Cuttlefish are well known for their brilliant colour changes, and are even known as the “chameleons of the sea”. Sometimes they produce a zebra stripe pattern that even appears to move across the body. They have several ways of doing this, and one of them has inspired scientists to design a new type of TV screen, with very low power requirements—under 1% of normal screens.
Cuttlefish have several types of structures that allow it to change colour rapidly
Chromatophore
This is a group of cells that include an elastic saccule that holds a pigment, plus 15–25 muscles attached to this saccule. When the muscle contracts, it stretches the saccule so it covers a wider area. Chameleon colour2 is also largely caused by chromatophores, but cuttlefish chromatophores each have a nerve ending. This allows finer control, so one saccule can be expanded while the neighbouring ones contract. This allows the cuttlefish to produce complex patterns that can change quickly.
Iridophore
These are tiny stacks of plates, which act as a diffraction grating, and produce iridescent colours. Iridescent blues in butterflies and birds are also produced by diffraction, i.e. splitting and spreading out the different colours of the spectrum. Depending on the spacing and the angle of the observer, different colours are seen. These are called structural colours, since they depend on the structure of the material rather than a pigment. In the cuttlefish, iridophore colours are relatively fixed, but hormones do cause some change.
Leucophore
These are similar to iridophores, but are flat and more orderly plates that reflect light rather than diffract it. Their colour matches the surrounding light: white light will produce a white shine, but if incoming light is a different colour, then that’s what will be reflected. This helps with camouflage.
Photophore
This actually emits light, rather than absorb (pigment), diffract (iridophore) or reflect (leucophore) already existing light. They use bioluminescence, or producing light from a chemical reaction with very little heat. Sometimes, these creatures have sacs containing bioluminescent bacteria in a symbiotic relationship.
A team led by Edwin Thomas at the Massachusetts Institute of Technology knows a good technology when they see it, and described it in the journal Advanced Materials. Dr Thomas explains, evidently talking about the iridophores:
Cuttlefish change their color by secreting different chemicals to change the spacing between membranes. … We have created an artificial electrical system to control the spacing between layers.3
The prototype screen is a few inches square, but only a micron thick (a thousandth of a millimetre). Filling this micron are very thin layers of alternating “dirt cheap polystyrene” with poly(2-vinylpyridine) (2VP). The former is inert, but the latter expands when a small voltage is applied. By increasing the voltage, the 2VP layer thickness increases, and with it the light wavelength reflected to the viewer.4 So low voltage produces violet and blue light while higher voltages move through the spectrum until red is seen at 10 V.
The screen doesn’t emit light, which is why it needs very low power. But this means it needs an external light source shining on it. And it needs to be viewed from right in front, because the image colour changes with angle.
According to Thomas, it’s very easy to assembly such a screen. So he is working with a local high school science teacher to make a variety that is simple, cheap and safe enough for schoolchildren to build in a chemistry class. But despite its simplicity, Stephen Foulger, of Clemson University, South Carolina, says:
The tunability of these systems is fantastic. There is a huge span of colors and applications.
This is hardly the first time that Nature—or rather, the Designer of Nature—has surprised scientists with sophisticated and efficient ways of producing exquisite colours and patterns (see articles below5).6
