Faster Microfiber Actuators Mimic Human Muscle

UCSD researchers make an artificial muscle prototype from liquid crystals

3 min read
A long white vertical piece of plastic attached to a horizontal piece holds a barbel shape like an arm. Affixed to the vertical and horizontal parts are two copper colored squares, connected by thin black fibers.

The researchers tested the electrospun liquid crystal elastomer microfiber actuator on microbotic systems.

University of California, San Diego/Science Robotics

Robotics, prosthetics, and other engineering applications routinely use actuators that imitate the contraction of animal muscles. However, the speed and efficiency of natural muscle fibers is a demanding benchmark. Despite new developments in actuation technologies, for the most past artificial muscles are either too large, too slow, or too weak.

Recently, a team of engineers from the University of California San Diego (UCSD) have described a new artificial microfiber made from liquid crystal elastomer (LCE) that replicates the tensile strength, quick responsiveness, and high power density of human muscles. "[The LCE] polymer is a soft material and very stretchable," says Qiguang He, the first author of their research paper. "If we apply external stimuli such as light or heat, this material will contract along one direction."

Though LCE-based soft actuators are common and can generate excellent actuation strain—between 50 and 80 percent—their response time, says He, is typically "very, very slow." The simplest way to make the fibers both responsive and fast was to reduce their diameter. To do so, the UCSD researchers used a technique called electrospinning, which involves the ejection of a polymer solution through a syringe or spinneret under high voltage to produce ultra-fine fibers. Electrospinning is used for the fabrication of small-scale materials, to produce microfibers with diameters between 10 and 100 micrometers. It is favored for its ability to create fibers with different morphological structures, and is routinely used in various research and commercial contexts.

The microfibers fabricated by the UCSD researchers were between 40 and 50 micrometers, about the width of human hair, and much smaller than existing LCE fibers, some of which can be more than 0.3 millimeters thick. "We are not the first to use this technique to fabricate LCE fibers, but we are the first…to push this fiber further," He says. "We demonstrate how to control the actuation of the [fibers and measure their] actuation performance."

Animation: A long white vertical oblong attached to a horizontal white oblong which holds a barbel shape, which is being flexed up and down. Affixed to the vertical and horizontal parts are two copper colored squares, connected by thin black fibers. University of California, San Diego/Science Robotics

As proof-of-concept, the researchers constructed three different microrobotic devices using their electrospun LCE fibers. Their LSE actuators can be controlled thermo-electrically or using a near-infrared laser. When the LCE material is at room temperature, it is in a nematic phase: He explains that in this state, "the liquid crystals are randomly [located] with all their long axes pointing in essentially the same direction." When the temperature is increased, the material transitions into what is called an isotropic phase, in which its properties are uniform in all directions, resulting in a contraction of the fiber.

The results showed an actuation strain of up to 60 percent—which means, a 10-centimeter-long fiber will contract to 4 centimeters—with a response speed of less than 0.2 seconds, and a power density of 400 watts per kilogram. This is comparable to human muscle fibers.

An electrically controlled soft actuator, the researchers note, allows easy integrations with low-cost electronic devices, which is a plus for microrobotic systems and devices. Electrospinning is a very efficient fabrication technique as well: "You can get 10,000 fibers in 15 minutes," He says.

That said, there are a number of challenges that need to be addressed still. "The one limitation of this work is…[when we] apply heat or light to the LCE microfiber, the energy efficiency is very small—it's less than 1 percent," says He. "So, in future work, we may think about how to trigger the actuation in a more energy-efficient way."

Another constraint is that the nematic–isotropic phase transition in the electrospun LCE material takes place at a very high temperature, over 90 C. "So, we cannot directly put the fiber into the human body [which] is at 35 degrees." One way to address this issue might be to use a different kind of liquid crystal: "Right now we use RM 257 as a liquid crystal [but] we can change [it] to another type [to reduce] the phase transition temperature."

He, though, is optimistic about the possibilities to expand this research in electrospun LCE microfiber actuators. "We have also demonstrated [that] we can arrange multiple LCE fibers in parallel…and trigger them simultaneously [to increase force output]… This is a future work [in which] we will try to see if it's possible for us to integrate these muscle fiber bundles into biomedical tissue."

The Conversation (0)

Can This DIY Rocket Program Send an Astronaut to Space?

Copenhagen Suborbitals is crowdfunding its crewed rocket

15 min read
Vertical
Five people stand in front of two tall rockets. Some of the people are wearing space suits and holding helmets, others are holding welding equipment.

Copenhagen Suborbitals volunteers are building a crewed rocket on nights and weekends. The team includes [from left] Mads Stenfatt, Martin Hedegaard Petersen, Jørgen Skyt, Carsten Olsen, and Anna Olsen.

Mads Stenfatt
Red

It was one of the prettiest sights I have ever seen: our homemade rocket floating down from the sky, slowed by a white-and-orange parachute that I had worked on during many nights at the dining room table. The 6.7-meter-tall Nexø II rocket was powered by a bipropellant engine designed and constructed by the Copenhagen Suborbitals team. The engine mixed ethanol and liquid oxygen together to produce a thrust of 5 kilonewtons, and the rocket soared to a height of 6,500 meters. Even more important, it came back down in one piece.

That successful mission in August 2018 was a huge step toward our goal of sending an amateur astronaut to the edge of space aboard one of our DIY rockets. We're now building the Spica rocket to fulfill that mission, and we hope to launch a crewed rocket about 10 years from now.

Copenhagen Suborbitals is the world's only crowdsourced crewed spaceflight program, funded to the tune of almost US $100,000 per year by hundreds of generous donors around the world. Our project is staffed by a motley crew of volunteers who have a wide variety of day jobs. We have plenty of engineers, as well as people like me, a pricing manager with a skydiving hobby. I'm also one of three candidates for the astronaut position.

Keep Reading ↓ Show less