We tend to think of invisibility cloaks as science fiction. But one group of scientists has taken a big step toward making them a reality.
For the first time, scientists at Pennsylvania State University have created synthetic replicas of brochosomes, naturally occurring nanoparticles that could one day be used to make invisibility cloaking devices.
Invisibility cloaking isn’t the only application for synthetic brochosomes. In the next few years, they could find their way into a range of commercial applications — from solar energy to pharmaceuticals, according to lead investigator Tak Sing Wong, professor of mechanical engineering and biomedical engineering at Penn State.
Solving a 70-year-long geometric mystery
Lin Wang and Tak-Sing Wong
Brochosomes are bucky-ball-shaped, hollow nanoparticles covered in holes — known as through-holes — that go all the way through them. This complex structure allows them to absorb or scatter certain wavelengths of light, depending on the size of the brochosome and its holes.
The only place in the world where you can find naturally occurring brochosomes is on the back of a leafhopper — a common backyard insect. Their brochosome coats were first discovered in the 1950s, and they probably help them blend into their surroundings.
Scientists aren’t sure why leafhoppers secrete and cover themselves in brochosomes. Until now, they didn’t even understand the purpose of the nanoparticles’ intricate geometry.
“This is really the first study to understand how the brochosome’s complex geometry interacts with light,” Wong said.
To reach that understanding, Wong and his colleagues had to figure out how to make a replica of a brochosome. After almost a decade of research, they managed to 3D print the world’s first synthetic brochosomes.
The invisibility properties of brochosomes
Lin Wang and Tak-Sing Wong
There are two important elements of brochosome geometry: the diameter of the particle, and the diameter of its through-holes.
If a wavelength of light is the same length as the diameter of the brochosomes, it will be scattered in all directions when it hits the particle. But if the wavelength of light is the same length as the diameter of the brochosomes’ through-holes, it will pass through the particle and get absorbed by it.
This absorption coupled with light-scattering means that brochosomes have very limited light reflection — and can be invisible over certain electromagnetic ranges. Covering an object in them could, in theory, work as an invisibility cloak.
The beauty of synthetic brochosomes is that they could be made at different sizes, and thus tailored to absorb and scatter different wavelengths across the electromagnetic spectrum. That means that engineers can customize brochosomes for specific functions, such as invisibility to infrared radiation to help with military defense.
In fact, Wong’s brochosomes are the right size to do that. They’re about 40 to 50 times larger than naturally occurring ones, and they only interact with infrared radiation. Wong’s future research will partly focus on making smaller synthetic brochosomes to target the shorter end of the electromagnetic spectrum.
The commercial potential of brochosomes
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Although Wong’s synthetic brochosomes mark a major step towards invisibility-cloaking technology, scientists are still decades away from bringing anything to market.
“I think in my lifetime, it’s possible,” said Hao Xin, a professor of electrical and computer engineering and physics at the University of Arizona who was not involved in the study. It will take at least 50 years, he said.
But in just three to five years, Wong hopes to produce brochosomes on a large enough scale to use them in pigments, pharmaceuticals, and solar panels.
For example, titanium oxide, a white pigment that’s used in everything from candy to sunscreen, was recently banned as a food additive by the European Union. Wong believes that brochosomes could eventually replace titanium oxide in foods like candy and coffee creamers.
“Depending on our imagination, I think there are many cool applications that can come out of brochosomes,” Wong said.
