A new two-dimensional electronic technology may be able to harvest radio energy to power a variety of devices, such as hearing aids, sensors, and other tools that make up the so-called “Internet of Things.”
It could be a flexible, flat, semiconductor material that can extract energy from radio signals penetrating urban environments, to be the power source for a new generation of electronics.
According to a team from MIT, a thin layer of molybdenum disulfide (MoS₂ ) , a two-dimensional material no more than three atoms thick, can act as an antenna to convert radio signals from Wi-Fi networks, mobile phones, and broadcast radio or television into power for wireless devices. This technology could bring energy-efficient devices—such as pacemakers, hearing aids, and sensors—into the realm of the Internet of Things.
The power generated by this technology won’t be enough to charge cell phones and tablets until the technology makes significant progress. Even the power requirements of a Fitbit fitness tracker will be slightly greater than the energy it generates. But a small step toward generating power from Wi-Fi signals may be within our grasp. “The future of electronics is bringing intelligence to everything in our lives, from our clothes to our offices to our infrastructure,” says Thomas Palacios, a professor of electrical engineering at MIT. “The key is how to power all these billions of devices.” Palacios believes that thin layers of molybdenum disulfide are a promising option because of their flexibility and the possibility of producing them cheaply using automated printing.
A team from MIT has developed a flexible material capable of extracting radio waves at frequencies up to 10 GHz, covering the common 2.4 GHz and 5 GHz bands used for Wi-Fi and other wireless communication signals. Flexibility is a key characteristic for wearable electronics, as well as many other sensing applications, but generally, other flexible materials absorb little radio wave energy at frequencies above 1.6 GHz, limiting their power extraction capabilities. Palacios says the two-dimensional semiconductor can extract between 30 and 50 microns of Wi-Fi signal in an environment with a diameter of about 100 microns, enough to power pacemakers, hearing aids, emotion and stress sensors, communication devices, and many low-power IoT devices. This system can operate without a battery, making it lightweight and avoiding problems like power leakage from an implanted medical device.
Diji Akinwando, an electrical and computer engineer at the University of Texas at Austin who was not involved in the work, speaks about the MIT team’s achievement. He says, “This is an important preliminary demonstration of technology for extracting energy from ambient Wi-Fi signals. It looked even more interesting because everything is integrated into the same flexible substrate. The next challenge is to increase the efficiency of the devices to produce more power to run modern mobile applications.”
Energy harvesting systems already exist that can power other remotely operated devices, eliminating the need for frequent battery replacements. “Most current systems derive their power from light, temperature differences, or kinetic energy,” says Andreas Schneider, CEO of EnOcean, a German manufacturer of battery-free autonomous devices, which was not involved in the research. He adds that photovoltaic cells can generate enough light energy to power indoor devices at illumination levels up to 100 lux—roughly the level of a typical arcade and less than a third of standard office lighting. The flick of a mechanical switch can also generate enough power to send a signal to turn on a light in a room or upstairs. Temperature gradients along hot water pipes can send signals to the heating system. However, the company found that ambient radio signal energy is insufficient to power current devices unless additional local radio transmitters are added, which Schneider says “are best kept out of reach” due to concerns that electromagnetic field levels could be harmful to health.
Looking ahead to the future of 5G wireless networks and the Internet of Things, Palacios says: “You can use solar cells to generate power, but sunlight is only available during the day. So the other option is to harness the power already present in radio frequency signals, such as Wi-Fi, which fills in most of the time.”
Radio energy harvesters and radio signal receivers collect energy when radio waves interact with antennas. Electromagnetic forces attract electrons in the conductive material back and forth, generating an electric current whose direction alternates with the alternating phase of the waves. Antennas that harvest signals for radio signal receivers send oscillating signals to electrical circuits, which then amplify the signals, converting them into audio or visual frequencies. Antennas that capture radio energy send an oscillating current to an electronic device called a rectifier, which allows the current to flow in only one direction, converting the incoming alternating current to direct current that can power electronic devices or charge batteries.
Current rectifiers are typically made of semiconductors, while antennas are usually made of metals and are characterized by high conductivity. Molybdenum disulfide is “a very good semiconductor,” says Palacios. It can be modified to be highly conductive, allowing it to function as both an antenna and a current rectifier. This device, called a rectenna—antenna and rectifier in one—was invented in the 1960s and is now used to identify devices and objects using radio frequencies (RFID) and in smart cards.
Most current Rectina devices consist of small, rigid wafers made of inflexible semiconductors, such as silicon. These wafers offer good frequency response but suffer from some drawbacks. A team at MIT is the first to create large, flexible Rectina devices capable of generating power from commonly used, unlicensed radio frequencies up to 10 GHz, without requiring a battery to start the process. Flexibility and thinness are essential for their use in wearable devices and “smart exoskeletons,” which can be used in infrastructure, aircraft, or other equipment for continuous monitoring, or as part of a distributed network of smart sensors. The wafers, only three atoms thick, can be fabricated using a common semiconductor manufacturing process: chemical vapor deposition. This process is inexpensive, scalable, and maintains the ability to operate at very high frequencies.
This technology is still in the lab. Production needs to be scaled up, and the thin layers need to be integrated with the devices that will power them. Another challenge is designing devices that operate at just 12 micrometers. But Palacios expects to see its first commercial applications within five to seven years. “The main thing we need is to increase production capacity,” he says. “We need to develop a methodology that allows us to manufacture sensors over a very large area, at a low cost.” Additional uses Palacios anticipates include powering small displays by providing 30 to 50 micrometers of energy and connecting implantable medical devices to external displays. For at least some applications, it may already be possible to extract energy from subtle signals in the surrounding air.
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