  Printed electronics rely on the deposition of conductive liquid inks,
typically onto polymeric or paper substrates. Among available conductive
fillers for use in electronic inks, carbon nanotubes (CNTs) have high
conductivity, low density, processability at low temperatures, and intrinsic
mechanical flexibility. However, the electrical conductivity of printed CNT
structures has been limited by CNT quality and concentration, and by the need
for nonconductive modifiers to make the ink stable and extrudable. This study
introduces a polymer-free, printable aqueous CNT ink, and presents the
relationships between printing resolution, ink rheology, and ink-substrate
interactions. A model is constructed to predict printed feature sizes on
impermeable substrates based on Wenzel wetting. Printed lines have conductivity
up to 10,000 S/m. The lines are flexible, with < 5% change in DC resistance
after 1,000 bending cycles, and <3% change in DC resistance with a bending
radius down to 1 mm. Demonstrations focus on (i) conformality, via printing
CNTs onto stickers that can be applied to curved surfaces, (ii) interactivity
using a CNT-based button printed onto folded paper structure, and (iii)
capacitive sensing of liquid wicking into the substrate itself. Facile
integration of surface mount components on printed circuits is enabled by the
intrinsic adhesion of the wet ink.
