Flexible device converts Wi-Fi signals into electricity

MIT scientists have developed the first fully flexible device that can convert energy from Wi-Fi signals into electricity, paving the way for wirelessly powering everyday electronics without batteries. Devices that convert AC electromagnetic waves into DC electricity are known as "rectennas." The researchers demonstrate a new kind of rectenna, described in a study appearing in the journal Nature, that uses a flexible radio-frequency (RF) antenna that captures electromagnetic waves -- including those carrying Wi-Fi -- as AC waveforms. The antenna is then connected to a novel device made out of a two-dimensional semiconductor just a few atoms thick. 

The AC signal travels into the semiconductor, which converts it into a DC voltage that could be used to power electronic circuits or recharge batteries. In this way, the battery-free device passively captures and transforms ubiquitous Wi-Fi signals into useful DC power. Moreover, the device is flexible and can be fabricated in a roll-to-roll process to cover very large areas. "What if we could develop electronic systems that we wrap around a bridge or cover an entire highway, or the walls of our office and bring electronic intelligence to everything around us? How do you provide energy for those electronics?" said Tomas Palacios, a professor at Massachusetts Institute of Technology (MIT) in the US. 

"We have come up with a new way to power the electronics systems of the future -- by harvesting Wi-Fi energy in a way that's easily integrated in large areas -- to bring intelligence to every object around us," said Palacios. Promising early applications for the proposed rectenna include powering flexible and wearable electronics, medical devices, and sensors for the "internet of things." Flexible smartphones, for instance, are a hot new market for major tech firms. In experiments, the researchers' device can produce about 40 microwatts of power when exposed to the typical power levels of Wi-Fi signals (around 150 microwatts). 

That is more than enough power to light up a simple mobile display or silicon chips. Another possible application is powering the data communications of implantable medical devices, said Jesus Grajal, a researcher at the Technical University of Madrid. For example, researchers are beginning to develop pills that can be swallowed by patients and stream health data back to a computer for diagnostics.

"Ideally you don't want to use batteries to power these systems, because if they leak lithium, the patient could die," Grajal said.  "It is much better to harvest energy from the environment to power up these small labs inside the body and communicate data to external computers," he said. 

The research provides blueprints for flexible Wi-Fi-to-electricity devices with substantial output and efficiency. The maximum output efficiency for the current device stands at 40 per cent, depending on the input power of the Wi-Fi input. At the typical Wi-Fi power level, the power efficiency of the rectifier is about 30 per cent. 

For reference, today's best silicon and gallium arsenide rectennas made from rigid, more expensive silicon or gallium arsenide achieve around 50 to 60 per cent. The team is now planning to build more complex systems and improve efficiency.