A single fiber actuator inspired by human muscles

To effectively replicate the movements of humans and animals, robots must incorporate muscle-like structures. These artificial muscles must achieve optimal performance for all relevant actuation parameters, including energy density, tension, stress and mechanical resistance.

Researchers from KAIST and Pusan ​​National University in South Korea have recently developed an actuator for robotic applications that is inspired by the skeletal and muscular structures of mammals. This actuator, presented in an article published in Nature’s nanotechnologyis based on soft fibers with strong contraction actuation properties.

“I learned about liquid crystal elastomer (LCE) actuators during a university meeting with Professor Suk Kyun Ahn, one of the co-authors of the paper,” said Sang Ouk Kim, one researchers who conducted the study. Phys.org. “LCEs are promising soft actuator materials with an unusually large reversible dimensional change (shrinkage/relaxation) upon actuation, which is rarely observed in other types of actuator materials but very significant to ideally mimic the natural behavior of skeletal muscles.”

Many actuators developed in the past are based on LCE materials, a class of polymers that can rapidly change shape in response to environmental stimuli. Despite their shape morphing advantages, LCE polymers are known to be associated with relatively poor mechanical properties and poor actuation behavior.

To overcome this limitation, Kim and Prof. Ahn decided to incorporate ultra-strong graphene fillers into the LCE actuators. In addition to improving their mechanical properties, the team expected the graphene fillers to allow light-driven, fast and remotely controllable actuation, thanks to graphene’s photothermal conversion capability.

“Pure LCE actuators typically require a temperature rise, which is usually a long process with no specific spatial controllability, to trigger actuation driven by the aligned state of the liquid crystals to the isotropic random coiled state of the LCE molecules,” explained Kim.

The actuators developed by the researchers are based on soft fibers and include finely exfoliated graphene fillers in the matrix of the LCE material. When laser light is applied to the fiber, the photothermal conversion effect associated with the graphene filler instantly increases the temperature of its surrounding LCE matrix. This results in LCE molecules changing from an aligned liquid-crystal state to a so-called isotropic random coiled state, ultimately causing fibers to shrink on a macroscopic scale.

“Once the laser illumination is removed, the fiber restores the original length while the LCE die is instantly cooled,” Kim said. “The synergistic incorporation of a minor portion (~0.3 wt%) of strong graphene fillers enhances the actuator material itself as well as its actuation performance. of graphene also achieves reversible and rapid actuation at high power, which can be easily controlled remotely by the manipulation of external light.”

One of the most valuable features of the actuator created by Kim and his colleagues is the reversible percolation of the graphene charge network inside. This process allows the fibers to be reversibly shrunk and relaxed back to their original size, while ensuring high mechanical strength over the entire actuation cycle.

“The reversible large shrinkage/relaxation of longitudinal fiber actuation induces reversible assembly and disassembly of the graphene filler network within the composite actuator volume,” Kim said.

“This unprecedented behavior dramatically reinforces the actuator, especially in the constricted actuated state, and results in the intriguing modulation of electrical conductivity as a function of actuation state, which is similar to the EMG signal generation of natural muscles. Remarkably, the inherent mechanical weakness of the LCE actuator especially in the actuated constricted state has been the long-standing critical challenge to the practical use of LCE actuators.”

The researchers evaluated their actuator in a series of tests and found that they obtained very promising results. In fact, they both exhibited the advantageous shape-morphing properties of actuators based on LCE materials, while allowing for robust and reversible actuation stress.

“Our actuator ultimately achieves virtually significant actuation performance, which surpasses that of natural animal muscles in many respects, including actuation tension, stress, energy density, and power,” Kim said. . “Artificial muscles shown in previous work have sometimes achieved superior performance in one or a few of these characteristics, but there has not yet been a report for this type of overall superior performance compared to natural muscle.”

Kim and his colleagues finally demonstrated the potential of their actuators by implementing them on soft robots and evaluating their performance on a series of tasks. They found that the robots were able to imitate different human and animal movements, for example lifting a 1 kg dumbbell, bending individual fingers on an artificial hand and replicating the movement of caterpillars.

Interestingly, the team tested a robotic caterpillar based on its actuator by “racing” it with a living caterpillar. Their system won the race, further highlighting the potential of their unique fiber-based actuator to create super-powered, high-performance robots, bionic prosthetic tools, and perhaps even reconfigurable smart clothes.

“The next big challenge will be to integrate our artificial muscle with neural activity,” Kim added. “If the individual actuator fiber is specifically controllable mimicking neutral control, natural, sophisticated animal-like movement and locomotion should be possible while interfaced with the human brain or AI. Currently, most actuators rely on hard mechanical systems.Our composite soft actuator would be a promising candidate for addressing the inherent limitations of the traditional mechanical actuation system, such as heavy weight and mechanical rigidity, and achieving a truly natural animal like robotics gentle.

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