Tag: biomimetic

Spider silk

Photo by Fir0002/flagstaffotos (GFDL license)

I’ve never worked on spider silk myself, but my work on synthetic polymers and biological physics took me to conferences where spider silk work was presented and it always struck me as a very interesting. Spider silk has a rather impressive set of material properties, yet it is produced rapidly at the back end of a spider under everyday conditions. This is a pretty electron micrograph of spider spinnarets from where the silk comes (warning: page includes creepy crawlies).

I introduced molecules, and proteins back in this post. Proteins are the key molecules used to make organisms, an organism’s DNA are the instructions to make a set of proteins. Spider silk is made from protein. A spider is able to produce a whole range of silks with different physical properties: dragline silk is used to make the outer-rim and spokes of a web and is strong and tough; capture-spiral silk is sticky, stretchy and tough; tubiliform silk is used for egg cases and is the stiffest; aciniform silk used for wrapping prey is the toughest; minor-ampullate silk used to make temporary scaffolding for building a web (it’s not as strong but very stretchy). From a technical point of view “strong” refers to how hard it is to stretch something, and “tough” refers to how hard it is to break something. Spider silk is similar to silkworm silk but it is stronger and more extensible.

The properties of spider silk arise from it’s microstructure, essentially the protein molecules make a very fine net held together with little crystals. The fact that crystals form is a function of the protein structure, exactly how many and what distribution of crystals form is influenced by how the spider treats the silk-protein solution as it comes out of it’s spinnarets. Precisely how the spider achieves this isn’t entirely clear, the protein starts off in a liquid solution, the spinnarets force the liquid out into the air whilst changing things such as the salinity, concentration and pH of the liquid and “Hey, presto” it turns into silk! It would be nice if we could farm spiders for their silk unfortunately this is difficult, they just don’t get on with each other.
The strength of natural materials is often compared to that of steel, but there is a trick to watch out for here: the comparison is often based on weight. Steel is about x10 denser than silk, so your strand of equivalent strength is rather fatter if it is made from silk.

The closest synthetic material to spider silk in terms of it’s strength per weight is Kevlar. Kevlar is processed using hot sulphuric acid under high pressure which as you might imagine is not very nice. Spider silk, on the other hand is made at room temperature and pressure from an aqueous solution of benign materials.  Not only this, a spider can eat the silk it’s already made and use it to make more silk. As scientists, this makes us more than a little bit jealous.

Not only is spider silk interesting of itself, but from a material scientist point of view, it really isn’t fun to make and use new polymers (you need to build expensive plant to make them, you need to work out your ingredient supply chain, you need to check for safety and environmental problems). If, on the other hand, you can get the properties you want from one of your pre-existing polymers by changing the microstructure then life is much easier. Spider silk may provide hints as to how this might be done.

The neat thing about this story is that it illustrates an important point: we can genetically engineer bacteria and goats to produce the protein in spider silk but not make nice silk-like stuff. Knowing the sequence of amino acids that a spider is making is not enough to make silk. In much the same way knowing the proteins that go up to make up a human is rarely enough to understand, let alone cure, a disease. 

Scientists have done research on the effect of different drugs on web spinning, filmmakers have made some fun of this experiment* (warning: contains spiders). Other interesting biomaterials include, mollusc adhesive and slug slime and I’ve already written about why butterflies are blue.

Update: Curtesy of @happymouffetard, the evolutionary origin of spider-silk spinnarets appears to be hair follicles, according to this article.

*Thanks to Stephen Curry for pointing me to the “spiders on drugs” video.

Why is that butterfly blue?

Some colours come from the properties of individual molecules, some colours come from the shape of things. This is a post about the colour from the shape of things – structural colour, like that found in the Morpho rhetenor butterfly pictured on the right.

To understand how this works, we first need to know that  light is a special sort of wave known as electromagnetic radiation, and that these waves are scattered by small structures.

For the purposes of this post the most important property of a wave is it’s wavelength, it’s “size”. The wavelengths of visible light fall roughly in the range 1/1000 of a millimetre to 1/2000 of a millimetre. (1/1000 of a millimetre is a micron). Blue light has a shorter wavelength than red light.

The Spectrum of visible light (Image from Wikipedia)

Things have colour either because they generate light or because of the way they interact with light that falls upon them. The light we see is made of many different wavelengths, the visible spectrum. Each wavelength has a colour, and the colour we perceive is a result of adding all of these colours together. Our eyes only have three different colour detectors, so in the eye a multiplicity of wavelengths is converted to just three signals which we interpret as colour. The three colour detectors are why we can get a full colour image from a TV with just three colours (red, green and blue) mixed together. Some other animals have more colour sensors, so they see things differently.

The problem with viewing the small structures that lead to the blue colour of the butterfly wings is that they have interesting features of a size about the same as the wavelength of light, and that means you can’t really tell much by looking at them under a light microscope. They come out blurry because they’re at the resolution limit. So you resort to an electron microscope, electrons act as a wave with a short wavelength so you can use an electron microscope to look at small things in much the same way as you would use a light microscope except the wavelength of the electrons is smaller than that of light so you can look at smaller things.

So how to explain resolution (how small a thing you can see) in microscopy. I would like to introduce you to a fresh analogy in this area. Summon up in your mind, a goat (tethered and compliant), a beachball (in your hands), and a ping-pong ball (perhaps in a pocket). Your task is to explore the shape of the goat, by touch, via the beachball, so proceed to press your beachball against the goat. The beachball is pretty big, so you’re going to get a pretty poor tactile picture of the goat. It’s probably going to have a head and a body but the legs will be tricky. You might be able to tell the goat has legs, but you’re going to struggle to make out the two front legs and the two back legs separately. Now discard the beachball and repeat the process with the ping-pong ball. Your tactile picture of the goat should now become much clearer. The beachball represents the longer wavelength of light, the ping-pong ball the shorter wavelengths of electrons in an electron microscope.

And now for scattering; retrieve your beachball; step back from the goat. You are now going to repeatedly throw beachball and ping-pong ball at the goat and examine where the balls end up having struck the goat. This is a scattering experiment. You can see that how the ball bounces off the goat will depend on the size of the ball, and obviously the shape of the goat. This isn’t a great analogy, but it gives you some idea that the shape of the goat can lead to different wavelengths being scattered in different ways.

So returning to the butterfly at the top of the page, the iridescent blueness doesn’t come from special blue molecules but from subtle structures on the surface of the wings. These are pictured below, because these features are smaller than the wavelength of light we need to take the image using an electron microscope (we are in ping-pong ball mode). The structures on the surface of the butterfly’s wing look like tiny Christmas trees.

Structures on the surface of a morpho butterfly wing (scale bar 1.8 micron)

These structures reflect blue light really well, because of their shape, but not other colours – so the butterfly comes out blue.

Another example of special structures that interact with light is this is a *very* white beetle:

Cyphochilus beetle (Image by Peter Vukusic)

The cunning thing here is that the beetle manages to make itself very white, meaning it reflects light of all wavelengths very efficiently, using a very thin scales (5 micron). This is much better than we can achieve with synthetic materials. The trick is in the detail, once again the scales have a complicated internal structure as you can see in this image from an electron microscope:
Cross-section of a beetle scale (scale bar is 1 micron, Image by Peter Vukusic )

It turns out that the details of the distribution of the scale material (keratin) and air in the scale conspire to make the scale highly reflective. Making things white is something important to a number of industries, for example those that make paint or paper. If we can work out how the beetle does this trick then we can make cheaper, thinner, better white coatings.

Finally, this is something a little different. If you’ve got eyes, then you want to get as much light into them as possible. The problem is that some light gets reflected from the surface of an object, even if it is transparent – think of the reflection of light from the front surface of a clear glass window. These structures:

The surface of a butterfly’s eye (scalebar 1micron, Image by Peter Vukusic)

known an “anti-reflective nipple array”, are found on the surface of butterfly eyes. The nipples stop the light being reflected from the surface of the eye, allowing it instead to enter the eye. Similar structures are found on the surface of transparent butterfly wings.

In these cases animals have evolved structures to achieve a colour effect, but more widely we see structural colours in other places like rainbows, opal, oil films and CDs. The sky is blue for a related reason…

The work on butterflies and beetles was done by a team led by Peter Vukusic at Exeter University: