The quest to create machines that move and interact like humans has long fascinated scientists and engineers. A key challenge in this pursuit is developing actuators, or motion-enabling components, that can replicate the physiological characters of human muscles such as variable stiffness. Traditional designs often lack the ability to adapt their rigidity to different tasks, limiting their performance and safety. Imagine a new kind of actuator that combines soft and rigid properties seamlessly, adapting its stiffness on demand to meet a variety of challenges. This vision is now closer to reality, bringing the potential for robots that work more harmoniously with humans and their environment.
Something remarkable in robotics has emerged, offering a new way to replicate human muscle function. Professor Ning Xi and his team from The University of Hong Kong have developed a shear-stiffening gel-enabled twisted string actuator. Their work, published in the well-regarded journal Scientific Reports, demonstrates an artificial muscle that dynamically adjusts its stiffness to meet varying demands, marking significant progress in bio-inspired robotics.
The actuator integrates a twisted string mechanism, which converts rotational movement into linear motion, with a specially formulated shear-stiffening gel that hardens when subjected to rapid force. This combination enables the system to adjust its stiffness and elasticity based on the speed of twisting. At higher speeds, the gel transitions from a soft to a stiff state, which significantly enhances the ability to transmit forces. For example, the elasticity of the actuator increased roughly threefold at higher twisting speeds compared to lower speeds. This flexibility allows these systems to mimic the broad range of stiffness exhibited by human muscles, enabling safe and efficient human-robot interactions.
Human physiology provided inspiration for the design, particularly the body’s ability to adjust muscle stiffness for tasks ranging from lifting heavy objects to performing delicate actions. “By integrating the shear-stiffening gel, we aimed to create actuators that not only replicate the natural stiffness modulation of muscles but also offer a practical solution for wearable robotics,” explained Professor Xi. Their unique design combines high-strength Kevlar and Dyneema fibers, materials known for their exceptional durability and lightness, coated with the gel, resulting in systems that are lightweight, flexible, and capable of generating considerable force.
Thorough testing revealed that the actuator significantly increased its elasticity at maximum twisting speeds compared to lower speeds. This ability to dynamically adjust stiffness highlights its potential for applications in prosthetics, exoskeletons, and rehabilitation devices. Prosthetics refer to artificial devices that replace missing body parts, while exoskeletons are wearable robotic systems designed to enhance human mobility and strength. Such systems could effectively support human muscles by compensating for motion loss, offering particular benefits to individuals with mobility challenges or age-related muscle weakening.
Research findings emphasize the adaptability and efficiency of the actuator. By adjusting twisting speeds, the system achieved varying stiffness and elasticity levels that closely resemble the mechanical behavior of human muscles. Additionally, it was observed that the force-generating capacity, or the ability to produce movement and support under strain, improved significantly at higher twisting speeds, making this technology a promising solution for tasks requiring both strength and precision.
Findings from the Professor Xi and colleague’s study underscore the potential for the actuator to revolutionize wearable robotics and assistive technologies. “This development bridges the gap between artificial systems and biological inspiration, offering a future where robots and humans can collaborate seamlessly,” noted Professor Xi. Its compact design and versatility make it suitable for a wide range of applications, including robotic limbs, wearable assistive robots, and rehabilitation devices .
Through the fusion of advanced material science, which studies the properties and applications of materials, and bio-inspired engineering, which draws ideas from nature to solve human challenges, this innovation sets the stage for a new era of robotics that aligns with human abilities. As the technology evolves, it holds the promise of transforming lives, particularly for those in need of enhanced mobility or physical assistance.
Journal Reference
Zhang Q., Xue Y., Zhao Y., Zou K., Yuan W., Tian Y., Chen J., Chen J., Xi N. “Shear stiffening gel-enabled twisted string for bio-inspired robot actuators.” Scientific Reports, 2024, 14(4710). DOI: https://doi.org/10.1038/s41598-024-55405-x
About the Author
Professor Ning Xi received D.Sc. degree in Systems Science and Mathematics from Washington University in St. Louis, Missouri, USA in December 1993. Currently he is the Chair Professor of Robotics and Automation, the Director of Advanced Technologies Institute, and the Head of Department of Data and Systems Engineering at the University of Hong Kong. Before joining the University of Hong Kong, he was a University Distinguished Professor, the John D. Ryder Professor of Electrical and Computer Engineering and Director of Robotics and Automation Laboratory at Michigan State University in United States. Prof. Xi is a fellow of IEEE. He also served as the President of IEEE Nanotechnology Council (2010-2011) and the President of IEEE Robotics and Automation Society (2018). His research interests include robotics, artificial intelligence, manufacturing automation, micro/nano manufacturing, nano-bio technology, sensors, and intelligent control and systems.
