Journal of Medical and Biological Engineering, 30(6): 355-360 355 A Tactile Display System for Hand Prostheses to Discriminate Pressure and Individual Finger Localization Christian Antfolk 1,* Christian Balkenius 2 Göran Lundborg 3 Birgitta Rosén 3 Fredrik Sebelius 1 1 Department of Measurement Technology and Industrial Electrical Engineering, Lund University, SE-221 00 Lund, Sweden 2 Department of Cognitive Science, Lund University, SE-222 22 Lund, Sweden 3 Department of Hand Surgery, Skåne University Hospital, SE-205 02 Malmö, Sweden Received 31 Mar 2010; Accepted 10 Oct 2010; doi: 10.5405/jmbe.766 Abstract No current commercially available myoelectrically controlled prosthetic hands provide conscious sensory feedback to the user. A system aiming at relocation of sensory input from a prosthetic hand equipped with force sensors to the forearm skin of amputees, a tactile display, has been developed and constructed. The system consists of five piezoresistive force sensors or, alternatively, a prosthetic hand equipped with force sensors, five digital servomotors with a lever and a circular plastic disk pushing on the skin, control electronics based on an MSP430 microcontroller and a test application implemented in LabVIEW running on a PC. The tactile display system is intended to be integrated into the socket of a hand prosthesis and used as a conscious sensory feedback system for hand amputees using a myoelectrically controlled hand prosthesis. The system will provide continuous force feedback from sensors in the fingertips of each prosthetic finger and will likely improve the users’ controllability and perception of the prosthetic hand. Here we report on tests made on “a five site” localization discrimination task and three pressure level discrimination tasks on the forearm of five healthy participants (non-amputees) using the LabView application to generate the stimulations. A mean five-finger discrimination accuracy of 86% and a mean three-level pressure discrimination accuracy of 93% were achieved, indicating the system to be a viable method of producing sensory feedback on the level of individual fingers. Keywords: Sensory feedback, Tactile display, Hand prosthesis 1. Introduction A tactile display is a device that mechanically stimulates a set of points on the skin to produce a specific sensory pattern. This sensory pattern can in principle represent different types of information, but here we are interested in how such a device, applied on the forearm in a future hand prosthesis, can represent pressure or force from an artificial hand and whether the stimulation produced by the tactile display on the forearm can give rise to a conscious perception of touch in a hand prosthesis and thus give a useful sensory feedback to the amputee during use of the prosthetic hand. Normally, in the intact hand, tactile feedback results from the stimulation and activation of mechanoreceptors in the skin responding to pressure (Merkels discs), vibration (Meissner and Pacini corpuscles) and stretching (Ruffinis corpuscles ) and is important for the control of the hand. Perception of the shape of a novel object stems from * Corresponding author: Christian Antfolk Tel: +46-46-2229786; Fax: +46-46-2224527 E-mail: christian.antfolk@elmat.lth.se signals from mechanoreceptors when working in an active exploratory procedure [1,2], i.e. merging sensory and motor information. Combining expectations from previous experiences [3] with contextual information and working memory forms representations of the shape of an object [4]. In addition, the mechanoreceptors of the hand and the sensory inflow are also necessary to produce the feeling that the hand is a part of the body. Lack of sensation from the hand can give rise to effects where the hand is no longer felt as a part of the body [5], much in the same way that a prosthesis is usually considered an extra weight or a tool rather than as a body part. Since current myoelectric prostheses provide no conscious sensory feedback, controlling the force of a grasp is not easily learnt. Users often rely on visual feedback when controlling the grasp force. A myoelectric prosthetic hand with slip sensors that provide feedback to a intrinsic control system exists [6]. However, prostheses that do not have the ability to provide sensory feedback to the user have a higher risk of not being used [7-9]. Having force sensors in the fingertips of a prosthetic hand and subsequently providing sensory feedback using a tactile display could reduce the risk of users not wearing and using their prosthetic hands.