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Bionic Fingers Give Partly Amputated Hands a Good Grip

by Brendon Nafziger, DOTmed News Associate Editor | December 10, 2009
ProDigits by
Touch Bionics
While full "bionic" hands have been available for some time, patients with partly amputated hands (missing fingers) tended to have limited prosthetic options. But all that could be about to change.

On Tuesday, Livingston, UK-based developer Touch Bionics announced that ProDigits (short for prosthetic digits) is now commercially available.

Testing on the technology, which offers fully functional electromechanical fingers, began in November 2000. Over the last two years, around 30 people have been outfitted with the device in early clinical applications, including a pianist in London and a former Navy man injured in a plane crash in Washington state.

The ProDigits device is meant primarily for people missing one or more fingers (including the thumb) past the knuckle. Touch Bionics estimates about 40,000 Americans, and 1.2 million people around the world, could potentially benefit from the product.

The bionic appendage slips over the surviving parts of the hand like a glove, the prosthetic digits sheathed in a protective silicone skin. It's battery-powered, with the battery usually housed on a band on the wrist or forearm. (To keep the device slim, the battery has a life of about one or two days.)

Two kinds of sensors

Because each person needing ProDigits has different injuries, the designers of the device ensured that it could work in one of two ways. The first involves electrodes on the skin which pick up the 20 millivolt signal generated when you flex a muscle. "This signal is detected, amplified and sent into the electronics," Hugh Gill, director of technology and manufacturing at Touch Bionics, tells DOTmed News. By reading these electrical impulses, the device allows the user to open and close the bionic digits at will, pinching fingers or closing for a full grip (squeezing up to 22 pounds of pressure, according to Gill).

But not all patients have the right anatomy for the electrodes to be optimal. For them, Touch Bionics came up with a pressure-sensitive pad, which they call the force sensitive resistor. "What happens is if you press on it," says Gill, "the voltage changes in proportion to the amount applied to it. It sends a signal to the hand, so you get proportional control."

The FSR fits on the palm or the surviving part of the finger, snuggling against the skin. Even if, say, the thumb is missing, the pressure from nearby muscles can be used to activate the pad and control the bionics. "If you lost your digits, you can still see skin rocking [if you flex your muscles], and that would be enough to apply pressure to the sensor," says Gill.