Spider silk: the new Kevlar?

A recent article in PLoS One (Agnarsson et al. 2010. Bioprospecting finds the toughest biological material: extraordinary silk from a giant riverine orb spider. PLoS One 5 e11234 doi:10.1371/journal.pone.0011234) claims to have discovered the toughest known biomaterial: silk from “Darwin’s bark spider”, Caerostris darwini.  The reason they looked at the properties of silk from this particular spider was the extraordinary size and shape of the webs it builds.  C. darwini likes to build its webs across streams, rivers and even small lakes, and this means that the lines supporting the webs have to be long enough to bridge from one bank to another.  The authors measured bridgelines 10-14 meters long, and webs of 2.8 m(2).  [How the spider gets across the river in the first place is an area of active investigation.]

C. darwini spider webs. From Agnarsson et al. doi:10.1371/journal.pone.0011234

It seemed likely to them that this lifestyle might have led to the evolution of even stronger silk than is normal for spiders, and so they captured several female spiders and forced them to extrude silk by pulling.  Then they measured the properties of the silk.

There are many ways to measure how strong a material is.  One measure is the force required to break a fiber (scaled to the size of the cross-section of the fiber); another is the breaking strain, how much the material deforms before breaking.  The authors focus primarily on toughness, the amount of energy a material absorbs before rupturing.  By this measure, C. darwini silk is truly extraordinary, absorbing approximately 10x as much energy as Kevlar before rupturing.  (Kevlar, famously, is tougher than steel).  It’s recently been reported that it’s possible to spin spider silk from protein produced in bacteria, and that the fibers produced have properties comparable to the natural silk.  That study used silk proteins from Nephila clavipes, which (according to Agnarsson et al.) produces silk that is significantly less tough than that of C. darwini.  It’ll be interesting to see to what extent the properties of the material are intrinsic to the protein monomer, rather than to higher-order matters such as crosslinking among chains.

Agnarsson et al. argue that it is very likely that yet stronger silks remain to be discovered.  The properties of spider silks have only been carefully investigated for a few species, covering only a tiny fraction of the genetic and ecological diversity of spiders.  The property you want to measure also depends on the application you have in mind; if you measure toughness at two different rates of pulling, the measurements may not match, and which silk is better may also change.  The authors suggest that thinking about the ecological niche a species inhabits may — as in this case — provide important clues to finding even more extraordinary biomaterials.

So, one day, police officers may wear (spider) silk shirts instead of Kevlar vests.  Much more stylish, if a little predatory in symbolism.

Agnarsson I, Kuntner M, & Blackledge TA (2010). Bioprospecting finds the toughest biological material: extraordinary silk from a giant riverine orb spider. PloS One, 5 (9) PMID: 20856804