The power of pork bellies

A miniature sensor device that could be swallowed for diagnostics or used to release medication and is powered by the chemistry of stomach acid would avoid the significant risk of using lithium batteries in such devices. Researchers at Massachusetts Institute of Technology (MIT) and Brigham and Women's Hospital have demonstrated a voltaic cell that could sit in the gastrointestinal tract for extended periods of time. Traverso et al., Nature Biomed Eng (2017), 1, 0022; DOI: 10.1038/s41551-016-0022]

"We need to come up with ways to power these ingestible systems for a long time," explains Giovanni Traverso, of the Koch Institute for Integrative Cancer Research. "We see the GI tract as providing a really unique opportunity to house new systems for drug delivery and sensing, and fundamental to these systems is how they are powered."

The same team has previously developed ingestible devices for measuring temperature, heart rate, and breathing rate, and to deliver drugs and suggest that the work is paving the way to electronic ingestible pills. Until now, the experimental devices rely on small lithium batteries, which come with certain risks but also self-discharge over time. To circumvent these problems, the team has taken inspiration from an old favorite of school science projects, the lemon battery, which consists of a galvanized (zinc-coated) nail and a copper penny jammed into a lemon and connected with wires to a low-voltage light bulb. The lemon's citric acid acts as the electrolyte to carry a current from one electrode to the other.

Instead of citric acid, the team exploits stomach acid and replaces the galvanized nail and copper penny with appropriate small-scale electrodes in a permeable plastic capsule that would hold sensor or drug dispenser. Enough current is generated in this setup to power a commercial temperature sensor and a 900 MHz transmitter the signal from which can be picked up two meters away. The team has demonstrated viability for six days or transition time in the GI tract of pigs. Even once in the small intestine, there is sufficient electrolyte to transmit data albeit on a less frequent schedule than every 12 seconds as was possible with stomach power.

The prototype currently being tested is a plastic cylinder 40 millimeters long and 12 millimeters in diameter. The team reckons they could make the device about a third of this size by integrating all the electronic components on to a single circuit. "A big challenge in implantable medical devices involves managing energy generation, conversion, storage, and utilization," MIT team leader Anantha Chandrakasan. "This work allows us to envision new medical devices where the body itself contributes to energy generation enabling a fully self-sustaining system," he says.

"As part of our future research we are aiming to develop novel sensors to detect a range of parameters from the body," Traverso told Materials Today. "Also we are aiming to investigate further safety aspects associated with long term residence of systems like these. Reassuringly, we did not observe any adverse events in our initial study but we will have to do further studies in chronic pre-clinical models."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".