If the invention takes off then electronics printed on paper could could lead to applications such as smart labels on foods and pharmaceuticals or as wearable medical sensors.
While engineers have managed to print transistors and solar cells on paper, in the past, they have been unable to do memory.
Paper is made of fibre making it difficult to lay down the thin, uniform layers of materials that typical memory technologies need.
To get around this problem, the team, led by Ying-Chih Liao, Si-Chen Lee, and Jr-Hau He of National Taiwan University decided to build resistive random access memory (RRAM), a relatively new type of memory with a structure simple enough to cope with such surface variations.
In an RRAM device, an insulator can be set to different levels of electrical resistance by applying a voltage across it; one level of resistance corresponds to the 1s of digital logic, the other to the 0s. So each bit in RRAM consists of an insulator sandwiched by two electrodes.
The device was built with silver, titanium dioxide, and carbon, although other combinations of a metal, an insulator, and a conductor could be used. They started by using a screen-printing process to coat a carbon paste onto the paper to form the bottom electrode. The process was repeated 10 times to reduce roughness, then the coated paper was cured at 100 °C for 10 minutes in a vacuum.
Ink was made by mixing TiO2 nanoparticles in acetyl acetone and used an ink-jet printer to deposit a layer of the particles on top of the carbon, where it would act as the insulator. Once that dried, the researchers used a solution of ethylene glycol and water containing silver nanoparticles, and they printed silver dots on top of the TiO2 layer to serve as top electrodes.
The memory paper was robust and could be bent at least 1,000 times with no degradation in performance.