Muscles, as nature's perfect actuators, outperform most synthetic counterparts in power and precision relative to their size, possessing the unique ability to repair and strengthen through exercise. This remarkable efficiency has inspired engineers to harness natural muscles to power robots with natural muscles, resulting in "biohybrid" robots capable of performing actions such as walking, swimming, pumping, and gripping through muscle-based actuators - devices that turn energy into motion. However, the diversity in robot designs has created the need for a universal approach to optimize muscle utilization across various robot models. Now, engineers have developed a spring-like device, termed a "flexure," which serves as a basic skeleton-like module suitable for almost any muscle-driven robot.

The new spring, or “flexure,” has been designed by engineers at MIT (Cambridge, MA, USA) to extract maximum work out of any attached muscle tissues. Similar to a perfectly weighted leg press machine, this device significantly enhances the amount of movement naturally produced by a muscle. Upon fitting a ring of muscle tissue onto the device, similar to a rubber band being stretched around two posts, they found that the muscle pulled on the spring, reliably and repeatedly, and stretched it five times more, in comparison with other earlier device designs. The device was also found to accurately measure muscle performance and endurance.

After experimenting with different muscle contraction frequencies (such as stimulating the bands to contract once versus four times per second), they found that the muscles became tired at higher frequencies and failed to generate as much pull. The engineers consider the flexure design as a new building block that can be combined with other flexures to build artificial skeletons of any configuration. These skeletons can then be fitted with muscle tissues to power their movements. The team is currently exploring ways to adapt and combine flexures to develop precise, articulated, and dependable robots powered by natural muscles.

“An example of a robot we are trying to build in the future is a surgical robot that can perform minimally invasive procedures inside the body,” said MIT’s Ritu Raman. “Technically, muscles can power robots of any size, but we are particularly excited in making small robots, as this is where biological actuators excel in terms of strength, efficiency, and adaptability.”

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