Creating soft robotic systems that move with biological precision is a major engineering challenge, especially for applications inside the human body where flexibility, delicacy, and wireless control are essential. Now, a new artificial muscle technology addresses these gaps: silicone membranes embedded with thousands of microbubbles that bend, arch, and generate complex motion when stimulated with ultrasound.

Researchers at ETH Zurich (Zurich, Switzerland) have introduced a new class of soft robotic actuators designed using a casting mold with a defined microstructure. The artificial muscles work by trapping microbubbles inside tiny pores on the underside of a silicone membrane. Each pore is roughly 100 micrometers deep and wide — about the thickness of a human hair. When submerged in water, these pores capture the microbubbles. Ultrasound causes the bubbles to oscillate, generating a directed flow that moves the membrane.

By adjusting the size, shape, and spatial arrangement of the microbubbles, the team could produce a wide variety of movements, from uniform bending to wave-like, undulatory motion. The muscles respond within milliseconds and can be stimulated wirelessly. In demonstrations, the researchers designed a miniature soft gripper capable of gently capturing and releasing a zebrafish larva underwater without causing harm. They also created a 4-centimetre “stingraybot” with two artificial muscles that act like pectoral fins.

The research, published in Nature, shows that when exposed to ultrasound, the bot produces undulatory motion and swims without any wires. The team built a wheel-like structure that moves through the narrow, irregular environment of a porcine intestine by stimulating microbubbles of specific sizes in sequence. Additionally, they created ultrasound-activated medical patches capable of adhering to curved surfaces and releasing therapeutics precisely, as shown in lab models where dye was delivered to a targeted location.

These soft, acoustically controlled muscles show potential for future medical applications, including drug delivery systems, gastrointestinal robots, implantable patches, and minimally invasive tools. Their flexibility, biocompatibility, and wireless actuation make them promising candidates for controlled movement inside confined or sensitive environments.

“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,” said Professor Daniel Ahmed, who led the research.

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