Biomacromolecules 2001, 2, 32-36
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Articles Plasma Polymerized N-Isopropylacrylamide: Synthesis and Characterization of a Smart Thermally Responsive Coating Y. Vickie Pan,† Roger A. Wesley,†,‡,§ Reto Luginbuhl,† Denice D. Denton,| and Buddy D. Ratner*,†,‡ University of Washington Engineered Biomaterials (UWEB), Department of Bioengineering, Department of Chemical Engineering, and Department of Electrical Engineering, University of Washington, Seattle, Washington 98195 Received July 3, 2000; Revised Manuscript Received September 27, 2000
A lower critical solution temperature (LCST) in an aqueous environment has been observed with poly(Nisopropylacrylamide) (pNIPAM) deposited onto solid surfaces from a plasma glow discharge of NIPAM vapor. The synthesis and spectroscopic data (ESCA, FTIR) for the plasma polymerized NIPAM (ppNIPAM) shows a remarkable retention of the monomer structure. The phase transition at 29 °C was measured by a novel AFM method. The phase transition was surprising because of the expectation that the plasma environment would destroy the specific NIPAM structure associated with the thermal responsiveness. The phase change of ppNIPAM is also responsible for the changes in the level of the meniscus when coated capillaries are placed in warm and cold water. Plasma polymerization of NIPAM represents a one-step method to fabricate thermally responsive coatings on real-world biomaterials without the need for specially prepared substrates and functionalized polymers. Introduction Poly(N-isopropylacrylamide) (pNIPAM) shows a lower critical solution temperature (LCST) of 31 °C in an aqueous environment.1,2 The well-hydrated polymer chains below the LCST have a random coil configuration. Above the LCST, the polymer chains take on a much more compact configuration by sudden dehydration and increased hydrophobic interaction between the polymer chains. In the literature, pNIPAM has been studied most frequently as a water-soluble polymer and a cross-linked network. When grafted onto solid surfaces, this phase change gives a “smart” surface with varying physical properties that can be controlled by applying an external stimulusstemperature.3-5 Below the phase transition temperature, the pNIPAM-grafted surfaces are hydrophilic, swollen, and nonprotein adsorptive (nonfouling.) As the temperature increases above the transition temperature, the grafted polymer chains collapse and the surface becomes hydrophobic and protein-retentive. Because of this thermal transition around the body temperature, the sharp property change, and the ability to immobilize the polymer onto a solid support, pNIPAM* Corresponding author. Box 351720, University of Washington, Seattle, WA 98195. E-mail:
[email protected]. † University of Washington Engineered Biomaterials (UWEB), Department of Bioengineering. ‡ Department of Chemical Engineering. § Current address: Aspen Technology, Inc., Bothell, WA 98011-8009. | Department of Electrical Engineering.
grafted surfaces offer possibilities for a number of novel applications. Examples include smart and thermally responsive coatings as cell culture substrates to control the attachment and detachment of cells,3,6 the recovery of cultured cells,7-12 a biofouling releasing coating,13 temperature responsive membranes,14,15controlled release of drugs and growth factor,16,17 and temperature responsive chromatography.18-22 Thus, a pNIPAM coating should be categorized as an enabling technology that can facilitate experiments and applications that were previously difficult or impossible. Different methods have been reported to graft pNIPAM on surfaces. These include activated substrates and functionalized polymers,18-27 e-beam irradiation,6,12,17,28 photoinitiated grafting of functionalized polymer,5,9,14-16,29,30 and plasma-induced grafting.7,29,31 Vapor-phase deposition of pNIPAM by plasma polymerization is described in this manuscript. Plasma polymerization is a one-step, solventfree, vapor-phase coating technique. Plasma deposited coatings generally have excellent physical properties: conformal, sterile, pinhole-free, complete surface coverage and excellent adhesion to different substrates. However, a concern with this technique has always been the significant monomer fragmentation in the plasma, and, consequently, the loss of chemical functional groups and material functionality in the coating. The plasma-polymerized NIPAM (ppNIPAM) coatings reported here show an exceptional retention of the monomer structure after plasma deposition. Consequently,
10.1021/bm0000642 CCC: $20.00 © 2001 American Chemical Society Published on Web 12/01/2000
Plasma Polymerized N-Isopropylacrylamide
Figure 1. Schematic diagram of a radio frequency plasma polymerization reactor utilized for the deposition of smart ppNIPAM coatings.
the phase transition property of pNIPAM is successfully retained in ppNIPAM. This manuscript describes the preparation of ppNIPAM and characterization by FTIR, ESCA and capillary rise measurements. A separate manuscript will fully address the phase transition measurements based on a novel atomic force microscopy (AFM) technique. Excellent chemical and physical properties achieved by this coating technique open the exciting prospect of preparing thermally responsive surfaces on real-world biomaterials, including those for tissue engineering applications. Materials and Methods Plasma Polymerization of NIPAM. The apparatus for plasma polymerization is shown schematically in Figure 1. There are four major components in the setup: monomer delivery system, radio frequency power components, reactor vessel, and the pumping and pressure control system. N-Isopropylacrylamide (99.5%+) was purchased from Aldrich and used as received. For each deposition experiment, 5 g of NIPAM was measured and placed in a 100 mL volume glass flask. The flask is connected to a metering needle valve used to control the monomer delivery rate. A shut-off valve isolated the cylindrical glass chamber from the metering valve and the monomer flask. Because of the low volatility of NIPAM, the monomer flask was maintained at 72 °C and the monomer delivery line at 80 °C for the deposition process. A number of different substrates were used to prepare samples for characterization experiments. They include Si wafers for ESCA and AFM analyses, IR transparent 3M Type 62 IR cards for FTIR analysis, and glass capillaries (5 cm long and 1.5 mm internal diameter) for the capillary rise experiment. Glass capillaries were placed on the sample stage parallel to the direction of gas flow in the reactor. Both Si wafers and capillaries were sonicated successively in methylene chloride, acetone, and methanol prior to use. The powered electrode is connected to a 13.56 MHz radio frequency power source and a manual impedance matching network. The distance between the two external electrodes is 5 in. After loading substrates for coating, a mechanical pump evacuates the reactor to the desired base pressure in the low 10-3 Torr range. A liquid nitrogen cooled cold trap between the vacuum pump and the reactor is used to condense organic materials. The monomer vapor is introduced to the chamber and the chamber pressure is controlled
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independently by a throttle valve and, in this case, maintained at 100 mT. As soon as the radio frequency power is applied to the powered electrode, a uniform glow discharge is observed primarily between the two electrodes. The plasma was first maintained at a high power of 120 W to form an adhesion-promoting layer between the substrates and the plasma polymer film. The plasma power was reduced successively to 60, 5, and finally 1 W for the deposition of a functional coating. The adhesion-promoting layer plays an important role for immobilizing the soft, functional lowpower coating on substrates. The lower-power coating could delaminate significantly in water without such an adhesionpromoting layer, particularly on nonpolymeric substrates. After the deposition process is completed and the residual organic vapors are pumped out of the reactor, the substrates are removed for subsequent analysis. Chemical Characterization. The FTIR measurements were performed using a Bio-Rad FTS-60 series spectrometer equipped with a liquid nitrogen cooled mercury cadmium telluride (MCT) detector. The transmission FTIR spectrum of ppNIPAM from 1300 to 4000 cm-1 was acquired at a resolution of 4 cm-1. ppNIPAM was deposited at 1 W on an IR transparent 3M Type 62 IR card (Teflon) for the IR measurement. ESCA analyses were performed on a Surface Science Instruments (SSI) X-Probe ESCA instrument with an aluminum KR1,2 monochromatic X-ray source and a hemispherical energy analyzer. This instrument permits analysis of the outermost 20-100 Å of a sample in an elliptical area whose short axis can be adjusted from 150 to 1000 µm. An electron flood gun set at 4 eV was used to minimize surface charging of the samples. Typical pressures in the analysis chamber during spectral acquisition were 10-9 Torr. Spectra were collected with the analyzer at 55° with respect to the surface normal of the sample, resulting in a data collection depth of 50-80 Å. LCST Characterization. The LCST of ppNIPAM thin films was determined using a technique based on shear force modulation atomic force microscopy (AFM) to measure the mechanical properties of coatings. AFM measurements were performed using a stand alone system (Explorer, ThermoMicroscopes, Inc.) equipped with a 100 µm X-Y scanner and a 10 µm Z scanner. A function generator (Stanford Research Systems) was used to add a sinusoidal signal (115 kHz) to the piezo signal input in the lateral direction for shear moduli measurements, and a dual-phase lock-in amplifier (Stanford Research Systems) measured the responses to the modulation. Bar-shaped Si cantilevers were used (NTMDT and Nanosensors GmbH). Measurements were performed on a ppNIPAM coating deposited on Si substrates immersed in an open 2 mL liquid cell. The cell temperature was systematically varied and allowed to equilibrate prior to each modulation measurement. Results and Discussion FTIR Characterization. The FTIR spectrum of ppNIPAM is shown in Figure 2. The peak assignments are as follows: 1366 and 1386 cm-1 (deformation of the two methyl groups on -C(CH3)2);32,33
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Figure 2. FTIR spectrum of ppNIPAM shows a high structural retention. The two bands of almost equal intensity at 1366 and 1386 cm-1 are assigned to the two methyl groups in the isopropyl functionality.
1458 cm-1 (-CH3 and -CH2- deformation);32 1538 cm-1 (secondary amide N-H stretching, aka amide II band), 1645 cm-1 (secondary amide CdO stretching, aka amide I band), 2874 cm-1 (-CH3 symmetric stretching), 2932 cm-1 (asymmetric -CH2- stretching), 2970 cm-1 (-CH3 asymmetric stretching);33,34 3070 cm-1 (amide II band overtone);32 3301 cm-1 (secondary amide N-H stretching).35 We believe that the presence of two IR bands of almost equal intensity at 1366 and 1386 cm-1 is indicative of the retention of structure in ppNIPAM. These two bands are associated with the deformation of the two methyl groups in the pendant isopropyl functionality that is believed to contribute to the phase transition of the polymer.33 The LCST of pNIPAM in solution is independent of both molecular weight and concentration, indicating that the phase transition of pNIPAM is a single chain phenomenon.36 The driving force is associated with temperature-dependent molecular interactions, mainly hydrogen bonding and hydrophobic interaction.37-39 More recently, detailed FTIR experiments were carried out with pNIPAM in water33,34 and pNIPAM hydrogels34 to study the molecular interactions during the phase transition. The hydrophobic interaction of the isopropyl functionality increased above the LCST,33 and the hydrogen bonding of the hydrophilic CdO and N-H groups changed from intermolecular to intramolecular above the LCST.33-34 It was concluded in these studies that the overall hydration state of pNIPAM has significant effect on the amide I and II regions. Furthermore, the amide I (CdO) and II (N-H) regions were found to be composed of three distinct bands arising from the three states of hydrogen bonding: intermolecular, intramolecular and the free form of the non-hydrogenbonded state.33-35 Because glow discharge is a dry fabrication process, the product, ppNIPAM, is in a dehydrated state. Atmosphere is the only source of limited moisture for ppNIPAM hydration at ambient environment. The relatively dehydrated state of bulk ppNIPAM at ambient humidity and temperature resulted in amide bands associated with the nonhydrogen-bonded states: 1645 cm-1 for amide I and 1538 cm-1 for amide II in Figure 2. In comparison, reported values for the free form of non-hydrogen-bonded states are 1643 cm-1 for amide I and 1535 cm-1 for amide II.33 ESCA Characterization. The ESCA survey scan of a ppNIPAM coating is shown in Figure 3. Three peaks were
Pan et al.
Figure 3. ESCA survey scan of a ppNIPAM coating and its elemental composition (excluding hydrogen). A good agreement is found between the result and the theoretical expectation.
Figure 4. ESCA high-resolution C 1s spectrum of a ppNIPAM coating and the three resolved peaks of C-C/C-H (peak 1 s-s-), C-N (peak 2 - - - ) and CdO (peak 3 s) carbons. Reduced data are shown as a solid line and the composite fit is shown as a bold dashed line.
observed: carbon at 285 eV, nitrogen at 400 eV, and oxygen at 531 eV. Excluding hydrogen, the elemental composition (atomic %) of ppNIPAM obtained from ESCA is 76.8% carbon, 12.8% nitrogen, and 10.4% oxygen. This compares well to the theoretical composition of conventionally polymerized NIPAM (75.0% carbon, 12.5% nitrogen, and 12.5% oxygen). The ESCA survey scan did not reveal any signal from the Si substrate, consistent with the visual observation of ppNIPAM as a thin solid film with complete surface coverage. A high surface coverage is an advantage to maximize the overall responsiveness in future devices, such as a cell culture surface for rapid cell-sheet release. Additional information on the molecular structure of ppNIPAM is obtained from the ESCA high-resolution C 1s scan (Figure 4). Three peaks of equal peak width (1.45 eV) are used for the peak resolution, and the fitted peaks are shown in Figure 4. The first peak at 285.0 eV corresponds to C-C and C-H carbons. The second peak at 286.1 eV corresponds to C-N carbons. The third peak at 287.8 eV is indicative of the CdO carbons. The peak fit demonstrates that 61.2% of carbons in ppNIPAM are in C-C and C-H environments, 21.6% are C-N carbons, and 17.1% are Cd O carbons. This also compares well to the theoretical composition of a conventionally polymerized NIPAM which should have a distribution of 66.7% C-C and C-H, 16.7% of C-N, and 16.7% of CdO. The excellent retention of molecular structure seen in ppNIPAM is surprising. One might expect to see consider-
Plasma Polymerized N-Isopropylacrylamide
Figure 5. AFM modulation measurement as a function of temperature. The modulation signal reflects the contact mechanical properties of ppNIPAM sample. A sharp change in response signal was found at 28.5 °C, which is the transition temperature of ppNIPAM.
able fragmentation after plasma deposition. Possibly, the monomer introduced into the plasma chamber at elevated temperature condenses on the cooler substrates, simultaneously with the deposition. Thus, intact monomer may be conventionally free radical polymerized on the surface from active species in the plasma. Structure retention in plasma deposition associated with this mechanism has been studied previously.40-42 The low power used in the final phase of the deposition also can account for the structural retention. LCST. The major impetus of this work is to fabricate smart surfaces that exhibit a defined, controllable phase transition. Preliminary data on the temperature transition of ppNIPAM coatings is obtained from a novel atomic force microscopy (AFM) technique and a capillary rise experiment. The sharp physical property change of the coating makes it well suited for AFM-based techniques that measure the mechanical properties of thin films. Glass transition temperatures of polymer thin films have been measured and reported using AFM.43 A typical AFM result is presented here, and a future manuscript will be devoted to describing in detail this measurement technique and the temperature transition of ppNIPAM coatings. Figure 5 shows one of the AFM response curves obtained from the ppNIPAM coating at different temperatures in water. A sharp transition is observed at 28.5 °C. The observed phase transition confirms that the specific NIPAM structure associated with thermal responsiveness was not destroyed in the plasma environment. The phase change is also demonstrated by the capillary rise method shown in Figure 6. Indeed, the chemical characterization of the coating has shown good retention of the polymer structure in the ppNIPAM coating so this result might be expected. On the other hand, the relatively short polymer chains expected from plasma deposition/polymerization could have inhibited the sharp transition. These experiments show this is not the case and a similar transition to that in the conventionally polymerized polymer is observed. Capillary Rise Experiment. The two phases of ppNIPAM can also be observed in the capillary rise experiment. The water meniscus level inside a capillary is a result of surface tension and the interaction between the capillary wall and water. When pNIPAM was photochemically grafted inside capillaries, a different meniscus was reported for the two states of the polymer.30 For the capillary rise experiment, 5 cm long glass capillaries with 1.5 mm internal diameter were treated in a NIPAM glow discharge.
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Figure 6. Capillary rise experiment of (a) untreated capillary in roomtemperature water, (b) ppNIPAM treated capillary in cold water, and (c) ppNIPAM treated capillary in warm water.
Figure 6 shows a photograph of the meniscus level of three capillaries: (a) an untreated glass capillary in water of room temperature, (b) a ppNIPAM-coated capillary in cold water, and (c) a second ppNIPAM-coated capillary in warm water. The ppNIPAM-coated capillaries were first allowed to equilibrate to the proper temperature before being placed in test tubes with 0.5 mL of cold (5 °C) and warm (55 °C) water. The water level in test tubes with 0.5 mL water is 1.0 cm. The untreated glass capillary has a hydrophilic surface, and the meniscus level inside the untreated capillary is 1.7 cm above the water level as shown in Figure 6a. Figure 6b with a ppNIPAM-coated capillary in cold water shows a meniscus level of 0.9 cm above the water level. The capillary in Figure 6c is ppNIPAM coated as well, but it is in a warm environment to induce the hydrophobic state of ppNIPAM. The meniscus level is 0.2 cm above the water level. By comparison of these two coated capillaries at different temperatures, a difference of 0.7 cm is observed. An ESCA experiment was performed on the interior (luminal) surface of a ppNIPAM-coated capillary to confirm the presence of ppNIPAM throughout the capillary interior. The capillary was first broken into five equal 1 cm sections, and each section was gently broken to expose the capillary lumen to the X-ray beam. The result of the ESCA survey scans is shown in Figure 7. For comparison, the elemental composition of an untreated glass capillary (excluding hydrogen), a ppNIPAM film on Si, and the theoretical composition of a conventionally polymerized NIPAM are outlined next to the capillary data. The elemental compositions from all five sections of the treated capillary are consistent with the presence of a thin ppNIPAM coating inside the capillary. Although a ppNIPAM coating of roughly half a micron was obtained on flat Si surfaces, the coating thickness inside the glass capillary is estimated to be 4080 Å from the presence of low levels of Si and Na, species unique to the glass substrate, in the ESCA analysis. Thus, significantly less ppNIPAM deposition occurred inside the glass capillary than on flat surfaces. The ability of the plasma deposit to penetrate so far into a narrow capillary is unexpected but potentially useful.
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Figure 7. Elemental composition (based on ESCA survey scans) of an untreated glass capillary, the interior surface of a ppNIPAM coated capillary at different lengths. For comparison, the composition of an untreated glass capillary, theoretical pNIPAM, and ppNIPAM deposited on a flat Si wafer are shown on the side.
Conclusions Plasma polymerization of N-isopropylacrylamide has been carried out and characterized using ESCA and FTIR. The resulting coating showed a striking resemblance to conventionally polymerized NIPAM. Because plasma polymerization-deposition is a one-step coating process and is relatively independent of the nature of the substrates, this technique has advantages over the methods reported to date for grafting pNIPAM onto solid supports and surfaces. The temperature responsiveness of the coating has been observed. Additional characterization and applications of the coating are currently under investigation, and a separate manuscript (in preparation) will describe the details of the AFM-based phase transition measurement technique. Acknowledgment. The authors are grateful for the assistance of Mr. Winston Ciridon, Prof. Rene Overney, and Ms. Cynthia Buenviaje at the University of Washington, and Ms. Deborah Leach-Scampavia and Dr. Stephen Golledge at the National ESCA and Surface Analysis Center for Biomedical Problems (NESAC/Bio). The surface analysis experiments done at NESAC/BIO were supported by NIH Grant RR-01296 from the National Center for Research Resources. This work was supported in part by NSFEngineering Research Center program Grant No. EEC9529161, the Washington Technology Center, and MesoSystems Technology, Inc., Richmond, WA. References and Notes (1) Priest, J. H.; Murray, S. L.; Nelson, R. J.; Hoffman, A. S. ACS Symp. Ser. 1987, 350, 255-264. (2) Kubota, K.; Hamano, K.; Kuwahara, N.; Fujishige, S.; Ando, I. Polym. J. 1990, 22, 1051-1057. (3) Yamada, N.; Okano, T.; Sakai, H.; Karikusa, F.; Sawasaki, Y.; Sakurai, Y. Makromol. Chem., Rapid Commun. 1990, 11, 571-576. (4) Takei, Y. G.; Aoki, T.; Sanui, K.; Ogata, N.; Sakurai, Y.; Okano, T. Macromolecules 1994, 27, 6163-6166. (5) Kubota, H.; Nagaoka, N.; Katakai, R.; Yoshida, M.; Omichi, H.; Hata, Y. J. Appl. Polym. Sci. 1994, 51, 925-929. (6) Okano, T.; Yamada, N.; Okuhara, M.; Sakai, H.; Sakurai, Y. Biomaterials 1995, 16, 297-303. (7) Okano, T.; Yamada, N.; Sakai, H.; Sakurai, Y. J. Biomed. Mater. Res. 1993, 27, 1243-1251. (8) von Recum, H.; Kikuchi, A.; Okuhara, M.; Sakurai, Y.; Okano, T.; Kim, S. W. J. Biomater. Sci., Polym. Ed. 1998, 9, 1241-1253.
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