Edible electronics are on the way
US team converts toast into graphene, opening the way for a new class of sensors. Lauren Fuge reports.
Graphene patterns can be written onto everyday materials such as food, paper, cloth and cardboard, say US scientists, potentially producing a new class of edible electronics.
Graphene is a revolutionary material made up of a single layer of carbon atoms arranged in a honeycomb lattice. It is almost completely transparent, extremely light and strong, and an efficient conductor of heat and electricity. Ongoing research is working to exploit its properties in diverse applications such as tissue engineering, water filtration, solar cells and glass-based electronics.
As described in a study published in the American Chemical Society journal ACS Nano, a team of scientists led by Yieu Chyan and Ruquan Ye of Rice University in Texas, US, used a commercial laser to create graphene patterns on a variety of materials, including paper, cardboard, cloth, coal, potatoes, coconuts, and toast.
“This is not ink,” says James Tour, Rice University chemist and co-author of the study. “This is taking the material itself and converting it into graphene.”
The materials used in the study have a common factor: lignin, a complex organic polymer that binds the cells, fibres and vessels of many plants and algae. Crucially, it is largely composed of carbon.
The team claim that any material with a high enough carbon content can be turned into graphene. In 2011 they made graphene out of insects, waste and even Girl Scout cookies, using a different technique involving carbon deposition on copper foil.
The team recently developed the new technique of laser-induced graphene (LIG), which uses a computer-controlled laser to transform a variety of materials into porous graphene foam. Instead of a conventional lattice, the foam consists of a jumble of microscopic, cross-linked graphene flakes about 20 microns thick.
To create the foam, a laser is passed over the target surface multiple times, first converting the lignin-rich surface into amorphous carbon and then into graphene. Defocusing the laser makes the beam of light wider, allowing for more speed and finer control.
Using this technique, the team made an LIG micro super-capacitor in the shape of an “R” (for Rice University) on a coconut, and etched a graphene owl on cloth.
This is a step up from the previous state-of-the-art method of making patterned graphene, which involved transferring a sheet of it onto the desired surface and then etching away the excess.
Tour is enthusiastic about the potential applications. “Perhaps all food will have a tiny RFID tag that gives you information about where it's been, how long it's been stored, its country and city of origin and the path it took to get to your table,” he says.
LIG tags could also be used as sensors with the ability to detect E. coli or other microorganisms in food. “They could light up and give you a signal that you don't want to eat this,” says Tour.
Shaun Hendy, a physicist at the University of Auckland who was not involved in the study, comments: “Modern electronics relies on materials that are not sourced sustainably, so this technique could help put the industry on a more environmentally-friendly footing.”
The technique may also have applications in biodegradable, edible and wearable electronics. But it is not quite ready for widespread use yet.
“The graphene produced still has many defects and is unlikely to have the properties required for transistors,” says Cameron Shearer, nanoscience researcher at the University of Adelaide who was also not in the research team. “A method to make perfect graphene, with a metal catalyst and in a patterned form, is a research goal in this area.”