Jennifer Lewis is intent on printing the power of the sun.

Professor Lewis, a Hans Thurnauer Professor of Materials Science and Engineering at the University of Illinois Urbana-Champaign, has developed a direct ink writing system which uses silver nanoparticle ink she thinks will enable the fabrication of flexible, stretchable microelectrodes for use in printed electronic and optoelectronic devices.

The conventional approaches to printed electronics – methods which use screen and ink-jet printing – facilitate low cost manufacturing of large-area, flexible devices.

These approaches are excellent for a variety of applications, but both techniques are limited to producing low aspect ratio features which require an underlying support. This limitation prevents the construction of "spanning structures."

Direct ink writing, a method based on the deposition of continuous, filamentary inks is Lewis' alternative as she says it allows "in-plane" 3D printing.

"We're interested in directing materials assembly for applications ranging from printed electronics to 3D microfluidics," Lewis said. "For printed electronics applications, you need to be able to store the ink for several months because silver is expensive. Since silver particles don't actually form until the ink exits the nozzle and the ammonia evaporates, our ink remains stable for very long periods. For fine-scale nozzle printing, that's a rarity."

Electrical sintering of silver nanoparticle inkIn Lewis' method, a conductive silver structure comprised of a silver salt and a short chain carboxylic acid or a complexing agent are combined to create an ink for writing.

Printed electronics made in this way are an alternative to conventional technologies as they enable the creation of applications which require high-conductivity materials – with fine-scale features – for use in modern electronics. Such applications might include solar cell electrodes, flexible displays, radio frequency identification tags and antenna. To make such high-technology devices increasingly affordable, the substrates used typically have relatively little temperature resilience and require low processing temperatures to maintain integrity.

That's where the research at UI Urbana-Champaign comes in:

As the vast majority of commercially produced conductive inks are specifically designed for inkjet, screen-printing, or roll-to-roll processing methods, the inks have disparate viscosities and synthesis parameters. Particle-based inks like those in development by Lewis and her team are based on conductive metal particles.

Those inks are then tuned for the specific particle process. While current particle- and precursor-based methods rely on high temperatures to create conductive coatings, it's those temperature levels which sometimes render them incompatible with substrates which require low processing temperatures to maintain integrity. Higher temperatures render the ink incompatible with most plastic and paper substrates currently used in flexible electronic and biomedical devices.

Via her method, Lewis has introduced factors which allow for the creation of low- resistance, patterned silver microelectrodes on a polyimide substrate with a bend radius of 14mm, wire bonded silver microelectrodes onto the surface of a spherical silicon shell, and fabricated interconnects for an LED array with spanning arches printed over an electrode junction.