A tiny ingestible sensor can measure temperature from inside the body

In a hospital or at home, temperatures are usually taken using an oral or forehead thermometer, but these do not always accurately reflect the core body temperature. Measuring core temperature from within the body could make it easier to determine whether someone is sick, and whether they’re at risk of spiking a dangerous fever.To make it more feasible to obtain core body temperature measurements, MIT engineers have developed an ingestible sensor that can send continuous temperature updates from the GI tract. The sensor is shaped like a tiny blueberry, 6 millimeters in diameter and 4 millimeters in height. That makes it much smaller than existing ingestible temperature sensors, which are more difficult to swallow and pose a potential risk of obstructing the GI tract.“A sensor like this gives us the ability to monitor infections and identify them early,” says Giovanni Traverso, an associate professor of mechanical engineering at MIT, a gastroenterologist at Brigham and Women’s Hospital, and an associate member of the Broad Institute of MIT and Harvard. “That’s very relevant, particularly for at-risk populations like people who are immunosuppressed from chemotherapy treatments or immunosuppressive drugs.”Ingestible sensors could also enable more accurate temperature measurements for fertility tracking, and for monitoring people during anesthesia.Traverso and Anantha Chandrakasan, MIT’s provost and the Vannevar Bush Professor of Electrical Engineering and Computer Science, are the senior authors of the new study. MIT postdoc Saransh Sharma is the lead author of the paper, which appears today in Nature Electronics.Ingestible electronicsA handful of ingestible temperature sensors have become commercially available in recent years, but most are the size of a multivitamin or slightly larger, making them more challenging to swallow. Their size can also increase the risk of obstructing the GI tract.Those capsules tend to be large due to the complex circuits they include, which require a great deal of power. That power is provided by relatively large, on-board batteries that make up much of the bulk of the capsule.The MIT team wanted to design sensors that could measure temperature accurately, but at a much smaller size.“The reason for them to be small is safety,” Traverso says. “We want something that is so small that the risk of any blockage or obstruction is highly mitigated, and also so that it can be easily ingested.”To create a smaller device, the researchers set out to reduce the size of all of the main components — the temperature-sensing circuit, the antenna that relays temperature data, and the battery.For the circuit, they created their own customized circuit that can fit onto a 1-square-millimeter silicon chip. To reduce the chip’s power consumption, the researchers designed an oscillator based on leakage current — the small current that flows through a circuit when it’s off. The frequency of this current varies depending on the temperature of the chip’s surroundings.This circuit, which can detect temperature with an accuracy of 0.01 degrees Celsius, requires very little power — about 10 nanowatts. This means that it can be powered with a 1.55-volt coin cell battery, which is 4.8 millimeters in diameter and about 1.6 millimeter thick.The new design further cuts energy consumption by using a communication strategy known as backscattering. This approach allows most of the power requirements to be outsourced to an external antenna that is located outside the body, within a foot or two of the sensor. The external antenna emits an ultra-high-frequency radio wave, which is then modulated by a tiny antenna within the sensor and sent back to the external antenna. By interpreting the changes in the radio wave, the external antenna can calculate the temperature value.“We combined all of these different pieces together — the silicon chip, the battery, and the antenna — and we made it into an ingestible capsule, which is the smallest ingestible capsule that we have seen for temperature-sensing paradigms,” Sharma says. The internal antenna sends out a temperature reading once every second, allowing for continuous monitoring of temperature.Tiny thermometersThe researchers envision that this kind of sensor could be useful in several scenarios, including monitoring infection and observing patients during and after anesthesia. Anesthesia often disrupts the body’s normal temperature regulation mechanisms, which can put patients at risk of hypothermia.This type of device could also be used at home, for monitoring fevers in children, or measuring core body temperature as a marker of ovulation, for fertility purposes. It could also be useful for monitoring athletes, soldiers, or anyone else who might be exposed to extreme temperatures. To explore these possible uses, the researchers tested the sensors in animals while they were under anesthesia, and found that they could accurately detect and transmit temperature information. They also obtained accurate readings from animals that were awake and actively moving.The researchers are now working on combining the temperature sensor with other sensors that could measure vital signs such as heart rate. They hope to begin testing these types of sensors in clinical trials within the next few years. If proven effective for people in high-risk situations, Traverso believes such sensors could become widely used by anyone who needs to monitor their temperature. “I think this could replace all thermometers, because it’s the most accurate way of taking temperature,” he says. “If we have miniature systems that can be easily swallowed and give very accurate data that’s superior to the current data, I think it can be helpful in so many ways.”Other authors of the paper include Yubin Cai, Injoo Moon, Zhenming Yang, Peter Chai, Niora Fabian, Kailyn Schmidt, Alison Hayward, Andrew Pettinari, Maria Platero, Benedict Laidlaw, and Ashley Guevara.The research was funded by the 711th Human Performance Wing, the Defense Advanced Research Projects Agency (DARPA), and the Advanced Research Projects Agency for Health (ARPA-H), which notes that the views and conclusions contained in this article are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the United States government.