Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 26752−26758
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Bioinspired Further Enhanced Dry Adhesive by the Combined Effect of the Microstructure and Surface Free-Energy Increase Emre Kizilkan* and Stanislav N. Gorb Department of Functional Morphology and Biomechanics, Zoological Institute, Kiel University, Am Botanischen Garten 1-9, 24118 Kiel, Germany
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S Supporting Information *
ABSTRACT: Silicone elastomers are known for having low surface free energies generally leading to poor adhesive performances. This surface characteristic can be enhanced by plasma treatments. The microstructured silicone elastomer surfaces can demonstrate superior adhesive performance that is more than 10 times higher in terms of pull-off forces, compared to their unstructured counterpart. Here, we have demonstrated that the combination of these two methods further enhances adhesive performance, especially when the surfaces are biomimetic micro/nanopatterned with, e.g., beetle-inspired mushroom-shaped adhesive microstructure (MSAMS). The plasma treatment time and pressure parameters were varied for the unstructured and MSAMS poly(vinylsiloxane) surfaces to find optimum parameters for maximum adhesion performance. Air plasma treatment induced average adhesive enhancement forces up to 30% on the unstructured surface, but the MSAMS surface demonstrated an enhancement of adhesive forces up to 91% higher than that of an untreated, microstructured control, despite the plasma-treated surface area of the structured surface being only 50% of that of the unstructured surface. High-speed video-recordings of individual microstructures in contact with a glass surface shows that the origin of the adhesion enhancement is due to the special detachment mechanism of individual microstructures that allows sustaining a wider contact area at detachment. We believe that this integration of the plasma treatment with MSAMS suggests a versatile way of functionalization that can further advance the adhesive ability of low-surface-energy polymer surfaces. KEYWORDS: biomimetics, adhesives, polymers, silicones, gecko
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INTRODUCTION The robust and strong attachment abilities of geckos, spiders, and beetles have been attracting researchers for many years.1,2 Their attachment organs, such as fibrillar adhesive pads, branch into single contact units down to a nanometer scale. These fibrillar units enable strong contact compliance on various substrates and generate adhesion due to both intermolecular short-ranged interactions and capillary forces.3−6 The attempts to mimic these animals’ fibrillar attachment systems are partially based on the use of a contact splitting principle, in which individual surface structures split into smaller units, allowing for better surface conformity to various substrates and for tolerance of substrate roughness.7−10 By using deposition, lithography, and other patterning techniques, micro/nanostructured surfaces were manufactured with substantial adhesive properties, compared to their unstructured counterparts, which paves the way to glue-free adhesives, such as bioinspired dry sticky tapes.11−15 Beetle-inspired mushroom-shaped adhesive microstructure (MSAMS) is one of such a prominent, reversible, dry adhesive structure providing superior attachment ability, even outperforming other biomimetic microstructures.16−19 The most used materials for the fabrication of bioinspired dry adhesives are the silicone elastomers, e.g., poly(dimethylsiloxane) or poly(vinylsiloxane) (PVS).17,20 Their © 2018 American Chemical Society
properties, such as ease of production, mechanical stability, flexibility, and compliance on the rough surfaces, have made them prime candidate materials for the development of complex, high-performance surface micro/nanofeatures.20−22 However, silicone elastomers have rather low surface free energy (SFE) that restricts their applicability due to the risk of adhesion failure. One of the main methods for activating such low SFE is the plasma treatment that incorporates polar groups with the depletion of hydrophobic groups onto the surface.23,24 In addition, plasma treatment on the silicone elastomers might modify the surface topography, which then can cause inadequate contact with a substrate.25,26 Through proper optimization of the plasma process parameters, high-SFE silicone elastomers can be obtained and presumably the performance of bioinspired microstructured adhesives can be further enhanced. In this work, we have combined plasma treatment and microstructuring as adhesion enhancement tools for PVS. The MSAMS PVS surfaces and their unstructured controls were subjected to low-pressure air plasma treatment. The time and process pressure parameters of the treatment were varied to Received: April 24, 2018 Accepted: July 16, 2018 Published: July 16, 2018 26752
DOI: 10.1021/acsami.8b06686 ACS Appl. Mater. Interfaces 2018, 10, 26752−26758
Research Article
ACS Applied Materials & Interfaces examine their influence on the surface morphologies and in turn on the adhesive performances. The obtained results were rather surprising because at some particular set of parameters, the MSAMS surface improved its already superior adhesive properties to a much higher extent than the unstructured surface, which was actually expected to demonstrate a higher improvement due to its larger apparent contact area. Here, we applied force measurement techniques, high-speed videorecordings, surface topography metrology, and SFE investigations to examine the role of the detachment process of the microstructured surface in the adhesive performance. Experimental observations were also confirmed by a detachment mechanism model and convinced us that the combination of surface topography with specific chemical functionalization can advance the adhesive ability of the silicone elastomer surfaces.
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MATERIALS AND METHODS
The individual MSAMSs made of PVS have a stalk diameter of 40 μm, length of 70 μm, and top contact layers with diameter and thickness of 45 and 2 μm, respectively (Figure 1A,B). The microstructured area
Figure 2. Force−time curve of the representative pull-off measurement of samples made of PVS in contact with the glass sphere substrate. The samples of unstructured surface (red line) and MSAMS surface (blue line) were brought into contact with a preload of 18 mN. The adhesive characterization was performed by separating the glass sphere from the sample surfaces and obtaining pull-off force values for the unstructured (Fun) and MSAMS (Fmush) surfaces. Air plasma treatments were performed with a power of 10 W, frequency of 40 kHz, and gas flow of 80 sccm in a low-pressure cold plasma system (Diener Plasma Zepto, Ebhausen, Germany) mounted on a vacuum pump (Pfeiffer Duo 2.5 Rotary Vane Vacuum Pump, Aßlar, Germany). Our trials using powers higher than 10 W damaged the microstructures made of PVS. The chamber was evacuated to a pressure of 0.2 mbar before the desired process pressures were adjusted. The contact angle and SFE measurements were performed with a high-speed optical contact angle measuring device OCAH 200 (DataPhysics Instruments, Filderstadt, Germany) with a Hamilton 1750 TLL syringe (Hamilton Bonaduz AG, Bonaduz, Switzerland) and a needle DataPhysics SNS 052/026 with an outer diameter of 0.52 mm, inner diameter of 0.26 mm, and length of 51 mm (DataPhysics Instruments, Filderstadt, Germany). By using the sessile drop method (drop volume: 1 μL), static contact angles of water (surface tension: 72.1 mN/m; dispersion component: 19.9 mN/m, polar component: 52.2 mN/m, Busscher et al., 1984), diiodomethane (surface tension: 50.0 mN/m; dispersion component: 47.4 mN/m, polar component: 2.6 mN/m, Busscher et al., 1984), and ethyleneglycol (surface tension: 48.0 mN/m; dispersion component: 29.0 mN/m, polar component: 19.0 mN/m, Erbil, 1997) were measured on test surfaces in 3 s when the liquid drops were stable on the solid samples. The contact angles were calculated using the circular-shape fitting and tangent method: the angles between solid−liquid interface and liquid−vapor interface were measured. The SFE measurements were performed according to the Owens−Wendt−Rabel−Kaelble (OWRK) method because it is able to demonstrate polar interactions in SFE calculations.28 For the atomic force microscopy (AFM) imaging, a NanoWizard atomic force microscope (JPK Instruments, Berlin, Germany) mounted on an inverted light microscope (Zeiss Axiovert 135, Carl Zeiss MicroImaging GmbH, Göttingen, Germany) was used. NanoWizard SPM software 4.0.31 (JPK Instruments, Berlin, Germany) was utilized to adjust scanning parameters. The samples were scanned using an AFM tapping mode with a silicon cantilever (SSS−NCHR; tip radius, 5.3 nm; NanoWorld AG, Neuchâtel, Switzerland). A scan size of 15 μm × 15 μm was used. Z-range of 15 μm was applied, having 0.2 nm resolution. The detachment areas of individual MSAMSs were recorded at 60 fps using a Photron Fastcam SA1.1 (Photron, San Diego, CA) camera mounted on an inverse optical microscope Observer.A1 (Carl Zeiss MicroImaging, Göttingen, Germany) with a 40× objective Zeiss A-Plan 40×/0.65 (Carl Zeiss Microscopy GmbH, Jena, Germany). Samples
Figure 1. Scanning electron microscopy (SEM) images of the MSAMS surface. (A) Top view. (B) Side view. The scale bar is 100 μm.
occupies 50% of the apparent surface area. As a control, the unstructured back sides of MSAMS samples were used. The samples of the MSAMS surfaces were obtained from Gottlieb Binder GmbH (Holzgerlingen, Germany). Scanning electron microscopy (SEM) images were acquired by a TM-3000 at 5 kV acceleration voltage (Hitachi Ltd, Tokyo, Japan). Prior to observations, the samples were coated with 10 nm of Au−Pd by a BAL-TEC SCD 500 sputter coater using a BAL-TEC QSG 100 quartz film thickness monitor (Bal-Tec AG, Balzers, Liechtenstein). The adhesive characterization was performed by a pull-off force test in a customized setup consisting of a glass sphere with a diameter of 3 mm attached to a force sensor (25 g capacity; World Precision Instruments, Inc., FL) similar to our previous work.27 The force sensor was connected to an amplifier (Biopac Systems, Inc., Goleta, CA). The glass sphere was brought into contact with the sample surfaces by applying a preload of 18 mN through using a micromanipulator DC3314R (World Precision Instruments, Inc., Sarasota, FL) (Figure 2). The average absolute pull-off force values of untreated samples were 1.49 ± 0.3 mN (Fun) and 15.18 ± 1.16 mN (Fmush) for the unstructured and MSAMS surfaces, respectively. The adhesion experiments were statistically analyzed with one-way ANOVA Shapiro-Wilk normality tests using the SigmaPlot 12.5 software (t-test; SPSS, Inc., San Jose, CA). 26753
DOI: 10.1021/acsami.8b06686 ACS Appl. Mater. Interfaces 2018, 10, 26752−26758
Research Article
ACS Applied Materials & Interfaces with a diameter of 1.5 mm were placed on a glass slide by hand to assure a parallel contact. Afterward, the samples’ back sides were mounted on a removable sample holder attached to a force transducer (10 g capacity; World Precision Instruments, Inc., FL) by using liquid dental silicone (Coltène President light body, Coltène Whaledent Dental Vertriebs Ltd., Konstanz, Germany). After 5 min, the silicone hardened and the experiment was started. The samples attached to the force transducer were retracted away from the glass slide surface with a speed of 1 μm/s. For subsequent plasma treatment, the samples were removed with the holder. After plasma treatments, the samples were placed in the same position on the force transducer. Using the camera and the force transducer, the contact between the sample and the glass slide was gently generated.
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RESULTS AND DISCUSSION The influence of low-pressure air plasma treatment on the surfaces was examined by changing the treatment time and the plasma process pressure parameters. During the experiment, the treatment times of 3.6, 5.5, 8.6, 13, 18.6, 23.6, 34.6, and 60 s were applied at plasma process pressures of 0.3, 0.8, and 2 mbar. The pull-off forces of each sample were measured before and after the particular treatment for comparison. The results of those pull-off force measurements were depicted as relative adhesive values (i.e., relative adhesion = pull-off forces of plasma-treated surface/pull-off forces of untreated surface) (Figures 3 and 4). For each data point, three samples were examined.
Figure 4. Relative adhesion values on the MSAMS surfaces. The values measured on unstructured surfaces obtained by changing time (3.6−60 s) and pressure (0.3, 0.8, and 2 mbar) parameters of the plasma treatment.
The applied plasma process pressures of 0.3, 0.8, and 2 mbar have demonstrated a similar relative adhesion change over 0−60 s of treatment. The comparison of maximum relative adhesion values for both the MSAMS and unstructured samples showed that the higher process pressures induced higher average relative adhesion. The maximum relative adhesion values were 1.15 ± 0.01, 1.26 ± 0.02, and 1.30 ± 0.08 for 0.3, 0.8, and 2.0 mbar process pressures for unstructured surfaces, respectively. For the MSAMS surfaces, the maximum relative adhesion values were 1.71 ± 0.3, 1.82 ± 0.14, and 1.91 ± 0.11 for 0.3, 0.8, and 2 mbar, respectively. In comparison to values obtained from 3.6 s with maximum relative adhesion of plasma treatment, unstructured and MSAMS samples demonstrated a significant relative adhesion increase (P = 0.093, P = 0.011, and P = 0.011 for unstructured samples; P = 0.01, P = 0.001, and P < 0.001 for MSAMS samples treated at 0.3, 0.8, and 2 mbar plasma process pressures, respectively). The obtained maximum in relative adhesion values is due to the increase in the SFE. These SFE changes were characterized by static water contact angle measurements (see Materials and Methods for details) (Figure 5). Before the plasma treatments, the unstructured and MSAMS surfaces had water contact angles of 112 and 134°, respectively. From 0 to 60 s of treatment, the unstructured surface demonstrated a decay of contact angle: from hydrophobic to hydrophilic state. At 60 s of treatment, the water contact angles after application of plasma process pressures of 0.3, 0.8, and 2.0 mbar were 16.1, 0, and 0°, respectively, for unstructured surface. The MSAMS surfaces did not demonstrate any contact angle change until 34.6 s of treatment for all applied plasma process pressure parameters. During measurements after treatment for 60 s with 0.8 and 2.0 mbar process pressures, the water contact angle could not be measured on the MSAMS surface. After those treatments, the water droplets used for contact angle measurements penetrated between the individual microstructures and wetted the complete area from the bottom to the top plates of MSAMS. After treatment for 60 s at a plasma process pressure of 0.3 mbar, the microstructured surface did not change the wetting state: the spaces between individual microstructures were not wetted, and the contact angle was the same as the one obtained after 34.6 s of treatment. These water contact angle measurements for both
Figure 3. Relative adhesion values on unstructured surfaces. The values measured on unstructured surfaces obtained by changing time (3.6−60 s) and pressure (0.3, 0.8, and 2 mbar) parameters of the plasma treatment.
As a result, the obtained relative adhesion values varied between 1.30 (maximum) and 0.73 (minimum) for unstructured surfaces (Figure 3) and between 1.91 (maximum) and 0.44 (minimum) for the MSAMS (Figure 4). The relative adhesion obtained for the unstructured surface increased for any process pressure applied up to 18.6 s of plasma treatment (relative adhesion > 1). Longer treatments, over 18.6 s, induced reduction of the relative adhesion to values below 1. For the MSAMS samples, the relative adhesion increased up to 23.6 s and then decreased below 1 at 34.6 and 60 s of treatments. The maximum relative adhesion values were obtained in the range of 13−23.6 s for the MSAMS surface. For the unstructured surfaces, the maximum relative adhesive values were obtained between 8.6 and 13 s. 26754
DOI: 10.1021/acsami.8b06686 ACS Appl. Mater. Interfaces 2018, 10, 26752−26758
Research Article
ACS Applied Materials & Interfaces
unstructured and microstructured surfaces display that the SFE increase at the 0.3 mbar plasma process pressure is lower than that at 0.8 and 2.0 mbar treatments. Therefore, the differences in average maximum relative adhesion of different process pressure conditions obtained might be associated with the SFE change. The relative adhesion reduction is due to a concurrence in the modification of the surface topographies and the SFE change through the plasma treatment. The surface roughnesses of untreated and plasma-treated samples were investigated by atomic force microscopy (AFM) imaging, and their values are depicted as Rrms (root mean squares of peaks and valleys) and Rz (distance between the highest peak and the lowest valley) (Figure 6). As representatives of the plasma-treated surfaces, samples treated at 0.8 mbar process pressure with treatment times of 13 and 60 s were used. The plasma-treated samples of 13 s did not demonstrate a distinctive change in roughness (Rrms: 1.97 nm; Rz: 22.09 nm) compared to the untreated surface (Rrms: 1.78 nm; Rz: 17.51 nm). However, the sample treated for 60 s demonstrated higher roughness values (Rrms: 6.66 nm; Rz: 57.25 nm) in comparison to the untreated one and to the plasmatreated one treated for 13 s. These higher surface roughness
Figure 5. Water contact angle measurement. On the unstructured (red) and MSAMS surfaces (blue).
Figure 6. Investigation of reduction of relative adhesion values on unstructured and MSAMS surfaces. (A) AFM images of unstructured surface before and after 13 and 60 s air plasma treatments (0.8 mbar). The 13 s treatment induced a small change of roughness, but the 60 s treatment caused higher surface roughness by the formation of cavities and protrusions. (B) SEM images of the MSAMS surface obtained after 13 and 60 s plasma treatment. The 13 s treatment did not induce a topographical change at the micrometer scale; however, the 60 s treatment led to the deformation of individual microstructures. The scale bar is 100 μm. 26755
DOI: 10.1021/acsami.8b06686 ACS Appl. Mater. Interfaces 2018, 10, 26752−26758
Research Article
ACS Applied Materials & Interfaces
Figure 7. Observation of the detachment of individual MSAMSs. (A−E) Untreated microstructures’ top plates shrink until the moment of detachment (32.8 s). (F−J) Plasma-treated microstructures maintain their initial contact area until the moment of detachment (50.4 s). They also sustain contact longer than the untreated ones. This longer contact maintenance leads to even stronger elongation and shrinkage of stalks of the microstructures. The scale bar is 50 μm.
To quantify the influence of the polar groups, the SFEs of the untreated and air plasma-treated surfaces were calculated by contact angle measurements with several liquids according to the Owens−Wendt−Rabel−Kaelble (OWRK) method (see Materials and Methods for details). As a representative of a plasma-treated sample, an unstructured surface treated for 13 s at 0.8 mbar plasma process pressure was used since there is a negligibly small topographical change observed in AFM measurements. The measured SFEs were found as 17.37 mN with a dispersive component of 17.33 mN/m and a polar component of 0.04 mN/m on the untreated surface and as 41.08 mN with a dispersive component of 17.8 mN/m and a polar component of 23.28 mN/m on the plasma-treated surface. Obviously, the increase in SFE is associated mainly with an increase in its polar component. To estimate the influence of the plasma treatment on the adhesion, the roles of the polar and dispersive components of SFEs can be delineated in terms of work of adhesion in contact with the glass sphere. The measured SFE of the glass sphere was 26.61 mN/m with a dispersive component of 23.79 mN/m and a polar component of 2.82 mN/ m. The work of adhesion, Wa, between unstructured PVS and the glass sphere, taking into consideration the SFE dispersive and polar component interactions, can be calculated as follows
values are due to the cavities and protrusions appearing during the treatment (Figure 6A). Thus, the reduction of relative adhesion down to minimum 0.73 can be attributed to the topographical change on the unstructured surface by the relatively longer plasma treatment. In comparison to the unstructured surface, the MSAMS surface demonstrated a minimum relative adhesion value of 0.44 at 60 s plasma treatment. This reduction cannot be explained only by the change in the surface roughness, but also by the modification of the microstructure features. The SEM images revealed that the individual microstructures maintained their position and topography after 13 s plasma treatment was applied (Figure 6B). However, after 60 s treatment, individual MSAMSs were deformed, i.e., the top contact layer’s edges were crumpled, partially etched, and the microstructures stuck to each other or collapsed because of the enhanced SFE. Thus, the overall contact area and the number of individual microstructures contributing to the attachment were decreased, and the relative adhesion was reduced down to minimum 0.44. The changes in SFE are due to the chemical modification of the surfaces. Those modifications induced by plasma treatment with different gases, including air, have been previously studied for silicone elastomers.24,29,30 It was demonstrated that the plasma treatment leads to the formation of polar groups, such as hydroxyl (−OH) at the expense of hydrophobic groups, such as methyl (−CH3) and/or hydroxymethyl (CH2OH).
p Wa = 2 (γ )dPVS (γ )Gd + 2 (γ )PVS (γ )Gp
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(1)
DOI: 10.1021/acsami.8b06686 ACS Appl. Mater. Interfaces 2018, 10, 26752−26758
Research Article
ACS Applied Materials & Interfaces where γdPVS and γdG are dispersive components and γpPVS and γpG are polar components of the SFEs of an unstructured PVS surface and glass sphere, respectively.31,32 By using eq 1, the Wa values obtained are 41.22 and 57.35 mN/m for untreated and treated surfaces, respectively, with 13 s time and 0.8 mbar process pressure parameters. During the detachment of the glass sphere from the unstructured surface, the crack that leads to the separation of the contacting surfaces initiates from the edges of the glass sphere and continues along the contact interface until complete detachment is reached. Previously, such detachment behavior had been modeled to estimate adhesive forces.33 In our experiments, since the surface topography of an unstructured surface was not distinctively modified by a plasma treatment using a treatment time of 13 s at 0.8 mbar of plasma process pressure, the relative adhesion can be estimated by using the Wa values obtained through the SFE dispersive and polar component interactions of two surfaces by using the detachment model as Fun ‐ pl Fun
=
unstructured surface, the top contact layer of the individual MSAMS eliminates the high stress at the outer edges. Until the very detachment point, the layer remains in contact. The crack initiates under the top contact layer and propagates toward the outer edges. This detachment type was previously modeled and also demonstrated experimentally.17,33 By using the model, the relative adhesion value of plasma-treated MSAMS can be estimated as Fmush ‐ pl Fmush
Wa ‐ pl Adet ‐pl Wa
Adet
(3)
where Fmush‑pl is the value of pull-off forces after plasma treatment of MSAMS surface and Adet and Adet‑pl are values of detachment areas of untreated and plasma-treated individual MSAMSs, respectively. In comparison, the estimated value of Fmush ‐ pl by eq 3 is found to be 1.84 (0.8 mbar, 13 s), which also Fmush
agrees very well with the experimentally obtained maximal relative adhesion values in the range of 1.71−1.91.
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Wa ‐ pl Wa
=
CONCLUSIONS In conclusion, we have analyzed the influence of air plasma treatment on the adhesive performance of the MSAMS surface and compared it to the unstructured control surfaces made of the same silicone elastomer. Plasma treatments longer than 23.6 s led to a reduction in adhesive performance of surfaces, whereas for the treatments in the range of 8.6−23.6 s, average relative adhesion was enhanced. The relative adhesion values of unstructured surfaces demonstrated an enhancement of a maximum of 30% after plasma treatment. On the contrary, the MSAMS demonstrated up to 91% of a relative adhesion increase by using the optimized process parameters for maximum adhesion. This superior attachment improvement is due to the combination of the SFE increase and its contribution to specific detachment behavior of MSAMSs. Previous works showed that plasma-treated silicone elastomer surfaces recover their hydrophobic character after varying times depending on the procedures done. However, by functionalization of the surfaces using methods such as grafting hydrophilic units through postchemical treatments with silane-based solutions,29,34 storing under polar liquids,35 and coating with 2-hydroxyethyl methacrylate,23 the hydrophilicity of the silicone elastomers can be preserved either permanently or for an extended amount of time. Presumably, by applying those methods, improved adhesive ability of MSAMS could find a broader range of application areas in which glue-free, reversible adhesion is needed.
(2)
where Fun‑pl is the value of pull-off forces after plasma treatment of the unstructured surface and Wa‑pl is the work of adhesion of contact between the glass sphere and plasma-treated unstructured surfaces. We have estimated the relative adhesion after the plasma treatment (0.8 mbar, 13 s) through eqs 1 and 2, considering the work of adhesion values for an unstructured F sample. As a result, the estimated un‐ pl is 1.18, which agrees well Fun
with the experimentally obtained maximum relative adhesion range of 1.15−1.30. Interestingly, the MSAMS surface demonstrated a higher relative adhesion increase, up to 91%, even though its plasmatreated apparent surface area is only 50% of the unstructured surface. To understand this increase, we observed the detachment of the individual, untreated, and plasma-treated MSAMS surfaces (Figure 7). The MSAMS sample was brought in contact with a glass slide, was retracted with a speed of 1 μm/s, and the contact area was simultaneously video-recorded at 60 fps. It was observed that the untreated individual microstructures demonstrated contact area shrinkage up to 64% until detachment (Figure 7A−E, see also Supporting Information Video 1). The plasma-treated (0.8 mbar, 13 s) individual microstructures, however, did not show such a change in their detachment contact area in comparison to the initial area (Figure 7F−J, see also Supporting Information Video 2). While maintaining contact on the glass slide, the stalk diameters of plasma-treated microstructures became narrower with the time the retraction force was applied. Compared to detachment time of the untreated MSAMS sample (33 s) from the glass slide, the plasma-treated MSAMS sample exhibited a longer (50.6 s) contact from the start of retraction to the complete detachment. This comparative observation suggests that the plasma-treated surface is able to sustain its initial contact area with the substrate surface longer and maintains stronger contribution of the intermolecular forces to the total adhesion. These forces were reported to be the main contribution to the dry adhesion exerted by MSAMS.17 The superior adhesive performance of MSAMS was elucidated by its crack entrapping ability. Different from the edge propagating crack observed in detachment behavior of the
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b06686.
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Detachment of untreated individual MSAMSs (Video 1) (MPG) Detachment of plasma-treated individual MSAMSs (Video 2) (MPG)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 26757
DOI: 10.1021/acsami.8b06686 ACS Appl. Mater. Interfaces 2018, 10, 26752−26758
Research Article
ACS Applied Materials & Interfaces ORCID
(18) Del Campo, A.; Greiner, C.; Arzt, E. Contact Shape Controls Adhesion of Bioinspired Fibrillar Surfaces. Langmuir 2007, 23, 10235− 10243. (19) Kizilkan, E.; Gorb, S. N. Combined Effect of the Microstructure and Underlying Surface Curvature on the Performance of Biomimetic Adhesives. Adv. Mater. 2018, 30, No. 1704696. (20) Boesel, L. F.; Greiner, C.; Arzt, E.; Del Campo, A. GeckoInspired Surfaces: A Path to Strong and Reversible Dry Adhesives. Adv. Mater. 2010, 22, 2125−2137. (21) Zhou, J.; Ellis, A. V.; Voelcker, N. H. Recent Developments in PDMS Surface Modification for Microfluidic Devices. Electrophoresis 2010, 31, 2−16. (22) Haines, S.; Beamson, G.; Williams, R.; Weightman, P. Changes in the Electronic Structure of Silicone Rubber Surfaces Induced by Oxygen Plasma Treatment. Surf. Interface Anal. 2007, 39, 942−947. (23) Bodas, D.; Khan-Malek, C. Formation of More Stable Hydrophilic Surfaces of PDMS by Plasma and Chemical Treatments. Microelectron. Eng. 2006, 83, 1277−1279. (24) Morent, R.; De Geyter, N.; Axisa, F.; De Smet, N.; Gengembre, L.; De Leersnyder, E.; Leys, C.; Vanfleteren, J.; Rymarczyk-Machal, M.; Schacht, E.; Payen, E. Adhesion Enhancement by a Dielectric Barrier Discharge of PDMS Used for Flexible and Stretchable Electronics. J. Phys. D: Appl. Phys. 2007, 40, 7392. (25) Williams, R. L.; Wilson, D.; Rhodes, N. Stability of PlasmaTreated Silicone Rubber and Its Influence on the Interfacial Aspects of Blood Compatibility. Biomaterials 2004, 25, 4659−4673. (26) Vlachopoulou, M.-E.; Tsougeni, K.; Kontakis, K.; Vourdas, N.; Tserepi, A.; Gogolides, E. Plasma Etching Technology for Fabrication and Surface Modification of Plastic Microfluidic Devices. Pure Appl. Chem. 2010, 86, 1−4. (27) Kizilkan, E.; Strueben, J.; Staubitz, A.; Gorb, S. N. Bioinspired Photocontrollable Microstructured Transport Device. Sci. Rob. 2017, 2, No. eaak9454. (28) Owens, D. K.; Wendt, R. Estimation of the Surface Free Energy of Polymers. J. Appl. Polym. Sci. 1969, 13, 1741−1747. (29) Sharma, V.; Dhayal, M.; Shivaprasad, S.; Jain, S. Surface Characterization of Plasma-Treated and PEG-Grafted PDMS for Micro Fluidic Applications. Vacuum 2007, 81, 1094−1100. (30) Olander, B.; Wirsén, A.; Albertsson, A. C. Silicone Elastomers with Controlled Surface Composition Using Argon or Hydrogen Plasma Treatment. J. Appl. Polym. Sci. 2003, 90, 1378−1383. (31) Fowkes, F. M. Attractive Forces at Interfaces. Ind. Eng. Chem. 1964, 56, 40−52. (32) Leite, F.; Herrmann, P. Application of Atomic Force Spectroscopy (AFS) to Studies of Adhesion Phenomena: A Review. J. Adhes. Sci. Technol. 2005, 19, 365−405. (33) Carbone, G.; Pierro, E.; Gorb, S. N. Origin of the Superior Adhesive Performance of Mushroom-Shaped Microstructured Surfaces. Soft Matter 2011, 7, 5545−5552. (34) Roth, J.; Albrecht, V.; Nitschke, M.; Bellmann, C.; Simon, F.; Zschoche, S.; Michel, S.; Luhmann, C.; Grundke, K.; Voit, B. Surface Functionalization of Silicone Rubber for Permanent Adhesion Improvement. Langmuir 2008, 24, 12603−12611. (35) Kim, B.; Peterson, E.; Papautsky, I. In Long-Term Stability of Plasma Oxidized PDMS Surfaces, Proceedings of the 26th Annual International Conference of the IEEE EMBS; IEEE, 2004; pp 5013− 5016.
Emre Kizilkan: 0000-0001-7666-8347 Author Contributions
E.K. and S.N.G. designed the experiments; E.K. performed the experiments and wrote the manuscript; and S.N.G. reviewed the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was funded by the Collaborative Research Center 677 “Function by Switching” (project C10). The authors are thankful to Lars Heepe for valuable discussions. They also thank Theresa Gödel, Janina Röckner, and Clemens F. Schaber for their efforts in the AFM investigations.
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REFERENCES
(1) Autumn, K.; Liang, Y. A.; Hsieh, S. T.; Zesch, W.; Chan, W. P.; Kenny, T. W.; Fearing, R.; Full, R. J. Adhesive Force of a Single Gecko Foot-Hair. Nature 2000, 405, 681−685. (2) Gorb, S. N. Functional Surfaces in Biology: Little Structures with Big Effects; Springer: Netherlands, 2009; Vol. 1. (3) Autumn, K.; Sitti, M.; Liang, Y. A.; Peattie, A. M.; Hansen, W. R.; Sponberg, S.; Kenny, T. W.; Fearing, R.; Israelachvili, J. N.; Full, R. J. Evidence for van der Waals Adhesion in Gecko Setae. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12252−12256. (4) Huber, G.; Mantz, H.; Spolenak, R.; Mecke, K.; Jacobs, K.; Gorb, S. N.; Arzt, E. Evidence for Capillarity Contributions to Gecko Adhesion from Single Spatula Nanomechanical Measurements. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 16293−16296. (5) Arzt, E.; Gorb, S.; Spolenak, R. From Micro to Nano Contacts in Biological Attachment Devices. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 10603−10606. (6) Glassmaker, N. J.; Jagota, A.; Hui, C.-Y.; Noderer, W. L.; Chaudhury, M. K. Biologically Inspired Crack Trapping for Enhanced Adhesion. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 10786−10791. (7) Kamperman, M.; Kroner, E.; del Campo, A.; McMeeking, R. M.; Arzt, E. Functional Adhesive Surfaces with “Gecko” Effect: The Concept of Contact Splitting. Adv. Eng. Mater. 2010, 12, 335−348. (8) Varenberg, M.; Murarash, B.; Kligerman, Y.; Gorb, S. N. Geometry-Controlled Adhesion: Revisiting the Contact Splitting Hypothesis. Appl. Phys. A 2011, 103, 933−938. (9) Huber, G.; Gorb, S. N.; Hosoda, N.; Spolenak, R.; Arzt, E. Influence of Surface Roughness on Gecko Adhesion. Acta Biomater. 2007, 3, 607−610. (10) Hui, C.-Y.; Glassmaker, N.; Jagota, A. How Compliance Compensates for Surface Roughness in Fibrillar Adhesion. J. Adhes. 2005, 81, 699−721. (11) Geim, A. K.; Dubonos, S.; Grigorieva, I.; Novoselov, K.; Zhukov, A.; Shapoval, S. Y. Microfabricated Adhesive Mimicking Gecko FootHair. Nat. Mater. 2003, 2, 461−463. (12) Lee, H.; Lee, B. P.; Messersmith, P. B. A Reversible Wet/Dry Adhesive Inspired by Mussels and Geckos. Nature 2007, 448, 338−341. (13) Sitti, M.; Fearing, R. S. Synthetic Gecko Foot-Hair Micro/NanoStructures as Dry Adhesives. J. Adhes. Sci. Technol. 2003, 17, 1055− 1073. (14) Bhushan, B.; Jung, Y. C. Natural and Biomimetic Artificial Surfaces for Superhydrophobicity, Self-Cleaning, Low Adhesion, and Drag Reduction. Prog. Mater. Sci. 2011, 56, 1−108. (15) Fang, X.-Q.; Bai, G.-X. Preparation and Service Performance Characterization of Ni/PMN-PT: Effect of Preparation Temperature. J. Alloys Compd. 2018, 735, 1131−1136. (16) Gorb, S.; Varenberg, M.; Peressadko, A.; Tuma, J. Biomimetic Mushroom-Shaped Fibrillar Adhesive Microstructure. J. R. Soc., Interface 2007, 4, 271−275. (17) Heepe, L.; Gorb, S. N. Biologically Inspired Mushroom-Shaped Adhesive Microstructures. Annu. Rev. Mater. Res. 2014, 44, 173−203. 26758
DOI: 10.1021/acsami.8b06686 ACS Appl. Mater. Interfaces 2018, 10, 26752−26758
