With the third generation of the Fluidhand, Schulz transferred the technology of flexible fluid actuators to a hand prosthesis. To achieve higher grasping forces, the drives were modified for grasping even heavy objects. The unfolded silicone tubes reinforced with fabric were replaced by miniature folded bellows, which in turn were encased in fabric and attached to aluminum joints in the folds by nylon threads to keep their shape. Three drive elements in each finger, with the two distal bellows coupled together, and two drives in the thumb allow 14 joint axes to move in this hand, equivalent to 14 DOF at 10 iDOF. The fluid actuators were driven by means of miniature hydraulics. The control system, consisting of pump, valve, electronics, sensors and tank, was connected to the prosthesis via a hose approximately 1 m long. The hydraulic unit was the size of a portable telephone and was worn on the belt.

 

The load-bearing structure of the prosthetic hand was modeled on the skeleton of the human hand. The radiating metacarpal bones, which merge into the long fingers, are moveably mounted in the carpus, and spring elements are located between the metacarpal bones to elastically stretch the metacarpus. This design achieves a naturally acting passive abduction of the long fingers, with a positive effect on the adaptability of the hand during grasping and on the natural feel of the hand. The bellows drives are inflated with fluid, usually water, at a pressure of up to 6bar when a finger joint is moved. The expanding bellows thereby flexes the finger joint. The extension of the joint is achieved partly by the suction of the drive, partly by an additional elastic restoring band. The weight of the prosthesis is 190g, the grasping force on the finger is approx. 5N. In this prosthesis functional sample, all fingers were simultaneously filled and deflated via the hydraulic hose to investigate hand function and adaptive grasping. As a cosmetic cover and to create a functional surface, a customized latex glove was fabricated. A first functional sample was successfully tested at the Orthopedic University Hospital in Heidelberg. Schulz, by now head of an interdisciplinary research group, was now able to establish work on a hand prosthesis as a program-oriented research priority.