A new kind of electronic device completely disintegrates within a month when exposed to a mild acid like watered-down vinegar, researchers from Stanford University reported May 1 in the Proceedings of the National Academy of Sciences
. The material reflects the latest advance in the field of “transient electronics,” which researchers hope will help address the massive buildup of electronic waste, or e-waste, that has accompanied rapid advances in technology. Almost 50 million tons of electronics will be discarded this year alone, the United Nations Environment Programme has projected—and most of this material will be non-decomposable and contain toxic materials. To find an alternative, study leader Zhenan Bao turned to biology for inspiration. “In my group, we have been trying to mimic the function of human skin to think about how to develop future electronic devices,” she says. Previously, her team developed a flexible, stretchable electrode; the new one is also biodegradable. Her team accomplished this trick with a new type of polymer: a molecule composed of repeated subunits. Essentially, the material is a type of flexible plastic that can conduct electricity. Polymer-based electronics, also known as organic electronics, have attracted lots of research interest in recent years because they are made of commonly available raw materials and their manufacturing process is more environmentally friendly than that of silicon-based electronics. The subunits of the new polymer that Bao’s team created are connected with imine bonds, which link atoms of carbon and nitrogen. Imine bonds can withstand high temperatures and immersion in water, but break apart when exposed to weak acid. “This is the first example of a semiconductive polymer that can decompose,” says Ting Lei, a postdoctoral fellow working with Bao. The team built circuits on the polymer out of iron, which is environmentally friendly and nontoxic to humans. And it fully disintegrates—unlike gold, which has previously been used in circuits for transient electronics. Finally, the researchers attached their polymer-plus-circuits to an ultrathin substrate made of cellulose. That’s the same plant-derived molecule of which paper is made, so of course it’s biodegradable, too. The substrate would enable the electronic device to be worn on the skin as a patch, or even potentially implanted inside the body. The resulting device has a lot of advantages: it’s less than a micrometer thick, very lightweight (one-fortieth the weight of a sheet of office paper of similar size), flexible, and able to conform to irregular surfaces. And it requires little power to operate. The entire apparatus degrades into nontoxic components, disappearing entirely within 30 days when exposed to an acid with a pH of 4.6. Comparably acidic conditions are common in the biological world (full-strength vinegar has a pH of 2 to 3, for example), so the researchers say the natural environment could likely handle the task of breaking down the device. The researchers envision their material could be used in disposable skin patches to monitor blood pressure, glucose, or other biological functions. Or, devices could be implanted inside the body and then resorbed once they were no longer needed. The electronics could also be useful for large-scale environmental surveys in remote locations, eliminating the need to either gather sensors back up after an experiment or leave plastic littering the environment.
Image: A newly developed flexible, biodegradable semiconductor developed by Stanford engineers shown on a human hair. Credit: Bao lab.