McCormick Magazine

Stretching electronics

Yonggang Huang explores ways to make circuits more flexible

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huangCircuits that can wrap around your arm. A camera based on the human eye. Electronic newspapers. These tantalizing technologies are not yet available to the public, but a partnership between Yonggang Huang, the Joseph Cummings Professor in civil and environmental engineering and in mechanical engineering, and John Rogers of the University of Illinois at Urbana-Champaign has brought them closer to reality.

Electronic components have historically been flat and unbendable because silicon, the principal component of all electronics, itself is inflexible: Any significant bending or compression renders an electronic device useless.

Huang and Rogers have new ideas on how to overcome this hurdle. The pair has created stretchable circuits that could be used for everything from wearable electronics to circuits that can wrap around an airplane’s wings. And they’ve created a camera with a curved sensor array that’s based on the design of the human eye, which gives a photograph a wide field of view with a greater range of focus. That technology — which uses “pop-up” wires to connect electrical components so they don’t degrade when bent — could lead to further advances in the area.

“Stretchable and bendable electronics have many applications that could be available to the public within a few years,” Huang says.

Bend, don’t break
Their research had its first breakthrough in December 2005, when Huang and Rogers developed a one-dimensional, stretchable form of single-crystal silicon that could be stretched in one direction without altering its electrical properties. This year they extended that concept to two dimensions. To create their fully stretchable integrated circuits, the researchers began by applying a layer of polymer to a substrate. They then deposited on top of this layer another very thin plastic coating, which supported the integrated circuit. The circuit components were then crafted on the surface using both conventional techniques and nanoscale printing methods, which create tiny nanoribbons of silicon that are used as the semiconductor. Researchers wash away the initial polymer layer, leaving the complete circuit system with the plastic coating as a flexible substrate. It has a total thickness of about 1/50th the diameter of a human hair.

Next, this flexible, ultrathin circuit is bonded to a piece of silicone rubber that is prestretched, like a drumhead. When released, the rubber springs back to its initial shape, compressing the circuit. That compression spontaneously leads to a complex pattern of buckling, creating a two dimensional “wavy” configuration that allows the circuit to be folded or stretched in different directions to conform to a variety of complex shapes or to accommodate mechanical deformations during use.

Using this method, researchers constructed integrated circuits consisting of transistors, oscillators, logic gates, and amplifiers. These circuits can wrap around complex shapes such as spheres, body parts, and aircraft wings and can operate during stretching, compressing, folding, and other types of extreme mechanical deformations, all while maintaining electronic properties comparable to those of similar circuits built on conventional silicon wafers. Huang’s research group is responsible for the mechanical analysis that guides the design of these circuits so they avoid mechanical failure and degradation of electrical behavior when stretched and bent.

That type of mechanical analysis also allowed Huang and Rogers to create a kind of camera based on the human eye — work featured on the cover of the journal Nature in August. This time around, the team created an array of silicon detectors and electronics that conformed to a curved surface. Like the human eye, the curved surface then acts as the focal plane array of the camera, which captures an image. On a normal digital camera, such electronics must lie on a flat surface, and the camera’s complex system of lenses must reflect an image several times before it hits the right spot on the focal plane.

“The advantages of curved detector surface imaging have been understood by optics designers for a long time and by biologists for an even longer time,” Huang says. “That’s how the human eye works — using the curved surface at the back of the eye to capture an image.”

Electronic eye
electronic-eye cameraExactly how to place those electronics on a curved surface to yield working cameras has stumped scientists, despite many attempts over the last 20 years. Huang and Rogers have established theoretical foundations and experimental methods, respectively, for an effective way to transfer the electronics from a flat surface to a curved one.

Huang and Rogers created a hemispherical transfer element made of a thin elastomeric membrane that can be stretched into the shape of a flat drumhead. In this form, planar (flat) electronics can be transferred onto the elastomer. Popping the elastomer back into its original hemispheric form enables the transfer of the electronics onto a hemispherical device substrate. Since silicon wafers can only be compressed 1 percent before they break and fail, a major challenge was finding a way to apply this process without catastrophic mechanical fracture in the brittle semiconductor materials.

Huang and Rogers got around this by creating an array of photodetectors and circuit elements that are so small — approximately 100 micrometers square — they aren’t affected when the elastomer pops back into its hemispheric shape. Think of them like buildings on the Earth: Though flat buildings are built on the curved Earth, the area they take up is so small that the curve isn’t felt. The tiny circuits on the array are connected by thin metal wires on plastic that form arc-shaped structures that Huang and Rogers call “pop-up bridges.” These bridges interconnect the circuits and allow for the strain associated with return of the elastomer to its curved shape.

The researchers designed the array so that the silicon component of each device is sandwiched between two other layers, the so-called natural mechanical planes. That way, when the top layer is stretched and the bottom layer is compressed as in bending, the middle layer experiences very little stress. When tested, more than 99.9 percent of the devices worked after the elastomer returned to its hemispherical shape. Researchers found that the silicon in the devices was only compressed .002 percent — far below the 1 percent point where silicon fails.

Early images obtained using this curved array in an electronic eye–type camera produce large-scale pictures that are much clearer than those obtained with similar planar cameras when simple imaging optics are used. “In a conventional planar camera, parts of the image that falls at the edges of the field of view are typically not imaged well using simple optics,” Huang says. “The hemispheric layout of the electronic eye eliminates this and other limitations, thereby providing improved imaging characteristics.”

Huang and Rogers will continue to optimize the camera by adding more pixels. “There is a lot of room for improvement, but early tests show how well this works,” Huang says. “We believe that this is scalable, in a straightforward way, to more sophisticated imaging electronics. It has been a very good collaboration between the two groups.”

—Emily Ayshford