TINY POWERHOUSE: The decomposable semiconductor is thinner than a human hair. (Photo: Linda A. Cicero/Stanford News Service)

A Solution to E-Waste

Chemical engineers develop a biodegradable electronic circuit.

In the not-so-distant future, the internet may connect to everything from your toaster to your T-shirt. But as we realize the Internet of Things, the number of sensors and other devices will only proliferate, bringing with them some serious environmental complications. Namely, what do you do with all the spent electronics?

For the past seven years, chemical engineering professor Zhenan Bao’s lab has been developing a solution, unveiled this past May: the world’s first fully biodegradable circuit.

According to Ting Lei, a postdoctoral scholar in the lab and lead author of the paper announcing completion of the project, one of the longstanding problems had been the creation of a decomposable semiconductor. To address this, the group started with the basics. Molecular basics.

“I searched through the literature to find chemical bonds that can be decomposed under certain situations,” says Lei. That led him to some natural-based dyes that could be readily broken down by acids. This chemistry would form the basis of the team’s new semiconducting polymer. “Actually,” says Lei of the discovery, “we are standing on a giant, right?”

Having previously developed a cellulose-based substrate — the nonconductive base of a circuit — the team then added its semiconductor and iron electrodes to create a simple logic circuit, the building block of computers. Although it’s resistant to heat, water and mild basic solutions, the fully functioning circuit dissolved when placed in a slightly acidic solution, with a pH about 100 times weaker than that of common vinegar or lemon juice — in other words, acidity levels that occur in nature. In such a solution, the circuit’s electrodes degraded in about an hour, with the rest of the device disintegrating within a month.

Even before its disappearance, the team’s circuit was tiny: thinner than a human hair, and about 40 times lighter in weight than a piece of office paper of the same surface area. Although they aren’t quite to the point of demonstrating applications, Lei imagines using these tiny devices to conduct environmental studies. “What we typically do is use a satellite or something to gather information,” he says. Now they might be able to blanket an area to be studied with low-cost biodegradable sensors that could measure “biological signals, or what’s the level of certain chemicals. We don’t need to gather them back.”

And because the device breaks down into elements and natural compounds, that gives it another important property: biocompatibility. Lei said the lab also envisions a future with more body-sensing electronics, whether they’re implanted or worn on the skin. Although there are already a number of biocompatible devices doctors can implant in people today, “typically you need a secondary surgery,” Lei explains, to remove them at some point.

But with the team’s advances in naturally degrading electronics, these parts could be broken down and reabsorbed by the body itself.

To that end, they’re working on decomposable antennas and a few other parts to measure and transmit information from within a person’s body. “The basic components have been demonstrated,” Lei says. “The next step is to integrate all the stuff together into a functional system.” •