For thousands of years people have worn shimmering silk to stand out in a crowd. Within the next few years people could wear silk to become invisible in a a crowd.
For the first time ever, scientists have created an invisibility cloak made from silk, and coated in gold.
The new metamaterial, as invisibility cloaks and their kin are technically called, only works on relatively long terahertz waves (a region of the electromagnetic spectrum between radio and infrared light), but the Boston-area scientists who developed the technology think that silk could work as an invisibility cloak at much smaller wavelengths, even in the visible range.
The research could lead to a wide range of optically unique materials for use in biomedicine or defense.
"This is an unusual angle for a metamaterial because of silk's ability to interface with the human body," something that no other metamaterial is currently capable of, said Fiorenzo Omenetto, a scientist at Tufts University who, along with colleagues at Boston University, helped develop the silk-based metamaterial and detail their new research in the journal Advanced Materials.
"On the sensing side it gives you a platform that is very adaptable."
Invisibility cloaks, along with their optically exotic cousins, perfect absorbers and perfect reflectors and others, belong to a special class of materials known as metamaterials. Unlike most materials, which derive optical properties like color from their chemical make up, metamaterials derive their properties from the physical structure.
A curly cue, or short spiral, is a common metamaterial structure. Scientists call them split ring resonators, or SSRs. Usually scrawled into metals, SSR can give ordinary materials extraordinary abilities, like absorbing or reflecting all the specific wavelengths of light, or bending a wavelength around an object.
To create their silk-based metamaterial, the Tufts and Boston University scientists, including Richard Averitt, started with a one-centimeter-square piece of silkworm silk. (In another recent paper, Omenetto's colleague and another co-author of the Advanced Material's paper, David Kaplan of Tufts, created silk-producing bacteria.) Onto that tiny piece of dielectric silk they stenciled 10,000 gold resonators.
Ordinarily when silk is exposed to terahertz waves they pass straight through it. When the new silk metamaterial was subjected to T-rays the scientists detected a resonance.
A metamaterial that works in the terahertz range is nothing new. But, unlike other metamaterials, silk is biocompatable -- the human body won't reject silk-based implants the way it does with most other materials. The scientists implanted the patterned silk into a muscle, and still detected a resonance.
The new silk research is "interesting," said Douglas Werner, a metamaterial scientist at Pennsylvania State University. "There is a lot of interest in using flexible substrates for metamaterials, and silk is a good candidate for that."
The potential applications of silk-based invisibility are huge. Omenetto and his colleagues at Tufts aren't even focused on Harry Potter or Star Trek-style invisibility materials, although he says that is one potential application.
Their main focus is in biomedical applications. One of the first biomedical uses could be as an implantable glucose sensor for diabetics. As the level of glucose changes inside the body, it changes the silk. Then as the silk changes, do does the metamaterial printed on the silk. That change would then be relayed to the person's cell phone; no needle prick necessary.
Silk-based invisibility would also allow doctors and radiologists to cloak various organs or tissues and see through them, said Omenetto, getting a better image of the organs or tissues usually hidden behind.
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