Mechanically Switchable Elastomeric Microfibrillar Adhesive Surfaces for Transfer Printing

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© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (1 of 5) 1300159wileyonlinelibrary.com

Mechanically Switchable Elastomeric Microfi brillar Adhesive Surfaces for Transfer Printing

Veikko Sariola and Metin Sitti *

Dr. V. Sariola, Prof. M. Sitti Carnegie Mellon University 5000 Forbes Avenue, 15213 Pittsburgh , PA , USA E-mail: [email protected] Dr. V. Sariola Aalto University P.O. Box 15500, 00076 Aalto , Finland

DOI: 10.1002/admi.201300159

the substrate. Having two distinct components allows the adhe-sive and the mesh to be designed independently. The relative distance Δz of the two surfaces (Figure 1 a) controls the adhe-sion: The adhesive surface is on when Δz > 0 and off when Δz < 0. Both the pillars and the mesh are individually actuated by separate motorized stages (Figure 1 a). To preload the adhe-sive surface on a substrate, the stage with the pillars moves down. To switch the adhesive from on to off state, the stage with the mesh moves down and the stage with the pillars moves up.

We fabricated the hexagonal elastomeric pillars from polyu-rethane (brown in Figure 1 b), by casting into laser cut molds (Figure S1). The pillars and mesh holes were hexagon shaped and laid out in a honeycomb pattern in order to maximize the surface area of the adhesive. The inner diameter of the hexagon pillars is 2 mm, the center-to-center distance is 3 mm, and the height is 2.8 mm. The prototype device consists of 19 pillars, forming a hexagonal pattern. The total surface area of the pillar tops is 0.7 cm 2 and they cover approximately 44% of the total area. The pillar tops were made signifi cantly more adhesive and compliant by covering them with a mushroom-shaped microfi -brillar polyurethane adhesive (Figure 1 c-d), using a dip casting process (Figure S2). [ 16,28 ] Many works [ 14,17,26,28–31 ] have explored different methods for making such microfi brillar adhesives. Our switching principle is not restricted to dry microfi brillar adhesives; any reversible adhesive coating could work. For example, we also tested covering the pillars with an ordinary pressure sensitive adhesive (3M Spray Mount Artist’s Adhe-sive), which functioned well as a switchable adhesive. That pres-sure sensitive adhesive worked initially just as well as the dry microfi brillar adhesive, but eventually contaminated the mesh, making switching diffi cult or impossible. For applications where reusability is less important, ordinary pressure sensitive adhesives could be a viable option. The non-adhesive release mesh was laser cut from a 1.6 mm thick polyoxymethylene sheet (Figure 1 e). The inner diameter of the hexagon holes was 2.6 mm. We used polyoxymethylene, because it is easy to pro-cess and had suffi cient characteristics for our applications. The mesh could be further improved by choosing a material with a higher yield strength, which would allow making the mesh thinner, a larger elastic modulus, which would allow switching with minimal retraction of the pillars, and lower surface energy, to further minimize the adhesion in the off state.

We measured the adhesive strength of the surface by pre-loading the pillars against a pre-cleaned fl at glass substrate and measuring the force on the pillars while slowly retracting the pillars away from the substrate. The pull-off force is defi ned as the maximal tensile force observed during the retraction ( Figure 2 a). The pillars were carefully aligned to the glass sub-strate so that the pull-off force was maximized, because the pull-off force is sensitive to misalignment (Figure S3). The pull-off

Many reversible adhesive applications such as transfer printing, [ 1,2 ] wall climbing, [ 3–5 ] wall mounting, [ 6 ] endoscopic capsule immobilization, [ 7 ] textile fastening [ 8 ] and medical skin patches, [ 9 ] require a detachable adhesive joint. Instead of typical mechanical peeling based release mechanisms, such applica-tions could benefi t signifi cantly from fully switchable adhe-sives [ 10 ] that have a triggering mechanism to go from a highly adhesive on state to a weak, non-adhesive off state. Current approaches to making switchable adhesives can be broadly cat-egorized into chemical and topographical methods, [ 10 ] where chemical methods modify the surface chemistry by heat, light, solvent or pH, while topographical switching methods rely on special topography or peeling motions for the release. Topographical adhesives have usually a microfi brillar struc-ture, inspired by the setae of the gecko feet. [ 11 ] Topographical adhesives have several important characteristics: leaving no residue, [ 12 ] being self-cleaning [ 13–15 ] and being reusable. [ 16 ] Topo-graphical switchable adhesives have been based on compres-sive loading and fi ber buckling, [ 17,18 ] magnetic fi ber tilting, [ 19 ] thermal fi ber alignment [ 20 ] and thermal fi ber stiffness con-trol. [ 21 ] Examples of topographical switchable adhesives without microfi brillar structure include wrinkling surfaces, [ 22,23 ] com-pressible elastomeric pyramid-shaped structures, [ 2 ] self-peeling two-layer adhesive based on shape memory polymers [ 24 ] and a surface with electronically controllable capillary adhesion. [ 25 ] However, achieving controllability, quick switching, reusability, and strong bonding simultaneously remains still a challenge. One of the major problems of the aforementioned methods is that designing a surface for maximal adhesion typically increases the adhesion in the off state. Another problem is that the switching is not completely decoupled from the adhe-sion mechanism, risking inadvertent release of the adhesive. Therefore, in this paper, we report a new type of mechanically switchable topographical adhesive surface, which consists of two distinct components: a) a thin non-adhesive mesh, and b) elastomeric pillars, covered with a gecko-inspired mushroom-shaped elastomer microfi brillar adhesive layer, [ 16,26,27 ] extending through the holes in the mesh ( Figure 1 a). The switching is achieved by actively retracting the pillars through the mesh holes, leaving the non-adhesive mesh surface in contact with

Adv. Mater. Interfaces 2014, 1, 1300159

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force depends on the preload (Figure 2 b). Initially, the pull-off force increases linearly with the preload, but starts saturating with higher preloads. The maximum pull-off force of 1.02 N was achieved with a preload of 0.8 N. Dividing this pull-off force by the surface area of the pillar tops we estimate a maximal pull-off pressure of 15 kPa. The maximum pull-off-to-preload ratio was 1.68 with a preload of 0.1 N, the smallest tested. To conclude, the dry microfi brillar adhesive was pressure sensitive and the preload should be controlled for repeatable pull-off forces.

We quantifi ed the switching capability of the proposed struc-tural adhesive by running a series of experiments where we actively moved both the mesh and pillars. The experiments used the following four-step protocol. First the pillars were lowered in contact with the substrate until a 0.5 N preload was achieved. Next, the mesh was compressed against the surface. Then the pillars were slowly retracted from the surface. Finally, the mesh was retracted from the surface. The displacement of the mesh was pre-calibrated so that the mesh alone created a compressive force close to 0.5 N. Figure 2 c shows the force acting on a glass substrate during the experiment. The force behavior during the retraction of the pillars is due to adhesion and friction between the pillars and the mesh. As the pillars retract from the sub-strate, the compressive force on the substrate drops. When the pillars are released from the mesh, the mesh can spring back and compress against the substrate, increasing the compres-sive force on the substrate. Nevertheless, the total force acting on the substrate is always strictly compressive. When the mesh was fi nally retracted, no pull-off force could be observed.

The microfi brillar surface is reusable and the switching is fully controlled by the relative displacement of the two surfaces. Figure 2 d shows the adhesive force from 12 cycles of adhesion on and off switching experiments (altogether 24 experiments). The very low pull-off force values for the off switching experi-ments in Figure 2 d were overestimates, as they were obtained by taking the minimum of the noisy load cell signal. Closer observation of the load cell signal revealed that no characteristic pull-off force peak could be observed in any of the 12 switching experiments. The load cell measurement noise level was 0.31 mN, suggesting a switching ratio, i.e. the ratio of on and off state adhesion, result of at least 2000-to-1, which is among the highest ever reported. [ 2 ]

To demonstrate the potential applications of the proposed switchable adhesive, we used it as a mini-gripper for picking and transferring various parts. The major benefi t of this type of a gripper is that it can transfer parts in a parallel manner. We have picked and placed four silicon chips simultaneously using the gripper. Each silicon chip was 3 mm × 3 mm × 380 µm. Schematics of the silicon chip pick-and-place are shown in Figure 3 a, pictures from the actual experiment are shown in Figures 3 b-c and a video is included in the supplementary material (Video S1). Another benefi t of this kind of a gripper is that it could pick and release very thin parts. Figure 3 d and Video S2 show the gripper picking a 25-µm thick polyimide sheet. The gripper is also not limited to picking only fl at parts: we also picked and placed a glass hemisphere (Figure 3 e and Video S3) and a thin, but long nylon wire (Video S4). The glass


Figure 1. Schematics and images of the mechanically switchable structured adhesive surface. a) Illustration of the switching principle: I : macroscopic elastomer pillars, II : microscopic dry microfi brillar adhesive, III : non-adhesive mesh, IV : substrate, and V : motorized stages. Dashed arrows indicate the movement of the surfaces during switching. b) Photograph of the macroscopic polyurethane elastomer pillars. c) Scanning electron microscope (SEM) micrograph of a pillar top. d) SEM micrograph of the microfi bers on top of the pillars. The microfi bers have mushroom shaped tips. The fi bers are 100 µm long. The stems have a diameter of 40 µm and the mushroom tips have a diameter of 90 µm. The tip is fl at and the outer edge of the tip has around 48˚ wedge angle. The center-to-center distance of the fi bers is 118 µm. e) Photograph of the non-adhesive polyoxymethylene mesh.


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hemisphere had a diameter of 6 mm and the nylon wire had a length of 12 mm and a diameter of 600 µm. The gripper could pick up parts with many different sizes and shapes, with the upper limit set only by the maximum weight that could be car-ried in the adhesion on state. Based on the preload experiments (Figure 2 b), the maximum weight is expected to be at least 1 N for fl at glass objects. Previously, transfer printing techniques [ 1 ] have been used to pick and place sub-millimeter-sized silicon chips, but were found less reliable for larger chips, as a single asperity between the printed object and the receiving substrate could impede transfer. [ 1 ] Our gripper could reliably pick, carry and release millimeter-sized objects. The lower limit of the part size is set by the size of the hexagonal holes in the mesh: at least one lateral dimension of the part has to be longer than the size of the hexagonal holes (inner diameter 2.6 mm) so that the mesh could release it. The size of the holes is limited by the diameter of the pillars, which is currently 2 mm. In principle, the pillars could be made much smaller. Also, the hierarchical adhesive surface could be made in a single casting process, by using microfabricated hierarchical fi ber molds. [ 30,32 ] Such molds should allow pillars with a diameter as small as 50 µm at

least. [ 32 ] Also, the mesh could be microfabricated, e.g. by using SU-8 lithography or deep reactive ion etching, and an addi-tional low surface-energy coating could be added to reduce the adhesion further.

In summary, we have reported a controllable mechanical switching method for a topographical adhesive surface. We have shown maximum pull-off pressures of 15 kPa in the adhesion on state on a large area and switching ratios of at least 2000-to-1. We have shown that the surface could be used as a robotic gripper for picking and releasing various parts, including picking several millimeter-sized silicon parts simulta-neously as well as manipulating thin parts. The surface consists of dry microfi brillar adhesive elastomeric pillars that extend through a non-adhesive mesh, and the switching is achieved by retracting the pillars through the mesh. The mechanism is expected to generalize well: in principle, any type of reversible adhesive coating could be used, depending on the application requirements on bonding strength and reusability. Also, the mesh material choice is not very important, as long as it is non-adhesive and can be fabricated to the desired shape. However, the current implementation of the switchable surface has three


Figure 2. Characterization of the switchable adhesive surface. a) Example of a pillar adhesion experiment on a fl at glass substrate: I : the pillars are preloaded against the glass substrate, II : the pillars start to retract from the substrate, III : the pull-off force is 0.62 N. b) Pull-off force as a function of preload, where each point is an average of fi ve experiments and error bars show standard deviation. c) Switching experiment: I : the pillars are preloaded against the glass substrate, II : the mesh is compressed against the substrate, III : the pillars start to retract from the substrate, IV : the mesh starts to retract from the substrate, V : no pull-off force within the resolution of the load cell can be observed. d) Switching repeatability demonstration. Pull-off force measured in 12 cycles of adhesion on and off switching experiments. The adhesion on state experiments are as in Figure 2 a and the off switching experiments are as in Figure 2 c.


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© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheimwileyonlinelibrary.com1300159 (4 of 5)

main limitations: 1) the current fabrication process limits the aspect ratio of the pillars and thickness of the release mesh, which sets a lower limit on the part size that can be released from the mesh; 2) having the mesh and the pillars as separate components requires accurate alignment of the pillars to the mesh holes on a large area; and 3) the actuation mechanism is still rather bulky. To overcome limitation 1, lithographic micro-fabrication techniques should enable miniaturization of the mesh and the pillars. Likewise, monolithic, wafer-level micro-fabrication techniques could potentially overcome limitation 2, so that the mesh and the pillars could be fabricated as an integrated structure. Limitation 3 could be solved by making the pillars from a smart material that achieve the retraction by an external thermal, electric or magnetic triggering. Such a smart material would be preferable in space-limited applica-tions, such as adhesive skin pads. Despite its limitations, we have shown that the current surface and actuation mechanism are suitable for transferring millimeter-sized parts in a parallel manner, which has been diffi cult with the previously developed methods. [ 1,2 ] As a practical application of our surface, we envi-sion using it for parallel microelectronic chip assembly and transfer printing. [ 1 ]

Experimental Section Fabrication of Macroscopic Elastomer Pillars : A master negative mold

was laser cut from a 2.8 mm thick acrylic sheet. The mold was dipped in a spin coated cyanoacrylate instant adhesive (Loctite 498) and pressed hard against the bottom of a polyethylene terephthalate petri dish. A washer was laser cut from a 2.8 mm thick acrylic sheet and similarly glued on top of the pillar mold. Making of the laser cut negative master is detailed in Figure S1. Spray-on mold release (Smooth-On Universal Mold Release) was applied to the mold. ST-1087 polyurethane (BJB Enterprises) was mixed in 100:50 ratio by weight A to B, degassed in a vacuum chamber for roughly 1 min or until it stopped bubbling, poured into the master and degassed again for another 3 min. Finally, an acrylic backing layer was carefully lowered onto the liquid polyurethane,

pressed hard against the washer to remove any excess polyurethane and left to dry in room temperature with weights on top, for at least 24 hours. The backing layer was removed and the cured polyurethane removed from the mold, resulting in polyurethane pillars with the length 2.8 mm on a 2.8 mm thick polyurethane backing layer. The initial positive polyurethane pillars were replicated by molding into silicone rubber (MoldMax® 20) and using the negative silicone rubber mold (instead of the laser cut mold) for subsequent polyurethane casts.

Fabrication of Dry Microfi brillar Adhesive : ST-1060 polyurethane (BJB Enterprises) was mixed in 100:55 ratio by weight A to B and degassed for approximately 1 min, until no bubbles were observed. A ∼3 cm diameter puddle was poured on a plastic petri dish lid and the puddle was degassed until no bubbles were observed. The puddle was spun in a spin coater at 1500 rpm for 30 s. The pillars were dipped in the resulting fi lm, using a manual micrometer stage. Polyurethane inked pillar tops were pressed into a mushroom tipped microfi ber mold and the structures were left to dry for at least 24 hours in room temperature. Figure S2 details the dip-casting process. Making of the mushroom-shaped microfi ber molds has been reported previously. [ 16 ]

Preload versus Pull-off Force Measurements : A microscope glass slide (Fisher Scientifi c) was cleaned with ethyl alcohol and mounted on top of an inverted microscope, which provided bottom view of the substrate. The adhesive pillars were glued using cyanoacrylate instant adhesive (Loctite 498) to an aluminum piece, which was mounted on a 250 g load cell (Transducer Techniques GSO-250). The load cell was on a vertical motorized stage (Newport MFA-CC) with two manual tilting stages (Newport GON40-L and GON40-U) for controlling the alignment. Custom made software controlled the motorized stage, while recording the measured loads at 1 kHz sampling rate and recording video from the inverted microscope. The pillars were compressed against the substrate with the speed of 50 µm s −1 , until the required preload force was reached. The force was maintained for at least 60 s to let any viscoelastic effects equilibrate, and then the pillars were retracted at a speed of 5 µm s −1 .

Adhesion on and off State Switching Experiments : The setup used for adhesion measurements was adapted to the adhesion on and off switching measurements. This time, a load cell was measuring the forces on a cleaned microscope glass substrate. The release mesh was added to the system, by mounting it on a vertical motorized stage (Newport VP-25XA). The no-switch experiments had the pillars moving down with a speed of 50 µm s −1 until 0.5 N preload was reached. This force was maintained for at least 60 s to let any viscoelastic effects equilibrate, and


Figure 3. Transferring part demonstrations using the switchable adhesive surface. a-c) Picking and placing four silicon chips using the switchable adhesive. Each chip is 3 mm × 3 mm × 380 µm. a) Schematics of the silicon chip pick-and-place. Dashed arrows indicate the movement of the pil-lars, the mesh and the substrate. Large arrows indicate the steps in the sequence. I : The silicon chips. b) Bottom view of picking the silicon chips ( I ). The pillars ( II ) are extended through the holes in the mesh ( III ). The substrate is marked with a black dot. The picture is a snapshot from Video S1. c) Bottom view of the silicon chips after releasing. The pillars have been retracted to release the parts. The picture is a snapshot from Video S1. d) Picking 25-µm thick yellow polyimide sheet ( I ) using the switchable surface. The lateral dimensions of the fi lm are 12 mm × 12 mm. The picture is a snapshot from Video S2. e) Picking a glass hemisphere ( I ). The diameter of the hemisphere is 6 mm. The pillars ( II ) can be seen extending through the mesh ( III ). The picture is a snapshot from Video S3.


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then the pillars were retracted at a speed of 10 µm s −1 . The switching experiments started with the calibration of the mesh displacement. The mesh was lowered until reaching to an approximately 0.5 N compressive force. The mesh position was recorded and the mesh was raised up. The pillars were then lowered with a speed of 50 µm s −1 , until 0.5 N preload was reached. The mesh was then moved down to the prerecorded position with a speed of 50 µm s −1 . The pillars were then raised 0.7 mm up with a speed of 10 µm s −1 . Finally, the mesh was raised with a speed of 10 µm s −1 .

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements V. S. was supported by Jenny and Antti Wihuri Foundation, Walter Ahlström Foundation, and the Academy of Finland (grant 268685), and M.S. was supported by the NSF CMMI-1130520 grant. We thank T. Novitsky and P. Glass for helping in the dry microfi brillar adhesive fabrication, J. Suhan for the SEM micrographs, and members of the NanoRobotics Lab for helpful comments.

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Received: December 29, 2013 Revised: January 16, 2014

Published online: February 8, 2014


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Mechanically Switchable Elastomeric Microfibrillar Adhesive Surfaces for Transfer Printing