A chameleon or cuttlefish can change color because its skin cells, known as chromatophores, have contractile fibers that shift pigments around. The pigments are either spread out to show color or squeezed together to make the cell clear.
Cambridge researchers built artificial chromatophores based on the same principle.
The team — somewhat unintentionally — developed an artificial “chameleon skin” that changes color when exposed to light. The material supports a range of possible applications, including active camouflage and large-scale dynamic displays.
Instead of using nature-like contractile fibers, the material's color-changing abilities rely on microscopic drops of water and light-powered nano-mechanisms.
The “skin” is made of tiny gold particles that are coated in a polymer shell and then squeezed into microdroplets of water in oil. When exposed to heat or light, the particles stick together, changing the color of the material.
“Putting these into small droplets of water was a curiosity-led idea,” said the study's lead author Dr. Andrew Salmon, who spoke with Tech Briefs. “We didn’t know what would happen.”
When the material is brought to above 32 °C, nanoparticles store large amounts of elastic energy in a fraction of a second, as the polymer coatings expel all the water and collapse. The sudden energy forces the nanoparticles to bind together into tight clusters. When the material is cooled, the polymers take on water and expand, and the gold nanoparticles are strongly and quickly pushed apart, like a spring.
“Loading the nanoparticles into the microdroplets allows us to control the shape and size of the clusters, giving us dramatic color changes,” said Dr. Salmon.
When the nanoparticles are spread apart, they appear red. When they cluster together, the particles are dark blue. Additionally, the droplets of water also compress the particle clusters, causing them to shadow each other and make the clustered state nearly transparent.
At the moment, the single-layer material is only able to change from red to blue. More colors are likely possible, however, as the Cambridge researchers experiment with different nanoparticle materials, shapes, and extra layers to make a fully dynamic material, like real chameleon skin.
In an interview with Tech Briefs, Dr. Salmon explains the kinds of applications he hopes to see in the future.
Tech Briefs: What inspired you to make this artificial skin?
Dr. Andrew Salmon: Actually it was not something we really planned! The basis for the work is the temperature switchable nanoparticles. Putting these into small droplets of water was a curiosity-led idea; we didn’t know what would happen.
When we tried it, we found that the color change was much larger than expected. The droplets help to localize the particles so that they can shadow each other. We realized that this was the same mechanism demonstrated by certain chromatophores, the skin cells in animals like cuttlefish and chameleons that allow them to change color. Specifically, it mimics the type called melanophores. Based on that, we made films of the droplets and it worked beautifully.
Tech Briefs: What is most exciting to you about this work? Which applications are most exciting to you?
Dr. Salmon: I think the most exciting thing is being able to use nanomachines to deliver a real visible effect. In movies you see nanomachines that are like tiny robots. In reality we are a long way from being able to do that, and anyway you can’t just scale down something big and expect it to work.
We really have to go the other way: Understand how things operate on that scale, and work upwards. I think we are just starting to get there in making nanomachines that can really do things. Being able to go from nanodevices not that much wider than a DNA helix to a dynamic-colored skin that you can hold in your hand is amazing, I think.
For applications, right now we are working on integrating the switching mechanism into tests for diseases. The idea is the color change can be used to clearly indicate the result of the test.
Tech Briefs: What kinds of color-changes are possible currently, and what can be done with this kind of color-changing control?
Dr. Salmon: We use round gold nanoparticles, which have a ruby red color when spaced apart. This is due to the interaction of the free electrons of the metal and the electric field of the light. The electrons oscillate with a natural frequency, like the surface of a drum.
Because we have used this type of nanoparticle, the color can change between ruby red and a more transparent blue. The color depends on the material and shape of the nanoparticles; there is nothing stopping us from using different types to get different colors. We want to try making multiple layers with different types, so that the color can be switched between them.
Tech Briefs: What was the most challenging part of the design process for you?
Dr. Salmon: If you see a color change, how do you know what is happening? The closest you can come to directly looking is electron microscopy, but then the sample has to be dried down, so it doesn’t really tell you what they are doing in the droplets. We have to use a mixture of tricks, tests, and computer modeling to figure it out. It is tricky.
Tech Briefs: What’s next regarding this research?
Dr. Salmon: I already mentioned that we want to couple the color-change mechanism to healthcare tests. That is going to be a big focus. Another thing we spotted is that the artificial cells can “swim” when light is shone on one side of them. This is really remarkable, and if we could generate collective behavior, like flocks of birds, that would be even more interesting. Potentially we could use this to move cargo around precisely on a tiny scale.
What do you think? Share your comments and questions below.
The results of Dr. Salmon's research are published in the journal Advanced Optical Materials.