Colorado makes breakthrough soft muscle for soft robots that is cheap, strong and effective

The University of Colorado has created next generation healing soft muscle actuators. It is inspired by biological muscle.

he newly developed hydraulically amplified self-healing electrostatic (HASEL) actuators eschew the bulky, rigid pistons and motors of conventional robots for soft structures that exceed or match the strength, speed and efficiency of biological muscle. Their versatility may enable artificial muscles for human-like robots and a next generation of prosthetic limbs.

“HASEL actuators synergize the strengths of soft fluidic and soft electrostatic actuators, and thus combine versatility and performance like no other artificial muscle before. Just like biological muscle, HASEL actuators can reproduce the adaptability of an octopus arm, the speed of a hummingbird and the strength of an elephant.”

One iteration of a HASEL device, described in Science, consists of a donut-shaped elastomer shell filled with an electrically insulating liquid (such as canola oil) and hooked up to a pair of opposing electrodes. When voltage is applied, the liquid is displaced and drives shape change of the soft shell. As an example of one possible application, the researchers positioned several of these actuators opposite of one another and achieved a gripping effect upon electrical activation. When voltage is turned off, the grip releases.

Another HASEL design is made of layers of highly stretchable ionic conductors that sandwich a layer of liquid and expands and contracts linearly upon activation to either lift a suspended gallon of water or flex a mechanical arm holding a baseball.

A third design, detailed in Science Robotics, and known as a Peano-HASEL actuator, consists of three small rectangular pouches filled with liquid, rigged together in series. The polymer shell is made from the same low-cost material as a potato chip bag and is thin, transparent and flexible. Peano-HASEL devices contract on application of a voltage, much like biological muscle, which makes them especially attractive for robotics applications. Their electrically powered movement allows operation at speeds exceeding that of human muscle.

The devices are also cheap and can be made for ten cents each.

The materials are low-cost, scalable and compatible with current industrial manufacturing techniques.

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


Soft robotic systems are well suited to unstructured, dynamic tasks and environments, owing to their ability to adapt and conform without damaging themselves or their surroundings. These abilities are crucial in areas such as human-robot interaction. Soft robotic systems are currently limited by the soft actuators that power them. To date, most soft actuators are based on pneumatics or shape-memory alloys, which have issues with efficiency, response speed, and portability. Dielectric elastomer actuators (DEAs) are controlled and powered electrically and excel with muscle-like actuation, but they typically require a rigid frame and prestretch to perform effectively. In addition, DEAs require complex stacks or structures to achieve linear contraction modes. We present a class of soft electrohydraulic transducers, termed Peano-HASEL (hydraulically amplified self-healing electrostatic) actuators, that combine the strengths of fluidic actuators and electrostatic actuators, while addressing many of their issues. These actuators use both electrostatic and hydraulic principles to linearly contract on application of voltage in a muscle-like fashion, without rigid frames, prestretch, or stacked configurations. We fabricated these actuators using a facile heat-sealing method with inexpensive commercially available materials. These prototypical devices demonstrated controllable linear contraction up to 10%, a strain rate of 900% per second, actuation at 50 hertz, and the ability to lift more than 200 times their weight. In addition, these actuators featured characteristics such as high optical transparency and the ability to self-sense their deformation state. Hence, this class of actuators demonstrates promise for applications such as active prostheses, medical and industrial automation, and autonomous robotic devices.

Science – Hydraulically amplified self-healing electrostatic actuators with muscle-like performance

Liquids show their strength
Dielectric elastomer actuators are electrically powered muscle mimetics that offer high actuation strain and high efficiency but are limited by failure caused by high electric fields and aging. Acome et al. used a liquid dielectric, rather than an elastomeric polymer, to solve a problem of catastrophic failure in dielectric elastomer actuators. The dielectric’s liquid nature allowed it to self-heal—something that would not be possible with a solid dielectric. The approach allowed the authors to exploit electrostatic and hydraulic forces to achieve muscle-like contractions in a powerful but delicate gripper.


Existing soft actuators have persistent challenges that restrain the potential of soft robotics, highlighting a need for soft transducers that are powerful, high-speed, efficient, and robust. We describe a class of soft actuators, termed hydraulically amplified self-healing electrostatic (HASEL) actuators, which harness a mechanism that couples electrostatic and hydraulic forces to achieve a variety of actuation modes. We introduce prototypical designs of HASEL actuators and demonstrate their robust, muscle-like performance as well as their ability to repeatedly self-heal after dielectric breakdown—all using widely available materials and common fabrication techniques. A soft gripper handling delicate objects and a self-sensing artificial muscle powering a robotic arm illustrate the wide potential of HASEL actuators for next-generation soft robotic devices.