Research ArticleSOFT ROBOTS

Peano-HASEL actuators: Muscle-mimetic, electrohydraulic transducers that linearly contract on activation

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Science Robotics  05 Jan 2018:
Vol. 3, Issue 14, eaar3276
DOI: 10.1126/scirobotics.aar3276
  • Fig. 1 Basic components of Peano-HASEL actuators and principles of operation.

    (A) Schematic side view showing the cross section of a three-unit Peano-HASEL actuator; each unit consists of a rectangular pouch made from an inextensible and flexible polymer shell, filled with a liquid dielectric. Electrodes are placed over a portion of the pouch on either side; when an increasing voltage (V) is applied, electrostatic forces displace the liquid dielectric, causing the electrodes to progressively close. This forces fluid into the uncovered portion of the pouch, causing a transition from a flat cross section to a more circular one, which leads to a contractile force, F. (B) Schematic side view of a three-unit Peano-HASEL actuator with voltage off and voltage on. The theoretical maximum strain for the contracting area of the pouch is Embedded Image or about 36%. L0, length of contracting area with voltage off. L, length of contracting area with voltage on. (C) Three-unit Peano-HASEL actuator shown lifting 20 g on application of 8 kV across the electrodes. This construction used transparent hydrogels as electrodes and fiberglass-reinforced tape for mounting connections.

  • Fig. 2 Fabrication process for Peano-HASEL actuators.

    (A) Basic components of a Peano-HASEL actuator. (B) Two BOPP sheets were placed between two layers of Kapton film and sealed using a heated brass-rod die. The die was designed to give pouches (2 cm by 4 cm) with 2-mm access ports for filling with liquid dielectric. A PDMS sheet was placed below the Kapton as a load-dispersing layer. Figure S1 describes the heat-press in more detail. (C) Pouches were filled with FR3 liquid dielectric using a syringe. (D) A heated aluminum rod was used to seal the filling ports. (E) PDMS-backed hydrogel electrodes were placed on each side of the pouches. Figure S2 describes the process for fabricating these electrodes. (F) A finished actuator is shown.

  • Fig. 3 Force-strain characteristics of Peano-HASEL actuators using hydrogel and aluminum electrodes.

    (A) Comparison of the force-strain curves for two Peano-HASEL actuators, one using hydrogel and the other using aluminum electrodes, revealing no difference in performance. A maximum of 10% strain was observed under a 20-g load at 10-kV applied voltage. (B) A hydrogel-electrode actuator was mounted on an acrylic stand for actuation tests and demonstrated contraction under 20-g load at 8 kV. (C) An aluminum-electrode actuator was mounted on an acrylic stand for actuation tests and demonstrated contraction under 20-g load at 8 kV.

  • Fig. 4 Scaling up forces with arrays of Peano-HASEL actuators.

    (A) Peano-HASEL actuators arranged in parallel to scale up force generation in a compact array. Actuators are stacked such that adjacent actuators are vertically offset by half of the pouch height. Electrode polarity alternates (as shown on the right) such that electrodes facing each other from adjacent actuators are always at the same potential. (B) Six actuators shown contracting 6.6% under a 200-g load at 8 kV. The white ovals show the offset pouches in the two rightmost actuators. (C) Comparison of the force-strain characteristics for one actuator to an array of six. Single-actuator data were projected upward by multiplying the load by six (dashed line) to estimate expected performance for an array of six actuators. The array of six actuators slightly outperforms expected results, demonstrating the ability to effectively scale up actuation force. (D) Six actuators shown contracting 4.6% under a 500-g load at 8 kV.

  • Fig. 5 High-speed performance of Peano-HASEL actuators.

    (A) Schematic of the test setup for determining contraction characteristics. The minimum cross section of the actuator used for testing was (40 mm by 0.042 mm) corresponding to a maximum static stress of 2.9 MPa with a 500-g load. (B) An 8-kV square wave was applied to the actuator. The resulting contraction response was measured, where ts and te correspond to the time of initial contraction and equilibrium contraction, respectively. Underdamped oscillations were observed after initial contraction. The small oscillations observed after 0.3 s correspond to out-of-plane swinging of the load and are not part of the characteristic response. (C) Peak strain rate during contraction as a function of load. (D) Peak and average specific power as a function of load. (E) Schematic of the test setup for frequency response. Elastic bands were attached to the bottom of the actuator and tensioned to provide a constant 1-N restoring force. (F) Frequency response curves for Peano-HASEL actuators filled with liquid dielectrics of different viscosities. The actuator filled with FR3 liquid dielectric showed a nearly flat response up to 20 Hz. The lower viscosity Drakeol 7 allowed maximum actuation at higher frequencies.

  • Fig. 6 Demonstration of high-speed and precise actuation.

    (A) A lever-arm setup was connected to two Peano-HASEL actuators in parallel for demonstrating fast and controllable actuation. (B) By applying a 13-kV voltage step, these actuators contracted fast enough to throw a ping-pong ball 24 cm into the air. Labeled times are measured from the start of contraction. (C) Incrementing voltage allowed controllable actuation of the arm, as shown in the progression of images with increasing voltage left to right. The yellow lines mark the position of the top of the ball for comparison. The ruler to the left of each picture shows 1-cm increments for scale.

  • Fig. 7 Self-sensing of actuator position.

    Plot of dimensionless capacitance and optically tracked position data for a single actuator under the influence of a varying voltage signal. Capacitance data were multiplied by a constant scale factor to provide agreement with position data; no other calibration was performed.

  • Fig. 8 Invisible Peano-HASEL actuators.

    (A) A Peano-HASEL actuator was suspended in an acrylic box with a colorful background (Claude Monet’s Water Lilies). (B) The acrylic box was filled with a liquid dielectric (Drakeol 19). The submerged portion of the actuator is nearly invisible. (C) Submerged actuator with a suspended 10-g weight and no applied voltage. On application of 8 kV, the actuator contracted and lifted the weight.

Supplementary Materials

  • robotics.sciencemag.org/cgi/content/full/3/14/eaar3276/DC1

    Materials and Methods

    Fig. S1. Heat-press used for sealing BOPP pouches.

    Fig. S2. Fabrication process for Peano-HASEL actuators with hydrogel electrodes.

    Fig. S3. Fabrication process for Peano-HASEL actuators with aluminum electrodes.

    Fig. S4. Voltage signal with reversing polarity used during force-strain tests.

    Fig. S5. Example of damage to aluminum electrodes after voltage cycling.

    Fig. S6. High-speed contraction of Peano-HASEL actuators.

    Fig. S7. Experimental setup used for frequency response tests of Peano-HASEL actuators.

    Fig. S8. Full actuation signal for lever arm tests.

    Fig. S9. Lifetime test for Peano-HASEL actuators.

    Fig. S10. Dielectric breakdown tests for KOH-etched BOPP film.

    Movie S1. Demonstration of actuation characteristics.

    Movie S2. Actuation using integrated aluminum electrodes.

    Movie S3. Scaling up forces with Peano-HASEL actuators.

    Movie S4. Frequency response of Peano-HASEL actuators.

    Movie S5. Demonstration of precise and rapid actuation.

    Movie S6. Transparent Peano-HASEL actuators.

    References (47, 48)

  • Supplementary Materials

    Supplementary Material for:

    Peano-HASEL actuators: Muscle-mimetic, electrohydraulic transducers that linearly contract on activation

    Nicholas Kellaris, Vidyacharan Gopaluni Venkata, Garrett M. Smith, Shane K. Mitchell, Christoph Keplinger*

    *Corresponding author. Email: christoph.keplinger{at}colorado.edu

    Published 5 January 2018, Sci. Robot. 3, eaar3276 (2018)
    DOI: 10.1126/scirobotics.aar3276

    This PDF file includes:

    • Materials and Methods
    • Fig. S1. Heat-press used for sealing BOPP pouches.
    • Fig. S2. Fabrication process for Peano-HASEL actuators with hydrogel electrodes.
    • Fig. S3. Fabrication process for Peano-HASEL actuators with aluminum electrodes.
    • Fig. S4. Voltage signal with reversing polarity used during force-strain tests.
    • Fig. S5. Example of damage to aluminum electrodes after voltage cycling.
    • Fig. S6. High-speed contraction of Peano-HASEL actuators.
    • Fig. S7. Experimental setup used for frequency response tests of Peano-HASEL actuators.
    • Fig. S8. Full actuation signal for lever arm tests.
    • Fig. S9. Lifetime test for Peano-HASEL actuators.
    • Fig. S10. Dielectric breakdown tests for KOH-etched BOPP film.
    • Legends for Movies S1 to S6
    • References (47, 48)

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Demonstration of actuation characteristics.
    • Movie S2 (.mp4 format). Actuation using integrated aluminum electrodes.
    • Movie S3 (.mp4 format). Scaling up forces with Peano-HASEL actuators.
    • Movie S4 (.mp4 format). Frequency response of Peano-HASEL actuators.
    • Movie S5 (.mp4 format). Demonstration of precise and rapid actuation.
    • Movie S6 (.mp4 format). Transparent Peano-HASEL actuators.

    Files in this Data Supplement:

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