science articles

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How to disappear completely

Animals use all sorts of optical trickery to make themselves invisible to predators. Hayley Birch finds out how the natural world can help develop new camouflage materials

In Short

• Materials scientists are looking to mimic optical techniques used by animals, such as cuttlefish, squid, butterflies and moths
  
• Squid skin is designed to mirror back its surrounding environment, rendering itself almost invisible
  
• Squid can also dynamically change their camouflage to match environment changes, and copying this is the ultimate aim for military camouflage
  

Wrap yourself in aluminium foil, take yourself off to the woods and you'll all but disappear, laughs camouflage expert Adam Shohet. But he's only half joking. Although it might only take one rustle of your shiny cloak to rumble you, there's nothing ridiculous about the idea of using reflective materials as camouflage. It's a strategy animals have been using for millions of years, and one that organisations like UK-based QinetiQ - where Shohet works - would like to be able to copy

 

Iridescent proteins in the skin of squid

© DANNY DEMARTINI, UNIVERSITY OF CALIFORNIA, SANTA BARBARA, US

Unsurprisingly, however, those involved in this intriguing area of research are almost as difficult to pin down as their study subjects. Cephalopods, including squid, are masters of disguise in the marine 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.

Even QinetiQ, as consultants to the defence industry, can offer only the briefest of glimpses at the work they do and the materials they are developing. More than once, Shohet, research and innovation manager for the company's Farnborough-based stealth materials group, has to apologise for his generalisations when pressed for details. Still, what scraps do exist in the public domain make for absorbing conversation

 

Tricks of the light

 

For a start, camouflage isn't all about green and brown paint. As Shohet explains, it's fairly easy to spot someone trying to hide themselves with chemical pigments or paints. 'In most cases, colour can only be seen at quite close range,' he says. 'So you might get away with not matching colour particularly well.' Cuttlefish, the camouflage kings, are colour blind, yet they match colour very effectively. According to Shohet, the fact that they don't need to be able to see their background to blend into it hints that it's not all about colour matching

 

Cuttlefish, despite being colour blind, blend well into their background

© RICHARD HERMANN/VISUALS UNLIMITED, INC.

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A better strategy, and one that the cephalopods employ successfully, is to try to become completely reflective. So like the foil cloak in the woods, the skin of a squid is mirrored to reflect back as much of its surrounding environment as possible. And in the featureless environment of the ocean, those mirrors become almost invisible. But what is it that enables the squid to do this? Its secret lies in soft optical materials and, more specifically, in the layer of cells called iridophores that lurk below the coloured pigment sacs in the squid's skin. These contain proteins with a very particular structure responsible for producing an iridescent sheen much like the structures in some birds' feathers and butterfly wings

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Similar principles are employed in thin-film optics to make photonic crystals and anti-reflective coatings for glasses, often by physical or chemical vapour deposition. These rely on creating periodically repeating structures or layers in the range of the wavelength of visible light; the reflective properties arise from interference with the incoming light waves. In the natural world, peacock feathers are examples of complex photonic crystal structures, ones that scientists have recently used as templates to make novel, tunable photonic materials - light emitting nanoparticles are embedded into the regular structure of the feather, which serves to control the light.1

 

One of the best known examples of 'structural colour' in the animal kingdom is the Blue Morpho butterfly, according to Mohan Srinivasarao, a physical chemist at Georgia Tech in Atlanta, US. Although it's true that the Blue Morpho's wings look more blue because they contain blue pigments, this is not the main reason we see blue. A close look at the wing scales under an electron microscope reveals a regularly spaced array of biopolymer.2 'The blue is because of the structure not because of the pigments that are there,' says Srinivasarao

.

 

Blue Morpho butterflies look blue to the human eye due to the regular structure of their wings

© PATRICK LANDMANN / SCIENCE PHOTO LIBRARY (INSET - PASIEKA / SCIENCE PHOTO LIBRARY

)

 

Although he's no squid expert, Srinivasarao does believe that inspiration for manmade materials can come from the natural world. One of the routes researchers at Georgia Tech are currently pursuing, he says, is trying to mimic the structures on butterfly wings. In work presented at an American Physical Society meeting in March this year,3 the team succeeded in replicating the green on the wings of the Emerald Swallowtail - formed from multiple layers of the polymer chitin and air - using self-assembly and deposition of titanium dioxide and aluminium oxide in very thin layers.

Squid switch

 

So does structural colour or iridescence work for camouflage as it does for the brightly coloured displays of butterflies? Sönke Johnsen, a biologist at Duke University, Durham, US, who last year received a US navy grant of $7.5 million (£5 million) to study cephalopod camouflage, explains what's special about the squid - they can do it dynamically; they realign the protein structures responsible for their iridescence in order to match their surroundings. In the rapidly fluctuating light fields near the sea surface, this can mean constant readjustment. 'They can change it on a dime,' says Johnsen. 'They switch from one optical characteristic to another, so they could be reflecting blue light and then they can tell their cells to change and all of a sudden they're reflecting green light.' This ability to adapt instantaneously to environmental changes requires softer, more flexible materials than the hard chitin found in butterfly scales and is clearly one that would be of interest to military funders. But Johnsen says his team work purely on the basic science level - his funders ask him not to think too deeply about possible military applications as all of their results are published in public journals

 

 

Squid reflectin proteins stack into plate-like structures, switching on iridescence

© NATURE MATERIALS

 

Last year, one of Johnsen's collaborators, Alison Sweeney, currently at the University of California at Santa Barbara, US, published a paper that reveals more details of the reflective structures in a cephalopod commonly fished around the Californian coast - the longfin inshore squid (see Chemistry World , November 2009, p28). 'There are iridescent cells and then darkly pigmented cells on top of those, and the two of those working in concert are responsible for these dynamic camouflage changes,' explains Sweeney. In the skin of the squid, she explains, the arrangement of the proteins in the iridescent cells is completely disordered. (This is fairly unusual since cell proteins tend to have definite architectures such as helices or sheets). But chemical stimulation by a neurotransmitter causes the polymers, which are otherwise repelled from each other by their positive charge, to gain negative phosphates that allow them to agglomerate. In more neutral conditions, aromatic interactions begin to dominate and the proteins organise themselves into stacked, plate-like structures - essentially, they 'switch on' iridescence.

 

More recently, Sweeney and co-workers proved that varying the thickness of the platelets could produce colour shifts right across the visible spectrum.4 They also went on to suggest that soft protein materials such as these could have biomedical applications, for instance in smart artificial lenses with self-correcting focal lengths. But this is not the first time the potential of so-called 'reflectin' proteins has been recognised. In a 2007 Nature paper, scientists at the Air Force Research Laboratory in Dayton, US, cast reflectin proteins - engineered to be manufactured in bacteria - in films of varying thickness, resulting in a range of different structural colours.5 They also showed it was possible to induce dynamic iridescence by exposing reflectins to water vapour, which makes them swell and changes their reflectance - shifting from one wavelength to another.

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colour or structure?

 

Back at QinetiQ in the UK, Shohet is cagey on the subject of controlling dynamic iridescence, but recognises 'it would be a problem for a manmade material'. Certainly, translating the way the squid do it into something workable in a synthetic material seems complex. But Shohet says there are already some materials that mimic, broadly speaking, the layers in a squid's skin. In the squid, the pigment - acting like a filter - resides in a layer of cells overlying the reflective plates in the iridophores. Similarly, says Shohet, displays currently being marketed by electronic giants like Sony and Sharp use liquid crystals to create a multilayered effect, except underneath is an absorption layer rather than a reflective layer. These kinds of low power displays are 'bi-stable', meaning they only require power if the image needs to be switched - an advantage that could be equally valuable in a military situation.

 

Alternatively, mimicking the pigment layer rather than the reflective layer, one might imagine a microfluidic system that could control the movement of tiny volumes of coloured chemicals to and from the surface of a material, enabling a colour change.6 Or even coloured electroactive polymers - polymers that change their shape when a current is applied - that would act like the muscles surrounding the pigment sacs, which stretch open when the squid wants to make larger dots of colour. 'Electroactive polymers are essentially very small scale actuators,' says Shohet. 'So this is an area that is quite interesting to look at because it is a way of making a sort of very small muscle.'

 

It's still the idea of completely reflective materials, though - the foil in the woods idea - that Shohet thinks is most promising. 'There's a very big difference between using pigments for camouflage and using reflective structures,' he says. 'One of the reasons we're particularly interested in using structural colours is because if you take a chemical pigment, there's a limit on how much light you can actually reflect, which is why the cuttlefish, octopus and squid use reflective elements underneath their absorption layer so that they can actually reflect more light back.' So no matter how good the colour match, he says, someone sitting in bright sunlight and only reflecting 50 per cent of the light that hits them is always going to be too dark

.

 

Adaptive camouflage also relies on making a useful assessment of your environment and this is one important aspect of cephalopod camouflage that materials scientists must rely on behavioural biologists to unpick. Fortunately, Johnsen, in collaboration with Sweeney, is preparing to build a Star Trek-style holodeck where they will observe the squid's responses to virtual surroundings captured by virtue of a six-headed camera. Johnsen is hopeful this will eventually lead to materials applications. 'It's tricky because animals have one big advantage in that they're alive and their cells are capable of changing things at tiny size scales that are still pretty challenging for the average engineer,' he says. 'We're hoping to find things that could then be converted over to human technology, but it's always a challenge

.'

Srinivasarao, on the other hand, suggests the feat of dynamic iridescence may already have been achieved. 'For camouflage, there are people doing this, but they are all funded by the government and so if they can do it very well, they are not going to tell you,' he says. 'My guess is that some of the guys actually know how to do it.' And that, says Shohet, is a fair comment.

Hayley Birch is a freelance science writer and editor based in Bristol, UK

Anti-reflection

Moth eyes inspired artificial materials

© DAVID SCHARF/SCIENCE FACTION/CORBIS

 

Tuning reflectivity is not just about turning it up. Anti-reflective materials are 'key to military camouflage', says Shohet, for instance in reducing glint on the lenses of binoculars used by the military. Inspiration for anti-reflective materials has come from moth eyes, one of the most widely cited examples of where natural optical materials have been translated into biomimetics. Compared to the human eye, which reflects back about four per cent of all the light that hits it, the moth eye only reflects back about 0.1 per cent. This is down to the fine nanostructure of the eye - a regular pattern of tiny bumps smaller than the wavelength of visible light and capable of suppressing reflection. The synthetic equivalent uses, for example, silicon nanotips etched into a silicon wafer.7 Such materials may also be useful in maximising absorption of light in photovoltaics .

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A new catalyst that generates hydrogen from sea water has been developed by scientists in the US. This new metal-oxo complex displays high catalytic activity and stability, whilst being low cost, the researchers say.

Hydrogen is very attractive as a clean source of power. Currently, it is produced by natural gas reforming - where steam is reacted with methane in the presence of a nickel catalyst to form hydrogen - but this method produces the greenhouse gas carbon dioxide

Jeffrey Long and colleagues from the University of California, Berkeley, prepared a simple molybdenum-oxo complex that can serve as an electrocatalyst, reducing the energy required to generate hydrogen from water on a mercury electrode. As an abundant metal, molybdenum is much cheaper than precious metal catalysts where the costs associated with large scale hydrogen production would be high

The team's molybdenum-oxo species generates hydrogen from sea water

© Nature

Long explains that the stability of the catalyst is due to a ligand that bonds to the molybdenum in five places (pentadentate) making it a very strong complex. 'The molecule is very robust and is stable in aqueous conditions for long periods of time so we don't see degradation of the catalytic activity over three days of running the reaction,' he says.

Significantly, Long's catalyst is also stable in the presence of impurities that can be found in the ocean, meaning that sea water can be used without pre-treatment. The team used a sample of California sea water in the system and found the results to be similar to the results obtained for water at neutral pH. In addition, no other electrolyte is necessary when using sea water, which helps reduce costs and removes any need for organic acids or solvents that could degrade the catalyst.

 

'The work clearly demonstrates that the molybdenum-oxo complex explored shows good catalytic activity, with at least an order of magnitude higher turnover frequency [the speed at which a catalytic cycle is completed] than alternative catalysts quoted,' says Bruce Ewan, an expert in hydrogen production and renewable energy at the University of Sheffield, UK. 'This new catalyst also opens up new possibilities as a catalytic agent in other proton reducing scenarios,' he adds.

Long and his team hope to develop this system so that 'in the future a catalyst like this could be used in conjunction with a solar cell to produce hydrogen,' he explains. The team is now working on modifying the catalyst to reduce the potential at which the electrochemical reaction proceeds and make the system more efficient.

Mike Brown

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There are plenty of reasons oxytocin is referred to as the cuddle chemical. Levels of the hormone surge during caresses, and researchers think it evolved as a way to reduce stress and fear of others long enough to enable contact necessary for procreation. It also helps facilitate bonding between mothers and newborns. But for the first time, scientists have found that Mom's innate ability to soothe — and to boost oxytocin levels — is as powerful whether she's offering a hug in person or consolation over the phone.

 

To understand how mothers can influence levels of oxytocin in their children, researchers at the University of Wisconsin-Madison's Child Emotion Lab recruited 61 girls, ages 7 to 12, and placed them in a stressful situation: they had to give an impromptu speech and solve math problems in front of strangers. Afterward, some girls were allowed to seek refuge in their mothers' arms, others talked to Mom on the phone, and the control group watched an emotion-neutral film (March of the that bored many participants to sleep.

 

The results, published in May in the biological-sciences journal Proceedings of the Royal Society B, showed that oxytocin levels jumped almost exactly as much in girls who were comforted in person as they did in girls who'd been calmed long distance.

 

The findings add to a growing body of research on the impact of oxytocin, which has been shown to promote such qualities as generosity and empathy.

 

Leslie Seltzer, the biological anthropologist who led the study, suggests an evolutionary reason for the soothing power of Mom's voice. When faced with a threat — say, members of a rival tribe — men could choose to fight or take flight, but women's options were complicated by having little ones in tow. Fleeing might expose the children to more danger. That's why, Seltzer speculates, women may have developed the ability to use social bonds to "tend and befriend" — to diminish stress either by touching or by talking. Seltzer's next study: to see if Mom can send some oxytocin love by instant message.

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Snowball Earth

 

According to Paul Hoffman from Harvard, some 700 million years ago all the planet’s landmasses formed a single continent – Rodinia - straddling the Equator. The polar seas began to freeze. Their brilliant white surface reflected sunlight, cooling the planet until the entire Earth froze over. Rather like Jupiter’s moon Europa today (pictured), our planet was a cosmic snowball. Life could only survive at hot volcanic vents on the ocean floor.

 

Hoffman believes that volcanoes eventually pumped enough carbon dioxide and other greenhouse gases into the atmosphere, to heat the Earth until the ice began to thaw. As more of the dark ocean surface was exposed, it absorbed more of the Sun heat: the seas became warmer and the ice rapidly melted.

Hammer from the Heavens

Mike Baillie, of Queen’s University, Belfast, has been checking out wood from ancient forts in Northern Ireland. Within the tree-rings of the ancient oak logs, he’s found one band where the rings are very narrow – a few years when the tree struggled hard to survive. Baillie dates it to AD 536.

 

That had to be a period when the climate was extremely bad. Indeed, chroniclers from around the world – from Italy to China – complained about the bad weather and consequent famine. Baillie links it to the appalling conditions that were supposed to follow the death at that time of Britain’s warrior hero, King Arthur.

 

Baillie blames the appalling conditions on a comet (like Hale-Bopp, pictured below) that impacted the Earth, and spread cooling dust around our planet – a bigger cousin of a comet or asteroid that was seen to explode over Siberia in 1908.

 

 

That’s not the only danger we face from the sky. Jan Veizer of the University of Ottawa has been looking inside meteorites, to see how the Solar System – including the Earth – is affected by cosmic radiation, as the Sun moves around the Milky Way Galaxy. He has found the radiation peaks every 143 million years – matching the changing temperature of the Earth. Veizer proposes that the cosmic rays would seed clouds on Earth, so reducing the planet’s temperature. He claims that – through the geological past – cosmic rays have been twice as effective as carbon dioxide in controlling Earth’s temperature.

 

Whatever effects the Cosmos has on us, it’s something we have no hope of controlling – and not even of predicting. As the dinosaurs found out 65 million years ago, if you’re unlucky enough to be in the path of a runaway asteroid, then the climate is going to be unimaginably extreme. When that asteroid hit the Earth, it created a fireball at over a thousand degrees that literally roasted the dinosaurs alive.

 

That was the ultimate “fry” in the Earth’s long history, just as Snowball Earth was the ultimate “freeze”. These geological perspectives prove that our current global warming is a mere blip as compared to the changes that nature can inflict on our planet.

But that’s no reason for us to be complacent. Though our planet has survived these extreme climate swings, and life has somehow pulled through, the Earth’s cycles of freeze and fry have always destroyed the dominant species of their timeâ €¦

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Are you unhappy with how much or how little time you spend with your partner? How much time should couples spend together? Here are some tips here which might help you decide

I have known people who feel that a good relationship means you have to spend all of your free time together. They socialize together, they spend all of their free time together, they even go grocery shopping or run errands together! I have to say that if I spent that much time with my husband on a regular basis, things would not be good in my household.

This type of relationship can be smothering. Spending more time together also means there are more opportunities to disagree and fight. Often, there are other issues at work as well. If one person is insisting on spending all of this time together when the other person truly does not want to, this is a sign of a controlling or abusive relationship

 

You are only one-half of a couple - you are an individual and you should not be defined solely as a part of a marriage or a pair. Your happiness should not be dependent primarily upon another person.

 

On the other hand, I also know couples who NEVER spend time together. They live in the same house but they are basically roommates. They don't do anything together except perhaps activities for their children. They never have one-on-one time to work on their relationship and never spend time together outside of their family. This is not healthy either. They are supposed to be committed to each other after all - whether it is being married or living together. The relationship is about them. If there is no "them", then there is no relationship.

 

Balance, of course, is the key. There are some couples where the scenarios above work for them. However, I doubt that they are truly happy together. There are some folks who just drift their life and don't think much about the details. This may work fine for you but you should want more than "fine" out of your life. You should find joy and satisfaction in your life and in your relationship. In order to have a happy and healthy couple, you need to be happy and healthy individuals. Just existing and getting through the day isn't necessarily the same thing as being happy.

So, what can you do? The best course of action is to change your relationship as it is right now. If you are spending all of your time together and that isn't working for you, try something different. Make an effort to engage life as an individual. Take some time for yourself - spend time away from the relationship. Reconnect with family and friends. Or, just do something by yourself - shopping, a spa day or take in a game. If your partner gets angry or can't understand, this is a sign that there is something seriously wrong with your relationship. If you can't break away, see a counselor so you can find the strength to do so.

 

If the other scenario is the case, then you need to reconnect as a couple. Go on a date or, even better, get away together. This should be something that occurs regularly - not just once in a while. Even if it is just putting the kids to bed early and having a romantic dinner at home - this is a step in the right direction. Set up a "date night" and commit to it. It may not happen every time - life does get in the way, after all. But it should happen more often than not. The laundry will still be there tomorrow and the kids will be fine for a couple of hours with a sitter. The goal is to remember that you are a couple and to make that a priority.

Remember - healthy couples are made up of healthy people. You are an individual outside of your relationship. At the same time, being a couple takes commitment. It doesn't just happen. Being together isn't necessarily the same as being a couple. Work on these goals even if it means taking a break from your life as it exists now.