44.6.3. PoNs Device

The current-generation PoNS device (Figure 44.1d) achieves localized electrical stimulation of afferent nerve fibers on the dorsal surface of the tongue via small surface electrodes. Because of the resulting tactile sensation, which, depending on stimulation waveform, typically feels like vibration, tingle, or pressure, it is certain that tactile nerve fibers are activated. Taste sensations are infrequently reported, although it is not known whether gustatory afferents are in fact stimulated, given the nonphysiological patterns of activation likely to result from PoNS-induced stimulation of these fibers.

All electrotactile systems, including the PoNS device, must adhere to a set of core principles to ensure comfortable and controllable tactile percepts, as well as safe operation. As these have already been extensively reviewed (Gibson, 1968; Kaczmarek and Bach-y-Rita, 1995; Kaczmarek et al., 1991; Rollman, 1973; Schaning and Kaczmarek, 2008; Szeto and Riso, 1990; Szeto and Saunders, 1982), we will focus on application of these principles specifically to the PoNS device. An expanded discussion of the waveform, electrode, and safety features appropriate for tongue stimulation has been previously published (Kaczmarek, 2011).

44.6.3.1. Physical Construction

The PoNS device is held lightly in place by the lips and teeth around the neck of a tab that goes into the mouth and rests on the anterior, superior part of the tongue. The paddle-shaped tab of the system has a hexagonally patterned array of 143 gold-plated circular electrodes (1.50-mm diameter on 2.34-mm centers) that is created by a photolithographic process used to make printed circuit boards. The board is an industry-standard polyimide composite that is USP Class VI compliant and meets ISO 10993 biocompatibility standards. The edges and nonelectrode surface of the array tab are coated with a rugged USP Class VI biocompatible epoxy. Therefore, the only materials that contact oral tissues are the gold electrodes and the biocompatible polymers. The remainder of the PCB and all electronic components, including battery, are in a sealed Delrin (USP Class VI compliant) enclosure that remains outside the mouth. Although the PoNS device is built using biocompatible materials, it is investigational and not approved by any regulatory agency. Device function is user-controlled by four buttons: On, Off, Intensity “Up,” and Intensity “Down.” The PoNS device is powered by an internal battery that may be recharged via an external power supply that plugs into a 120-V or 240-V AC electric mains outlet, similarly to a mobile phone.

44.6.3.2. Electrical Stimulation

The tongue electrodes deliver 19-V positive voltage-controlled pulses that are capacitively coupled both to limit maximal charge delivered under any rare circuit failure and also to ensure zero DC to the electrodes, minimizing potential tissue irritation from electrochemical reactions. (Such irritation has never been observed or reported.) Tongue sensitivity to positive pulses is greater than that for negative pulses. The pulse width is adjustable in 64 unequal steps from 0.3 to 60 µs by the intensity buttons. This intensity control scheme takes advantage of the steep section of the strength–duration relationship for electrical stimulation of neural tissue (Reilly, 1998). These pulses repeat at a rate of 200 per second, within the typical physiological firing rate for tactile afferents. Because of the neural refractory period, and extrapolated from earlier single-fiber median nerve response to similar electrotactile stimuli on a rhesus monkey fingerpad (Kaczmarek et al., 2000), it is presumed that at most one action potential results in any given afferent fiber for each stimulation pulse. To minimize sensory adaptation (Kaczmarek and Tyler, 2000) and to ensure a good quality of sensation (Kaczmarek et al., 1992), every fourth pulse is removed from the pulse train, so that each electrode delivers a burst of three pulses every 20 ms (Figure 44.3). This combination of pulse amplitude and width results in an electrotactile stimulus that may be varied by the user from well below sensory threshold to a perceived sensation at the upper limit of comfortability.