Researchers at MIT have developed a battery-free underwater communications systems that uses near-zero power to transmit sensor data.

The sensor could be used to create a submerged network of interconnected sensors that send data to the surface to help monitor sea temperatures to study climate change or track marine life over long periods of time. Eventually, the sensor could even sample waters on distant planets.

This underwater internet of things (IoT) uses two key features, one called the “piezoelectric effect” that occurs when vibrations in certain materials generate an electrical charge and “backscatter,” a communication technique typically used for RFID tags to transmit data by reflecting modulated wireless signals off a tag and back to a reader.

The Piezo-Acoustic Backscatter System uses a transmitter to send acoustic waves through water toward a piezoelectric sensor. The sensor uses stored energy to reflect a wave back to a receiver, alternating between reflection in such a way as to correspond to the bits in the transmitted data. In the reflected wave, the receiver decodes a 1. For no reflected wave, the receiver decodes a 0.

The sensor created to enable the underwater internet of things. Source: MIT“Once you have a way to transmit 1s and 0s, you can send any information,” said Fadel Adib, an assistant professor in the MIT Media Lab and the Department of Electrical Engineering and Computer Science 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.”

MIT demonstrated the system in an MIT pool where it collected water temperature and pressure measurements. The system was found to transmit 3 kilobytes per sound of data from two sensors simultaneously at a distance of 10 meters between the sensor and receiver.

With data being collected on Saturn’s largest moon, Titan, indicating subsurface water and a mission set to send a rover to that moon in 2026, the sensor could be used to collect samples, researchers said.

“How can you put a sensor under the water on Titan that lasts for long periods of time in a place that’s difficult to get energy?” said Adib. “Sensors that communicate without a battery open up possibilities for sensing in extreme environments.”

How it works

Piezoelectric materials, which are commonly used in microphones and other portable devices, produce a small voltage in response to vibrations and the effect is reversible as applying voltage causes the material to deform. When placed underwater, this effect produces a pressure wave that travels through the water, so they are used to find sunken vessels, marine life or other underwater objects.

“That reversibility is what allows us to develop a very powerful underwater backscatter communication technology,” Adib said.

Allowing the sensor to communicate underwater requires that the piezoelectric resonator doesn’t naturally deform due to strain. The heart of the system is a submerged node, a circuit board with the piezoelectric resonator, an energy-harvesting unit and a microcontroller. The sensor can be integrated into the node by programming the microcontroller while the transmitter and receiver are placed some distance away, researchers said.

The transmitter and receiver require power, but can be on ships, buoys and places where batteries can easily be swapped or connected to outlets and where they can receive information from multiple sensors across many areas — an important feature if tracking a marine animal.

The next steps are to make the system work at greater distances and to communicate with more sensors simultaneously. Researchers will also seek to test the system to transmit sound as well as low-resolution images.