Published: 28 March, 2002

Strength in numbers…and zippers: The secrets of spider silk

What makes spider silk so strong?

What makes spider silk so strong? There are lots of different creatures which make fibers we call silks. Spiders even make several different silks; some are sticky—which makes it hard for dinner to escape from the web. But the strongest silk is dragline silk—the stuff holding the spider suspended as it drops, slowly, from the ceiling onto your pillow and which makes the main part of the spider's web. The best dragline silks are as strong as steel. How do spiders do it? It's a puzzle combining chemistry and engineering—which is what makes it hard, and interesting, to study, as we don't entirely know which piece of the puzzle is most important.

The basic idea requires that we imagine a silk molecule; it looks like a whole lot of zippers connected together by strings. Not just any kind of zipper, either; these are special zippers, which zip on two sides, not the ordinary one-sided zipper. Take a whole bunch of these basic zipper and string molecules and set them down in a pile and get a bunch of people to start zipping them together and you'll end up with a pile of sheets of zippers connected together by strings. Now we can yank on all the strings so the ends are pointing in the same direction and voila, we have a fiber of silk. And it's strong because lots and lots of zippers are holding everything together.

Of course, nature doesn't make zippers and strings—here's where the chemistry comes into action. Silks are examples of proteins, made from a basic set of twenty molecules known as amino acids. When the spider wants to make something that acts like a zipper, it puts a string of similar sized amino acids—typically glycine or alanine, the two smallest amino acids—together. When stretched out straight, the amine group from one amino acid can form a weak bond known as a hydrogen bond to the acid group found right next door—forming one of the links in the zipper; get enough hydrogen bonds all in a row and you have the whole zipper. If the sizes of the amino acids are similar, then lots of them can be stacked next to each other. Different silks have different numbers of links per zipper and different amino acids forming the links, so not all the zippers are exactly the same strength.

The stringy portions of the silks are important too; in the best silks, the connections between zippers are made of amino acid combinations that are stretchy, like a small piece of rubber band. So that when a fly comes blasting into the web, there's a little give; the web stretches but doesn't break. That's, again, where the chemistry comes in—as different combinations of amino acids can change the properties of either the zippers or of the strings. But that's all that silk is, molecular zippers connected by strings.