FIG* I • Diagrammatic representation of current waveforms used in neural stimulation. (A) sinusoidal. (B) Square wave, pulsatile (may be unidirectional as shown, or bidirectional). (G) Charge-balanced, symmetrical biphasic (bidirectional). (D) Charge-balanced, biphasic with brief delay (d) between phases. (E) Charged-balanced, asymmetric biphasic. (F) "Anapol." (G) Charge-balanced, alternating bidirectional. (H) Monophasic, capacitively coupled. Note that T = time of one cycle, or period; t - pulse duration or pulse width; lp = peak current of sinusoidal waveform; da - delay before anodization phase of Anapol waveform; ia - anodization current following stimulation pulse of Anapol waveform; ic = current due to capacitor discharge in monophasic, capacitively coupled waveform. [Reproduced, with permission, from Robblee and Rose (1990), Chapter 2, Fig. 2.5, p. 38, and legend, p. 39.1

developed by Borgens and colleagues (e.g., 1987) and subsequently by Kerns et al. (1987). These devices essentially isolate a metal electrode (usually Ag-AgCl) from the tissues of the body by passing current through a long insulated "wick" such as cotton thread. Over time, however, toxic silver by-products may be able to diffuse through the length of the wick into the body, or the wicks may become blocked.

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