Foldable and Stretchable Silicon Circuits Conform to Many Shapes

April 2, 2008

Scientists have developed a new form of stretchable silicon integrated circuit that 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, without a reduction in electrical performance.

Yonggang Huang, Joseph Cummings professor of Civil and Environmental Engineering and professor of mechanical engineering at McCormick, collaborated with researchers at the University of Illinois Urbana-Champaign and with researchers in Korea and Singapore to develop this new kind of circuit. Their paper on the research, of which Huang is a corresponding author, was published online March 27, 2008 on the Science Express web site in anticipation of its publication in the journal Science.

“It is possible now to design electronics and optoelectronics that are flexible and stretchable, and that go well beyond what the current technology can offer,” Huang said.

The new designs and fabrication strategies could produce wearable systems for personal health monitoring and therapeutics or systems that wrap around mechanical parts such as aircraft wings and fuselages to monitor structural properties.

In December 2005, Huang and John Rogers, a Founder Professor of Materials Science and Engineering at the University of Illinois at Urbana-Champaign, and their research groups reported the development of a one-dimensional, stretchable form of single-crystal silicon with micron-sized, wave-like geometries. That configuration allows reversible stretching in one direction without significantly altering the electrical properties, but only at the level of individual material elements and devices.

Now, Huang and Rogers’ groups have extended this concept to two dimensions, and at a much more sophisticated level to yield fully functional integrated circuit systems.

“We’ve gone way beyond just isolated material elements and individual devices to complete, fully integrated circuits in a manner that is applicable to systems with nearly arbitrary levels of complexity,” Rogers said.

To create their fully stretchable integrated circuits, the researchers begin by applying a sacrificial layer of polymer to a rigid carrier substrate. On top of the sacrificial layer they deposit a very thin plastic coating, which will support the integrated circuit. The circuit components are then crafted using conventional techniques for planar device fabrication, along with printing methods for integrating aligned arrays of nanoribbons of single-crystal silicon as the semiconductor. The combined thickness of the circuit elements and the plastic coating is about 50 times smaller than the diameter of a human hair.

Next, the sacrificial polymer layer is washed away, and the plastic coating and integrated circuit are bonded to a piece of prestrained silicone rubber. Lastly, the strain is relieved, and as the rubber springs back to its initial shape, it applies compressive stresses to the circuit sheet. Those stresses spontaneously lead to a complex pattern of buckling, to create a geometry that then 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.

The researchers constructed integrated circuits consisting of transistors, oscillators, logic gates and amplifiers. The circuits exhibited extreme levels of bendability and stretchability, with electronic properties comparable to those of similar circuits built on conventional silicon wafers.

The new design and construction strategies represent general and scalable routes to high-performance, foldable and stretchable electronic devices that can incorporate established, inorganic electronic materials whose fragile, brittle mechanical properties would otherwise preclude their use, the researchers report.

Huang’s research group at Northwestern University is responsible for the mechanical analysis that guides the design of stretchable and foldable silicon integrated circuits, which prevents the mechanical failure and the degradation of electrical behavior of these circuits during stretching and bending.

The work was funded by the National Science Foundation and the U.S. Department of Energy.