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Magnetically Actuated Patterns for Bioinspired Reversible Adhesion (Dry and Wet) Dirk-Michael Drotlef, Peter Blümler, Aránzazu del Campo* Over the last decade the unique “strong but reversible” characteristics of gecko’s and tree frog’s adhesive pads have been intensively investigated both in the natural systems and artificial mimics.[1,2] Whereas adhesive strength in the natural systems has been matched and even surpassed with synthetic micro and nanostructured surfaces,[3,4] strategies to effectively switch between adhesive and non adhesive states, drastically or gradually, are still scarce and limited either in performance or by the complexity of the preparation method.[5–9] Reversibility in patterned adhesives relies on a significant surface pattern reorganisation via application of an external stimulus. In artificial systems this has been achieved by temperature changes using patterns of responsive polymer materials (shape memory polymers[5] and liquid crystalline polymers[6]) or by mechanical stretching of wrinkled patterns supported by elastomeric films.[7–9] Switching adhesion with temperature based methods is slow and cannot be tuned. Adhesion changes by mechanical forces only work on stretchable films and the principle is not applicable when these are supported by rigid solids.[7–9] Methods for reversible and tunable adhesion controlled by noncontact external stimuli (temperature, light, etc.) remain a scientific and technical challenge. Polymer-based magnetically actuated microcomponents have been reported to undergo predesigned, complex two- and threedimensional motions upon application of magnetic fields.[10–12] Elastomeric materials filled with magnetic nanoparticles shaped in different microgeometries by soft moulding methods have been reported, including pillar patterns.[11,13–15] These structures have been applied in microfluidics,[15] for inducing localized traction forces to cells[11,13] and to generate anisotropic motion of microsized objects.[10] However, all these examples are based on either small movements or movement of isolated components. None of the reported cases allow homogeneous, robust and strong magnetic-driven movement of microcomponents over large areas, which is a prerequisite for efficient switching of adhesion on structured surfaces. Here we report on a facile strategy to obtain magnetically actuated arrays of micropillars able to undergo reversible, homogeneous, drastic and tunable

D.-M. Drotlef, Dr. A. del Campo Max-Planck Institut für Polymerforschung Ackermannweg, 10. 55128, Mainz, Germany Tel: +496131379563 E-mail: [email protected] Dr. P. Blümler Institute of Physics University Mainz Staudingerweg 7, 55099, Mainz, Germany

DOI: 10.1002/adma.201303087

Adv. Mater. 2014, 26, 775–779

geometrical changes upon application of a magnetic field with variable strength. We demonstrate, for the first time, a magnetically tunable adhesive that works under dry and wet conditions. Arrays of magnetic micropillars with 47 μm height and 18 μm diameter were obtained by soft moulding[16–18] using PDMS precursors containing NdFeB microparticles (see Figure 1 and Experimental Section for details). A drop of magnetic PDMS precursor was cast on the mould and a suitable magnetic field gradient was applied by placing the mould on top of a permanent magnet. In this way, the microparticles accumulated inside the pillars, where the magnetic field gradient was stronger (Figure 1, step 1). This step was crucial for a homogeneous and strong magnetic response of the pillars across the pattern. The residual magnetic PDMS layer on the mould was scraped off and a previously cured PDMS thin film was pressed against the mould (Figure 1, step 2). A non magnetic backing layer is important in order to avoid interfering magnetic interaction with the pillars. The sandwich PDMS mould/PDMS-NdFeB/PDMS film (Figure 1) was cured in an oven and then placed in a strong homogeneous magnet for magnetizing the embedded NdFeB particles (Figure 1, step 3). Control patterns were also prepared where the magnetization step was omitted. After demoulding, homogeneous arrays of magnetic micropillars supported by a non magnetic backing layer were obtained (Supporting Information (SI) Figure SI-1). The microparticles were homogeneously distributed along the pillars, as observed by optical microscopy (Figure 1). Alternatively the sandwich PDMS mould/PDMS-NdFeB/PDMS film was magnetized before curing and then placed on a magnet in order to attract the highly magnetized microparticles in the fluid PDMS to the top of the pillars (Figure SI-2). However, in this case irreversible stick of neighbouring pillars was typically observed after demoulding. This was a consequence of the high concentration of magnetized particles at the top of the pillars, which makes neighbouring pillars to behave like two interacting micromagnets (Figure SI-3). For this reason this method was rejected in the following studies. In order to test the magnetic response of the micropillars, a cylindrical NdFeB permanent magnet mounted on a micromanipulator was approached to the sample (Figure SI-4) and the response of the micropillars was followed by optical microscopy. Micropillars bended and rotated about their own axes when the magnet approached the sample and was moved around it (Figure 2A, SI-movie 1). The movement was homogeneous across the pattern (SI-movie 2). At stronger field gradients, a more pronounced bending and contact between pillars (Figure 2B1, SI-movie 3) or contact between the upper part of the pillar and the backing layer of the array (Figure 2B2, SI-movie 4 and 5) was observed depending on the bending direction of the pillars. When the magnet was removed, the pillars returned to

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www.MaterialsViews.com 1. Casting PDMS/ NdFeB mixture

4. Demoulding, inking

7. Peeling

PDMS/ Nd Fe B PDMS mould

PDMS film

Magnet

2. Press PDMS backing layer, curing

5. Transfer

90°C

3. Magnetization

6. Printing, curing 90° C

Figure 1. Scheme showing the fabrication steps for obtaining arrays of magnetic micropillars (steps 1–3) and their modification with T-shape terminals for adhesion enhancement (steps 4–6, see text for additional explanation and references[16,17] for additional details on the fabrication). The SEM image shows the obtained magnetic PDMS patterns with T-shape (25 μm diameter, 48 μm height and 15 μm separation). The microscopic image shows the magnetic microparticles homogeneously distributed inside the micropillar. Scale bar corresponds to 20 μm.

their original position. In some cases sticking of neighbouring pillars was observed after demoulding (Figure SI-5). However, pillars recovered their upright position when a magnet was applied again (Figure SI-5). Using an alternating magnetic field (electro-magnet connected to a pulse generator or an ordinary magnetic stirrer), switching between the pillar’s tilting direction was achieved (Figure SI-6, movie 6). The tilting angle in this case was low (15°) due to the weaker field gradients when compared to the micromanipulated permanent magnets used above. The strength and homogeneity of the response of the micropillars was dependent on the initial content of NdFeB

A1

B1

A2

microparticles, the time in contact with the magnet before curing (i.e. the final particle concentration inside the pillars), and the magnetization time. These parameters were optimized in our process for maximizing the magnetic response and actuation of the patterns. A 20% concentration of microparticles in the original suspension was necessary to achieve tilting of the micropillars down to the backing layer. Lower concentrations rendered only partial tilting of the pillars and higher concentrations did not significantly improve the pillar’s response to the magnet (Figure SI-7). It is important to note that the actual concentration and size distribution of particles in the pillars

A3

B2

Figure 2. Actuation of magnetic micropillars under the microscope by approaching a magnet with a micromanipulator. A) Tilting of magnetic pillars to opposite directions when the magnet is approached from opposite sides. B) Tilting of magnetic pillars at stronger field gradients. Scale bar corresponds to 20 μm.

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of iron nanoparticles (diameter 0.1-1 μm) showed ca. 9° tilting upon magnetic action.[13] In contrast to these examples, our NdFeB/PDMS micropillar arrays are mechanically robust, can be easily prepared and show strong and homogeneous response due to the higher magnetic moment of the NdFeB microparticles compared to iron oxide ones. All these factors are prerequisites for their application as actuated adhesive structures. The strong and reversible tilting of the single micropillars under the action of an external magnetic field gradient caused a drastic change in the surface topography. We tested if this property could be applied to tune the adhesive performance of the patterned surface on-demand using arrays of magnetic microposts decorated with a T-shape tip (Figure 1 steps 4–6 and Figure SI-8). This geometry mimics the spatular terminal of gecko’s setae and has been demonstrated to improve adhesion performance of patterned adhesives under dry[16,17] and wet[20] conditions. The adhesion performance of the surface in the presence and absence of the magnet was characterized by recording force vs. distance curves using a home-made adhesion tester.[20] Non magnetized patterns were also characterized as reference. The patterned sample was mounted on a glass sample holder and pressed against a spherical probe. The sphere was retracted until pull-off occurred. The experimental value of the pull-off force, A B Foff, was extracted from the minimum of the retraction curves and was taken as adhesion force. Figure 3A shows characteristic forcedisplacement curves obtained from magnetic patterns. In the absence of a magnetic field, a clear pull-off event was detected and an adhesive force, Foff = 11 mN was obtained. When a magnetic field gradient was applied, a drop by one order of magnitude in the pull-off force was observed (0.7 mN). This result indicates that only a weak contact was established between the probe and the tilted pillars as a consequence of the reduced contact area. C This value was maintained independently D of the applied preload. The bending of the pillars also affected the slope of the loading curve, which reflects the effective Young’s modulus of the patterned surface.[18] The slope of the loading curve was lower in the presence of the magnetic field gradient, indicating that the partially bended pillar array can be deformed more easily than the array of vertical pillars during loading and, therefore, the surface appears effectively softer. The adhesive and non-adhesive states were reversibly cycled by switching the magnetic field on and off (Figure 3B), demonstrating Figure 3. A) Force-distance curves recorded on patterned surfaces with T-shaped tips (pillar the first case of magnetoswitchable adhesion. diameter 25 μm, height 48 μm and spacing 15 μm) with and without magnetic field. B) Changes Note that the adhesion force of magnetic flat in the adhesion force (Foff ) measured on arrays of magnetic T-shaped micropillars when the terminated pillar patterns can be switched as magnetic field was turned on and off. C) Change of the adhesion force with the bending of well, but the adhesion force is significantly the pillars caused by higher field gradients and corresponding microscopic pictures. Measuresmaller (Figure SI-9). Control measurements ments were performed under dry and wet conditions. D) Net-force acting on a single magnetized particle in a distance of 1 mm from the surface of a cylindrical NdFeB permanent magnet on non magnetized PDMS samples (flat and (BHmax = 35 MGOe) with radius 4 mm and height 5 mm as a function of the radius. The data patterned) did not show adhesion changes were obtained from a FEM-simulation. upon magnetic actuation either (Figure SI-10).

is different than the microparticle concentration and particle size distribution in the original suspension. This is due to the enrichment of microparticles inside the pillars by the action of the applied magnetic field gradient before curing. TGA analysis of the magnetic micropillars (see experimental information for more details) gave an average particle concentration inside the pillars of 82 weight%. The optimum enrichment and magnetization times were 10 and 2 minutes respectively. At this point it is interesting to compare the magnetic-driven actuation of our micropillars with the few examples reported by other groups. High aspect ratio Fe2O3/PDMS nanopillars (0.5 μm diameter, 90° did not further decrease Foff. These results constitute the first demonstration of a gecko-inspired tunable reversible adhesive. The area of tilted pillars was around 0.4 × 0.4 mm2. Outside of this area, pillar bending gradually decreased. We theoretically estimated the net magnetic force acting on the pillars and the expected deflection and compared it with the experimental observations. The applied force F to a single particle inside a pillar was estimated by   − → → − → → − F = ∇ − m· B (1) Which is the gradient of the dot product of the magnetic moment, m, of a particle and the magnetic flux density, B, produced by the magnet at the particles location. The magnitude of the magnetic moment can be estimated for the magnetically very hard NdFeB alloy as V m = BR :0

(2)

Where V is the volume of the particle (5 μm diameter) and BR = 0.8 T is the remanence of the material as provided by the supplier, resulting in m = 4.17 · 10−9 Am2. Theoretically, the polarization step (Figure 1, step 3) fixes the direction of the magnetic moment normal to the surface. Therefore, only the normal field component generates a force on the magnetized parti− → → → J =− m × B, cles when analyzing Equation (1). The torque, − however depends also on the radial field component causing radial forces. To estimate the net magnetic force on a pillar, the magnitude of the gradient field tensor was determined by a FEM-simulation (FEMM 4.2) for a cylindrical magnet with the same properties as the one used for the experiments. Figure 3D shows the net magnetic force on a single particle located 1 mm above the surface of the magnet. The magnetic force has a 778

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maximum of ∼16 nN very close to the edge of the magnet, as observed experimentally too. With the force of 16 nN acting on a single particle and a particle content of 82 weight%, which corresponds to 67 particles per column, the total force on each pillar was estimated to be Fpil = 1.1 μN. From this value, the defection of the pillar, ν, can be theoretically predicted by

Magnetically actuated patterns for bioinspired reversible adhesion (dry and wet).

A facile strategy to obtain magnetically actuated arrays of micropillars able to undergo reversible, homogeneous, drastic, and tunable geometrical cha...
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