Developed using responsive materials and innovative fiber structures, these actuators offer a powerful alternative to traditional motors and fluid-driven systems. By mimicking the way real muscles move and respond to stimuli, fiber-type artificial muscles bring a new level of control and adaptability to machines designed to interact with people and complex environments.
Replacing Rigid Hardware with Responsive Fibers
Traditional actuators (gears, motors, and hydraulics) are strong but stiff, often too bulky or inflexible for wearable or soft robotic systems. Even more advanced actuators, like pneumatic or ionic devices, tend to be hard to control or difficult to miniaturize.
Fiber-type artificial muscles offer a different approach. They’re thin, lightweight, and capable of movement when exposed to specific stimuli such as heat, light, or electricity. What sets them apart is their ability to generate motion through internal changes in material structure, rather than external mechanical components.
This review highlights this shift as a turning point in actuator design, moving from rigid mechanics to systems that are soft, scalable, and adaptable by design.
Mimicking Nature, One Fiber at a Time
Biological muscle functions through a hierarchical system of contraction, with individual sarcomeres generating tensile force that scales to produce complex motion (bending, rotation, or extension) via coordinated muscle groups. Fiber-type artificial muscles apply this principle by using structural pre-conditioning, such as twisting or coiling, to convert material-level changes into macroscopic actuation.
In many cases, torsional actuation forms the basis: a pre-twisted fiber untwists in response to internal expansion, generating rotational torque. This torsional motion can then be harnessed or transformed into tensile contraction or bending, depending on how the fiber is integrated into a system.
Other fibers are engineered to bypass torsion entirely, producing direct tensile or bending actuation through material anisotropy or asymmetric structural design.
The core mechanism involves the material’s internal response to stimuli - molecular reorientation, phase transitions, or volumetric changes - within smart components like carbon nanotubes, liquid crystal elastomers, or graphene composites. These microstructural changes, when guided by fiber geometry and architecture, result in precise, controllable deformation, enabling lifelike motion in synthetic systems.
How Movement Happens: Stimuli and Structure
The review outlines four primary actuation modes:
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Torsional actuation leverages the untwisting of a pre-twisted fiber upon stimulation. The rotational output, often driven by thermal, electrical, or humidity-induced expansion, can exceed 11,000 rpm in optimized systems, demonstrating high power density and speed.
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Tensile actuation involves linear extension or contraction along the fiber axis. While some materials contract intrinsically, tensile strains are typically amplified through geometric transformation - twisting fibers into coils, which can then undergo secondary coiling. Certain configurations have achieved contraction strokes exceeding 8600 %.
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Bending actuation enables more complex and joint-like movement. Strategies include building asymmetric bilayer structures, inducing localized stiffness gradients through laser patterning, or translating torsional energy into bending via controlled fiber anchoring and geometry.
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Isometric actuation produces substantial force without altering length, a requirement in applications such as static gripping or wound compression. Electrochemical and moisture-powered designs have demonstrated isometric stresses of up to 28 MPa, more than 80 times that of mammalian skeletal muscle.
This modular categorization reflects how structural design and material selection enable programmable motion across multiple axes and functional modes.
Beyond Actuation: Integrated Functionality
Beyond mechanical output, many fiber-based artificial muscles incorporate multifunctional properties. Some designs enable self-sensing (embedding strain or pressure sensitivity directly into the actuator), while others support energy harvesting, electrical conductivity, or optical feedback through color change.
These hybrid functionalities allow a single fiber to serve multiple roles, such as simultaneously sensing external loads and adjusting shape in response. This convergence of sensing and actuation is particularly valuable for autonomous systems where external feedback loops may be impractical or slow. Such integration represents a shift toward more efficient, compact, and adaptable soft robotic systems.
Applications Across Scales and Systems
Fiber-type artificial muscles are already being applied in a wide range of domains, from human-assistive devices to surgical robotics:
- In wearable systems, they act as humidity-responsive smart textiles, sleeves that self-adjust, or compression garments for rehabilitation.
- In soft robotics, they power biomimetic machines that crawl, swim, jump, or fly - mimicking the movement strategies of worms, insects, and aquatic organisms.
- In bionic systems, tendon-driven limbs, color-adaptive soft grippers, and shape-changing appendages demonstrate how these fibers can replicate human or plant-like responsiveness.
- At the micro- and mesoscale, fiber muscles enable fine control in tools like microfluidic mixers, soft tentacles for grasping, or wireless surgical scissors and drills, where low-profile actuation is essential for delicate or minimally invasive procedures.
Emerging examples also include wireless actuators activated via RF heating, underwater grippers with spatiotemporal programmability, and dual-bundle tendon systems that deliver high-fidelity bionic motion.
Scaling Challenges and Future Directions
Despite their promise, fiber-type artificial muscles face practical hurdles before widespread deployment. Key challenges include scalable manufacturing, durability under repeated cycling, and the sustainability of materials and fabrication methods.
The review highlights promising developments in textile-based manufacturing routes, fiber-bundle architectures that integrate electrodes and sensing layers, and the use of bio-derived materials such as lotus and cotton fibers. These directions suggest that large-scale, eco-conscious production is possible, particularly if aligned with advances in automated fiber processing and modular assembly.
Ultimately, the transition from high-performance lab prototypes to commercially viable systems will depend on solving engineering bottlenecks in scalability and reliability, while preserving the core advantages of responsiveness, adaptability, and multifunctionality that define fiber-type artificial muscles.
Journal Reference
Kim, S. H., & Kim, S. J. (2025). Fiber-type artificial muscles for robotic actuation. Npj Robotics, 3(1). DOI:10.1038/s44182-025-00059-8. https://www.nature.com/articles/s44182-025-00059-8
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