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Plastic Sheet of Power

Printing flexible electronics on plastic provides a way to wirelessly power gadgets.
December 14, 2006

Researchers at the University of Tokyo have demonstrated a prototype that could offer a new way to power gadgets. The prototype, which consists of plastic and flexible electronics, can wirelessly supply power to any device that touches its surface.

These sheets of flexible electronics can wirelessly transmit power to gadgets. The sheet in hand has copper coils for power transmission; the black and brown squares are switches for turning the current on and off; the next set of copper coils senses the position of a gadget; and the black and white grid is an array of organic transistors that detect the position of the gadget and direct current flow.

The power sheet, says Takao Someya, professor of engineering at the University of Tokyo, relies on the well-known physical principle of electromagnetic induction, used to charge electric toothbrushes and some RFID tags. However, he says, his system is designed in a way that overcomes the limitations of common induction schemes. Traditional induction systems can only spread small amounts of power over a relatively large area, and fairly large amounts of power can only be supplied to precise locations (such as a toothbrush mount). Someya’s power sheets, in contrast, can be large, and they can still supply a large amount of power to gadgets placed near them.


  • Video: See a model system in action

This new capability, he says, is enabled by a novel design and by advances in the fabrication of flexible electronics. The power system actually consists of two types of sheets: one sheet senses the position of an object, and the other sheet supplies power to the object’s point of contact, but not to the rest of the sheet. “In this way, the system selectively feeds power as high as 30 watts to electronic objects placed upon it,” Someya says.

The position-sensing sheet relies on two types of flexible electronics. Using a technique similar to silk screening, the researchers printed an array of copper coils 10 millimeters in diameter. In addition, they used a modified inkjet printer to print an array of organic transistors. Both devices are thin and flexible enough to bend with a sheet of plastic.

Gadgets would need to be equipped with a coil and special power-harvesting circuitry to use the power pad. As the gadget gets closer to the pad, the electrical resistance of the pad’s coils decreases. The array of transistors detects the exact position of the change in resistance and effectively directs the subsequent power flow, which is provided by devices on the second sheet of plastic.

This second power-supplying sheet has an array of switches and copper coils. The switches, made of silver and plastic, turn the electric current on and off, mediating its flow to the adjacent copper coil.

When a laptop is placed on the combined sheets, its position is sensed, and current flows through a coil, enabling electromagnetic induction. The flow of the current creates a magnetic field. If a laptop with a power-harvesting coil comes close enough to the field, it will induce an electric current to flow through the laptop’s coil, wirelessly supplying power.

The researchers’ combining of a number of types of flexible electronics is notable, says John Rogers, professor of materials science at the University of Illinois. Rogers is a pioneer in the field of flexible and stretchable electronics. The new system “appears to represent a very interesting and new application of organic or flexible electronics,” he says.

The sheet of power is research that’s “well worth highlighting,” says Sigurd Wagner, professor of electrical engineering at Princeton. Someya is taking advantage of technology that allows electronics to be printed on a sheet with a large area, he says, and in a way that lets large amounts of power transmit to a selectively small area when needed.

Still, the flexible electronics used to make this prototype is still in its infancy. “There’s a lot of space to improve,” Someya says. The devices aren’t quite reliable enough. They change their characteristics in a period of months, he says, due to oxygen and humidity, which attack organic semiconductors and electrodes. However, he says, he is optimistic because some commercial displays, called organic electroluminescence (OEL) displays, use similar materials, and in recent years the display market has helped drive improvements in these organic devices.

Someya estimates that it will take about five years to overcome the remaining technical issues. Ultimately, he hopes to create a rollable, portable, and reliable power system that could be built into furniture and homes. “Our final goal is to implement the device as infrastructure,” he says, “embedded [in walls and tables] from the beginning.” Imagine, he says, moving a flat-screen television from wall to wall, without needing to worry about plugging it into an electrical outlet.

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