Electroactive Polymers

dielectric elastomer actuators in action

Electroactive Polymer (EAP) is a generic term for a multitude of syntetics which share the common characteristics of changing their shape under an applied electrical voltage or charge. The underlying mechanism of the shape change can differ from material to material.

The following systems can be distinguished:

  • wet EAP (intermolecular ion transaction)
  • dry EAP:
    • electrostrictive polymers
    • piezoelectric polymers
    • dielectric polymers

Dielectric Elastomer Actuators

Motion is caused by electrostatic forces, applied to the elastic dielectric between two compliant electrodes. With a maximum electric field strength of 30 V/µm a strain ratio of up to 20% is possible.

further advantages of the actuator principle:

  • light, flexible, silent actuators
  • very high energy-density (> 0,2 J/cm³) compared to piezoelectric principle (0,1 J/cm³)
  • enclosed actuators, depending on the choosen elastomer they can work in nearly every environment
  • low-cost actuator

Functional principle

Electrostatic solid-state actuators can be realised by an elastic dielectric layer between compliant electrodes. Applying a voltage at the electrodes the dielectric contracts due to electrostatic forces and expands in perpendicular direction (Fig. 1).

Using high elastic silicone elastomer with thin graphite powder electrodes a relative thickness strain up to 20 % can be achieved.The electrostatic pressure causing the deformation is given by the permittivity, the thickness of the elastomer film and the applied voltage.

Since polymers are nearly incompressible, the volume remains constant during deformation.

By reducing the voltage the dielectric returns to its initial shape and can produce forces due to the stored elastic energy. To realise a sufficient strain with applicable voltages the thickness of the dielectric has to be in the range of a few microns. The electrodes have to be very compliant not to constrain the deformation.

Research and Applications

Despite the large strain of about 20%, the deflection of a single 20µm thick layer is not applicable. As it is known from piezoelectric actuators, stacking severla layers increases the absolute value of deflection without increasing the driving voltage. This multilayer principle was developed for the application in tactile displays as a high density of actuators is necessary for these displays. Further investigations concerning the fabrication and the used materials have to be done. A theoretical model has been developed for describing the performance of simple structures. To design more complicated actuators this model has to be improved.

Two different applications are actually focused

Tactile Display

Peristaltic Pump

Contact persons

Researchers

Dipl.-Ing. Henry Haus

Dipl.-Ing. Roman Karsten

Holger Mößinger, M.Sc.

Dipl.-Wirtsch.-Ing. Florentine Förster

Dipl.-Ing. Tanja Grotepaß

Davide Gatti, M.Sc

Contact

Technische Universität Darmstadt

Institute of Electromechanical Design

Microtechnology

Prof. Dr.-Ing. Helmut F. Schlaak

S3/06 128
Merckstraße 25
64283 Darmstadt

+49 6151 16-4696
+49 6151 16-4096

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