6.2.1 Neurostimulation

The purpose of neural stimulation is to deliver the electric charge to a nerve bundle, a

neuron, or generally, to neural tissue. For example, in patients suffering from epilepsy,

upon detecting the onset of a seizure via a neural recording front end, a neuro­

stimulator is used to deliver charge to the brain tissue via a set of electrodes [2]. When

enough charges are delivered, the tissue’s membrane depolarizes to below its

threshold; this produces a unidirectional action potential that prevents the seizure. The

neurostimulator accomplishing this task must be designed such that it delivers enough

charges to cause the depolarization. Secondly, the neurostimulator must deliver the

stimulation such that there is no charge imbalance or residual average direct current

(DC) at the membrane.

Charge imbalance can cause tissue damage [2]. For instance, Aran et al. reported tissue

damage occurring in guinea pig cochleae at residual average DC levels in the range of

20–40 μA [3]. Similarly, Hurlbert et al. reported that when the average residual DC was

maintained at 1.5 μA for rat spinal cords, there was no pathological change around the

electrodes even after prolonged stimulation. This indicated that such an amount of

average residual DC was safe for the tissue. Therefore, achieving charge balance, in ad­

dition to achieving the correct current amplitude and temporal characteristics, is a central

aspect of neurostimulator design [4]. Current-mode stimulation is used to achieve bi­

phasic stimulation and ensures that charge balance is maintained.

Figure 6.1 illustrates a general configuration for charge balanced-neurostimulation

along with its associated stimulation regimen. A cathodic pulse with pre-defined am­

plitude Icath and duration tcath is applied to the nerve to deliver the required electric

charge on the tissue membrane. This pulse is maintained until the threshold membrane

potential is reached and the action potential is elicited. With a brief delay, the anodic

pulse (Ianod, tanod) is initiated to offset the previously supplied charge to maintain charge

balance. To guarantee a complete charge balance or at least a safe average residual DC

after each cycle, the current source and current sink arms of the circuit need to be ac­

curately matched. Unfortunately, achieving such matching is not feasible in practice due

FIGURE 6.1

Schematic of an electrical stimulation scheme of a nerve. (a) Switched current sources are connected to the nerve

via an electrode. (b) Typical biphasic pattern showing switch operation and the current delivered to the nerve as

a function of time.

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