Communicating with Plastic

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The technology is part of a wider effort to produce large-area electronics using inexpensive manufacturing processes. The semiconductor industry has excelled at packing hundreds of millions of transistors into small chips. But some applications, such as displays the size of walls or structural sensors for airplane wings, require distributing electronics over large areas. The Tokyo researchers are taking the approach of developing various inks and printing the electronics. Others are developing ways to transfer transistors and other parts made using conventional techniques to larger substrates--a strategy called pick and place.

Since the communications sheet is made with inkjet printing, it could be inexpensive, compared with conventional electronics, to pattern over large areas, such as a desk, a floor, or a wall. Eventually, similar sheets could be used for communications between thousands of devices, potentially for health-care applications, Someya says. With so many devices, conventional wireless communications might not be practical for a number of reasons, he says. Wireless communications can be insecure, and there is limited allocation of radio bandwidth. What's more, radio communications can consume a lot of power. The extremely short-range wireless communications used in the sheet would take little power--and that power could come from coupling the communications sheet with the power sheet that the researchers developed last year. This would give designers added flexibility in designing sensors and other electronics.

The two key components of the device are the plastic MEMs switches and the memory cells. In the MEMs switches, electrostatic forces attract two electrodes, which closes a circuit, either triggering a coil of copper to act as a radio transmitter and receiver, or making a connection to a neighboring cell.

Each of the memory cells is composed of three transistors: a memory transistor, a transistor for writing, and one for erasing. The memory transistor acts as a type of nonvolatile memory, retaining information without the need for a continuous supply of power. At its heart is a ferroelectric polymer: an electric signal switches the polarity of the polymer, which in turn creates a change in the conductivity of the transistor. The transistor has two distinct conductivities that serve as the 1 and 0 of memory. The memory cell is special because it allows a continuous voltage to be applied to the MEMs switches to hold them in the required position.

To make the communications sheets, the researchers first print a layer of memory cells, then print separate layers for the MEMs, the wiring between the cells, and the communications coils. Then they laminate the layers together.

For practical applications, the sheet will probably need to be bigger and more rugged--especially if it's to be used on the floor. The researchers are also working on improving the switching speeds, which would be important for communications between thousands of devices.