Washington, Mar 28 : University of Illinois scientists have developed new foldable and stretchable silicon integrated circuits that can be wound around various complex shapes without a reduction in their electrical performance.
The circuits can be wrapped 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.
The new designs and fabrication strategies developed by John Rogers, a Founder Professor of Materials Science and Engineering at the University of Illinois, 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.
"The notion that silicon cannot be used in such applications because it is intrinsically brittle and rigid has been tossed out the window. Through carefully optimized mechanical layouts and structural configurations, we can use silicon in integrated circuits that are fully foldable and stretchable," said Rogers.
In a previous study, the researchers developed a one-dimensional, stretchable form of single-crystal silicon with micron-sized, wave-like geometries that allowed reversible stretching in one direction without altering the electrical properties, but only at the level of individual material elements and devices.
In the new study, the researchers extended this basic wavy 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. The wavy concept now incorporates optimized mechanical designs and diverse sets of materials, all integrated together in systems that involve spatially varying thicknesses and material types. The overall buckling process yields wavy shapes that vary from place to place on the integrated circuit, in a complex but theoretically predictable fashion," said Rogers.
He added that achieving high degrees of mechanical flexibility, or foldability, is important to sustaining the wavy shapes.
"The more robust the circuits are under bending, the more easily they will adopt the wavy shapes which, in turn, allow overall system stretchability. For this purpose, we use ultrathin circuit sheets designed to locate the most fragile materials in a neutral plane that minimizes their exposure to mechanical strains during bending," he said.
In order to create their fully stretchable integrated circuits, first a sacrificial layer of polymer was applied to a rigid carrier substrate. On top of the sacrificial layer a very thin plastic coating was deposited, to support the integrated circuit. Then, the circuit components were 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.
Later, the sacrificial polymer layer was washed away, and the plastic coating and integrated circuit are bonded to a piece of prestrained silicone rubber. Finally, the strain was relieved, and as the rubber regained its initial shape, it applied compressive stresses to the circuit sheet. Those stresses spontaneously led to a complex pattern of buckling, to create a geometry that then allowed 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.
Integrated circuits consisting of transistors, oscillators, logic gates and amplifiers were then created. The circuits exhibited extreme levels of bendability and stretchability, with electronic properties comparable to those of similar circuits built on conventional silicon wafers.
The researchers said that 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 study is published in the journal Science, and posted on its Science Express Web site.