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Posted: Oct 14, 2009
High-performance artificial muscles from spider silk can be powered by humidity only
(Nanowerk Spotlight) Spider silk is a fascinating biopolymer that is stronger than steel and more elastic than rubber. Most of the world's 40,000 species of spiders produce a silken thread that possesses a unique combination of mechanical properties: strength (its tensile strength is about five times as strong a steel of the same density), extensibility (up to 30%) and toughness (its ability to absorb a large amount of energy without breaking). Researchers are experimenting with spider silk to design better adhesives; advanced materials that are both stretchy and strong; and to get clues for protein engineering (see: "Protein engineering - from the humble spider to the nanotechnology future of material design").
Yet the impressive performance of spider silk is not limited solely to tensile mechanics. Researchers have now shown that silk also exhibits powerful cyclic contractions that are precisely controlled by changes in humidity, allowing it to act as a high performance mimic of biological muscles.
"My colleague from Biology, Todd Blackledge, was discussing his recent results on spider silk," Ali Dhinojwala tells Nanowerk. "He was restricting a very thin thread of spider silk (5 micron diameter) in a force machine and was cycling humidity and was observing a cyclic force exerted by the silk. This cyclic behavior was not discussed in the literature."
Dhinojwala, the H.A. Morton Professor in the Department of Polymer Science at the University of Akron was very intrigued by this observation because materials scientists have been looking for synthetic materials that can work similar to biological muscles.
"I felt that this cyclic humidity response could be an interesting approach to design biomimetic muscles using spider silk" he says. "We made a slightly modification in our experiments. Instead of restraining the spider silk on a force machine, we decided to hang weights. Interestingly, as we cycled the humidity, the weight was lifted up and down in response to changes in humidity."
The calculations by Dhinojwala's and Blackledge's team showed that silk generates work 50 times greater than the equivalent mass of human muscle. The researchers point out that these numbers are also much better than most of the synthetic materials developed so far.
Lifting performed by spider dragline silk during
repeated cycles of wetting and drying. A plastic weight is suspended from a single dragline silk thread and subjected to repeated changes in humidity. The relative humidity is indicated in each frame. The initial increase in humidity results in a large displacement during supercontraction. Subsequently, the single 5.5 µm diameter silk thread repeatedly lifted the 9.5 mg weight through seven cycles of drying. The average displacement during each contraction was 0.65 mm or 1.7% of the thread’s total post-supercontraction length (enlarged views for two cycles are shown at the bottom). (Reprinted with permission from The Company of Biologists)
"It is intriguing that we can do this by changing only humidity instead of all the complex electrical power based muscles that researchers have been working on" says Dhinojwala.
The researchers hypothesize that water molecules cause a general swelling of the silk and their removal during drying results in contraction.
"This is strikingly similar to the mechanism proposed to explain how plant tissues can act as motors – actively expelling seeds from the parent plant and even burying seeds in the ground," explains Dhinojwala. "For instance, differential expansion and contraction on opposite sides of the cellulose awns of wheat seeds causes them to bend under daily fluctuations of humidity thereby burying the seeds in the ground. Thus, cyclic contraction of spider silk may result from a relatively general response of biological tissues to humidity."
For their experiments, the team began the tests at high humidity. Initial drying induces stress (contraction) of ∼40 MPa (ranging from 10–140 MPa across 35 independent tests). The fiber then relaxes back to its original tension when humidity is subsequently increased. This cyclic response – independently of a phenomenon of silk called supercontraction – generates high forces in the silk.
It appears that the magnitude of the stress generated is directly proportional to the change in humidity, thereby providing a
precise mechanism to control the stress generated by the spider silk. The total force generated increases as fibers are bundled together. The cyclic contraction of spider silk can produce work that is sufficient for a single 40 mm long, 5µm diameter fiber to lift at least 100mg. The lifting response occurs within 3 seconds of the change in humidity.
Currently, the main limitation of silk as a biomimetic muscle fiber is the small maximum displacement that can be achieved.
"We have been achieving a strain (or change in length) of about 2% and we need to design strategies to amplify this strain" says Dhinojwala. "But we are confident that the potential limitation of lower strain can be overcome by using larger lengths of silk or through strain amplification."
The scientists point out that their focus in this work is on the potential applicability of silk as a biomimetic muscle, rather than on the causes of the cyclic contraction itself. They see silk engineering as an attractive technology for lightweight biomimetic muscles.
"We are thinking of using this for designing actuators for robots and micromachines, sensors, drug delivery, control valves that respond to humidity, and even power generations," says Dhinojwala. "This could result in a very environmentally friendly and energy efficient mimic of biological muscles that generates impressive power density. Silk thus emerges as an attractive model for biomimetic muscle fibers that could be used for a range of applications in industry and the biomedical sciences."