Conductive copper lines were directly written on paper through inkjet printing of a copper salt and a reducing agent sequentially from a multi-color printhead. The copper ink was an aqueous copper citrate solution and the reducing agent was a solution of sodium borohydride (NaBH4). The two inks were loaded in two separate compartments of a traditional HP color cartridge, which enabled the generation of two droplet streams from the two separate compartments. The cartridge was fixed above an X-Y positioning table and conductive copper lines prepared using multiple printing passes. The estimated conductivity obtained on paper (1.8 ′ 106 S·m-1), is about 1/30 that of bulk metal copper (59.6 ′ 106 S·m-1 at room temperature). Oxidation of the printed copper lines was studied using EDS elemental analysis of lines printed onto polyvinylidenefluoride (PVDF) membranes. The Cu/O ratio of copper lines decreased over 400 hours in air, due to oxidation, but leveled off afterwards. The same approach has also been applied to the printing of nickel where oxidation is less marked.
Dapeng Li, David Sutton, Andrew Burgess, Derek Graham, Paul D. Calvert, "Conductive Copper and Nickel Lines via Reactive Inkjet Printing" in Proc. IS&T Int'l Conf. on Digital Printing Technologies and Digital Fabrication (NIP26), 2010, pp 305 - 308, https://doi.org/10.2352/ISSN.2169-4451.2010.26.1.art00081_1