Muscles are extremely effective systems for generating controlled force, and engineers developing hardware for robots or prosthetics have long strived to create analogues that can achieve their unique combination of force, rapid response, scalability and control. But now scientists at the MIT Media Lab and the Politecnico di Bari in Italy have developed artificial muscle fibers that come closer to many of these characteristics.
Like fibers that connect to each other to form biological muscles, these fibers can be arranged in different configurations to meet the demands of a given task. Unlike conventional robotic actuation systems, they are compatible enough to work comfortably with the human body and operate quietly, without motors, external pumps or other bulky supporting equipment.
A recent paper described novel electrofluidic fiber optic muscles – electrically driven actuators built in a fiber optic format published in . The work is led by Media Lab PhD student Ozgun Kilic Afsar; Vito Cacucciolo, professor at the Polytechnic University of Bari; and four co-authors.
The novel system combines two technologies, explains Afsar. One is a fluid-powered artificial muscle known as a slim McKibben actuator, and the other is a miniaturized electrohydrodynamics (EHD)-based solid-state pump that can create pressure inside a closed fluid chamber without moving parts or an external fluid source.
Until now, most fluid-driven supple actuators have relied on external “heavy, bulky, and often noisy hydraulic infrastructure,” says Afsar, “which makes them difficult to integrate into systems where portability or a compact, lightweight design is important.” This has created a fundamental bottleneck in the practical application of jet actuators in real-world applications.
The key to overcoming this bottleneck was the utilize of integrated pumps based on electrohydrodynamic principles. These millimeter-scale, electrically driven pumps create pressure and flow by injecting charge into a dielectric fluid, creating ions that pull the fluid along with them. Weighing just a few grams each and barely thicker than a toothpick, they can be produced continuously and are easily scaled. “We integrated these fiber optic pumps in a closed hydraulic circuit with thin McKibben actuators,” says Afsar, noting that this was not a straightforward task given the different dynamics of the two components.
A key design strategy was to pair these fibers in so-called antagonist configurations. Cacucciolo explains that this is where “one muscle contracts and the other lengthens,” such as when you flex your arm and your biceps contract when you extend your triceps. In their system, a millimeter-scale fiber optic pump is placed between two similarly scaled McKibben actuators, forcing fluid into one actuator to contract while relaxing the other.
“It is very similar to the configuration and organization of biological muscles,” Afsar says. “We didn’t choose this configuration simply for biomimicry, but because we needed a way to store fluid within the muscle structure.” The need to have an external tank open to the atmosphere has been one of the main factors limiting the practical utilize of EHD pumps in robotic systems outside the laboratory. By pairing two McKibben fibers in-line with a fiber pump between them, creating a closed circuit, the team completely eliminated this need.
Another key finding was that muscle fibers required pre-compression, not just filling. “There is a minimum internal system pressure that the system can tolerate,” says Afsar, “below which the pump may deteriorate or temporarily stop working.” This occurs due to cavitation, where vapor bubbles are formed when the pressure at the pump inlet drops below the vapor pressure of the liquid, ultimately leading to dielectric breakdown.
To prevent cavitation, they applied “deflection” pressure from the very beginning so that the pressure at the fiber pump inlet never dropped below the vapor pressure of the liquid. The amount of this bias pressure can be adjusted depending on the application. “To achieve the maximum contraction that a muscle can generate, we found that there is an optimal range of deflection pressure,” he says. “If you want to configure the system for faster response, you can increase the yaw pressure, although with some reduction in maximum contraction.”
Cacucciolo adds that most contemporary robotic limbs and hands are built based on electric servomotors, the configuration of which is fundamentally different from the configuration of natural muscles. Servo motors produce rotational motion on a shaft that must be converted to linear motion, while muscle fibers naturally contract and extend linearly, much like electrofluidic fibers.
“Most robotic arms and humanoid robots are designed around servo motors that power them,” he says. “This creates integration limitations because servo motors are difficult to pack densely and tend to concentrate mass near the joints they drive. In contrast, artificial muscles in the form of fibers can be packed tightly inside a robot or exoskeleton and spread throughout the structure, rather than being concentrated near a joint.”
These electrofluidic muscles may be particularly useful in wearable applications, such as exoskeletons that support a person lift heavier weights or assistive devices that restore or raise dexterity. However, the basic principles may also have broader applications. “Our findings apply to fluid-driven robotic systems in general,” says Cacucciolo. “Wherever jet actuators are used or where engineers want to replace external pumps with internal pumps, these design principles can be applied to a wide range of fluid-driven robotic systems.”
The work “represents a significant advance in soft actuation in a fiber-optic format” that “addresses several long-standing obstacles in the field, particularly regarding portability and power density,” says Herbert Shea, a professor in the supple actuation laboratory at Ecole Polytechnique Federale de Lausanne in Switzerland, who was not associated with this research. “The lack of moving parts on the pump means these muscles work quietly, which is a major advantage of prosthetic devices and supportive clothing,” he says.
Shea adds that “this high-quality and rigorous work bridges the gap between fundamental fluid dynamics and practical applications of robotics. The authors deliver a complete system-level solution – characterizing individual components, developing a predictive physical model, and validating it with a series of demonstrators.”
In addition to Afsar and Cacucciolo, the team also included Gabriele Pupillo and Gennaro Vitucci from the Politecnico di Bari, and Wedyan Babatain and Professor Hiroshi Ishii from the MIT Media Lab. The work was supported by the European Research Council and the multi-sponsored Media Lab consortium.