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Communicating with Plastic

A new inkjet-printed technology enables a different kind of wireless connectivity between devices.
December 12, 2007

University of Tokyo researchers have developed a plastic pad that allows electronic devices placed on it to communicate with each other. This communications sheet could provide a more secure and lower-energy alternative to short-range wireless communications, such as Bluetooth.

Bright idea: Simple electronic devices–in this image, robots illuminated by LEDs–can communicate securely through electronics embedded in a plastic sheet.

“The first application might be an ‘intelligent table’ ” that would allow a few devices to communicate with each other without the need to wire them together, says Takao Someya, professor of engineering at the University of Tokyo. Someya says that the team’s long-term goal is to develop a system that connects thousands of devices, as might one day be required. The amount of energy needed to wirelessly connect such an array of devices “would be huge,” he says, and wiring them all together would be cumbersome. Someya’s approach uses a combination of extremely short-range wireless communications and wires to provide a low-energy alternative.

The sheet, which is one millimeter thick, is made by inkjet-printing various insulating and semiconducting polymers, as well as metal nanoparticles, to make transistors, plastic microelectromechanical (MEMs) switches, communications coils, and memory cells. It’s designed to be used in combination with another sheet developed by the Tokyo researchers last year that can sense the location of an electronic device placed on it and deliver power to it. (See “Plastic Sheet of Power.”) The new technology was presented this week at the International Electron Devices meeting in Washington, DC.

Each communications sheet, which is not quite as wide as a sheet of paper, is made of an eight-by-eight-inch grid of cells. Each cell contains a coil for transmitting and receiving signals, and plastic MEMs switches for turning the coils on and off and for connecting to adjacent cells. Once someone places two electronic devices on the sheet, sensors register their location. A control chip (made using conventional silicon processes) at the edge of the sheet decides on the best way to route signals between them through the sheet.

Multimedia

  • See schematics of the system.

Communication involves two processes. First, information is transmitted wirelessly between the device and the sheet using extremely short-range radio signals (on the order of millimeters). Then a series of MEMs switches are closed to form a wired connection between adjacent cells forming a path between the two devices. This forms a wired connection between a receiving coil under the first device and a transmitting coil under the second. When the devices are moved or removed, or when new devices are added, new communications links can be formed on the fly.

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.

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