Chemical and Topographical Modification of Polycarbonate Surfaces

Article pubs.acs.org/Langmuir

Chemical and Topographical Modification of Polycarbonate Surfaces through Diffusion/Photocuring Processes of Hydrogel Precursors Based on Vinylpyrrolidone Alberto Gallardo,*,† Noelia Lujan,† Helmut Reinecke,† Carolina García,† Adolfo del Campo,‡ and Juan Rodriguez-Hernandez*,† †

Instituto de Ciencia y Tecnología de Polímeros (ICTP), Consejo Superior de Investigaciones Científicas (CSIC), C/Juan de la Cierva 3, 28006 Madrid, Spain ‡ Instituto de Cerámica y Vidrio (ICV-CSIC), C/Kelsen 5, 28049 Madrid, Spain S Supporting Information *

ABSTRACT: Facile procedures capable of simultaneously conferring hydrophilicity and tailored topography to surfaces of hydrophobic supports, such as polycarbonate (PC), are very attractive but rare. In this work, we describe a simple methodology to wrinkle PC surfaces after a process of (a) contacting with a photopolymerizable vinylic solution, (b) UV curing of such solutions, and (c) detachment of the formed polymer network, upon swelling in ethanol. The influence of different parameters such as contact lag time between the PC surface and the polymerizable solution, the monomer concentration and type of solvents, as well as the cross-linking degree on the formation of wrinkles, has been studied. The dimensions of the wrinkles can be tailored to some extent by altering the different parameters. Surface chemistry has been analyzed by contact angle measurements and by confocal Raman microscopy. The results are consistent with a chemical alteration of the surface and the formation of an outer hydrogel layer, which is interpenetrated into the PC structure. A mechanism of monomer diffusion and PC swelling that produces surface instabilities and wrinkling is proposed.

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INTRODUCTION Surface chemical and topographical modifications of mechanically robust polymers may modulate the interaction with the environment while the block mechanical properties are preserved. This seems to be particularly relevant in fields such as tribology or biomedicine where resistant materials with tailored surface properties (both chemical and topographical) are desired.1−4 Usually, the two surface modifications, chemical or topographical, are addressed separately. Regarding surface patterning, special attention is currently paid to those approaches that resort to surface instabilities to produce surface patterns without the requirement of expensive equipment.5 As a function of the driving force inducing the surface instability, different strategies have been reported including the breath figure methodology, controlled dewetting, or electrically induced instabilities.5,6 In addition to these methodologies, different groups have reported the formation of singular surface patterns, known as wrinkles, by different external stimuli. For instance, elastic substrates that are predeformed and surfacetreated permitted the fabrication of aligned micrometer-sized waves.7 Other groups used rigid substrates to deposit a material that can either swell8 or undergo a volume contraction,9,10 also leading to out-of-plane deformations with variable morphologies ranging from peanut to labyrinth surface patterns.11 Wrinkled surfaces have found applications in multiple areas © 2017 American Chemical Society

such as templates to create ordered surface arrays, for the fabrication of flexible electronics, or the design of surfaces with controlled wettability and adhesion/friction properties.6,12 However, one of the still remaining issues in the fabrication of wrinkled surfaces involves the variation in their functionality and their extension to other neither elastic nor inorganic supports. In particular, owing to the widespread applications of thermoplastic polymers based on their easy transformation, processability, and low cost, preparation of functional wrinkled structures at their surface would be interesting. In this context, polycarbonate (PC) is an interesting model because it provides processability together with high impact strength, heat resistance, and toughness.13 Besides, and focusing on the chemical modifications, the surface modification of hydrophobic substrates such as PC with hydrophilic coatings is extremely attractive. Surface functionalization of PC is actually a requirement for some of the many biomedical applications of PC (it has been actually used in the design of different biomedical devices such as renal dialysis units, surgical instruments, or cardiac surgery products).14,15 The surface modification of PC has been accomplished by different surface Received: November 17, 2016 Revised: January 18, 2017 Published: January 26, 2017 1614

DOI: 10.1021/acs.langmuir.6b04143 Langmuir 2017, 33, 1614−1622

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Table 1. Photosensitive Precursor Solutions Used Comprising 6 mol of VP (1.265 mL), 2 mol of SPM (492 mg), 0.1 mol % of C2 (3.1 mg), and Different Amounts of Both C1 and Solvents (Mixture of Water and Ethanol)a name C1 (mol %) solvent (mL) a

EtOH water

HYD_1 2.0

HYD_2 2.0

0.735

1.000

HYD_3 2.0 0.245 0.490

HYD_4 2.0 0.490 0.245

HYD_5 0.5

HYD_6 1.0

HYD_7 4.0

0.735

0.735

0.735

The contact times used before photopolymerization were 0, 5, 10, 20, and 30 min.

Figure 1. Setup for the fabrication of PC surfaces modified with VP-based hydrogels, together with illustrative optical profile images of original PC films and a wrinkled PC surface obtained upon photopolymerization of a precursor solution (HYD_1) placed onto the PC surface. radical photopolymerization using HCPK as initiator (0.5 wt %). The reaction mixtures were bubbled with N2 and transferred to PC/ polypropylene molds via syringe. The molds were separated by silicone spacers of 0.5 mm thick. The polymerization was carried out for 40 min under UV radiation (λ = 365 nm) in a UVP ultraviolet lamp (model CL-1000L, 230 V). A summary of the hydrogel types prepared is given in Table 1. The polypropylene covers were removed after photocuring, and the networks formed on the PC substrates were allowed to swell in ethanol until equilibrium was reached. Subsequently, the residual PC substrates were exhaustively washed with ethanol and water to remove any soluble material, and finally they were dried for the analysis. Characterization. Cross-sectional profiles and 3D images of the wrinkled surfaces were characterized using a Zeta-20 True Color 3D Optical Profiler from Zeta Instruments. Static contact angle measurements were recorded using a contact angle goniometer (Tetha, KSV instruments) using the sessile-drop method. In addition to the static contact angle values, advancing and receding contact angle values were measured. A motorized syringe was set to a specific speed to control the volumetric flow rate of the liquid to or from the sessile drop. The mechanism pushed the syringe plunger during the advancing procedure and pulled it during the receding procedure, leading to an increase and decrease in the drop size, respectively. Images of the growing and shrinking drop were then recorded using the computer, typically at a rate of one picture every second. In this study, the advancing and receding processes were repeated at least seven times, taking the system through seven cycles. PC swelling experiments in the different liquid components were performed gravimetrically. The samples were allowed to swell for 3 days to allow equilibrium swelling. The measurements were taken in triplicates. The swelling degree was determined according to the following expression

activation protocols. Pioneer studies take advantage of the use of UV/ozone treatment to create carboxyl groups13 or exposure of the surface to an ammonia plasma to create ammonium groups.14 All mentioned strategies to pattern or chemically modify the surface have advantages and drawbacks, but none of them allows for the combination, in one single step, of both types of modifications to modulate the hydrophilicity and topography of the surface.16 In this contribution, we describe a general approach that simultaneously permits surface wrinkling and hydrogel surface immobilization on PC.

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EXPERIMENTAL SECTION

Materials. Vinylpyrrolidone (VP), ethylene glycol dimethacrylate (C1), sulphopropylmethacrylate (SPM), 1-hydroxyl cyclohexyl phenyl ketone (HCPK), and azobisisobutyronitrile (AIBN) were purchased from Sigma. VP was distilled before use and stored at 4 °C. AIBN was recrystallized from ethanol and stored at 4 °C. The divinylic compound C2 used as a second cross-linker, which is a VP derivative, was synthesized in our laboratory as previously described.17,18 The PC used throughout this study was provided by Orbi-Tech in the form of a 3 mm filament. The molecular weight measured using gel permeation chromatography in dimethylformamide (LiBr) was Mn 3100 g/mol, PD:1.67. Preparation of the PC Films. The PC films were prepared using PC pellets (obtained by cutting the PC filament) that were melted and pressed to form the film using a P200P Collin platen press. The protocol of pressure and time followed to prepare the films was 6 min at 2 bars, 10 min at 70 bars, 15 min at 130 bars, and 4 min at 50 bars. The temperature was maintained at 180 °C during the entire process. Roughness (Sa) of the films was measured using a 3D optical profiler (ZETA-20) and was calculated to be Sa = 0.06. Networks Synthesis. A previously optimized formulation17 was used as a starting point in all cases: water as a solvent, VP and SPM concentrations of 6 and 1 mol/L, respectively, and C2 and C1 molar percentages (with respect to the monomers) of 2.0 and 0.1, respectively. Hydrogels were synthesized in a conventional one-step

Swelling (S , %) =

Wt − W0 × 100 Wt

(1)

where Wt and W0 are the weights of the swollen and dried sample, respectively. 1615


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Figure 2. Evolution of the PC surface topography leading to wrinkle formation as a function of the time elapsed between the HYD_1 solution contact with the PC substrate and the initiation of the photopolymerization step: (a) 0, (b) 5, (c) 10, (d) 20, and (e) 30 min. WAMP: wrinkle amplitude (μm); WP: wrinkle period (μm). White bars indicate the areas selected for the cross-sectional profiles (right). The chemical composition and depth profiles of the polymeric films were determined using confocal Raman microscopy integrated with atomic force microscopy on a CRM-Alpha 300 RA microscope (WITec, Ulm, Germany) equipped with a Nd/YAG dye laser (maximum power output of 50 mW power at 532 nm). The Raman spectra were recorded point by point with a step of 100 nm.

The solution HYD_1 was confined between a transparent cover and the PC substrate using a spacer. The hydrogel, resulting after a lag time of 10 min and a UV−vis photopolymerization step, was submerged in an EtOH solution. Upon swelling, the hydrogel detaches from the PC support, leaving a substrate in which the topography has been significantly modified. Illustrative 3D optical profile images of a nontreated PC surface and after the UV-photopolymerization step are depicted in Figure 1. In contrast to the pristine planar PC surface, the surface of the films obtained after hydrogel removal shows randomly distributed wrinkles. In particular, under the conditions described before, wrinkles with homogeneous dimensions (around ∼27 μm in wavelength and amplitudes of around ∼4 μm) over the entire surface were observed. This particular surface topography on the PC surface must be related to the hydrogel formation and postpolymerization detachment upon immersion in ethanol. It has to be noted that control experiments carried out without the small percentage of 0.1 mol % of the divinyl compound C2 did not result in a homogeneous hydrogel swelling/detachment neither in wrinkles. As it was addressed previously,17 C2 plays a key role not only because of the network properties but also because of its possible participation in links between chains rich in SPM and chains rich in VP. Analogous heterolinks have been described to enhance mechanical properties of true DNs.20 Besides this, an increase in C1 to 4 mol % leads to either a partial detachment or a complete anchoring of the hydrogel to the PC. This influence of C1 on the hydrogel detachment will be addressed later. Finally, control experiments without SPM lead to the formation of weak hydrogels easily broken during the swelling, forming nonuniform and unshaped coatings. The wrinkled surface of Figure 1 resembles the structures described by Guvendiren et al.8 for hydrogels supported on inorganic solid substrates. According to these authors, wrinkle formation is the consequence of a gradual swelling occurring

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RESULTS In a previous work of our group,17 a VP-based polymerizable formulation was optimized to produce, in a single step, highly swellable (90 wt % of water) but robust pseudo-double network (pseudo-DN) hydrogels, which exhibited a unique performance as cell culture supports. They were capable of hosting cells to confluence and afterward induce a rapid cell detachment or cell sheet transplantation through simple mechanical agitation without the need for a superstrate. This formulation included sulfopropylmethacrylate as the second main component as well as two cross-linkers, a dimethacrylate (C1) and a divinyl compound (C2). A narrow compositional window of the four components where the performance as supports for cell culture as well as the mechanical properties was clearly superior was found: a VP/SPM molar ratio of 6:1 and a molar percentage of C1 and C2 of 2 and 0.1%, respectively. Formulations out of these optimized ratios were easily breakable during manipulation. The term “pseudo-DN” refers to the structural tendency of these materials to form DNs, which can be described as interpenetrating polymer networks (IPNs) comprised of two highly asymmetric cross-linked networks.19 In this work, this particular precursor formulation (that has been labeled as HYD_1) is shown to be capable of inducing microstructuration of PC substrate surfaces. The latter was obtained in one single step by UV-initiated polymerization of the monomeric HYD_1 solution deposited on a PC substrate (using the setup depicted in Figure 1), followed by the detachment of the hydrogel upon swelling in ethanol. 1616


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Figure 3. (a) Morphological evolution of the wrinkles as a function of the solvent mixture used (image size: 350 × 250 μm2). Variation in the wrinkle period (b) and amplitude (c) as a function of the exposition time of the monomer solution to the PC substrate.

from the top surface (highly swollen) to the inner material (poorly swollen). With this precedent, it was hypothesized here that the formation of the wrinkles as shown in Figure 1 is somehow related to a PC swelling and therefore to a penetration of the monomer mixture into the outer PC layers. PC actually dissolves in VP and swells 40 ± 1% in C1 (ethylenglycoldimethacrylate). The detachment of the hydrogel upon swelling in ethanol would reveal the wrinkles. To address this hypothesis of wrinkling caused by the initial monomer diffusion into PC, the influence of different parameters that may influence on that process such as solvent type or the exposure time between the substrate and the photopolymerizable solution before UV irradiation, have been studied. Besides, the surface chemical composition has been analyzed: First, the eventual changes on surface wettability were analyzed by advancing and receding contact angle measurements. Second, the eventual changes on chemical surface composition were investigated using confocal Raman microspectroscopy. Influence of Contact Time. To analyze the influence of contact time, the monomer mixture HYD_1 was brought in contact with the PC surface, and the lag time between the contact established and the initiation of UV light irradiation time was varied between 0 and 30 min (note that the sample depicted in Figure 1 was obtained after a lag time of 10 min). As observed in Figure 2, the surface topography gradually varies from a rather planar substrate to a wrinkled surface by increasing the contact time. Short contact times produced only a slight increase in the surface roughness (Figure 2a). However, the surface topography significantly changes when the

precursor solution is kept in contact with the substrate for 5 min or longer. In this situation (5 min), the surface resulting in complete hydrogel detachment revealed the formation of wrinkles with periods around ∼20 μm and amplitudes below 3 μm (Figure 2b). Interestingly, in the period of time observed, that is from 0 up to 30 min, a gradual increase in the wrinkle dimensions was observed. As a result, wrinkles with periods ranging between 19 and 40 μm and amplitudes comprised between 2.6 and 17 μm could be easily prepared just by increasing the time elapsed between the contact of the precursor solution and the UV−vis photopolymerization step. Influence of Solvent. In addition to the contact time, the nature and ratio of the solvents used for the photosensitive mixture may have a strong influence on the surface swelling process of PC. Water and ethanol have been selected for this study. Although PC does not swell in these solvents (less than 1%), this parameter may influence the diffusion process of the compatible components VP and C1. HYD_1 used a small and optimized amount of water as solvent. To address this issue, samples HYD_2 to HYD_4 in Table 1 have been studied, by varying the amount and the nature of the solvent. Wrinkle dimensions (amplitude and period) clearly varied depending on the solvent used. The 3D optical profile images and two additional graphs representing the variation in the wrinkle characteristics as a function of the contact time for different precursor solutions are depicted in Figure 3. Using the HYD_2 solution with a larger amount of water, the wrinkles observed are clearly smaller than those observed using HYD_1. This behavior proved that wrinkle formation is probably related 1617


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Figure 4. (a) Static contact angle measurements and (b) advancing−receding contact angles for seven cycles on a treated and nontreated PC. (c) Optical profile image of an advancing water droplet at the wet−dry interface. The wrinkled PC surface was obtained upon photopolymerization of a precursor solution HYD_1 placed onto the PC surface with an elapsed time of 10 min between the contact and UV irradiation.

values of around 82°, the values measured for the following cycles were around 10°−12°. This interesting observation indicates that in the first cycle the treated interface requires a first wetting to become highly hydrophilic. This high hydrophilicity, observed also during the following cycles, may indicate the presence of hydrogel at the interface. The advancing of the water front during the advancing angle measurements has been imaged using an optical profiler (Figure 4c). The water droplet wets the valleys formed by the wrinkles and advances, forming a thin water layer. Equally, as expected, the receding contact angle on the treated films is very low with values below 10°. Raman Confocal Spectroscopy. Contact angle experiments proved the formation of a hydrophilic surface layer, but they do not provide any information on the surface chemical composition and the depth profile of the treatment. Information on these two aspects was achieved by Raman confocal. Before the investigation of the modified substrates, the differences between the Raman spectra of the polyvinylpyrrolidone (PVP)-based hydrogel and the PC substrate were evaluated (Figure S1). By comparing these two spectra, we observed several characteristic signals. First, the signal found at 1675 cm−1 corresponds to the CO groups of the PVPbased hydrogel. However, the carbonyl functional groups present in the PC provide a Raman signal at 1613 cm−1. In addition, the bands found at 1495, 1457, and 1425 cm−1 correspond to the main-chain methylene deformation of the PVP material. Finally, the band at 944 cm−1 is due to the pyrrolidone-ring breathing mode and those observed at 860 and 768 cm−1 are due to the ring modes.21,22

to the swelling produced by the migration of the monomers to the PC surface. On the other hand, partial substitution of water by EtOH in the prehydrogel solution leads to wrinkles with larger dimensions. Moreover, an increase in the amount of EtOH in the solvent mixture resulted in wrinkles with larger periods and amplitudes. Thus, beyond water addition, incorporation of a further solvent with higher affinity to the substrate allowed us to finely tune the resultant wrinkle dimensions. In particular, as depicted in Figure 3, structured wrinkled surfaces with periods between 10 and 100 μm and amplitudes ranging between 1 and 20 μm were straightforwardly obtained. Contact-Angle Measurements. Regarding the chemical nature of the surface, static contact angle measurements recorded on both unmodified (planar) and modified PC indicated an increase in their surface wettability, that is, the surface upon treatment became more hydrophilic (Figure 4a). A sample of type HYD_1 with 10 min contact time was chosen for this study. More interestingly, advancing and receding angle measurements proved significant surface changes. In Figure 4b, the advancing and receding contact angles of seven cycles observed for treated and untreated substrates are presented. The precursor substrate, that is, pure PC, exhibits advancing contact angle values of around 90°−93° and receding contact angle values of 15°−18°. These values remained constant during all cycles explored. However, for the surfaces treated with the photosensitive mixture and upon hydrogel detachment, the advancing contact angle exhibited significant differences between the first and the following cycles. Whereas, in the first cycle, the advancing water contact angle exhibited 1618


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Figure 5. Above: Representation of the intensity ratio between the signals at 1676 cm−1 observed in the hydrogel and the band at 1615 cm−1 assigned to the PC as a function of the depth for HYD_2 (a−e) and HYD_1 (f−j). Below: Evolution of the Raman spectra as a function of the depth for HYD_2 (k) and HYD_1 (l).

indicating the extent of the surface modification. An increase in the contact time of the photopolymerizable solution on the substrate and the UV−vis irradiation step leads to an increase in the wrinkle characteristics. Finally, to further support the idea that surface wrinkling occurs before the photopolymerization step, the following tests have been carried out. On the one hand, as has been stated above, VP, which is the main component, dissolves PC and ethylene glycol dimethylacrylate (EGDMA) swells PC to 40%. The solvents did not produce any swelling, and the SPM was not tested because of its solid form. In summary, both VP and EGDMA can drive the incorporation of the hydrogel components inside of the PC. On the other hand, the PC substrate was in contact with the monomer solution for 30 min. Then, the liquid monomers and the solvents used were removed from the reservoir, and the UV-photopolymerization was carried out. The surface obtained using this approach clearly proved the formation of surface instabilities in the form of wrinkles that can come only from the fact that monomers are able to diffuse into the PC (see Figure S6). These analyses are consistent with the initial hypothesis of monomer penetration into the outer layers of the PC and the formation of wrinkles because of a surface deformation caused by the gradient swelling, similar to that described by Guvendiren et al. As the contact time increases, the monomer

Raman spectra of the treated and nontreated surfaces (HYD_1 with 10 min of contact time) can be found in Figure S2. By comparison with the Raman spectrum observed for the PC substrate, we clearly observed the formation of a top layer in which the chemical composition is a mixture of PC- and VPbased hydrogels.23 A further interesting feature of confocal Raman microspectroscopy is related to the possibility of obtaining depth profiles that show the variation in the chemical composition from the surface to the PC bulk. For this analysis, the top of a wrinkle hill was used as reference and Raman spectra were recorded at different depths up to 30 μm. The Raman spectra of HYD_1 obtained at different depths at different contact times show a gradual variation in the spectra from a pure hydrogel mainly composed of PVP to pure PC at a depth of 30 μm (see Figure S3). By normalizing the signal at 1615 cm−1 assigned to the CO of the PC, a gradual decrease can be easily observed following, for instance, the bands at 1495, 1457, and 1425 cm−1 due to the main-chain methylene deformation or the band at 1678 cm−1 due to the CO groups of the hydrogel formed. As a result, a comparison of the bands at 1676 cm−1 observed in the hydrogel and the band at 1615 cm−1 characteristic of PC allows us to estimate the variation in the chemical composition and the depth of the modified layer. As depicted in Figure 5, plotting the relationship between the two signals permits the construction of cross-sectional profiles 1619


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Figure 6. Formation of hydrogels based on VP and situations observed upon hydrogel swelling. Two main different possibilities were observed: (a) hydrogels are completely removed, leaving a thin layer of hydrogels on top of the surface and (b) hydrogels remain partially/totally anchored to the surface. (c) Illustrative optical images and 3D optical images of a PC surface after treatment (left) and the complementary hydrogel surface obtained upon drying (right).

between the hydrogel and the PC substrate. This number of anchoring points is strongly related to the cross-linking degree. As mentioned before, an increase in C1 leads to either a partial detachment or a complete anchoring of the hydrogel to the PC substrate (depending on the contact time). However, a value of C1 between 0.5 and 1 mol % (decrease of the cross-linking agent compared to original HYD_1) resulted in a complete detachment of the hydrogel from the PC, leading to the wrinkling described here. Interestingly, the surface of the hydrogel (Figure 6c) also exhibits the formation of wrinkles that are complementary to the wrinkles observed in the PC surface. A hypothetic model has been developed according to these results (see Figure 6). Networks of higher cross-linking density are able to absorb a limited amount of solvents and thus exhibit a reduced swelling (see Figure S4) and minor tensions related to the swelling phenomenon. The number of anchoring points between the interface and the hydrogel increases as well. Therefore, a critical cross-linking degree is proposed, below which there is a bond breaking at the interface and a hydrogel detachment. This is due to the reduced number of anchoring points and to the higher extension of tensions during swelling when compared with cross-linking degrees above the critical point, which are able to keep the full hydrogel anchored.

mixture penetrates deeper into the PC, swelling occurs to a larger extent, and surface instabilities appeared. Photopolymerization forms the VP-based network (actually the integrated hydrogel/PC outer layers form a semi-interpenetrated structure) and “freezes” the surface deformation. The detachment of the hydrogel, finally, reveals the wrinkles at the interface. According to the observations described above using the 3D optical profiler, the wrinkle size increases as the modified layer increases. Thus, the process of wrinkle formation is directly related to the extent of swelling. Figure 5 also shows the relevance of the monomer concentration by comparing HYD_1 with HYD_2. Whereas HYD_1 is prepared using 0.735 mL of water, HYD_2, containing 1 mL of water, is less concentrated. First of all, as depicted in the cross-sectional profiles and summarized in the graphs (Figure 5k,l), the profiles indicate a larger diffusion of the hydrogel precursor components by increasing the monomer concentration. Whereas HYD_2 exhibited monomer penetration profiles with thicknesses below 15 μm, HYD_1 proved that the surface region affected is above 25 μm thick. Hydrogel Detachment. In the previous scenario of gradient swelling and surface deformation, wrinkles become visible if the hydrogel is able to detach at the deformed interface. On the basis of the explanation provided by Yuk et al.,24 hydrogel detachment may be related to the tensions that originated at the surface upon hydrogel swelling and is influenced by the number of anchoring points established

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CONCLUSIONS The fabrication of wrinkled surfaces decorated with VP-based hydrogels has been described. A selected mixture of monomer/ 1620


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cross-linking agents/solvent when in contact with PC diffuses and swells the polymer surface. As a result, upon photopolymerization and further hydrogel swelling in EtOH, a controlled hydrogel detachment takes place, leaving a thin hydrogel layer at the PC surface. This thin hydrogel layer is consequence of the mentioned partial diffusion, and as a result, swelling of the PC surface by the hydrogel precursor. The diffusion observed has two simultaneous consequences. First, the surface chemical composition of the PC is altered and a surface with a larger hydrophilicity is obtained. Second, diffusion and swelling of the PC surface induce surface instabilities that finally result in the formation of wrinkled surfaces. Interestingly, by modifying the composition of the precursor solution and the contact time, a reasonable control over the wrinkle characteristics (period and amplitude) is obtained. This fabrication strategy in which thermoplastic supports can be modified with hydrogels is currently being evaluated for different applications including the fabrication of platforms with reduced friction and their use as cell culture templates. Complementary wrinkling on the detached hydrogel and its influence on in vitro cell response will be studied next.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b04143. Raman spectra of the PC used as a substrate for the hydrogel formation, a pure hydrogel constructed from VP, and the modified surface resulting upon photopolymerization and hydrogel detachment. Raman confocal spectra of a PC surface treated with a photopolymerizable monomer mixture. Hydrogel swelling as a function of the solvent used and the amount of crosslinking agent used. Optical image and 3D image of a pristine PC surface together with roughness measurement (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.G.). *E-mail: [email protected] (J.R.-H.). ORCID

Alberto Gallardo: 0000-0003-4614-4299 Juan Rodriguez-Hernandez: 0000-0003-2464-1040 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge support from the Consejo ́ Superior de Investigaciones Cientificas (CSIC). Equally, this work was financially supported by the Ministerio de Economiá y Competitividad (MINECO) through MAT2013-47902-C2-1R, MAT2013-42957-R, and MAT2016-78437-R.

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REFERENCES

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Chemical and Topographical Modification of Polycarbonate Surfaces

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