In the world of robotics, where the boundaries of what is possible are constantly shifting, scientists and engineers are constantly looking for ways to make machines as similar as possible to living organisms. One of the latest and most exciting breakthroughs in this direction comes from Northwestern University, where innovative, soft artificial muscles have been developed. This revolutionary technology opens the door to a new generation of robots – those that will be capable of more fluid movement, better interaction with the environment, and, most importantly, autonomous functioning without the need for constant external power.

These new actuators, as they are professionally called, represent a key step toward building robotic musculoskeletal systems that mimic the complexity and efficiency of the human body. Their performance and mechanical properties promise to transform the way robots walk, run, interact with people, and navigate the dynamic world around them. Imagine robots moving with the grace and power of an athlete, capable of absorbing shocks but also generating sufficient force to perform demanding tasks.

Inspiration from nature: The path to more flexible robots

For a long time, robots were synonymous with rigid, mechanical structures, designed for precise but often limited movements in controlled environments. Although such robots are extremely efficient in industrial plants, their rigidity is a significant drawback in the unpredictable and complex real world. The human body, with its bones, muscles, and tendons, offers a perfect example of a system that is simultaneously strong, flexible, and adaptable. It was this bio-inspiration that guided the team of engineers from Northwestern University.

Dr. Ryan Truby, senior author of the study and a professor of materials science and mechanical engineering at the McCormick School of Engineering, emphasizes the importance of this approach. His goal is to create robotic bodies that are flexible, adaptable, and able to cope with the uncertainty of the physical world. This includes not only practical artificial muscles but also components that mimic bones, tendons, and ligaments. Through such an approach, robots not only become more resilient and adaptable, but they can also leverage the mechanics of softer materials to become more energy-efficient.

Taekyoung Kim, a postdoctoral researcher in Truby's lab and the first author of the study, emphasizes that it is extremely difficult to create robots without physical compliance that can react smoothly to external changes and interact safely with humans. For future robots that will move more naturally and safely in unstructured environments, it is crucial to design them based on the human body – with hard skeletons and soft, muscle-like actuators.

Overcoming challenges in muscle replication

Previous attempts to develop soft actuators with mechanical properties similar to muscles have often encountered significant obstacles. Many required bulky and heavy power equipment, and even then, they were not durable enough nor could they generate sufficient force to perform real tasks. Dr. Truby explains that it is extremely difficult to engineer soft materials to function like muscles. Even if a material can be made to move like an artificial muscle, there are numerous other challenges, such as transmitting sufficient force with adequate power. Connecting such muscles to rigid, bone-like elements presents additional problems.

The team overcame these challenges by relying on an actuator previously developed in Truby's lab. The heart of this actuator is a 3D-printed cylindrical structure called a "handed shearing auxetic" (HSA). The HSA possesses a complex structure that allows for unique movements and properties, such as elongating and expanding when twisted. The twisting motion required to move the HSA can be generated by a small, integrated electric motor. Kim developed a method for 3D printing HSA structures from a common, inexpensive rubber, similar to that often used for making mobile phone cases.

Innovative artificial muscle design

In the new design, the team coated the HSA structure with a rubber origami-like bellows structure. This innovative combination allows a rotating motor to drive the extension and contraction of the assembled actuators. The result is artificial muscles that can push and pull with impressive force. What is particularly fascinating is the muscle's ability to dynamically stiffen when activated – just like a human muscle. This characteristic is crucial for stability and movement control.

Each of these artificial muscles weighs about as much as a football and is slightly larger than a soda can. It can stretch to 30% of its length, contract, and lift objects 17 times its own weight. Perhaps most importantly for their application in robotic bodies is the fact that the muscles can be powered by batteries, eliminating the need for heavy, external equipment. This autonomy opens the way for truly independent robots that are not tethered to power sources.

A humanoid leg that "kicks" and "feels"

To demonstrate the real potential of these muscles, Truby, Kim, and their team used 3D printing to create a life-sized robotic leg. The "bones" of the leg were constructed from rigid plastic, while the tendon-inspired connectors were made of rubber. Elastic tendons connect the quadriceps and hamstring muscles to the shin bone, and the calf muscle to the foot structure. These tendons and muscles help to dampen movements and absorb shocks, similar to a biological musculoskeletal system. This integration of soft and rigid components allows for more fluid and natural movement, reducing the risk of damage to the robot or its environment.

Additionally, the team integrated a flexible, 3D-printed sensor that allows the leg to "feel" its own muscle. Designed like a sandwich, a conductive layer of flexible plastic is squeezed between two non-conductive layers. When the artificial muscle moves, the sensor also moves. As it stretches, its electrical resistance changes, allowing the robot to sense how much its muscle is extending or contracting. This ability of proprioception – the sense of one's own position and movement – is crucial for fine control and adaptation in complex tasks. It enables the robot to perform movements more precisely, maintain balance, and react to unexpected obstacles.

The resulting leg is compact and battery-powered. A single charge of a portable battery provided enough energy for the leg to bend at the knee thousands of times in one hour. Achieving similar capabilities with other soft actuator technologies would be extremely difficult, if not impractical. This energy efficiency and autonomy make these artificial muscles extremely promising for a wide range of applications.

Broader context and future applications

The development of these "bony muscles" represents a significant step forward in the field of soft robotics, a branch of robotics that focuses on creating robots from materials that are inherently flexible and adaptable. Unlike traditional robots, soft robots can work safely alongside humans, manipulate delicate objects, and navigate through complex, unstructured environments. The potential applications are vast and varied.

In medicine, these muscles could revolutionize prosthetics, creating limbs that are not only functional but also sensitive and more natural for the user. They could be used in the development of exoskeletons to assist people with disabilities or for rehabilitation. In industry, robots equipped with these muscles could take on tasks that require gentle handling, such as packing delicate products or working in confined spaces. Their ability to absorb shocks makes them ideal for work in dynamic and unpredictable environments, reducing the risk of equipment damage or injury.

Also, these muscles open up new possibilities for search and rescue. Robots with such muscles could move through rubble, narrow passages, or dangerous terrains with greater agility and resilience. In the field of space exploration, flexible robots could be ideal for manipulating samples on other planets or for repairs in space, where precision and adaptability are key. Dr. Truby looks forward with enthusiasm to how these artificial muscles can inspire new directions for humanoid and animal-like robots, paving the way for machines that are not only intelligent but also physically capable of interacting with the world in a way that was previously reserved only for living beings.

The research was published in the prestigious journal Advanced Materials, which confirms its scientific importance and innovativeness. In addition to Ryan Truby and Taekyoung Kim, the study also involved Eliot Dunn, a high school research intern at the Robotic Matter Lab, and Melinda Chen, a participant in the Research Experience for Undergraduates program at the Northwestern University Materials Research Science and Engineering Center. The work was supported by the Office of Naval Research and by Leslie and Mac McQuown through the Center for Engineering Sustainability and Resilience at Northwestern University. This collaboration of researchers at different levels and the support of key institutions underscore the multidisciplinary approach and the significance of this discovery for the future of robotics.

Source: Northwestern University