flexible electronics

Abstract: Flexible electronic devices often require hermetic coatings that can withstand applied strains. This paper calculates the critical strains for various configurations of channel cracks in a coating consisting of organic and inorganic layers. We show that the coating can sustain the largest strain when the organic layer is of some intermediate thicknesses.

Flexible electronics are promising for diverse applications, such as rollable displays, conformal sensors, and printable solar cells. These systems are thin, rugged, and lightweight. They can be manufactured at low costs, for example, by roll-to-roll printing. The development of flexible electronics has raised many issues concerning the mechanical behavior of materials. This paper examines a particular issue: channel cracks in hermetic coatings.

Electronic devices (e.g., organic light-emitting devices, OLEDs) often degrade when exposed to air. Developing hermetic coatings has been a significant challenge. Organic films are permeable to gases, and inorganic films inevitably contain processing flaws, so that neither by themselves are effective gas barriers. These considerations have led to the development of multilayer coatings consisting of alternating organic and inorganic films. To be used in flexible electronics, these coatings must also withstand applied strains without forming channel cracks...

Teng Li, Zhenyu Huang, Zhichen Xi, Stephanie P. Lacour, Sigurd Wagner, Zhigang Suo, Mechanics of Materials, 37, 261-273 (2005).

Under tension, a freestanding thin metal film usually ruptures at a smaller strain than its bulk counterpart. Often this apparent brittleness does not result from cleavage, but from strain localization, such as necking. By volume conservation, necking causes local elongation. This elongation is much smaller than the film length, and adds little to the overall strain. The film ruptures when the overall strain just exceeds the necking initiation strain, ε_N , which for a weakly hardening film is not far beyond its elastic limit. Now consider a weakly hardening metal film on a steeply hardening polymer substrate. If the metal film is fully bonded to the polymer substrate, the substrate suppresses large local elongation in the film, so that the metal film may deform uniformly far beyond ε_N. If the metal film debonds from the substrate, however, the film becomes freestanding and ruptures at a smaller strain than the fully bonded film; the polymer substrate remains intact. We study strain delocalization in the metal film on the polymer substrate by analyzing incipient and large-amplitude nonuniform deformation, as well as debond-assisted necking. The theoretical considerations call for further experiments to clarify the rupture behavior of the metal-on-polymer laminates.

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R. Huang, Applied Physics Letters 87, 151911 (2005).

Gold films on an elastomeric substrate can be stretched and relaxed reversibly by tens of percents. The films initially form in two different structures, one continuous and the other containing tri-branched microcracks. We have identified the mechanism of elastic stretchability in the films with microcracks. The metal, which is much stiffer than the elastomer, forms a percolating network.

Here is one answer from Nokia.

Nokia 888 communicator, a concept design which recently won the Nokia's Benelux Design Award. It uses liquid battery, flexible touch display, speech recognition, touch sensitive body cover which lets it understand and adjust to the environment. It has a simple programmable body mechanism so that it changes forms in different situations. Don't forget to enjoy a video demo of this cell phone of future.
Yet one more future application of flexible electronics, it's clear there're great mechanics and materials challenges in making electronic devices flexible. It will be great mechanicians can help accelerate the advance of this emerging technology.

The SES 2007 Conference, Oct. 21-24, 2007, Texas A&M University campus in College Station, Texas, home to the George Bush Presidential Library and Museum.

Call for abstract

Symposium: Mechanics of micro/nano structures on soft substrates: applications for flexible and stretchable electronics

Teng Li, Zhigang Suo, Stephanie P. Lacour, Sigurd Wagner

Journal of Materials Research, 20, 3274-3277 (2005)