Battery-Free System Could Power Underwater Internet of Things

An underwater “internet of things” is being developed to help scientists investigate the marine environment, and researchers from MIT have developed a battery-free underwater communication system that uses near-zero power to transmit sensor data. 

The system could be used to monitor sea temperatures and track marine life over long periods, or even to sample waters on distant planets.

The system makes use of two key phenomena. One, called the “piezoelectric effect,” occurs when vibrations in certain materials generate an electrical charge. The other is “backscatter,” a communication technique commonly used for RFID tags, that transmits data by reflecting modulated wireless signals off a tag back to a reader.

In the researchers’ system, a transmitter sends acoustic waves through water toward a piezoelectric sensor that has stored data. When the wave hits the sensor, the material vibrates and stores the resulting electrical charge. Then the sensor uses the stored energy to reflect a wave back to a receiver — or it doesn’t reflect one at all. Alternating between reflection in that way corresponds to the bits in the transmitted data: for a reflected wave, the receiver decodes a one; for no reflected wave, the receiver decodes a zero.

“Once you have a way to transmit ones and zeros, you can send any information,” says co-author Fadel Adib, assistant professor in the MIT Media Lab and founding director of the Signal Kinetics Research Group. “Basically, we can communicate with underwater sensors based solely on the incoming sound signals whose energy we are harvesting.”

The researchers demonstrated their Piezo-Acoustic Backscatter System in an MIT pool, using it to collect water temperature and pressure measurements. The system was able to transmit three kilobytes per second of accurate data from two sensors simultaneously at a distance of 10 meters between sensor and receiver.

Piezoelectric materials have been used in microphones and other devices for about 150 years. They produce a small voltage in response to vibrations. But that effect is also reversible: applying voltage causes the material to deform. If placed underwater, that effect produces a pressure wave that travels through the water. They’re often used to detect sunken vessels, fish and other underwater objects.

The transmitter and receiver must have power but can be planted on ships or buoys, where batteries are easier to replace, or connected to outlets on land. One transmitter and one receiver can gather information from many sensors covering one area or many areas.

Adib says the system could also be used to collect data in the recently discovered subsurface ocean on Saturn’s largest moon, Titan. In June, NASA announced the Dragonfly mission to send a rover in 2026 to explore the moon, including sampling water reservoirs.

The MIT work is sponsored, in part, by the U.S Office of Naval Research.