A recent study published in Nature reveals an innovative type of artificial muscle that operates using ultrasound rather than traditional power sources like wires or batteries. This technology, developed by researchers at ETH Zürich, allows a soft gel infused with microscopic bubbles to mimic the behavior of natural muscle, contracting and lifting with remarkable strength when stimulated by ultrasound waves.
The breakthrough introduces exciting possibilities for wireless control and rapid responsiveness, which could lead to advancements in soft robotics. This includes applications such as robots that can navigate through narrow spaces with agility, surgical instruments that can flex within the human body, and delicate grippers capable of handling fragile items without causing damage. Ryan Truby, a materials scientist at Northwestern University, commented on the significance of the research, stating, “From a medical perspective, it”s really cool. They”re using relatively simple approaches, but they”re integrating them in clever new ways.”
Creating effective artificial muscles has been a long-standing challenge in robotics. Traditional motors and hydraulic systems deliver force but often lack the finesse necessary for safe use in medical applications. Conversely, soft actuators powered by heat, air, or chemical reactions can be bulky or inefficient. Daniel Ahmed, a nanoroboticist at ETH Zürich, took a novel route by embedding thousands of tiny bubbles within a biocompatible gel. These bubbles are arranged in lattice patterns that allow them to spring into action when exposed to ultrasound.
The researchers found that varying the size of the bubbles and the frequency of the ultrasound waves enables precise control over the gel”s movements, allowing it to flex, rotate, or deform. Ahmed noted, “By activating different sets of frequencies, you can actually get programmable muscle.”
Several prototypes demonstrate the capabilities of these bubble muscles. One example is a claw-like gripper that successfully closed around live zebrafish larvae without harming them. Another prototype, resembling a stingray, features fins equipped with bubbles of different sizes that undulate under ultrasound, allowing it to glide smoothly through water, even within the stomach of a pig—though not from a living specimen.
Utilizing pig tissue from a local abattoir, Ahmed”s team also tested the muscle”s gripping ability. A patch of the bubble-patterned gel adhered tightly to the surface of a pig heart, remaining in place for over an hour while responding to ultrasound stimulation. In a separate experiment, the researchers enclosed their artificial muscle in a biodegradable capsule and placed it within a pig bladder. Once the capsule dissolved, ultrasound triggered the device to unfold and attach to the bladder”s inner wall, hinting at future uses for targeted medical treatments.
One notable advantage of this ultrasound-driven artificial muscle is that the microbubbles can be monitored using standard ultrasound imaging techniques. The operational frequencies of the bubbles are significantly lower than those used in clinical imaging, meaning these two processes can coexist without interference. However, all validation tests so far have been conducted on non-living tissues, and further research is needed to assess functionality in living organisms, such as rats or pigs, where bone and other tissues may disrupt ultrasound signals.
W. Hong Yeo, a bioengineer at Georgia Tech who did not participate in the study, emphasized the importance of in vivo testing. He stated, “You can”t tell if this is really working or not without in vivo evidence.” Additionally, prolonged use of the artificial muscles can lead to bubble expansion, which may compromise functionality after approximately thirty minutes of operation.
Despite these challenges, Yeo highlighted the rapid responsiveness and small size of the bubble muscles as factors that could make them particularly appealing for biomedical implants. “That catches my eye,” he remarked. “It”s very unique and it makes sense.”
