Squid Inspired Camouflage Coatings

The pencil squid, uncamouflaged. Photo by Steve Dunleavy.
The pencil squid, on the rare occasion it chooses to stand out. Photo by Steve Dunleavy.

By Kate Wheeling
BU News Service

Camouflage patterned uniforms help soldiers vanish into their surroundings during the day, but at night, under infrared imaging, those same uniforms stand out against the environments they were fashioned to resemble. Now researchers at the University of California-Irvine and Caltech found a novel solution to this problem in the animal kingdom’s master of camouflage – the pencil squid.

Squid use a combination of reflective and pigmented cells to control skin coloration and blend into the background. The reflective cells create red, orange, yellow, green and blue color based on the angle that light hits them – the same process makes soap bubbles appear to change color in sunlight. Brown, red and yellow pigmented cells overlay the reflective cells and filter the light that reaches them. Squid use these two cell types in concert to produce colors than span the entire visible spectrum. Researchers zeroed in on a protein called reflectin, a main component of squid reflective cells and demonstrated its practical applications for stealth technologies in a study published July 30 in Advanced Materials.

The team, led by Alon Gorodetsky, Assistant Professor at UCI, created reflectin-coated thin films with tunable reflectance properties – they could manipulate the films to make them appear and disappear under infrared light. To begin, the team engineered E. coli to express a copy of the reflectin protein – a common strategy in protein engineering to produce and purify large quantities of a desired protein. They integrated purified reflectin onto glass by a process akin to spackling a wall. An adhesive layer between the glass and protein coating ensured the protein stuck to the glass and spread out uniformly. Glass is one of many materials options for this technology. This adhesive layer could be used to integrate the protein with virtually anything – plastic, paper, or even cloth. Once assembled, the thin films appeared orange under visible light, but it was their color under infrared light that interested the researchers.

The team then used a chemical trigger, acetic acid vapor, to tune the reflectance of the films over a large wavelength range – beyond what squid can even do, according to Lydia Maethger, an Assistant Research Scientist at the Marine Biological Laboratory in Woods Hole. Researchers compared the infrared reflectance of the films to leaves, which reflect in the infrared and thus appear red when viewed with infrared cameras. When the researchers looked at the reflectin-coated glass with infrared cameras it appeared black. But when they exposed it to acetic acid vapor it appeared red. These changes were reversible, meaning that Gorodetsky and his team could alter the reflectance to match multiple environments.

This is only the first step towards the application of biological camouflage coatings. “Now we need to modify our approach to develop something that’s a little bit more robust and easier to use,” explains Gorodetsky. Acetic acid vapor triggers a reflectance shift well in the lab, but it’s not the best option for the real world. The group plans to look for other chemical or mechanical approaches to induce the same reflectance changes.

Camouflage coatings also need to stand up to the elements. While Maethger is impressed with the tunable range the researchers achieved, she cautions, “If this kind of thing is to be used in the field, it would have to be able to handle a lot of stress.” This isn’t a hard problem to solve according to Gorodetsky. Reflectin is already fairly tough and the thin films could be strengthened by cross linking the proteins.

But robustness won’t be a problem at all if the technology develops the way Gorodetsky envisions it now, “Really I see it as a disposable coating, maybe even something you could put in an aerosol can and spray yourself with and then once you no longer need camouflage, you just get rid of it” by wiping it off or changing clothes.

Fish Depth May Affect Mercury Content

A fish auction in Hawaii sells  local a local shallow swimmer called moonfish, or Opah. (Photo: C. Anela Choy).
A fish auction in Hawaii sells local a local shallow swimmer called moonfish, or Opah. (Photo: C. Anela Choy).

By Poncie Rutsch
BU News Service

For years, scientists have noticed that fish swimming the deep seas contain more mercury than their shallow swimming friends. Now, a recent study from the University of Michigan and the University of Hawaii shows why that discrepancy exists.

The researchers collected nine different fish species and measured the mercury accumulated in the fish tissue. The fish they collected ranged from a lanternfish, which swims as deep as five thousand feet below the surface, to flying fish, which leap out of the water and glide through the air.

Researchers used a large net to catch the nine species of fish tested for mercury in the study.  The net weighs 2000 pounds when it's dry, and consists of different bundles that can be selectively opened to catch fish at specific depths. (Photo: Jeff Drazen).
Researchers used a large net to catch the nine species of fish tested for mercury in the study. The net weighs 2000 pounds when it’s dry, and consists of different bundles that can be selectively opened to catch fish at specific depths. (Photo: Jeff Drazen).

The researchers used isotope analysis to determine what kinds of chemical reactions the mercury had undergone. They found that sunlight may help mercury to degrade and published their findings in Nature Geoscience at the end of August.

Fish eat mercury every day when they consume sea plants containing mercury-consuming bacteria. In a pristine environment, their mercury levels remain constant. But because of human activity, fish mercury levels have increased in the past hundred years.

Scientists worried about mercury in fish for decades and have been studying its origins. Mercury in the atmosphere exists in a vaporous, inorganic state that does not cause significant damage to humans. It settles on the sea’s surface, and collects on sea plants. Here tiny microbes digest the mercury and convert it to methylmercury, or organic mercury, which impairs human brain development. The microbes can convert mercury as deep as two thousand feet below the surface. Fish eat the sea plants with methylmercury accruing microbes, and the mercury accumulates in their tissue.

Because the mercury from each source undergoes different chemical reactions, each has a different chemical fingerprint. This makes it fairly easy for scientists to trace mercury in the atmosphere back to its source. The researchers started with the mercury in fish tissue and determined each reaction that had happened to it on its way to the fish.

Although mercury can cause significant damage to developing human brains, lead author Joel Blum said that the increasing mercury doesn’t seem to harm the fish themselves. “Yet,” he said, “if mercury levels double, will there even be fish?”

Previous research linked the mercury in Pacific fish to coal-fired power plants in Asia. According to co-author Brian Popp, mercury levels in Pacific fish are on the rise as Asian manufacturing continues to increase.

“But in the Atlantic,” Popp said, “we see the opposite, probably because of new regulations in North America and Europe.”

The research connects the mercury cycle to the fish we eat. Popp and Blum agree that because even tiny amounts of mercury can cause so much damage, understanding the cycle is vital for choosing which fish to eat and which to avoid.