Scientists have used inexpensive materials to develop foldable electronic switches and sensors directly onto paper. The researchers from the University of California, Berkeley in the US used the technique to create prototype generators, supercapacitors and other electronic devices for a range of applications. Besides its availability and low cost, paper offers an intriguing potential: simply folding it could switch circuits on and off or otherwise change their activity - a kind of electronic origami, researchers said.
However, most efforts to fabricate electrodes onto paper with sufficient conductivity for practical use have employed expensive metals such as gold or silver as the conducting material, swamping the potential savings of paper as a substrate. The technology uses the inexpensive element molybdenum as the source of the conducting metal. It is added to gelatin in solution and binds to carbon in the gelatin.
The paper is then coated with the solution and dried. A laser beam precisely "writes" the desired circuitry patterns, heating the molybdenum to about 1,000 degrees Celsius and forming conductors of durable molybdenum carbide. The laser-written circuits are about 100 microns wide - about the diameter of a human hair. All the unheated portions of the paper remain non-conductive, researchers said.
The gelatin coating not only provides the carbon for the conductive compound but also prevents the laser beam from burning the paper, they said. "Without the gelatin, the paper would turn into ashes," said Liwei Lin, senior author of the paper published in the journal Advanced Materials.
The engineers envision widespread potential for the new, disposable paper electronics. For example, circuitry to detect heavy metal contamination could be "written" on paper to economically monitor toxins.
A sensor made of several electrodes integrated onto a paper circuit could detect unsafe lead levels in a drop of water - or in a drop of a patient's blood, said Xining Zang, who led the research as a Berkeley mechanical engineering graduate student in Lin's lab.
"Our work provides a versatile and easy path to define conductive areas on the paper. We have now shown both the practicality of writing versatile conductive patterns on paper, and the durability of folding the electronic paper many hundreds of times for switching circuits on and off," Lin said.
