Researchers at MIT and other places have contributed to building the first fully flexible device capable of converting energy from Wi-Fi signals into electricity that has the capability to power electronic devices.
In a study appearing in Nature Today, the researchers have demonstrated a new kind of ‘rectenna’ (devices that convert AC electromagnetic waves into DC electricity) – this rectenna makes use of a flexible radio-frequency (RF) antenna that captures electromagnetic waves and uses it as AC waveforms.
Consequently, this antenna is connected to a device made out of a two-dimensional semiconductor with a thickness of just a few atoms. The AC signal travels into the semiconductor, which converts it into a DC voltage. This then comes in handy in powering electronic circuits or recharging batteries.
The rectenna can thus capture and transform active Wi-Fi signals into DC power that can be utilized. Additionally, the device is flexible and can be assembled in a roll-to-roll process to cover very large areas.
Tomás Palacios, a professor in the Department of Electrical Engineering and Computer Science and Director of the MIT/MTL Center for Graphene Devices and 2D Systems in the Microsystems Technology Laboratories, who has co-authored the study paper, stated:
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? 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
The proposed rectenna has been proven to produce about 40 microwatts of power when exposed to the Wi-Fi signals of around 150 microwatts, which can sufficiently power a LED or drive silicon chips. It is anticipated that this device can initially power flexible and wearable electronics, medical devices, and sensors for the “internet of things.”
Powering data communications of implantable medical devices is another anticipated application of the rectenna. Pills that can be swallowed by patients and stream health data back to a computer for diagnostics are gradually being developed by researchers.
Ideally you don’t want to use batteries to power these systems, because if they leak lithium, the patient could die. 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 – Jesús Grajal, Co-author and researcher at the Technical University of Madrid
The component known as the “rectifier,” that converts the AC input signal into DC power is a key component of the rectenna. Traditional rectennas use either silicon or gallium arsenide for the rectifier. Even though such materials can cover the Wi-Fi band, they are rigid. Moreover, using these materials for small devices would be relatively inexpensive; using them to cover vast areas, such as the surfaces of buildings and walls, would be cost-prohibitive.
Unfortunately, the few rectennas reported by researchers up until now operate at such low frequencies that they are unable to capture and convert signals in gigahertz frequencies, where most of the relevant cell phone and Wi-Fi signals are.
The researchers have utilized an innovative 2-D material called molybdenum disulfide (MoS2) for the device in question. The material is three atoms thick, making it one of the thinnest semiconductors in the world.
The team of researchers has found that when exposed to certain chemicals, the atoms of the MoS2 readjust in a way that acts as a switch, forcing a phase transition from a semiconductor to a metallic material, the resulting structure being known as the Schottky diode, which is the junction of a semiconductor with a metal.
According to the first author and EECS postdoc Xu Zhang, “By engineering MoS2 into a 2-D semiconducting-metallic phase junction, we built an atomically thin, ultrafast Schottky diode that simultaneously minimizes the series resistance and parasitic capacitance.”
Parasitic capacitance, where certain materials store a little electrical charge and slow down the circuit, is an unavoidable situation in electronics. So lower parasitic capacitance would mean increased rectifier speeds and higher operating frequencies, with a capability of capturing and converting up to 10 gigahertz of wireless signals.
Such a design has allowed a fully flexible device that is fast enough to cover most of the radio-frequency bands used by our daily electronics, including Wi-Fi, Bluetooth, cellular LTE, and many others – Xu Zhang, first author and EECS postdoc
Regarding output and efficiency, the work states that depending upon the input power of the Wi-Fi input, the current rectenna’s maximum efficiency is at 40 percent. While rectennas made from rigid, more expensive silicon or gallium arsenide achieve around 50 to 60 percent, the power efficiency of the MoS2 rectifier at the typical Wi-Fi power level stands at about 30 percent.
The team includes 15 other researchers and paper co-authors from MIT, Technical University of Madrid, the Army Research Laboratory, Charles III University of Madrid, Boston University, and the University of Southern California, who plan to build more complex systems and improve efficiency.
