The technology could let users control wireless devices when their hands are full - answering the phone while cooking, for instance.
The device, called Nail, developed by the Massachusetts Institute of Technology researchers was inspired by the colourful stickers that some women apply to their nails.
The researchers envision that a commercial version of the device would have a detachable membrane on its surface, so that users could coordinate surface patterns with their outfits.
To build their prototype, the researchers needed to find a way to pack capacitive sensors, a battery, and three separate chips - a microcontroller, a Bluetooth radio chip, and a capacitive-sensing chip - into a space no larger than a thumbnail.
"The hardest part was probably the antenna design," said Artem Dementyev, a graduate student in media arts and sciences.
"You have to put the antenna far enough away from the chips so that it doesn't interfere with them," Dementyev said.
For their initial prototype, the researchers built their sensors by printing copper electrodes on sheets of flexible polyester, which allowed them to experiment with a range of different electrode layouts.
But in ongoing experiments, they're using off-the-shelf sheets of electrodes like those found in some track pads.
Researchers have also discussed with battery manufacturers and identified a technology that they think could yield a battery that fits in the space of a thumbnail, but is only half a millimetre thick.
A special-purpose chip that combines the functions of the microcontroller, radio, and capacitive sensor would further save space.
As the site for a wearable input device, the thumbnail has advantages: It's a hard surface with no nerve endings, so a device affixed to it wouldn't impair movement or cause discomfort.
It is easily accessed by the other fingers - even when the user is holding something in his or her hand, researchers said.