Iridescence in nature How to disappear completely Animals use all sorts of optical trickery to make themselves invisible to predators. So what can we learn from the natural world in developing new camouflage materials? Quite a lot, says Hayley Birch

Wrap yourself in aluminium foil, the defence industry, can offer only scraps do exist in the public domain take yourself off to the woods and In short the briefest of glimpses at the work make for absorbing conversation. you’ll all but disappear, laughs  Materials scientists they do and the materials they are camouflage expert Adam Shohet. are looking to mimic developing. More than once, Shohet, Tricks of the light But he’s only half joking. Although optical techniques research and innovation manager For a start, camouflage isn’t all about it might only take one rustle of your used by animals, such for the company’s Farnborough- green and brown paint. As Shohet shiny cloak to rumble you, there’s as cuttlefish, squid, based stealth materials group, has explains, it’s fairly easy to spot nothing ridiculous about the idea butterflies and moths to apologise for his generalisations someone trying to hide themselves of using reflective materials as  Squid skin is when pressed for details. Still, what with chemical pigments or paints. camouflage. It’s a strategy animals designed to mirror have been using for millions of years, back its surrounding and one that organisations like environment, rendering UK-based QinetiQ – where Shohet itself almost invisible works – would like to be able to copy.  Squid can also Unsurprisingly, however, those dynamically change their involved in this intriguing area of camouflage to match research are almost as difficult to environment changes, pin down as their study subjects. and copying this is the Cephalopods, including squid, are ultimate aim for military masters of disguise in the marine camouflage environment, and attracting interest from the sort of funders that prefer to keep their findings under wraps – most obviously, the military. From an outsider’s perspective, it’s only a matter of time before one of these undercover research teams works out how to disappear completely. Iridescent proteins in the

Even QinetiQ, as consultants to skin of squid US BARBARA, SANTA CALIFORNIA, OF UNIVERSITY DEMARTINI, DANNY 42 | Chemistry World | June 2010 www.chemistryworld.org RICHARD HERMANN/VISUALS UNLIMITED, INC. UNLIMITED, HERMANN/VISUALS RICHARD

‘In most cases, colour can only be producing an iridescent sheen much Cuttlefish, despite being Although it’s true that the Blue seen at quite close range,’ he says. like the structures in some birds’ colour blind, blend well Morpho’s wings look more blue ‘So you might get away with not feathers and butterfly wings. into their background because they contain blue pigments, matching colour particularly well.’ Similar principles are employed this is not the main reason we see Cuttlefish, the camouflage kings, in thin-film optics to make photonic blue. A close look at the wing scales are colour blind, yet they match crystals and anti-reflective coatings under an electron microscope colour very effectively. According to (see box, p45) for glasses, often reveals a regularly spaced array of Shohet, the fact that they don’t need by physical or chemical vapour biopolymer.2 ‘The blue is because to be able to see their background deposition. These rely on creating of the structure not because of to blend into it hints that it’s not all periodically repeating structures or the pigments that are there,’ says about colour matching. layers in the range of the wavelength Srinivasarao. A better strategy, and one that the of visible light; the reflective Although he’s no squid expert, cephalopods employ successfully, properties arise from interference Srinivasarao does believe that is to try to become completely with the incoming light waves. inspiration for manmade materials reflective. So like the foil cloak in In the natural world, peacock can come from the natural world. the woods, the skin of a squid is feathers are examples of complex One of the routes researchers at mirrored to reflect back as much photonic crystal structures, ones Georgia Tech are currently pursuing, of its surrounding environment that scientists have recently used he says, is trying to mimic the as possible. And in the featureless as templates to make novel, tunable structures on butterfly wings. In environment of the ocean, those photonic materials – light emitting work presented at an American mirrors become almost invisible. But nanoparticles are embedded into Physical Society meeting in March what is it that enables the squid to the regular structure of the feather, this year,3 the team succeeded in do this? Its secret lies in soft optical which serves to control the light.1 replicating the green on the wings of materials and, more specifically, in One of the best known examples the Emerald Swallowtail – formed the layer of cells called iridophores of ‘structural colour’ in the animal ‘Camouflage from multiple layers of the polymer that lurk below the coloured kingdom is the Blue Morpho isn’t all about chitin and air – using self-assembly pigment sacs in the squid’s skin. butterfly, according to Mohan and deposition of titanium dioxide These contain proteins with a very Srinivasarao, a physical chemist green and and aluminium oxide in very thin particular structure responsible for at Georgia Tech in Atlanta, US. brown paint’ layers. www.chemistryworld.org Chemistry World | June 2010 | 43 Strap in here like this

Squid switch Blue Morpho butterflies So does structural colour or look blue to the human iridescence work for camouflage eye due to the regular as it does for the brightly coloured structure of their wings be of interest to military funders. have definite architectures such displays of butterflies? Sönke But Johnsen says his team work as helices or sheets). But chemical Johnsen, a biologist at Duke purely on the basic science level stimulation by a neurotransmitter University, Durham, US, who last – his funders ask him not to think causes the polymers, which are year received a US navy grant of too deeply about possible military otherwise repelled from each other $7.5 million (£5 million) to study applications as all of their results are by their positive charge, to gain cephalopod camouflage, explains published in public journals. negative phosphates that allow them what’s special about the squid – Last year, one of Johnsen’s to agglomerate. In more neutral they can do it dynamically; they collaborators, Alison Sweeney, conditions, aromatic interactions realign the protein structures currently at the University of begin to dominate and the proteins responsible for their iridescence in California at Santa Barbara, US, organise themselves into stacked, order to match their surroundings. published a paper that reveals more plate-like structures – essentially, In the rapidly fluctuating light details of the reflective structures they ‘switch on’ iridescence. fields near the sea surface, this in a cephalopod commonly fished More recently, Sweeney and can mean constant readjustment. around the Californian coast – the co-workers proved that varying ‘They can change it on a dime,’ says longfin inshore squid (see Chemistry the thickness of the platelets could Johnsen. ‘They switch from one World, November 2009, p28). ‘There produce colour shifts right across optical characteristic to another, are iridescent cells and then darkly the visible spectrum.4 They also so they could be reflecting blue pigmented cells on top of those, and went on to suggest that soft protein light and then they can tell their ‘Squids can the two of those working in concert materials such as these could have cells to change and all of a sudden constantly are responsible for these dynamic biomedical applications, for instance they’re reflecting green light.’ This camouflage changes,’ explains in smart artificial lenses with self- ability to adapt instantaneously to readjust their Sweeney. In the skin of the squid, correcting focal lengths. But this environmental changes requires camouflage she explains, the arrangement of is not the first time the potential of softer, more flexible materials than the proteins in the iridescent cells is so-called ‘reflectin’ proteins has the hard chitin found in butterfly to match their completely disordered. (This is fairly been recognised. In a 2007 Nature scales and is clearly one that would environment’ unusual since cell proteins tend to paper, scientists at the Air Force 44 | Chemistry World | June 2010 www.chemistryworld.org Research Laboratory in Dayton, US, very big difference between using cast reflectin proteins – engineered pigments for camouflage and using to be manufactured in bacteria – in reflective structures,’ he says. ‘One films of varying thickness, resulting of the reasons we’re particularly in a range of different structural interested in using structural colours MATERIALS NATURE colours.5 They also showed it is because if you take a chemical was possible to induce dynamic pigment, there’s a limit on how

PASIEKA / SCIENCE PHOTO LIBRARY LIBRARY PHOTO SCIENCE / PASIEKA iridescence by exposing reflectins much light you can actually reflect, to water vapour, which makes them which is why the cuttlefish, octopus swell and changes their reflectance and squid use reflective elements – shifting from one wavelength to underneath their absorption layer another. so that they can actually reflect more light back.’ So no matter how good Colour or structure? the colour match, he says, someone Back at QinetiQ in the UK, sitting in bright sunlight and only Shohet is cagey on the subject of reflecting 50 per cent of the light controlling dynamic iridescence, but that hits them is always going to be recognises ‘it would be a problem too dark. for a manmade material’. Certainly, Adaptive camouflage also relies translating the way the squid do on making a useful assessment of it into something workable in a your environment and this is one synthetic material seems complex. important aspect of cephalopod But Shohet says there are already a microfluidic system that could Squid reflectin proteins camouflage that materials scientists some materials that mimic, broadly control the movement of tiny stack into plate-like must rely on behavioural biologists speaking, the layers in a squid’s skin. volumes of coloured chemicals to structures, switching on to unpick. Fortunately, Johnsen, In the squid, the pigment – acting and from the surface of a material, iridescence in collaboration with Sweeney, is like a filter – resides in a layer of enabling a colour change.6 Or preparing to build a Star Trek-style cells overlying the reflective plates even coloured electroactive holodeck where they will observe in the iridophores. Similarly, says polymers – polymers that change the squid’s responses to virtual Shohet, displays currently being their shape when a current is surroundings captured by virtue marketed by electronic giants like applied – that would act like the of a six-headed camera. Johnsen Sony and Sharp use liquid crystals muscles surrounding the pigment is hopeful this will eventually to create a multilayered effect, sacs, which stretch open when the lead to materials applications. except underneath is an absorption squid wants to make larger dots ‘It’s tricky because animals have layer rather than a reflective layer. of colour. ‘Electroactive polymers one big advantage in that they’re These kinds of low power displays are essentially very small scale alive and their cells are capable of are ‘bi-stable’, meaning they only actuators,’ says Shohet. ‘So this is an changing things at tiny size scales require power if the image needs area that is quite interesting to look that are still pretty challenging to be switched – an advantage at because it is a way of making a sort for the average engineer,’ he says. that could be equally valuable in a of very small muscle.’ ‘We’re hoping to find things that

PATRICK LANDMANN / SCIENCE PHOTO LIBRARY LIBRARY PHOTO SCIENCE / LANDMANN PATRICK military situation. It’s still the idea of completely could then be converted over to Alternatively, mimicking the reflective materials, though – the human technology, but it’s always a pigment layer rather than the foil in the woods idea – that Shohet challenge.’ reflective layer, one might imagine thinks is most promising. ‘There’s a Srinivasarao, on the other hand, suggests the feat of dynamic iridescence may already have been Anti-reflection achieved. ‘For camouflage, there are people doing this, but they are all Tuning reflectivity is not just funded by the government and so if about turning it up. Anti-reflective they can do it very well, they are not materials are ‘key to military going to tell you,’ he says. ‘My guess camouflage’, says Shohet, for is that some of the guys actually instance in reducing glint on the know how to do it.’ And that, says lenses of binoculars used by Shohet, is a fair comment. the military. Inspiration for anti- reflective materials has come Hayley Birch is a freelance science from moth eyes, one of the most writer and editor based in Bristol, UK DAVID SCHARF/SCIENCE FACTION/CORBIS SCHARF/SCIENCE DAVID widely cited examples of where natural optical materials have References been translated into biomimetics. 1 J Han et al, Nanotechnology, 2008, 19, Compared to the human eye, which Moth eyes inspired artificial materials 365602 reflects back about four per cent ‘If the military 2 M Srinivasarao, Chem. Rev., 1999, 99, 1935 3 C J Summers, American Physical Society of all the light that hits it, the moth capable of suppressing reflection. have achieved meeting, March 2010, http://absimage.aps.org/ eye only reflects back about 0.1 The synthetic equivalent uses, for image/MWS_MAR10-2009-001691.pdf per cent. This is down to the fine example, silicon nanotips etched dynamic 4 A R Tao et al, Biomaterials, 2010, 31, 793 nanostructure of the eye – a regular into a silicon wafer.7 Such materials iridescence, 5 R M Kramer et al, Nature Mater., 2007, 6, 533 pattern of tiny bumps smaller than may also be useful in maximising 6 A Shohet and C Lawrence, J. Defence Sci., they are not 2006, 10, 252 the wavelength of visible light and absorption of light in photovoltaics. 7 Y-F Huang et al, Nature Nanotech., 2007, 2, going to tell you’ 770 www.chemistryworld.org Chemistry World | June 2010 | 45