Silky Muscles

You're running through the cool woods on a hot day, barefoot as dead leaves rustle underfoot and the cold flint tickles beneath. The green leaves and kudzu blur past as you dodge beaming shafts of sunlight and the hot ground they illuminate. You scan the earth ahead for sinkholes and patches of poison ivy, but still, the chilled, humid air coiled around the trees flowing in your ears feels joyous in comparison to the sauna of the open field. You dart between two trees, then suddenly stop and gyrate wildly, windmilling arms about your face as you splutter and ick; swiping instinctually at your face to pull away the clinging threads of a spider web.

To many of us, spiders are mostly nuisances, either by dangling from a single invisible thread in the most inconvenient places or by stumbling into webs and getting their sticky strands stuck in our eyebrows such that we look like a surprised Gandalf. However, a recent publication suggests that spider silk, the material they spin webs and drag lines out of, may turn out to be much more useful that we previously thought.

Compared to contractile biological muscles, mechanical rotary motors are rather inefficient. Getting a micro-servo to function correctly in a robot arm is a difficult art of soldering and fine-tuning. In prosthetic limbs, robots, and industrial applications, there is a current need for a small, reliable, lightweight, and dependable actuator. It turns out that when spider silk is exposed to alternating extremes of ambient humidity*, it contracts much like a biological muscle and does so repeatedly. Many biological fibers (cotton, wool, etc.) can also contract in high humidity, but they can only do so once before becoming inert. This occurs because different fibers are composed of repetitive hydrophilic materials that suck up water and collapse into lower net energetic states when the water is available, as it is during humid conditions.

The really cool part of this research was the force generated by the spider silk. On a basis of equivalent mass, spider silk was found to be capable of doing 500X the work of a human biological muscle. Agnarsson et al calculated that, based on their scaling experiments with combining individual silk fibers, a 2cm diameter strand of spider silk would be capable of lifting 2tons of mass! Similar, though weaker, effects were observed in silkworm silk (which is already commercially availble).

The caveat to this, because there's always a caveat, is the degree to which the spider silk contracts. Human muscle is capable of elastic modulus (how much it can bunch up without breaking) of 30-40%, while spider sillk was found to be capable of a modulus of only ~2%. This is considerably less useful, but still cool. The researchers noted that this was all done in one particular species of spider, Nephila clavipes, and that the silk of other spider species may turn out to have more useful modulus while preserving greater scaling strength and simple humidity switch.

Let's hope that these tests are done and something found, because robots everywhere are itching for a change.

*Steps of 10% differences. Contraction was found to be irreversible after exposure to <70%>

Agnarsson, I., Dhinojwala, A., Sahni, V., & Blackledge, T. (2009). Spider silk as a novel high performance biomimetic muscle driven by humidity Journal of Experimental Biology, 212 (13), 1990-1994 DOI: 10.1242/jeb.028282