Friday, May 29, 2009

The new CRT? (Cuttlefish-Related Technology)

Wouldn't it be great if reading your computer screen were just like reading a Kindle, which is just like reading a piece of paper? Think of the reduced eye fatigue. No more late nights lit only by the glow of the monitor. You'd have to turn the light on to be able to read your screen. (Seems almost anachronistic, doesn't it? But that's technology--two steps forward--whoops, that second step wasn't so good--one step back--wait, one step forward?)

But for now, ambient-light screens are just so much science fantasy. We'll be screen-bathing in front of our LCDs for years to come. Right?

Maybe not. Folks at MIT have been working on ambient-light screens that run on orders of magnitude less power than a traditional backlit screen. Their article "Bioinspired Electrochemically Tunable Block Copolymer Full Color Pixels" is a pretty full meal, but here's the quick snack version, as near as I can figure:

They made a new kind of pixel, whose color can be changed ("tuned") by firing an electrical charge into it. It doesn't generate any of its own light, so its color range is only as good as the ambient light it can reflect. If it's exposed to full-spectrum white light, it can be any color of the rainbow. Which wavelength (=color) it reflects depends on the actual physical structure of the pixel.

This sounds weird, right? Why should the structure of an object determine its color? Cubes, spheres, and plates are all equally capable of being vivid red or yucky green.

That is true on a macroscopic level, true for objects that you can see and handle. But on a microscopic level, the shape, size, and structure of an object are integral to its color. That's because visible colors are just wavelengths of light, between 380 and 750 nanometers in length. So a nanometer-scale object can get all up in a color's business, and have an actual physical altercation with blue, if it's feeling feisty.

The new pixels created by Walish et al. are just that tiny. They're a type of object called a 1D photonic crystal, which consists of lots of very thin layers. When the layers are closer together, they reflect shorter wavelengths (read: blue) and when they are further apart, they reflect longer wavelengths (read: red). Fine. The truly amazing part is that these guys figured out is how to spread a gel (that's the "block copolymer") between these thin crystal layers, like a multi-decker jelly sandwich using a miraculous kind of jelly than expands and contracts when you jolt it with varying voltages. (And if there's anything scientists are really good at, it's got to include jolting systems with varying voltages!)

As far as I can tell by wading through this study, which is by the way the first article in Advanced Materials that I have ever read, they made a single pixel. One. And they can change its color from red to green over the course of a few seconds. Normally you watch movies at 29.97 frames per second. So, it's kind of a far cry from a TV screen.

So why the big honking deal? Why do I even know about this, and why did I feel compelled to wade through the article at all? The answer, as always, is that there's a cephalopod involved.

One of the most wonderful aspects of being known far and wide (read: to
friends and family) as a cephalopodiatrist is that I no longer have to
read the news. Oh, no. It comes to me. And you know which cephalopod has been in the news lately? Cuttlefish. And TV screens.

To put it kindly, the media are doing some pretty advanced yoga to be able to stretch that far. Here's the connection, as worded by Walish et al:

Fish such as the blue damselfish, neon tetra, and paradise whiptail all employ 1D photonic crystals in which the lamellar spacing is controlled through chemical secretion by the sympathetic nervous system. Color change in the squid Lolliguncula brevis is also thought to be controlled by secretion of a chemical trigger causing changes in the thickness of the high refractive index protein platelets within the iridophore resulting in a change in reflected color.

Despite the fact that this mechanism (electrical signals changing the spacing in a layered crystal) is known to be employed by fish and thought to be employed by squid, the only picture of an animal in the paper is, in fact, of a squid (Figure 1a). I guess cephalopods are just that much more photogenic than fish. Sorry, vertebrates!

Anyway, the cephalopods horned their way into the paper in the form of the lovely brief squid (really, that's its common name!). So where do the cuttlefish come in? reported:

"Cuttlefish change their color by secreting different chemicals to
change the spacing between membranes," explains MIT-professor Edwin
Thomas. "We have created an artificial electrical system to control the
spacing between layers."
Who's to say what happened in the interview--maybe the interviewer had never heard of squid, only cuttlefish, so Prof. Thomas tried to put it in terms the poor guy would understand. We'll never know.

In any case, I now feel obligated to explain that cuttlefish (and squid, and octopuses) use a lot more than just 1D photonic crystals (which are called iridophores when they're living in cephalopod skin, by the way) to change colors. In fact, the more prominent cephalopod color-changing mechanism is the chromatophore system.

This is where cuttlefish really shine (only, not really, because "shining" would be a better description of the iridescent structural colors produced by iridophores, and . . . shut up, self!) Anyway, cuttlefish have the highest DPI of any cephalopod. That is to say: they have more chromatophores per square inch of their skin than any squid or octopus. How many? We're talking up to 500 chromatophores per square millimeter. That's over five hundred DPI*.

And each chromatophore is considerably more complicated than just a "dot." A chromatophore is a tiny elastic sac of pigment surrounded by muscle fibers, like spokes on a wheel. When the muscles are relaxed, the chromatophore is just a small dark pinprick at the center of the "wheel." When the muscles contract, they pull on all sides of the naturally elastic chromatophore, and the sac has no choice but to expand, becoming a spot of color on the skin--black, brown, red, or yellow, depending on the color of the pigment filling the sac. (Cephalopods seem to have partitioned their chromatophore pigments to take care of the warm colors, while the iridophore structures produce cool colors.)

How does the octopus, squid, or cuttlefish contract the chromatophore muscles? The same way you or I contract muscles: by sending a nervous signal, a natural jolt of electricity. You may remember that's how the iridophores are controlled, too. This means that each of the animal's millions of chromatophores and iridophores is under direct nervous control. That's the secret. That's why cephalopods are the color-change masters of the world. Chameleons? Forget them. They actually change their colors through chemical diffusion, a painfully slow process in comparison with electrical signals zipping down the nerves of a squid or cuttlefish. Chameleons take multiple seconds to change colors. Cephalopods can flash multiple complex patterns across their entire bodies in the space of a second.

If you are feeling jealous of the cuttlefish's incredible resolution and frame rate, I cannot help you. Not yet. There are no dynamically changing cephalopod-inspired clothes that I am aware of. (Materials scientists? Can you get on that please?) But if you can be content with static designs, there are some really wonderful wearable cephalopods. I have a list! And I finally figured out how to move it over to my blog! So, check it out, over there on the left under "pages": Ceph Gear.

* I have just learned that dpi is a 1D measurement, so, literally dots per inch, not per inch squared. I was always a little fuzzy on that. Now I know! Thanks, live-in tech support!

Tangential note: As far as we know, cephalopods are color-blind. Isn't that crazy? The evidence for color-blindness is both mechanistic and behavioral. We can't find any mechanism whereby they could distinguish colors, since they have only one visual pigment. And we can trick them behaviorally by placing them on patterns with different colors of exactly matched intensity (so the only way to tell them apart is by wavelength, just like those red-green bubble tests). And the cephalopods can't color-match. But . . . it's still so hard to believe.


  1. and all this without neural crest! I think people tend to study evolution like a game of chess (or psychedelic croquet?) but it is in fact more like Calvinball. How do you study Calvinball??

  2. I think that's one reason I like the Red Queen Hypothesis so much. It carries the connotation that the rules of the game are changing just as fast as the players and the plays.

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  10. That's because visible colors are just wavelengths of light, between 380 and 750 nanometers in length.