Bioinspired further enhanced dry adhesive by the combined effect of

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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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06686 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 17, 2018

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ACS Applied Materials & Interfaces

Bioinspired further enhanced dry adhesive by the combined effect of the microstructure and surface free energy increase AUTHOR NAMES Emre Kizilkan*, Stanislav N. Gorb CORRESPONDING AUTHOR Emre Kizilkan; E-mail: [email protected] AUTHOR ADDRESS Department of Functional Morphology and Biomechanics, Zoological Institute, Kiel University, Am Botanischen Garten 1-9, 24118 Kiel, Germany KEYWORDS Biomimetics, adhesives, polymers, silicones, gecko

ACS Paragon Plus Environment

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ABSTRACT

The 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, if 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 PVS 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 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 videorecordings 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.


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Introduction The robust and strong attachment abilities of geckos, spiders and beetles have impressed researches 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, if compared to their unstructured counterparts and pave 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 and even outperforming other biomimetic microstructures 16-19

.

The most used materials for the fabrication of bioinspired dry adhesives are the silicone elastomers e.g. polydimethylsiloxane (PDMS) or polyvinylsiloxane (PVS) 17, 20. Their 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 / nano features

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 the plasma treatment on


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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, the 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 in order to 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 video-recordings, surface topography metrology, and SFE investigations, in order 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.

Materials and Methods The individual MSAMSs made of PVS have stalk diameters of 40 µm, a length of 70 µm, top contact layers having a diameter of 45 µm and 2 µm thick (Figure. 1A and B). The microstructured area occupies 50 % of the apparent surface area. As a control, the unstructured


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backsides of MSAMS samples were used. The samples of the MSAMS surfaces were obtained from Gottlieb Binder GmbH, (Holzgerlingen, Germany). The scanning electron microscopy (SEM) images were acquired by 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, Lichtenstein). The adhesive characterization was performed by a pull-off force test in a customized set-up consisting of a glass sphere with a diameter of 3 mm attached to a force sensor (25 g capacity; World Precision Instruments, Inc., FL, USA) similar to our previous work

27

. The force sensor

was connected to an amplifier (Biopac Systems Ltd, Goleta, CA, USA). A 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, USA) (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. Air plasma treatments were performed with a power of 10 W, a frequency of 40 kHz and a gas flow of 80 sccm in a low pressure cold plasma system (Diener Plasma Zepto, Ebhausen, Germany) mounted with 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.


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The contact angle and SFE measurements were performed with a high-speed optical contact angle measuring device OCAH 200 (Data-Physics 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 solidliquid 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 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 of resolution.


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The detachment areas of individual MSAMSs were recorded at 60 fps using a Photron Fastcam SA1.1 (Photron, San Diego, CA, USA) camera mounted on an inverse optical microscope Observer.A1 (Carl Zeiss MicroImaging, Göttingen, Germany) with a 40x objective Zeiss A-Plan 40x / 0.65 (Carl Zeiss Microscopy GmbH, Jena, Germany). The samples with a diameter of 1.5 mm were placed on a glass slide by hand to assure a parallel contact. After, the samples' backsides were mounted to a removable sample holder attached to a force transducer (10 g capacity; World Precision Instruments, Inc., FL, USA) by using liquid dental silicone (Coltène President light body, Coltène Whaledent Dentalvertriebs 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. With the help of the camera and the force transducer, the contact between the sample and the glass slide was gently generated. 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, USA).

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 a plasma process pressure of 0.3, 0.8 and 2 mbar. The pull-off forces of each sample were measured before and


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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 plasmatreated surface / pull-off forces of untreated surface) (Figure. 3 and 4). For each data point, three samples were examined. As a result, the obtained relative adhesion values varied at a range between 1.30 (maximum) to 0.73 (minimum) for unstructured surfaces (Figure. 3) and at a range 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). The 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 s and 60 s of treatments. The maximum relative adhesion values were obtained in the range of 13 s to 23.6 s for the MSAMS surface. For the unstructured surfaces, the maximum relative adhesive values were obtained between 8.6 to 13 s. 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 mbar, 0.8 mbar 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 in 0.3 mbar, 0.8 mbar 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 =


8

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0.001 and P < 0.001 for MSAMS samples treated in 0.3 mbar, 0.8 mbar and 2 mbar of 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 also Materials and methods section 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 applied plasma process pressures of 0.3 mbar, 0.8 mbar 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. At measurements done after the treatment of 60 s with 0.8 and 2.0 mbar of process pressures, the water contact angle was not able to be measured on the MSAMS surface. After those treatments, the water droplets used for contact angle measurements penetrated in between the individual microstructures and wetted the complete area; from the bottom to the top plates of MSAMS. After the treatment of 60 s with 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 unstructured and microstructured surfaces display that the SFE increase at the 0.3 mbar of plasma process pressure is lower than those in 0.8 and 2.0 mbar of 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 roughness of


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untreated and plasma-treated samples was investigated using atomic force microscopy (AFM) imaging and their values are depicted as Rrms (root mean squares of peaks and valleys values) and Rz (distance values between the highest peak to 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 s and 60 s were used. The plasma-treated samples of 13 s did not demonstrate a distinctive change in roughness (Rrms: 1.97 nm and Rz: 22.09 nm), if compared to the untreated surface (Rrms: 1.78 nm and 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 plasma-treated one treated for 13 s. These higher surface roughness 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 relative adhesion value of minimum 0.44 at 60 s of 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 of plasma treatment were applied (Figure. 6B). However, after 60 s of 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 reduced 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


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silicone elastomers 24, 29-30. It was demonstrated that the plasma treatment leads to a formation of polar groups, such as hydroxyl (-OH) at the expense of hydrophobic groups, such as the methyl (-CH3) and/or hydroxymethyl (CH2OH). 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 section for details). As a representative of a plasma-treated sample, an unstructured surface treated for 13 s in 0.8 mbar of 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; 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 role of the polar and dispersive components of SFEs can be delineated in terms of work of adhesion in contact to 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 dispersive and polar components the interactions of SFE can be calculated as follows:

p (γ)Gp Wa = 2 (γ)dPVS (γ)Gd + 2 (γ)PVS

(1)


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p and γ Gp are polar components of the where γ dPVS and γ Gd are dispersive components and γ PVS

SFEs of an unstructured PVS surface and glass sphere, respectively

31-32

. By using the above

equation (1), the Wa values obtained are 41.22 mN/m and 57.35 mN/m for an untreated and treated surface with 13 s of time and in 0.8 mbar of process pressure parameters, respectively. 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 in 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

=

Wa − pl

(2)

Wa

where Fun − pl is the value of pull-off forces after plasma treatment of the unstructured surface. 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 the equations (1) and (2) considering the work of adhesion values for an unstructured sample. As a result, the estimated

Fun − pl

Fun

is 1.18 which is agrees well with the

experimentally obtained maximum relative adhesion range of 1.15 - 1.30.


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Interestingly, the MSAMS surface demonstrated a higher relative adhesion increase, up to 91 %, even though its plasma-treated apparent surface area is only 50 % of the unstructured surface. In order 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 videorecorded 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 Supplementary 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 Supplementary 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 maintain 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 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 towards the outer edges. This detachment type was previously


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modeled and also demonstrated experimentally

17, 33

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. By using the model, the relative adhesion

value of plasma-treated MSAMS can be estimated as:

Fmush − pl Fmush

=

Wa − pl Adet − pl Wa

(3)

Adet

where Fmush − pl is the value of pull-off forces after plasma treatment of MSAMS surface. Adet and Adet − pl are values of detachment area of untreated and plasma-treated individual MSAMSs,

respectively. In comparison, the estimated values of

Fmush − pl Fmush

by equation (3) is found to be 1.84

(0.8 mbar, 13 s), which also agrees very well with the experimentally obtained maximal relative adhesion values in the range of 1.71 to 1.91.

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 at 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


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MSAMSs. Previous works showed that plasma-treated silicone elastomer surfaces recover their hydrophobic character after varying times depending on the procedures done. However, functionalization of the surfaces by such methods as grafting hydrophilic units by post chemical treatments with silane-based solutions

29, 34

, storing under polar liquids

35

and coating with

HEMA (2-hydroxyethyl methacrylate) 23, the hydrophilicity of the silicone elastomers can either permanently or for an extended amount of time be preserved. 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.

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(11) Geim, A. K.; Dubonos, S.; Grigorieva, I.; Novoselov, K.; Zhukov, A.; Shapoval, S. Y. Microfabricated Adhesive Mimicking Gecko Foot-Hair. Nat. Mater. 2003, 2 (7), 461-463. (12) Lee, H.; Lee, B. P.; Messersmith, P. B. A Reversible Wet/Dry Adhesive Inspired by Mussels and Geckos. Nature 2007, 448 (7151), 338-341. (13) Sitti, M.; Fearing, R. S. Synthetic Gecko Foot-Hair Micro/Nano-Structures as Dry Adhesives. J. Adhes. Sci. Technol. 2003, 17 (8), 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), 1-108. (15) Fang, X.-Q.; Bai, G.-X. Preparation and Service Performance Characterization of Ni/PMNPT: 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 (13), 271-275. (17) Heepe, L.; Gorb, S. N. Biologically Inspired Mushroom-Shaped Adhesive Microstructures. Annu. Rev. Mater. Res. 2014, 44, 173-203. (18) Del Campo, A.; Greiner, C.; Arzt, E. Contact Shape Controls Adhesion of Bioinspired Fibrillar Surfaces. Langmuir 2007, 23 (20), 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 (19), 1704696. (20) Boesel, L. F.; Greiner, C.; Arzt, E.; Del Campo, A. Gecko-Inspired Surfaces: A Path to Strong and Reversible Dry Adhesives. Adv. Mater. 2010, 22 (19), 2125-2137. (21) Zhou, J.; Ellis, A. V.; Voelcker, N. H. Recent Developments in PDMS Surface Modification for Microfluidic Devices. Electrophoresis 2010, 31 (1), 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 (12‐13), 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 (4), 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. Adhesion Enhancement by a Dielectric Barrier Discharge of PDMS Used for Flexible and Stretchable Electronics. J. Phys. D: Appl. Phys. 2007, 40 (23), 7392. (25) Williams, R.; Wilson, D.; Rhodes, N. Stability of Plasma-Treated Silicone Rubber and Its Influence on the Interfacial Aspects of Blood Compatibility. Biomaterials 2004, 25 (19), 46594673. (26) Vlachopoulou, M.; 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 (6), 1-4. (27) Kizilkan, E.; Strueben, J.; Staubitz, A.; Gorb, S. N. Bioinspired Photocontrollable Microstructured Transport Device. Sci. Robot. 2017, 2 (2), eaak9454. (28) Owens, D. K.; Wendt, R. Estimation of the Surface Free Energy of Polymers. J. Appl. Polym. Sci. 1969, 13 (8), 1741-1747. (29) Sharma, V.; Dhayal, M.; Shivaprasad, S.; Jain, S. Surface Characterization of PlasmaTreated and PEG-Grafted PDMS for Micro Fluidic Applications. Vacuum 2007, 81 (9), 10941100.


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(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 (5), 1378-1383. (31) Fowkes, F. M. Attractive Forces at Interfaces. Ind. Eng. Chem. 1964, 56 (12), 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 (3-5), 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 (12), 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 (21), 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, San Francisco, CA, USA, Sept 1-5, 2004; IEEE: 2004; pp 5013-5016.

FIGURES


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Figure. 1. SEM images of the MSAMS surface. A. Top view. B. Side view. Scale bars = 100 µm.

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.


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Figure. 3. Relative adhesion values on unstructured surfaces. The values measured on unstructured surfaces obtained by changing time (3.6 to 60 s) and pressure (0.3 mbar, 0.8 mbar and 2 mbar) parameters of the plasma treatment.


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Figure. 4. Relative adhesion values on the MSAMS surfaces. The values measured on unstructured surfaces obtained by changing time (3.6 to 60 s) and pressure (0.3 mbar, 0.8 mbar and 2 mbar) parameters of the plasma treatment.


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Figure. 5. Water contact angle measurement. On the unstructured (red) and MSAMS surfaces (blue).


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Figure. 6. Investigation of reduction of relative adhesion values on unstructured and MSAMS surfaces. A. AFM images of unstructured surface before, after 13 s and 60 s of air plasma treatments (0.8 mbar). The 13 s of treatment induced a small change of roughness but the 60 s of treatment caused higher surface roughness by formation of cavities and protrusions. B. SEM images of the MSAMS surface obtained after 13 s and 60 s of plasma treatment. The 13 s of treatment did not induce a topographical change at the micrometer scale, however, the 60 s of treatment led to the deformation of individual microstructures. Scale bars = 100 µm.


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Figure. 7. Observation of the detachment of individual MSAMSs. A-E. The untreated microstructures' top plates shrink until the moment of detachment (32.8 s). F-J. The plasmatreated 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. Scale bars = 50 µm.


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ASSOCIATED CONTENT Supporting Information. Supplementary video 1. Detachment of untreated individual MSAMSs. Supplementary video 2. Detachment of plasma-treated individual MSAMSs. AUTHOR INFORMATION Corresponding Authors Emre Kizilkan*, Stanislav. N. Gorb* Present Addresses Department of Functional Morphology and Biomechanics, Zoological Institute, Kiel University, Am Botanischen Garten 1-9, 24118 Kiel, Germany Author Contributions E.K. and S.N.G. designed the experiments; E.K. performed the experiments and wrote the manuscript; S.N.G. reviewed the manuscript. ACKNOWLEDGMENT This work was funded by the Collaborative Research Center 677 “Function by Switching” (project C10). The authors are thankful to Lars Heepe for valuable discussions. We would also like to acknowledge Theresa Gödel, Janina Röckner and Clemens F. Schaber for their efforts in the AFM investigations.


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Bioinspired further enhanced dry adhesive by the combined effect of

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