Dielectric elastomers

A DEA is a compliant capacitor (see image), where a passive elastomer film is sandwiched between two compliant electrodes. When a voltage ${\displaystyle U}$ is applied, the electrostatic pressure ${\displaystyle p_{el}}$ arising from the Coulomb forces acts between the electrodes. The electrodes squeeze the elastomer film. The equivalent electromechanical pressure ${\displaystyle p_{eq}}$ is twice the electrostatic pressure ${\displaystyle p_{el}}$ and is given by:

${\displaystyle p_{eq}=\varepsilon _{0}\varepsilon _{r}{\frac {U^{2}}{z^{2}}}}$

where ${\displaystyle \varepsilon _{0}}$ is the vacuum permittivity, ${\displaystyle \varepsilon _{r}}$ is the dielectric constant of the polymer and ${\displaystyle z}$ is the thickness of the elastomer film. Usually, strains of DEA are in the order of 10–35%, maximum values reach 300% (the acrylic elastomer VHB 4910, commercially available from 3M, which also supports a high elastic energy density and a high electrical breakdown strength.)

Films of carbon powder or grease loaded with carbon black were early choices as electrodes for the DEAs. Such materials have poor reliability and are not available with established manufacturing techniques. Improved characteristics can be achieved with liquid metal, sheets of graphene, coatings of carbon nanotubes, surface-implanted layers of metallic nanoclusters and corrugated or patterned metal films.[4][5]

These options offer limited mechanical properties, sheet resistances, switching times and easy integration. Silicones and acrylic elastomers are other alternatives.

The requirements for an elastomer material are:

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

Mechanically prestretching the elastomer film offers the possibility of enhancing the electrical breakdown strength. Further reasons for prestretching include:

• Film thickness decreases, requiring a lower voltage to obtain the same electrostatic pressure;
• Avoiding compressive stresses in the film plane directions.

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

Materials used in research include graphite powder, silicone oil / graphite mixtures, gold electrodes. The electrode should be conductive and compliant. Compliance is important so that the elastomer is not constrained mechanically when elongated.[4]

Films of polyacrylamide hydrogels formed with salt water can be laminated onto the dielectric surfaces, replacing electrodes.[4]

DEs based on silicone (PDMS) and natural rubber are promising research fields.[6] Properties such as fast response times and efficiency are superior using natural rubber based DEs compared to VHB (acrylic elastomer) based DEs for strains under 15%.[7]

Dielectric elastomer actuators are to be designed so as to avoid the phenomenon of dielectric breakdown in their whole course of motion. In addition to the dielectric breakdown, DEAs are susceptible to another failure mode, referred to as the electromechanical instability, which arises due to nonlinear interaction between the electrostatic and the mechanical restoring forces. In several cases, the electromechanical instability precedes the dielectric breakdown. The instability parameters (critical voltage and the corresponding maximum stretch) are dependent on several factors, such as the level of prestretch, temperature, and the deformation dependent permittivity. Additionally, they also depend on the voltage waveform used to drive the actuator. [8]

Configurations include:

• 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 is mounted around the film. Examples are expanding circles and 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 the axial direction. The actuators can be rolled around a compression spring or without a core. Applications include 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.
• Shell-like actuators: Planar elastomer films are coated at specific locations in the form of electrode segments. With a well-directed activation, the foils assume complex three-dimensional shapes. Examples may be utilized for propelling vehicles through air or water, e.g. for blimps.
• Stack actuators: Stacking planar actuators can increase deformation. Actuators that shorten under activation are good candidates.
• Thickness Mode Actuators: The force and stroke moves in the z-direction (out of plane). Thickness mode actuators are a typically a flat film that may stack layers to increase displacement.
• Bending actuators:The in-plane actuation of dielectric elastomer (DE) based actuator is converted into out-of-plane actuation such as bending or folding using unimorph configuration where one or multiple layers of DE sheets are stacked on top of one layer of inactive substrate.[9]
• Balloon actuators: Plane elastomer is attached to an air chamber and inflated with a constant volume of air, then the stiffness of the elastomer can be varied by applying electrical load; hence resulting in voltage-controlled bulging of the elastomeric balloon.[10]

Dielectric elastomers offer multiple potential applications with the potential to replace many electromagnetic actuators, pneumatics and piezo actuators. A list of potential applications include:

• Pelrine, R.; Kornbluh, R.; Pei, Q.; Joseph, J. (2000). "High-Speed Electrically Actuated Elastomers with Strain Greater Than 100%". Science. 287 (5454): 836–839. Bibcode:2000Sci...287..836P. doi:10.1126/science.287.5454.836. PMID 10657293.
• Carpi; De Rossi; Kornbluh; Pelrine; Sommer-Larsen (2008). "Dielectric elastomers as electromechanical transducers: Fundamentals, materials, devices, models & applications of an emerging electroactive polymer technology". Elsevier. {{cite journal}}: Cite journal requires |journal= (help)

• Smart Materials & Structures (EAP/AFC) program at Empa
• European Scientific Network for Artificial Muscles
• EuroEAP – International conference on Electromechanically Active Polymer (EAP) transducers & artificial muscles
• WorldWide Electroactive Polymer Actuators * Webhub: Yoseph Bar-Cohen's link compendium at JPL
• Loverich, J. J.; Kanno, I.; Kotera, H. (2006). "Concepts for a new class of all-polymer micropumps". Lab on a Chip. 6 (9): 1147–1154. doi:10.1039/b605525g. PMID 16929393.
• Danfoss PolyPower
• The Biomimetics Laboratory at The University of Auckland
• Dielectric Elastomer Stack Actuators (DESA) Archived 12 March 2012 at the Wayback Machine at Technische Universität Darmstadt
• PolyWEC EU Project: New mechanisms and concepts for exploiting electroactive Polymers for Wave Energy Conversion