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Underwater photo of an colourful squid at night

Scientists engineer human cells with squid-like transparency

Image credit: Ribe | Dreamstime

Researchers in the US have engineered human cells to have similar transparent abilities to those found in sea creatures such as squids and octopuses.

Deep-sea creatures such as squids and octopuses can switch on a “camouflage mode” to hide from predatory fish and have cephalopod skinBoth animals are covered in sacs of a red pigment, called chromatophores, which they can expand or contract at will. Under normal ambient light, the sacs are shrivelled and the animals are largely transparent.

Squid and octopuses do this by using specialised tissues in their bodies to manipulate the transmission and reflection of light. Drawing inspiration from this ability, scientists at the University of California, Irvine (UCI) have endowed mammalian cells with tunable transparency and light-scattering characteristics.

“For millennia, people have been fascinated by transparency and invisibility, which have inspired philosophical speculation, works of science fiction, and much academic research,” said Atrouli Chatterjee, a UCI doctoral student in chemical & biomolecular engineering. “Our project – which is decidedly in the realm of science – centres on designing and engineering cellular systems and tissues with controllable properties for transmitting, reflecting and absorbing light.”

For the study, the researchers drew inspiration from the way female Doryteuthis opalescens squid can evade predators by dynamically switching a stripe on their mantle from nearly transparent to opaque white. They then borrowed some of the intercellular protein-based particles involved in this biological cloaking technique and found a way to introduce them into human cells to test whether the light-scattering powers are transferable to other animals.

The black and white phase microscope image above helped UCI researchers identify where the squid reflectin protein nanostructures were present in human cells (dark regions, with some indicated by white arrows). The panel in color shows the associated pathlength for light traveling through a given area (red corresponds to longer pathlengths and blue corresponds to shorter pathlengths).

The black and white phase microscope image above helped UCI researchers identify where the squid reflectin protein nanostructures were present in human cells (dark regions, with some indicated by white arrows). The panel in color shows the associated pathlength for light traveling through a given area (red corresponds to longer pathlengths and blue corresponds to shorter pathlengths).

Image credit: Atouli Chatterjee / UC

Such species of squid have specialised reflective cells called leucophores which can alter how they scatter light. Within these cells are leucosomes, membrane-bound particles that are composed of proteins known as reflectins, which can produce iridescent camouflage.

In their experiments, the team at UCI cultured human embryonic kidney cells and genetically engineered them to express reflection. Here, they found that the protein would assemble into particles in the cells’ cytoplasm in a disordered arrangement. They also saw through optical microscopy and spectroscopy that the introduced reflectin-based structures caused the cells to change their scattering of light.

“We were amazed to find that the cells not only expressed reflectin but also packaged the protein in spheroidal nanostructures and distributed them throughout the cells’ bodies, said Alon Gorodetsky, UCI associate professor of chemical & biomolecular engineering. 

“Through quantitative phase microscopy, we were able to determine that the protein structures had different optical characteristics when compared to the cytoplasm inside the cells; in other words, they optically behaved almost as they do in their native cephalopod leucophores.”

The team also tested whether the reflectance could potentially be toggled on and off through external stimuli. To determine this, they sandwiched cells in between coated glass plates and applied different concentrations of sodium chloride. By measuring the amount of light that was transmitted by the cells, they found that the ones exposed to higher sodium levels scattered more light and stood out more from the surroundings.

“Our experiments showed that these effects appeared in the engineered cells but not in cells that lacked the reflectin particles, demonstrating a potentially valuable method for tuning light-scattering properties in human cells,” said Chatterjee.

While invisible humans are still firmly in the realm of science fiction, Gorodetsky said his group’s research can offer some tangible benefits in the near term.

“This project showed that it’s possible to develop human cells with stimuli-responsive optical properties inspired by leucophores in celphalopods, and it shows that these amazing reflectin proteins can maintain their properties in foreign cellular environments,” he explained, adding the new knowledge could also open the possibility of using reflectins as a new type of biomolecular marker for medical and biological microscopy applications.

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