HASEL Artificial Muscles for a New Generation of Lifelike Robots-Recent Progress and Future Opportunities

Abstract

Future robots and intelligent systems will autonomously navigate in unstructured environments and closely collaborate with humans; integrated with our bodies and minds, they will allow us to surpass our physical limitations. Traditional robots are mostly built from rigid, metallic components and electromagnetic motors, which make them heavy, expensive, unsafe near people, and ill-suited for unpredictable environments. By contrast, biological organisms make extensive use of soft materials and radically outperform robots in terms of dexterity, agility, and adaptability. Particularly, natural muscle-a masterpiece of evolution-has long inspired researchers to create "artificial muscles" in an attempt to replicate its versatility, seamless integration with sensing, and ability to self-heal. To date, natural muscle remains unmatched in all-round performance, but rapid advancements in soft robotics have brought viable alternatives closer than ever. Herein, the recent development of hydraulically amplified self-healing electrostatic (HASEL) actuators, a new class of high-performance, self-sensing artificial muscles that couple electrostatic and hydraulic forces to achieve diverse modes of actuation, is discussed; current designs match or exceed natural muscle in many metrics. Research on materials, designs, fabrication, modeling, and control systems for HASEL actuators is detailed. In each area, research opportunities are identified, which together lays out a roadmap for actuators with drastically improved performance. With their unique versatility and wide potential for further improvement, HASEL actuators are poised to play an important role in a paradigm shift that fundamentally challenges the current limitations of robotic hardware toward future intelligent systems that replicate the vast capabilities of biological organisms.

Keywords: HASEL actuators; artificial muscles; electrostatics; robotics; soft actuators.

Figure 1

Selected sources of inspiration for hydraulically amplified self‐healing electrostatic (HASEL) actuators. The timeline shows key sources of inspirations for the invention of the HASEL technology. Photo of the hummingbird and elephant by Günter Oesterling and reproduced with permission. Photo of the octopus by Jeahn Laffitte on Unsplash and reproduced with permission. “Soft, electrostatic actuator studied by Röntgen” image: Reproduced with permission.^{[ 137 ]} Copyright 2010, National Academy of Sciences USA. “McKibben actuator” image: Reproduced from ref. ^{[ 10 ]}. “Pneumatically driven flexible microactuators” image: Reproduced with permission.^{[ 59 ]} Copyright 1996, Elsevier Ltd. “Dielectric elastomer actuators with strains > 100%” image: Reproduced with permission.^{[ 9 ]} Copyright 2000, AAAS. “Self‐sensing in dielectric elastomer actuators” image: Reproduced with permission.^{[ 38 ]} Copyright 2008, Elsevier Ltd. “Hydrostatically coupled dielectric elastomer actuators” image: Reproduced with permission.^{[ 39 ]} Copyright 2010, IEEE. “Embedded pneumatic networks (PneuNets) of channels in elastomers” image: Reproduced with permission.^{[ 60 ]} Copyright 2011, Wiley‐VCH. “Zipping dielectric elastomer actuators” image: Reproduced with permission.^{[ 40 ]} Copyright 2012, SPIE. “PneuNet tentacle” image: Reproduced with permission.^{[ 61 ]} Copyright 2013, Wiley‐VCH. “Self‐healing dielectric elastomer actuators” image: Reproduced with permission.^{[ 57 ]} Copyright 2014, American Institute of Physics. “Pneumatic pouch motors” image: Reproduced with permission.^{[ 63 ]} Copyright 2014, IEEE. “HASEL actuators” top row images; two left images: Reproduced with permission.^{[ 14 ]} Copyright 2018, The Authors, published by AAAS; Two right images: Reproduced with permission.^{[ 15 ]} Copyright 2018, The Authors, published by AAAS. “HASEL actuators” bottom row, left four images: Reproduced under the terms of the CC‐BY Creative Commons Attribution 4.0 International license (https://creativecommons.licenses/by/4.0).^{[ 89 ]} Copyright 2019, The Authors, published by Wiley‐VCH. “HASEL actuators” bottom row, right two images: Reproduced with permission.^{[ 97 ]} Copyright 2019, Wiley‐VCH.

Figure 2

Basic operating principles of HASEL actuators. a) HASEL actuators consist of polymer shells that are coated with opposing electrodes and that are filled with a liquid dielectric. When a voltage is applied to the actuator an electric field arises between the electrodes. The electric field causes a Maxwell stress in the actuator, which leads to redistribution of the liquid dielectric and deformation of the actuator. b) HASEL actuators use the principles of hydraulic amplification to generate forces. c) After a dielectric breakdown event, the liquid dielectric redistributes and returns to its insulating state. This property leads to electrical self‐healing in HASEL actuators. d) The electrodes of HASEL actuators form a capacitor. Shape changes due to applied voltages or external forces can be detected by measuring the capacitance. a–d) Adapted with permission.^{[ 14 ]} Copyright 2018, The Authors, published by AAAS.

Figure 3

The elastomeric donut HASEL actuator. a) The elastomeric donut HASEL actuator consists of a circular elastomeric pouch that is filled with a liquid dielectric and partially covered on both sides with circular electrodes. When a voltage is applied to the actuator, it deforms into a donut shape (see also Figure 2). The strain–voltage curve of a donut HASEL actuator shows a pronounced pull‐in instability which leads to distinct off‐ and on‐states. The output force and actuation strain of an elastomeric donut HASEL actuator follow the principles of hydraulic amplification: actuators with large electrodes exhibit larger actuation strains but generate lower forces than actuators with small electrodes. b) Multiple actuators can be stacked to increase the total actuation stroke. c) Photographs of a stack of five elastomeric donut HASEL actuators in the off‐ and on‐states. d) Photographs of a soft gripper, whose design was based on elastomeric donut HASEL actuators. The gripper is gentle enough to handle raspberries without damaging them. a–d) Reproduced with permission.^{[ 14 ]} Copyright 2018, The Authors, published by AAAS.

Figure 4

The planar HASEL actuator. a) The planar HASEL actuator consists of a planar elastomeric shell which encloses a chamber that is filled with a liquid dielectric. Stretchable electrodes cover both sides of the elastomeric shell over the entire area of the chamber. The actuator is prestretched perpendicular to the direction of actuation. When a voltage is applied to the electrodes, the thickness of the actuator decreases, and the actuator elongates. b) Demonstration of quasi‐static linear actuation with a planar HASEL actuator. c) Six planar HASEL actuators dynamically lifting a gallon of water. a–c) Reproduced with permission.^{[ 14 ]} Copyright 2018, The Authors, published by AAAS.

Figure 5

The Peano‐HASEL actuator. a) The Peano‐HASEL actuator is comprised of rectangular, inextensible but flexible shells. The shells are filled with a liquid dielectric and partially coated on both sides with flexible electrodes. When a voltage is applied to the actuator, the electrodes zip together from the edges of the shell where the electric field is highest, displacing the liquid dielectric. The liquid‐filled region of the shell takes a more cylindrical shape and the actuator contracts in length. b) Photographs of a Peano‐HASEL actuator lifting a weight. c) Prototypical Peano‐HASEL actuators demonstrated up to 10% linear contraction on activation, with both hydrogel and aluminum electrodes. d) Peano‐HASEL actuators allow for high‐speed actuation; two actuators connected to a lever arm throw a table tennis ball up into the air. e) When the actuator is made with transparent materials and submerged in an index matched fluid it becomes invisible. a–e) Reproduced with permission.^{[ 15 ]} Copyright 2018, The Authors, published by AAAS.

Figure 6

Rapid prototyping of HASEL actuators made from thermoplastic polymers. a) Two thermoplastic sheets are bonded together with a CNC heat‐sealing machine to form a shell, b) which allows the rapid fabrication of different designs. c) The shells are filled with the liquid dielectric with a syringe through a fill port. d) The fill ports of the shell are sealed with a hot soldering iron. e) Electrodes are applied to the filled actuator (alternatively electrodes may also be applied to the shell before filling). f) To rapidly fabricate a stack of actuators, two thermoplastic films are bonded together to form a strip of multiple interconnected actuators. g) Then, electrodes are screen‐printed onto the strip, and h) all actuators are filled in a single step with a liquid dielectric. i) Finally, the fill port and the connections between the individual actuators are sealed, and j) the strip is folded to form a stack of actuators. a–j) Reproduced under the terms of the CC‐BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).^{[ 89 ]} Copyright 2019, The Authors, published by Wiley‐VCH.

Figure 7

Quadrant donut HASEL actuator with progressive zipping. a) The quadrant donut HASEL actuator consists of a circular shell, which is subdivided into four equal quadrants. Circular electrodes cover the center of the actuator. When a voltage is applied to the actuator, the electrodes progressively zip from the center of the shell outward and the actuator takes a donut shape. b) The strain–voltage curves of a stack of three quadrant donut actuators do not show a pull‐in instability. c) Frequency response of a stack of three quadrant donut actuators to a sinusoidal excitation signal. d) Photographs of a stack of 11 quadrant donut HASEL actuators during actuation. e) The stack of quadrant donut HASEL actuators has a high power to weight ratio and thus can jump without requiring a power amplification mechanism. a–e) Reproduced under the terms of the CC‐BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).^{[ 89 ]} Copyright 2019, The Authors, published by Wiley‐VCH.

Figure 8

Curling HASEL actuator. a) A curling HASEL actuator consists of a reservoir of liquid dielectric connected to a corrugated shell. The reservoir is covered with electrodes. When the electrodes zip, the liquid dielectric is displaced into the corrugated shell, which contracts. Attaching a strain limiting layer to one side of the actuator leads to curling. b) Photographs of a biomimetic scorpion tail. c) Curling HASEL actuators can be used for soft grippers capable of grasping objects. d) A curling actuator shaped like a Fibonacci spiral simultaneously curls and twists. e) Using a coiled spring as a strain limiting layer creates an actuation mode that is the inverse to curling; upon activation, liquid dielectric forces the coiled spring to unravel, which causes the actuator to elongate. a,b,d) Reproduced under the terms of the CC‐BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).^{[ 89 ]} Copyright 2019, The Authors, published by Wiley‐VCH. c) Reproduced with permission.^{[ 94 ]} Copyright 2020, Mary Ann Liebert, Inc. e) Reproduced under the terms of the CC‐BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).^{[ 95 ]} Copyright 2019, The Authors, published by MDPI.

Figure 9

The high‐strain Peano‐HASEL actuator. a) The high‐strain Peano‐HASEL actuator consists of rectangular shells with electrodes placed on the sides of the shells. When the electrodes zip, the liquid dielectric is displaced to the center of the shells, leading to contraction of the actuator. b) Photographs of a contracting high‐strain Peano‐HASEL actuator. c) Because the electrodes are placed orthogonally to the actuation direction, the high‐strain Peano‐HASEL actuator achieves larger actuation strains than a Peano‐HASEL actuator. However, it has a comparatively smaller blocking force. d) Photographs of a cylindrical pump based on a high‐strain Peano‐HASEL actuator, which mimicked the actuation of circular muscle layers found in nature. a–d) Reproduced with permission.^{[ 97 ]} Copyright 2019, Wiley‐VCH.

Figure 10

A HASEL actuator with a single shell and multiple, individually addressable pairs of electrodes. a) Schematic of an actuator that consists of a single shell that is covered with two pairs of electrodes of different size. When a voltage is applied to one electrode, the liquid dielectric is displaced causing the region of the other electrode to expand. Because the electrodes have different sizes, the force–strain characteristics depend on which pair of electrodes is activated. b) Photograph of rotary device controlled by four actuators shown in (a); the device rolls a ball in a circle. a,b) Reproduced with permission.^{[ 98 ]} Copyright 2020, IEEE.

Figure 11

Modeling of HASEL actuators. a) The electrical structure of a HASEL actuator is a complex capacitive architecture that includes solid and liquid dielectrics of different thicknesses. A mechanical description needs to include the large deformations of the shell, contact between the walls of the shell, and fluid‐structure interaction. b) Finite element model of an elastomeric donut HASEL actuator. The model included nonlinearities due to large deformations, the hyperelastic behavior of the shell, and contacts. The electric field was calculated in the shell and the liquid dielectric. The liquid was modeled as a hydrostatic medium. c) Analytical quasi‐static model of Peano‐HASEL actuators. The force–strain curves of the actuators are calculated by minimizing the Helmholtz free energy of the system, which includes the electrical energy of the voltage supply, the electrical energy stored in the actuator, and the potential energy of the applied load. Without relying on a fitting factor, the model agrees very well with experimental results. d) At high loads near the blocking force, Peano‐HASEL actuators exhibit an instability that leads to inhomogeneous zipping (dashed rectangle indicates zipped region of the electrodes). Inhomogeneous zipping leads to an increase in the blocking force (F _b) of Peano‐HASEL actuators, an effect that becomes stronger with higher electrode coverage (f _E = length of electrode/length of shell). a) Reproduced with permission.^{[ 104 ]} Copyright 2018, SPIE. b) Reproduced with permission.^{[ 105 ]} Copyright 2018, The Authors. c) Adapted with permission.^{[ 96 ]} Copyright 2019, Elsevier Ltd. d) Reproduced with permission.^{[ 108 ]} Copyright 2019, Elsevier Ltd.

Figure 12

Strategies to improve the performance of HASEL actuators using the Peano‐HASEL as a model system. a) The weight of a Peano‐HASEL actuator can be decreased by replacing a single long shell with multiple short shells in series while maintaining the same force–stroke characteristics, which leads to an increase in specific energy of the actuator. b) Decreasing the shell‐length in parallel stacks of Peano‐HASEL actuators increases the actuation stress (output force divided by the cross‐sectional area of the stack) as the packing density can be increased. c) Decreasing the shell‐length increases the specific energy of Peano‐HASEL actuators until the bending stiffness of the shell reduces the actuation strain. Decreasing the film‐thickness allows the use of smaller actuation voltages but requires smaller pouch‐lengths to maintain the specific energy (indicated by black lines). d) The use of high‐performance materials with high dielectric constants and dielectric breakdown strengths may drastically improve the specific energy of HASEL actuators. a–d) Reproduced with permission.^{[ 96 ]} Copyright 2019, Elsevier Ltd.

Figure 13

Electrostatic transducers related to HASEL actuators. a) Zipping dielectric elastomer actuators use the electrostatic zipping of a stretchable membrane onto a rigid substrate to generate large out‐of‐plane deformation. Zipping dielectric elastomer actuators have been used in peristaltic pumps. b) Electrostatic microhydraulic actuators are integrated with a rigid substrate and use electrostatic attraction to deform liquid‐filled parylene shells. Like HASEL actuators, these actuators use the principles of hydraulic amplification. Electrostatic microhydraulic actuators have been used as microvalves and micropistons. c) Electrohydraulic actuation principles have been used to create a tactile display. As a voltage is applied, electrostatic zipping of an elastomeric membrane onto a rigid substrate causes expansion of a deformable tactile dot. d) Electrohydraulic transduction principles have been used to create generators that harvest electrical energy from a periodic flow of liquid. For these devices, a stretchable membrane covers a circular chamber that is filled with a liquid dielectric. When a voltage is applied, the membrane zips onto the floor of the chamber. Pumping a liquid dielectric into and out of the chamber, these devices operate as electrostatic generators. e) In electroribbon actuators, electrostatic zipping deforms flexible ribbons to generate motion. A thin film of liquid dielectric covers the ribbons in the zipping region to improve force output. Since electroribbons are open structures, where the space between ribbons is mainly filled with air, they are lightweight and have the potential for high specific energies. a) Reproduced with permission.^{[ 41 ]} Copyright 2013, IOP Publishing Ltd. b) Reproduced with permission.^{[ 115 ]} Copyright 2016, IEEE. c) Reproduced with permission.^{[ 117 ]} Copyright 2019, SPIE. d) Reproduced under the terms of the CC‐BY Creative Commons Attibution 3.0 Unported license https://creativecommons.org/by/3.0).^{[ 119 ]} Copyright 2017, IOP Publishing Ltd. e) Reproduced with permission.^{[ 120 ]} Copyright 2018, The Authors, published by AAAS.

Figure 14

High‐voltage driving electronics for untethered soft robots based on HASEL actuators. a) A proof‐of‐concept high‐voltage power supply for untethered operation of HASEL actuators. This circuit used only off‐the‐shelf components; input power was provided by a lithium‐ion battery and the circuit was controlled with an Arduino microcontroller. The high voltages required for actuation were generated by a miniature high‐voltage amplifier, and optocouplers were used as high‐speed, high‐voltage switches to charge and discharge the actuator. b) The weight of the power supply including the battery was ≈100 g, so it was easily lifted by a stack of quadrant donut HASEL actuators operated at 8 kV. c) The power supply was used to actuate an array of three stacks of 22 actuators, which achieved 40% strain under a 1 kg load under an applied voltage of 8 kV. d) Photographs of an untethered, multiple‐degree‐of‐freedom manipulator controlled by a joystick. The soft continuum actuator consisted of three stacks of 55 quadrant donut HASEL actuators, and the design of the gripper was based on curling HASEL actuators. a–d) Reproduced under the terms of the CC‐BY Creative Commons Attribution 4.0 International license (https://creativecommons/licenses/by/4.0).^{[ 89 ]} Copyright 2019, The Authors, published by Wiley‐VCH.

Figure 15

Deformation sensing and control for HASEL actuators. a) As HASEL actuators are deformable capacitors, monitoring their capacitance can be used to self‐sense deformation (see Figure 2). b) Example of closed‐loop control of a planar HASEL actuator for displacement control using self‐sensing to measure deformation. c) An external, stretchable, capacitive sensor is used to measure the deformation of a stack of expanding HASEL actuators. d) Example of closed‐loop control for displacement steps, of the actuators shown in (c) using the external capacitive sensor to measure deformation. a) Adapted with permission.^{[ 14 ]} Copyright 2018, AAAS. b) Reproduced with permission.^{[ 126 ]} Copyright 2018, IEEE. c) Reproduced with permission.^{[ 127 ]} Copyright 2020, IEEE. d) Reproduced with permission.^{[ 127 ]} Copyright 2020, IEEE.