Artificial muscles use ultrasound-activated microbubbles to move

Researchers at ETH Zurich have developed artificial muscles that contain microbubbles and can be controlled with ultrasound. In the future, these muscles could be deployed in technical and medical settings as gripper arms, tissue patches, targeted drug delivery, or robots.

It might look like a simple material experiment at first glance, as a brief ultrasound stimulation induces a thin strip of silicone to start bending and arching. But that’s just the beginning. A team led by Daniel Ahmed, Professor of Acoustic Robotics for Life Sciences and Healthcare, has developed a new class of artificial muscles: flexible membranes that respond to ultrasound with the help of thousands of microbubbles.

The work is published in the journal Nature.

Microbubble patterns facilitate flexible movement

The researchers created the artificial muscles using a casting mold with a defined microstructure. The silicone membrane produced in this mold has tiny pores on its underside, each around 100 micrometers in depth and diameter—around the width of a human hair. When the researchers submerge the membrane in water, tiny microbubbles become trapped in these pores.

When subjected to sound waves, these microbubbles begin to oscillate and produce a directed flow that moves the muscle. The size, shape and positioning of these microbubbles can be precisely controlled, which makes it possible to produce movements ranging from uniform curving to wave-like patterns. The muscles respond within milliseconds and can be controlled wirelessly.

Gentle gripping and smooth movement

The researchers have demonstrated several applications for these artificial muscles, one of which is a soft, miniature gripper arm. In an experiment, they were able to gently trap a zebrafish larva in water and then release it again.

“It was fascinating to see just how precisely yet gently the gripper functioned; the larva swam away afterwards unharmed,” recalls Zhiyuan Zhang, a former doctoral student under Ahmed and one of the lead authors of the study.

The researchers also constructed a robot that resembles a tiny stingray to demonstrate undulatory movements. It is about four centimeters wide. Two artificial muscles mimic the function of pectoral fins. When the researchers apply ultrasound stimulation, it induces undulatory motion in the muscle, enabling the miniature robot to glide through water without any cabling.

“Undulatory locomotion was a real highlight for us,” says Ahmed. “It shows that we can use the microbubbles to achieve not only simple movements, but also complex patterns, like in a living organism.”

Long-term prospects for these devices—dubbed “stingraybots” by the researchers—include deployment in the gastrointestinal tract, possibly to release medication with absolute precision or support minimally invasive procedures. In fact, the researchers have already considered how a stingraybot could be transported into the stomach: They propose rolling the robot up and placing it in a specially developed capsule that could be swallowed before dissolving in the patient’s stomach.

Suitable for confined spaces and sensitive surfaces

The researchers also produced a small, wheel-like silicone structure, featuring microbubbles of different sizes, which can also be driven using ultrasound. In experiments with a porcine intestine, the researchers demonstrated their ability to navigate through intestinal convolutions by sequentially stimulating microbubbles of different sizes.

“The intestine is a particularly complex environment because it is narrow, curved and irregular,” explains Zhan Shi, a former doctoral student under Ahmed and the study’s other lead author. “It was, therefore, particularly impressive that our wheel robot was actually able to move in there.”

The researchers have also developed medical patches that—through ultrasound activation—are capable of adhering to curved structures. These patches can be specifically tailored to different tissue types and release medication in precise locations, such as to treat scars or tumors. In lab experiments, the team has already successfully delivered dye to a specific location in 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,” summarizes Ahmed.

While the technology remains limited to laboratory trials for now, it holds vast potential for future medical and technical applications. In the long term, these soft artificial muscles could help to administer medication more precisely and make procedures less invasive. By combining biocompatibility with flexibility and wireless control, they represent a promising tool for medical applications. For the researchers, the journey toward acoustically controlled muscles is only at its beginning.