Electrical currents in metallic conductors such as the BION circuits and electrodes are carried by free electrons. Electrical currents in biological tissues are carried by the motion of ions. There are two basic mechanisms for passing current from the one medium to the other: capacitive "double-layer" charging and electrochemical reactions. Electrochemical reactions occur whenever the voltage across the metal-electrolyte junction reaches the free energy barrier for the reaction in question. For "noble" metals such as platinum, this tends to occur at about ±0.8 VDC, resulting in electrolysis of water, denaturation of proteins, and gradual dissolution of the electrode into heavy metal ions.6 All of these reactions are irreversible electrochemically, and they tend to generate toxic products that are severely deleterious to the adjacent tissue. Capacitive charging permits alternating currents to be passed indefinitely across the metal-electrolyte junction as long as the charge density (coulombs per square centimeter of electrode surface area) in any single phase of the waveform does not polarize the interface to this ±0.8 VDC threshold. In conventional clinical stimulators, this is usually accomplished by using charge-regulated AC waveforms with symmetrical alternating phases and by incorporating a capacitor between the stimulus generator and the electrodes to block any residual DC leakage. Such circuitry is not practical for the BION.
In order to maximize the power efficiency and safety of the BION within the constraints of its unusually small package, we developed a novel electrode-tissue interface system based on the special properties of tantalum9 and iridium.10 The instantaneous power required during a single stimulus pulse at maximal intensity (30 mA through 0.5 kilohms = 0.45 W) is far higher than can be transmitted over the relatively weak inductive link between the external RF coil and the tiny implant coil. The BION takes advantage of the low duty cycle required for muscle stimulation (pulse width <0.5 ms times pulse rate <20 pps yields <1% duty cycle). During interpulse intervals, it stores energy from the RF field on an electrolytic capacitor. However, the BION package has little room available for such a discrete component. Furthermore, a component failure resulting in DC leakage would result in severe electrolysis if the system were powered. Instead, the BION employs a fail-safe electrolytic capacitor consisting of its output electrodes and the body fluids (Chapter 3, Color Figure 2*).
The Ta electrode is made from a sintered powder, resulting in a porous structure with a very high surface area. It is anodized to +70 VDC (four times the maximal operating voltage of +17 VDC) in the last step of BION fabrication, resulting in an electrolytic capacitor with about 4 pF capacitance and less than 1 pA leakage current. The resulting tantalum pentoxide surface is biocompatible and capable of self-healing by reanodization in saline if damaged.9 The counter-electrode is iridium, which develops a porous, electrically conductive oxide layer in its "activated" state.10 Iridium exhibits a range of positive valence states with essentially no polarization, allowing each layer of the oxide coating to absorb and release hydroxyl ions from solution while simultaneously shifting electrons out of and into the metal, respectively. Thus,
* Chapter 3, Color Figure 2 follows page 112. © 2001 by CRC Press LLC
it acts like a metal-electrolyte capacitance with an extremely large surface area and with a tendency to dissipate any net polarization by shifting the distribution of valence states among the constituent iridium atoms in the oxide layers.11
The normal operating mode of the BION is to charge continuously the output electrodes to the regulated high voltage supply powered by the RF field (+17 VDC Ta vs. Ir) and to generate stimulation pulses by discharging this capacitor through a current-regulated circuit for the desired number of clock cycles. As long as the mean rate of charge dissipation (stimulus current times pulse duration times pulse rate) is lower than the recharging capability, the electrodes stay charged to the full 17 V compliance voltage. This compliance voltage limits the maximal stimulus current that can be generated through a given load, which tends to be dominated by the resistance of the tissue in which the implant is located. This is generally in the range of 500-1000 ohms for a long term implant in muscle.12,13
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