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Surface Mobility of 2-Methacryloyloxyethyl Phosphorylcholine-co-Lauryl Methacrylate Polymers Stuart Clarke, Martyn C. Davies,* Clive J. Roberts, Saul J. B. Tendler, and Philip M. Williams Laboratory of Biophysics and Surface Analysis, School of Pharmaceutical Sciences, The University of Nottingham, Nottingham NG7 2RD, U.K.
Vincent O’Byrne, Andrew L. Lewis, and Jeremy Russell† Biocompatibles Ltd., Farnham Business Park, Weydon Lane, Farnham, GU9 8QL U.K. Received November 9, 1999. In Final Form: February 15, 2000 To design new polymers for use in vivo it is necessary to characterize the surface of the material to understand the interactions that occur when it is exposed to biological environments. Incorporation of phosphorylcholine (PC) into polymers has been shown to improve biocompatibility by suppressing unfavorable responses which occur on contact with body fluids. Here, a series of copolymers with varying ratios of the monomers 2-methacryloyloxyethyl phosphorylcholine (MPC) and lauryl (dodecyl)methacrylate (LMA) have been synthesized. The composition of the copolymers were analyzed using nuclear magnetic resonance spectroscopy (NMR), and coatings of these materials characterized using angle-resolved X-ray photoelectron spectroscopy (ARXPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The interaction of the copolymer coatings with protein was investigated using surface plasmon resonance (SPR), while dynamic contact angle (DCA) was used to monitor the surface hydrophobicity of the copolymers. The combination of the analytical techniques applied to the study of these copolymers has shown that the surfaces are extremely mobile and are able to rearrange depending on the environment in which the polymer is placed. SPR analysis has shown that the plasma protein fibrinogen, known to initiate the clotting cascade, does not adsorb to the surface of the copolymers once they are hydrated.
Introduction Synthetic polymers are used extensively in biomedical fields as materials from which medical devices can be fabricated. Their mechanical and chemical properties can be tailored to suit the application for which they are intended, and they can be manufactured on a large scale with relative ease. However, many of the polymers currently in use as biomaterials were originally designed for other applications and despite having excellent physical and mechanical properties are seldom found to have an acceptable level of biocompatibility.1 When the polymer is placed into a biological environment, unfavorable responses may be elicited. In the case of blood, protein adsorption and platelet adhesion initiate the activation of the clotting cascade which results in the production of a potentially fatal thrombus. This can be particularly problematic in procedures involving blood contact with the large surface area of a medical device, for example, a cardiopulmonary bypass operation. The main method used to address this problem is to systematically administer an anticoagulant such as heparin, but this can cause further complications such as uncontrolled bleeding,2 and heparin only halts thrombus formation; protein adsorption and platelet adhesion still occur. In addition to blood, components of other body fluids can also react adversely when in contact with synthetic surfaces: proteins will * Telephone: +44 (0) 115 9515063. Fax: +44 (0) 115 9515110. E-mail:
[email protected]. † SIMS Portex Ltd., Hythe, Kent, CT21 6JL. (Formerly of Biocompatibles Ltd.) (1) Biomaterials Science; Ratner, B. D., Hoffman, A. S., Schoen, F. J., Lemons, J. E., Eds.; Academic Press: New York, 1996. (2) Van Oeveren, W.; Kazatchine, M. D.; Descamps-Latscha, B.; Maillet, F.; Fischer, E.; Carpentier, A. J. Thorac. Cardiovasc. Surg. 1985, 89, 888.
adhere to contact lenses, and urinary catheters can be blocked by mineral deposition. Although these problems are less severe than those associated with blood clotting, they can cause discomfort to the user and lower the quality of life by resulting in the need to replace the device. This increases the cost per patient and, in some cases, means that more surgery is required. One approach for improving the biocompatibility of biomaterials is based on a form of “biomimicry”. This concept gained much attention in the 1980s3,4 and involves reproducing the surface properties of the outer surface of cell membranes. This is achieved by use of chemical entities that are similar in structure to phosphorylcholine (PC), the major lipid headgroup present in the extracellular lipid bilayer. PC-containing surfaces have been shown to suppress protein adsorption,5,6 thrombus formation,7 platelet adhesion8 and cell adhesion.9 The polymers being investigated in this paper are methacrylate-based copolymers containing PC. Similar materials have been studied by several groups and have been reported to exhibit high levels of biocompatibility.5,8,10-12 Another class of (3) Fukishima, S.; Kadoma, Y.; Nakabayashi, N. Kobunshi Ronbunshu 1983, 40, 785. (4) Hayward, J. A.; Chapman, D. Biomaterials 1984, 5, 135. (5) Campbell, E. J.; O’Byrne, V.; Stratford, P. W.; Quirk, I.; Vick, T.; Wiles, M. C.; Yianni, Y. P. Am. Soc. Artif. Intern. Organs J. 1994, 40, M853. (6) Ishihara, K.; Ziats, N. P.; Tierney, B. P.; Nakabayashi, N. J. Biomed. Mater. Res. 1991, 25, 1397. (7) von Segesser, L. K.; Tonz, B.; Leskosek, M.; Turina, M. Am. Soc. Artif. Intern. Organs J. 1994, 17, 294. (8) Ishihara, K.; Iwasaki, Y.; Nakabayashi, N. Mater. Sci. Eng. 1998, C6, 253. (9) Ueda, T.; Oshida, H.; Kurita, K.; Ishihara, K.; Nakabayashi, N. Polym. J. 1992, 24, 1259. (10) Iwasaki Y.; Ishihara, K.; Nakabayashi, N.; Khang, G.; Jeon, J. H.; Lee, J. W.; Lee, H. B. J. Biomater. Sci. Polym. Ed. 1998, 9, 801.
10.1021/la991472y CCC: $19.00 © 2000 American Chemical Society Published on Web 04/21/2000
Surface Mobility of 2-MPC-co-LMA Polymers
polymer to which PC has been attached is the polyurethanes, and these materials have also been shown to improve blood compatibility.13 Other research groups have synthesized polymers containing PC-analogous groups to study their potential as biomaterials.14,15 Since it is the surface which is of most importance in defining the in vivo performance of a material, it is important to characterize the surface as well as the bulk properties. Two of the most established analytical techniques used to probe the surface of polymeric materials are X-ray photoelectron spectroscopy (XPS)16 and timeof-flight secondary ion mass spectrometry (ToF-SIMS).17 When used in conjunction, these techniques are able to provide chemical and molecular information concerning the sample under investigation. Surface plasmon resonance (SPR) is an analytical technique which can probe surface interactions in real time and has been used to study a variety of surface events including erosion of biodegradable polymer blends18 and interactions of proteins with surfaces.19,20 Dynamic contact angle (DCA) measurements provide additional information on the wetting behavior of surfaces, and this technique has been applied to many polymers,21,22 including those intended for use as biomaterials.23 In addition to measuring the contact angle formed by a liquid on a surface, DCA has also been used to monitor surface rearrangements of polymers.24-26 This paper describes the characterization of a series of 2-methacryloyloxyethyl phosphorylcholineco-lauryl methacrylate polymers (for monomer structures, see Figure 1). The bulk composition of the copolymers has been estimated using 1H NMR spectroscopy, and a comparison is made with the results obtained from surfacesensitive techniques. The protein-resistant nature of the polymers has been investigated using SPR. Materials and Methods Preparation of Polymer Films. Poly(lauryl methacrylate) (PLMA) was purchased from Aldrich (Poole, Dorset, U.K.) as a 25% (w/v) solution in toluene. The solvent was removed by heating to 60 °C for approximately 16 h. The preparation of the PLMA thin film was performed at room temperature by spin casting 130 µL of a 0.5% (w/v) polymer solution in chloroform onto either silver (for SPR analysis) or aluminum foil (XPS and ToF-SIMS (11) Ruiz, L.; Hilborn, J. G.; Le´onard, D.; Mathieu, H. J. Biomaterials 1998, 19, 987. (12) Zhang, S. F.; Rolfe, P.; Wright, G.; Lian, W.; Milling, A. J.; Tanaka, S.; Ishihara, K. Biomaterials 1998, 19, 691. (13) van der Heiden, A. P.; Willems, G. M.; Lindhout, T.; Pijpers, A. P.; Koole, L. H. J. Biomed. Mater. Res. 1998, 40, 195. (14) Oishi, T.; Uchiyama, H.; Onimura, K.; Tsutsumi, H. Polym. J. 1998, 30, 17. (15) Korematsu, A.; Murakami, T.; Sakuri, I.; Kodama, M.; Nakaya, T. J. Mater. Chem. 1999, 9, 647. (16) Beamson, G.; Briggs, D. High-Resolution XPS of Organic Polymers: The Scienta ESCA300 Database; Wiley & Sons: New York, 1992. (17) Briggs, D.; Brown, A.; Vickerman, J. C. Handbook of Secondary Ion Mass Spectrometry (SIMS); Wiley & Sons: New York, 1989. (18) Shakesheff, K. M.; Chen, X.; Davies, M. C.; Domb, A.; Roberts, C. J.; Tendler, S. J. B.; Williams, P. M. Langmuir 1995, 11, 3921. (19) van Delden, C. J.; Lens, J. P.; Kooyman, R. P. H.; Engbers, G. H. M.; Feijen, J. Biomaterials 1997, 18, 845. (20) Green, R. J.; Davies, J.; Davies, M. C.; Roberts, C. J.; Tendler, S. J. B Biomaterials 1997, 18, 405. (21) Hogt, A. H.; Gregonis, D. E.; Andrade, J. D.; Kim, S. W.; Dankert, J.; Feijen, J. J. Colloid Inerface. Sci. 1985, 106, 289. (22) Mera, A. E.; Goodwin, M.; Pike, J. K.; Wynne, K. J. Polymer 1999, 40, 419. (23) Holly, F. J.; Refojo, M. F. J. Biomed. Mater. Res. 1975, 9, 315. (24) Van Damme, H. S.; Hogt, A. H.; Feijen, J. J. Colloid Interface Sci. 1986, 114, 167. (25) Lee, S. H.; Ruckenstein, E. J. Colloid Interface Sci. 1987, 20, 529. (26) Xu, M. X.; Liu, W. G.; Wang, J.; Gao, W.; Yao, K. D. Polym. Int. 1997, 44, 421.
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Figure 1. Chemical structure of the monomers which are used to produce the copolymer MPC/LMA (1:6) and the homopolymer PLMA. analysis) at approximately 2000 rpm. For DCA analysis, the polymers were dip-coated onto a cleaned glass slide from a 3% (w/v) solution in chloroform at a speed of 19.6 µm/s. The copolymers and the poly(MPC) were provided by Biocompatibles Ltd., Farnham, U.K. Thin film preparation for poly(MPC) was performed as for the PLMA except that the casting solvent was methanol. The copolymer films were produced in the same manner as the homopolymers except that solvent systems of chloroform/methanol were required to allow complete solvation. NMR Analysis. 1H NMR experiments were carried out using a 400 MHz JEOL spectrometer. Samples were dissolved in deuterated chloroform (CDCl3) or a solvent mixture of CDCl3 and deuterated methanol (CD3OD), at an approximate concentration of 40-50 mg/mL. The exact amount of each solvent required was determined by the solubility of the polymer being analyzed, which was affected by the monomer ratio, but a mixture close to 1:1 (CDCl3/CD3OD) was aimed for. The signal from residual protons in the CDCl3 at δ ) 7.25 was used as a standard for the chemical shifts. XPS Analysis. XPS experiments were carried out using an ESCA300 spectrometer (Scienta, Uppsala, Sweden) employing a monochromated Al KR source. The instrument has been described in detail in the literature.16 Takeoff angles were 10° and 45° relative to the sample surface. Films to be analyzed were cast onto cleaned aluminum foil from solutions which provided a film sufficiently thick to mask the substrate (>10 nm). Charge compensation was provided by an electron flood gun. All samples were analyzed at a pressure of 10-9 Torr or less. Data were analyzed using Scienta WinESCA software. ToF-SIMS Analysis. ToF-SIMS analysis was carried out using a Physical Electronics model 7200 spectrometer employing a cesium ion source. A pulsed current of 0.7 pA was employed, and the total ion dose was kept below 5 × 1012 ions/cm to ensure that analysis was carried out in the static regime. Samples were cast onto aluminum stubs which had previously been cleaned by sonication in chloroform for 10 min. The solution concentration was chosen to provide a sufficiently thick film to prevent interference from the Al substrate (>2-3 nm). Data were analyzed with Physical Electronics WinCadence software. SPR Analysis. The SPR experiments were carried out using an instrument employing the Kretchmann configuration and a monochromatic laser source of 780 nm (Ortho Clinical Diagnostics, Buckinghamshire, U.K.). The instrument has been described in detail previously.22 The SPR sensors consisted of glass slides with thin films of silver (approximately 50 nm thick) coated on one side. The polymers under investigation were cast onto the silver-coated side of the slides which were then index matched to the hemicylindrical prism of the SPR instrument by the application of immersion oil (BDH). Phosphate buffer, pH 7.4, was flowed over the polymer surface at a rate of 0.24 mL/ min. The temperature of the SPR sensor was allowed to stabilize at approximately 34.5 °C before a solution of fibrinogen (Sigma, Poole, Dorset, U.K.) in phosphate buffer (0.05 mg/mL) was introduced via the flow cell. Interaction of the protein with the polymer surfaces was detected by measuring the angle at which minimum reflectivity of the SPR laser on the sensor slide occurs, as recorded by a photodiode array. Dynamic Contact Angle Analysis. DCA was carried out using a Cahn Instruments model 322 dynamic contact-angle
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Figure 2. 1H NMR spectrum of a typical MPC/LMA copolymer showing the signals attributed to the protons on the PC headgroup and the protons from the methylene group adjacent to the oxygen on the lauryl tail. Table 1. Results from the Compositional Analysis of the Copolymer Series Using 1H NMR Spectroscopy Showing that the Polymers Produced Have a Composition Close to the Feed Ratio Used in the Synthesis
Figure 3. Negative-ion SIMS spectrum of PLMA spun cast onto an aluminum substrate. Mass range is 30-200 amu. Typical methacrylate secondary ions can be seen at m/z 55, 85, and 183.
compositional ratio nomenclature
MPC
LMA
MPC/LMA (1:6) MPC/LMA (1:5) MPC/LMA (1:3) MPC/LMA (1:2) MPC/LMA (1:1.5) MPC/LMA (1:1) MPC/LMA (1.5:1) MPC/LMA (2:1)
1 1 1 1 1 1.2 1.7 3.7
6.1 4.4 2.8 2 1.3 1 1 1
analyzer. Glass slides were cleaned with acetone prior to coating. An immersion speed of 19.6 µm/s was employed for analysis, and the slide was permitted to dwell in the liquid medium for 2 h between successive cycles.
Results and Discussion NMR Analysis. Figure 2 shows a typical 1H NMR spectrum of a copolymer of MPC-co-LMA (spectrum shown is MPC/LMA 1:2). Through the choice and integration of a signal from each of the monomers, the bulk composition of the copolymers can be estimated. The signal at approximately δ ) 3.3 ppm is characteristic of the trimethylammonium protons N+(CH3)3, while the LMA component of the copolymer can be identified by either the signal at δ ) 1.7 ppm, attributed to the protons which are β to the ester on the C12 tail or the signal at ∼0.9 ppm, characteristic of the terminal methyl of the alkyl chain. Table 1 shows the composition of the copolymers calculated from the 1H NMR data. All of the copolymers have compositions close to that of the feed ratio of the monomers used during synthesis, indicating that the polymerization reaction proceeded as expected. ToF-SIMS Analysis. For polymers containing carbon, hydrogen, and oxygen only, positive-ion SIMS spectra are often uninformative. It is typically the negative-ion spectra which provide more information. Figure 3 is the negativeion spectrum of the homopolymer PLMA, it shows ions typically formed by methacrylate polymers,17 including the secondary ion at m/z 183 which has been assigned as the C12H25O- lauryl tail after the loss of two protons. For nitrogen-containing polymers, the positive-ion spectrum can be informative as positively charged secondary ions containing protonated nitrogen atoms are often sputtered. Figure 4 is the positive-ion spectrum of the poly(MPC) and shows ions characteristic of the PC headgroup11 and of the specific monomer unit. Table 2 lists the mass of the
Figure 4. Positive-ion SIMS spectrum of poly(MPC) spun cast onto an aluminum substrate. Mass range is 0-300 amu. Secondary ions typical of the PC headgroup can be seen along with the molecular ion at m/z 296.
ions sputtered with the secondary ion assignment. The negative ion spectrum of the poly(2-MPC) has not been shown but was dominated by the phosphate ions PO2(m/z 63) and PO3- (m/z 79). The spectra of the copolymers contained ions derived from both monomers with the ion intensities changing with composition as expected. XPS Analysis. Methacrylate polymers have been extensively studied and characterized with both SSIMS and XPS. The XPS spectra obtained for PLMA were found to be comparable with that in the literature,16 and quantification of the data showed no difference in composition to the reported values. Figure 5 shows the peakfitted C 1s region of poly(MPC). Literature values for individual chemical shifts were applied,16 and an excellent fit to the envelope was attained. As for PLMA, the ratios of the areas of the peaks attributed to the different chemical environments of the carbon atoms were very close to those calculated theoretically (see Table 3). Measurement of the peak areas in the peak-fitted C 1s spectra of the copolymers recorded at 45° takeoff angles also showed good agreement with the NMR data previously obtained, suggesting that the surface composition of the copolymers reflects that of the bulk. Analysis at different takeoff angles results in the ejection and analysis of photoelectrons from different depths within the sample, giving an indication of changing chemical composition throughout the top 1-10 nm of the sample being analyzed. XPS analysis using a 45° takeoff angle with respect to the sample surface provides a sampling depth of 5-6 nm for
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Table 2. Assignments for the Secondary Ions Sputtered from the Surface of Poly(MPC) Using ToF-SIMS Showing Typical Ions from a PC Headgroup
Table 4. Comparison of the Surface Composition of 2:1, 1:2, and 1:6 MPC/LMA by XPS Using 10° and 45° Takeoff Anglesa takeoff angle (with respect to sample surface) 10° sample
45°
%C
MPC/LMA (2:1) 84.7 MPC/LMA (1:2) 86.8 MPC/LMA (1:6) 90.5
67.8 78.3 84.5
10°
45°
%O 13.6 11.5 8.8
25.3 17.8 13.7
10°
45°
%N 0.7 0.6 0.2
3.0 1.7 0.9
10°
45°
%P 1.0 1.1 0.5
3.9 2.2 0.9
a The amount of nitrogen and phosphorus is less at the lower angle.
Figure 5. Peak-fitted C 1s region of poly(MPC) spun cast onto an aluminum substrate with the assignments for the different carbon environments present in the sample. Table 3. Comparison of the Theoretically Calculated and Experimentally Found Areas of the Peak-Fitted C 1s Region of Poly(MPC) Recorded by XPS carbon type
exp. area
theor. area
O-CdO R-C-O R-C-N+ R-C-CO2 R-C-R
8.8 25.2 37.5 8.8 19.7
9.1 27.3 36.4 9.1 18.1
most common elements.28 When the angle is lowered to 10°, the escape depth of the photoelectrons is reduced to approximately 1.5 nm, and it is the extreme surface of the sample which is being analyzed. A comparison of the results obtained for three of the copolymers is shown in Table 4 and confirms the compositional variance with sampling depth. When a 10° takeoff angle is applied, the (27) Weisel, J. W. Biophys. J. 1986, 50, 1080. (28) Briggs, D. Surface Analysis of Polymers by XPS and Static SIMS; Cambridge University Press: Cambridge, 1998; Chapter 2.
amount of nitrogen and phosphorus detected at the surface is greatly reduced in all three of the samples and is barely detectable in MPC/LMA (1:6). As these two elements are present in the MPC only, this suggests that the amount of MPC present at the surface of the copolymer is far less than that found in the bulk material. Figures 6 and 7 show the survey scans of MPC/LMA (2:1) at both 10° and 45° takeoff angles and illustrate the difference in atomic composition between the two data sets. The four elements present in the copolymer can be identified from their XPS peaks and are phosphorus (binding energy, BE, ∼ 133 and 191 eV), carbon (BE ∼ 285 eV), nitrogen (BE ∼ 400 eV) and oxygen (BE ∼ 533 eV). The binding energy scale has been corrected, using the C 1s as a reference at BE ) 285 eV. This adjustment was necessary due to the spectra being shifted to lower binding energy compared to normal due to the charge compensation from the electron flood gun. Figures 6b and 7b are the higher resolution scans of the C 1s regions of the spectra which highlight the difference in shape between the two C 1s peak envelopes. On moving from 10° to 45°, there is a large increase in the size of the shoulder visible at BE ) 286.3 eV. From peak fitting of the C 1s region of poly(MPC), this peak has been attributed to the R-C-N+ group. This type
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Figure 8. SPR trace of fibrinogen flowing over the surface of PLMA. As the protein adsorbs to the surface, a large increase in θSPR is observed.
Figure 6. (a) Survey spectrum of MPC/LMA (2:1). The spectrum was recorded using a takeoff angle of 45° with respect to the sample surface. (b) C 1s region, showing a shoulder at approximately 283 eV.
Figure 9. SPR trace of fibrinogen passing over MPC/LMA (1:1), representative of the whole series of copolymers studied. No increase in θSPR is observed on injection of the fibrinogen showing the protein-resistant nature of the material. The initial decrease in θSPR is caused by slight hydration of the material.
Figure 7. (a) Survey spectrum of MPC/LMA (2:1). The spectrum was recorded using a takeoff angle of 10° with respect to the sample surface. (b) C 1s region. The shoulder previously seen at 283 eV has almost disappeared.
of carbon environment is absent from LMA, providing further evidence that the LMA dominates the extreme surface of the polymer, while the MPC is located mainly in the bulk of the materials. SPR Analysis. Prior to SPR analysis, the polymercoated slides were incubated in ultrapure water to allow hydration and swelling of the polymer films to occur. In SPR, protein adsorption is accompanied by an increase in the angle of minimum light reflection (termed θSPR). Since
θSPR is also affected by the refractive index of the polymer which coats the SPR sensor and swelling of the polymer will alter this index, it was decided that prehydration of the films would remove any inconsistencies caused by swelling of the film during the SPR experiment. Figure 8 shows the adsorption profile of the plasma protein fibrinogen as it is passed over PLMA. Passing fibrinogen over PLMA caused an increase in θSPR of 286 ( 9 mDA, which is equivalent to a monolayer coverage of the polymer surface. PLMA is a very hydrophobic polymer due to the C12H25 side chain and the long hydrocarbon backbone. It has been hypothesized that the hydrophilicity of a surface has a major influence on the amount of protein that will adsorb.25 The XPS analysis of the copolymers showed the surface region to be dominated by the LMA component. This would suggest that the materials containing predominantly LMA would exhibit interfacial properties similar to those of the PLMA and not prevent proteins from adhering to their surface. However, in contrast to PLMA, no increase in θSPR was found to occur for any of the copolymers on passing fibrinogen over the polymer surface. Figure 9 shows fibrinogen passing over MPC/ LMA (1:1) and is representative of the whole series of copolymers (the initial decrease in θSPR is attributed to slight hydration of the material which results in swelling of the film). For the samples rich in MPC such as the MPC/LMA (2:1), this is not surprising since XPS analysis showed that there was still a fairly high percentage of PC present when using a 10° takeoff angle. However, in MPC/
Surface Mobility of 2-MPC-co-LMA Polymers
Figure 10. DCA profile of a PLMA-coated glass slide being lowered into water at a speed of 19.6 µm/s. Dwell time in the water is 2 h. The four cycles show that, although there is a slight change over time, the traces are almost identical.
LMA (1:6), the nitrogen signal was barely detectable, suggesting that little PC was present at the extreme surface of the materials. As the PC headgroup is believed to be responsible for the protein-resistant qualities of these materials, it might be expected that this would be the predominant surface moeity. DCA Analysis. Although DCA is typically used to calculate the angle formed between a solid material and a liquid wetting medium, it can also be used to monitor dynamic surface events including protein adsorption and desorption from a surface29 and polymeric surface rearrangements.24-26 In this paper, no contact angle values will be reported as DCA was employed as a technique to observe changes in the interfacial behavior of the copolymers when exposed to different external environments. Although DCA traces are different depending on the actual material being analyzed, it is possible to distinguish between samples which are overall hydrophobic or hydrophilic. This can be explained by the interaction that will occur as the sample enters liquid. For all DCA experiments described in this paper, the wetting medium was ultrapure water. The technique involves suspending the sample from a microbalance which measures the force exerted on the sample as it enters the wetting medium. If a hydrophilic material is lowered into water, the resultant force will be positive as a result of the attraction between the two. If a hydrophobic material enters water, repulsion will occur, and the measured force will be negative. The XPS analysis showed the surface of the copolymers to be dominated by LMA, which would result in a material interacting with the aqueous wetting medium, and it is reasonable to assume that the DCA trace obtained would be comparable to that of the homopolymer PLMA. Figure 10 shows the DCA trace of PLMA being lowered into water at a velocity of 19.6 µm/s. The slide was allowed to dwell in the water for 2 h before being raised then lowered again. This was repeated until the PLMA had been through four cycles. The anticipated trace was produced, with a large negative force being exerted on the hydrophobic PLMA on entering the water. Repeated cycles showed concurrent results, differing only in a small decrease in the negative force measured. This can be explained by the fact that once the slide has been submerged in the water the surface becomes inevitably wet; thus during the subsequent cycle, a small amount of water will be present on the slide. This can result in a slight increase in the attraction between the slide and wetting medium. The results for the MPC/LMA (1:2)coated slide are shown in Figure 11 and explain the discrepancies between the XPS and SPR results. The
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Figure 11. DCA profile of a MPC/LMA (1:2)-coated glass slide being lowered into water at a speed of 19.6 µm/s. Dwell time in the water is 2 h. The first cycle produces a trace similar to that found for PLMA, whereas the second cycle produces a trace typical of an extremely hydrophilic surface.
initial lowering of the slide into the water produced the expected negative force and produced a trace comparable to that of PLMA. The sample was allowed to remain in the aqueous environment for 2 h, and the cycle was repeated. The appearance of the trace was extremely different to that previously observed, with a large positive force being exerted on the slide as it contacts the water. It seems unlikely that the previously described wetting effect, although undoubtedly present, could have such a marked effect on the results. This suggests that the surface of the MPC/LMA (1:2) has changed from highly hydrophobic to highly hydrophilic. A possible explanation for this phenomenon is that the surface of the MPC/LMA (1:2), and the surface of the MPC/LMA (x:y) copolymers in general, are able to rearrange depending on the environment in which the material is placed. It is wellknown that the surface of polymeric materials cannot be treated in the classical way, which assumes that surfaces are immobile and fixed. Indeed, as far back as 1975, Holly and Refojo23 suggested that in materials containing hydrophobic and hydrophilic segments the surface can change to preferentially express the moeity that would result in a minimum surface energy. The LMA and MPC monomers which form the copolymers represent extremes in hydrophobicity and hydrophilicity, and so exposure of LMA to an aqueous environment or MPC to a hydrophobic environment would result in a large surface energy. The combination of the flexible C12 backbone of LMA and mobile PC moeity in MPC provides an ideal material to allow surface rearrangements to occur. By use of such a thesis, the UHV analysis chambers required for ToFSIMS and XPS provide an environment in which the hydrocarbon LMA is presented at the material’s surface to minimize surface energy, while on contact with water, MPC becomes the predominant surface group. As the DCA experiments were carried out in ambient conditions, it can be assumed that the water vapor content of air does not provide a sufficiently attractive environment for expression of the PC at the polymer surface. Indeed, after the MPC/LMA (1:2) slide was allowed to dry in air, a DCA trace resembling that of PLMA was once again observed. Conclusions A combination of complementary surface-sensitive techniques have been used to investigate the surface mobility of copolymers containing LMA and MPC in differing ratios. Angle-resolved XPS has shown that the composition of the copolymers changes with analysis depth. In UHV conditions, the hydrophobic LMA com-
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ponent dominates the extreme surface region (1-2 nm). All the copolymers studied exhibited excellent protein resistance, with no fibrinogen adsorbing to the surface of the hydrated polymers as measured by SPR. DCA has been used to study the ability of this copolymer system to alter its surface composition in order to minimize surface energy. It has shown that in environments such as air and UHV the LMA component is preferentially expressed at the surface, while in aqueous media the hydrophilic PC headgroup becomes dominant. This highlights the potential danger of applying UHV techniques such as XPS and ToF-SIMS to the characterization of biomaterials. The surface properties of a material intended for use in vivo may depend on the medium with which it is in contact, and UHV conditions are rarely encountered as working environments. The extreme contrast in hydrophilic nature
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of the two components of the copolymers has allowed the true mobility of polymer surfaces to be demonstrated and, combined with the high level of biocompatibility produced by incorporation of only a small amount of the PC headgroup into a material, highlights the important role that it can play in the future of biomaterial design. Acknowledgment. S.C. thanks the EPSRC and Biocompatibles Ltd. for funding. Additional thanks is due to the EPSRC for providing access to both the Scienta ESCA300 spectrometer at the RUSTI facility, Daresbury, U.K., and to the Phi 7200 ToF-SIMS spectrometer at ICI, Wilton, U.K. LA991472Y
