It can hold a credit card, use a keyboard with the index finger, and lift a bag weighing up to 20 kg—the new prosthetic Fluidhand can move each finger separately and its construction allows for an unprecedented range of grip configurations and forces.
A patient at the Orthopedic University Hospital in Heidelberg, Germany, was recently the first to have tested the Touch Bionics i-LIMB hand in comparison with the Fluidhand. Eighteen-year-old Sören Wolf, who was born with only one hand, is enthusiastic about the capabilities of these devices.
The Fluidhand has advantages over previous models. For example, current models on the market now allow only a pinch grip using thumb, index, and middle finger, and not a grip using all five fingers. This does not allow a full-wrap grip of an object.
In the i-LIMB, myoelectric signals from the stump of the arm control the prosthesis. Complex electronics and five motors contained in the fingers enable every digit of i-LIMB to be powered individually. A passive positioning of the thumb enables various grip configurations to be activated. The myoelectric signals from the stump control the prosthetic hand; muscle signals are picked up by electrodes on the skin and transferred to the control electronics in the prosthetic hand. Batteries provide the necessary power.
The Fluidhand from Karlsruhe, thus far developed only as a prototype that is also being tested in the Orthopedic University Hospital, is based on a somewhat different principle. Unlike its predecessors, the new hand can close around objects, even those with irregular surfaces. A large contact surface and soft, passive form elements greatly reduce the gripping power required to hold onto such an object. The hand also feels softer, more elastic, and more natural than conventional hard prosthetic devices.
The Fluidhand device also offers better finishing and better grip function. The flexible drives are located directly in the movable finger joints and operate on the biological principle of the spider leg—to flex the joints, elastic chambers are pumped up by miniature hydraulics. In this way, index finger, middle finger and thumb can be moved independently. The prosthetic hand gives the stump feedback, enabling the amputee to sense the strength of the grip.
Thus far, Wolf has been the only patient in Heidelberg who has tested both models.
“This experience is very important for us,” says Simon Steffen, director of the Dept. of Upper Extremities at the Orthopedic University Hospital. The two new models were the best of those tested, with a slight advantage for Fluidhand because of its better finishing, the programmed grip configurations, power feedback, and the more easily adjustable controls. However, this prosthetic device is not in serial production.
“First the developers have to find a company to produce it,” says Alfons Fuchs, director of orthopedics engineering at the Orthopedic University Hospital, but the costs of manufacturing it are comparatively high. Thus far, only one patient in the world has received a Fluidhand for everyday use. A second patient will soon be fitted with this innovative prosthesis in Heidelberg.
