They're called button cells, coin cells or watch batteries.
By any name, these tiny, round batteries pose a choking danger to small kids.
And if a child succeeds in swallowing a button cell, the battery may short-circuit in the moist esophageal environment, burning the tissue.
A few thousand kids wind up in emergency rooms each year after swallowing a button battery.
But a team of Harvard and M.I.T. researchers that includes prolific inventor Robert Langer thinks they have a partial solution: a protective coating.
The scientists covered batteries with a material—technically a quantum-tunneling composite—in which microparticles of conductive metal are suspended in an insulating layer.
Under most circumstances, including inside of a child, the layer is nonconductive.
But when the material is subjected to high pressure, the microparticles are squeezed close enough together to carry a current.
One such pressurized environment is the typical battery compartment in a small device—you often have to force the battery into place.
So the same battery that remains inert when swallowed works just fine when it's jammed into its slot in a hearing aid.
The waterproof design would also protect batteries from corrosion in high humidity.
The research is in the Proceedings of the National Academy of Sciences USA.
Tests with pigs found the coated batteries to be gentle on the porcine esophagus.
Next step: figure out a way to keep kids from putting the batteries in their mouths in the first place.
Can a quantum tunneling composite be made to taste terrible?