ARTICLE pubs.acs.org/Biomac
Highly Hydrophobic Electrospun Fiber Mats from Polyisobutylene-Based Thermoplastic Elastomers Goy Teck Lim,† Judit E. Puskas,*,†,‡ Darrell H. Reneker,‡ Antal Jakli,§ and Walter E. Horton, Jr.||
)
Departments of †Chemical and Biomolecular Engineering and ‡Polymer Science, University of Akron, Akron, Ohio 44325, United States § The Liquid Crystal Institute, Kent State University, Kent, Ohio 44242, United States Department of Anatomy and Neurobiology, Northeastern Ohio Universities Colleges of Medicine and Pharmacy, Rootstown, Ohio 44272, United States ABSTRACT: This paper is the first report of electrospinning neat polyisobutylene-based thermoplastic elastomers. Two generations of these materials are investigated: a linear poly(styrene-b-isobutylene-b-styrene) (L_SIBS) triblock copolymer and a dendritic poly(isobutylene-b-p-methylstyrene) (D_IB-MS), also a candidate for biomedical applications. Cross-polarized optical microscopy shows birefringence, indicating orientation in the electrospun fibers, which undergo large elongation and shear during electrospinning. In contrast to the circular cross section of L_SIBS fibers, D_IB-MS yields dumbbell-shaped fiber cross sections for the combination of processing conditions, molecular weight, and architecture. Hydrophobic surfaces with a water contact angle as high as 146 ( 3° were obtained with D_IB-MS that had the noncircular fiber cross section and a hierarchical arrangement of nano- to micrometer-sized fibers in the mat. These highly water repellent fiber mats were found to serve as an excellent scaffold for bovine chondrocytes to produce cartilage tissue.
1. INTRODUCTION A linear poly(styrene-b-isobutylene-b-styrene) triblock copolymer (L_SIBS), a thermoplastic elastomer (TPE) is used in clinical practice as the drug-eluting polymeric coating on the TAXUS coronary stent.1 We have developed a new family of polyisobutylene-based block copolymers (Arbomatrix) with new molecular architectures comprising a branched (arborescent or dendritic) polyisobutylene (PIB) core and end blocks of polystyrene (PS) or its derivatives. Because of the outstanding biocompatibility and biostability of PIB, Arbomatrix is also a candidate for indwelling implant applications. The arborescent PIB core is synthesized by inimer-type living carbocationic polymerization of isobutylene by the 4-(2-methoxy-isopropyl)styrene and 4-(1,2-oxirane-isopropyl)styrene inimers with titanium tetrachloride, after which styrene or its derivatives are added to form the end blocks of the final Arbomatrix polymer.2,3 These novel self-assembling “double” networks have unique properties and were also shown to be biocompatible in a rabbit model.4 ElectroNanospray, a proprietary technology developed at the University of Minnesota, was used to coat coronary stents with selected Arbomatrix polymers loaded with Dexamethasone (DXM), a model drug.5,6 We demonstrated that drug release profiles could be influenced by both the molecular weight of the branched PIB midblock and the spraying conditions of the polymer-drug mixture. Specifically, a smooth coating obtained with an Arbomatrix polymer having Mn = 70000 g/mol and r 2011 American Chemical Society
Mw/Mn = 4.5 had drug particles exposed at the surface (Figure 1a), resulting in a burst release followed by slow release, similar to that reported for the TAXUS coatings.7 Changing the spray conditions led to a particulate coating in which the drug particles were fully encapsulated by the polymer (Figure 1b). This particulate coating did not have an initial burst release, but exhibited slow continuous release over time. Increasing the molecular weight of the branched PIB midblock led to complete encapsulation of the drug particles. In this case, smooth films were not obtained, and the more “continuous” (Figure 1c) and the more “particulate” (Figure 1d) films both had slow continuous release profiles. Based on these encouraging results, we explored the use of electrospun fibers for drug encapsulation and release. Electrospinning produces polymeric fibers with diameters ranging from micrometers to as low as tens of nanometers. In electrospinning, a high voltage applied to the polymer solution creates a Taylor cone8 from which a fluid jet carrying electric current emerges. During its flight in space, this electrically charged fluid jet elongates, thins, bends into coils with a multitude of turns, and finally solidifies to form micro- or nanofibers as the solvent evaporates.8 Over the last two decades, the simple equipment for electrospinning and its applicability to many Received: February 1, 2011 Revised: March 18, 2011 Published: March 31, 2011 1795
dx.doi.org/10.1021/bm200157b | Biomacromolecules 2011, 12, 1795–1799
Biomacromolecules
ARTICLE
Table 1. Materials and Polymer Solutions Used for Electrospinning hard phase content materials L_SIBS
(wt%) 34
polymer viscositya concentrationb
Mn (g/mol) Mw/Mn 78300
1.74
(mL/g) 38.6
(wt%) 5.0
(SIBSTAR
10.0
103T)
15.0 20.0 25.0 30.0
D_IB-MS
31
302100
2.56
129.0
2.5 5.0 7.5 10.0 12.5 15.0
a
Figure 1. (a) Smooth and (b) particulate coatingzs of Arbomatrix loaded with DXM (Mn = 70000 g/mol, PS content = 33 wt%); (c) “more continuous” and “more particulate” coatings of Arbomatrix loaded with DXM (Mn = 220000 g/mol, PS content = 29 wt%).
different polymers have gained widespread acceptance as an inexpensive and versatile nanotechnology to make nanofibers and fiber mats. The state of the electrospinning nanotechnology has evolved from a pure research laboratory endeavor to a commercial level. An exciting area of application for electrospun fibers lies in the biomedical industry: selected synthetic biocompatible polymers can be spun into fiber mats to make stretchable yet breathable wound dressings, flexible scaffolds for cell growth and tissue engineering, and implantable membranes with controlled drug delivery capability. PIB-based block copolymers could be excellent candidates for the preparation of biocompatible electrospun fiber mats. However, Liu et al.9 reported that neat L_SIBS could not be electrospun, attributing this to the nonconductivity of the polymer solution. Single-walled carbon nanotubes (SWNT)9 and iron(III) p-toluenesulfonate10 were introduced to the polymer solutions of L_SIBS to increase the electrical conductivity for spinnability and cell stimulation. Favorable cell adhesion was observed and growth of L-929 mouse fibroblast cells occurred on a fiber mat spun from a solution containing 0.3% (w/v) SWNT and 13% (w/v) L_SIBS.9 The same researchers used a vapor-phase polymerization technique to coat L_SIBS fibers with conductive polypyrrole and demonstrated the cytocompatibility of these coated mats for the neuronal differentiation and growth of PC12 rat adrenal medulla cells.10 The goal of our study was to develop conditions for the electrospinning of neat PIB-based block copolymers to study the encapsulation and release of from the fiber mats. We successfully electrospun L_SIBS and D_IB-MS, which is an Arbomatrix polymer with p-methylstyrene end blocks, producing highly hydrophobic fiber mats that supported the growth of bovine cartilage chondrocyte cells.
2. EXPERIMENTAL SECTION 2.1. Materials. Tetrahydrofuran (THF, Sigma-Aldrich) was distilled from calcium hydride before use, and toluene (99.8%, Sigma-Aldrich) was used as received. D_IB-MS was synthesized and
From size exclusion chromatography. b Solvent: THF/toluene 95/5 (w/w).
characterized as reported11 with 31 wt% (p-methylstyrene) content and Mn = 302100 g/mol with Mw/Mn = 2.56. L_SIBS (SIBSTAR 103T) was obtained courtesy of Kaneka Corporation, Japan. SIBSTAR 103T had 34 wt% polystyrene with Mn = 78300 g/mol with a polydispersity index of 1.74.12 Table 1 outlines the material properties of the two polymers. 2.2. Preparation of Polymer Solutions. A solvent mixture of THF and toluene in 95:5 (w/w) ratio was used to prepare the polymer solutions (L_SIBS: 5, 10, 15, 20, 25, and 30 wt%, and D_IB-MS: 2.5, 5.0, 7.5, 10.0, 12.5, and 15.0 wt%). The details are summarized in Table 1. 2.3. Electrospinning. A positive potential of 20 kV was applied between the polymer solution in a glass pipet and a grounded metal collector covered with aluminum foil or a rotating disk for the collection of a fiber bundle. The distance between the tip of the glass pipet, and metal collector or rotating disk was maintained at 20 cm. During the electrospinning experiment, the measured current at the collector was in the range of 10 15 nA. 2.4. Scanning Electron Microscopy (SEM) imaging. SEM imaging was conducted with a field emission scanning microscope (JEOL JSM-7401F) at an accelerating voltage of 10 kV. The aluminum foil with the fibers was attached onto a SEM stub. The sample was kept in a vacuum oven at room temperature overnight. To improve electron conductivity, the fibers on the stubs were coated, before imaging, with silver sputtered in an argon atmosphere for 1 min. 2.5. Polarizing Optical Microscopy (POM) Studies. POM evaluation of the fiber mats was performed using an Olympus BX60 polarizing microscope both in transmission and reflection modes. 2.6. Water Contact Angle (WCA) Measurement. A contact angle goniometer, Model 500, of the F1 series from Rame-Hart Instrument Co., was employed for the WCA measurement using deionized water. A water droplet of about 4 6 μL was dispensed from a microsyringe onto the sample surface. The profile of a water droplet on the sample was captured with a high resolution camera as a bitmap image that was subsequently postprocessed using the ImageJ software to determine the contact angle. The data reported is the average of three contact angle measurements. 2.7. Cell Culture Study. Chondrocyte cells were isolated from bovine cartilage by enzymatic digestion and used fresh. Chondrocytes (24 106) were seeded onto the aluminum disks of 20 mm in diameter deposited with electrospun D_IB-MS fibers. We needed to use more cells than the usual 5 106 cells to ensure that the hydrophobic fiber surface was covered with the cells. The scaffolds with the chondrocytes 1796
dx.doi.org/10.1021/bm200157b |Biomacromolecules 2011, 12, 1795–1799
Biomacromolecules were maintained for 7 weeks in a 12-well culture dish. Opti-MEM medium (with 50 μg/mL of ascorbate and 100 μg/mL of primocin) were changed thrice in weeks 1 and 2 and twice in subsequent weeks. The scaffolds with the tissue were detached from the aluminum disk, processed, embedded in paraffin wax, microtomed and stained with thionin (0.1%) for 1 min, and finally dehydrated to prepare for gross examination and histology.
3. RESULTS AND DISCUSSION 3.1. Fiber Morphology. Figure 2 shows SEM micrographs of fibers electrospun from L_SIBS and D_IB-MS solutions. The fiber diameter versus polymer concentration plots are shown in Figure 3.
Figure 2. SEM images of (a) L_SIBS (540 ( 60 nm) and (b) D_IB-MS fibers (1490 ( 280 nm) spun from 15 wt% polymer solutions (THF/ toluene = 95:5 w/w).
Figure 3. Effect of polymer concentration on the average fiber diameter.
ARTICLE
The average fiber diameter increased with polymer concentration in both cases, with D_IB-MS producing thicker fibers at the same concentration (Figure 2b) because of its higher molecular weight. At lower polymer concentrations (5 wt% for L_SIBS and 2.5 wt% for D_IB-MS), beaded fibers were produced. Figure 4a is a SEM micrograph of a D_IB-MS fiber at a higher magnification, showing a cleft along the fiber axis. The optical micrograph of a larger diameter fiber (Figure 4b) shows a similar cleft. Additionally, spots are evident in Figure 4b, indicating phases with optical contrast or “dimples” in the skin. Similar “dumbbell-shaped” electrospun fibers of poly(ether imide) (Ultem 1000) in hexafluoro-2-propanol (10 wt%) and PS (Mn = 280000 g/mol) in dimethylformamide (30 wt%) have been observed earlier.8,13,14 It was concluded that fluid mechanical effects, electrical charge carried with the jet, and evaporation of the solvent all contributed to the formation of odd-shaped fibers. The formation of a distinct skin was observed, creating an inward pressure that collapsed the skin. The dumbbell-shape can form when diametrically opposite parts of the skin come into contact, resulting in the formation of two smaller parallel tubes connected by a “bar”, as illustrated by the cartoon in Figure 4c.8,13,14 In these examples from the literature, the “bar” segment of the dumbbell was longer and the “tubes” were more regular than in our case.8,13,14 In contrast, circular cross sections were observed in the electrospun L_SIBS fibers (Figure 4d). We theorize that cleft formation was due to uneven solvent evaporation because of the higher molecular weight (Mn = 302100 g/mol) of the D_IB-MS. Because L_SIBS cannot be produced with Mn higher than 150000 200000 g/mol due to inherent limitations in the polymerization process,15 this theory remains unproven. Figure 5a shows the polarized optical image of a D_IB-MS fiber mat collected on aluminum foil at 20 magnification. With a polarizer inserted between the light source and the sample, bright birefringence can be observed in the fibers with the brightest areas situated close to the fiber surfaces. The aluminum background underneath the fiber mat is seen as black. The fiber mats spun from L-SIBS showed similar birefringence. Figure 5b shows the polarized image of a free-standing electrospun fiber bundle of L_SIBS collected, as described in the Experimental Section. The image is dark within the dotted ellipsoid when the fibers in the bundle are aligned with the axes of either the polarizer or the analyzer. Rotating the L_SIBS fiber bundle around the optical axis of the microscope led to the appearance of bright regions within the dotted ellipsoid. The largest number
Figure 4. (a) SEM micrograph of a ∼0.5 μm D_IB-MS fiber spun from a 10 wt% solution; (b) optical image of a ∼5 μm D_IB-MS fiber spun from a 15 wt% solution; (c) cartoon of the formation of dumbbell-shaped cross section after drying; (d) optical image of a ∼2 μm L_SIBS fiber spun from a 30 wt% solution. Nonpolarized transmitted light was used for images (b) and (d). 1797
dx.doi.org/10.1021/bm200157b |Biomacromolecules 2011, 12, 1795–1799
Biomacromolecules
ARTICLE
Figure 5. (a) Cross-polarized optical images of D_IB-MS fibers spun from a 15 wt% solution. (b) Cross-polarized transmission optical image of a free-standing L_SIBS fiber bundle spun from a 30 wt% solution. The dotted ellipsoids represent the same spot; after rotation, the birefringence appears.
Figure 6. Water droplet on a silver ragwort leaf that has hierarchical fibrous surface morphology.21 Reproduced with permission from ref 21. Copyright 2006 IOP Publishing Ltd. doi: 10.1088/0957-4484/17/20/019.
Table 2. WCA Data water contact material D_IB-MS
classification bulk polymer
L_SIBS
108 electrospun mats
silver ragwort (Senecio cineraria)21
146 ( 3 137 ( 2
L_SIBS fiberb sacred lotus (Nelumbo nucifera)20
95 95
PTFE (untreated)19 D_IB-MS fibera
angle (deg)
natural material
162 147
Average fiber diameter = 1490 ( 280 nm. b Average fiber diameter = 2720 ( 1040 nm. a
of bright spots appeared when the fiber bundle axis was pointed in directions between the axis of the polarizer and the analyzer. Bright spots that appeared at other angles of rotation were associated with local curves, for example, at points where one fiber crossed another. This is the characteristic behavior of a birefringent material. The variation of the brightness of the birefringent spots is consistent with variations in the amount of elongation and the associated local diameter variations along the fiber, which occurred during the electrospinning process. The birefringence indicates orientation/chain alignment. PIB-based polymers undergo strain-induced crystallization, as reported earlier.16,17 These crystallites disappear at room temperature when the deformation is removed. Electrospinning produces a significant amount of elongation and shear in the solidifying polymer jet during its flight in space, which may induce temporary strain-induced crystallization. When the polymer solidifies and the PIB chains will be anchored by the hard phases, local orientation/alignment may be “frozen in” and can thereby contribute to the birefringence observed in these fibers. This explanation is consistent with the findings of Fong and Reneker who observed elongated PS domains (10 nm wide and 100 nm long) in electrospun poly(styrene-b-butadiene-bstyrene) SBS block copolymer fibers by transmission electron microscopy (TEM).18 3.2. Water Contact Angle Measurements. Water contact angles (WCA) were measured on the fiber mats and Table 2 lists the results. The highest WCA was 137° ((2°) for L_SIBS fiber mats spun from a 30 wt% solution (average fiber diameter = 2720 ( 1040 nm), and 146° ((3°) for D_IB-MS mats spun from a 15 wt% solution (average fiber diameter = 1490 ( 280 nm). Both L_SIBS and D_IB-MS fiber mats repel water better than untreated PTFE (WCA = 108°)19 and compare well with
Figure 7. (a) WCA versus average fiber diameter of L_SIBS fiber mats spun from solutions of various polymer concentrations, and water droplet profile on (b) D_IB-MS fiber mat, spun from a 15 wt% solution, and (c) L_SIBS fiber mat, spun from a 30 wt% solution.
naturally occurring sacred lotus leaf (Nelumbo nucifera),20 silver ragwort leaf (Senecio cineraria),21 and the wing of insects.22 These naturally occurring surfaces can support water droplets with a WCA of 150° and higher20 and are referred to as “superhydrophobic”.23 Interestingly, the dumbbell-shaped fiber cross section of the D_IB-MS fibers resembles that of the fibers on the silver ragwort leaf (compare Figure 4a to Figure 6).21 A compendious review of superhydrophobic surfaces based on electrospun polymeric micro- and nanofibers can be found in an article by Ma et al.24 The hierarchical arrangement of micro- and nanostructured surface topology created by the random orientation, shape, and surface texture of collected electrospun polymeric fibers is believed to be the key factor to the hydrophobicity and superhydrophobicity achieved in some cases.21,24 26 In line with this, the electrosprayed surfaces of Arbomatrix materials with 100 500 nm feature sizes shown in Figure 1 had the same WCA as the bulk polymer (95°). We found that the WCA slightly increased with average fiber diameter, a representative plot for L_SIBS is shown in Figure 6a. Interestingly, the WCA of the L_SIBS fiber mat spun from a 5 wt% solution (134° ( 6°) was higher than those spun from 10 25 wt% 1798
dx.doi.org/10.1021/bm200157b |Biomacromolecules 2011, 12, 1795–1799
Biomacromolecules
Figure 8. Optical images of a thin section of thionin-stained D_IB-MS fiber mat after 7 weeks of cell culture and a series of histological treatment at (a) 4 and (b) 20 magnification.
solutions, most likely due to fiber beading. Ma and his co-workers reported a similar phenomenon.24,25 Figure 7b,c presents the profiles of water droplets on these electrospun fiber mats. 3.3. Cell Culture Study. Figure 8 presents the optical images of bovine cartilage chondrocyte cells cultured on a D_IB-MS fiber mat after 7 weeks. This fiber mat was electrospun from a 7.5 wt% polymer solution with an average fiber diameter of 610 ( 170 nm and a WCA of 135 ( 3°. Based on the SEM images, the fiber mat was about 5 μm thick when the cells were seeded on it. As mentioned before, more than a usual number of cells had to be used to let the cells infiltrate into the hydrophobic mat, otherwise the droplets rolled off the mat. After 7 weeks the tissue grew to ∼500 μm. Thionin specifically stains chondrocyte cells, and the tone of the purple color observed in Figure 8 indicates the extent of cell proliferation on the fiber mat. The top layer, shown at higher magnification in Figure 8b, indicates the development of extracellular matrix. The chondrocyte cells, with the nuclei stained as blue dots are encapsulated within the extracellular matrix. The excellent cell growth demonstrates the potentials of using electrospun D_ IB-MS mats as scaffolds for tissue engineering applications.
4. CONCLUSIONS Neat PIB-based block copolymers were electrospun successfully for the first time and yielded fiber mats with highly hydrophobic surfaces. D_IB-MS fibers were thicker than L_SIBS fibers spun from solutions of the same polymer concentration, on account of the much higher molecular weight of D_IB-MS. Optical microscopy revealed birefringent and highly oriented regions in the fibers. Water contact angles of 146 ( 3° for D_IB-MS and 138 ( 3° for L_SIBS were measured on the mats, with the former being comparable to that of the silver ragwort leaf (147°). A 7-week bovine cartilage chondrocyte cell culture study showed excellent cell proliferation and the beginning of extracellular matrix formation. Given the excellent biocompatibility and biostability of these polymers, the findings of this study indicate a high potential of the electrospinning technology coupled with PIB-based biomaterials for biomedical applications. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: 1-330-972-6203. Fax: 1-330-972-5290. E-mail: jpuskas@ uakron.edu.
’ ACKNOWLEDGMENT This work is supported by the National Science Foundation under NSF Grants DMR-0509687 and DMR-0804878 and by
ARTICLE
the Austen BioInnovation Institute in Akron. The content is solely the responsibility of the authors and does not necessarily represent the official views of Austen BioInnovation Institute in Akron. The contributions of Mr. John Friess, an 11th grade high school student from St. Vincent St. Mary High School, Akron, OH, who performed some of the electrospinning trials and water contact angle measurements, and Mr. Paul Pavka, from the University of Akron, who performed some of the microscopy work, are gratefully acknowledged. The authors would like to thank the authors and the publisher (IOP Publishing Ltd.) of ref 21 for granting permission to use the figure in this article.
’ REFERENCES (1) U.S. FDA TAXUS Express2TM Paclitaxel-eluting coronary stent system (monorail and over the wire), 2004; PO30025. (2) Puskas, J. E.; Kwon, Y.; Antony, P.; Bhowmick, A. K. J. Polym. Sci., Part A: Polym. Chem. 2005, 43 (9), 1811–1826. (3) Foreman, E. A.; Puskas, J. E.; Kaszas, G. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 5847–5856. (4) Puskas, J. E.; Foreman-Orlowski, E. A.; Lim, G. T.; Porosky, S. E.; Evancho-Chapman, M. M.; Schmidt, S. P.; El Fray, M.; Piatek, M.; Prowans, P.; Lovejoy, K. Biomaterials 2010, 31 (9), 2477–2488. (5) Puskas, J. E.; Mu~ noz-Robledo, L. G.; Hoerr, R. A.; Foley, J.; Schmidt, S. P.; Evancho-Chapman, M. M.; Dong, J. P.; Frethem, C.; Haugstad, G. WIRE Rev. 2009, 1 (4), 451–462. (6) Puskas, J. E.; Hoerr, R. A. Macromol. Symp. 2010, 291 292, 326–329. (7) Kamath, K. R.; Barry, J. J.; Miller, K. M. Adv. Drug Delivery Rev. 2006, 58 (3), 412–436. (8) Reneker, D. H.; Yarin, A. L. Polymer 2008, 49 (10), 2387–2425. (9) Liu, Y.; Gilmore, K. J.; Chen, J.; Misoska, V.; Wallace, G. G. Chem. Mater. 2007, 19 (11), 2721–2723. (10) Liu, Y.; Liu, X.; Chen, J.; Gilmore, K. J.; Wallace, G. G. Chem. Commun. 2008, 2008 (32), 3729–3731. (11) Puskas, J. E.; Dos Santos, L. M.; Sen, M. Y.; Kaszas, G. Rubber Chem. Technol. 2007, 80 (4), 661–671. (12) Puskas, J. E.; Dos Santos, L. M.; Fischer, F.; G€otz, C.; El Fray, M.; Altst€adt, V.; Tomkins, M. Polymer 2009, 50 (2), 591–597. (13) Koombhongse, S.; Liu, W. X.; Reneker, D. H. J. Polym. Sci., Part B: Polym. Phys. 2001, 39 (21), 2598–2606. (14) Liu, W. X.; Wu, Z. Q.; Reneker, D. H. Polymer Reprints 2000, 41 (2), 1193–1194. (15) Puskas, J. E.; Antony, P.; El Fray, M.; Altst€adt, V. Eur. Polym. J. 2003, 39 (10), 2041–2049. (16) Tanaka, T.; Chatani, Y.; Tadokoro, H. J. Polym. Sci., Part B: Polym. Phys. 1974, 12 (3), 515–531. (17) Kaszas, G. Polym. Mater. Sci. Eng. 1993, 67, 325–326. (18) Fong, H.; Reneker, D. H. J. Polym. Sci., Part B: Polym. Phys. 1999, 37 (24), 3488–3493. (19) Zhang, J. L.; Li, J. A.; Han, Y. C. Macromol. Rapid Commun. 2004, 25 (11), 1105–1108. (20) Neinhuis, C.; Barthlott, W. Ann. Bot. 1997, 79 (6), 667–677. (21) Miyauchi, Y.; Ding, B.; Shiratori, S. Nanotechnology 2006, 17 (20), 5151–5156. (22) Wagner, T.; Neinhuis, C.; Barthlott, W. Acta Zool. 1996, 77 (3), 213–225. (23) Adamson, A. W.; Gast, A. P., Physical chemistry of surfaces, 6th ed.; John Wiley & Sons: New York, 1997. (24) Ma, M.; Hill, R. M.; Rutledge, G. C. J. Adhes. Sci. Technol. 2008, 22 (15), 1799–1817. (25) Ma, M. L.; Mao, Y.; Gupta, M.; Gleason, K. K.; Rutledge, G. C. Macromolecules 2005, 38 (23), 9742–9748. (26) Ma, M. L.; Hill, R. M.; Lowery, J. L.; Fridrikh, S. V.; Rutledge, G. C. Langmuir 2005, 21 (12), 5549–5554. 1799
dx.doi.org/10.1021/bm200157b |Biomacromolecules 2011, 12, 1795–1799
