Flexible circuits created through 3D printing

German researchers have developed a process suitable for 3D printing that can be used to produce transparent and mechanically flexible electronic circuits. Described in the journal Scientific Reports, the process involves a mesh of silver nanowires that can be printed in suspension and embedded in various flexible and transparent plastics (polymers).

“The aim of this study was to functionalise 3D-printable polymers for different applications,” said Michael Rübhausen from the Center for Free-Electron Laser Science (CFEL), a cooperation between Deutsches Elektronen-Synchrotron (DESY), the University of Hamburg and the Max Planck Society. “With our novel approach, we want to integrate electronics into existing structural units and improve components in terms of space and weight.”

Rübhausen led the project together with DESY researcher Stephan Roth, who is also professor at the Royal Institute of Technology in Stockholm. Using the bright X-ray light from DESY’s research light source PETRA III and other measuring methods, the team has precisely analysed the properties of the nanowires in the polymer.

“At the heart of the technology are silver nanowires, which form a conductive mesh,” said Tomke Glier from the University of Hamburg. These nanowires are typically several tens of nanometres (millionths of a millimetre) thick and 10 to 20 micrometres (thousandths of a millimetre) long.

Detailed X-ray analysis shows that the structure of the nanowires in the polymer does not change, but the conductivity of the mesh improves thanks to the compression by the polymer, as the polymer contracts during the curing process. The nanowires are then applied to a substrate in suspension and dried.

“For cost reasons, the aim is to achieve the highest possible conductivity with as few nanowires as possible. This also increases the transparency of the material,” said Roth, who is head of the P03 measuring station at PETRA III. “In this way, layer by layer, a conductive path or surface can be produced.”

A flexible polymer is applied to the conductive tracks, which in turn can be covered with conductive tracks and contacts. Depending on the geometry and material used, various electronic components can be printed in this way, such as printable light-emitting diodes, solar cells or tools with integrated circuits. In this study, the researchers produced a flexible capacitor.

Example of a flexible and transparent electronic component: a flexible capacitor. Image credit: University of Hamburg, Tomke Glier.

“In the laboratory, we carried out the individual work steps in a layering process, but in practice they can later be completely transferred to a 3D printer,” said Glier.

Rübhausen added that “the further development of conventional 3D printing technology, which is usually optimised for individual printing inks, is also essential for this. In inkjet-based processes, the print nozzles could be clogged by the nanostructures.”

Next, the researchers now want to test how the structure of the conductive paths made of nanowires changes under mechanical stress, with Roth asking, “How well does the wire mesh hold together during bending? How stable does the polymer remain? X-ray investigation is very suitable for this because it is the only way we can look into the material and analyse the conductive paths and surfaces of the nanowires.”

Top image caption: A mesh of silver nanowires forms the flexible electronics. The wires are typically 0.01 to 0.02 millimetres long and a few dozen nanometres thick. Image credit: University of Hamburg, Ferdinand Otto.

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