Dielectric elastomers

Dielectric elastomers (DEs) are smart material systems which produce large strains (up to 300%^{[citation needed]}) and belong to the group of electroactive polymers (EAP). Based on their simple working principle dielectric elastomer actuators (DEA) transform electric energy directly into mechanical work. DE are lightweight, have a high elastic energy density and have been investigated since the late 90’s. Many potential applications exist as prototypes. Every year in spring a SPIE conference takes place in San Diego where the newest research results concerning DEA are exchanged.

Working principle of dielectric elastomer actuators. An elastomeric film is coated on both sides with electrodes. The electrodes are connected to a circuit. By applying a voltage U the electrostatic pressure p_el acts. Due to the mechanical compression the elastomer film contracts in the thickness direction and expands in the film plane directions. The elastomer film moves back to its original position when it is short-circuited.

Working principle

A DEA is a compliant capacitor (see image), where a passive elastomer film is sandwiched between two compliant electrodes. When a voltage U is applied, the electrostatic pressure p_el arising from the Coulomb forces acting between the electrodes. Therefore the electrodes squeeze the elastomer film. The equivalent electromechanical pressure p_eq is twice the electrostatic pressure p_el and is given by the following equation:

$p_{eq}=\varepsilon_0\varepsilon_r\frac{U^2}{z^2}$

where ε₀ is the vacuum permittivity, ε_r is the dielectric constant of the polymer and z is the thickness of the elastomer film. Usually, strains of DEA are in the order of 10–35%, maximum values are up to 300%.

Materials

For the elastomer often silicones and acrylic elastomers are used. In particular, the acrylic elastomer VHB 4910, commercially available from the company 3M has shown the largest activation strain (300%), a high elastic energy density and a high electrical breakdown strength. Basically, the requirements for an elastomer material for DEA are

• The material should have a low stiffness (especially when large strains are required);
• The dielectric constant should be high;
• The electrical breakdown strength should be high.

A possibility to enhance the electrical breakdown strength is to prestretch the elastomer film mechanically. Further reasons for prestretching the elastomer are the following:

• The thickness of the film decreases. A lower voltage has to be applied to obtain the same electrostatic pressure;
• The prestrain avoids compressive stresses in the film plane directions which might be responsible for failure.

The elastomers show a visco-hyperelastic behavior. Models which describe large strains and viscoelasticity are required for the calculation of such actuators.

Several different types of electrodes are used in the research (e.g. graphite powder, silicone oil / graphite mixtures, gold electrodes, etc.). The electrode should be conductive and compliant. The compliance of the electrode is important in order that the elastomer is not constrained mechanically in its elongation by the electrode.

Configurations

Several configurations exist for dielectric elastomer actuators:

• Framed/In-Plane actuators: A framed or in-plane actuator is an elastomeric film coated/printed with two electrodes. Typically a frame or support structure will be mounted around the film. Examples are expanding circles, planars (single and multiple phase)
• Cylindrical/Roll actuators: Coated elastomer films are rolled around an axis. By activation, a force and an elongation appear in axial direction. The roll actuators can be rolled around a compression spring or rolled without a core. The application of such cylindrical actuators are artificial muscles (prosthetics), mini- and microrobots, and valves.
• Diaphragm actuators: A diaphragm actuator is made as a planar construction which is then biased in the z-axis to produce out of plane motion. Artificial Muscle, Inc. offers a Universal Muscle Actuator (UMA) that is a double diaphragm. [1]
• Shell-like actuators: Planar elastomer films are coated at specific locations in form of different electrode segments. With a well-directed activation of the cells with the voltage, the foils assume complex three-dimensional shapes. Such shell-like actuators may be utilized for the propulsion of vehicles through air or water, e.g. for blimps.
• Stack actuators: By stacking up several planar actuators the force and the deformation can be enlarged. Especially an actuator which shortens by activation can be realized.
• Thickness Mode Actuators: The force and stroke of DEA's can be taken in the z-direction. Thickness mode are a typically a flat film that may have a stack of multiple layers to increase displacement.

Applications

Dielectric elastomers offer a wide variety of potential applications as a novel actuator technology that can replace many electromagnetic actuators, pneumatics, and piezo actuators. And dielectric elastomers can enable actuators to be integrated into applications that were previously infeasible. A list of some applications for dielectric elastomers are:

• Haptic Feedback
• Pumps
• Valves
• Robotics
• Prosthetics
• Power Generation
• Optical Positioners such for auto-focus, zoom, image stabilization
• Sensing of force and pressure
• Active Braille Displays
• Speakers
• Deformable surfaces for optics and aerospace

References

"High-Speed Electrically Actuated Elastomers with Strain Greater Than 100%". Ron Pelrine, Roy Kornbluh, Qibing Pei, Jose Joseph. Science Vol. 287. no. 5454, pp. 836–839. 4 February 2000 [2]
Carpi, De Rossi, Kornbluh, Pelrine and Sommer-Larsen "Dielectric elastomers as electromechanical transducers. Fundamentals, materials, devices, models & applications of an emerging electroactive polymer technology" Elsevier (2008).

External links

• "Artificial Muscle, Inc. - High-Volume EPAM Actuator Developer and Manufacturer
• http://empa.ch/plugin/template/empa/*/39286/---/l=2
• http://esnam.eu
• http://ndeaa.jpl.nasa.gov/nasa-nde/lommas/eap/EAP-web.htm
• Loverich, Kanno, and Kotera, "Concepts for a new class of all-polymer micropumps". Lab Chip, 2006, 6, 1147–1154, DOI: 10.1039/b605525g
• Danfoss PolyPower
• Biomimetics Laboratory