'Edible optics' could make food safer

Bryn Nelson Columnist

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.”

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