  We present the design, implementation, and experimental evaluation of a
low-cost, customizable, easy-to-use transradial hand prosthesis capable of
adapting its compliance. Variable stiffness actuation (VSA) of the prosthesis
is based on antagonistically arranged tendons coupled to nonlinear springs
driven through a Bowden cable-based power transmission. Bowden cable-based
antagonistic VSA can, not only regulate the stiffness and the position of the
prosthetic hand, but also enables a light-weight and low-cost design, by
opportunistic placement of motors, batteries and controllers on any convenient
location on the human body, while nonlinear springs are conveniently integrated
inside the forearm. The transradial hand prosthesis also features tendon driven
underactuated compliant fingers that allow natural adaption of the hand shape
to wrap around a wide variety of object geometries, while the modulation of the
stiffness of their drive tendons enables the prosthesis to perform various
tasks with high dexterity. The compliant fingers of the prosthesis add inherent
robustness and flexibility, even under impacts. The control of the variable
stiffness transradial hand prosthesis is achieved by an sEMG based natural
human-machine interface.
