Spider Webs May Mimic Muscles and Serve as Future Medical Device MaterialSpider Webs May Mimic Muscles and Serve as Future Medical Device Material

spider-web

A plastic weight is suspended from a single spider silk thread and subjected to the changes in relative humidity shown in each frame. The initial increase in humidity resulted in a large displacement during supercontraction. Subsequently, the silk thread lifted the 9.5 mg weight through seven drying cycles. The average displacement during each contraction was 0.65 mm, or 1.7% of the thread's total postsupercontraction length. (Image courtesy of the Company of Biologists.)

With toughness, extensibility up to 30%, and tensile strength about five times stronger than that of steel of the same density, spider silk is being investigated by researchers interested in harnessing the material's mechanical properties to make a range of future medical products--from adhesives and sensors to drug-delivery devices. But researchers are also interested in spider silk's cyclic contractions, which are controlled by changes in humidity. This property may eventually enable scientists to use the material to mimic muscles.Todd Blackledge, a researcher in the biology department at the University of Akron (OH), found that by restricting a 5-µm-diameter spider silk thread in a force machine and cycling humidity levels, the silk could exert a cyclic force. "I felt that this cyclic humidity response could be an interesting approach to design biomimetic muscles using spider silk," remarks Ali Dhinojwala, professor in the department of polymer science at the University of Akron. "We made a slight 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 and Blackledge's team indicate that silk generates work 50 times greater than the equivalent mass of human muscle--a much better result than most synthetic materials developed so far. "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," Dhinojwala says.The researchers believe that the magnitude of the stress generated by spider silk is directly proportional to changes in humidity. 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, 5-µm diameter fiber to lift at least 100 mg. The lifting response occurs within 3 seconds of the change in humidity. "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."Demonstrating the use of dry and wet air to control the contraction and relaxation of spider silk and silkworm fibers, the scientists 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," Dhinojwala explains. "Thus, cyclic contraction of spider silk may result from a relatively general response of biological tissues to humidity."The scientists emphasize that they are more interested in the potential applicability of silk as a biomimetic muscle than in the causes of the cyclic contraction. They foresee that silk engineering could perhaps serve as an attractive technology for future lightweight biomimetic muscles.