Taking your temperature from the inside

Oral and forehead thermometers may not accurately capture a person’s core body temperature, and the few ingestible temperature sensors on the market are so big they are hard to swallow and risk obstructing the GI tract. But MIT engineers created one that can send continuous temperature updates at a size of just six by four millimeters. To create it, the researchers built a circuit that can fit on a one-square-­millimeter silicon chip and designed an oscillator based on leakage current—the small current that flows through a circuit when it’s off. Its frequency varies depending on the temperature of the chip’s surroundings. The circuit detects temperature within 0.01 °C and can be powered with a 1.55-volt coin cell battery. A strategy known as backscattering further cuts energy consumption, allowing most of the power requirements to be outsourced to an antenna outside the body. It emits an ultra-high-frequency radio wave, which is modulated by an antenna within the sensor and sent back to the external one. By interpreting the changes in the radio wave, the external antenna can calculate the temperature value. The result is “the smallest ingestible capsule that we have seen for temperature-­sensing paradigms,” says Saransh Sharma, a former MIT postdoc now at the University of Cambridge, who is lead author of a paper on the work.  The researchers envision using it to monitor infection, observe patients during and after anesthesia, track fevers in children, help mark ovulation, and more. It could also be used to monitor athletes, soldiers, or anyone else exposed to extreme temperatures.  “A sensor like this gives us the ability to monitor infections and identify them early,” says Giovanni Traverso, an associate professor of mechanical engineering and one of the paper’s senior authors, along with MIT provost Anantha Chandrakasan. “That’s very relevant, particularly for at-risk populations like people who are immunosuppressed.” Ultimately, he hopes, it could replace other types of thermometers for everyone.