Birefringence of Spider Silk

As planned last week, today I investigated the birefringence of spider silk and its possible applicability in medical technology, etc.

First off, let me explain birefringence. It's the property of a substance where the index of refraction changes based on the polarization and direction of propagation of light.

Basically, polarization is the orientation of an electromagnetic wave, in this case visible light. See, these transverse waves have their E (electric field vector) and B (magnetic field vector) perpendicular (90°) to the direction that the wave is moving. So, a vertically polarized wave traveling in the z direction would look like this:


Where λ is one wavelength. (Note how E oscillates in the x direction and B in the y.)


Okay, so birefringent substances change their index of refraction as the orientation and direction of light changes.

This means that the speed of light through the substance is no longer constant. And if we wanted to know the index of refraction we'd need some math. (This is really simple, but just to get an idea of what's happening. There's a bunch of del's and multivariable calculus in the actual theory.)

We'll say that the index of refraction (n) is equal to v/c. But this time, v will be a function of x, a function of position of a substance on the x axis.

n = v(x) / c

Now, the index is dependent on where you are in the substance.

So what can you do with a birefringent substance?

Some researchers in EPFL's group for Fibre Optics believe that spider silk could potentially be used in chemical detection. Because of the way it can conduct light, they think it could be useful in medical devices to be implanted in a living body. This is also possible because silk is biodegradable.

Birefringence is also used to diagnose diseases, but silk doesn't have an application there. Next time, I will probably look more at applications, and then move on to the different levels/domains of silk's structure.