Proton NMR data were obtained on a QE300 instrument. Angle-dependent electron spectroscopy for chemical analysis (ESCA) data were obtained using a Physical (20) Regen, S. L.; Kirszensztejn, P.; Singh, A. Macromolecules 1983, 16, 335. (21) Hayward, J. A.; Chapman, D. Biomaterials 1984, 4, 135. (22) Sells, T. D.; O’Brien, D. F. Macromolecules 1994, 27, 226. (23) Laschewsky, A.; Ringsdorf, H.; Schmidt, G.; Schneider, J. J. Am. Chem. Soc. 1987, 109, 788. (24) MacDonald, R. C.; MacDonald, R. I.; Menco, B. Ph. M.; Takeshita, K.; Subbarao, N.; Hu, L. Biochim. Biophys. Acta 1991, 1061, 297.
Electronics (PHI) Model 5100 spectrometer equipped with a Mg/ Ti dual-anode source and an Al/Be window. Spectra were obtained at the following takeoff angles: 15, 45, and 90°. GPC results were obtained using a Waters 590 progammable HPLC pump, a Waters 410 differential refractometer maintained at 40 °C, a Waters 745 data module, and two narrow-bore Phenogel columns (linear pore size and 500 Å, Phenomenex) in series maintained at 35 °C. Molecular weights are relative to monodisperse polystyrene standards and the solvent was THF. Ellipsometry data was obtained on a PlasMos ellipsometer, Model SD2300. Chromium (∼50 Å) and then gold (∼200 Å) were evaporated onto the silicon wafer using an CVC Products e-beam evaporator, Model SC-5000. Gold surfaces were cleaned using a Plasma-Therm RIE, Model Waf’r/Batch 720/740.
Results and Discussion Stabilized phospholipid monolayer surfaces were prepared utilizing a strategy based upon the fusion of unilamellar vesicles with an alkylated substrate followed by in-situ polymerization. Briefly, the phospholipid monomer was prepared by the esterification of 12(acryloyloxy)-1-dodecanoic acid and 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine.2,22 The polymerization of the phospholipid occurs in aqueous solution at 70 °C utilizing a water-soluble, free radical initiator (Scheme 1). Previously, we have reported the polymerization of this phospholipid at a solid-liquid interface on the alkylated substrate OTS/glass.2 In this study, we investigated the postulate that vesicle fusion and in-situ polymerization would proceed more efficiently on a molecularly mobile alkylated surface. A sulfur-containing terpolymer was bound to a gold-coated silicon wafer, and the phospholipid was fused and polymerized onto the terpolymer (Figure 1). The terpolymer consists of two commercially available monomers, 2-hydroxyethyl acrylate (HEA) and 2-(methylthio)ethyl methacrylate (MTEM), and a third monomer, AOD, that was synthesized (Scheme 2). The molar ratio of the terpolymer is 6:3:1, HEA/AOD/ MTEM. The polymer is based upon a similar terpolymer designed by Spinke et al.3 and was obtained by an AIBNinitiated free radical polymerization (Scheme 3). The sulfur-containing methacrylate monomer binds to gold as an anchor, whereas the hydrophobic monomer AOD is expected to migrate to the surface, exposing a layer of alkyl chains for vesicle fusion. The hydrophilic HEA component acts as a “cushion”, facilitating self-assembly of the alkyl chains at the solid-liquid interface. Fusion of the lipid vesicles was executed following established methods.25,26 In order to achieve vesicle/surface fusion, experiments were performed at 40 °C, above the known (25) Plant, A. L. Langmuir 1993, 9, 2764. (26) Lamparski, H.; Lee, Y.-S.; Sells, T. D.; O’Brien, D. F. J. Am. Chem. Soc. 1993, 115, 8096.
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Langmuir, Vol. 13, No. 21, 1997 5699 Scheme 3. Preparation of Terpolymer
Table 1. Static Stability Contact Angles for All Polymers (deg) (Advancing/Receding)
Figure 1. Schematic of polymerized phospholipid on terpolymer surface. Scheme 2. Preparation of Hydrophobic Monomer
polymer substrate
prior to fusion
initiala
one day
one week
OTS ODT terpolymer
111/105 ((2) 107/104 ((2) 102/82 ((3)
65/45 ((4) 76/58 ((6) 58/31 ((5)
77/58 ((5) 84/64 ((6) 64/36 ((5)
80/56 ((4) 88/71 ((5) 68/40 ((5)
a Contact angles were taken immediately after fusion and subsequent polymerization of phospholipid monomer.
Table 2. Ellipsometry Results (Å)
sample
initial thickness
terpolymer substrate 45.9 ( 3.5 polymer/terpolymer 144.7 ( 23.2
Tm for the acrylate-functionalized lipid monomer.27 After fusion, a water-soluble free radical initiator, AAPD, was added directly to the film in the buffer solution and polymerization was initiated by heat (65-70 °C). The polymerized film was rinsed copiously with water and surface characterization was performed. We have previously optimized polymerization parameters on OTS glass, and we have utlilized those conditions in these investigations.2 Vesicle size and concentration were 600 nm and 1200 µM, respectively, with a fusion time of 24 h. Polymerization was performed with a monomer/initiator ratio of 10/1 and allowed to proceed overnight at 70 °C. Initial surface characterization, including alylated substrates, was performed using contact angle goniometry (Table 1). As anticipated, base substrates are hydrophobic, while the resulting phospholipid polymeric surfaces are hydrophilic. Average advancing and receding water contact angles of 58° and 31°, respectively, were observed for the polymer fused onto the terpolymer. When ODT on Au was utilized as a substrate for vesicle fusion and polymerization, we observed advancing and receding water contact angles of 76° and 58°, respectively (Table 1). In a prior study, advancing and receding water contact angles of 64° and (27) Bain, C. Ph.D. Thesis, Harvard University, 1989.
thickness thickness after 24 h of after 24 h of H2O immersion vacuum drying 73.1 ( 5.3 166.5 ( 13.2
50.7 ( 8.5 135.6 ( 3.2
44°, respectively, were obtained for this phospholipid polymer supported on an OTS/glass substrate.2 Ellipsometry measurements were obtained both for alkylated substrates, as well as for supported, polymerized PC monolayers. Film thickness for the ODT/Au substrate was 18.1 ( 0.3 Å, as expected, indicating a close-packed, self-assembled monolayer of octadecyl chains.27 The film thickness for the PC-polymer on the ODT/Au substrate was 65.5 ( 15.0 Å, which is comparable to the theoretical thickness based upon a consideration of standard bond lengths and angles. Film thickness measurements of the terpolymer substrate with or without the supported, polymerized PC monolayer are summarized in Table 2. The observed thickness of the terpolymer substrate increased by approximately 30 Å after storage for 24 h in water. Likely, this represents both water absorption by the HEA segment of the terpoylmer and associated chain expansion. Notably, a 30 Å increase in film thickness was also seen under similar conditions when the terpolymer substrate supported the PC-polymer (Table 2). Further, after the substrate was dried under vacuum, film thickness approached the prehydration baseline measurement. Nonetheless, film thickness was greater than that expected for a polymerized PC monolayer supported on a terpolymer substrate. A plausible explanation may be the refractive index value assumed for this system. Since this a novel system and no refractive indices are reported in the literature, a typical value of 1.4500 was utilized. Another possibility is that the phospholipid has formed multilayers on the surface. If that were the case, contact angle measurements would likley not be stable over time, as was observed and noted in Table 1.
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Marra et al.
Table 3. ESCA Results for ODT/Au Substrate atomic percentages ODT
15°
45°
90°
theoretical %
C S Au
91.9 ( 1.0 2.1 ( 1.3 6.0 ( 2.1
73.9 ( 1.7 2.3 ( 0.2 23.9 ( 1.9
63.2 ( 0.9 2.5 ( 0.9 34.2 ( 0.2
87.1 12.9
Table 4. ESCA Results for PC-Polymer on ODT/Au atomic percentages Ringsdorf
15°
45°
90°
theoretical %
C O N P S Au
71.8 ( 7.1 18.7 ( 0.9 1.2 ( 0.2 0.9 ( 0.4 2.4 ( 2.1 1.7 ( 0.6
65.1 ( 5.0 12.9 ( 7.0 0.9 ( 0.6 1.0 ( 0.3 3.4 ( 1.3 11.4 ( 7.6
63.5 ( 8.4 13.6 ( 7.3 1.6 ( 0.5 1.0 ( 0.9 2.9 ( 1.7 13.1 ( 13.6
77.9 12.4 1.1 2.4 6.2
Table 5. ESCA Results for Terpolymer/Au Substrate atomic percentages Ringsdorf
15°
45°
90°
theoretical %
C O N S Au
71.7 ( 7.7 19.5 ( 2.2 1.4 ( 0.6 1.0 ( 0.6 6.1 ( 8.3
72.5 ( 1.5 16.2 ( 0.4 1.3 ( 0.1 1.2 ( 0.5 8.8 ( 0.5
68.8 ( 7.5 17.1 ( 3.9 2.2 ( 1.2 1.1 ( 0.6 10.7 ( 8.8
76.6 18.2 1.6 3.6
Table 6. ESCA Results for PC-Polymer on Terpolymer/Au atomic percentages Ringsdorf
15°
45°
90°
theoretical %
C O N P S Au
73.6 ( 4.2 21.9 ( 1.9 1.3 ( 0.1 1.1 ( 0.9 0.8 ( 0.2 0.2 ( 0.2
73.0 ( 1.3 20.9 ( 1.6 1.8 ( 0.6 1.2 ( 0.3 0.8 ( 0.4 1.5 ( 1.0
70.5 ( 1.3 21.8 ( 1.6 2.2 ( 0.7 1.0 ( 0.3 0.6 ( 0.1 2.9 ( 1.7
73.5 20.6 1.8 2.0 2.1
Moreover, ESCA results would indicate an excess of phosphorus. As will be described, that was not the case. Angle-dependent ESCA measurements were carried out to further define atomic level surface properties (Tables 3-6). Three angles were utilized (15°, 45°, and 90°). The results for the ODT substrate are given in Table 3 and are as expected.27 The results for the PC-polymer on ODT/ Au are provided in Table 4 (assuming 2.5 OTS chains per phospholipid unit). The results, especially at the topmost surface (15°), with very little gold detected, are near theoretical predictions. The results for the terpolymer/ Au substrate are summarized in Table 5. Likewise, at the shallowest depth (15°), the atomic percentages approximate theoretical predictions, based on the number of atoms in the smallest repeating structural unit. Assuming 2.5 OTS chains per phospholipid unit, the atomic percent surface concentrations were calculated for the PC-polymer on terpolymer/Au substrate (Table 6). Phosphorus and nitrogen were identified, providing additional confirmation of a polymerized lipid film. There is considerably less gold detected in the PC-terpolymer system, confirming that this is a thicker film as determined by ellipsometry. The ratio of phosphorus to sulfur is in good agreement with the theoretical ratio at all three angles (1.4, 1.5, 1.7 at 15°, 45°, and 90°, respectively, compared to a theoretical ratio of 2.0). Overall, ESCA has been used infrequently in the characterization of PCbased surfaces. However, our results are consistent with those of Hayward et al.28 and Ko¨hler et al.29 of PCderivatized glass. (28) Hayward, J. A.; Durrani, A. A.; Lu, Y.; Clayton, C. R.; Chapman, D. Biomaterials 1986, 7, 252.
Short term water stability tests were executed (Table 1). The results for the three substrates are given. Water contact angles were measured intermittently by removing the polymer films from their water storage, air-drying, and measuring contact angles. After 1 week, the phospholipid polymer on the terpolymer substrate remains hydrophilic, indicating a stable PC surface. Differences in substrates lead to noticeable differences in contact angles with the terpolymer yielding the lowest values. The terpolymer provides a more flexible surface than the OTS/glass or ODT/gold substrates and, thereby, may lead to enhanced vesicle fusion or more efficient polymerization. It is our conclusion that a molecularly mobile alkylated surface provides a better support for the creation of polymerized lipid films. The binding of a sulfur-containing polymer to a gold substrate has been reported previously. As noted, Spinke et al. reported the fusion of phospholipid vesicles onto a multifunctional amphiphilic terpolymer bound to gold.3 The polymers were composed of HEA, a disulfide-containing methacrylate, and a hydrophobic acrylate containing octadecyl side chains. Recently Grainger and his associates have reported several noteworthy studies.30-32 Extending Ringsdorf’s initial investigations, Sun et al. characterized the film properties of terpolymers containing HEA, as well as a disulfide-containing acrylate and methoxyethyl acrylate bound to gold.30 ESCA results indicated an exposed acrylate backbone and water contact angles were 45-68°. Ellipsometry results, however, were not as expected, which was attributed to inherent errors associated with assumed refractive index values. In a subsequent report, Sun et al. observed that the average film thicknesses varied as a function of the density anchoring dithioalkyl chains.31 Presumably, more backbone loops and longer segments in the HEA segments lead to a larger hydrophilic “cushion” and, consequently, greater thickness values.31 Most recently, his research group has prepared disulfide-containing, siloxane terpolymers.32 One siloxane segment contained fluorocarbon side chains. The terpolymer was shown to bind to gold and phase segregate to obtain a low-energy surface. Contact angle and ESCA results confirmed a fluorocarbon surface and ellipsometry revealed a film thickness of approximately 30 Å. Despite more than 4 decades of research, a clinically durable blood compatible surface remains an elusive goal. The biological membrane appears to be an ideal starting point for the generation of synthetic blood-compatible substrates for medical implants or biosensors.9 In-situ polymerization of a monoacrylate phospholipid is a convenient means of stabilizing a monolayer at a solidliquid interface with potentially promising blood contacting properties. Conclusion A stabilized, phosphatidylcholine-containing polymeric surface was produced by in-situ polymerization of 1-palmitoyl-2-[12-(acryloyloxy)dodecanoyl]-sn-glycero-3-phosphorylcholine at a solid-liquid interface. The phospholipid monomer was synthesized, prepared as unilamellar vesicles, and fused onto close-packed octadecyl chains as (29) Ko¨hler, A. S.; Parks, P. J.; Mooradian, D. L.; Rao, G. H. R.; Furcht, L. T. J. Biomed. Mater. Res. 1996, 32, 237. (30) Sun, F.; Castner, D. G.; Grainger, D. W. Langmuir 1993, 9, 3200. (31) Sun, G.; Grainger, D. W.; Castner, D. G. J. Vac. Sci. Technol. A 1994, 12 (4), 2499. (32) Sun, F.; Castner, D. G.; Mao, G.; Wang, W.; McKeown, P.; Grainger, D. W. J. Am. Chem. Soc. 1996, 118, 1856.
Cytomimetic Biomaterials
part of an amphiphilic terpolymer. Free-radical polymerization was carried out in aqueous solution at 70 °C using the water-soluble initiator, 2,2′-azobis(2-methylpropionamidine) dihydrochloride (AAPD). The supported monolayer displayed advancing and receding water contact angles of 58° and 31°, respectively and ESCA data were consistent with theoretical predictions for lipid membrane. Ellipsometry results indicate relatively thick surfaces. In the absence of network formation, polymeric films demonstrated acceptable short-term stability.
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Acknowledgment. This work was supported by the Whitaker Foundation (E.L.C.) and HL56819 (E.L.C.). E.L.C. is a Clinician-Scientist of the American Heart Association. The authors wish to acknowledge the Emory University Mass Spectrometry Center provided by grants from the NIH and NSF, Professor Joseph A. Gardella, Jr., and Dr. Richard Nowak at SUNY Buffalo for ESCA measurements, and Ms. Janine Orban at the University of Pittsburgh for GPC measurements. LA970473S
