We fabricated single- and two-unit planar HASEL actuators, where a unit is defined as a discrete region of liquid dielectric (figs. S8 and S9) (24). Linear actuators were oriented vertically with the load applied in the direction of gravity, but they can be operated in any orientation as long as the liquid dielectric regions are sufficiently small to limit uneven distribution of liquid dielectric (fig. S10). A single-unit planar HASEL actuator (Fig. 3C) was activated by increasing DC voltage in discrete steps and achieved a maximum of 79% linear actuation strain under a load of 250 g (actuation stress ~32 kPa), exceeding typical values of strain observed for biological muscle (26).

Linear actuation can be achieved with planar HASEL actuators by implementing a fixed prestretch in one planar direction and applying a load in the perpendicular planar direction (29).

To compare the actuation response of HASEL and DE actuators, we measured area strain as a function of voltage for two circular actuators with the same total dielectric thickness, t (Fig. 3A). Both were fabricated from Ecoflex 00-30 (Smooth-on); however, one-third of the thickness of the HASEL actuator was liquid dielectric, tliq (24). At 11 kV, the area strain of the HASEL actuator exceeded the area strain of the DE actuator by a factor of ~4 (Fig. 3A and fig. S7).

We modified two stacks of donut HASEL actuators to operate as a soft gripper, a common application for soft robotics (8, 28). Actuators within the stacks were constrained on one side to produce a tilting motion (Fig. 2, C to G, and fig. S6).

The stacked actuators readily showed large actuation response up to a frequency of 20 Hz (movie S3).

The ability of HASEL actuators to self-heal from electrical damage provides the means to scale up devices to produce a large actuation stroke by stacking multiple actuators (Fig. 2A)

Conversion efficiency was 21%,

(F) The use of a liquid dielectric confers self-healing capabilities to HASEL actuators.

(D and E) Strain and force of actuation can be tuned by modifying the area of the electrode. The minimum electric field to trigger the pull-in transition was ~2.7 kV/mm; the maximum field applied was ~33 kV/mm.

(C) The actuator deforms into a donut shape with application of voltage. This voltage-controlled deformation can be used to apply force F onto an external load.

A) Schematic of a HASEL actuator shown at three different applied voltages, where V1 < V2 < V3.

The higher actuation strain is attributed to the layer of liquid dielectric, which effectively reduces the modulus of the HASEL actuator

A stack of five donut HASEL actuators achieved 37% linear strain, which is comparable to linear strain achieved by biological muscle (26) and corresponds to an actuation stroke of 7 mm (Fig. 2B).

which is comparable to linear strain achieved by biological muscle (26)