The inside of a computer or a mobile phone contains a stiff circuit board that is green in color, is crammed with chips, resistors, capacitors, and sockets, and is interconnected by a suburban sprawl of printed wiring. What if the circuit board was not rigid, but flexible enough to fold?
Now, Michael Drack of Johannes Kepler University in Austria, T. Sekitani of the University of Tokyo and their colleagues have designed a highly reliable, flexible, and stretchable sub-2-μm sensor using organic conductors with similar electrical resistance as metals. Metals are excellent conductors, but are mechanically mismatched to polymer substrates. Polymer conductors, on the other hand, have elastic moduli comparable to those of thin-film poly(ethylene terephthalate) (PET) substrates, making them interesting for stretchable and elastic conductors.
The electronic foil is transferred onto a pre-stretched elastomer (left, middle) forming out-of-plane wrinkles upon release (right). Reprinted with permission from Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. © 2014.
As reported in the January 7 issue of Advanced Materials (DOI: 10.1002/adma.201403093; p. 34), the researchers used a 1.4-µm-thick PET foil temporarily attached to a reusable support substrate foil—125-µm-thick PET covered with a thin poly(dimethylsiloxane) adhesion layer (see Figure)—to allow all-planar device fabrication and avoid subsequent defect-free peeling or transfer of the completed device to a pre-stretched elastomer. The researchers prepared samples of thin-film conductor strips of an organic conductor made of poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) and various metals (silver, gold, and copper) on the ultrathin PET foil with 3 nm adhesion layers of various metals and ultrathin PET. The samples were stretched up to 50% with a strain rate of 0.94%/s while continuously recording the resistance. All the metal conductors where cycled up to 1000 times (Ag, Au, and Cu) or until failure (Al, ca. 400 times); the conducting polymer PEDOT:PSS was fatigued up to 10,000 cycles. Au, Cu, and PEDOT:PSS were found to be highly reliable, withstanding 1000 stretch cycles for the metals, and 10,000 for the polymer conductor without failure. In addition, the resistance slightly increased by 30% over the course of 10,000 cycles. However, the researchers found no indication of fracture of the PEDOT:PSS. The resistance increase may be attributed to the operation in ambient air and the potential water uptake during fatigue.
“The main advantage of our approach is the planar fabrication of our stretchable interconnects and the easy transfer on mechanically stretchable substrates,” says Drack. This work “is just a first step toward a novel technology platform with the thinnest and most flexible circuit boards,” he says.
Such highly reliable transparent electrodes form the basis of new avenues for the design of complex, hybrid rigid-island stretchable-interconnect electronic devices such as light-emitting diode strips that can be stretched and twisted without impairing their function. Such materials are in demand in applications in textiles, wearable as glasses, and inner organs like hearts where flexibility, compliance, weight, and softness are important to next-generation electronic devices.
