Dielectric elastomers

Dielectric elastomers (DEs) are smart material systems that produce large strains and are promising for Soft robotics, Artificial muscle, etc.

DE actuators (DEA) transform electric energy into mechanical work and vice versa.

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.)

Aqueous ionic hydrogels can deliver potentials of multiple kilovolts, despite the onset of electrolysis at below 1.5 V.[3][4] The difference between the capacitance of the double layer and the dielectric leads to a potential across the dielectric that can be millions of times greater than that across the double layer.

The hydrogel's stiffness can be thousands of times smaller than the dielectric's, allowing actuation without mechanical constraint across a range of nearly 100% at millisecond speeds.

[3][4] Remaining issues include drying of the hydrogels, ionic build-up, hysteresis, and electrical shorting.

[3][4] Early experiments in semiconductor device research relied on ionic conductors to investigate field modulation of contact potentials in silicon and to enable the first solid-state amplifiers.

Ionic gels can also serve as elements of high-performance, stretchable graphene transistors.

[4][5] These options offer limited mechanical properties, sheet resistances, switching times and easy integration.

Models that describe large strains and viscoelasticity are required for the calculation of such actuators.

[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%.

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.

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 the electrostatic pressure 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.