TOWARDS ARRAY MICROMANIPULATORS BASED ON DIELECTRIC ELASTOMERS

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1 TOWARDS ARRAY MICROMANIPULATORS BASED ON DIELECTRIC ELASTOMERS Martin ŘEZÁČ, Zdeněk HURÁK Department of Control Engineering, Faculty of Electrical Engineering (FEL), Czech Technical University in Prague (ČVUT), Karlovo náměstí 13/e, Praha 2, Česká republika Abstract The paper describes the authors' initial steps in the domain of advanced microactuators based on dielectric elastomers. Namely, local deformation of a thin (~10um) elastomer film as a result of Maxwell pressure on the surface is analyzed. The pressure results from attractive forces between the opposite charges on the conducting electrodes deposited on both sides of the polymer film. When at least one of the two electrodes is compliant, repulsive forces among the alike charges also contribute to the surface deformation. Compared to the conventional materials such as piezoelectric ceramics, DEA promise much higher strain (nearly 10%) while keeping reasonable values of other parameters such as efficiency and time response. The ultimate goal of this initiating research is to develop a deformable surface that could be used for planar manipulation with flat objects placed on top of it. Induced surface traveling wave, for instance, could be used to achieve simple transportation of samples in the style of common linear ultrasonic motors. The early achievements presented in this paper include trivial yet tedious fabrication consisting in sputtering Cr+Au electrodes on the substrate, then spincoating a ~10um-thick film from a commercially available elastomer known as PDMS (polydimethyl siloxane, branded as Sylgard 184 by Dow Corning) and finally depositing the top (compliant) gold electrode, and laboratory experiments with high voltage (around 1kV) applied to the electrodes. Deformation of the film observed with an optical microscope partially agrees with the computational model developed in a FEM modeler/solver. The deviations from the model are discussed. 1 INTRODUCTION TO PLANAR ARRAY MICROMANIPULATION The ultimate goal of the research outlined in this paper is to develop a distributed planar micromanipulator for steering small (meso or microscale) objects over a flat substrate. The fact that the manipulation will be spatially distributed promises an increased throughput for various assembly procedures because it allows for high degree of parallelism compared to inherently traditional high precision microrobotic manipulation schemes. Among the most prominent researchers in the field of distributed manipulation is Karl Bohringer. His monograph [4] gives a good survey. In a long series of papers from the late 1990s he introduces a concept of a programmable force field [6, 8]. Loosely speaking, it is a mathematical formalism that allows engineers to calculate control signals for large (dense) arrays of actuators to accomplish a given mission. A distinguished contribution was made by Luntz and Messner [14] as they were the first to consider some discreteness issued of the actuator field. Murphey and Burdick elaborated on these mathematically difficult problems a bit more [15].

2 A screenshot of the output from a numerical simulation of such a force field is given in Fig.1. A spatially discrete array of actuators exerts forces and moments on object placed onto the surface of such actuator. Fig. 1: Simulation of motion of objects in a planar spatially discrete force field. 2 INTRODUCTION TO DIELECTRIC ELASTOMER ACTUATORS The previous section on array micromanipulators did not give much hint as for the link of this paper with the field of material engineering and nanotechnology. Here it comes. The idea for our research is to create a film of an elastic polymer, so called elastomer, and sandwich it between two conductive electrodes. The effect which is well known for parallel plate capacitors pulls the two electrodes together and deforms the surface. In addition, some pillars (bristles, legs, cilias) can be fabricated on top of this deformable surface to make the troubles with stiction less significant. Locally deforming (squeezing) the surface will result in some kind of transversal motion of the particle as illustrated in Fig.2. Fig. 2: Principle of a planar manipulator based on a dielectric elastomer film. Cross-section of the sandwich structure. The pillars (legs, bristles, cilia) on the top electrode magnify the motion and help avoid problems with stiction of small particles. The behaviour sketched above is actually well known since late 1980s [7] and the community uses the name dielectric elastomer actuators for it (to distinguish it from a broader class of electroactive polymers) but interest of the community arose after publication of some marvelous features of these materials in Science journal by Pelrine in the new millennium [16]. Actually, vast majority of key results come from this group at SRI International. The major two monographs covering the development in the last decade are [1] and [2]. In our reseach, we focus on one particular mode, so called thickness mode, where we aim to change the thickness by squeezing the film locally using electric (Maxwell) pressure on the surface. Some tricks relying on transversal deformation of compliant electrodes (due to repulsion of alike charges) are in [5]. Among the applications of local change of thickness published so far are spatial light modulators [13] and deformable micromirrors [17]. Application to manipulation has been mentioned in [5] with no details, though.

3 3 FEM SIMULATIONS In order to be able to use the deformation of the elastomer film toward our goals, the response of the system to voltage applied to the electrodes must be modeled and simulated. Even though dielectric elastomers exhibit strain up to several hundred percent, which calls for nonlinear models of elasticity (so called rubber elasticity due to volume incompressibility of elastomers) [10, 11], in thickness mode the deformation is usually just a few percent (compare with 0.1% for piezos) which justifies to use linear solvers to get some initial idea. We used the coupled mode from Comsol Multiphysics where plain stran and electrostatics were solved using finite elements method. In Fig.3 a crossection of a film with one electrode at the bottom and the other on the top is visualized. Fig. 3: FEM simulation of electrical potential between two electrodes, including deformation of dielectrics allowed by compliance of the upper electrode. 4 FABRICATION 4.1 Design of simple thickness mode DEA actuator For the first experiments with fabrication of DEA actuator we have used mainly PDMS (Polydimethylsiloxane) elastomer provided commercially by Dow Corning under the name Sylgard 184. Gold electrodes were sputtered on the bottom and top of it. A full account of alternative commercially available elastomers is in [2], we also experimented with HS - III, which has similar performance but a bit more troublesome fabrication properties (more viscous). The recipe used for fabrication was as follows.

4 On the glass substrate create gold electrodes by sputtering gold (possibly chromium first to improve adhesion). Thickness of gold on substrate should be about 40 nm, which provides small enough resistance even for electrodes a few tens of microns wide. The sputtered gold layer should not be much thicker because roughness of substrate affects homogeneity of elastomer film very much. With the final gold of measuring spatial deformation of the film, we fabricated very narrow electrodes, about 100 µm. This was done by laser ablation using an eximer laser available in a lab or our research partner. Typical outcome is in Fig.3. Next step is to create an elastomer film on top of this patterned gold electrode. The easiest way to deposit a thin layer of PDMS (about 10 µm) is using a spincoater (iterating a bit on the right spinning profile). The PDMS comes in two parts (base and curing agent) which must be mixed immediately before spincoating, because the crosslinking process starts immediately. After spincoating, the sample is placed into an oven for about 1 hour to finish the crosslinking process during which the elastomers becoms a solid film (no longer flowing). Finally the top electrode needs to be deposited. We used the same process as with the bottom (substrate) electrode using sputtering gold. The only difference is that now thickness of sputtered gold matters a bit more, because it affects elasticity of the resulting system. Fig. 4: Photo of simple laboratory sample demonstrating the deformation of a PDMS film. The substrate is just a piece of glass on which a (vertical) gold strip is sputtered. Then an (almost) transparent PDMS film is spincoated (the edges of the glass were protected by a tape, hence the square layout of the PDMS film) and a top (horizontal) electrode is sputtered. The wide lower electrode is structured using an excimer laser to obtain a very narrow electrode in the middle. 4.2 Design and fabrication of pillars First idea how to create pillars is to cast PDMS into polycarbonate mold, which could be prepared using the same excimer laser used for ablating electrodes (in fact, the gaps). In Fig.5 you can see a screen shot of an array of pillars during measurement process and the resulting height profile. The measurement was done using a confocal microscope. Because these pillars act as a motion multiplier (the higher the pillars the stronger the generated in-plane actuation) the aim is to create them as high as posible and as thin as possible.

5 Fig. 5: The cast for a matric of pillars on a PDMS film. On the left there is a screenshot from measurement using a confocal microscope, on the right there is a graph of the resulting profile. Each pillar is 100x100x40 µm. The maximum ratio we have achieved so far is about 2. Pillars on Fig.5 have ratio 0.4 only. The major limitation comes from the need to peel the cured elastomer film from a polycarbonate mold. When some other technology is used, for instance, when the mold is ablated into a photoresist which is chemically dissolved after being filled with an elastomer, ratios up to 10 could be achievable. 4.3 Fabrication of a sample with electrodes and pillars In order to prove the concept, we tried to fabricate both the elastomer film with electrodes and the elastic pillars. Fig.6 shows the result. Fig. 6: DEA sample with three elestrodes and pillars (14x25 small dark points). Each of 3 electrodes is 100µm thick. During pulling the cast off from the sample some air bubble gets in and destroys this sample. Comparing the size of pillars to size of electro Three electrodes were ablated on the substrate (note that the black areas in the figure show the interelectrode gaps). Then we fabricated a cast with 14x25 small holes and squeezed the uncured PDMS

6 layer between substrate and the cast. Unfortunately during this operation some air got in between the substrate and the cast and therefore when peeling the film off the cast, part of the sample wad damaged. Apparently the fabrication procedures described in this section are rather simple, but they only served to prepare a sample for a proof-of-concept. More specialized research partner will be needed for further joint research. 5 LAB EXPERIMENTS TO MEASURE DEFLECTION In order to have a model of deformation accurate enough both in time as well as spatial domain, we have used two different types of measurements of the surface deformation. First, the surface deformation was measured using a Sensofar profilometer. This confocal microscope has a precision of the order of tens to hundreds of nanometers. In Fig.7, you can see the results of measuring the surface deformation of DEA with a single thin electrode from Fig.4. Larger deflection is observed around the edges of electrodes (on both sides) where the eletric field is strongest. There is no surprise that since elastomers are incompresible, there is some kind of bulge next to the electrode. On the right on Fig.7 you can see a colored deflection map (viewed from above). This picture shows an interesting fact that even in the middle of the electrode there are some hills and valleys when a voltage is applied. Possible explanation of this phenomenon comes from having less intensity of electric field in the middle of the electrode. Thus instead of seeing a positive deflection only, some negative deflection is also observed alongside roughness of the top gold electrode. Fig. 7: On the left - shape of deformation when applied four different voltages. On the right 2D (top view) deflection map when 1300V applied. Blue colour means the valley and red the hills acording to the picture and scale on the left.

7 Because the measurement process using Sensofar profilometer is very slow (the profile in Fig.6 was took about 120 s) it is not the way to go to get time response of DEA. For this purpose we have realized contactless measurement using CD-ROM drive sensor. Principle of this sensor can be found for example in the thesis [3]. Maximum deflection amplitude of CD-ROM drive is about 10µm. In Fig.8 there is a time response of the deflection with the driving voltage in the same figure. On the left, some short term switching measurement has been done. The right half shows in calibration of the sensor during the first 200s. Between 200 s and 340 s there was four times applied voltage on the sample in one point. After that the position of measured point was changed and the experiment has been done again. Now the response is completely different, which is because the first time the point of interest was in the hill of deflection and the other in the valley. Interesting is period between 480 s to 550 s, where the stall voltage was applied the whole time, which was leading to continuous deflection all the time. This probably means that elastomer in our sample was not cured enough and shows viscoelastic behavior and that's why we observe two in range very different time constants. Fig. 8: On the left: time response of a surface deflection at one point. On the right: similar measurement, but also with a calibration phase and measuring at two different points. 6 CONCLUSIONS AND FUTURE WORK The paper reports on initial research steps of the authors as well as key long-term motivation in the domain of dielectric elastomers as modern smart materials for advanced micromanipulators. The major objective of the paper is to inform the local research community about these initiated activities (perhaps surprisingly in a control systems and robotics oriented group). The authors believe that these can offer opportunities in material research as well since, for instance, design and implementation of compliant electrodes can benefit much from technologies of conductive flexible layers based on carbon nanotubes. Similarly, optimizing the electric and mechanical properties of the currently available elastomers by adding nanoparticles turns out an attractive research direction as well. See the recent monograph [2] for detailed information on material engineering aspects.

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