Normally fragile and brittle silicon chips have been made to bend and fold, paving the way for a new generation of flexible electronic devices.

The stretchy circuits could be used to build advanced brain implants, health monitors or smart clothing.
The complex devices consist of concertina-like folds of ultra-thin silicon bonded to sheets of rubber.
The chip's performance is similar to conventional electronics.
Silicon microelectronics has been a spectacularly successful technology that has touched virtually every part of our lives. The rigid and fragile nature of silicon made it very unattractive for many applications, such as biomedical implants.
In many cases you'd like to integrate electronics conformably in a variety of ways in the human body - but the human body does not have the shape of a silicon wafer.
Completely integrated, extremely bendable circuits have been talked about for many years but have not been demonstrated before.This is the first one.

Silicon wave

In 2005, the team demonstrated a stretchable form of single-crystal silicon.
That demonstration involved very thin narrow strips of silicon bonded to rubber.
At a microscopic level these strips had a wavy structure that behaved like "accordion bellows", allowing stretching in one direction.
The silicon is still rigid and brittle as an intrinsic material but in this accordion bellows geometry, bonded to rubber, the overall structure is stretchable.
Using the material, the researchers were able to show off individual, flexible circuit components such as transistors.
The new work features complete silicon chips, known as integrated circuits (ICs), which can be stretched in two directions and in a more complex fashion.
In order to do this, they had to figure out how to make the entire circuit in an ultra-thin format.
The team has developed a method that can produce complete circuits just one and a half microns (millionths of a metre) thick, hundreds of times thinner than conventional silicon circuits found in PCs. That thinness provides a degree of bendability that substantially exceeds anything we or anyone else has done at circuit level in the past.

Rubber wrinkle

The slim line circuits, like conventional chips, are made of sandwiches of multiple materials to form the wires and different components. The depth and relative position of the different layers, including chromium, gold and silicon, is crucial.
You have to design the thicknesses of those materials in such a way that you put what is called the neutral mechanical plane so that it overlaps with the most brittle material.
The neutral mechanical plane is the layer in a material where there is zero strain.
In a homogenous substance, this plane occurs exactly half way between the top and bottom surface, where there is equal compression and tension as it bends.
This is where the silicon - the most brittle material - is usually positioned.
If you locate your circuits there, you can bend your overall system to a very tight radius of curvature, but your circuit doesn't experience any strain. To create the foldable chips, these circuit layers are deposited on a polymer substrate which is bonded in turn to a temporary silicon base.
Following the deposition of the circuits, the silicon base is discarded to reveal delicate slivers of circuitry held in plastic.
These are then bonded to a piece of pre-strained rubber. When the strain is removed, the rubber snaps back into shape, causing the circuits on the surface to wrinkle accordingly.
This leads to the wavy geometry that allows the overall circuit system to be stretched in any direction you want. The complete circuits are still relatively crude compared to top-end computer chips but have typical "silicon wafer performance" for the size of the component.

Brain pad

Other companies and researchers are working on different approaches to flexible electronics.
One approach is to make so-called "organic" electronics, also known as plastic electronics.
These rugged devices are made from organic polymers and have been built into flexible "electronic paper" displays.
However, they are relatively slow and therefore of limited use in high performance devices. The new work offers an alternative. One collaboration seeks to develop a smart latex glove for surgeons which would measure vital signs, such as blood oxygen levels, during an operation.
Another aims to develop a sheet of electronics which could lie on the surface of the brain to monitor brain activity in epileptics.