Of Slug Slime, Spider Silk, and Mollusk Shells

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Several UW researchers are taking their inspiration from nature in designing new materials. In a revolution called "biomimetics"--mimicking biological systems--they are unlocking the secrets of natural materials and applying that knowledge in the hope of producing man-made substances that are stronger, lighter, cheaper than conventional materials, and easier on the environment to boot. In one instance, this research has yielded an important clue to a long-standing medical mystery as well.

[Electron micrograph of abalone shell]
Electron micrograph of abalone shell

Consider the case of the common banana slug. You might not think the slug could be a boon to science. But UW bioengineering professors Pedro Verdugo and Christopher Viney, together with zoology professor Ingrith Deyrup-Olsen, have been studying the properties of slug mucus, and have found it's quite a remarkable substance. Slugs use it for protection and as a sort of slithery railway for transportation. Previously, researchers thought that at the molecular level, slug slime was like a bowl of spaghetti--the more tangled the strands, the thicker the mucus. But in 1993, Verdugo and Viney showed that slug mucus is not random; it is a highly organized polymeric material. Furthermore, when secreted, the polymer absorbs water extremely rapidly—up to 100 times its initial volume.footnote 1 "Add water and stir" takes on explosive proportions when it comes to mucus.

The UW researchers discovered that the thickness of mucus in general is controlled by the swelling of the polymer network, and that swelling, in turn, is governed by the saltiness of the water it picks up. This discovery was the first direct link between thick mucus and the defective transport of chloride ion that are both characteristic of the disease cystic fibrosis.footnote 2

Deyrup-Olsen has been studying the basic mechanisms by which slug mucus is secreted. The slug cells cleverly package up the mucus in granules, coating the mucus with a layer of cell membrane material, thereby keeping it dry until it is well outside the cell. Then these packets break open (apparently upon contact with extracellular ATP), exposing the mucus to water.

The researchers foresee many potential applications of slug slime technology in materials science and bioengineering: new drug delivery systems, pollutant traps for sewage treatment plants, and water-based lubricants, for example.

UW materials science and engineering professor Mehmet Sarikaya describes another avenue of exploration in the burgeoning field of biomimetics. The abalone, for instance, produces a shell so tough it can be run over by a truck without breaking. The organism manufactures its beautiful and lustrous abode in the most benign of conditions: at sea-water temperature, using readily available starting materials--mainly calcium carbonate, or chalk. Studying the structure under an electron microscope, Sarikaya and colleagues have discovered that the part of the shell called nacrethe shiny inner "mother-of-pearl"is made of thin layers of laminate. Tiny stacks of overlapping, six-sided calcium carbonate "bricks" are bound together by a mortar of protein and sugars. The structure is as much as 30 times stronger than calcium carbonate made in the laboratory.footnote 3 Sarikaya and former UW professor Ilhan Aksay, now at Princeton, initiated a project with medical geneticist Clement Furlong and microbiologist James Staley to develop techniques of artificially "growing" novel materials patterned after the way an abalone produces its shell.

Viney and colleagues have brought their expertise in liquid crystals to bear on the subject of silk spun by spiders and silkworms. Liquid crystals are liquids in a highly ordered state, something between a liquid and a solid crystal. They are most familiar as the display screens for digital watches.

In a 1991 paper published in the scientific journal Nature, Viney and colleagues reported that spider silk owes its exceptional strength to temporarily becoming a liquid crystal. The researchers found that as the spider secretes its silk, molecules in the droplets align themselves in rod-like structures, passing through a semi-ordered liquid crystal phase. The resultant solid material can support far more weight for its size than steel. Can new bulletproof fabrics and stronger suspension bridges be far away?

  1. "Doing What Comes Naturally," L. G. Blanchard, Columns, September 1993, p. 20.
  2. "Brighter Future Ahead for Cystic Fibrosis Patients," Medicine Northwest, Summer 1987, p. 18.
  3. "Science Takes a Lesson From Nature, Imitating Abalone and Spider Silk," Warren E. Leary, The New York Times, Aug. 31, 1993, p. C1.

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