Materials for stretchable electronics
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e electronics, the newest class of large-area electronics Stretchable electronics is the newest class of large-area electronics. Quite literally large-area electronics has become a success story: flat panel displays are manufactured at the rate of 100 and solar cells at 200 square kilometers a year. Many of these products are made with thin films. Liquid-crystal displays1 are driven by active matrices of amorphous silicon transistors,2 and 10% of all solar cells are made of amorphous silicon3 or chalcogenide films.4,5 Like any other stiff material, circuits become flexible and rollable when their thickness is reduced to 1/1000 of the desired radius of curvature.6–9 Thinfilm circuits are made on flat surfaces by standard microfabrication techniques. When made on plastic substrates and with plastically deformable interconnects, they can be shaped to surfaces that need expansion out of the plane, for instance spherical caps.10 On plastic substrates, this deformation is permanent. Elastomeric substrates and elastic interconnects let circuits go a shape beyond: to reversible deformation and near-arbitrary dimensions.11–16 Sizes and shapes of elastomeric circuits can be changed reversibly by applying mechanical force,17 by gas pressure,18,19 or by application of an electric field.20,21 Now we can make electronic skin,22 conformable sensors and displays (see the Kim et al. and Sekitani and Someya articles in this issue), electronic biointerfaces,12,23–25 electronic
muscles,20,21,26 and energy harvesters (see the article by Kornbluh et al. in this issue). Bending,7–9 shaping,10,27 stretching (see the Suo article),11 and electroactuation20,21 or energy harvesting are illustrated in Figure 1. The latest progression is circuits made on a biodegradable substrate,28,29–31 which disappears to leave the bare circuit conforming to a living organism; the goal is a circuit that envelopes the heart fully to sense and stimulate it with high spatial resolution while expanding and shrinking with the heartbeat.31 Stretchable electronics is the research frontier of large-area electronics. A materials scientist first will notice the elastic, soft nature of experimental stretchable electronics: a polymeric electroactuator (see the article by Kornbluh et al. in this issue), a wraparound pressure sensing skin (see the article by Sekitani and Someya), or a neural interface in the folds of the cerebral cortex (see the article by Kim et al.). The materials scientist will then ask: How does this work? How can I make this? Stretchable electronics are fabricated with planar technologies that evolved from the materials and processes of classical microfabrication to those of large-area electronics on rigid substrates, and then they are branched out to a diverse range of fabrication techniques. Functionalities never seen before can be realized by combining electrical with mechanical properties drawn from all conceivable classes of materials: liquid, gel, solid, organic and inorganic, insulator, semiconductor, and metal.
Sigurd Wagner, Department of Electrical
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