GERMANY – In the same way that today texts and images are printed on paper, in the future you can use printers to produce all kinds of electronic circuits. Such technology will require new specialized inks, such as the one presented at the industrial fair in Hanover, the world’s largest, by researchers from the Leibniz Institute for New Materials (INM, Germany).

“With this ink cartridge can be loaded from a fountain pen and draw an electronic circuit to illuminate an LED” said Lola González García, the Spanish scientist who has been involved in its development, in her interview with SINC.  The details are published in the journals Chemical Science.

The components of this innovative product are gold nanoparticles coated with a conductive organic polymer. The resulting nanostructures are very stable diluted in alcohols and water, the usual ingredients in conventional inks.  In fact, the idea is to apply the method in inkjet printers.

“The nanometric size of the metal particles and good stability makes it to print lines very thin ink (a few microns),” Gonzalez said, who recalled that minimize the maximum width of these lines is one of the key issues in the current printed electronics.

Another advantage of the ink, which is conductive done when dry, is that it allows drawing the electrical circuits on flexible materials such as paper or plastic, using common tools such as a pen without any additional process.

This is achieved thanks to the properties of polymers, which have a triple function. On the one hand, they exert the ‘ligands’ which stabilize and secure the metal particles remaining suspended in the solvent (if it agglomerates it would damage the printing process).  Furthermore, they help package them during drying, improving the quality of the printed lines. As well, they act as a ‘hinge’: when the material is bent it maintains connectivity between the metallic particles and therefore its electrical conductivity.

There were other inks with metallic nanoparticles and organic ligands, but the fundamental problem is that organic molecules are insulated (not conduct electricity) which precludes its use unless a sintering process is applied (heat treatment applied or powder metal compacted to increase its resistance) after printing the material.

This process involves a number of drawbacks, such as having to use high temperatures which does not support all substrates (paper, plastics or textiles, for example), and the deterioration of the quality of the printed lines.

“But our new inks do not require sintering, in addition to being particularly flexible and be conductive as soon as they dry,” concludes Tobias Kraus, director of the group INM Structure Formation.

Another research on gold nanowires

This study is part of NanoSpekt, a project for the development of materials for printed electronics, within which researchers have also made another product: gold nanowires which are extremely thin (less than 2 nanometers) and very asymmetric (4 to 8 microns long). The technique has allowed embossing motifs with a width of less than 1 micron line, another advance in miniaturizing the electronic printed circuit.

The materials presented in another article, published in the journal Nano Letters., are very transparent (up to 92% transparency) and maintain good conductivity against bending. “In other words, they are flexible, transparent and drivers, ideal for markets such as medical implants or flexible displays,” says Gonzalez.