What on earth have spiders got to do with printed electronics? Well, as is often the case, nature shows us how to do really difficult tasks in quite spectacular ways. Spiders are well known for their webs, approximately 40,000 species of spider exist many of which produce thin silk fibres spun from pressurized abdominal sacs containing a polymeric solution. These sacs eject tiny jets that dry in-flight to produce the fibres with diameters of approximately 2.5 to 4.0 microns and are used to make the intricate patterned webs we see all around us. Studying these natural processes has led to the development of microfluidic liquid jet printing and hydrodynamic printing concepts.
A recent patent application US20160129634 was published on 12 May 2016 by inventors David Keicher and Adam Cook of the Sandia Corporation and describes how this process can be used to create very thin inkjets suitable for use in direct printed electronic applications.
Summary: The present invention is directed to an apparatus for two-fluid hydrodynamic printing, comprising a coaxial tube assembly, comprising an inner tube having an exit orifice for flowing an ink stream therethrough and an annular outer tube for flowing a sheath fluid therethrough wherein the sheath fluid has a higher velocity than the ink stream such that the ink stream is hydrodynamically focused by the outer sheath fluid upon exiting from the exit orifice of the inner tube. The apparatus can further comprise a focusing nozzle downstream from the exit orifice of the inner tube for further focusing of the ink stream therein. The apparatus can further comprise means for removing the sheath fluid from the ink stream downstream from the exit orifice of the inner tube and a recirculation channel for recirculating the removed sheath fluid. For example the ink can comprise polyvinylpyrrolidone and the sheath fluid can comprise water, alcohol, or a combination thereof. The focused ink stream can be deposited on a substrate.
Print results using two-fluid hydrodynamic focusing yielded a 30 μm wide by 0.5 μm tall line that suggests that the cross-section of the printed feature from the print head was approximately 2 μm in diameter. Printing results also demonstrated that complete removal of the sheath fluid is not necessary for all material systems. For example, hydrodynamic printing of two fluids enables printing of insulated conductors and clad optical interconnects.
The inventor D. Keicher also has a granted patent US8919899 which describes the same basic principle and in my view is relevant prior art to this application.
Printed electronics requires some very tight specifications for line width and resolution if the resultant printed tracks are to work reliably and meet the designs required by modern electronic circuits and components. The conductive tracks often require liquids with specific properties which are not always suitable for inkjet nozzles or the typical methods of ink droplet formation. The technology described here produces very narrow liquid lines on the substrate and is an interesting alternative for direct printed electronics.