Artificial skin and electrotactile stimulation for advanced tactile feedback in myoelectric prostheses

Modern prosthetic arms and hands are sophisticated robotic devices that can provide some of the motor functions lost due to an amputation. However, none of the commonly used commercial systems restores somatosensory feedback to its user. In principle, the latter can be achieved by recording data from prosthesis sensors and conveying this information by stimulating the sensory structures of the amputee using invasive and noninvasive interfaces. Many such systems have been presented in the literature with promising results; however, they all rely on using only a few discrete stimulation points to transmit information and hence suffer from a limited information bandwidth. In this chapter, we propose a novel concept of a high-bandwidth feedback interface that relies on advanced sensing and stimulation to convey a large amount of information to the prosthesis user. The interface comprises an artificial skin covering the prosthesis with a dense network of tactile sensors (taxels) and a compact stimulation device delivering electrical current pulses through a matrix electrode with many conductive pads. The state-ofthe- art in the two technologies that are required for the implementation of the proposed concept are reviewed. This includes biomimetic e-skins that are suitable for the application in a wearable scenario, stimulation systems integrating a demultiplexing circuit to distribute electrical pulses, and flexible electrodes that can be produced with arbitrary shape, size, and distribution of conductive pads. Finally, the challenges in selecting feedback variables (raw signals versus high-level features) and mapping of these variables into stimulation parameters are addressed.