Skin stretch haptic feedback to convey closure information in anthropomorphic, under-actuated upper limb soft prostheses

Restoring hand function in individuals with upper limb loss is a challenging task, made difficult by the complexity of human hands from both a functional and sensory point of view. Users of commercial prostheses, even sophisticated devices, must visually attend to the hand to know its state, since in most cases they are not provided with any direct sensory information. Among the different types of haptic feedback that can be delivered, information on hand opening is particularly likely to reduce the requirement of constant visual attention. In recent years there has been a trend of using underactuated, compliant multi-fingered hands as upper limb prostheses, in part due to their simplicity and ease of use attributed to low degree-of-freedom actuation. The trend towards underactuation encourages the design of one degree of freedom (d.o.f.) haptic devices to provide intuitive sensory feedback from the prosthesis. However, mapping the closure of a multi-d.o.f. prosthetic hand to a simple and intuitive haptic cue is not a trivial task. In this paper we explore the use of a one d.o.f. skin stretch haptic device, the Rice Haptic Rocker, to provide intuitive proprioceptive feedback indicating overall hand closure of an underactuated prosthesis. The benefits and challenges of the system are assessed in multi-tasking and reduced vision scenarios for an object-size discrimination task, in an effort to simulate challenges in daily life, and are compared against the haptic resolution of the device using the Just Noticeable Difference. Finally, an evaluation done with a prosthesis user, in the form of a truncated version of the AM-ULA, shows possible benefits of the addition of haptic feedback in tasks with reduced visual attention.