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Nanofibrillar Surfaces via Reactive Ion Etching Heather M. Powell and John J. Lannutti* Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210 Received May 29, 2003. In Final Form: July 29, 2003 The reactive ion etching of poly(ethylene terephthalate) using various gas compositions was examined. A radio frequency power supply operating at 30 kHz was used to produce plasmas from the following gases and 50:50 gas mixtures: Ar, N2, O2, Ar-N2, Ar-O2, and N2-O2. Exposure times were typically 10, 20, and 30 min. The relatively inert gases and gas mixtures (Ar, N2, Ar-N2) produced a polygonal pattern of protrusions surrounding shallow cavities. In contrast, the presence of oxygen or oxygen-containing plasmas invariably imposed a fibrillar structure on these polygons in which “nanofibrils” typically originated at the triple points of several neighboring polygons. The dimensions of these fibrils varied with the exposure time but were up to 300 nm in length and approximately 20 nm in diameter. Atomic force microscopy was used to quantify the surface roughness and show that the inert gas compositions (Ar, N2, Ar-N2) produced statistically indistinguishable values of Ra (14.1 ( 1.7, 15 ( 2.5, 14.2 ( 2.9 nm) significantly larger than those of the as-received film. The pure oxygen-etched films have Ra values approximately twice as large as those of the other gas compositions. Prior work by Nie et al. suggests that films consisting of etchingresistant, low-molecular-weight fragments can form under these conditions. We find that surface physics normally associated with thin polar films on apolar substrates adequately describes the origin of the observed nanofibrils. Prior surface deformation has an additional influence on fibril spacing and density.
Introduction In the early 1930s, Weiss demonstrated that cells grown on a substrate having an oriented surface assume a corresponding orientation and migrate in that direction.1 Since then many investigations have examined the effect of surface characteristics on cell behavior.2-8 Techniques such as mechanical abrasion, treatment with solvents, caustic solutions, acids, graft polymerization, and micromachining have been employed to alter material surfaces to elicit specific cellular behavior.9-14 Gadegaard et al. prepared nickel replicas of bovine type I collagen via physical vapor deposition of a small-grained metal layer followed by galvanic plating. Thermoplastic polymers were injection-molded into the nickel replica to create surfaces having biomimetic topography.15 Quirk et al. altered the * Author to whom correspondence should be addressed: e-mail
[email protected], phone (614)292-3926, fax (614)292-1537. (1) Weiss, P. J. Exp. Zool. 1934, 68, 393-448. (2) Anselme, K.; Bigerelle, M.; Noel, B.; Dufresne, E.; Judas, D.; Iost, A.; Hardouin, P. J. Biomed. Mater. Res. 2000, 49, 155-166. (3) Campbell, C. E.; von Recum, A. F. J. Invest. Surg. 1989, 2, 51-74. (4) Degasne, I.; Bsale, M. F.; Demais, V.; Hure, G.; Lesourd, M.; Grolleau, B.; Mercier, L.; Chappard, D. Calcif. Tissue Int. 1999, 64, 499-507. (5) Hatano, K.; Inoue, H. K. T.; Matsunaga, T.; Tsujisawa, T.; Uchiyama, C.; Uchida, Y. Bone 1999, 25, 439-445. (6) Meyle, J.; Wolberg, H.; von Recum, A. F. J. Biomater. Appl. 1993, 7, 362-375. (7) Turner, A. M. P.; Dowell, N.; Turner, S.; Kam, L.; Isaacson, M.; Craighead, H. G.; Shain, W. J. Biomed. Mater. Res. 2000, 51, 430-441. (8) von Recum, A. F.; van Kooten, T. G. J. Biomater. Sci., Polym. Ed. 1995, 7, 181-198. (9) Stepien, E.; Stanisz, J.; Korohoda, W. Cell Biol. Int. 1999, 23, 105-116. (10) Eisenbarth, E.; Meyle, J.; Nachtigall, W.; Breme, J. Biomaterials 1996, 17, 1399-1403. (11) Kononen, M.; Hormia, M.; Kivlahti, J.; Hautaniemi, J.; Thesleff, I. J. Biomed. Mater. Res. 1992, 26, 1325-1341. (12) Mirzadeh, H.; Dadsetan, M.; Sharifi-Sanjani, N. J. Appl. Polym. Sci. 2002, 86, 3191-3196. (13) Brunette, D. M. Exp. Cell Res. 1986, 164, 11-26. (14) Green, A. M.; Jansen, J. A.; van der Waerden, J. P. C. M.; von Recum, A. F. J. Biomed. Mater. Res. 1994, 28, 647-653. (15) Gadegaard, N.; Mosler, S.; Larsen, N. B. Macromol. Mater. Eng. 2003, 288, 76-83.
cell binding capability of poly(lactic acid) (PLA) by physically entrapping modifying species via reverse gelation.16 Localization of cells has also been achieved by microprinting a pattern of fibronectin and bovine serum albumin on a culture surface.17 Derivatization of poly(acrylamide) with arginine-glycine-aspartate-containing nonapeptides has also been shown to increase cell spreading.18 In contrast, reactive ion etching has been used for many years in the plastics industry to induce a surface topography sufficient to enable printing operations19,20 and other applications involving adhesives, coatings, composites, and electronics. Modification via reactive ion etching has been used extensively as a result of its ability to modify large areas easily without changing bulk properties.19 Plasma etching has been used in biological applications to provide improved cell adhesion and hydrophilicity.19,21,22 Reactive plasmas contain charged particles (ions and electrons), excited neutrals, radicals, and UV radiation,19,23,24 which can react with a polymeric surface to remove contamination, introduce chemical functionalities, and induce chain scission and cross-linking.19,25-28 The (16) Quirk, R. A.; Davies, M. C.; Tendler, S. J. B.; Chan, W. C.; Shakesheff, K. M. Langmuir 2001, 17, 2817-2820. (17) Nishizawa, M.; Takoh, K.; Matsue, T. Langmuir 2002, 18, 36453649. (18) Reinhart-King, C.; Dembo, M.; Hammer, D. A. Langmuir 2003, 19, 1537-1539. (19) Egitto, F. D.; Matienzo, L. J. IBM J. Res. Dev. 1994, 38, 423439. (20) Kaplan, S. L.; Rose, P. W. Int. J. Adhes. Adhes. 1991, 11, 109. (21) Nakamatsu, J.; Delgado-Aparicio, L. F.; Da Silva, R.; Soberon, F. J. Adhes. Sci. Technol. 1999, 13, 753-761. (22) Ramires, P. A.; Mirenghi, L.; Romano, A. R.; Palumbo, F.; Nicolardi, G. J. Biomed. Mater. Res. 2000, 51, 535-539. (23) France, R. M.; Short, R. D. J. Chem. Soc., Faraday Trans. 1997, 93, 3173-3178. (24) Denes, F. Trends Polym. Sci. 1997, 5, 23-31. (25) Beake, B. D.; Ling, J. S.; Leggett, G. J. J. Mater. Chem. 1998, 8, 1735-1742. (26) Weikart, C. M.; Yasuda, H. K. J. Polym. Sci., Polym. Chem. Ed. 2000, 38, 3028-3042. (27) Bourceanu, G. D.; Gheorghiu, M. D.; Moisa, C. Rev. Roum. Phy. 1985, 30, 145-150.
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predominant reactive species in radio frequency (RF) plasmas are positive ions and photons capable of breaking primary chemical bonds to form reactive radicals.25,27,29,30 These reactive radicals usually interact with the surrounding gas to produce new surface functionalities such as carbonyl, hydroxyl, or carboxylic acid groups created from radical-gas interactions. Both the chemical structure of the polymer and the type of plasma determine the extent and character of degradation.26 Polymeric structures containing oxygen are known to be more susceptible to oxygen plasmas, whereas structures containing benzene rings are more resistant to oxidation and etching.26 On the other hand, the ability of argon plasmas to etch the oxygen-containing polymer poly(ethylene terephthalate) (PET) is controversial. Fischer et al. reported that argon plasma alone does not etch PET,31 while Beake et al. reported that it does.25 The effects of oxygen plasma etching are, in contrast, quite consistent. Etching rates typically increase with oxygen atom concentration in the plasma.32,33 The addition of N2 and N2O amplifies the oxygen atom concentration and etch rates.19 Rates of material removal appear to be faster at lower gas pressures.25 Linear etching rates have been reported in some gases such as argon and nitrogen, but the rates are parabolic in the presence of oxygen or air.27 Bourceanu et al. reported that the etch rates varied with gas identity; the highest etch rates involved pure oxygen followed by air, nitrogen, and argon gases.27 Exposure to the plasma also results in the loss of C-H bonds as a result of hydrogen abstraction.34 Chain scission results in a surface rich in low-molecular-weight fragments (LMWFs), which then either are removed via the vacuum system or remain on the surface.35-37 Etching and chemical modification often occur simultaneously and are competitive processes.19,25 In this study, the ability of these different plasma “media” (nitrogen, argon, oxygen, and 50:50 vol % mixtures) to etch PET in a manner that we anticipate will alter the behavior of adherent cells8,38-40 is investigated. High-resolution scanning electron microscopy (SEM) and atomic force microscopy (AFM) are used to analyze the resultant surface and provide an estimate of their biomimetic nature. Materials and Methods Polymer Films. Rectangular PET coverslips (Thermanox, Nalgene Nunc International, Rochester, NY) were used in this study. The thickness, width, and length of these PET films were 200 µm, 22 mm, and 66 mm, respectively. (28) Beake, B. D.; Ling, J. S.; Leggett, G. J. J. Mater. Chem. 1998, 8, 2845-2854. (29) Kaempf, G.; Orth, H. J. Macromol. Sci., Phys. 1975, 11, 151164. (30) Rapp, R. S.; Briglia, D. J. Chem. Phys. 1965, 43, 1480. (31) Fischer, G.; Haemeyer, A.; Dembowski, J.; Hibst, H. J. Adhes. Sci. Technol. 1994, 8, 151-161. (32) Cook, J. M.; Hannon, J. J.; Benson, B. W. Proc. 6th Int. Symp. Plasma Chem. 1983, 616-620. (33) Spencer, J. E.; Jackson, R. L.; Hoff, A. Proc. Symp. Plasma Process. 1987, 86-87, 186-200. (34) Meichsner, J.; Nitschke, M.; Rochotzki, R.; Zeuner, M. Surf. Coat. Technol. 1995, 74-75, 227-231. (35) Ko, H.-S.; Nah, J.-W.; Paik, K. W.; Park, Y. J. Vac. Sci. Technol., B 2002, 20, 1000-1007. (36) Nie, H.-Y.; Walzak, M. J.; McIntyre, N. S. Appl. Surf. Sci. 1999, 144-145, 627-632. (37) Boyd, R. D.; Kentwright, A. M.; Badyal, J. P. S. Macromolecules 1997, 30, 5429-5436. (38) Xie, Y.; Sproule, T.; Li, Y.; Powell, H.; Lannutti, J. J.; Kniss, D. A. J. Biomed. Mater. Res. 2002, 61, 234-245. (39) Den Braber, E. T.; de Ruijter, J. E.; Smits, H. T. J.; Ginsel, L. A.; Von Recum, A. F.; Jansen, J. A. J. Biomed. Mater. Res. 1995, 29, 511-518. (40) Ma, T.; Li, Y.; Yang, S. T.; Kniss, D. A. Biotechnol. Prog. 1999, 15 (4), 715-724.
Powell and Lannutti
Figure 1. SEM image of the surface of as-received PET. The slight cracks visible are an artifact within the Au-Pd coating. The scale bar is 200 nm. Reactive Ion Etching. In the reactive ion etcher (Technics Micro-RIE Series 800-II, Concord, CA) we used, the platen (cathode) is connected to a RF power supply operating at 30 kHz. The chamber itself acts as the anode, causing the gas entering the chamber to be ionized when an electric field is applied. Gases were supplied to the system at a rate of 25 cm3/min under 50 W of power with a pressure of 135 mTorr in the chamber. To investigate the effect of gas media and time on etching ability, a test matrix was designed that utilized three exposure periods in combination with the different gases selected on the basis of previous literature. Beake et al. suggest that the most significant change in the surface topography occurs after 20 min of exposure to oxygen plasma; therefore, 10-, 20-, and 30-min time intervals were investigated.25 Different gases are known to possess varying etching abilities, and the presence of multiple gas species can have an effect. Thus, high-purity gases and 1:1 mixtures of each gas (high-purity argon, oxygen, or nitrogen or a 1:1 mixture of argon-oxygen, argon-nitrogen, or nitrogenoxygen) were examined. The PET films were placed flat in the center of the plasma chamber floor. The surface modification process consisted of continuous exposure to the various plasmas for 10, 20, or 30 min. The films were then placed upright in a microslide container to prevent contact with any other surface. Each experiment was conducted four times to ensure reproducibility. SEM. The surface topography of the etched and nonetched films was qualitatively examined using SEM (FEI Sirion). Both the etched and the untreated films were sputter-coated with a 10-nm layer of gold-palladium and then analyzed in the ultrahigh-resolution mode with a 5-keV accelerating voltage. Image analysis of the SEM micrographs was preformed to estimate the feature size. For each sample, a total of 20 representative protrusions or fibrils from several images were measured and the feature-size ranges were recorded. AFM. The surface topography of the films was quantified using AFM (Digital Instruments Nanoscope IIIa, Veeco Metrology, Tucson, AZ) in the tapping mode with a frequency of 225 kHz, an amplitude of 1.225 V, and a scan rate of 0.500 Hz. Several areas ranging from 1 to 5 µm2 on each surface were imaged and analyzed with Digital Instruments software version 4.22r2. The topographical roughness parameters Ra (average deviation from the arithmetic centerline), Rmax (maximum peak height), and Rq (root-mean-square deviation of the surface) were determined for each surface.
Results SEM. SEM revealed that each etched surface has a different surface topography than the unetched PET film (Figure 1). Films etched with pure gases exhibit distinct surface morphologies characteristic of the gas used. PET films etched with argon display the smallest amount of topographic change and are characterized by a polygonally patterned surface made up of many small protrusions.
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Figure 2. High-resolution SEM images of PET surfaces after etching (a) 10 min in Ar, (b) 30 min in Ar, (c) 10 min in N2, (d) 30 min in N2, (e) 10 min in O2, and (f) 30 min in O2.
After 10 min, the surface displays small (10-20 nm in diameter) protrusions in a polygonal pattern alternating with shallow divots (Figure 2a). After 20 min, the protuberances become larger (15-30 nm in diameter) and the polygonal pattern becomes less dense. The protrusions increase in size (40-50 nm in diameter) and decrease in density following the final 30-min interval (Figure 2b). Plasma etching with nitrogen is classified by larger protrusions arranged in a less regular polygonal pattern. At 10 min, the surface exhibits 20-40-nm diameter protrusions with shallow depressions between the raised protrusions at the edges of the polygons (Figure 2c). As the etching exposure increases, these protuberances decrease in number and those remaining increase in length
until they form short (60-80 nm) fibrils emerging from the triple points of the polygons (Figure 2d). In contrast to the nitrogen and argon results, oxygen etching produces a distinctive fibrillar structure even at the shortest time interval; only 10 min of etching produces fibrils 65-100-nm long and 20 nm in diameter (Figure 2e). Increasing the exposure increases the fibril length (up to 300-nm long at 30 min) and decreases the fibril density (Figure 2f). Gas mixtures produce topologies intermediate to those of the pure gas morphologies. In all oxygen-containing mixtures, the fibrillar structure predominates. For example, oxygen-argon mixtures contain small protrusions at the triple points of the polygonal features at the 10 min
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Figure 3. High-resolution SEM image of the PET surface etched with an O2-N2 gas mixture for 30 min.
Figure 5. (a) Ra (average deviation from the arithmetic centerline) and (b) Rmax (maximum peak height) values from films exposed to 30 min of the indicated plasmas. Figure 4. AFM image of PET etched with argon for 30 min.
interval and grow into thin (15-20-nm diameter), long (200-250 nm) fibrils after 30 min of etching. Oxygennitrogen-etched surfaces are also characterized by tiny protrusions at their triple points following 10 min of exposure. In contrast, only 100-200-nm long fibrils are visible after 30 min of exposure (Figure 3). Argon-nitrogen mixtures produce a polygonal pattern consisting of very fine protrusions (5-15 nm in diameter) visible at the bottom of the polygonal divots. As the etching time increases, larger protrusions are produced and increase in density until the surface contains protrusions typically 35 nm in diameter. AFM. To better quantify the surface topography, tapping mode AFM was used to analyze these etched surfaces following 30 min of exposure. Surfaces imaged by AFM closely resemble the corresponding SEM images for surfaces etched with Ar (Figure 4), N2, and Ar-N2. However, the surface fibrils appear much flatter than those in SEM. This is in part due to the complex fibril geometries (which include many 90° angles) and the inability of the reverse tapping probe to image angles greater than roughly 80°. Thus, the surface parameters calculated for surfaces etched with oxygen and oxygen-containing mixtures are clearly less than their actual values. Despite this deficiency, the AFM results indicate that only the oxygenetched surface has significantly different average roughness and maximum peak-height values (Figure 5a,b) when compared to those of the other surfaces. The average roughness values for the oxygen-, argon-, nitrogen-, argon-nitrogen-, argon-oxygen-, and nitrogen-oxygenetched films were 31.4 ( 4.6 nm, 14.1 ( 1.7 nm, 15 ( 2.5 nm, 14.2 ( 2.9 nm, 12.4 ( 2.8 nm, and 11.5 ( 1.6 nm,
respectively. These values are all significantly larger than those of the as-received film (3.5 ( 0.8 nm). The maximum peak values of pure oxygen-etched films were roughly twice as large as the maximum values of all the other gas combinations. Films etched with gas media other than pure oxygen all had statistically indistinguishable roughness values. Discussion Reactive plasmas produce a broad spectrum of effects on polymer surfaces, including greater surface roughness and an increased affinity for water through the addition of polar groups to the underlying macromolecules. The hydrophilicity of the surface is not stable with time.21,25,27 Upon storage in air for 1 day, the wetting angle of PET returns to its original value.25 This hydrophobic recovery is believed to result from the movement of surface macromolecules into the bulk, which homogenizes the overall surface character. In contrast to these chemical changes, surface analysis using SEM and AFM has shown that the increase in the surface roughness is stable over time. In agreement with previous literature, our SEM and AFM results indicate that oxygen plasmas have the greatest potential to etch PET.25,27,31 Positive ions are thought to be responsible for the degradation of the surface, and, thus, a higher concentration of positive ions can lead to an increased etching ability. On the basis of differences in the chamber pressure before and during plasma etching, Bourceanu et al. determined that the concentration of dissociated ions in nitrogen plasmas was roughly 7% while for oxygen it was approximately 50%.27 This could lead to a greater etch rate and surface roughness for films exposed
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Figure 6. Schematic of morphological development on oxygenetched PET surfaces. In part a, the initial PET surface is shown. As etching initiates, part b illustrates the formation of a thin continuous film of relatively polar LMWFs. At a subsequent stage in the etching process, part c shows that the larger scale polygonal instability has nucleated while smaller scale instabilities continue to exist. The final morphological stage is shown in part d, in which the smaller scale instabilities are no longer apparent and only the rim and nanofibrillar features are still evident.
to oxygen plasma simply because there are more dissociated species (positive and negative) present. Films etched with oxygen exhibited the largest surface topography, followed by nitrogen then argon; the surface roughness parameters for nitrogen and argon are statistically indistinguishable. This is in contrast with previous reports31 that conclude that argon plasmas do not possess the ability to etch PET. The SEM and AFM results presented here indicate that under these conditions all gas and gas mixtures used in this study can etch PET. While etch rates and surface chemistry alterations have been reported for a number of materials,21,23,34,41 few studies describe the morphological results of the process. Nanofibril Formation by Dewetting. At its most basic level, the process of surface roughening occurs by the breaking of primary chemical bonds.25,27-30 Exposure to plasma will induce both chain scission and reaction resulting in relatively polar LMWFs.36 We can consider the product of the etching process to be an unstable, nonwetting thin film of liquid on the PET substrate (Figure 6). At the very limits of our resolution, Figure 2f shows that the base of each fibril in the final etched surface appears to be well-connected to the substrate. Multiple rims branch out into not only the major rim defining the polygon but also into the “crater” of the polygon itself. We can rationalize the development of this morphology in the (41) Friedrich, J. F.; Rohrer, P.; Saur, W.; Gross, T.; Lippitz, A.; Unger, W. Surf. Coat. Technol. 1993, 59 (1-3), 371-393.
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context of Figure 6 as a special case of the dewetting phenomenon observed in the two-phase [polystyrene (PS) films on heated silicon substrates] system of Sharma and Reiter.42 Sharma and Reiter showed that the addition of heat (135-170 °C) to a thin film (
