There have been two different types of stimulators: electrotactile or vibrotactile. Electrotactile stimulators use direct electrical stimulation of the nerve ending in the skin to initiate the action potentials; the sensation triggered, burn, itch, pain, pressure etc. depends on the stimulating voltage. Vibrotactile stimulators use pressure and the properties of the mechanoreceptors of the skin to initiate action potentials. There are advantages and disadvantages for both these stimulation systems. With the electrotactile stimulating systems a lot of factors affect the sensation triggered: stimulating voltage, current, waveform, electrode size, material, contact force, skin location, thickness and hydration. Electrotactile stimulation may involve the direct stimulation of the nerves (percutaneous), or through the skin (transcutaneous). Percutaneous application causes additional distress to the patient, and is a major disadvantage of this approach. Furthermore, stimulation of the skin without insertion leads to the need for high voltage stimulation because of the high impedance of the dry skin, unless the tongue is used as a receptor, which requires only about 3% as much voltage. This latter technique is undergoing clinical trials for various applications, and been approved for assistance to the blind in the UK. Alternatively, the roof of the mouth has been proposed as another area where low currents can be felt.