A new "near-zero-power" temperature sensor developed by scientists at the University of California, San Diego requires just 113 picowatts -- an infinitesimal amount energy -- to operate.

Engineers believe the sensor could make wearable and implantable devices, as well as other environmental monitoring technologies, much more energy efficient. The sensor could also allow such devices to derive power exclusively from energy created by the body or the surrounding environment.

Researchers detailed their breakthrough in a new paper published this week in the journal Scientific Reports.

"Our vision is to make wearable devices that are so unobtrusive, so invisible that users are virtually unaware that they're wearing their wearables, making them 'unawearables,'" senior study author Patrick Mercier, an electrical engineering professor at UCSD's Jacobs School of Engineering, said in a news release. "Our new near-zero-power technology could one day eliminate the need to ever change or recharge a battery."

The new sensor requires 628 times less power than the most energy efficient sensor currently used temperature-monitoring technologies, including implantable medical devices and smart thermostats.

The sensor is powered by what are known as "gate leakage" transistors -- transistors so thin that they're unable to totally block the flow of electrons. For tiny transistors inside microprocessors, gate leakage is a problem. In the newest sensor, gate leakage is the main source of power.

"Many researchers are trying to get rid of leakage current, but we are exploiting it to build an ultra-low power current source," said Hui Wang, an electrical engineering Ph.D. student at UCSD.

Researchers compounded the low-power current's energy savings by making the temperature digitization process more efficient.

Most sensors pass electricity though a temperature sensitive resistor. As the resistor reacts to the temperature, the passing voltage is affected. An analog to digital converter translates the voltage change to a temperature reading.

The new sensor uses two low-power current sources, one synced with temperature and another with time. A digital feedback loop ensures both flow at the same rate by altering the size of the capacitor used by the temperature-sensitive current. As the temperature drops, the current slows. As a result, the feedback loop switches to a smaller capacitor to keep the currents at the same pace. This capacitor change digitizes the temperature, which is recorded on a small chip.

The trade-off for energy efficiency is that the sensor registers only a single temperature reading per second. But such a rate is plenty fast for devices deployed in the human body and the home.

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