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Switchable Adhesion in Vacuum Using Bio-Inspired Dry Adhesives

Switchable Adhesion in Vacuum Using Bio-Inspired Dry Adhesives This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Research Article www.acsami.org †,‡,§ †,‡,§ ,‡,§ Julia Purtov, Mareike Frensemeier, and Elmar Kroner* Department of Materials Science and Engineering, Saarland University, Campus D2 2, 66123 Saarbrücken, Germany INM − Leibniz Institute for New Materials, Campus D2 2, 66123 Saarbrücken, Germany * Supporting Information ABSTRACT: Suction based attachment systems for pick and place handling of fragile objects like glass plates or optical lenses are energy-consuming and noisy and fail at reduced air pressure, which is essential, e.g., in chemical and physical vapor deposition processes. Recently, an alternative approach toward reversible adhesion of sensitive objects based on bioinspired dry adhesive structures has emerged. There, the switching in adhesion is achieved by a reversible buckling of adhesive pillar structures. In this study, we demonstrate that these adhesives are capable of switching adhesion not only in ambient air conditions but also in vacuum. Our bioinspired patterned adhesive with an area of 1 cm provided an adhesion force of 2.6 N ± 0.2 N in air, which was reduced to 1.9 N ± 0.2 N if measured in vacuum. Detachment was induced by buckling of the structures due to a high compressive preload and occurred, independent of air pressure, at approximately 0.9 N ± 0.1 N. The switch in adhesion was observed at a compressive preload between 5.6 and 6.0 N and was independent of air pressure. The difference between maximum adhesion force and adhesion force after buckling gives a reasonable window of operation for pick and place processes. High reversibility of the switching behavior is shown over 50 cycles in air and in vacuum, making the bioinspired switchable adhesive applicable for handling operations of fragile objects. KEYWORDS: gecko, responsive, switchable, dry adhesive, reversible, vacuum 1. INTRODUCTION The first systematic investigations of the adhesive mecha- nisms and the interactions of gecko toe pads with a broad 1 2 3 Animals like flies, ants, or beetles have developed versatile variety of substrates were made in the early 20th century. attachment systems which enable them to attach quickly and Weitlaner performed adhesion experiments with living and reversibly to surfaces of varying chemistry and topography, i.e., dead geckos to understand whether the gecko uses a smooth and rough surfaces. Their contact elements are covered “pneumatic mechanism” for attachment. Despite his very with millions of fine fibrils, which, often in combination with limited experimental equipment, he found that amputated 4 5 secretions, play a crucial role in adhesion. One of the most and shear loaded gecko feet did not lose their sticking capability complex and efficient adhesion systems is found in geckos, the to various surfaces even at reduced air pressure. He concluded 6,7 largest known animals with hairy attachment pads. Their that the, at that time assumed, “pneumatic mechanism” does attachment system is a “dry” system and does not rely on not have an essential impact on the extraordinary adhesive adhesion enhancing secretions. Although some phospholipids properties of the gecko toe pad but may only have a minor were found in gecko footprints, the function of these lipids contribution to adhesion. In summary, the gecko’s adhesion seem to be irrelevant for adhesion. The adhesive interaction of system combines the following properties: high adhesive forces, gecko toe pads with a surface is mainly based on van der Waals quick and easy detachment, dry “residue-free” contact, and 7 9,10 forces, likely enhanced by capillary forces due to humidity. operational in vacuum. Geckos can generate large forces, reaching a surprisingly high Hence, it is not surprising that this attachment system gains shear strength of up to 100 kPa. This performance is assumed growing attention, not only from the scientific community but to be related to good adaptability of the hairy attachment pads also from industry, especially as its properties may lead to new to roughness, an improved stress distribution, an increased artificial attachment devices, which could replace current state defect tolerance, and size effects. Besides the outstanding of the art systems such as suction cups. Consequently, artificial adhesive properties, a quick and easy release of the adhesive bioinspired adhesive systems have been extensively stud- 11,14−17 ied, and comparably high adhesive performance was pads is crucial for locomotion and, ultimately, the survival of the gecko. Detachment is controlled by the anisotropy of the adhesive structures and the biomechanics of the gecko’s Received: August 7, 2015 motion, which consists of simultaneous shear and peel Accepted: October 12, 2015 movement. Published: October 12, 2015 © 2015 American Chemical Society 24127 DOI: 10.1021/acsami.5b07287 ACS Appl. Mater. Interfaces 2015, 7, 24127−24135 ACS Applied Materials & Interfaces Research Article reached, even exceeding the so-called “gecko-limit” of 100 2. EXPERIMENTAL SECTION kPa. In extension to Weitlaner’s results on the adhesion of 2.1. Mold Preparation. Aluminum molds were fabricated using a 45,46 geckos, recent studies suggest that adhesion of (synthetic) process similar to the one reported in earlier studies. An array of holes with 2 mm depth, 0.4 mm width, and a center−center spacing of bioinspired surfaces relies, in addition to van der Waals 0.8 mm was milled. The geometrical parameters were chosen to yield interactions, to a small part on suction. It was found samples which possess a mechanical instability at high compressive experimentally that a small suction effect is present for 40,42 loading, known to lead to detachment. The array contained 203 mushroom-shaped patterned adhesives if adhesion is tested in 2 holes and covered an area of approximately 1 cm . The mold was 19−21 vacuum. It has also been predicted theoretically that thoroughly cleaned in acetone, ethanol, isopropanol, and deionized suction effects may become relevant in patterned surfaces as water in an ultrasonic bath and subsequently silanized. For this, the 22,23 mold was placed together with a glass vial containing 10 μLof soon as a certain critical contact size is exceeded. trichloroperfluorooctysilane (Sigma-Aldrich) into a desiccator and Many of these bioinspired systems have been applied to grip evacuated to a pressure below 10 mbar for at least 45 min until the and release objects; most approaches function close to the silane evaporated completely. Afterward, the mold was kept in an oven directional, shear induced adhesion found for geckos. There, in air at 95 °C for 2 h. switching adhesion mainly relies on asymmetric adhesive 2.2. Sample Preparation. Samples were prepared from structures, which exhibit high adhesion if sheared into one polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning) by soft molding of the previously prepared aluminum molds. The PDMS direction, while adhesion drops significantly if sheared in the prepolymer and cross-linker were mixed in a 10:1 ratio, poured onto opposite direction. The frequently occurring lateral displace- the silanized mold, and degassed in a desiccator. The filled mold was ment of the object during attachment and detachment may be then placed in an oven and cured at 75 °C for 4 h. After cooling to circumvented by gripper designs, where two or more room temperature, the PDMS sample was carefully peeled from the anisotropic adhesive pads are sheared in opposite directions mold, resulting in a PDMS array of cylindrical pillars. The backing so that the lateral forces cancel out. Other approaches layer was approximately 3 mm thick. 2.3. Tip Modification. Pillars with mushroom-shaped tips were combine electrostatic adhesion and bioinspired adhesives to reported to significantly increase adhesion compared to pillars having maintain a compressive preload on the adhesive structures or 18,45,47 flat or spherical tips. Thus, the tips were modified using a even use biological structures obtained from gecko toes for variation of a previously described process. Briefly, PDMS was mixed handling of small objects. An approach to handle objects with and degassed as described above. A thin metal rod was dipped into the a more complex geometry is based on a balloon-like gecko liquid PDMS and gently brought into contact with the pillars, resulting in deposition of a small droplet of liquid PDMS on the tip of each adhesive tape, which can be adapted to curved surface pillar. Afterward, the droplet-covered tips were placed face-down onto geometries by “inflating” and “deflating” the balloon. a smooth, silanized glass plate (silanization protocol as above). The Besides fabrication and characterization of bioinspired tips were squashed, resulting in a flattened mushroom-shape. The adhesives with high and robust adhesion and adhesion control patterned sample was fully cured in an oven at 75 °C for 4 h and, after using “passive” peeling or shearing, the control of adhesion by cooling to room temperature, carefully removed from the glass plate. 29−33 an external stimulus has been studied and improved to Three samples were chosen for adhesion experiments; an exemplary obtain switchable adhesives even in extreme environments like sample is shown in Figure 1a. outer space. While the complex detachment motion works efficiently for geckos and has already been mimicked relatively close to the natural archetype, other approaches have emerged to switch adhesion by using external triggers. Shape 36,37 memory materials, active polymeric materials such as liquid crystal elastomers, injection of liquids in subsurface micro- channels, application of magnetic fields to orient magnetic structures, or mechanical loading of rubber elastic patterned samples was applied to obtain switchability. The latter has been investigated in detail and shows detachment of rubber elastic pillars due to mechanical instability at high compressive 40−43 load, leading to a preload responsive switchable adhesive. Figure 1. (a) The photograph shows an exemplary bioinspired Due to the simplicity of the operation mode and the fast and switchable dry adhesive PDMS sample with an array of mushroom- reversible response, this approach shows significant potential shaped pillars. The inset exhibits a side view of a single mushroom for pick and place processes. In a more recent publication, this shaped tip. (b) The experimental adhesion tester setup is built in a approach of pressure activated switchable adhesion was vacuum oven for experiments at ambient air pressure and at low extended by using structures of different length to switch pressure condition (<10 mbar). between three adhesive states, namely, low, high, and very low adhesion. 2.4. Adhesion Testing Setup. An adhesion measurement setup To obtain pick and place handling in vacuum conditions, as shown in Figure 1b, inspired by the macroscopic adhesion robust adhesion in vacuum and reliable switchability need to be measurement device (MAD), was built in a vacuum oven. It consisted of a linear z-positioning system and a load-cell based force linked. In the present study, the adhesive performance of a measurement. Each patterned PDMS sample was fixated to a glass preload responsive, bioinspired adhesive was tested both in backing by applying oxygen plasma to the backside of the sample and vacuum and in air, and the influence of air pressure on adhesion bringing it into contact with the smooth cleaned glass plate. The was quantified. Further, pick and place processes were sample was mounted to a load cell with a stiffness of >100 kN/m. performed and the reversibility was demonstrated over 50 Prior to the adhesion measurements, the sample was aligned using a loading cycles. manual alignment stage and applying the alignment process published 24128 DOI: 10.1021/acsami.5b07287 ACS Appl. Mater. Interfaces 2015, 7, 24127−24135 ACS Applied Materials & Interfaces Research Article Figure 2. (a) Schematic of different phases during adhesion experiments; (1) the sample with the rubber elastic pillars is moved toward the glass slide, (2) attachment occurs and a preload P < P is applied, (3) the sample is moved upward, lifting the glass plate with the lifting force F , (4) a b L preload P > P is applied, where buckling of the pillars occurs, and (5) pull-off from the glass plate with the force F. The different sequences are schematically given in (b, c, and d). by Kroner et al. Force and displacement were recorded during all These phases can be grouped into different sequences to represent specific adhesion measurements. The following sequences were measurements. applied: 2.5. Adhesion Measurements. All experiments were performed with a testing velocity of 80 μm/s. Each sample was tested at least − Sequence 1 (Figure 2b): The preload P is chosen so that three times and thereby rotated by ∼120° along the vertical axis in contact is formed with the glass plate, but no buckling of the between the experiments to avoid misalignment. Adhesion measure- structures occurs. The glass plate is fixated to prevent it from ments were conducted in ambient atmosphere, called “air” in the lifting. This experiment corresponds to the phases (1) (2) (5). subsequent text, and at reduced pressure <10 mbar, called “vacuum” in − Sequence 2 (Figure 2c): In this sequence, a preload P above the the subsequent text. A glass plate was used as test substrate which was, critical buckling load P is applied. The glass plate is also depending on the testing mode, either fixated or loosened. To obtain fixated. This experiment is represented by the phases (1) (2) an equilibrium surface state, more than 300 contacts were made (4) (5). between a smooth PDMS sample and the glass plate prior to adhesion − Sequence 3 (Figure 2d): A pick and place process is imitated 50,51 measurements. The experimental error for all adhesion measure- using a loosened glass plate. A preload P below the buckling ments was ±0.1 N. load P is applied and the glass plate is lifted, which Different phases occurred during an adhesion experiment with corresponds to the phases (1) (2) (3). In the next step, the preload responsive dry-adhesive samples, which are schematically glass plate is lowered and detached using a preload P above the shown in Figure 2a and can be described as follows: buckling load P , which corresponds to the phases (2) (4) (5). − Phase (1): The aligned sample is moved toward the smooth The complete pick and place process is described by phases (1) (2) (3) (2) (4) (5). glass plate. − Phase (2): The sample forms contact with the glass plate, and a 2.6. Applied Measurement Sets. To determine the adhesive properties of the switchable bioinspired adhesive and its applicability compressive preload P below the critical buckling preload P is for pick and place processes, the following measurement sets and applied. The load is kept for at least three seconds. analyses were conducted in air and vacuum conditions: − Phase (3): The sample is retracted and, due to adhesion, the (i) Force−time curves were recorded for different preloads up to 7 glass plate is lifted with a force F . This phase only applies if the N, allowing determination of the preload dependent pull-off glass plate is loosened. force behavior of the switchable adhesive. The pull-off force F − Phase (4): A preload above the critical buckling preload P is (absolute value of the maximum detachment force) was plotted applied, causing the structures to buckle and the structure tips as a function of preload P, leading to the identification of the to detach from the glass probe. buckling preload P . These measurements correspond to − Phase (5): The sample is retracted from the glass plate, and the sequence 1 for P < P and sequence 2 for P > P . b b pull-off force F,defined as the absolute value of the maximum (ii) Exemplary force−time curves from (i) were analyzed for two negative force of the recorded force−time curves, is measured. selected measurements, one having a preload P < P according 24129 DOI: 10.1021/acsami.5b07287 ACS Appl. Mater. Interfaces 2015, 7, 24127−24135 ACS Applied Materials & Interfaces Research Article to sequence 1, and one having a preload P > P according to in air, while experiments in vacuum resulted in pull-off forces sequence 2. between 1.8 and 1.9 N. It can be clearly seen that, for lower (iii) Adhesion experiments with sequence 1 directly followed by preloads, the pull-off force depends on the air pressure; sequence 2 were repeated 50 times to test for reversibility. adhesion was reduced by about 30% in vacuum. For P > P , the (iv) Force−time curves were recorded for a pick and place process pull-off force was found to be 0.9 N and was independent of air represented by sequence 3. The glass plate with a weight of 65 pressure. g was lifted for at least 10 s during phase (3). Reversibility was 3.2. Measurement Set (ii): Force−Time Curves. again tested by repeating this sequence for 50 times. Exemplary force−time curves with preload P < P and P > P , respectively, measured in air and vacuum, are plotted in 3. RESULTS Figure 4. The experimental results of the different measurement sets are Figure 4a,b depicts exemplary force−time curves of adhesion described in the following four subsections. measurements with a preload P < P , which corresponds to 3.1. Measurement Set (i): Preload Dependent Pull-off sequence 1. The sample was moved toward the fixated glass Force. Pull-off forces F were measured as a function of preload plate, formed contact, and was loaded, until the preload P was and are given in Figure 3. The pull-off force was found to be reached. In these examples, the preload P was 3.7 N for testing in air (Figure 4a) and 4.0 N for testing in vacuum (Figure 4b). The load was applied for at least 3 s. Then, the sample was retracted again, leading to an adhesive (tensile) force, ultimately reaching the maximum pull-off force F. The negative sign indicates the direction of force measurement. For these measurements, a pull-off force F of 2.6 N was found in air (Figure 4a) and of 1.9 N (Figure 4b) in vacuum. Representative force−time curves, where a preload P > P was applied corresponding to sequence 2, are shown in Figure 4c,d. The sample was approached, formed contact with the glass probe, and was loaded. The compressive force increased during loading until a local maximum occurred at a critical load P of 5.8 N. Subsequently, the compressive force decreased Figure 3. Absolute values of the pull-off forces are plotted as a b rapidly with ongoing compression and buckling of the pillars function of applied preload, measured in air and in vacuum. At a critical preload P , indicated by the dashed line within the gray area, was optically observed. As the pillars were bent further with buckling of the pillars was observed. increasing displacement, the compressive force increased again until the predefined preload P was reached and kept for at least almost preload independent at low preloads. As soon as a 3s.In Figure 4c, the preload was 6.6 N, and in Figure 4d, the critical preload was applied, the pull-off force dropped preload was 6.3 N. The reverse force−time behavior was significantly which corresponded to the optically observed observed during retraction; the occurring maximum corre- elastic buckling of the pillars. The critical buckling preload P sponded to an optically observed “unbuckling” of the pillars. A was highly reproducible for each sample but showed some pull-off force F of 0.9 N was recorded during retraction, both variation in a range from 5.6 to 6.0 N for different samples. For for measurements in air and in vacuum. These force−time preloads P < P , the pull-off forces were between 2.5 and 2.7 N curves for a preload P > P are characteristic for the buckling b b Figure 4. Representative force−time curves for adhesion experiments of bioinspired dry adhesives on a fixated glass plate. Measurements using sequence 1 with a preload P below the buckling preload P in (a) air and (b) in vacuum. Measurements using sequence 2 with a preload P above the buckling preload P (c) in air and d) in vacuum. The phases from Figure 2a are indicated. 24130 DOI: 10.1021/acsami.5b07287 ACS Appl. Mater. Interfaces 2015, 7, 24127−24135 ACS Applied Materials & Interfaces Research Article behavior and have been reported and characterized in earlier studies. The force−time curves are very similar for experiments in air and in vacuum. A significant difference was found only in the pull-off force F; experiments with a preload P < P resulted in a change in pull-off force F from 2.6 to 1.9 N, which is equal to a loss in adhesion of approximately 30%. For a preload P > P , the pull-off force F was substantially lower, reaching only 0.9 N, and did not differ between air and vacuum condition. 3.3. Measurement Set (iii): Reversibility. To evaluate the reversibility of the switching behavior between high and low pull-off force, alternating preloads below and above P were applied. 50 cycles of the sequence 1, directly followed by sequence 2, were performed in air and in vacuum according to the measurement set (iii). Figure 5 shows the recorded forces, Figure 6. Force−time curves of a pick and place process, where a glass plate of 65 g is lifted and released again. The experiment corresponds to the sequence 3; the numbers indicate the respective phases from Figure 2. The measurements were performed (a) in air and (b) in vacuum. the glass plate were comparable in both air and vacuum conditions. An exemplary video of a pick and place process is shown in the Supporting Information, using a silicon wafer instead of a glass plate for the sake of better visibility. 4. DISCUSSION On the basis of the experimental results, the following properties of the pressure actuated adhesive system in air and vacuum conditions were analyzed: the adhesive properties, the reversibility of switching, and the adaptability for pick and place processes. Figure 5. Reversibility tests of adhesion measurements with alternating The adhesive properties, represented by the force−time preload below and above the buckling load, described by sequence 1, directly followed by sequence 2. Each plot shows the applied preload P curves, are given in Figure 4. The curves exhibit a characteristic and the resulting pull-off force F over 50 test cycles (a) in air and (b) shape which is typical for patterned bioinspired adhesives such in vacuum. Note that the pull-off force F is not given as absolute value as the tested samples. Low preload leads to a comparably high here but has a negative sign for clarity of the diagram. pull-off force, while high preload results in buckling of the structures at a certain buckling preload P , which reduces namely, the preload P and the pull-off force F, as a function of adhesion significantly. The mechanism of adhesion loss has testing cycles. Note that the pull-off force F is not given as been investigated in an earlier study, where it was found that absolute value but has a negative sign for clarity of the diagram. the unbuckling during unloading does not allow reformation of In air (Figure 5a), the alternating preloads P of 2.9 and 5.8 N intimate contact between the pillar tips and the probe. The resulted in adhesive forces F of 2.6 and 0.9 N, respectively. In lack of intimate contact between pillar tips and probe causes a vacuum (Figure 5b), the applied preloads P were 3.3 and 5.8 N, reduction in adhesion. The adhesive behavior in air and vacuum which resulted in adhesive forces F of 1.9 and 0.9 N, conditions is qualitatively similar, indicating that the mecha- respectively. No notable changes in pull-off force were found nism of adhesion loss by buckling is unaffected by air pressure. within the 50 test cycles. The main influence of air pressure on the adhesive properties is 3.4. Measurement Set (iv): Pick and Place. A pick and found in the magnitude of the pull-off force, which can be seen place process, corresponding to the sequence 3 in Figure 2d, in Figures 3 and 4a,b (indicated as ΔF). The pull-off force F was simulated. As described in the measurement set (iv), an was found to be between 2.5 and 2.7 N in air and 1.8 and 1.9 N alternating preload below and above the buckling load P was in vacuum, respectively, exhibiting that the application of applied with the glass plate being loosened to allow lifting. vacuum reduces adhesion by ΔF of 0.7 N ± 0.2 N, which Representative force−time curves in air and in vacuum are corresponds to a loss in adhesion of approximately 30%. This given in Figure 6a,b, respectively. The graphs show no notable reduction becomes obvious by considering Figure 3; all pull-off differences, indicating that the lifting process and the release of forces obtained in vacuum lie below the ones obtained in air if 24131 DOI: 10.1021/acsami.5b07287 ACS Appl. Mater. Interfaces 2015, 7, 24127−24135 ACS Applied Materials & Interfaces Research Article the preload was chosen to be below the critical buckling Table 1. Geometric Parameters of Commercial Suction Cups preload P . and Their Adhesive Performance after Attaching Them with 2 a We identified two factors which may be responsible for the a Compressive Load of 9.0 N/cm varying adhesion with changing air pressure, namely, humidity diameter, contact area, force, strength, corrected strength, N/ and suction. We tend to exclude humidity and favor suction as 2 2 2 mm mm N N/cm cm the main mechanism for the change in pull-off force for the 2.6 5.3 0.15 2.8 0.9 following reasons. PDMS is a hydrophobic material which does 3.8 11.3 0.65 5.7 1.7 not tend to absorb water in larger quantities. In addition, it was 5.0 19.6 1.3 6.6 2.0 found that no measurable humidity effect is present at humidity 7.0 38.5 2.5 6.5 2.0 between 2% and 90% for smooth PDMS surfaces and for pillar 9.0 63.6 3.9 6.1 1.8 arrays with diameters of 25 μm. Huber et al. found an 11.0 95.0 6.9 7.3 2.2 additional adhesion effect in the presence of humidity and 16.5 213.8 11.0 5.1 1.5 explained it by a smoothening effect of the water on rough 22.0 380.1 16.0 4.2 1.3 surfaces, but the tested surfaces in this study are expected to 32.0 804.2 30.0 3.7 1.1 be smooth. These points indicate that capillarity effects may 41.0 1320.3 49.0 3.7 1.1 have a minor influence on the adhesion in our experiments. 51.4 2075.0 92.0 4.4 1.3 In contrast, suction effects on mushroom shaped pillars are The corrected strength assumes a packing density of 30%, data after expected from theoretical considerations and were also found 19,20 ref 55. in earlier experimental studies. It was shown that suction is present for adhesive pillars with mushroom-shaped tips and can suction force of 0.85 N. As the mushroom shaped structures in contribute considerably to adhesion with up to 10% of the pull- our study were not specifically optimized for suction, these off force. In our case, the suction component even exceeds results fit astonishingly well to the values provided for this percentage, reaching approximately 30%. While adhesion commercial suction cups. experiments in air result in pull-off forces between 2.5 and 2.7 It was reported that suction and van der Waals interactions N, the same set of experiments in vacuum exhibits pull-off have a different size effect. Thus, we expect that the suction forces between 1.8 and 1.9 N. This difference may be explained effect becomes more prominent with increasing size of the by the size of the pillars. Suction based forces scale with the contact elements, while reducing the size of the contact area of the contact, while adhesion of patterned surfaces due to elements diminishes the influence of suction. van der Waals forces was theoretically and experimentally 53,54 Consequently, if suction caused the change in adhesion of shown to scale with length. the present experiments, it would be strongly influenced by the Theoretically, the suction force F of a perfect suction suction contact geometry. During the buckling process, the mushroom cup, disregarding other adhesive interactions than suction, is tips detach and the pillars form side contact with the glass plate. given by the contact area A and the pressure difference ΔP contact This contact geometry does not allow building up a difference caused by the suction effect: in air pressure, thereby diminishing the suction component of FP =Δ ×A (1) suction contact the pull-off force. Our experiments show that the application of a load exceeding P leads to a pull-off force F of 0.9 N, both in Consequently, the pull-off strength of a perfect suction cup is air and in vacuum. This phenomenon is also reflected in Figure directly proportional to the pressure difference inside the 3; while the pull-off forces at a preload below P differ for contact area and outside the suction cup. For ideal vacuum (0 measurements in air and in vacuum, similar pull-off forces are bar) and atmospheric pressure (∼1 bar), a suction force of ∼10 found if the buckling preload P is overcome. These N/cm can be achieved using eq 1. Such high values are usually observations support the assumption that air pressure enhances not obtained using typical suction cups. adhesion due to suction in patterned bioinspired surfaces with To compare our experimentally derived pull-off strength structure sizes in the macroscopic range, while detachment values to the performance of typical suction cups, we have events after buckling of the pillars are not affected by air converted given data from commercial macroscopic silicone 55 pressure, since suction cannot be maintained after buckling has suction cups. The performance of the analyzed suction cups occurred. This leads to the conclusion that the difference in with diameters between 2.6 and 51.4 mm lie between 2.8 and 55 pull-off force of the bioinspired adhesive is not a result of 2 2 7.3 N/cm if a compressive stress of 9.0 N/cm is applied; see changing humidity but is caused most likely by a suction effect. also Table 1. For comparing these pull-off strength data with For a better description of the switching behavior, a our results, it is important to consider that the strength data switching efficiency S is introduced in eq 2, which is defined from the present study reflects the apparent contact strength. by the ratio of the pull-off forces at a preload above and below Thus, reduction in “real” contact area due to the pillar packing the buckling preload P : density of ∼30% has to be taken into account. A comparable pull-off performance between conventional suction cups and FP() > P S=− 1 the experiments from our studies would then result in a FP() < P (2) corrected strength, which is calculated by multiplying the given pull-off strength of the commercial suction cups with the pillar . packing density of ∼30% from our samples. These values are A value of S = 0 indicates no switching behavior, and S =1 also given in Table 1. resembles a perfect switch where adhesion can be completely As can be seen from the corrected pull-off strengths, values turned on and off.If eq 2 is applied to the obtained between 0.9 and 2.2 N/cm can be considered as typical for experimental data, the switching efficiency S is approximately commercial suction cups. Our experimentally derived suction 0.65 ± 0.07 in air, while a value of S = 0.50 ± 0.1 is obtained in component to the pull-off force lies slightly below the lowest vacuum. Thus, applying vacuum reduces the switching 24132 DOI: 10.1021/acsami.5b07287 ACS Appl. Mater. Interfaces 2015, 7, 24127−24135 ACS Applied Materials & Interfaces Research Article efficiency by a mean value of ΔS = 0.15. These calculated ASSOCIATED CONTENT efficiencies indicate that the switch in adhesion may be further * Supporting Information improved. Still, the reached values allow a significant change in The Supporting Information is available free of charge on the adhesion in air and in vacuum, opening a sufficiently large ACS Publications website at DOI: 10.1021/acsami.5b07287. window of operation for pick and place applications. Video showing an exemplary pick and place process These promising results are promoted by the reversibility test using a switchable bioinspired adhesive to handle a shown in Figure 5, which indicates that the switch in adhesion silicon wafer (AVI) is highly reversible in air and in vacuum. No change in adhesive performance or damage of the dry adhesive structures was AUTHOR INFORMATION detected after 50 testing cycles. Finally, pick and place Corresponding Author processes were conducted using a glass plate with a weight of *E-mail: elmar.kroner@leibniz-inm.de. Phone: +49 (0) 681 65 g. The glass plate was securely lifted and released in air and 9300 369. in vacuum. No significant difference is observed in the adhesion Author Contributions curves given in Figure 6 for operation at both air pressure J.P., M.F., and E.K. contributed equally. conditions. Hence, the pick and place process is not notably Funding influenced by the reduction of air pressure. The research leading to these results was conducted within a It should be mentioned that the pick and release process has Grant of the European Research Council under the European two restrictions for operation if a reliable switch in adhesion is Union’s Seventh Framework Program (FP/2007-2013)/ERC required. First, if the object to be lifted is too light, the pull-off Grant Agreement No. 340929, awarded to E. Arzt. force after application of a preload above the buckling preload Notes P may be too high for reliable detachment, representing a The authors declare no competing financial interest. minimum weight threshold, and second, if the object to be lifted is too heavy, it will detach prior to lifting, representing a ACKNOWLEDGMENTS maximum weight threshold. It follows that an optimum range The authors thank the Mechanical Workshop of the INM for of operation can be defined on the basis of the pull-off forces mold fabrication, Susanne Selzer and Ina Kothe for their help in measured for a preload below, and above, the buckling preload sample preparation, and Joachim Blau for building up the P . For the tested samples, the range of operation can be adhesion measurement setup. determined to be between approximately 0.9 and 2.5 N in air or between 0.9 and 1.8 N in vacuum. It further has to be ABBREVIATIONS considered that the viscoelasticity of the applied material may CVD = chemical vapor deposition have a significant influence in the buckling of the structures and PVD = physical vapor deposition may shift both the lower and the higher boundary of the range PDMS = polydimethylsiloxane of operation. This window of operation may be tuned MAD = macroscopic adhesion measurement device according to the envisaged application, for example, by changing the number of pillars, by their packing density, by REFERENCES further modification of their tip geometry, or by a different (1) Langer, M. G.; Ruppersberg, J. P.; Gorb, S. Adhesion Forces choice of sample material. Measured at the Level of a Terminal Plate of the Fly’s Seta. Proc. R. Soc. London, Ser. B 2004, 271 (1554), 2209−2215. (2) Federle, W.; Rohrseitz, K.; Holldobler, B. Attachment Forces of 5. CONCLUSIONS Ants Measured with a Centrifuge: Better ″Wax-Runners″ have a The present study showed that bioinspired switchable adhesion Poorer Attachment to a Smooth Surface. J. Exp. Biol. 2000, 203 (3), 505−512. based on reversible buckling of elastic pillars is applicable in (3) Stork, N. E. Experimental Analysis of Adhesion of Chrysolina vacuum. At low compressive load, the pull-off force for samples 2 Polita (Chrysomelidae: Coleoptera) on a Variety of Surfaces. J. Exp. with sizes of 1 cm was between 2.5 and 2.7 N (±0.1 N) in air Biol. 1980, 88 (1), 91−108. and was reduced to 1.8 to 1.9 N (±0.1 N) if measured in (4) Federle, W. Why Are So Many Adhesive Pads Hairy? J. Exp. Biol. vacuum. This indicates that a suction component was present 2006, 209, 2611−2621. in the attachment state, since an influence of humidity may be (5) Jagota, A.; Bennison, S. J. Mechanics of Adhesion Through a excluded. Application of a compressive load above the buckling Fibrillar Microstructure. Integr. Comp. Biol. 2002, 42 (6), 1140−1145. (6) Autumn, K.; Liang, Y. A.; Hsieh, S. T.; Zesch, W.; Chan, W. P.; preload P between 5.6 and 6.0 N (±0.1 N) caused a reversible Kenny, T. W.; Fearing, R.; Full, R. J. Adhesive Force of a Single Gecko buckling of the pillars and resulted in pull-off forces of 0.9 N Foot-Hair. Nature 2000, 405 (6787), 681−685. (±0.1 N), which were similar for experiments in air and in (7) Autumn, K.; Sitti, M.; Liang, Y. A.; Peattie, A. M.; Hansen, W. R.; vacuum. This indicates the absence of a suction component Sponberg, S.; Kenny, T. W.; Fearing, R.; Israelachvili, J. N.; Full, R. J. after buckling of the pillars occurred. Our experiments Evidence for Van Der Waals Adhesion in Gecko Setae. Proc. Natl. exhibited that the transition between the two adhesive states Acad. Sci. U. S. A. 2002, 99 (19), 12252−12256. (8) Hsu, P. Y.; Ge, L.; Li, X.; Stark, A. Y.; Wesdemiotis, C.; was sharp and the switching behavior was independent of air Niewiarowski, P. H.; Dhinojwala, A. Direct Evidence of Phospholipids pressure. Further, the switch in adhesion exhibited high in Gecko Footprints and Spatula−Substrate Contact Interface reversibility; we showed that the system works reliably in air Detected using Surface-Sensitive Spectroscopy. J. R. Soc., Interface and in vacuum for 50 pick and place cycles without any signs of 2012, 9 (69), 657−664. wear or change in adhesion performance. The functionality of (9) Buhl, S.; Greiner, C.; del Campo, A.; Arzt, E. Humidity Influence the switchable adhesive at low air pressure makes it applicable on the Adhesion of Biomimetic Fibrillar Surfaces. Int. J. Mater. Res. for handling operations of fragile objects in vacuum. 2009, 100 (8), 1119−1126. 24133 DOI: 10.1021/acsami.5b07287 ACS Appl. Mater. Interfaces 2015, 7, 24127−24135 ACS Applied Materials & Interfaces Research Article (10) Huber, G.; Gorb, S. N.; Hosoda, N.; Spolenak, R.; Arzt, E. (32) Kim, S.; Sitti, M.; Xie, T.; Xiao, X. Reversible Dry Micro- Influence of Surface Roughness on Gecko Adhesion. Acta Biomater. Fibrillar Adhesives with Thermally Controllable Adhesion. Soft Matter 2007, 3 (4), 607−610. 2009, 5 (19), 3689−3693. (11) Kamperman, M.; Kroner, E.; del Campo, A.; McMeeking, R. M.; (33) Nadermann, N.; Ning, J.; Jagota, A.; Hui, C. Y. Active Switching Arzt, E. Functional Adhesive Surfaces with “Gecko” Effect: The of Adhesion in a Film-Terminated Fibrillar Structure. Langmuir 2010, Concept of Contact Splitting. Adv. Eng. Mater. 2010, 12 (5), 335−348. 26 (19), 15464−15471. (12) Autumn, K.; Dittmore, A.; Santos, D.; Spenko, M.; Cutkosky, (34) Henrey, M.; Tellez, J. P. D.; Wormnes, K.; Pambaguian, L.; M. Frictional Adhesion: a New Angle on Gecko Attachment. J. Exp. Menon, C. Towards the Use of Mushroom-Capped Dry Adhesives in Biol. 2006, 209 (18), 3569−3579. Outer Space: Effects of Low Pressure and Temperature on Adhesion (13) Kroner, E.; Davis, C. S. A Study of the Adhesive Foot of the Strength. Aerosp. Sci. Technol. 2013, 29 (1), 185−190. Gecko Translation of a Publication of Dr. F. Weitlaner. J. Adhes. 2015, (35) Kwak, M. K.; Pang, C.; Jeong, H.-E.; Kim, H.-N.; Yoon, H.; 91 (6), 481−487. Jung, H.-S.; Suh, K.-Y. Towards the Next Level of Bioinspired Dry (14) Heepe, L.; Gorb, S. Biologically Inspired Mushroom-Shaped Adhesives: New Designs and Applications. Adv. Funct. Mater. 2011, 21 Adhesive Microstructures. Annu. Rev. Mater. Res. 2014, 44, 173−203. (19), 3606−3616. (15) Pattantyus-Abraham, A.; Krahn, J.; Menon, C. Recent Advances (36) Frensemeier, M.; Kaiser, J. S.; Frick, C. P.; Schneider, A. S.; Arzt, in Nanostructured Biomimetic Dry Adhesives. Front. Bioeng. E.; Fertig, R. S.; Kroner, E. Temperature-Induced Switchable Adhesion Biotechnol. 2013, 1 (22); DOI: 10.3389/fbioe.2013.00022 using Nickel−Titanium−Polydimethylsiloxane Hybrid Surfaces. Adv. (16) Sameoto, D.; Menon, C. Recent Advances in the Fabrication Funct. Mater. 2015, 25 (20), 3013−3021. and Adhesion Testing of Biomimetic Dry Adhesives. Smart Mater. (37) Reddy, S.; Del Campo, A.; Arzt, E. Bioinspired Surfaces with Struct. 2010, 19 (10), 103001. Switchable Adhesion. Adv. Mater. 2007, 19, 3833−3837. (17) Zhou, M.; Pesika, N.; Zeng, H.; Tian, Y.; Israelachvili, J. Recent (38) Cui, J.; Drotlef, D.-M.; Larraza, I.; Fernandez-Bla ́ zquez, ́ J. P.; Advances in Gecko Adhesion and Friction Mechanisms and Boesel, L. F.; Ohm, C.; Mezger, M.; Zentel, R.; del Campo, A. Development of Gecko-Inspired Dry Adhesive Surfaces. Friction Bioinspired Actuated Adhesive Patterns of Liquid Crystalline 2013, 1 (2), 114−129. Elastomers. Adv. Mater. 2012, 24 (34), 4601−4604. (18) Del Campo, A.; Greiner, C.; Arzt, E. Contact Shape Controls (39) Northen, M. T.; Greiner, C.; Arzt, E.; Turner, K. L. A Gecko- Adhesion of Bioinspired Fibrillar Surfaces. Langmuir 2007, 23 (20), Inspired Reversible Adhesive. Adv. Mater. 2008, 20 (20), 3905−3909. 10235−10243. (40) Paretkar, D.; Kamperman, M.; Schneider, A. S.; Martina, D.; (19) Heepe, L.; Varenberg, M.; Itovich, Y.; Gorb, S. N. Suction Creton, C.; Arzt, E. Bioinspired Pressure Actuated Adhesive System. Component in Adhesion of Mushroom-Shaped Microstructure. J. R. Mater. Sci. Eng., C 2011, 31 (6), 1152−1159. Soc., Interface 2011, 8 (57), 585−589. (41) Paretkar, D.; Kamperman, M.; Martina, D.; Zhao, J.; Creton, C.; (20) Davies, J.; Haq, S.; Hawke, T.; Sargent, J. P. A Practical Lindner, A.; Jagota, A.; McMeeking, R.; Arzt, E. Preload-Responsive Approach to the Development of a Synthetic Gecko Tape. Int. J. Adhes. Adhesion: Effects of Aspect Ratio, Tip Shape and Alignment. J. R. Soc., Adhes. 2009, 29 (4), 380−390. Interface 2013, 10 (83), 20130171; DOI: 10.1098/rsif.2013.0171 (21) Sameoto, D.; Sharif, H.; Menon, C. Investigation of Low- (42) Paretkar, D.; Schneider, A. S.; Kroner, E.; Arzt, E. In Situ Pressure Adhesion Performance of Mushroom Shaped Biomimetic Observation of Contact Mechanisms in Bioinspired Adhesives at High Dry Adhesives. J. Adhes. Sci. Technol. 2012, 26 (23), 2641−2652. Magnification. MRS Commun. 2011, 1 (1), 53−56. (22) Afferrante, L.; Carbone, G. The Mechanisms of Detachment of (43) Varenberg, M.; Gorb, S. Close-Up of Mushroom-Shaped Mushroom-Shaped Micro-Pillars: From Defect Propagation to Fibrillar Adhesive Microstructure: Contact Element Behaviour. J. R. Membrane Peeling. Macromol. React. Eng. 2013, 7 (11), 609−615. Soc., Interface 2008, 5 (24), 785−789. (23) Spolenak, R.; Gorb, S.; Gao, H.; Arzt, E. Effects of Contact (44) Isla, P. Y.; Kroner, E. A Novel Bioinspired Switchable Adhesive Shape on the Scaling of Biological Attachments. Proc. R. Soc. London, with Three Distinct Adhesive States. Adv. Funct. Mater. 2015, 25 (16), Ser. A 2005, 461 (2054), 305−319. 2444−2450. (24) Mengüc,̧ Y.; Yang, S. Y.; Kim, S.; Rogers, J. A.; Sitti, M. Gecko- (45) Kroner, E.; Arzt, E. Single Macropillars as Model Systems for Inspired Controllabe Adhesive Structures Appied to Micromanipula- Tilt Angle Dependent Adhesion Measurements. Int. J. Adhes. Adhes. tion. Adv. Funct. Mater. 2012, 22 (6), 1246−1254. 2012, 36,32−38. (25) Zhou, M.; Tian, Y.; Sameoto, D.; Zhang, X.; Meng, Y.; Wen, S. (46) Kroner, E.; Arzt, E. Mechanistic Analysis of Force-Displacement Controllable Interfacial Adhesion Applied to Transfer Light and Measurements on Macroscopic Single Adhesive Pillars. J. Mech. Phys. Fragile Objects by Using Gecko Inspired Mushroom-Shaped Pillar Solids 2013, 61 (6), 1295−1304. Surface. ACS Appl. Mater. Interfaces 2013, 5, 10137−10144. (47) Micciche,́ M.; Arzt, E.; Kroner, E. Single Macroscopic Pillars as (26) Ruffatto, D.; Parness, A.; Spenko, M. Improving Controllable Model System for Bioinspired Adhesives: Influence of Tip Dimension, Adhesion on Both Rough and Smooth Surfaces with a Hybrid Aspect Ratio, and Tilt Angle. ACS Appl. Mater. Interfaces 2014, 6 (10), Electrostatic/Gecko-Like Adhesive. J. R. Soc., Interface 2014, 11 (93), 7076−7083. 20131089. (48) Kroner, E.; Blau, J.; Arzt, E. Note: An Adhesion Measurement (27) Jeong, J.; Kim, J.; Song, K.; Autumn, K.; Lee, J. Geckopringint: Setup for Bioinspired Fibrillar Surfaces using Flat Probes. Rev. Sci. Assembly of Microelectronic Devices on Unconventional Surfaces by Instrum. 2012, 83 (1), 106101. Transfer Printing with Isolated Gecko Setal Arrays. J. R. Soc., Interface (49) Kroner, E.; Paretkar, D.; McMeeking, R.; Arzt, E. Adhesion of 2014, 11 (99), 20140627. Flat and Structured PDMS Samples to Spherical and Flat Probes: A (28) Song, S.; Sitti, M. Soft Grippers Using Micro-Fibrillar Adhesives Comparative Study. J. Adhes. 2011, 87 (5), 447−465. (50) Kroner, E.; Maboudian, R.; Arzt, E. Effect of Repeated Contact for Transfer Printing. Adv. Mater. 2014, 26 (28), 4901−4906. (29) Arul, E. P.; Ghatak, A. Control of Adhesion via Internally on Adhesion Measurements Involving Polydimethylsiloxane Structural Pressurized Subsurface Microchannels. Langmuir 2012, 28 (9), 4339− Material. IOP Conf. Ser.: Mater. Sci. Eng. 2009, 5, 012004. 4345. (51) Kroner, E.; Maboudian, R.; Arzt, E. Adhesion Characteristics of (30) Kier, W. M.; Smith, A. M. The Structure and Adhesive PDMS Surfaces During Repeated Pull-Off Force Measurements. Adv. Mechanism of Octopus Suckers. Integr. Comp. Biol. 2002, 42 (6), Eng. Mater. 2010, 12 (5), 398−404. 1146−1153. (52) Huber, G.; Mantz, H.; Spolenak, R.; Mecke, K.; Jacobs, K.; (31) Jeong, H. E.; Kwak, M. K.; Suh, K. Y. Stretchable, Adhesion- Gorb, S. N.; Arzt, E. Evidence for Capillarity Contributions to Gecko Tunable Dry Adhesive by Surface Wrinkling. Langmuir 2010, 26 (4), Adhesion from Single Spatula Nanomechanical Measurements. Proc. 2223−2226. Natl. Acad. Sci. U. S. A. 2005, 102 (45), 16293−16296. 24134 DOI: 10.1021/acsami.5b07287 ACS Appl. Mater. Interfaces 2015, 7, 24127−24135 ACS Applied Materials & Interfaces Research Article (53) Arzt, E.; Gorb, S.; Spolenak, R. From Micro to Nano Contacts in Biological Attachment Devices. Proc. Natl. Acad. Sci. U. S. A. 2003, 100 (19), 10603−10606. (54) Greiner, C.; del Campo, A.; Arzt, E. Adhesion of Bioinspired Micropatterned Surfaces: Effects of Pillar Radius, Aspect Ratio, and Preload. Langmuir 2007, 23 (7), 3495−3502. (55) P. V. GmbH Product database. https://www.piab.com/ Products/suction-cups/shape/universal/u---universal-2-50-mm/ (ac- cessed Sep 21, 2015). 24135 DOI: 10.1021/acsami.5b07287 ACS Appl. Mater. Interfaces 2015, 7, 24127−24135 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png ACS Applied Materials & Interfaces Pubmed Central

Switchable Adhesion in Vacuum Using Bio-Inspired Dry Adhesives

ACS Applied Materials & Interfaces , Volume 7 (43) – Oct 12, 2015

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Copyright © 2015 American Chemical Society
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1944-8244
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1944-8252
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10.1021/acsami.5b07287
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Abstract

This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Research Article www.acsami.org †,‡,§ †,‡,§ ,‡,§ Julia Purtov, Mareike Frensemeier, and Elmar Kroner* Department of Materials Science and Engineering, Saarland University, Campus D2 2, 66123 Saarbrücken, Germany INM − Leibniz Institute for New Materials, Campus D2 2, 66123 Saarbrücken, Germany * Supporting Information ABSTRACT: Suction based attachment systems for pick and place handling of fragile objects like glass plates or optical lenses are energy-consuming and noisy and fail at reduced air pressure, which is essential, e.g., in chemical and physical vapor deposition processes. Recently, an alternative approach toward reversible adhesion of sensitive objects based on bioinspired dry adhesive structures has emerged. There, the switching in adhesion is achieved by a reversible buckling of adhesive pillar structures. In this study, we demonstrate that these adhesives are capable of switching adhesion not only in ambient air conditions but also in vacuum. Our bioinspired patterned adhesive with an area of 1 cm provided an adhesion force of 2.6 N ± 0.2 N in air, which was reduced to 1.9 N ± 0.2 N if measured in vacuum. Detachment was induced by buckling of the structures due to a high compressive preload and occurred, independent of air pressure, at approximately 0.9 N ± 0.1 N. The switch in adhesion was observed at a compressive preload between 5.6 and 6.0 N and was independent of air pressure. The difference between maximum adhesion force and adhesion force after buckling gives a reasonable window of operation for pick and place processes. High reversibility of the switching behavior is shown over 50 cycles in air and in vacuum, making the bioinspired switchable adhesive applicable for handling operations of fragile objects. KEYWORDS: gecko, responsive, switchable, dry adhesive, reversible, vacuum 1. INTRODUCTION The first systematic investigations of the adhesive mecha- nisms and the interactions of gecko toe pads with a broad 1 2 3 Animals like flies, ants, or beetles have developed versatile variety of substrates were made in the early 20th century. attachment systems which enable them to attach quickly and Weitlaner performed adhesion experiments with living and reversibly to surfaces of varying chemistry and topography, i.e., dead geckos to understand whether the gecko uses a smooth and rough surfaces. Their contact elements are covered “pneumatic mechanism” for attachment. Despite his very with millions of fine fibrils, which, often in combination with limited experimental equipment, he found that amputated 4 5 secretions, play a crucial role in adhesion. One of the most and shear loaded gecko feet did not lose their sticking capability complex and efficient adhesion systems is found in geckos, the to various surfaces even at reduced air pressure. He concluded 6,7 largest known animals with hairy attachment pads. Their that the, at that time assumed, “pneumatic mechanism” does attachment system is a “dry” system and does not rely on not have an essential impact on the extraordinary adhesive adhesion enhancing secretions. Although some phospholipids properties of the gecko toe pad but may only have a minor were found in gecko footprints, the function of these lipids contribution to adhesion. In summary, the gecko’s adhesion seem to be irrelevant for adhesion. The adhesive interaction of system combines the following properties: high adhesive forces, gecko toe pads with a surface is mainly based on van der Waals quick and easy detachment, dry “residue-free” contact, and 7 9,10 forces, likely enhanced by capillary forces due to humidity. operational in vacuum. Geckos can generate large forces, reaching a surprisingly high Hence, it is not surprising that this attachment system gains shear strength of up to 100 kPa. This performance is assumed growing attention, not only from the scientific community but to be related to good adaptability of the hairy attachment pads also from industry, especially as its properties may lead to new to roughness, an improved stress distribution, an increased artificial attachment devices, which could replace current state defect tolerance, and size effects. Besides the outstanding of the art systems such as suction cups. Consequently, artificial adhesive properties, a quick and easy release of the adhesive bioinspired adhesive systems have been extensively stud- 11,14−17 ied, and comparably high adhesive performance was pads is crucial for locomotion and, ultimately, the survival of the gecko. Detachment is controlled by the anisotropy of the adhesive structures and the biomechanics of the gecko’s Received: August 7, 2015 motion, which consists of simultaneous shear and peel Accepted: October 12, 2015 movement. Published: October 12, 2015 © 2015 American Chemical Society 24127 DOI: 10.1021/acsami.5b07287 ACS Appl. Mater. Interfaces 2015, 7, 24127−24135 ACS Applied Materials & Interfaces Research Article reached, even exceeding the so-called “gecko-limit” of 100 2. EXPERIMENTAL SECTION kPa. In extension to Weitlaner’s results on the adhesion of 2.1. Mold Preparation. Aluminum molds were fabricated using a 45,46 geckos, recent studies suggest that adhesion of (synthetic) process similar to the one reported in earlier studies. An array of holes with 2 mm depth, 0.4 mm width, and a center−center spacing of bioinspired surfaces relies, in addition to van der Waals 0.8 mm was milled. The geometrical parameters were chosen to yield interactions, to a small part on suction. It was found samples which possess a mechanical instability at high compressive experimentally that a small suction effect is present for 40,42 loading, known to lead to detachment. The array contained 203 mushroom-shaped patterned adhesives if adhesion is tested in 2 holes and covered an area of approximately 1 cm . The mold was 19−21 vacuum. It has also been predicted theoretically that thoroughly cleaned in acetone, ethanol, isopropanol, and deionized suction effects may become relevant in patterned surfaces as water in an ultrasonic bath and subsequently silanized. For this, the 22,23 mold was placed together with a glass vial containing 10 μLof soon as a certain critical contact size is exceeded. trichloroperfluorooctysilane (Sigma-Aldrich) into a desiccator and Many of these bioinspired systems have been applied to grip evacuated to a pressure below 10 mbar for at least 45 min until the and release objects; most approaches function close to the silane evaporated completely. Afterward, the mold was kept in an oven directional, shear induced adhesion found for geckos. There, in air at 95 °C for 2 h. switching adhesion mainly relies on asymmetric adhesive 2.2. Sample Preparation. Samples were prepared from structures, which exhibit high adhesion if sheared into one polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning) by soft molding of the previously prepared aluminum molds. The PDMS direction, while adhesion drops significantly if sheared in the prepolymer and cross-linker were mixed in a 10:1 ratio, poured onto opposite direction. The frequently occurring lateral displace- the silanized mold, and degassed in a desiccator. The filled mold was ment of the object during attachment and detachment may be then placed in an oven and cured at 75 °C for 4 h. After cooling to circumvented by gripper designs, where two or more room temperature, the PDMS sample was carefully peeled from the anisotropic adhesive pads are sheared in opposite directions mold, resulting in a PDMS array of cylindrical pillars. The backing so that the lateral forces cancel out. Other approaches layer was approximately 3 mm thick. 2.3. Tip Modification. Pillars with mushroom-shaped tips were combine electrostatic adhesion and bioinspired adhesives to reported to significantly increase adhesion compared to pillars having maintain a compressive preload on the adhesive structures or 18,45,47 flat or spherical tips. Thus, the tips were modified using a even use biological structures obtained from gecko toes for variation of a previously described process. Briefly, PDMS was mixed handling of small objects. An approach to handle objects with and degassed as described above. A thin metal rod was dipped into the a more complex geometry is based on a balloon-like gecko liquid PDMS and gently brought into contact with the pillars, resulting in deposition of a small droplet of liquid PDMS on the tip of each adhesive tape, which can be adapted to curved surface pillar. Afterward, the droplet-covered tips were placed face-down onto geometries by “inflating” and “deflating” the balloon. a smooth, silanized glass plate (silanization protocol as above). The Besides fabrication and characterization of bioinspired tips were squashed, resulting in a flattened mushroom-shape. The adhesives with high and robust adhesion and adhesion control patterned sample was fully cured in an oven at 75 °C for 4 h and, after using “passive” peeling or shearing, the control of adhesion by cooling to room temperature, carefully removed from the glass plate. 29−33 an external stimulus has been studied and improved to Three samples were chosen for adhesion experiments; an exemplary obtain switchable adhesives even in extreme environments like sample is shown in Figure 1a. outer space. While the complex detachment motion works efficiently for geckos and has already been mimicked relatively close to the natural archetype, other approaches have emerged to switch adhesion by using external triggers. Shape 36,37 memory materials, active polymeric materials such as liquid crystal elastomers, injection of liquids in subsurface micro- channels, application of magnetic fields to orient magnetic structures, or mechanical loading of rubber elastic patterned samples was applied to obtain switchability. The latter has been investigated in detail and shows detachment of rubber elastic pillars due to mechanical instability at high compressive 40−43 load, leading to a preload responsive switchable adhesive. Figure 1. (a) The photograph shows an exemplary bioinspired Due to the simplicity of the operation mode and the fast and switchable dry adhesive PDMS sample with an array of mushroom- reversible response, this approach shows significant potential shaped pillars. The inset exhibits a side view of a single mushroom for pick and place processes. In a more recent publication, this shaped tip. (b) The experimental adhesion tester setup is built in a approach of pressure activated switchable adhesion was vacuum oven for experiments at ambient air pressure and at low extended by using structures of different length to switch pressure condition (<10 mbar). between three adhesive states, namely, low, high, and very low adhesion. 2.4. Adhesion Testing Setup. An adhesion measurement setup To obtain pick and place handling in vacuum conditions, as shown in Figure 1b, inspired by the macroscopic adhesion robust adhesion in vacuum and reliable switchability need to be measurement device (MAD), was built in a vacuum oven. It consisted of a linear z-positioning system and a load-cell based force linked. In the present study, the adhesive performance of a measurement. Each patterned PDMS sample was fixated to a glass preload responsive, bioinspired adhesive was tested both in backing by applying oxygen plasma to the backside of the sample and vacuum and in air, and the influence of air pressure on adhesion bringing it into contact with the smooth cleaned glass plate. The was quantified. Further, pick and place processes were sample was mounted to a load cell with a stiffness of >100 kN/m. performed and the reversibility was demonstrated over 50 Prior to the adhesion measurements, the sample was aligned using a loading cycles. manual alignment stage and applying the alignment process published 24128 DOI: 10.1021/acsami.5b07287 ACS Appl. Mater. Interfaces 2015, 7, 24127−24135 ACS Applied Materials & Interfaces Research Article Figure 2. (a) Schematic of different phases during adhesion experiments; (1) the sample with the rubber elastic pillars is moved toward the glass slide, (2) attachment occurs and a preload P < P is applied, (3) the sample is moved upward, lifting the glass plate with the lifting force F , (4) a b L preload P > P is applied, where buckling of the pillars occurs, and (5) pull-off from the glass plate with the force F. The different sequences are schematically given in (b, c, and d). by Kroner et al. Force and displacement were recorded during all These phases can be grouped into different sequences to represent specific adhesion measurements. The following sequences were measurements. applied: 2.5. Adhesion Measurements. All experiments were performed with a testing velocity of 80 μm/s. Each sample was tested at least − Sequence 1 (Figure 2b): The preload P is chosen so that three times and thereby rotated by ∼120° along the vertical axis in contact is formed with the glass plate, but no buckling of the between the experiments to avoid misalignment. Adhesion measure- structures occurs. The glass plate is fixated to prevent it from ments were conducted in ambient atmosphere, called “air” in the lifting. This experiment corresponds to the phases (1) (2) (5). subsequent text, and at reduced pressure <10 mbar, called “vacuum” in − Sequence 2 (Figure 2c): In this sequence, a preload P above the the subsequent text. A glass plate was used as test substrate which was, critical buckling load P is applied. The glass plate is also depending on the testing mode, either fixated or loosened. To obtain fixated. This experiment is represented by the phases (1) (2) an equilibrium surface state, more than 300 contacts were made (4) (5). between a smooth PDMS sample and the glass plate prior to adhesion − Sequence 3 (Figure 2d): A pick and place process is imitated 50,51 measurements. The experimental error for all adhesion measure- using a loosened glass plate. A preload P below the buckling ments was ±0.1 N. load P is applied and the glass plate is lifted, which Different phases occurred during an adhesion experiment with corresponds to the phases (1) (2) (3). In the next step, the preload responsive dry-adhesive samples, which are schematically glass plate is lowered and detached using a preload P above the shown in Figure 2a and can be described as follows: buckling load P , which corresponds to the phases (2) (4) (5). − Phase (1): The aligned sample is moved toward the smooth The complete pick and place process is described by phases (1) (2) (3) (2) (4) (5). glass plate. − Phase (2): The sample forms contact with the glass plate, and a 2.6. Applied Measurement Sets. To determine the adhesive properties of the switchable bioinspired adhesive and its applicability compressive preload P below the critical buckling preload P is for pick and place processes, the following measurement sets and applied. The load is kept for at least three seconds. analyses were conducted in air and vacuum conditions: − Phase (3): The sample is retracted and, due to adhesion, the (i) Force−time curves were recorded for different preloads up to 7 glass plate is lifted with a force F . This phase only applies if the N, allowing determination of the preload dependent pull-off glass plate is loosened. force behavior of the switchable adhesive. The pull-off force F − Phase (4): A preload above the critical buckling preload P is (absolute value of the maximum detachment force) was plotted applied, causing the structures to buckle and the structure tips as a function of preload P, leading to the identification of the to detach from the glass probe. buckling preload P . These measurements correspond to − Phase (5): The sample is retracted from the glass plate, and the sequence 1 for P < P and sequence 2 for P > P . b b pull-off force F,defined as the absolute value of the maximum (ii) Exemplary force−time curves from (i) were analyzed for two negative force of the recorded force−time curves, is measured. selected measurements, one having a preload P < P according 24129 DOI: 10.1021/acsami.5b07287 ACS Appl. Mater. Interfaces 2015, 7, 24127−24135 ACS Applied Materials & Interfaces Research Article to sequence 1, and one having a preload P > P according to in air, while experiments in vacuum resulted in pull-off forces sequence 2. between 1.8 and 1.9 N. It can be clearly seen that, for lower (iii) Adhesion experiments with sequence 1 directly followed by preloads, the pull-off force depends on the air pressure; sequence 2 were repeated 50 times to test for reversibility. adhesion was reduced by about 30% in vacuum. For P > P , the (iv) Force−time curves were recorded for a pick and place process pull-off force was found to be 0.9 N and was independent of air represented by sequence 3. The glass plate with a weight of 65 pressure. g was lifted for at least 10 s during phase (3). Reversibility was 3.2. Measurement Set (ii): Force−Time Curves. again tested by repeating this sequence for 50 times. Exemplary force−time curves with preload P < P and P > P , respectively, measured in air and vacuum, are plotted in 3. RESULTS Figure 4. The experimental results of the different measurement sets are Figure 4a,b depicts exemplary force−time curves of adhesion described in the following four subsections. measurements with a preload P < P , which corresponds to 3.1. Measurement Set (i): Preload Dependent Pull-off sequence 1. The sample was moved toward the fixated glass Force. Pull-off forces F were measured as a function of preload plate, formed contact, and was loaded, until the preload P was and are given in Figure 3. The pull-off force was found to be reached. In these examples, the preload P was 3.7 N for testing in air (Figure 4a) and 4.0 N for testing in vacuum (Figure 4b). The load was applied for at least 3 s. Then, the sample was retracted again, leading to an adhesive (tensile) force, ultimately reaching the maximum pull-off force F. The negative sign indicates the direction of force measurement. For these measurements, a pull-off force F of 2.6 N was found in air (Figure 4a) and of 1.9 N (Figure 4b) in vacuum. Representative force−time curves, where a preload P > P was applied corresponding to sequence 2, are shown in Figure 4c,d. The sample was approached, formed contact with the glass probe, and was loaded. The compressive force increased during loading until a local maximum occurred at a critical load P of 5.8 N. Subsequently, the compressive force decreased Figure 3. Absolute values of the pull-off forces are plotted as a b rapidly with ongoing compression and buckling of the pillars function of applied preload, measured in air and in vacuum. At a critical preload P , indicated by the dashed line within the gray area, was optically observed. As the pillars were bent further with buckling of the pillars was observed. increasing displacement, the compressive force increased again until the predefined preload P was reached and kept for at least almost preload independent at low preloads. As soon as a 3s.In Figure 4c, the preload was 6.6 N, and in Figure 4d, the critical preload was applied, the pull-off force dropped preload was 6.3 N. The reverse force−time behavior was significantly which corresponded to the optically observed observed during retraction; the occurring maximum corre- elastic buckling of the pillars. The critical buckling preload P sponded to an optically observed “unbuckling” of the pillars. A was highly reproducible for each sample but showed some pull-off force F of 0.9 N was recorded during retraction, both variation in a range from 5.6 to 6.0 N for different samples. For for measurements in air and in vacuum. These force−time preloads P < P , the pull-off forces were between 2.5 and 2.7 N curves for a preload P > P are characteristic for the buckling b b Figure 4. Representative force−time curves for adhesion experiments of bioinspired dry adhesives on a fixated glass plate. Measurements using sequence 1 with a preload P below the buckling preload P in (a) air and (b) in vacuum. Measurements using sequence 2 with a preload P above the buckling preload P (c) in air and d) in vacuum. The phases from Figure 2a are indicated. 24130 DOI: 10.1021/acsami.5b07287 ACS Appl. Mater. Interfaces 2015, 7, 24127−24135 ACS Applied Materials & Interfaces Research Article behavior and have been reported and characterized in earlier studies. The force−time curves are very similar for experiments in air and in vacuum. A significant difference was found only in the pull-off force F; experiments with a preload P < P resulted in a change in pull-off force F from 2.6 to 1.9 N, which is equal to a loss in adhesion of approximately 30%. For a preload P > P , the pull-off force F was substantially lower, reaching only 0.9 N, and did not differ between air and vacuum condition. 3.3. Measurement Set (iii): Reversibility. To evaluate the reversibility of the switching behavior between high and low pull-off force, alternating preloads below and above P were applied. 50 cycles of the sequence 1, directly followed by sequence 2, were performed in air and in vacuum according to the measurement set (iii). Figure 5 shows the recorded forces, Figure 6. Force−time curves of a pick and place process, where a glass plate of 65 g is lifted and released again. The experiment corresponds to the sequence 3; the numbers indicate the respective phases from Figure 2. The measurements were performed (a) in air and (b) in vacuum. the glass plate were comparable in both air and vacuum conditions. An exemplary video of a pick and place process is shown in the Supporting Information, using a silicon wafer instead of a glass plate for the sake of better visibility. 4. DISCUSSION On the basis of the experimental results, the following properties of the pressure actuated adhesive system in air and vacuum conditions were analyzed: the adhesive properties, the reversibility of switching, and the adaptability for pick and place processes. Figure 5. Reversibility tests of adhesion measurements with alternating The adhesive properties, represented by the force−time preload below and above the buckling load, described by sequence 1, directly followed by sequence 2. Each plot shows the applied preload P curves, are given in Figure 4. The curves exhibit a characteristic and the resulting pull-off force F over 50 test cycles (a) in air and (b) shape which is typical for patterned bioinspired adhesives such in vacuum. Note that the pull-off force F is not given as absolute value as the tested samples. Low preload leads to a comparably high here but has a negative sign for clarity of the diagram. pull-off force, while high preload results in buckling of the structures at a certain buckling preload P , which reduces namely, the preload P and the pull-off force F, as a function of adhesion significantly. The mechanism of adhesion loss has testing cycles. Note that the pull-off force F is not given as been investigated in an earlier study, where it was found that absolute value but has a negative sign for clarity of the diagram. the unbuckling during unloading does not allow reformation of In air (Figure 5a), the alternating preloads P of 2.9 and 5.8 N intimate contact between the pillar tips and the probe. The resulted in adhesive forces F of 2.6 and 0.9 N, respectively. In lack of intimate contact between pillar tips and probe causes a vacuum (Figure 5b), the applied preloads P were 3.3 and 5.8 N, reduction in adhesion. The adhesive behavior in air and vacuum which resulted in adhesive forces F of 1.9 and 0.9 N, conditions is qualitatively similar, indicating that the mecha- respectively. No notable changes in pull-off force were found nism of adhesion loss by buckling is unaffected by air pressure. within the 50 test cycles. The main influence of air pressure on the adhesive properties is 3.4. Measurement Set (iv): Pick and Place. A pick and found in the magnitude of the pull-off force, which can be seen place process, corresponding to the sequence 3 in Figure 2d, in Figures 3 and 4a,b (indicated as ΔF). The pull-off force F was simulated. As described in the measurement set (iv), an was found to be between 2.5 and 2.7 N in air and 1.8 and 1.9 N alternating preload below and above the buckling load P was in vacuum, respectively, exhibiting that the application of applied with the glass plate being loosened to allow lifting. vacuum reduces adhesion by ΔF of 0.7 N ± 0.2 N, which Representative force−time curves in air and in vacuum are corresponds to a loss in adhesion of approximately 30%. This given in Figure 6a,b, respectively. The graphs show no notable reduction becomes obvious by considering Figure 3; all pull-off differences, indicating that the lifting process and the release of forces obtained in vacuum lie below the ones obtained in air if 24131 DOI: 10.1021/acsami.5b07287 ACS Appl. Mater. Interfaces 2015, 7, 24127−24135 ACS Applied Materials & Interfaces Research Article the preload was chosen to be below the critical buckling Table 1. Geometric Parameters of Commercial Suction Cups preload P . and Their Adhesive Performance after Attaching Them with 2 a We identified two factors which may be responsible for the a Compressive Load of 9.0 N/cm varying adhesion with changing air pressure, namely, humidity diameter, contact area, force, strength, corrected strength, N/ and suction. We tend to exclude humidity and favor suction as 2 2 2 mm mm N N/cm cm the main mechanism for the change in pull-off force for the 2.6 5.3 0.15 2.8 0.9 following reasons. PDMS is a hydrophobic material which does 3.8 11.3 0.65 5.7 1.7 not tend to absorb water in larger quantities. In addition, it was 5.0 19.6 1.3 6.6 2.0 found that no measurable humidity effect is present at humidity 7.0 38.5 2.5 6.5 2.0 between 2% and 90% for smooth PDMS surfaces and for pillar 9.0 63.6 3.9 6.1 1.8 arrays with diameters of 25 μm. Huber et al. found an 11.0 95.0 6.9 7.3 2.2 additional adhesion effect in the presence of humidity and 16.5 213.8 11.0 5.1 1.5 explained it by a smoothening effect of the water on rough 22.0 380.1 16.0 4.2 1.3 surfaces, but the tested surfaces in this study are expected to 32.0 804.2 30.0 3.7 1.1 be smooth. These points indicate that capillarity effects may 41.0 1320.3 49.0 3.7 1.1 have a minor influence on the adhesion in our experiments. 51.4 2075.0 92.0 4.4 1.3 In contrast, suction effects on mushroom shaped pillars are The corrected strength assumes a packing density of 30%, data after expected from theoretical considerations and were also found 19,20 ref 55. in earlier experimental studies. It was shown that suction is present for adhesive pillars with mushroom-shaped tips and can suction force of 0.85 N. As the mushroom shaped structures in contribute considerably to adhesion with up to 10% of the pull- our study were not specifically optimized for suction, these off force. In our case, the suction component even exceeds results fit astonishingly well to the values provided for this percentage, reaching approximately 30%. While adhesion commercial suction cups. experiments in air result in pull-off forces between 2.5 and 2.7 It was reported that suction and van der Waals interactions N, the same set of experiments in vacuum exhibits pull-off have a different size effect. Thus, we expect that the suction forces between 1.8 and 1.9 N. This difference may be explained effect becomes more prominent with increasing size of the by the size of the pillars. Suction based forces scale with the contact elements, while reducing the size of the contact area of the contact, while adhesion of patterned surfaces due to elements diminishes the influence of suction. van der Waals forces was theoretically and experimentally 53,54 Consequently, if suction caused the change in adhesion of shown to scale with length. the present experiments, it would be strongly influenced by the Theoretically, the suction force F of a perfect suction suction contact geometry. During the buckling process, the mushroom cup, disregarding other adhesive interactions than suction, is tips detach and the pillars form side contact with the glass plate. given by the contact area A and the pressure difference ΔP contact This contact geometry does not allow building up a difference caused by the suction effect: in air pressure, thereby diminishing the suction component of FP =Δ ×A (1) suction contact the pull-off force. Our experiments show that the application of a load exceeding P leads to a pull-off force F of 0.9 N, both in Consequently, the pull-off strength of a perfect suction cup is air and in vacuum. This phenomenon is also reflected in Figure directly proportional to the pressure difference inside the 3; while the pull-off forces at a preload below P differ for contact area and outside the suction cup. For ideal vacuum (0 measurements in air and in vacuum, similar pull-off forces are bar) and atmospheric pressure (∼1 bar), a suction force of ∼10 found if the buckling preload P is overcome. These N/cm can be achieved using eq 1. Such high values are usually observations support the assumption that air pressure enhances not obtained using typical suction cups. adhesion due to suction in patterned bioinspired surfaces with To compare our experimentally derived pull-off strength structure sizes in the macroscopic range, while detachment values to the performance of typical suction cups, we have events after buckling of the pillars are not affected by air converted given data from commercial macroscopic silicone 55 pressure, since suction cannot be maintained after buckling has suction cups. The performance of the analyzed suction cups occurred. This leads to the conclusion that the difference in with diameters between 2.6 and 51.4 mm lie between 2.8 and 55 pull-off force of the bioinspired adhesive is not a result of 2 2 7.3 N/cm if a compressive stress of 9.0 N/cm is applied; see changing humidity but is caused most likely by a suction effect. also Table 1. For comparing these pull-off strength data with For a better description of the switching behavior, a our results, it is important to consider that the strength data switching efficiency S is introduced in eq 2, which is defined from the present study reflects the apparent contact strength. by the ratio of the pull-off forces at a preload above and below Thus, reduction in “real” contact area due to the pillar packing the buckling preload P : density of ∼30% has to be taken into account. A comparable pull-off performance between conventional suction cups and FP() > P S=− 1 the experiments from our studies would then result in a FP() < P (2) corrected strength, which is calculated by multiplying the given pull-off strength of the commercial suction cups with the pillar . packing density of ∼30% from our samples. These values are A value of S = 0 indicates no switching behavior, and S =1 also given in Table 1. resembles a perfect switch where adhesion can be completely As can be seen from the corrected pull-off strengths, values turned on and off.If eq 2 is applied to the obtained between 0.9 and 2.2 N/cm can be considered as typical for experimental data, the switching efficiency S is approximately commercial suction cups. Our experimentally derived suction 0.65 ± 0.07 in air, while a value of S = 0.50 ± 0.1 is obtained in component to the pull-off force lies slightly below the lowest vacuum. Thus, applying vacuum reduces the switching 24132 DOI: 10.1021/acsami.5b07287 ACS Appl. Mater. Interfaces 2015, 7, 24127−24135 ACS Applied Materials & Interfaces Research Article efficiency by a mean value of ΔS = 0.15. These calculated ASSOCIATED CONTENT efficiencies indicate that the switch in adhesion may be further * Supporting Information improved. Still, the reached values allow a significant change in The Supporting Information is available free of charge on the adhesion in air and in vacuum, opening a sufficiently large ACS Publications website at DOI: 10.1021/acsami.5b07287. window of operation for pick and place applications. Video showing an exemplary pick and place process These promising results are promoted by the reversibility test using a switchable bioinspired adhesive to handle a shown in Figure 5, which indicates that the switch in adhesion silicon wafer (AVI) is highly reversible in air and in vacuum. No change in adhesive performance or damage of the dry adhesive structures was AUTHOR INFORMATION detected after 50 testing cycles. Finally, pick and place Corresponding Author processes were conducted using a glass plate with a weight of *E-mail: elmar.kroner@leibniz-inm.de. Phone: +49 (0) 681 65 g. The glass plate was securely lifted and released in air and 9300 369. in vacuum. No significant difference is observed in the adhesion Author Contributions curves given in Figure 6 for operation at both air pressure J.P., M.F., and E.K. contributed equally. conditions. Hence, the pick and place process is not notably Funding influenced by the reduction of air pressure. The research leading to these results was conducted within a It should be mentioned that the pick and release process has Grant of the European Research Council under the European two restrictions for operation if a reliable switch in adhesion is Union’s Seventh Framework Program (FP/2007-2013)/ERC required. First, if the object to be lifted is too light, the pull-off Grant Agreement No. 340929, awarded to E. Arzt. force after application of a preload above the buckling preload Notes P may be too high for reliable detachment, representing a The authors declare no competing financial interest. minimum weight threshold, and second, if the object to be lifted is too heavy, it will detach prior to lifting, representing a ACKNOWLEDGMENTS maximum weight threshold. It follows that an optimum range The authors thank the Mechanical Workshop of the INM for of operation can be defined on the basis of the pull-off forces mold fabrication, Susanne Selzer and Ina Kothe for their help in measured for a preload below, and above, the buckling preload sample preparation, and Joachim Blau for building up the P . For the tested samples, the range of operation can be adhesion measurement setup. determined to be between approximately 0.9 and 2.5 N in air or between 0.9 and 1.8 N in vacuum. It further has to be ABBREVIATIONS considered that the viscoelasticity of the applied material may CVD = chemical vapor deposition have a significant influence in the buckling of the structures and PVD = physical vapor deposition may shift both the lower and the higher boundary of the range PDMS = polydimethylsiloxane of operation. This window of operation may be tuned MAD = macroscopic adhesion measurement device according to the envisaged application, for example, by changing the number of pillars, by their packing density, by REFERENCES further modification of their tip geometry, or by a different (1) Langer, M. G.; Ruppersberg, J. P.; Gorb, S. Adhesion Forces choice of sample material. Measured at the Level of a Terminal Plate of the Fly’s Seta. Proc. R. Soc. London, Ser. B 2004, 271 (1554), 2209−2215. (2) Federle, W.; Rohrseitz, K.; Holldobler, B. Attachment Forces of 5. CONCLUSIONS Ants Measured with a Centrifuge: Better ″Wax-Runners″ have a The present study showed that bioinspired switchable adhesion Poorer Attachment to a Smooth Surface. J. Exp. Biol. 2000, 203 (3), 505−512. based on reversible buckling of elastic pillars is applicable in (3) Stork, N. E. Experimental Analysis of Adhesion of Chrysolina vacuum. At low compressive load, the pull-off force for samples 2 Polita (Chrysomelidae: Coleoptera) on a Variety of Surfaces. J. Exp. with sizes of 1 cm was between 2.5 and 2.7 N (±0.1 N) in air Biol. 1980, 88 (1), 91−108. and was reduced to 1.8 to 1.9 N (±0.1 N) if measured in (4) Federle, W. Why Are So Many Adhesive Pads Hairy? J. Exp. Biol. vacuum. This indicates that a suction component was present 2006, 209, 2611−2621. in the attachment state, since an influence of humidity may be (5) Jagota, A.; Bennison, S. J. Mechanics of Adhesion Through a excluded. Application of a compressive load above the buckling Fibrillar Microstructure. Integr. Comp. Biol. 2002, 42 (6), 1140−1145. (6) Autumn, K.; Liang, Y. A.; Hsieh, S. T.; Zesch, W.; Chan, W. P.; preload P between 5.6 and 6.0 N (±0.1 N) caused a reversible Kenny, T. W.; Fearing, R.; Full, R. J. Adhesive Force of a Single Gecko buckling of the pillars and resulted in pull-off forces of 0.9 N Foot-Hair. Nature 2000, 405 (6787), 681−685. (±0.1 N), which were similar for experiments in air and in (7) Autumn, K.; Sitti, M.; Liang, Y. A.; Peattie, A. M.; Hansen, W. R.; vacuum. This indicates the absence of a suction component Sponberg, S.; Kenny, T. W.; Fearing, R.; Israelachvili, J. N.; Full, R. J. after buckling of the pillars occurred. Our experiments Evidence for Van Der Waals Adhesion in Gecko Setae. Proc. Natl. exhibited that the transition between the two adhesive states Acad. Sci. U. S. A. 2002, 99 (19), 12252−12256. (8) Hsu, P. Y.; Ge, L.; Li, X.; Stark, A. Y.; Wesdemiotis, C.; was sharp and the switching behavior was independent of air Niewiarowski, P. H.; Dhinojwala, A. Direct Evidence of Phospholipids pressure. Further, the switch in adhesion exhibited high in Gecko Footprints and Spatula−Substrate Contact Interface reversibility; we showed that the system works reliably in air Detected using Surface-Sensitive Spectroscopy. J. R. Soc., Interface and in vacuum for 50 pick and place cycles without any signs of 2012, 9 (69), 657−664. wear or change in adhesion performance. The functionality of (9) Buhl, S.; Greiner, C.; del Campo, A.; Arzt, E. Humidity Influence the switchable adhesive at low air pressure makes it applicable on the Adhesion of Biomimetic Fibrillar Surfaces. Int. J. Mater. Res. for handling operations of fragile objects in vacuum. 2009, 100 (8), 1119−1126. 24133 DOI: 10.1021/acsami.5b07287 ACS Appl. Mater. Interfaces 2015, 7, 24127−24135 ACS Applied Materials & Interfaces Research Article (10) Huber, G.; Gorb, S. N.; Hosoda, N.; Spolenak, R.; Arzt, E. (32) Kim, S.; Sitti, M.; Xie, T.; Xiao, X. Reversible Dry Micro- Influence of Surface Roughness on Gecko Adhesion. Acta Biomater. Fibrillar Adhesives with Thermally Controllable Adhesion. Soft Matter 2007, 3 (4), 607−610. 2009, 5 (19), 3689−3693. (11) Kamperman, M.; Kroner, E.; del Campo, A.; McMeeking, R. M.; (33) Nadermann, N.; Ning, J.; Jagota, A.; Hui, C. Y. Active Switching Arzt, E. Functional Adhesive Surfaces with “Gecko” Effect: The of Adhesion in a Film-Terminated Fibrillar Structure. Langmuir 2010, Concept of Contact Splitting. Adv. Eng. Mater. 2010, 12 (5), 335−348. 26 (19), 15464−15471. (12) Autumn, K.; Dittmore, A.; Santos, D.; Spenko, M.; Cutkosky, (34) Henrey, M.; Tellez, J. P. D.; Wormnes, K.; Pambaguian, L.; M. Frictional Adhesion: a New Angle on Gecko Attachment. J. Exp. Menon, C. Towards the Use of Mushroom-Capped Dry Adhesives in Biol. 2006, 209 (18), 3569−3579. Outer Space: Effects of Low Pressure and Temperature on Adhesion (13) Kroner, E.; Davis, C. S. A Study of the Adhesive Foot of the Strength. Aerosp. Sci. Technol. 2013, 29 (1), 185−190. Gecko Translation of a Publication of Dr. F. Weitlaner. J. Adhes. 2015, (35) Kwak, M. K.; Pang, C.; Jeong, H.-E.; Kim, H.-N.; Yoon, H.; 91 (6), 481−487. Jung, H.-S.; Suh, K.-Y. Towards the Next Level of Bioinspired Dry (14) Heepe, L.; Gorb, S. Biologically Inspired Mushroom-Shaped Adhesives: New Designs and Applications. Adv. Funct. Mater. 2011, 21 Adhesive Microstructures. Annu. Rev. Mater. Res. 2014, 44, 173−203. (19), 3606−3616. (15) Pattantyus-Abraham, A.; Krahn, J.; Menon, C. Recent Advances (36) Frensemeier, M.; Kaiser, J. S.; Frick, C. P.; Schneider, A. S.; Arzt, in Nanostructured Biomimetic Dry Adhesives. Front. Bioeng. E.; Fertig, R. S.; Kroner, E. Temperature-Induced Switchable Adhesion Biotechnol. 2013, 1 (22); DOI: 10.3389/fbioe.2013.00022 using Nickel−Titanium−Polydimethylsiloxane Hybrid Surfaces. Adv. (16) Sameoto, D.; Menon, C. Recent Advances in the Fabrication Funct. Mater. 2015, 25 (20), 3013−3021. and Adhesion Testing of Biomimetic Dry Adhesives. Smart Mater. (37) Reddy, S.; Del Campo, A.; Arzt, E. Bioinspired Surfaces with Struct. 2010, 19 (10), 103001. Switchable Adhesion. Adv. Mater. 2007, 19, 3833−3837. (17) Zhou, M.; Pesika, N.; Zeng, H.; Tian, Y.; Israelachvili, J. Recent (38) Cui, J.; Drotlef, D.-M.; Larraza, I.; Fernandez-Bla ́ zquez, ́ J. P.; Advances in Gecko Adhesion and Friction Mechanisms and Boesel, L. F.; Ohm, C.; Mezger, M.; Zentel, R.; del Campo, A. Development of Gecko-Inspired Dry Adhesive Surfaces. Friction Bioinspired Actuated Adhesive Patterns of Liquid Crystalline 2013, 1 (2), 114−129. Elastomers. Adv. Mater. 2012, 24 (34), 4601−4604. (18) Del Campo, A.; Greiner, C.; Arzt, E. Contact Shape Controls (39) Northen, M. T.; Greiner, C.; Arzt, E.; Turner, K. L. A Gecko- Adhesion of Bioinspired Fibrillar Surfaces. Langmuir 2007, 23 (20), Inspired Reversible Adhesive. Adv. Mater. 2008, 20 (20), 3905−3909. 10235−10243. (40) Paretkar, D.; Kamperman, M.; Schneider, A. S.; Martina, D.; (19) Heepe, L.; Varenberg, M.; Itovich, Y.; Gorb, S. N. Suction Creton, C.; Arzt, E. Bioinspired Pressure Actuated Adhesive System. Component in Adhesion of Mushroom-Shaped Microstructure. J. R. Mater. Sci. Eng., C 2011, 31 (6), 1152−1159. Soc., Interface 2011, 8 (57), 585−589. (41) Paretkar, D.; Kamperman, M.; Martina, D.; Zhao, J.; Creton, C.; (20) Davies, J.; Haq, S.; Hawke, T.; Sargent, J. P. A Practical Lindner, A.; Jagota, A.; McMeeking, R.; Arzt, E. Preload-Responsive Approach to the Development of a Synthetic Gecko Tape. Int. J. Adhes. Adhesion: Effects of Aspect Ratio, Tip Shape and Alignment. J. R. Soc., Adhes. 2009, 29 (4), 380−390. Interface 2013, 10 (83), 20130171; DOI: 10.1098/rsif.2013.0171 (21) Sameoto, D.; Sharif, H.; Menon, C. Investigation of Low- (42) Paretkar, D.; Schneider, A. S.; Kroner, E.; Arzt, E. In Situ Pressure Adhesion Performance of Mushroom Shaped Biomimetic Observation of Contact Mechanisms in Bioinspired Adhesives at High Dry Adhesives. J. Adhes. Sci. Technol. 2012, 26 (23), 2641−2652. Magnification. MRS Commun. 2011, 1 (1), 53−56. (22) Afferrante, L.; Carbone, G. The Mechanisms of Detachment of (43) Varenberg, M.; Gorb, S. Close-Up of Mushroom-Shaped Mushroom-Shaped Micro-Pillars: From Defect Propagation to Fibrillar Adhesive Microstructure: Contact Element Behaviour. J. R. Membrane Peeling. Macromol. React. Eng. 2013, 7 (11), 609−615. Soc., Interface 2008, 5 (24), 785−789. (23) Spolenak, R.; Gorb, S.; Gao, H.; Arzt, E. Effects of Contact (44) Isla, P. Y.; Kroner, E. A Novel Bioinspired Switchable Adhesive Shape on the Scaling of Biological Attachments. Proc. R. Soc. London, with Three Distinct Adhesive States. Adv. Funct. Mater. 2015, 25 (16), Ser. A 2005, 461 (2054), 305−319. 2444−2450. (24) Mengüc,̧ Y.; Yang, S. Y.; Kim, S.; Rogers, J. A.; Sitti, M. Gecko- (45) Kroner, E.; Arzt, E. Single Macropillars as Model Systems for Inspired Controllabe Adhesive Structures Appied to Micromanipula- Tilt Angle Dependent Adhesion Measurements. Int. J. Adhes. Adhes. tion. Adv. Funct. Mater. 2012, 22 (6), 1246−1254. 2012, 36,32−38. (25) Zhou, M.; Tian, Y.; Sameoto, D.; Zhang, X.; Meng, Y.; Wen, S. (46) Kroner, E.; Arzt, E. Mechanistic Analysis of Force-Displacement Controllable Interfacial Adhesion Applied to Transfer Light and Measurements on Macroscopic Single Adhesive Pillars. J. Mech. Phys. Fragile Objects by Using Gecko Inspired Mushroom-Shaped Pillar Solids 2013, 61 (6), 1295−1304. Surface. ACS Appl. Mater. Interfaces 2013, 5, 10137−10144. (47) Micciche,́ M.; Arzt, E.; Kroner, E. Single Macroscopic Pillars as (26) Ruffatto, D.; Parness, A.; Spenko, M. Improving Controllable Model System for Bioinspired Adhesives: Influence of Tip Dimension, Adhesion on Both Rough and Smooth Surfaces with a Hybrid Aspect Ratio, and Tilt Angle. ACS Appl. Mater. Interfaces 2014, 6 (10), Electrostatic/Gecko-Like Adhesive. J. R. Soc., Interface 2014, 11 (93), 7076−7083. 20131089. (48) Kroner, E.; Blau, J.; Arzt, E. Note: An Adhesion Measurement (27) Jeong, J.; Kim, J.; Song, K.; Autumn, K.; Lee, J. Geckopringint: Setup for Bioinspired Fibrillar Surfaces using Flat Probes. Rev. Sci. Assembly of Microelectronic Devices on Unconventional Surfaces by Instrum. 2012, 83 (1), 106101. Transfer Printing with Isolated Gecko Setal Arrays. J. R. Soc., Interface (49) Kroner, E.; Paretkar, D.; McMeeking, R.; Arzt, E. Adhesion of 2014, 11 (99), 20140627. Flat and Structured PDMS Samples to Spherical and Flat Probes: A (28) Song, S.; Sitti, M. Soft Grippers Using Micro-Fibrillar Adhesives Comparative Study. J. Adhes. 2011, 87 (5), 447−465. (50) Kroner, E.; Maboudian, R.; Arzt, E. Effect of Repeated Contact for Transfer Printing. Adv. Mater. 2014, 26 (28), 4901−4906. (29) Arul, E. P.; Ghatak, A. Control of Adhesion via Internally on Adhesion Measurements Involving Polydimethylsiloxane Structural Pressurized Subsurface Microchannels. Langmuir 2012, 28 (9), 4339− Material. IOP Conf. Ser.: Mater. Sci. Eng. 2009, 5, 012004. 4345. (51) Kroner, E.; Maboudian, R.; Arzt, E. Adhesion Characteristics of (30) Kier, W. M.; Smith, A. M. The Structure and Adhesive PDMS Surfaces During Repeated Pull-Off Force Measurements. Adv. Mechanism of Octopus Suckers. Integr. Comp. Biol. 2002, 42 (6), Eng. Mater. 2010, 12 (5), 398−404. 1146−1153. (52) Huber, G.; Mantz, H.; Spolenak, R.; Mecke, K.; Jacobs, K.; (31) Jeong, H. E.; Kwak, M. K.; Suh, K. Y. Stretchable, Adhesion- Gorb, S. N.; Arzt, E. Evidence for Capillarity Contributions to Gecko Tunable Dry Adhesive by Surface Wrinkling. Langmuir 2010, 26 (4), Adhesion from Single Spatula Nanomechanical Measurements. Proc. 2223−2226. Natl. Acad. Sci. U. S. A. 2005, 102 (45), 16293−16296. 24134 DOI: 10.1021/acsami.5b07287 ACS Appl. Mater. Interfaces 2015, 7, 24127−24135 ACS Applied Materials & Interfaces Research Article (53) Arzt, E.; Gorb, S.; Spolenak, R. From Micro to Nano Contacts in Biological Attachment Devices. Proc. Natl. Acad. Sci. U. S. A. 2003, 100 (19), 10603−10606. (54) Greiner, C.; del Campo, A.; Arzt, E. Adhesion of Bioinspired Micropatterned Surfaces: Effects of Pillar Radius, Aspect Ratio, and Preload. Langmuir 2007, 23 (7), 3495−3502. (55) P. V. GmbH Product database. https://www.piab.com/ Products/suction-cups/shape/universal/u---universal-2-50-mm/ (ac- cessed Sep 21, 2015). 24135 DOI: 10.1021/acsami.5b07287 ACS Appl. Mater. Interfaces 2015, 7, 24127−24135

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Published: Oct 12, 2015

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