Ultrasound Makes Artificial Muscles Bubble to Life

Artificial Muscles that Use Ultrasound to Move

A soft gel filled with tiny bubbles may not look like much, but when pulsed with ultrasound waves, the material behaves like natural muscle: contracting, gripping, and lifting with surprising strength. This discovery, reported in the journal Nature, introduces a new kind of artificial muscle – one powered not by wires, batteries, or pumps, but by sound.

The Acoustic Trick Behind Bubble Muscles

The acoustic trick behind these "bubble muscles" opens the door to wireless control, quick responsiveness, and even deep-tissue operation. This could lead to soft robots that wriggle through tight spaces with lifelike agility, surgical tools that bend and flex inside the body, or gentle grippers that can manipulate fragile objects without breaking them.

Challenges in Artificial Muscle Design

The robotics community has long struggled to design artificial muscles that rival the flexibility and suppleness of living tissue. Motors and hydraulics can deliver force but lack finesse and may pose safety risks inside the body, while soft actuators – driven by heat, air, or chemical reactions – tend to be bulky, inefficient, or too slow for practical use.

A New Approach to Artificial Muscle Design

Daniel Ahmed, a nanoroboticist at ETH Zürich, took a different approach. Harnessing the power of acoustic resonance, he and his colleagues embedded thousands of microscopic bubbles into a soft, biocompatible gel, arranging the air sacs in lattice-like patterns so they leap into motion when struck by ultrasound. Different bubble sizes respond to different ultrasound frequencies, allowing control over which parts of the material bend.

# Example of how bubble size and ultrasound frequency can be controlled
bubble_sizes = [10, 20, 30]  # micrometers
ultrasound_frequencies = [10, 20, 30]  # kilohertz

Adjusting Bubble Size and Ultrasound Frequency

Adjusting both the ultrasound frequency and the size of the bubbles in their muscle-mimicking arrays enabled the researchers to direct the gel to flex, rotate, or deform – in effect, turning invisible vibrations into controllable motion. "By activating different sets of frequencies," Ahmed says, "you can actually get programmable muscle."

Bubble Muscles in Soft Robotics

Several prototype devices showcase the bubble muscles in action. In one demonstration, the researchers fashioned a claw-like gripper that snapped shut around live zebrafish larvae without damaging the delicate animals. In another, they built a stingray-shaped soft robot whose fins, studded with tiny bubbles of three distinct sizes, undulated under ultrasound, propelling it smoothly through water – even within the stomach of a pig.

// Example of how the bubble muscles can be used in a soft robot
int main() {
  // Initialize the bubble muscles
  init_bubble_muscles();
  
  // Propulse the soft robot through water
  propulse_robot();
  
  return 0;
}

Applications of Bubble Muscles

Making the most of their pig tissue from the local abattoir, Ahmed's team also showed how the material could grip. On the surface of a pig heart, for example, a patch of the bubble-patterned gel clung tightly and stayed in place for more than an hour while flexing in response to ultrasound. A bandage-sized patch sticks firmly to the outside of a pig heart.

Ultrasound Imaging in Biomedical Implants

One notable feature of the ultrasound-driven artificial muscle is that the microbubbles involved can be tracked with standard ultrasound imaging. And because the actuation frequencies (between 1 and 100 kilohertz) are far below those used for clinical imaging (between 1 and 20 megahertz), the two functions don't interfere.

Challenges and Future Directions

However, all the proof-of-concept demonstrations have been trialed on dead tissues, and it remains to be seen how well the system performs inside a living rat or pig, much less in a human body – especially as bones and other irregular tissues may scatter and weaken the ultrasound signal, or fluids flowing inside the body might interfere with controlled movement. "You can't tell if this is really working or not without in vivo evidence," says W. Hong Yeo, a bioengineer at Georgia Tech who was not involved in the study.

Conclusion

Nonetheless, Yeo points to their tiny scale and rapid responsiveness as features that could make the bubble muscles especially attractive for biomedical implants. "That catches my eye," he says. "It's very unique and it makes sense."

Source: https://spectrum.ieee.org/artificial-muscle-ultrasound-waves-robots