Robust and Tailored Wet Adhesion in Biopolymer Thin Films

Oct 21, 2014 - KTH Royal Institute of Technology, †Fibre and Polymer Technology and ‡Wallenberg Wood Science Centre, Teknikringen 56, SE-100 44 St...
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Robust and Tailored Wet Adhesion in Biopolymer Thin Films Torbjörn Pettersson,*,†,‡,§ Samuel A. Pendergraph,*,†,§ Simon Utsel,† Andrew Marais,† Emil Gustafsson,†,‡ and Lars Wågberg*,†,‡ KTH Royal Institute of Technology, †Fibre and Polymer Technology and ‡Wallenberg Wood Science Centre, Teknikringen 56, SE-100 44 Stockholm, Sweden S Supporting Information *

ABSTRACT: Model layer-by-layer (LbL) assemblies of poly(allylamine hydrochloride) (PAH) and hyaluronic acid (HA) were fabricated in order to study their wet adhesive behavior. The film characteristics were investigated to understand the inherent structures during the assembly process. Subsequently, the adhesion of these systems was evaluated to understand the correlation between the structure of the film and the energy required to separate these LbL assemblies. We describe how the conditions of the LbL fabrication can be utilized to control the adhesion between films. The characteristics of the film formation are examined in the absence and presence of salt during the film formation. The dependence on contact time and LbL film thickness on the critical pull-off force and work of adhesion are discussed. Specifically, by introducing sodium chloride (NaCl) in the assembly process, the pull-off forces can be increased by a factor of 10 and the work of adhesion by 2 orders of magnitude. Adjusting both the contact time and the film thickness enables control of the adhesive properties within these limits. Based on these results, we discuss how the fabrication procedure can create tailored adhesive interfaces with properties surpassing analogous systems found in nature.

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INTRODUCTION Layer-by-layer (LbL) assembly has been demonstrated as a versatile method to tailor interfacial properties. The parameters dictating the film growth, intermolecular interactions, surface charge, and swelling in LbL thin films have been well characterized over the past two decades.1 These material characteristics can be controlled by the molecular weight and the composition of the electrostatic functional groups. One unique attribute of LbL films versus conventional polymer films is the ability to tune the strength of intermolecular interactions within the film as well as the surface charge through modification of the solvent (e.g., change in electrolyte concentration or pH).1−11 The range of control possessed in the material properties and geometry of LbL assemblies has subsequently motivated research in a range of fields including electronics, gas barriers, and drug delivery vehicles.12−17 Naturally occurring biopolymers have been extensively studied as a source for polyions in LbL assemblies.3,18,19 Some of the advantages of natural polyions are their abundance and compatibility for biomedical applications. Hyaluronic acid (HA) is one that has been given significant attention due to its in vivo compatibility. This polysaccharide has been implemented and demonstrated to be an effective polyanion toward biomedical applications.3,18 The majority of all LbL studies for biomedical applications have, however, been focused on the use as scaffolds and drug releasing components and not toward other structural applications. © 2014 American Chemical Society

Several interesting studies that have been conducted with the use of naturally occurring polyelectrolytes were focused on the adhesion between bone substrates using collagen by varying the electrolyte content and type of cationic counterions of the surrounding solution.20,21 The proposed mechanism for the increase in critical adhesive force and work of adhesion was the formation of sacrificial bonds, which can easily break, dissipate energy, and subsequently reform. Adhesive forces between these natural polyelectrolyte coatings acted over a significantly larger distance than the length of the polymer chain used or the thickness of the thin films. The literature on wet adhesion between LbLs is limited but the importance of charge interactions between oppositely charged LbLs has been identified by colloidal probe AFM measurements.22 Previous work from our group has also demonstrated that the macroscopic critical tensile stress and strain of fibrous networks can be increased significantly when formed from LbL treated cellulose fibers modified with poly(allyl amine) hydrochloride (PAH) and HA.19 These fibrous networks are formed under wet conditions and then dried, which means that the wet adhesive properties of the LbL films are important for the formation of strong dry fiber/fiber joints and for superior properties of the dry fibrous networks. However, the mechanism behind the improved wet adhesion is not entirely Received: August 15, 2014 Revised: October 19, 2014 Published: October 21, 2014 4420

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U.K.) was used to study the multilayer build-up in the wet state.24 All polymer solutions had a concentration of 100 mg/L and a continuous flow of 100 μL/min was used. Nitrogen-doped silicon chips were used as deposition substrates. Quartz Crystal Microbalance. A quartz crystal microbalance with dissipation monitoring (QCM-D) E4 (Q-Sense AB, Västra Frölunda, Sweden) was used to study the multilayer build-up with a continuous flow of 100 μL/min.25 The substrates were AT-cut quartz crystals with an active surface of sputtered silicon oxide and they were cleaned in the same way as the silicon wafers, described above. The change in frequency is equivalent to the adsorbed layer that contains both polymer and water coupled to the adsorbed layer.26 Earlier work has shown that this model is comparable to more advanced models for layers with higher dissipation.27 Atomic Force Microscopy. An atomic force microscope (AFM) MultiMode IIIa (Veeco Instruments Inc., Santa Barbara, CA) was used both for imaging and for adhesion measurements. For tapping mode imaging in air, an EV scanner was employed using standard noncontact mode silicon cantilevers with a spring constant in the range of 2.5 to 10 N/m (TAP150, Bruker, Camarillo, CA). The colloidal probe force measurements were performed by capturing normal force curves in aqueous solution28,29 using a PicoForce extension and a PF scanner. For the force measurements, tipless rectangular cantilevers (NSC12, MicroMasch, Madrid, Spain) approximately 110 μm in length and 35 μm in width, and with normal spring constants in the range of 3.5−12.5 N/m, were used. The exact values of the spring constants were determined by a method based on thermal noise with hydrodynamic damping using the AFM tune IT v 2.5 software (Force IT, Sweden).30 The thermal frequency spectra of the cantilevers were measured at room temperature with no particles attached.31 A silica particle (Thermo scientific, CA) with a diameter of approximately 10 μm was attached with the aid of a manual micromanipulator and an Olympus reflection microscope, to the end of the tipless cantilever, using a small amount of a twocomponent epoxy adhesive (Strong epoxy rapid, Casco). Time in contact between LbL-treated surfaces was varied by changing the delay time between approach and separation in the range of 0−10 s. The force and displacement measurement data were analyzed with AFM Force IT version 2.6 (ForceIT, Sweden) and with a custom MatLab program.

clear,23 and the wet adhesive properties of one LbL system, consisting of PAH/HA assembled in two different electrolyte concentrations, have been studied in this work to clarify the mechanism behind the extensive strength improvement. Lutkenhaus and co-workers have recently shown that by introducing a critical electrolyte concentration into a polyelectrolyte solution, LbL assemblies can exhibit a more viscoelastic behavior.2 This study also highlighted the ability for the chains in the LbL assembly to transition from a brittle material with no detectable thermal phase transition to a material with a detectable glass transition temperature (Tg) by adding a higher salt concentration to the assembling polyelectrolyte solutions.2 In the present work, we studied the properties of a (PAH/ HA) LbL assembly. Specifically, the multilayers were characterized to observe the increase in adsorbed amount per added layer, dissipation and film morphology through dual polarization interferometry (DPI), quartz crystal microbalance with dissipation (QCM-D), and atomic force microscopy (AFM). Next, we tested the wet adhesion of these films for different dwell times and number of polyelectrolyte layers. We demonstrate the parameters that affect the wet adhesion of two LbL-coated surfaces. By adjusting the assembling electrolyte concentration, the critical pull-off force and the work of adhesion could be increased by one and two orders of magnitude, respectively. Through the addition of electrolyte during the film formation, we achieved work of adhesion to separate the treated surfaces to be 20× greater than the work of adhesion found in previous studies with collagen and bone.20,21

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EXPERIMENTAL SECTION

Materials. PAH and HA were both provided by Sigma-Aldrich, and the molecular weights were 15 kDa and 1.6 MDa, respectively. Ethanol (96%) was provided by VWR and sodium chloride (NaCl) (analytical grade) was provided by Merck. Ultrapure water (Milli-Q) was used in all experiments. Layer-by-Layer Deposition. Silicon wafers, with naturally occurring silicon oxide surfaces, were used as substrates for AFM imaging and force measurements and were prepared and cleaned according to the following procedure. A silicon wafer (p-type, MEMC Electronic Materials, Novara, Italy) was cut into pieces which were rinsed with Milli-Q water, ethanol, and Milli-Q water and then blown dry with N2. The silicon wafers were then placed in an air plasma cleaner (Model PDC 002, Harrick Scientific Corporation, NY, U.S.A.) under reduced air pressure for 120 s at high effect (30 W), after which the wafers were ready for use. The AFM imaging was performed with silicon wafers dipped in polymer solutions with or without NaCl and rinsed with Milli-Q water between the deposition steps. All polymer solutions had a polymer concentration of 100 mg/L and the system with salt had a NaCl concentration of 10 mM. The unadjusted pH of the polymer solutions was 4.9 for PAH and 5.8 for HA. Deposition times were 10 minutes in polymer solutions with 5 min rinsing in Milli-Q water after each adsorption step. The surfaces were dried using nitrogen after the final step. The AFM force measurements were performed with silicon wafers as flat substrates, and silica spheres 10 μm in diameter (Thermo Scientific, CA) were used as probes. The multilayers were built-up in situ in the AFM simultaneously on both the colloidal probe and the silica wafer surfaces using the same adsorption procedure as described above, except that the rinsing was performed with a solution of 10 mM NaCl in Milli-Q water for the system with salt and no drying was performed during the build-up. Great care was taken not to have any free polyelectrolyte in solution during the force measurements and only symmetric LbLs were evaluated in the wet adhesion measurements. Dual Polarization Interferometry. A dual polarization interferometer (DPI) Analight Bio200 (Farfield Sensors Ltd., Manchester,

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RESULTS AFM Imaging. AFM imaging was performed with dry LbLtreated silicon wafers and typical height images of different layer numbers are seen in Figure 1. The pH values of the polyelectrolytes were 4.9 and 5.8 for PAH and HA, respectively, which corresponded to an unmodified solution of 100 mg polymer to 1 L of water. There was a difference in the surface structure between the PAH/HA systems assembled with and without salt. In the absence of NaCl, there was no visible surface structure after one layer of PAH was deposited. As more polyelectrolyte layers were added, an increasing density of structures appeared. The amount of structures increased with the number of polyelectrolyte layers but they still appeared as discrete features without any coalesced structures on the surface. When the films were formed in the presence of a supporting electrolyte concentration of 10 mM NaCl, there was no visible structure after one deposited layer of PAH. When the second layer (HA) was deposited, individual structures appeared, different from the prior assembly without NaCl and after the third layer a connected structure of polymeric structures was detected on the surface. These structures appeared to fully merge when the adsorption was continued from 4 to 10 layers. Smaller height differences were observed when PAH was in the outermost layer compared to when HA was in the outermost layer. This variation in film asperities indicated a change in the film structure. 4421

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However, there was a clear decreasing trend in RI with increasing number of layers. The incremental decrease per assembled layer was lower with each added layer approaching an RI value of 1.40. An odd−even effect was also observed from five layers and upward with 10 mM NaCl addition. A lower value in the RI was correlated with the increasing thickness of the film. QCM-D was used to determine the change in the frequency and dissipation associated with the adsorbed films, shown in Figure 3. The decrease in frequency provided information on the adsorption of the polyelectrolyte and the immobilized liquid in the film, whereas the dissipation measured viscoelasticity of the film. QCM-D data revealed a relatively low and linear decrease in frequency with increasing number of layers for films prepared in the absence of NaCl, indicating a linear growth of the films. The dissipation was relatively stable at a low value after the rinsing steps throughout the build-up, which indicated a rigid film with low viscous losses.2 Assembling the films in 10 mM NaCl induced a substantially larger decrease in frequency caused by greater adsorption onto the surface. When 10 layers were deposited, the change in frequency was 22 times higher compared to the system without NaCl, despite the still relatively low ionic strength. There was also an increase in dissipation with layer number, which was not observed for the films assembled without salt. An odd−even effect was again observed for the 10 mM NaCl experiments where the layers with PAH in the outermost layer had higher dissipation values than those with HA in the outermost layer. AFM Force Measurements. AFM force measurements were performed to evaluate the wet adhesion properties of the LbL films. Multilayers were assembled in situ in the AFM, simultaneously on both the silicon wafer and the silica particle that was used as the adhesion probe. All excess polyelectrolyte was removed before initializing the force measurements. The dwell time (i.e., the time the surface was purposely left in contact before separation) was varied between the measurements and typical force curves during separation at different positions on the samples are shown in Figure 4. Based on the earlier results in Figures 1−3, two different systems and two different ionic strengths were selected for the adhesion measurements at different dwell times. One consisted of five and six layers with PAH and HA in the outermost layer, respectively. In addition to evaluating the contact time, one series with a different number of layers at a constant dwell time of 10 s was investigated for both ionic strengths. In Figure 4a,b, the normalized pull-off force curves are shown for five layers assembled without and with 10 mM NaCl,

Figure 1. AFM height images of up to 10 layers (layer number denoted above or below the picture) without NaCl (a) and with 10 mM NaCl (b). The images are 2 × 2 μm2 and the z-range indicated in the scale bar to the right in the figure is 30 nm. The imaging was performed in air under ambient conditions.

Polyelectrolyte Adsorption and Film Formation. Adsorption studies were performed using DPI and QCM-D to observe the LbL build-up. Specifically we wanted to quantify the amount of both solid polyelectrolyte and immobilized water at the interface. DPI measurements result in refractive index (RI) and thickness of the adsorbed layers and the experimental results are shown in Figure 2. Films assembled without NaCl showed a linear growth of the film thickness as a function of layer number throughout the assembly. Furthermore, the RI was relatively constant at higher layer numbers. For films formed in 10 mM NaCl, there was a nonlinear increase in thickness with the number of deposition steps. After 10 layers had been deposited, the film thickness was over five times higher than the system without NaCl. For more than three layers there was an odd−even effect for the film growth in the presence of NaCl, where multilayers containing PAH in the outermost layer had a lower thickness and conversely HA had a larger thickness. The measured RI was initially higher for the system with 10 mM NaCl than for the one without NaCl.

Figure 2. DPI data (a) showing thickness and RI (b) for 10 layers of PAH/HA with and without salt. The lines in the left graph are intended merely as a guide to the eye, demonstrating a linearly growing LbL and a nonlinearly growing LbL. 4422

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Figure 3. (a) QCM-D data from the third overtone showing the normalized frequency change. The fitting is presented to be a guide to the eye to differentiate the growth of the films. (b) Dissipation (right graph) for 10 layers of PAH/HA at 0 M and 10 mM NaCl.

Figure 4. Force/displacement curves during separation of the colloidal probe and flat surfaces using different numbers of LbL treatment steps. Graphs (a) and (b) show the influence of contact time for 5 layers in the systems, that is, with PAH in the outermost layer, without (a) and with salt (b), respectively. Graphs (c) and (d) show the influence of contact time for six layers in the systems, that is, with HA in the outermost layer, without (c) and with 10 mM NaCl (d), respectively. Graphs (e) and (f) show the influence of layer number at 10 s contact time in the systems without (e) and with 10 mM NaCl (f), respectively.

10% compared to the five-layer film with PAH on the periphery for assemblies without NaCl and with NaCl, respectively (Figure 5). The force−separation curves in Figures 4e,f and 6 were aimed at evaluating the effect of number of adsorbed layers on pull-off adhesion force for a constant contact time. In Figure 4e (no NaCl addition), the force increased until five layers were added. Adding another layer decreased the film pull-off force by 25%. The pull-off force then showed a minor increase when 9 and 10 layers were added; however, the adhesion was not

respectively. In both examples, the adhesion increased with increasing contact time up to 10 s. When NaCl was present during the assembly, the adhesion at a contact time of 10 s contact time increased by a factor of 8 compared to the same number of layers without salt. In Figure 4c,d, the procedure was repeated for the six-layer LbL assemblies formed in no salt and in 10 mM NaCl, respectively. Similarly, the six-layer film had a factor of 10 greater adhesion than the same film made without salt given equal contact time. The average critical pull-off force for the film capped with HA decreased, on average, by 40 and 4423

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Figure 5. (a) Maximum normalized pull-off force as functions of contact time for five and six layers. (b) Corresponding plots for work of adhesion vs contact time.

Figure 6. (a) Maximum normalized pull-off force as a function of layer number for a contact time of 10 s. (b) Work of adhesion as functions of layer number for a contact time of 10 s.

Figure 7. (a) Displacement at 10% of maximum pull off force vs contact time. (b) Displacement at 10% of maximum pull off force vs number of polyelectrolyte layers, with a constant dwell time of 10 s.

higher than for five assembled layers (Figures 4e,f and 6). Contrary to this trend, when 10 mM NaCl was added to the solutions during LbL formation, the pull-off forces continued to increase with increasing number of layers. The maximum normalized force required for separation was the highest at nine layers, 18.9 ± 0.9 mN/m (Figure 5). The distances over which adhesive interactions were observed were also strongly dependent on electrolyte concentration. In order to consistently measure the distance where adhesive forces were significant, we set a cutoff displacement value where 10% of the maximum separation force was obtained on separation after the maximum pull-off force was passed. In Figure 7, this value is presented as a

function of layer number and contact time, with and without salt. Force values that approached 10% of the maximum force were found at much greater distances when HA was in the outermost layer (>2 μm). In previous literature, similar behavior in the force−displacement curves has been reported as reattachment of electrostatic linkages of single chains.20 However, by setting this limit, a consistent criterion could be applied to where there was overall separation of the LbL assembly rather than points where the top layer would reattach. There was little change in displacement for samples without salt as a function of contact time and number of layers. The presence of 10 mM NaCl in the system resulted in significantly larger displacement values for 10% of the maximum pull-off 4424

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different topographical characteristics. The surface morphologies suggested differences in the structure of the film. The RI showed a relatively linear, slightly decreasing, trend for the system without salt. This showed that the structure of the multilayer did not change significantly with increasing number of layers. For the system with salt, a nonlinear decrease was observed, indicating structural changes during the build-up. In the first layer, the RI was relatively high, compared to the system without salt, indicating a more compact polymer structure adsorbed on the surface. When HA was added to the system, the RI was suppressed indicating a more porous network within the film. In Figure 2a, the thickness increase per layer is reported for the LbL assemblies. The assembly without NaCl produced a linear film growth, which was consistent with other reports on LbL films at similar electrolyte concentrations.2,7 When the film formation was performed with 10 mM NaCl, a nonlinear growth was observed. High mobility of at least one component inside these multilayers has been suggested as a mechanism for nonlinear growth.1,2,34,35 However, it has also been proposed that increasing roughness on the surface of multilayers is the main cause of nonlinear growth and different models for this have been suggested, for example, the “Island model”.36 The initial nonlinear region then transferred into a linear region at a certain point due to a leveling-off in roughness induced by steric limitations. This behavior has also been observed in earlier experimental data for nonlinear LbL systems.34,35 The formation of a more complexlike structure at higher ionic strengths at the interface was also in accordance with Kovacevic et al.7 This study suggested that the lower interaction between the polyelectrolytes, at this ionic strength (10 mM), enabled rearrangement of the polyelectrolytes. This created a structure more similar to polyelectrolyte complexes.37 These types of similarities between assembled polyelectrolyte layers and polyelectrolyte complexes have also been discussed and described by Sukhishvili et al. who found large changes in the mobility of the layers as a function of a change in pH of the solution during LbL and complex assembly.38 The odd−even effect was observed for LbL films assembled in the presence of NaCl. After two layers, there was a difference in the film roughness and RI and relatively high differences in the frequency change, film thickness and dissipation were observed when the top polymer layer was varied. The film roughness, film thickness, and frequency all increased when HA was in the capping layer, while the RI decreased. We attribute this change with the top HA layer containing water which leads to this increased in mass, change in frequency and a subsequent decrease in RI. When then PAH was added, the roughness and frequency change was attenuated and the RI increased, again indicating desorption of water from the LbL assembly and the formation of a more compact assembly. Previous work from our group on thermoresponsive materials have shown that when thermoresponsive polymers were able to absorb water, higher dissipation in QCM-D was observed due to the increased mobility of the polymer chain.39 Conversely, when the polymer was heated above the lower critical solution temperature (LCST), the dissipation decreased due to removal of water and the reduced mobility of the chains.39 Recently, the thermal transitions of dried and hydrated LbL assemblies were also examined after assembling films in varying electrolyte concentrations.2 Similar to our observations, when LbL films were assembled without NaCl, the film growth was linear while it was nonlinear with NaCl.

force. The displacements observed for the samples with salt did increase as a function of contact time. For five-layer films, the average distance to where 10% of the critical pull-off force was observed increased by 75% from a contact time of 0−10 s. When the subsequent layer was added (HA), the film showed a similar dependence on contact time, where the average distance increased by 85% from 0−10 s. The displacement over which adhesive forces acted increased by almost a factor of 4 and a factor of 10 for the LbL films with salt compared to without salt for five and six layers, respectively. The same observation was made when the contact time was held constant and the number of layers was varied. In the examples without salt, there was relatively no change in the displacements as a function of number of layers in the film (Figure 7). However, when the films were assembled in the presence of 10 mM NaCl, the displacements increased substantially. There was a significant jump in the adhesion between one and two layers. The adhesion was relatively constant when more layers were added to five and six layers. After nine and ten layers were added, the displacement at 10% of the maximum force was over double the displacements at five and six layers. One difference in this series was the slight decrease in adhesion when PAH was the top layer, compared to six layers when the PAH had the higher displacement. Although at 10%, the force decreased from nine to ten layers, the sample with ten layers had adhesive interactions with breakage and reforming of adhesive contacts at distances of over 8 μm. The work of adhesion, calculated from the separation curves, contrasted significantly between the two samples. Similar to the critical pull-off force, there was little increase in the work of adhesion for the samples prepared in the absence of NaCl as a function of contact time for both five and six layers (Figure 5). Films assembled with 10 mM NaCl, on the other hand, displayed an increase in work of adhesion for increasing time, up to 10 s. The effect of the addition of NaCl was observed as well for LbL assemblies of different thicknesses (Figure 6). For one and two layers, the samples had comparable work of adhesion. However, when the samples were grown to five and six layers, there was over an order of magnitude difference in the work of adhesion. The trend continued, similar to the increasing displacements at 10% of the maximum force where the work of adhesion increased to 100.6 ± 4.7 × 10−15 J and 83.0 ± 8.7 × 10−15 J for nine and ten layers, respectively.

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DISCUSSION Adsorption and Structure During Multilayer Build-Up. The AFM height images showed a difference in structure between the films assembled with and without NaCl. Roughness was apparent in both cases with increasing layers attached. This surface-induced structuring has been observed previously for polyelectrolyte complexes from LbL systems.32,33 In our study, there was no initial roughness for the LbL assembly from a solution without NaCl. Small, particulate roughness appeared in the film with higher layer numbers. The roughness (indicated by Rq in Figure S1) of the film was discrete and did not coalesce with increasing thickness. Furthermore, the roughness of the film increased with layer numbers until about eight layers, where the Rq appeared to be independent of number of layers (Figure S1). Films assembled in the presence of salt had structures that were aggregated from three layers and higher, similar to previous studies.7 The films with salt showed higher roughness (Figure S1) over all and had 4425

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Conversely, the films were found to display a brittle, glassy behavior when assembled without NaCl and contain more free volume, according to QCM-D, when NaCl was present in the film preparation. The increased free volume and extrinsic bonding in the films assembled with NaCl, observed from QCM-D, was expected to allow chains to create better contact with another similarly fabricated surface. Therefore, we anticipated differences in the adhesive behavior from the different assembling conditions, due to the discrepancy in the film mobility. Wet Adhesion between LbL-Treated Surfaces. AFM force measurements were performed in order to investigate the adhesive properties of the different multilayers. The contact time was first evaluated to investigate the effect of contact on adhesion of these surfaces. Regardless of salt concentration, both LbL assemblies exhibited an increase in adhesion with increased contact time. This was expected, as an increased time in contact will allow polymer chains to flow and initiate contact with the adjacent surface.40 The multilayers assembled without NaCl displayed little increase in pull-off force when the contact time was increased beyond one second. We suggest that this was due to the film possessing lower tack, indicated by the low dissipation in QCM-D when the salt concentration is low in the assembling solution.7 In the presence of salt, the adhesion increased significantly with increasing contact time, where the longest contact time resulted in the largest pull-off force. The adhesion increased for both the five and six-layer films, demonstrating that despite the larger surface roughness compared to films not formed in NaCl, assemblies in 10 mM NaCl were relatively similar for the fifth and sixth layer. The maximum pull-off force for the film was also up to an order of magnitude higher than for the films that were created without salt. Varying the contact time gave controllable pull-off strength over an order of magnitude within a time interval of 10 s. After evaluating the time in contact for five and six layers thick films, we also examined the effect of the film thickness on the adhesion of the LbL assemblies with a constant contact time of 10 s. The multilayers assembled without NaCl had an increased adhesion with increasing thickness up to five layers. The highest adhesion of the films was observed at five layers, where the Rq of the film was less than 2 nm. When the film with six layers was tested without salt, the adhesion began to decrease, which was commensurate with a 90% increase in the measured Rq of the film. Beyond these layer numbers, the critical pull-off force was relatively unaffected by increasing film thickness. LbL films assembled in 10 mM NaCl displayed a dependence on the film thickness to the critical pull-off force and work of adhesion. For the second layer, the normalized pull-off force was 1.7 ± 0.4 mN/m. However, the normalized pull-off force increased by nearly an order of magnitude to 16.4 ± 0.3 and 14.9 ± 1.1 mN/m for five and six layers, respectively. Despite the thicker films becoming rougher compared to the assembly without salt, higher adhesive strengths were present. A final layer of PAH resulted in higher adhesive force and this observation suggested that PAH was the more mobile component of the film. We also suggest that this mobility along with the more labile chains from the nonlinear growth allow films to interpenetrate into each other and form electrostatic and nonelectrostatic proton donor/proton acceptor contacts, as described by Leibler et al.41 Both hydrogen bonding hydroxyl groups and electrostatic moieties were present to participate in similar intermolecular interactions.

At higher salt concentrations, these nonionic interactions have a larger influence on the properties of the formed films due to the electrostatic screening than at low salt concentrations where the electrostatics will dominate. The thickest films (nine and ten layers) gave the strongest adhesion of the multilayers. The maximum pull-off force was obtained when PAH was in the peripheral layer. When HA addition followed, the critical pulloff force decreased 4% to 18.1 ± 1.4 mN/m. In nine and ten layers, the nonlinear growth created a higher critical pull-off force of the LbL assemblies. Films assembled in NaCl not only displayed higher critical pull-off forces, they also possessed larger displacements where adhesive forces were present. The maximum displacements over which the 10% adhesive interactions were observed were 2.09 ± 0.15 μm, which is approximately half of the contour length of the HA used in this study.42,43 The films assembled without NaCl were adhesive with 10% displacement lengths less than the radius of gyration of an HA molecule.42,43 It was expected that the displacements would not change substantially for the samples not assembled in salt since there was little increase in the pull-off force and the displacement over which adhesive forces were acting on the sample. The QCM-D data showed low dissipation, which was suggestive that there was less interpenetration of the chains from one LbL film to another in the absence of NaCl. This diminished interpenetration also resulted in a lower degree of intermolecular overlap between electrostatic functional groups. The mobility of the polymer chains appeared to be an important factor for determining the distance where adhesive forces were active. However, the normalized force−displacement data suggests that the molecular weight of the components was also important. When HA was the outermost layer in the examples with 10 mM NaCl, the long-range chain pulling interactions (>2 μm) were more pronounced. Furthermore, when the final polyelectrolyte added was HA, the adhesive chain pulling pattern acted over 100% farther than when PAH was the last polymer in the film. Modification of the molecular weight in the outermost layer allows for the molecular interactions to act over a different range of distances. The work of adhesion for the samples followed a similar trend where the addition of salt increased the adhesion. This was expected since the separation force and the displacement of samples to 10% of the separation force, both increased with the addition of salt. The film with nine layers had the highest work of adhesion, which was commensurate with the largest displacement and pull-off force. In the films up to six layers, the work of adhesion appeared to correlate stronger with the increase in the pull-off force of the interface. However, when subsequent layers were added, the increase in work of adhesion was increasingly related to the ability of the film to undergo longer displacements. More studies are needed, however this could suggest that there is a transition between thinner films, where the work of adhesion is dominated mainly by the pull-off force, and thicker films, where the critical force approaches a limit where the displacement of the interfaces controls the work of adhesion. Previously, examinations into the addition of salt to LbL systems have shown that salt also increases the work of adhesion in dried samples.44 However, the adhesive behavior in the study was substantially influenced by the roughness of the sample, contrary to our study when the adhesion was tested in water without salt. Furthermore, more variables have been highlighted in our present results between the two processing 4426

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Author Contributions

conditions, which enable a more tunable adhesive. The work of adhesion for nine and ten layers formed in the presence of 10 mM NaCl was more than 20× higher than the maximum work of adhesion reported earlier for sacrificial bonds in bone structures.20,21,45 When equal contact times were considered (10 s), the work of adhesion in our system was almost 35× greater than between collagen and bone.21 Furthermore, adhesive chain pulling interactions were recorded over a distance twice as long (>8 μm) as the furthest measured for the chains to rupture with collagen and bone.21 The adhesive strengths demonstrated in the present work are promising for in vivo use of biomedical devices where moist environments are present. Modification of the thickness of the film and the salt concentration in the assembly process presents the opportunity to tune segments of the film where release is more ideal and others where adhesion is desired, while using the same material system. Recent investigations have also shown that the wet properties of the LbL film to a large extent were preserved in the dried state, since this LbL treatment was successful in dramatically increasing the tensile work at rupture of fibrous networks where the fibers were pretreated with the PAH/HA system before network formation.19 We anticipate the use of HA multilayer films as a robust wet adhesive that competes with and exceeds adhesive forces and displacements of naturally occurring processes.

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Equally contributing authors to the final manuscript (T.P. and S.A.P.)

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS BiMaC Innovation is gratefully acknowledged for financial support. Wallenberg Wood Science Centre (WWSC) is also acknowledged for financial support. S.A.P. would like to thank the KAMI Research Foundation for financial support.

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CONCLUSIONS Films comprising of PAH and HA were fabricated through LbL assembly. By adding 10 mM NaCl in the solution during assembly, the polymer films could have their inherent characteristics and subsequent wet adhesive properties dramatically changed. Films exhibiting linear growth and low dissipation in QCM-D displayed low adhesive strength and low work of adhesion for separating two similar, LbL-treated surfaces. On the contrary, films displaying nonlinear growth showed substantially larger critical pull-off forces and greater work of adhesion. The capping layer of the LbL film altered the adhesion slightly; however, the dominant factor was whether the layers were assembled with or without extra electrolyte. Through the evaluation of the assembly parameters (electrolyte concentration, film thickness), we demonstrated that strong and tunable wet adhesion can be obtained through (PAH/HA) multilayers. The molecular mass of the top layer controlled the distance for the long-range separation interactions that are associated with electrostatic linkages and possibly also hydrogen bonds breaking and reforming. Furthermore, through adjusting the contact time and the number of multilayers, our structures had adhesive strengths that were over an order of magnitude stronger than collagen binding with bone from previous literature.20,21

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ASSOCIATED CONTENT

* Supporting Information S

A plot of the surface roughness vs layer number for assemblies with and without salt is included. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

(1) Decher, G. Layer-by-Layer Assembly (Putting Molecules to Work), in Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials, 2nd ed.; Decher, G., Schlenoff, J. B., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2012; pp 1−21. (2) Vidyasagar, A.; Sung, C.; Gamble, R.; Lutkenhaus, J. L. ACS Nano 2012, 6, 6174−6184. (3) Burke, S. E.; Barrett, C. J. Biomacromolecules 2005, 6, 1419−1428. (4) Decher, G. Science 1997, 277, 1232−1237. (5) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153− 8160. (6) Illergard, J.; Enarsson, L. E.; Wagberg, L.; Ek, M. ACS Appl. Mater. Interfaces 2010, 2, 425−433. (7) Kovacevic, D.; van der Burgh, S.; de Keizer, A.; Stuart, M. A. C. Langmuir 2002, 18, 5607−5612. (8) Jang, Y.; Seo, J.; Akgun, B.; Satija, S.; Char, K. Macromolecules 2013, 46, 4580−4588. (9) Feng, X.; Pouw, K.; Leung, V.; Pelton, R. Biomacromolecules 2007, 8, 2161−2166. (10) Lvov, Y.; Caruso, F. Anal. Chem. 2001, 73, 4212−4217. (11) von Klitzing, R. Phys. Chem. Chem. Phys. 2006, 8, 5012−5033. (12) Priolo, M. A.; Holder, K. M.; Greenlee, S. M.; Grunlan, J. C. ACS Appl. Mater. Interfaces 2012, 4, 5529−5533. (13) Priolo, M. A.; Holder, K. M.; Greenlee, S. M.; Stevens, B. E.; Grunlan, J. C. Chem. Mater. 2013, 25, 1649−1655. (14) Jeon, J.-W.; O’Neal, J.; Shao, L.; Lutkenhaus, J. L. ACS Appl. Mater. Interfaces 2013, 5, 10127−10136. (15) Mike, J. F.; Lutkenhaus, J. L. ACS Macro Lett. 2013, 2, 839−844. (16) Buck, M. E.; Lynn, D. M. ACS Appl. Mater. Interfaces 2010, 2, 1421−1429. (17) Wood, K. C.; Boedicker, J. Q.; Lynn, D. M.; Hammond, P. T. Langmuir 2005, 21, 1603−1609. (18) Lee, K. Y.; Mooney, D. J. Chem. Rev. 2001, 101, 1869−1879. (19) Marais, A.; Utsel, S.; Gustafsson, E.; Wagberg, L. Carbohydr. Polym. 2014, 100, 218−224. (20) Thompson, J. B.; Kindt, J. H.; Drake, B.; Hansma, H. G.; Morse, D. E.; Hansma, P. K. Nature 2001, 414, 773−776. (21) Fantner, G. E.; Hassenkam, T.; Kindt, J. H.; Weaver, J. C.; Birkedal, H.; Pechenik, L.; Cutroni, J. A.; Cidade, G. A. G.; Stucky, G. D.; Morse, D. E.; Hansma, P. K. Nat. Mater. 2005, 4, 612−616. (22) Gong, H.; Garcia-Turiel, J.; Vasilev, K.; Vinogradova, O. I. Langmuir 2005, 21, 7545−7550. (23) Johansson, E.; Blomberg, E.; Lingstrom, R.; Wagberg, L. Langmuir 2009, 25, 2887−2894. (24) Swann, M. J.; Peel, L. L.; Carrington, S.; Freeman, N. J. Anal. Biochem. 2004, 329, 190−198. (25) Marx, K. A. Biomacromolecules 2003, 4, 1099−1120. (26) Sauerbrey, G. Z. Phys. 1959, 155, 206−222. (27) Aulin, C.; Varga, I.; Claesson, P. M.; Wågberg, L.; Lindström, T. Langmuir 2008, 24, 2509−2518. (28) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239−241. (29) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831−1836.

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Biomacromolecules

Article

(30) Sader, J. E.; Chon, J. W. M.; Mulvaney, P. Rev. Sci. Instrum. 1999, 70, 3967−3969. (31) Pettersson, T.; Nordgren, N.; Rutland, M. W. Rev. Sci. Instrum. 2007, 78, 093702. (32) Ondaral, S.; Ankerfors, C.; Odberg, L.; Wagberg, L. Langmuir 2010, 26, 14606−14614. (33) DeLongchamp, D. M.; Kastantin, M.; Hammond, P. T. Chem. Mater. 2003, 15, 1575−1586. (34) Lavalle, P.; Gergely, C.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J. C.; Picart, C. Macromolecules 2002, 35, 4458−4465. (35) Picart, C.; Mutterer, J.; Richert, L.; Luo, Y.; Prestwich, G. D.; Schaaf, P.; Voegel, J. C.; Lavalle, P. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12531−12535. (36) Haynie, D. T.; Cho, E. H.; Waduge, P. Langmuir 2011, 27, 5700−5704. (37) Ankerfors, C.; Pettersson, T.; Wagberg, L. Soft Matter 2012, 8, 8298−8301. (38) Xu, L.; Pristinski, D.; Zhuk, A.; Stoddart, C.; Ankner, J. F.; Sukhishvili, S. A. Macromolecules 2012, 45, 3892−3901. (39) Utsel, S.; Malmstrom, E. E.; Carlmark, A.; Wagberg, L. Soft Matter 2010, 6, 342−352. (40) Creton, C.; Leibler, L. J. Polym. Sci., Part B: Polym. Phys. 1996, 34, 545−554. (41) Cordier, P.; Tournilhac, F.; Soulie-Ziakovic, C.; Leibler, L. Nature 2008, 451, 977−980. (42) Hayashi, K.; Tsutsumi, K.; Nakajima, F.; Norisuye, T.; Teramoto, A. Macromolecules 1995, 28, 3824−3830. (43) Takahashi, R.; Kubota, K.; Kawada, M.; Okamoto, A. Biopolymers 1999, 50, 87−98. (44) Nolte, A. J.; Chung, J. Y.; Walker, M. L.; Stafford, C. M. ACS Appl. Mater. Interfaces 2009, 1, 373−380. (45) Tang, Z. Y.; Kotov, N. A.; Magonov, S.; Ozturk, B. Nat. Mater. 2003, 2, 413−418.

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