Self-organized minimum energy elastomer actuators

Dielectric elastomer actuators show impressive area expansions when activated with large electric fields. They are lightweight, inexpensive and can be easily shaped into a large variety of actuator configurations. These attractive features make dielectric elastomer actuators suitable for applications in artificial muscles, in adaptive structures, robotics etc. Dielectric elastomer actuators consist of a deformable capacitor, where the elastomer is sandwiched between highly compliant electrodes. The actuators are driven by applying a high voltage to the electrodes. The high voltage causes a Maxwell stress acting on the elastomer material; thereby the elastomer is compressed in the film direction and expanded in the film plane direction. A large number of actuator configurations are known which we have extended to self-organized minimum energy actuators recently. Since dielectric elastomer actuators are driven by electric fields, the corresponding electric current should contain information on the actuation state of the actuator. The electrical response of the actuator is highly nonlinear, due to the large changes in the elastomer during actuation. Though it seems natural to use dielectric techniques for the characterization of actuators, only few studies attempted to perform such measurements. Elastomers are model systems for entropy elasticity; they consist of a network of entangled and cross-linked polymer chains, able to sustain spatially varying stress and strain fields, accompanied by stored elastic energy. The high configurational entropy of elastomers is lowered by mechanical stretching, thereby increasing the free energy. Stretching ratios in elastomers are usually very large, area expansions of more than 300 % have been obtained experimentally, so these materials are described by hyperelasticity models. Hyperelasticity models can be based on purely phenomenological arguments or on statistical mechanics. Models for describing the actuation of elastomer actuators are essential for designing and optimizing actuator configurations. Models are best tested on simple actuator configurations, like the circular actuator. Due to the electrostatic nature of elastomer actuators, electromechanical pull-in instabilities may be observed. Only few studies are available on driving actuators into bistable or pull-in instability regions. In this project, we intend to introduce nonlinear dielectric spectroscopy techniques for the characterization of elastomer actuators. The area expansion of elastomer actuators will be measured electrically and compared with optical extensometer measurements. Thereby not only an electrical control of the actuation state seems possible; we also expect new insights when driving the actuators in instability regions, where wrinkles may appear in the elastomer film. It is also expected that the high sensitivity of dielectric measurements enables the determination of the quality of the highly compliant electrodes during actuation. We further intend to demonstrate new actuation schemes by employing piezoelectric and electrostrictive polymers as frame in minimum energy actuators. Modeling of elastomer actuators will be based on a thermodynamic approach, in order to analyze their limiting behavior caused by dielectric breakdown and pull-in instabilities. In order to gain insights into the material properties governing elastomer actuators and for providing guidelines for materials development and actuation, statistical mechanical models including effects of cross-links and entanglements will be used in describing experimental results. In summary, we intend to introduce new characterization techniques and actuation schemes, to investigate bi- and instabilities in actuators, to analyze the results by means of statistical mechanical models and to provide guidelines for materials and device optimization.

Dielectric elastomers change thickness and expand in area when an electric field is applied. This robust actuation mechanism is at the heart of an emerging technology with applications ranging from soft robots, tactile and haptic interfaces to adaptive optics. The project P20971-N20 contributed several new research avenues to the science of dielectric elastomers, addressing fundamental problems in dielectric elastomer technology. A critical problem in dielectric elastomer technology is electromechanical instability. When an actuator is subject to a voltage, the elastomer thins down, the electric field increases, and so do the attractive forces between the electrodes on the actuator. At the pull-in instability this positive feedback causes the elastomer to thin down dramatically until dielectric breakdown occurs. We have shown that electrode-free actuators, proposed by W. C. Röntgen in 1880 are not subject to the pull-in instability. We have also summarized the work progress in the dielectric elastomer transducer field in a Science perspective in 2010, indicating latest frontiers in the field. Reducing the operating voltage is one of these latest frontiers; here we have contributed by demonstrating large area expansion in a dielectric membrane triggered by a liquid gaseous phase change. Our latest research was devoted to make electromechanical instabilities safe. We have shown a giant voltage-induced area deformation of roughly 1700 % by employing the snap-through instability in balloon actuators. Besides, also new research avenues in related fields were identified. In the project we have developed the first mechanically stretchable battery for stretchable electronics. In this work, dielectric elastomers serve as substrate for battery fabrication. During the course of the project we were able to establish a currently ongoing fruitful scientific cooperation with the research team of Prof. Zhigang Suo from the School of Engineering and Applied Sciences at Harvard University. Christoph Keplinger, who worked in the project and finished his PhD in September 2011 is currently a post-doc at Harvard University. The work on stretchable batteries led to an ongoing cooperation with Prof. Takao Someya from the Electrical Engineering Department at Tokyo University, where Martin Kaltenbrunner, a PhD student who also contributed significantly to the success of the project and finished his PhD in March 2012 is currently working as a post-doc.