'Edible optics' could make food safer

Mukerrem Cakmak, a professor of polymer engineering at the University of Akron in Akron, Ohio, said sensor-related research and development has become a booming business, and the market is only expected to grow. The ultimate strength of an approach like the one proposed by Omenetto, he said, will depend in large part upon its relative cost and how widely usable it is. For polymers incorporating sensors, he said, “the name of the game is throwaway — use it once and throw it away. In other words, make it very cheap.”

If silk-based biopolymers haven’t yet met that requirement, the new research suggests that scientists have made strides in stretching the material far beyond its traditional confines.

Tufts collaborators began with silkworm cocoons from Japan or raw silk fiber from Brazil. Eventually left with a solution of pure silk fibroin protein, as it’s called, the scientists poured the silk into casts and then air-dried it to create the thin membranes.

To use structures like silk, gelatin and glass for photonics, researchers typically perturb the surfaces. The rainbow-hued holographic eagle appearing on many credit cards, for example, appears as light bounces off tiny, carefully spaced waves and crevices on the card’s surface, known as holographic diffraction gratings.

If a silk membrane embedded with biosensors boasts a similar diffraction grating, anything the biosensors latch onto will change how light interacts with the waves and crevices — thus changing the membrane’s appearance.

Omenetto and his colleagues used nanopattern-filled casts to etch the silk membranes with holographic diffraction gratings so fine that they contained up to 3,600 grooves in a space roughly the diameter of a pinhead. While the silk was still in the solution, the scientists also demonstrated that it could be embedded with the biologically active hemoglobin, horseradish peroxidase or phenol red compounds.

“Imagine that you have seas of these lines and I’m starting to put biological stuff on it and the lines are perturbed,” Omenetto said. “I’m not going to get the same rainbow as before.”

The result: a colorful indicator that a silk-embedded sensor has found and bound to its target, whether pathogenic E. coli in a supermarket, an environmental pollutant or glucose in a diabetic’s bloodstream.

As another example, Omenetto pointed out that oxygenation levels in the blood are routinely measured through a finger cuff that detects the darker read of deoxygenated blood versus the brighter hues indicating higher oxygen levels. Glucose in the blood, on the other hand, is transparent and much harder to detect.

One solution, he said, would be to create an artificial color signature via a silk biosensor implanted just beneath a patient’s skin. The silk would be embedded with an enzyme that binds glucose.

The more glucose is bound, the more the silk’s surface would be disrupted, creating a defined color change that could be easily monitored by a similar finger cuff — and give patients a respite from the daily hassle of pricking their fingers.

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