A research team headed by Daniel Ahmed, a Professor of Acoustic Robotics for Life Sciences and Healthcare, has developed a novel category of artificial muscles: flexible membranes that react to ultrasound through the assistance of thousands of microbubbles. Their study was published in Nature.
The researchers developed the artificial muscles using a casting mold with a specific microstructure. The silicone membrane formed within this mold features small pores on its underside, each approximately 100 µm in both depth and diameter, around the thickness of a human hair. When the researchers immersed the membrane in water, small microbubbles were trapped within these pores.
When exposed to sound waves these microbubbles start to oscillate, generating a directed flow that propels the muscle. The size, shape, and positioning of these microbubbles can be meticulously regulated, enabling the production of movements that range from smooth curving to undulating patterns. The muscles react within milliseconds and can be operated wirelessly.
Gentle Gripping and Smooth Movement
The researchers have demonstrated several applications for these artificial muscles, including a soft, compact gripper arm. In one experiment using the technology, researchers successfully captured and released a zebrafish larva in water, without causing any damage.
It was fascinating to see just how precisely yet gently the gripper functioned; the larva swam away afterwards unharmed.
Zhiyuan Zhang, Study Lead Author, ETH Zurich
The team has also developed a robot resembling a tiny stingray to demonstrate undulatory movements, only four centimeters in width. Two artificial muscles mimic the function of pectoral fins.
When ultrasound stimulation is applied, undulatory motion in the muscle is triggered, enabling the miniature robotic stingray to navigate through water without any cables.
Undulatory locomotion was a real highlight for us. It shows that we can use the microbubbles to achieve not only simple movements but also complex patterns, like in a living organism.
Daniel Ahmed, Professor, Acoustic Robotics for Life Sciences and Healthcare, ETH Zurich
The long-term potential of these 'stingraybots', as the researchers fondly refer to them, includes their use in the gastrointestinal tract, with potential applications in the precise delivery of medication or as an assistant in minimizing procedural invasiveness.
The researchers have also explored how a stingraybot could be transported into the stomach, proposing that it could be coiled up and placed in a specially designed capsule that later dissolves in the patient's stomach after ingestion for application.
Suitable for Confined Spaces and Sensitive Surfaces
The team has also created a compact, wheel-shaped silicone structure that incorporates microbubbles of varying sizes, similarly activated by ultrasound. In trials conducted with a porcine intestine, the researchers were able to maneuver through intestinal twists by sequentially stimulating microbubbles of different sizes.
The intestine is a particularly complex environment because it is narrow, curved, and irregular. It was, therefore, particularly impressive that our wheel robot was actually able to move in there.
Zhan Shi, Former Doctoral Student, ETH Zurich
The research doesn't stop there. The scientists have also created medical patches that can adhere to curved structures through ultrasound activation. These patches can be customized for various tissue types and are designed to release medication at specific sites, such as scars or tumors. In laboratory experiments, the team has already successfully administered dye to a targeted area within a tissue model.
Soft Muscles With Potential Medical Applications
“We started by conducting fundamental research before demonstrating the versatility of these artificial muscles, with applications ranging from drug delivery to locomotion in the gastrointestinal tract to cardiac patches,” Ahmed summarized.
Currently, the technology is confined to laboratory trials, but it possesses significant potential for future medical and technical uses. In the future, these soft artificial muscles may facilitate more precise medication delivery and reduce the invasiveness of procedures.
Ultraschall und künstliche Muskeln
The video shows how the researchers propel the "Stingraybot" with wave-like movements using ultrasound and how they capture a zebrafish larva with grippers and then release it again. Video Credit: Shi Z., et al. Nature 2025
Journal Reference:
Shi, Z., et al. (2025) Ultrasound-driven programmable artificial muscles. Nature. doi.org/10.1038/s41586-025-09650-3