To open up the smartphone screens to commands from other parts of the hand, Chris Harrison, from Carnegie Mellon University in Pittsburg, Pennsylvania, has built the prototype.
A modified smartphone, fitted with a small vibration sensor and running Harrison's FingerSense software, listens for the acoustic and vibrational differences between the three different types of touch, the New Scientist reported.
A fingertip could select an object while a knuckle tap could work like the right-click on a computer mouse and open up a submenu, for example.
"A big problem with touchscreens right now is that they are very simplistic, relative to the capability of our hands. We could do so much more," Harrison said.
"The more ways you have of expressing input into smartphones the better," said Joseph Paradiso at the MIT Media Lab in Boston, who has worked on similar technology.
Harrison said the sensor is a standard piece of electronics that can be added to the main circuit board of any smartphone, and he is already in talks with major phone manufacturers to do just that.
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"The real magic is in the software, this artificial intelligence that lives in the heart of the phone," he said.
Harrison has just launched a company called Qeexo to sell his device.