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'Edible optics' could make food safer

You’re in the produce aisle doubtfully eyeing those serrano peppers and fresh spinach. Are they really safe to eat?

You’re in the produce aisle doubtfully eyeing those serrano peppers and fresh spinach. Are they really safe to eat?

A clear answer could soon come in the form of a color-changing hologram patterned onto an edible membrane of pure silk, according to a new study.

The translucent membranes, dubbed “edible optics” by their creators at Massachusetts’ Tufts University, have been studded with tiny grooves and crests to create the kind of light-diffracting holograms more often associated with credit cards and passports. Unlike your Visa or MasterCard, however, you can eat silk with no ill effects, suggesting a future in which pure fibers fashioned into films or membranes are embedded with biological sensors and incorporated into produce bags to warn of E. coli or salmonella contaminants.

Because silk is biocompatible and already used for surgical sutures, the Tufts researchers also are envisioning an implant whose changing colors would correspond with varying blood glucose levels in diabetics. And unlike plastic, silk membranes biodegrade over time, meaning disposable biosensors could be deployed en masse as environmental (and environmentally friendly) monitors.

Several tech blogs have run wild with other potential applications for the material, suggesting everything from E. coli-sensitive meat wrappers to edible silk underwear.

Fiorenzo Omenetto, an associate professor of biomedical engineering and physics at Tufts, is a bit more restrained with his projections. Nevertheless, he said silk’s unique properties are providing an exciting bridge between the fields of biopolymers and photonics, or the science of manipulating light.

Nature's solution
The fibers spun by the humble silkworm are sturdy enough that a single membrane can be scored with miniscule patterns producing the same kind of light interference that produces holograms, causes butterfly wings to appear iridescent and gives opals their opalescence.

“In a nutshell, you have this unique aggregate of proteins so you can play the properties off each other,” Omenetto said. “You can have something you can eat and something that can sense at the same time.”

Gelatin and other biopolymers have been used as optical materials for decades, and Omenetto cited efforts in the ‘70s to make gelatin lasers. But a gelatin lens is too wobbly to be well-suited for applications requiring patterning at the nanoscale level. An optical device based on glass or semiconductors (typically silicon-based) would do the trick, but it’s “not something you want to eat too much of,” he said. Nor could such a device host biologically active components because of the harsh chemicals and high temperatures normally required to construct it. And semiconductors, glass and other polymers aren’t exactly biodegradable.

Nature, however, seems to have provided an ideal solution. For years, researchers in China and Japan have been working on creating new biomaterials from silk, and scientists have figured out how to immobilize enzymes such as glucose oxidase and horseradish peroxidase onto thin silk membranes. By combining that know-how with the ability to convert silk fibers into translucent films and etch the surface with ultra-fine patterns, Omenetto’s team has been able to further expand the material’s potential.

“You don’t have to refrigerate it, you don’t have to cook it or raise the pH,” he said of the membrane.

The relatively gentle processing of silk fibers from silkworm cocoons, in fact, is another key advantage.  In the September issue of the journal Biomacromolecules, Omenetto and his collaborators described embedding the compounds hemoglobin, horseradish peroxidase, and a pH indicator called phenol red into silk membranes. Months after being stored on a shelf, all three still retained their biological activity.

Although the research is preliminary, Omenetto said his team has more results on the way that suggest readouts from silk-embedded biosensors could be efficient enough to be seen with the naked eye. That success would embolden one of the group’s favorite “what if” scenarios: adding silk films embedded with bacterial sensors to bags of spinach, peppers or other produce.

Every time the sensors latch onto bacterial proteins, for example, the interaction would obscure part of the highly patterned silk surface. The resulting color change due to light interference could instantly warn consumers of any contamination.

Despite the recent high-profile bacterial outbreaks linked to serrano peppers and spinach, Omenetto conceded that bringing a silk-based warning system to the nation’s supermarkets could take years. Nevertheless, he said, “we can dream of a membrane that displays a certain logo or message. If the logo vanishes, you don’t eat the spinach.”

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.