Sublime Slime
Evan Zabawski | TLT From the Editor June 2013
Slugs trail a unique lubricant.
Slug slime is a non-Newtonian, hygroscopic, highly organized polymeric fluid.
On a recent hike through the forest, my wife pointed out a slug to my daughters, complete with a trail of slime. When we started talking about the slug and some of its features, I thought to myself the answer to the inevitable question about the slime would be this month’s column.
Slug slime performs a myriad of functions, including preventing the slug from drying out, making it less appetizing for predators with its anesthetic properties, offering sustenance and bedding during the act of reproduction and, of course, providing a means to travel. The general supposition is that the slime creates a slippery path for the slug, but it is actually much more complex than that.
Water, mucus and salts are the basic ingredients of slug slime, which belies the fact that it is a non-Newtonian, hygroscopic, highly organized polymeric fluid. The mucus is made up of mucins, which are proteins with attached carbohydrates that are able to form sticky, moisture-trapping gels when they are added to water.
The slime becomes very sticky (like rubber cement) at rest but liquefies under application of stress greater than the apparent yield stress. When the stress is removed, the mucus resolidifies. This thixotropic property allows the slug to use a rippling muscle action and its slime to actually grip vertical surfaces with alternating parts of its body and move upwards with the rest of its body.
Slugs do not have protective shells like snails and are much more prone to drying out, so they typically remain out of view in damp places to keep themselves moist and hidden from predators. Slugs will roll up into a little ball covered in protective slime while they lay dormant, waiting for the next rain or heavy fog. The hygroscopic nature of their mucus helps retain their moisture due to its ability to absorb up to 100 times its initial volume in water. Anyone who has gotten slug slime on their hands and tried to wash it off with water may now appreciate why it didn’t seem to come off readily.
University of Washington bioengineering professors Pedro Verdugo and Christopher Viney, together with zoology professor Ingrith Deyrup-Olsen, have discovered that the thickness of the 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.
The thought of all these properties together may lead one to wonder why slugs don’t just explode in the rain or simply start sliding uncontrollably downhill. The answer is that the mucus is ingeniously packaged in granules coated with a cell membrane material that keeps it dry. The membrane apparently breaks open upon contact with extracellular Adenosine triphosphate—a coenzyme used as an energy carrier in the cells of all known organisms. This mechanism allows the slug to carefully control the amount of slime at any given moment.
It would appear that the stickiness of slug slime would preclude it from being defined as a lubricant since lubricants are supposed to reduce friction, but that might not entirely be the case. Slugs do use the slippery nature of their slime as a defense mechanism to slip out of the mouth of a predator when captured.
So the next time you see a slug, you can marvel at the multifaceted functionality of its slime. In fact, the medical community is eyeing the stickiness of slug slime as an alternative bioadhesive to help set broken bones, use during surgery or to deliver drugs.
Evan Zabawski, CLS, is a reliability specialist in Calgary, Alberta, Canada. You can reach him at evan.zabawski@gmail.com.