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Yonggang Huang Writes Review Paper for Science

Throughout their history, electric components have been flat, unbendable, and unstretchable because silicon, the metalloid element that is the principal component of all electronics, itself is inflexible.

Bend it and it will break.

But recently, researchers have made strides to overcome this rigid problem, and the outcomes could mean everything from wearable health monitors to diagnostic implements that naturally integrate with the human body.

A principal innovator in this research is Yonggang Huang, Joseph Cummings Professor in civil and environmental engineering and in mechanical engineering at the McCormick School of Engineering and Applied Science at Northwestern University. Huang, along with collaborator John Rogers at the University of Illinois- Urbana Champaign and Takao Someya at the University of Tokyo, has published an invited review article in the journal Science summarizing the state of the art of stretchable electronics and speculating on possible commercial uses for the technology.

Currently, stretchable electronics — still in the academic phase — fall into two categories: stretchable inorganic (e.g., silicon-based) electronics, and organic electronics.

Huang and Rogers work with silicon-based stretchable electronics. In 2005, 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.

They then created fully stretchable integrated circuits 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.

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 and Rogers also created an "electronic eye" by fabricating an array of photodetectors and circuit elements that are so small — approximately 100 micrometers square — that they are 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" that allow for strain when the material is bent or stretched.

Many new medical applications could be the outcome of this technology. "We feel this technology has a strong potential for integrating intimately with the human body," Huang says. "The human body is round, and in order for electronics to be integrated, they need to be stretchable and flexible."

One application already demonstrated by these researchers and a research group led by Professor Brian Litt at the University of Pennsylvania is the cardiac electrophysiology. The stretchable and flexible electronics can fit conformally the shape of the heart, and therefore records accurately the electrical activities of heart.

"By working with doctors we can develop innovative devices that improve medicine and health monitoring," Huang says.

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