A berry-sized thermometer measures body temp. But you have to eat it.

Body temperature is considered a crucial vital sign of general health, but getting a reading with an oral or forehead thermometer does not always tell the whole story. And getting an internal by putting a thermometer up the rectum is not exactly the most pleasant experience.

Those rectal reading days could be numbered. A new ingestible sensor developed at MIT can send doctors continuous body temperature readings right from the GI tract, and could be heading to clinical trials in only a few years. Continuous monitoring like this could make it easier to determine whether a patient is sick and if they’re at risk of developing a dangerously high fever. It can also help for people tracking their fertility or those under anesthesia.

The sensor is shaped like a small berry, only four millimeters high and six millimeters around and is smaller than existing ingestible temperature sensors. The sensor is detailed in a study published today in the journal Nature Electronics.

“A sensor like this gives us the ability to monitor infections and identify them early,” said Giovanni Traverso, a study co-author, engineering professor at MIT, and a gastroenterologist at Brigham and Women’s Hospital, said in a statement. “That’s very relevant, particularly for at-risk populations like people who are immunosuppressed from chemotherapy treatments or immunosuppressive drugs.”

The wild world of ingestible electronics

Several ingestible temperature sensors are currently on the market. However, most of them are the size of a multivitamin or slightly larger. The size makes these sensors more challenging for patients to swallow and can also increase the risk that they obstruct the GI tract.

The sensors’ large size comes from the complex internal circuits inside of them, which require a lot of power. The power comes from on-board batteries that take up the bulk of the capsule’s real estate.

The MIT team’s goal was to design smaller ingestible sensors that can measure temperature accurately.

“The reason for them to be small is safety,” said Traverso. “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.”

The first step towards a smaller device was reducing the size of the three main components, the temperature-sensing circuit, the antenna that relays temperature data, and the battery.

To shrink the circuit, the team built one of their own that can fit onto a 1-square-millimeter silicon chip. They also designed an oscillator based on leakage current to reduce power consumption. Leakage current is a small current that flows through a circuit when it’s switched off. The temperature of the chip’s surroundings affects the current’s frequency.

The circuit can detect temperature with an accuracy of 0.01 degrees Celsius and only requires about 10 nanowatts of power. It can be powered with a small 1.55-volt coin cell battery, that is only 4.8 millimeters around. In one of the trials, the sensor was deployed endoscopically into a sedated pig’s stomach, and then traveled through the GI tract until it was excreted with the pig’s feces.

The new sensor design also used a communication strategy called backscattering to cut energy consumption even further. Backscattering allows most of the sensor’s power requirements to be outsourced to an external antenna located outside the body, generally within a foot or two of the sensor. That external antenna emits an ultra-high-frequency radio wave. The radio wave is then modulated by a tiny antenna within the sensor and sent back to the antenna outside of the body. That external antenna then can calculate the temperature value by interpreting the changes to the radio wave.

“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,” added study lead author and MIT postdoctoral researcher Saransh Sharma.

The internal antenna then sends out a temperature reading once per second, so temperature can be continuously monitored.

Wide applications

According to this team, this type of sensor could be used in several scenarios. Anesthesia often disrupts the body’s normal temperature regulation mechanisms, which can put patients at risk of hypothermia. These sensors could monitor for infection and observe patients both during and after anesthesia. It also could be used at home, monitoring children’s fevers, reading the core body temperature that signals ovulation for couples trying to conceive, and could monitor athletes, soldiers, or anyone else who risks being exposed to extreme temperatures.

The researchers tested the sensors in a pig under anesthesia and also when the animal was awake and moving. The sensors could accurately detect and transmit temperature information in both scenarios.

The team is now working on combining the temperature sensor with different sensors that could measure other vital signs such as heart rate. They hope to begin testing their sensors in clinical trials within a few years.

Traverso believes that sensors like these could become widely used by anyone who needs to monitor their temperature, as long as they are effective for people in high-risk situations.

“I think this could replace all thermometers, because it’s the most accurate way of taking temperature,” he said. “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.”

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