- Apr 2020
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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).
Single and two-unit planar HASEL actuators are built as explained in Figures S8 and S9. Linear actuators are vertical, with the load applied downward in the same direction of gravity. DC voltage is increased in small quantities, activating the planar actuator. The maximum strain detected is a 79% strain under a 250 g load.
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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).
Previous work done by S. J. A. Koh et al. tested how to achieve linear actuation with planar actuators using this method, allowing these authors to replicate these trials using their single and two-unit actuators.
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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).
To test the effectiveness of their design, the authors fabricated a dielectric (DE) actuator and a HASEL actuator with same dimensions. They tested the amount of in-plane expansion as a function of voltage. HASEL actuators expanded more (46% compared to 12%) under same conditions.
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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).
Two stacks of HASEL actuators are tilted and fixed to form a gripper for other items, as seen in Figure 2 C-G. When a voltage is applied, the grippers are able of grabbing fragile objects including an egg and a raspberry.
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The stacked actuators readily showed large actuation response up to a frequency of 20 Hz (movie S3).
Watch this video to view a stack of five donut HASEL actuators with an electrode diameter of 2.5 cm actuated with a 15 kV reversing square wave at 0.5, 5, 10, 15, and 20 Hz.
The HASEL actuators can be actuated (expand and contract) up to 20 times per second.
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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)
Stacking actuators allows for generating larger movement in comparison to one actuator. Since these actuators self heal from electrical damage, the actuation stroke can be increased without permanent damage to the system.
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Conversion efficiency was 21%,
The percentage of the electrostatic energy fed into the actuator that is converted into mechanical energy is 21%.
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(F) The use of a liquid dielectric confers self-healing capabilities to HASEL actuators.
Why do you think the Large actuator causes high strain with lower force and the small actuator causes lower strain with more force? What would be the difference between the two sizes?
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(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.
When voltage is applied, the actuator undergoes stress, causing it to deform into a donut. This idea can be used in other applications to apply force onto a load in D and E.
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(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.
When the voltage is off, you can se that the value of t, which is the thickness, is much smaller than wen the voltage is on (1C). This is because as the voltage increases, there is stress applied to the liquid inside, causing the pressure to increase inside the shell until breaking down.
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A) Schematic of a HASEL actuator shown at three different applied voltages, where V1 < V2 < V3.
Because the HASEL actuators can self-heal, a larger result can be found by stacking multiple actuators on top of each other (2A)
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- Mar 2020
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The higher actuation strain is attributed to the layer of liquid dielectric, which effectively reduces the modulus of the HASEL actuator
The soft nature of the liquid dielectric material helps HASEL actuators to move more (higher actuation strain) under the same actuation voltage.
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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).
With 5 HASEL actuators stacked on top of each other, the authors found that the strain achieved is 37%, which is the similar to the strain of actual muscle.
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which is comparable to linear strain achieved by biological muscle (26)
J. D. W. Madden et al. previously reported strain percentages of 20%, which is comparable to the experimental strain of 37% achieved in this paper.
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