Science has been trying to learn the secrets of spider silk – and imitate it – for a long time. It’s a worthy goal. Nothing else, artificial or natural, is quite like it. It has five times the tensile strength of steel and triple the strength of the best synthetic fibers. Scientists are beginning to understand the layered crystalline structure that makes spider silk unusually strong [See SciTechStory: New study: Why spider silk is so strong] and are working on artificial materials to mimic it. However, as described and explained by a study conducted at the Technical University of Munich (TUM) and the University of Bayreuth (Germany) there is an aspect to mimicking natural spider silk that presents a problem – storage.
The problem faced by the spider is somewhat like the problem of using epoxy glue. Epoxy requires two components: resin and hardener, but the instant the two components are brought together they start to react, so they have to be stored separately until used. Anyone who has worked with epoxy knows how timing is of the essence when mixing the components. Similarly, the spider doesn’t create its web material from ready-mix. It uses a very clever bit of organic chemistry for storing the long strands of protein that make up the silk.
If the spider stored the finished product, as silk, it would tend to crystallize and form clumps, leading to clogging of the silk duct. So the spider stores the silk protein in a water mixture within its silk gland in a very clever configuration. Using nuclear magnetic resonance spectroscopy (NMR), Franz Hagn and colleagues at the Institute for Advanced Study at the Technical University of Munich (Germany) discovered the silk protein chains are stored with their charged polar areas (hydrophilic) facing outside and the hydrophobic parts on the inside, which ensures solubility of the molecules in the watery environment. Crucially, this hydrophilic/hydrophobic combination (which is quite similar to the way channels in cell membranes work) keeps apart the segments of the silk molecule that would naturally bind to form the finished crystalline structured silk.
Then when the spider spins out the silk, the protein molecules enter the spinning duct where they encounter an entirely different salt concentration. This more highly ionized solution breaks apart the salt ions of the protein chains and the chains unfold. Then the unfolded chains are forced through the narrow spinning duct, and the long protein chains are squeezed into a parallel formation that puts the linking molecules side by side; the crystalline connections are made and the result is stable (linked) spider silk.
Using microsystem technology (that is, working on the micro scale), the TUM researchers have created an artificial spinning duct. Their partners at Bayreuth University are now working on other elements of the spinning apparatus. The goal is to present a complete artificial ‘spider silk’ system, with silk protein and the mechanisms to produce the ultra-strong silk in a variety of sizes. This will have very large number of potential applications, from surgical material to fibers for specialized cloth – all of which are still several years down the development road.