Highly conductive circuits are fabricated on nanopapers composed of densely packed 15-60 nm wide cellulose nanofibers. Conductive materials are deposited on the nanopaper and mechanically sieved through the densely packed nanofiber networks. As a result, their conductivity is enhanced to the level of bulk silver and LED lights are successfully illuminated via these metallic conductive lines on the nanopaper. Under the same deposition conditions, traditional papers consisting of micro-sized pulp fibers produced very low conductivity lines with non-uniform boundaries because of their larger pore structures. These results indicate that advanced, lightweight and highly flexible devices can be realized on cellulose nanopaper using continuous deposition processes. Continuous deposition on nanopaper is a promising approach for a simple roll-to-roll manufacturing process.

Cellulose nanopapers have been shown to maintain high optical transparency after high temperature heating at 150 °C. High temperature heating to around 150 °C is inevitable in electronic device processing. If a polyethylene terephthalate film is held at 150 °C for tens of minutes, cyclic oligomers migrate to the film surface, causing surface roughness that decreases the film transparency. However, because cellulose nanofibers have high thermal stability, the transparent nanopapers maintained their smooth surfaces and high optical transparency, even after heating to 150 °C for tens of minutes. These findings indicate the suitability of cellulose nanofiber papers for continuous roll-to-roll processing.

Polyimide films are the most promising substrates for use in printed electronics because of their high thermal stability. However, the high wettability of polyimide films by conductive inks often produces thin inkjet-printed lines with splashed and wavy boundaries, resulting in high electrical resistance of the lines. To overcome these disadvantages, we fabricated repellent pore structures composed of polyamideimide with high thermal stability on a polyimide film. Using this film, the inkjet-printed line thickness was increased without penetration of silver nanoparticles into the pore structures, thus resulting in very sharp edges without any splashing. This was because the repellent treatment restricted the spreading of the silver nanoparticles into the pore structures and the pore structures prevented ink splashing upon impact on the film. As a result, the electrical resistance of these lines decreased to one-fifth that of the lines on the pristine polyimide film. The inkjet printing of conductive inks onto repellent pore structures would contribute to the future of printed electronics because this technique enables printing closely packed line patterns while maintaining high conductivity within a limited space.

Conductive silver lines of various widths (0.04-40 mm) were fabricated with dilute silver-nanoparticle ink on polyimide films using an inkjet printer. The electrical properties of the lines were found to vary in width. In particular, wider lines (>0.4 mm) exhibited low resistivity (3.6-5.4 μ cm), approaching that of bulk silver (1.6 μ cm). On the other hand, narrower lines (<0.3 mm) exhibited much higher resistivity (14.6-16.5 μ cm), presumably because of the so-called coffee-ring effect. This effect, known to strongly influence nanoparticle deposition, is caused by convection flow, during which nanoparticles segregate at the line edge. However, when the narrower lines were heated slowly from 20 • C to 200 • C at a heating rate of 3 • C min −1 to reduce convection flow, the nanoparticles redistributed uniformly, after which the lines exhibited low resistivity (3.9-4.2 μ cm). Therefore, gradual heating appears to be an excellent method for enabling inkjet printing technology to yield narrow highly conductive lines.