Wirelessly Powered Ingestible Electronic Devices to Track Vital Information

Researchers at MIT, Brigham and Women's Hospital, and the Charles Stark Draper Laboratory have devised a way to wirelessly power small electronic devices in the digestive tract indefinitely after being swallowed. Such devices could be used to sense conditions in the gastrointestinal tract, or carry small reservoirs of drugs to be delivered over an extended period.

Finding a safe and efficient power source is a critical step in the development of such ingestible electronic devices. This new strategy is based on the wireless transfer of power from an antenna outside the body to another one inside the digestive tract. This method yields enough power to run sensors that could monitor heart rate, temperature, or levels of particular nutrients or gases in the stomach.

Right now there is no way of measuring things like core body temperature or concentration of micronutrients over an extended period of time, and with these devices you could start to do that kind of thing.

The research team has been working for several years on different types of ingestible electronics, including sensors that can monitor vital signs, and drug delivery vehicles that can remain in the digestive tract for weeks or months. To power these devices, the team has been exploring various options, including a galvanic cell that is powered by interactions with the acid of the stomach.

However, one drawback to using this type of battery cell is that the metal electrodes stop working over time. In their latest study, the team wanted to come up with a way to power their devices without using electrodes, allowing them to remain in the GI tract indefinitely.

The researchers first considered the possibility of using near-field transmission, that is, wireless energy transfer between two antennas over very small distances. This approach is now used for some cell phone chargers, but because the antennas have to be very close together, the researchers realized it would not work for transferring power over the distances they needed i.e. about 5 to 10 centimeters.

Instead, they decided to explore midfield transmission, which can transfer power across longer distances. Researchers at Stanford University have recently explored using this strategy to power pacemakers, but no one had tried using it for devices in the digestive tract. Using this approach, the researchers were able to deliver 100 to 200 microwatts of power to their device, which is more than enough to power small electronics. A temperature sensor that wirelessly transmits a temperature reading every 10 seconds would require about 30 microwatts, as would a video camera that takes 10 to 20 frames per second.

In a study conducted in pigs, the external antenna was able to transfer power over distances ranging from 2 to 10 centimeters, and the researchers found that the energy transfer caused no tissue damage.

For this study, the researchers used square antennas with 6.8-millimeter sides. The internal antenna has to be small enough that it can be swallowed, but the external antenna can be larger, which offers the possibility of generating larger amounts of energy. The external power source could be used either to continuously power the internal device or to charge it up, Traverso says.

This work, combined with exciting advancements in subthreshold electronics, low-power systems-on-a-chip, and novel packaging miniaturization, can enable many sensing, monitoring, and even stimulation or actuation applications.

The researchers are continuing to explore different ways to power devices in the GI tract, and they hope that some of their devices will be ready for human testing within about five years.