Optotune

This section gives a brief overview of some of the best studied electronic EAP actuators.

Dielectric elastomer actuators (DEAs) are electric field driven: when a voltage V is applied between two compliant electrodes that sandwich a dielectric elastomer, an electromechanical thickness strain

is induced by electrostatic forces (Maxwell’s stress). Here, t is the thickness of the dielectric elastomer, Y represents the modulus of elasticity, ε₀ denotes the permittivity of free space and ε_r is the dielectric constant of the elastomer (e.g. ε_VHB = 3.21). Using the constant volume approximation (1 + s_x)(1 + s_y)(1 + s_z) = 1 (Poisson’s ratio = 0.5) and assuming that the thickness compression results in an equi-bi-axial planar strain (s_planar = s_x = s_y), we obtain

 

Working principle of dielectric elastomer actuators

DEAs are simple in construction, achieve large strains of up to 380 %, high efficiencies, moderate bandwidths of more than 1 kHz in silicones and can operate over a large temperature range (-100 °C to 250 °C for silicones and -10 °C to 90 °C for acrylic elastomers). Therefore, DEAs are the prefered choice for many applications including electroactive fluid pumps, spring roll actuators in insect-like robots, heel-strike generators, a blimp and ultraflat loudspeakers.

DEA application examples: Stack actuator lifting 600x its own weight and blimp (both courtesy of EMPA), heel-strike generator and FLEX 2 six-legged robot (both courtesy of SRI)

Currently, the main limitation of DEAs is the high drive voltage of typically > 1 kV. According to Eq. 1.2 reducing the thickness of the actuator appears particularly promising to overcome this issue. Furthermore, increasing the dielectric constant of the elastomer will also help to lower the drive voltage.

 

Electrostrictive relaxor ferroelectric polymers

Ferroelectric polymers consist of dipoles attached to the polymer backbone. These dipoles can be aligned by an electric field to produce polarized domains. This reversible alignment of polar groups produces a contraction of up to 10 % in the direction of the electric field polarization. To avoid hysteresis, imperfections are introduced by either irradiation or a small mass of bulky monomers. These imperfections reduce the Curie point below room temperature, eliminating the formation of large ferroelectric domains. Furthermore, they reduce the energy barrier of the phase change between the nonpolar (alpha) and polar (ferroelectric beta) phase (see figure). These almost hysteresis free materials are called relaxor ferroelectric polymers. Relaxor ferroelectric polymers achieve moderate strains of up to 10 %, good electromechanical coupling of 0.42, high stresses (45 MPa blocking), and energy densities (1 MJ/m3) similar to DEAs. The main disadvantages are high voltages (typically > 1 kV), large energy dissipation and the limited temperature range. Similar to DEAs, reducing the film thickness or increasing the dielectric constant will lead to lower drive voltages.

Illustration of the field induced shape change of a (relaxor) ferroelectric molecule (e.g. PVDF) when switched between the nonpolar alpha phase (no field) and the polar beta phase (field applied)

Liquid crystal elastomers

Liquid crystals can not only be used as refractive index adjustable material but also as actuator material . Liquid crystal elastomers (LCEs) consist of liquid crystal molecules (mesogens) which are appended to a compliant polymer backbone. The soft backbone allows the mesogens to reorient while preventing the free flow of molecules. The thermally or electric field driven reorientation of the mesogens induces stresses on the backbone, resulting in a bulk strain of the LCE.

Liquid crystal elastomers. (a) Schematic of a ferroelectric liquid crystal elastomer. The mesogens (green; core of the chiral mesogen, red; crosslinkable end group of the mesogen) are attached to the flexible backbone (blue). (b) Illustration of the field induced conformational change in ferroelectric liquid crystal elastomers. (c) Thermal LC elastomer at 40 °C (upper panel) and 120 °C (lower panel)

Thermally actuated LCEs  are relatively slow due to their limited heat transfer. Especially the relaxation process is slow (∼ 10 seconds) and requires active cooling. Recently, novel photo-responsive materials, including dye-doped LCEs with fast photochemically induced strains of up to 20 % were developed. Electrically actuated ferroelectric liquid crystals have an intrinsic polarization of their mesogens or an anisotropic dielectric constant . An electric field quickly induces a realignment of the mesogens, causing bulk stresses and strains. Ferroelectric LCEs have a fast response time (∼ 10 ms) that is only limited by the time required for the reorientation of the mesogens. Furthermore, they need up to two orders of magnitude smaller electric fields (1.5 to 25 MV/m) than DEAs or ferroelectric polymers. However, the achieved stains of 4 % are much smaller than in thermally actuated LCEs (45 % linear strain). Applications for LCEs are artificial muscles, mechanically tunable optical elements and piezoelectric sensors.