Electrode–tissues interface: modeling and experimental validation

Abstract

The electrode-tissues interface (ETI) is one of the key issues in implantable devices such as stimulators and sensors. Once the stimulator is implanted, safety and reliability become more and more critical. In this case, modeling and monitoring of the ETI are required. We propose an empirical model for the ETI and a dedicated integrated circuit to measure its corresponding complex impedance. These measurements in the frequency range of 1 Hz to 100 kHz were achieved in acute dog experiments. The model demonstrates a closer fitting with experimental measurements. In addition, a custom monitoring device based on a stimuli current generator has been completed to evaluate the phase shift and voltage across the electrodes and to transmit wirelessly the values to an external controller. This integrated circuit has been fabricated in a CMOS 0.18 microm process, which consumes 4 mW only during measurements and occupies an area of 1 mm(2).

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“…The onset of non-linearity may therefore be gradual, and start very early at very limited areas on the electrode surface (e.g., it has been shown that very weak non-linearity is measurable at voltages than 100 mV). Hence, it may be difficult to differentiate between the non-linearity of the electrode processes and the tissue processes [61].…”

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“…Cs is all the shunt capacitance to the ground comprises the capacitance from the metal of the electrode to the solution through the insulation (Figure 3.17) [31,160].…”

Wireless monitoring of electrode-tissues interfaces for long term characterization

“…Changes also depend on different parameters such as the electrode material and surface, and the amount of charge injection, as well as the physiological conditions of tissues. The ETI variations were the subject of several characterisation studies [2] which are usually based on different equivalent circuit models that describe its electrical behaviour [3]. However, measurement of these parameters requires bulky instrumentations, computers, scopes, etc [4].…”

“…It is necessary then to monitor its long term evolution in order to adjust the stimulation parameters to increase safety and efficiency during chronic experiments. Recently, implantable measurement devices have been developed to monitor AC and DC impedances and stimulation thresholds, [1,3,5]. However, restrictions of power and space on implants make the ETI monitoring difficult and insufficient.…”

“…In order to evaluate the ETI environment, several electrical models were proposed [3,[6][7][8][9][10]. The latest model, that takes into account most of the electrochemical processes, involved nearby the interface is illustrated on Fig.…”