Printed ICs that can flex and stretch

February 20, 2017 // By Julien Happich
A team of researchers from the Michigan State University and the Florida State University has successfully demonstrated intrinsically stretchable thin-film transistors (TFTs) and integrated logic circuits, directly printed on elastomeric substrates.

Publishing their results in the ACS Nano letter under the title "Fully Printed Stretchable Thin-Film Transistors and Integrated Logic Circuits", the researchers detail how they came to use a solution of unsorted carbon nanotubes (CNTs) to print source/drain/gate electrodes), a solution of high-purity semiconducting single-walled carbon nanotubes (sSWCNTs) for the channel semiconductor, and barium titanate (BaTiO3) nanoparticles dispersed in a solution of polydimethylsiloxane (PDMS) to enable the printing of a novel type of stretchable hybrid gate dielectric.

Using a direct printing process with these solutions, the researchers first fabricated thin-film transistors (TFTs) and characterised the devices under thousands of stretching cycles beyond 50%. While the use of CNTs for making stretchable conductive parts has already been reported (because the one-dimensional CNTs form a mesh structure with overlapping strands), the paper focused largely on the novel hybrid gate dielectric developed.

Fully printed and intrinsically stretchable carbon nanotube thin-film transistors (TFTs) and integrated logic circuits. (a) The structure showing unsorted carbon nanotubes for the source/drain/gate electrodes, high-purity semiconducting single-walled carbon nanotubes (sSWCNT) for the channel semiconductor, and BaTiO3/PDMS composite as the gate dielectric. (b) Micrograph of a TFT printed on a PDMS substrate.

Choosing the right solvent and blending cubic phase BaTiO3 nanoparticles (about 50nm in diameter) with PDMS, the researchers were able to obtain a relative permittivity high enough (about 9 at a BaTiO3 volume content of 26%) to ensure a good gating strength, while endowing the printed dielectric blend good mechanical robustness and intrinsic stretchability.