Yonggang Huang
(Photo by Andrew Cambell)
Circuits 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 and 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.
To create their fully stretchable integrated circuits, the researchers began by applying a layer of polymer and then a coating of plastic to a substrate. The integrated circuit was then crafted on the surface using both conventional techniques and nanoscale printing methods. Researchers wash away the initial polymer layer, leaving the complete circuit system with the plastic coating as a flexible substrate.
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 leads to a complex pattern of buckling 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. Huang's research group is responsible for the mechanical analysis that guides the design of these circuits.
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 2008. This application uses an array of silicon detectors and electronics on a curved surface created using stretchable circuits.
First, the researchers created a hemispherical thin elastomeric membrane that can be stretched into the shape of a flat drumhead. In this form, flat electronics can be transferred onto the elastomer. In order to avoid fracture of the brittle semiconductor materials when the elastomer pops back into its hemispheric shape, the silicon components are made to be extremely small approximately 100 micrometers square - so that they aren't affected when the elastomer changes shape. 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 allow for the strain associated with return of the elastomer to its curved shape.
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.
