Biodegradable battery made from food-safe ingredients can power ingestible medical devices before safely dissolving in the gut

A new type of edible battery can operate inside the body for a limited period before being fully broken down in the gut, offering a way to power temporary internal medical devices.

Ingestible and implantable electronics can be used to monitor the gut, track whether patients have taken their medications or deliver electrical stimulation and are increasingly seen as a less invasive alternative to surgery. But these devices need a power source and conventional batteries often contain materials that are toxic. Alkaline and lithium-ion cells can leak corrosive electrolytes or generate gas if their casing fails and their non-degradable components either need retrieving or are left to accumulate as electronic waste in the gastrointestinal tract.

Bioresorbable batteries – that are designed to safely degrade in the body after use – offer a way around this. However, earlier attempts have struggled to be both powerful and reliable enough to work well inside the body.

Now Giovanni Traverso and colleagues at the Massachusetts Institute of Technology, US, have developed a paper battery using materials considered safe for human consumption in small amounts. ‘We used magnesium as the anode and molybdenum trioxide for the cathode,’ explains Mehmet Girayhan Say, a materials scientist at Chalmers University of Technology, Sweden, who led the study. They then used plant-derived cellulose fibres to hold the two metal layers together. This made thinner, sturdier electrodes with better electrical contact, while still allowing the material to break down naturally once inside the body. Finally, they swapped the usual water-based electrolyte for a biodegradable liquid made by melting together two food-safe ingredients, choline chloride and lactic acid. This gave more stable voltage output than the salt water of urea-based liquids used previously. ‘This performance is good [as it’s] directly providing sufficient voltage and relatively okay energy density to drive basic integrated circuits,’ comments Kourosh Kalantar-Zadeh at the University of Sydney, Australia, who was not involved in the work.

The battery is encapsulated in natural waxes – beeswax for short term protection and a plant wax for longer retention – and can be made in two formats: a small circular cell that fits inside a standard capsule and a larger version for more power-hungry devices. Both degrade progressively over weeks to months when tested in simulated stomach acid. ‘The electrode thicknesses and cathode loading were chosen so that even under complete dissolution, the quantities of magnesium and molybdenum released remain below established daily tolerable intake limits,’ says Say.

The researchers tested two devices powered by this battery in pigs. The first was a tracking tag that uses a wireless signal to convey when a capsule has been swallowed. Powered by the new battery, the tag could communicate over much longer distances than typical passive tags and its signal weakened as soon as it moved from open air into body tissue – a clear sign of being swallowed. The second device was an ingestible hormone stimulator that delivered several days of continuous electrical stimulation to the stomach wall. This raised levels of the hunger hormone ghrelin, without damaging the surrounding tissue. The batteries could stay in place in the stomach for at least three days, after which they degraded over a period of a few weeks. ‘The importance is not just the battery itself but what it enables: a fully bioresorbable, retrieval-free electronics platform for the gastrointestinal tract,’ notes Say. ‘It is motivating to show it powering a functional electroceutical device inside a living stomach.’

Say sees the technology’s real destination as full device integration. ‘Pairing ingestible electronics that are powered with a bioresorbable power source would give us a fully self-contained, dissolving biomedicine capsule,’ he says. ‘That is the direction I find most compelling from both a scientific and a clinical standpoint.’