A small voltaic cell was developed that is sustained by the acidic fluids in the stomach. The system can generate enough power to run small sensors or drug delivery devices that can reside in the gastrointestinal tract for extended periods of time. This type of power could offer a safer and lower-cost alternative to the traditional batteries now used to power such devices. This work could lead to a new generation of electronic ingestible pills that could someday enable novel ways of monitoring patient health and/or treating disease.
These devices are usually powered by small batteries, but conventional batteries self-discharge over time and pose a possible safety risk. To overcome those disadvantages, researchers took inspiration from a very simple type of voltaic cell known as a lemon battery that consists of two electrodes — often a galvanized nail and a copper penny — stuck in a lemon. The citric acid in the lemon carries a small electric current between the two electrodes.
To replicate that strategy, researchers attached zinc and copper electrodes to the surface of the ingestible sensor. The zinc emits ions into the acid in the stomach to power the voltaic circuit, generating enough energy to power a commercial temperature sensor and a 900-mega-hertz transmitter. While in the stomach, the voltaic cell produced enough energy to power a temperature sensor, and to wirelessly transmit the data to a base station located two meters away, with a signal sent every 12 seconds. Once the device moved into the small intestine, which is less acidic than the stomach, the cell generated only about 1/100 of what it produced in the stomach.
The current prototype of the device is a cylinder about 40 millimeters long and 12 millimeters in diameter, but the researchers anticipate that they could make the capsule about one-third that size by building a customized integrated circuit that would carry the energy harvester, transmitter, and a small microprocessor. A challenge in implantable medical devices involves managing energy generation, conversion, storage, and utilization. This work will enable researchers to envision new medical devices where the body itself contributes to energy generation, enabling a fully self-sustaining system.
Once the researchers miniaturize the device, other types of sensors can be added, and it could be developed for applications such as long-term monitoring of vital signs. For example, a self-powered pill could monitor vital signs from inside the body for a couple of weeks, making measurements and transmitting them to a mobile phone. Such devices could also be used for drug delivery. The power generated by the voltaic cell could release drugs encapsulated by a gold film. This could be useful for situations in which doctors need to try different dosages of a drug, such as medication for controlling blood pressure.