By Marlene Cimons, National Science Foundation
Anyone who brushes against a spider web knows how sticky it is. Scientists have long known that spiders make a glue-like substance that causes this stickiness, but until recently, they did not understand its mechanical behavior.
Researchers at the University of Akron believe they have figured out the nature of the microscopic glue-like substance that orb-weaving spiders produce along the round rings of silk they spin as part of their webs. These droplets—three times tinier than the diameter of a single hair—capture flies and other insects that spiders eat.
The drops are composed of highly entangled polymers, which are physically or chemically cross-linked and transmit forces very efficiently. Under a microscope, the researchers pulled on individual glue drops while measuring their force-extension behavior--not easy to do using a tiny probe.
Video: Silk's Many Strengths
They found the material to be both viscous and elastic, properties very valuable in catching fast-flying incoming insects, and in keeping them trapped long enough for the spider to subdue—and devour—them.
Its simplest description, however, is that it’s not like liquid or honey, or even silly putty.
“It feels like chewing gum,’’ said Ali Dhinojwala, a professor and chair of the university’s department of polymer science. “It just keeps stretching and stretching.”
It is also water resistant, a useful feature since spiders work in humid conditions, including rain. In fact, the material loses its stickiness without moisture, “unlike scotch tape, which isn’t sticky anymore once you put water on it,’’ Dhinojwala said. “This glue needs water to be sticky.’’
The information could have widespread applications in the development of new bio-adhesives, particularly in bandages and other products that must retain their stickiness when in contact with water.
“Sometimes you want your bandages to work under water,’’ Dhinojwala said. “Also, there are quite a number of times we want to attach things under water. Water is always a problem with adhesives. But this can hold under water.’’
The researchers, who also included Todd Blackledge, professor of biology, and doctoral candidate Vasav Sahni, studied only orb-weaving spiders, which are commonly found in trees and grasses. As their name suggests, they spin an orb-like web, that is, a web in the shape of a circle, with spokes and rings.
The spiders use different glands in their stomachs to secrete proteins that make up the web.
The thread of the spokes, which provides the web’s strength, comes from a gland known as the “major ampullate.’’ Each spider has two of these. “That thread is well-known for its strength,’’ Dhinojwala said. “It supports a lot of weight. By weight, that silk is stronger than steel.’’
The circles, where the glue is deposited, come from the “minor ampullate’’ gland. Each spider has two of these as well.
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