Stretchable Electrohydraulic Artificial Muscle

Abstract

Artificial muscles are central to musculoskeletal robots and prosthetic devices, but many practical actuator technologies can only pull or contract. In antagonistic joints, this non-stretchable behavior either blocks motion or requires slack tendons that reduce usable range. This work introduces a stretchable artificial muscle system that combines HASEL electrohydraulic actuators with electrostatic clutches. The clutch units selectively lock, release, and extend the muscle path, allowing coordinated contraction and extension in an antagonistic joint. The resulting system preserves smooth bidirectional motion, supports transitions up to 3.2 Hz, and demonstrates a route for adapting other non-stretchable muscle technologies to full-range musculoskeletal actuation.

Overview

Conventional antagonistic joints need each artificial muscle path to lengthen while the opposite side contracts. Non-stretchable actuators make that difficult: taut tendons resist joint motion, while slack tendons sacrifice range and responsiveness. The proposed HASEL-clutch unit acts as a contractile muscle when engaged and as an extendable element when released.

Full-range antagonistic motion without relying on permanent tendon slack.
Electrostatic clutch synchronization coordinates contraction, extension, braking, and release states.
Reusable muscle-clutch concept that can extend beyond HASELs to other non-stretchable actuator families.

Graphical overview of the stretchable electrohydraulic artificial muscle concept — The clutch-muscle system restores bidirectional joint motion by combining contractile electrohydraulic muscles with releasable electrostatic clutches.

Methods at a Glance

HASEL-Clutch Unit

A non-stretchable HASEL actuator provides contraction, while an electrostatic clutch controls when the tendon path is held or allowed to extend.

Antagonistic Joint

Opposing muscle-clutch units are integrated around a musculoskeletal joint so one side can contract as the other side releases and lengthens.

State Synchronization

A discrete control strategy coordinates muscle and clutch voltages to generate smooth motion profiles, directional switching, and braking.

Problem setup and proposed HASEL-clutch solution — Non-stretchable muscles either block antagonistic motion or require slack tendons; the proposed series clutch lets the muscle path extend when needed.

Results

82°

Maximum Joint Excursion

The clutch-muscle joint reached ±82°, compared with ±52° for the HASEL-only slack-tendon configuration.

58%

Range Increase

Adding electrostatic clutches increased joint motion range while using the same HASEL actuation principle.

3.2 Hz

Smooth Switching

Coordinated state transitions enabled seamless alternation between antagonistic muscles at operating frequencies up to 3.2 Hz.

Configuration	Motion Range	Key Mechanism	Observed Behavior
HASEL-only joint	±52°	Slack tendons	Motion possible, but displacement is lost to tendon slack.
HASEL-clutch joint	±82°	Release-lock clutch synchronization	Fuller bidirectional motion with controllable extension and contraction.

Comparison of joint range of motion with and without clutches — The integrated clutch-muscle mechanism expands the measured joint range from ±52° to ±82°.

Demonstrations

HASEL-only actuation shows the baseline behavior when the antagonistic system relies on slack tendons.

HASEL-clutch actuation demonstrates coordinated contraction and extension at 2.5 Hz.

Joint-angle comparison illustrates the measured motion difference between the two configurations.

Biological motivation highlights the antagonistic muscle behavior that motivates the robotic implementation.

Paper Figures

Antagonistic HASEL-clutch joint states — Antagonistic joint setup with coordinated top and bottom HASEL-clutch units.

State diagram for HASEL and clutch control — State-machine view of muscle and clutch activation for motion and braking.

Maximum motion range versus actuation frequency — Range of motion across actuation frequency for the HASEL-only and clutch-muscle cases.

Citation

@inproceedings{kazemipour2025stretchable,
  author    = {Kazemipour, Amirhossein and Hinchet, Ronan and Katzschmann, Robert K.},
  title     = {Stretchable Electrohydraulic Artificial Muscle for Full Motion Ranges in Musculoskeletal Antagonistic Joints},
  booktitle = {2025 IEEE International Conference on Robotics and Automation (ICRA)},
  year      = {2025},
  doi       = {10.1109/ICRA55743.2025.11128116},
  url       = {https://arxiv.org/abs/2409.11017}
}