A new artificial fiber spun from a polymer called liquid crystal elastomer (LCE) using high-voltage electricity replicates the strength, responsiveness, and power density of human muscle fibers, scientists report. When powered by heat or near-infrared light, the fibers pulled upward and downward or oscillated back and forth.
"Our work may open up an avenue to build soft robotics or soft machines using liquid crystal elastomers as the actuator," the authors write in their paper, published in the August 25 issue of Science Robotics.
When applied to a variety of potential applications, the fiber actuators successfully controlled the pinching motion of a micro-tweezer, directed the movement of a microswimmer and a tiny artificial arm, and pumped fluids into a light-powered microfluidic pump.
Inspired by the utility of tiny fibers in nature, scientists sought to create artificial fibers that could also serve as ubiquitous tools in robotics, as sensors or assistive devices, for example. In the past few years, researchers succeeded in constructing fiber actuators driven by heat or light that are as strong and flexible as natural fibers. However, many of these artificial threads respond to their stimulus very slowly, due to their large size or complex actuation processes. When fibers can respond quickly, there's a trade-off in size or quality; for example, micro-yarns made of carbon nanotubes are fast actuators, but aren't as strong as other fibers.
"Animal muscle fiber exhibits superior mechanical properties and actuation performance," said senior author Shengqiang Cai, associate professor of mechanical and aerospace engineering at University of California, San Diego. "Only a few existing materials show similar actuation behaviors as animal muscle, and the fabrication of fibers from those materials with a size and quality comparable to muscle fiber is not easy."
Electrospinning Fibers
Cai's team fabricated microfiber actuators out of LCE using electrospinning, a technique developed a few decades ago, used to construct materials for applications like tissue regeneration and smart textiles.
During electrospinning, a solvent-suspended polymer — LCE, in this case — loaded inside a metallic needle or syringe-like container is zapped with high voltage, causing the polymer to shoot out from the needle in strings. The strings are attracted to a conductive "fiber collector" — usually shaped like a spool — placed nearby the opening of the needle. As a result, strands and strands of wet polymer only tens of micrometers in diameter collect on the fiber collector. When the solvent dries, LCE fibers remain.
"The electrospinning technique adopted in the study is low cost and extremely simple to set up," said Cai.
The LCE microfibers measured around 10 to 100 micrometers in diameter but could pull itself with great force in less than 0.2 seconds in response to heat or near-infrared light. However, varying the temperature of the fiber very quickly may be difficult in real-world applications — something to consider for future work, the authors note.
Comparable to Human Muscle Fibers
Overall, the average actuation stress, strain, and power density of the LCE microfibers were comparable to the qualities of human muscle fiber, allowing the microfibers to maintain their fibrousness even after one million cycles of lifting at maximum strain at 90 degrees Celsius, or 194 degrees Fahrenheit.
"We are very excited about the facile and low-cost fabrication method developed by us to make fiber actuators, with comparable performance as animal muscle fiber," said Cai. Robots made from these fibers could be used in a range of fields, from medicine to wearable electronics, he added.
His lab is now working to further improve the artificial fiber's performance by reducing the microfiber's diameter to enhance its response speed, among other efforts.