Spider silk is renowned for its lightweight yet stronger-than-steel fibers, and now scientists are finding a new use for the fine, strong thread: as an artificial muscle.
Scientists at the University of Akron have developed two new ways to apply spider silk, and normal silk from silkworms, to artificial muscles. If refined and commercialized, scientists say the silk could be used in everything from robotics to microchip systems.
"No one thought about using spider silk as an artificial muscle," said Todd Blackledge, a professor at the University of Akron who studies spider silk. "But wrap a finger of drag line spider silk around your finger and dip it in water, and your finger will turn blue from the contraction of the fiber."
Scientists have known for years that some spider silk contracts dramatically, up to 50 percent, when it gets wet. The phenomena is known as super contraction. Scientists speculate that spiders use super contraction as a way to help tighten their webs when dew appears in the morning.
While powerful, super contraction generally only happens once; for a second contraction the fiber must be dried and manually stretched back to its original length.
Like human muscle, spider silk muscles would have degrees of movement, depending on the amount of water in the air around the muscle. High humidity would mean maximum contraction, low humidity would loosen the treads. Water breaks apart the hydrogen bonds inside the thin strands of protein, causing new, contracting bonds, to form. As the water evaporates the hydrogen bonds reform in their original positions.
While studying super contraction, the scientists found a second type of movement, known as cyclic contraction. Unlike super contraction, which generally is a one-time occurrence, cyclic contraction can instantly expand and contract as the humidity around the fiber changes, no drying necessary. While faster and more responsive than super contraction, cyclic contraction isn't nearly as dramatic; the fiber only shortens by 1 to 2 percent instead of 50 percent.
Those small numbers are fine with Blackledge, who notes that human muscle, despite its seemingly wide range of motion, only contracts between 10 and 20 percent. The new artificial muscle could be used in microelectromechnical systems (or MEMS) — tiny devices that operate on very small scales to, for example, separate individual cells. All a spider silk, cyclic contraction-based spider silk artificial muscle would have to do in a MEMS system is open or close a tiny channel to separate individual cells.
A MEMS system would be a great first use for spider silk artificial muscles because not much silk would be needed. Despite decades of research and dozens of attempts to create artificial spider silk, no one has succeeded in creating an effective replacement. All the silk for an artificial muscle would have to be gathered by hand from real spiders, a fairly labor intensive task.
Luckily, says Blackledge, the same silk worm-based silk that goes into dresses and medical products also exhibits similar contraction properties as spider silk and could also be used for artificial muscles at tiny scales.
Scaling up spider silk for human limb-sized artificial muscles could be problematic, says Brent Opell, a professor at Virginia Tech. A single thread of drag line spider silk is very narrow, about five microns in diameter, and has a high surface area that lets it quickly absorb large amounts of water to quickly contract.
Stacking multiple fibers next to each other will likely slow the diffusion of water through the fiber and slow the speed of contraction.
While there are still multiple issues that still need to be resolved before any actual device using spider silk as an artificial muscle is built, the idea does have its merits, especially when compared to other artificial muscles out there, says Adam Summers, a professor at the University of Washington.
"Spider silk is a remarkably long-lived polymer that would last for tens of thousands of cycles," said Summers.
Other artificial muscles exist that are superior to spider silk in terms of speed or amount of contraction, says Summers, but often require high amounts of electricity or toxic chemicals for activation and break down after a couple hundred cycles.
Blackledge also only studied one of the seven different types of silk produced by one of the more than 40,000 species of silk-producing spider species. Other spider species could produce silk with much higher cyclic contraction percentages or super contraction that has a faster response time.