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.

Time to catch up

I haven't posted in about a week, so this is to account both for this week (which was really one day because I was absent Wednesday) and last week. 

I been mostly looking at the optical properties of spider silk. As I discussed among the many applications of spider silk was the possibility of using it in place of fiber optic cable (FOC). At the time of my presentation, I had yet to learn more about it since I was looking in to other things. I have now been investigating this. What I have found so far is very promising. 

First let me explain the index of refraction. The index of refraction is defined as:

n ≡ c/v 

where n is the index, c is the speed of light in a vacuum, and v is the speed of light through a particular substance. The index of space is obviously 1, and air is very close to that. Normal FOC typically has an index of about 1.44 which is pretty good. There is only about a 31% decrease in the speed of light, which is still ridiculously fast. (~2.083X10⁸ m/s) 

Now, spider silk has been found to have an index of about 1.55. That's still good, but not as good as FOC. Spider silk reduces the speed of light by ~36%. So, between FOC and spider silk, there's only a 5% difference in speed. While it would take light traveling the distance to the moon 1.85 seconds through FOC, it would take 1.98 seconds through spider silk. So the question I have to answer now is: is it worth it to reduce the speed?

FOC typically has a diameter of 50 microns (50 millionths of a meter) not including the layers of insulation and protection around it. Spider silk has a diameter close to 7 microns. This means that you have more channels to transport information in the same space, ~7 times as much. 

Also I've considered too the production of FOC vs. silk in making it available for common and universal use. FOC can now be made at 50 m/s. Silk likely can't compete with that speed. At best, you can only produce a few meters per second. 

The process of manufacturing FOC is a very complex process that involves chemical vapor deposition, drawing, and coating. The manufacturing of spider silk is also complex, as discussed in my presentation. However, silk has no bi-products and the lab setup is simple.

FOC also has some practical issues. It's glass, so it's not nearly flexible as silk. It also has issues with installation because of its maximum tensile strength. 

 

Thinking holistically, you have to make the best use of time, resources, money, and space for this to be a feasible option. Spider silk as a product itself would reduce the normal speed by 5% and is manufactured at a slower rate. However, it is smaller in diameter, is cheaper to make, is clean, and more durable. 

Of course I may be biased because this is my thesis and I want silk to be used anywhere possible, but I think silk could be a viable alternative to fiber optic cable despite any cons. I will continue to investigate this one application as I move on to others. 

Next week, I'll look at applications based on its birefringence.

Birefringence is the property of a material where the index of refraction changes based on the polarization and propagation direction of light. Think liquid crystal displays in alarm clocks. The display changes as you angle it away or towards you. This property is made use of in numerous medical instruments etc. 

Should be interesting...

- Noah

Back in the swing

Today, since we finished all the presentations, I went back to working on my thesis.

the feedback that I got for my presentation was helpful, and I'll use it for next time.

So, today I took a closer look at the quaternary structure of the proteins (in aqueous solution anyway). Apparently it doesn't have any "considerable" secondary or tertiary structure. "Particularly in their repetitive core domains, however, the long repetitive sequences permit weak but numerous intra- and intermolecular interactions between neighboring domains and proteins upon passage through the spinning duct." And it's because of these interactions through the extrusion, that the secondary through tertiary structures emerge as the proteins polymerize.

Cool stuff.

Also, "the high electron density regions comprise crystalline sub-structures with high β-sheet content.39 These sub-structures are thought to be responsible for the mechanical strength of the silk thread. The elasticity of silk is based on the areas with low electron density, which are characterized by amorphous structures with few defined elements of secondary or supersecondary structure.40,41 Such arrangement closely resembles that of protein hydrogels.42 Upon tensile loading, the hydrogel-like areas can partially deform, contributing to the elasticity and flexibility of the thread." 

This gives me a better understanding of the chemical/physical reasons for the rigidity/elasticity.

I'm really enjoying this topic, and I can't wait to present my finished project! I should probably start that paper soon...

All in a day's work,

- Noah