Motion design of a starfish-shaped gel robot madeof electro-active polymer gelMihoko Otakea,Yoshiharu Kagami,Masayuki Inaba,Hirochika InoueDepartment of Mechano-Informatics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-Ku, Tokyo 113-8656, JapanDivision of Systems and Information Engineering, Hokkaido University, N13-W8, Kita-ku, Sapporo 060-8628, JapanAbstractThis paper proposes a method to generate novel motions of mollusk-type deformable robots made of electro-active polymer gel. Simulation and experimental results show that large transformations can be obtained with multiple electrodes in a planar configuration. We have designed a starfish-shaped gel robot that can turn over using spatially varying electric fields.2002 Published by Elsevier Science B.V.Keywords: Motion design; Electro-active polymer; Gel robot; Starfish; Deformable1 .Introduction Deformable robots, such as slug, eel, and snake-like robots can adapt to situations in which conventional artifacts consisting of rigid links and joints often fail. With their flexible bodies, eels can, for example, navigate through extremely cluttered sea floor environments inaccessible to more rigidly constructed animals. Actuators for robots of the future are expected to be artificial muscles. Electro-active polymer gels are a type of candidate material for building deformable robots like mollusk because their shapes and sizes are controllable by electric field. Such gels represent an open system, capable of exchanging matter and energy with the external environment. Our research goal is to develop a system for artificial muscles. Towards this end, we have been building prototype Gel Robots, which are made entirely of electro-active polymer gel. The purpose is to explore the mechanical design and control methodology required for such robots. Initially, we focused on the relationship between the shape and the bending response of the gel. We proposed a method to design functions through shape design of gel robots 1.Based on those experiments, we proposed a kinematic model that describes the transformation process of the gel. We found that the shape of the gel can be predicted based on local chemical reactions between the gel material and the surrounding electric field 2. We presented a modeling framework that divides electro-active polymer systems into two parts: the polymer itself and an electric field. We focused on the electric field rather than the gel material itself by using the same gel to make a variety of shapes. We conducted experiments using a spatially varying electric field generated by electrodes in a linear arrangement ,and derived a method to calculate the electric field 3. However, we have not yet generated dynamic motion in spatially varying electric fields generated by electrodes. In this paper, we create a novel motion of gel robots by combining the kinematic modeling method 2 and calculation method of the electric field 3.We simulate shape changes of the gel in spatially varying electric fields generated by multiple electrodes. By simultaneously driving real gel material under similar conditions, we can compare the results. Spatially varying electric fields generated by linear array, or a matrix arrangement of electrodes can bring out interesting motions of the gel, such as a starfish-shaped gel robot that turns over by deforming its whole body.2. Simulation framework for electro-active polymer gel robots We selected a typical electro-active polymer gel, poly(2-acrylarmido-2-methyl- propane sulfonic acid) gel (PAMPS gel) 4,5 and its co-polymer gel from among the variety of electro-active polymers because its ability to undergo large transformations. The gel bends in a surfactant solution when an electric field is applied. In general, electro-active polymer systems consist of polymer and electrodes either separate or composite. Both types can be modeled by considering the local electrical and chemical interactions of a polymer, the solution, and an electric field. In this uniform framework (polymers and electric fields), we analyze the mechanism of transformation of the gel (Fig. 1). PAMPS gel is driven by an electric field which was generated by parallel electrodes in a surfactant n-dodecyl pyridinium chloride solution containing sodium sulfate. The gel shows significant and rapid bending toward the anode. If the polarity of the electric field is altered repeatedly, the gel bending direction oscillates.The bending motion is based on molecular adsorption 4,6. The cationic molecules which are driven by the electric field adsorb on the surface of the anionic gel 7. Adsorbed molecules propagate and generate stress on the surface which causes strain on the surface 8,9. Thus we model the amount of adsorbed molecules as a function of the normal vector of the gel and current density at the surface of the gel and 2, (1)This approximately represents the adsorption proc