Electronics in the Round: Mixing plastic and silicon yields form-fitting circuitry

It’s already possible to make circuitry that flexes and can even roll up like a scroll. What’s not yet available are circuits that can conform to more-complicated surfaces, like robotic bodies and eyeball-shape cameras.

OVER THE HUMP. A circuit fabricated on a transparent, yellow-green plastic sheet and then shaped into a curve casts shadows on the flat surface below. R. Battacharya/Princeton

Pai-Hui I. Hsu and her colleagues at Princeton University now report taking steps toward that goal. Aiming to make a circuit that could fit closely to a spherical lens, the investigators used ordinary microcircuit-fabrication methods to pattern arrays of silicon-based transistors onto a flat sheet of polyimide plastic that they then deformed to give it a bowl-like shape. They describe the work in the Aug. 26 Applied Physics Letters.

Such curvaceous electronic circuits may eventually lead to compact cameras with extra-wide fields of view that could be used for spying and for preventing aircraft collisions, says study coauthor Sigurd Wagner. Or, if formed into sensitive artificial skins, the new technology could lead to improved prosthetic limbs as well as robots that would be aware of their environments in more humanlike ways.

These circuits would be more versatile than today’s flexible electronics. Many devices today, from cell phones and laptop computers to automobile dashboards and aircraft instruments, contain wires and components bonded to plastic substrates that can bend without damaging the electronics. Researchers also are developing flexible displays ranging from pliable liquid crystals that may one day adorn fabrics (SN: 6/1/02, p. 349: Paint-on displays get closer to reality) to sheets of “electronic paper” (SN: 4/28/01, p. 262: New device opens next chapter on E-paper), which are made of bendable plastic covered with electrically controlled black-and-white dots that can form patterns of letters and images.

Even so, the most flexible circuitry now available can’t form shapes that require deformation of the sheet, Wagner says. Getting circuits to conform to arbitrary shapes is “actually a very tricky problem,” he notes.

Wagner and his coworkers came up with a trick of their own to solve it. By heating the transistor-studded polyimide films to about 200C while inflating the softened surface from underneath, they created a curve. As a final step, the researchers deposited metal wires between the transistors.

In essence, the Princeton team divided the circuit into relatively strainfree transistor islands separated by very compliant “moats,” explains R. Fabian W. Pease of Stanford University. “It is very original work.”

The technique still needs improvement, Wagner notes. For instance, the bowed surface shrinks a little when it cools. Also, the wires deposited in the last step don’t always work.

Nonetheless, among various approaches to the problem of making curvy electronics, the Princeton work “seems practical and a good direction to pursue in further development,” comments George M. Whitesides, a microfabrication pioneer at Harvard University.

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