'Horrific' injuries
Two years ago, one of Dr Karp's colleagues saw the statistics on the problem - more than 3,000 ingestions per year in the US - and his team started looking for a solution.
Their new pressure-sensitive design makes use of a property called quantum tunnelling, which is also used in touch pads and screens.
"That would be superb, if something this simple could solve a major problem
Dr Kate Parkins, North West Paediatric Transport Service
The negative terminal of a battery was covered with a 1mm-thick layer of a material called a "quantum tunnelling composite" (QTC). It is mostly silicone, but is laced with tiny particles of metal.
When it is squeezed firmly, the metal particles get closer together which allows electrons to "tunnel" between them: a process that can only be explained by quantum mechanics.
"Quantum tunnelling is a wild phenomenon," said Dr Karp. "It essentially achieves the impossible."
Baffling though their details may be, QTCs are already found in many applications. A Yorkshire-based company, Peratech, developed QTCs and licensed them to smartphone manufacturers and to Nasa.
So Dr Karp's team didn't have to look far once they hit upon the idea of making batteries touch-sensitive.
"As a first attempt, we thought, what if we just purchased some of this from a catalogue and place it onto the battery and see if it works?" he told the BBC.
"And it worked quite well!"
With QTC stuck on one side, the edges of the battery were then covered with a sealant, so that the whole thing was waterproof.
Now, the battery would only deliver current if it was under pressure. The rest of the time it was completely inert.