Safety studies with the University of Melbourne multichannel electrotactile speech processor

Results of safety investigations conducted as an integral part of the development of a multichannel electrotactile speech processor (Tickle Talker) are reported. Electrical parameters of the stimulus waveform, design of the electrode handset and cabling, and the electrical circuitry of the speech processor/stimulator and programming interface have been analyzed for potential risks. Constant current biphasic square pulses delivered to electrodes positioned on the skin surface over the digital nerve bundles were chosen to optimize the safety, comfort, and function of the electrotactile stimulus. The device was battery-powered, and the user circuit was isolated from earth-referenced sources. Each electrode was isolated by capacitive coupling, preventing DC leakage of current to the user circuit. Studies of finger temperature showed slight cooling of the skin on the fingers of both stimulated and unstimulated hands for individual subjects following electrotactile stimulation through the Tickle Talker. Subsequent analysis of finger and hand vascular circulation in five subjects showed slight reductions in hand blood flow in some individuals. The results did not demonstrate a significant mean decrease in hand or finger blood flow following electrotactile stimulation. No evidence of sympathetic involvement was found, nor were any changes in vascular structure of the hand such as those associated with Raynaud's disease found. Evidence suggests that the decrease in temperature found in the initial study may be due to a change in the ratio of blood flow between arteriovenous anastomoses and nutritive capillary beds. Studies of: 1) changes in mean threshold and comfortable pulse widths over time; and, 2) changes in tactual sensitivity as measured by hot/cold, sharp/dull, and two-point difference limen discrimination, did not detect any systematic change in peripheral nervous system function following electrotactile stimulation. Analysis of electroencephalogram (EEG) recordings taken during electrotactile stimulation, and after relatively long periods of experience with the device did not show any pathological changes which might be associated with epileptic foci. In summary, no contraindications to long-term use of the Tickle Talker were detected in the studies performed.