When you stretch a rubber band, all of the parts of the rubber band move in unison. When you stretch a more complicated material, this is not necessarily the case.
Below is a video of a 3D collagen gel (which you cannot see) and the fibers of the gel are decorated with small fluorescent particles (bright dots). The video is a minimally artistic compilation of 4 different parts of the gel. Instead of stretching the gel, I have applied a shear deformation by moving a boundary, causing the network to move. In the video, you can imagine the top edge moving to the right, while the bottom edge moves to the left. During this deformation, I am using confocal microscopy to image inside the network.
While this collagen network is deformed, you can see that the particles attached to the fibers move both along the direction of shear and in bursts; sometimes these even move in and out of the plane. This is very different than what is seen in a rubber band. Hopefully by understanding how these particles move, we can gain insight into how structural changes in collagen affect its strength.
Many people ask me, “what is the difference between spider and silkworm silk?” Most commonly my answer involves how spiders have many different kinds of silk (specialized for either web building, egg protection, etc.) while silkworms only have one. I am involved in silkworm silk research mostly because silkworm silk is significantly easier to acquire in bulk, and you don’t need a room full of spiders (Kate already dislikes being in the lab as is). But when people study spider silk, it is often the dragline because its the strongest version of silk created by the spider.
Kraig Biocraft Laboratories has received a US Army contract to use their “dragon silk” to make a bulletproof material.
“Dragon silk,” completely unrelated to dragons, is a genetic insertion of a spider’s dragline silk DNA into the silkworm DNA. So these special silkworms will make cocoons out of a hybrid “dragon silk” that is just as strong as spider silk, but easy to access.
However, I am interested but a bit skeptical about using silk as a replacement for Kevlar. Naively you can draw comparisons between the toughness of silk and Kevlar, but the purpose of Kevlar is to absorb the bullet’s energy in a short amount of time – silk needs to stretch significantly before it can achieve the same energy storage, but it is still possible.
I look forward to seeing how silk does in this next challenge. Maybe American soldiers will be wearing silk in the near future à la 13th century Mongolio.