Substrate-Versatile Direct-Write Printing of Carbon Nanotube-Based Flexible Conductors, Circuits, and Sensors

TL;DR

This paper presents a new polymer-free aqueous CNT ink for printed electronics, enabling high-conductivity, flexible, and conformal printed circuits and sensors with improved resolution and mechanical stability.

Contribution

Introduction of a novel polymer-free aqueous CNT ink and a predictive model for printing resolution, enhancing flexibility, conductivity, and substrate compatibility in printed electronics.

Findings

Printed CNT lines achieve conductivity up to 10,000 S/m.

Printed lines maintain <5% resistance change after 1,000 bending cycles.

Demonstrated conformal printing and capacitive sensing on curved and folded surfaces.

Abstract

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.