The impact of ion drift in a Transcutaneous Electrical Stimulation model

Abstract: A major area of research in neuroscience is the effect of stimulating nerves into activation from extrinsic stimuli. This is commonly done with electric currents and external skin-contact electrodes. Research has shown that this method is efficient for producing muscle activation and that different current waveforms produce differing levels of activation and discomfort. This report aims to augment existing transcutaneous electric stimulation models with the effects of ion drift in the electric field caused by the electrodes and investigate how it affects nerve activation. This was done by creating a representative FEM model of the human forearm and directly simulating the effects of introducing ion drift into the model. Models for ionic effects on both electrical conductivity and charge density were included. Simulations showed a very limited effect on nerve activation and that the contributions from changes in ion concentration due to drift was small. Thereafter a 2D-axis symmetric model with the more accurate and costly Nernst Planck Poisson equations showed that the size of charge accumulation and its screening effect on the potential field were both small. Lastly it was studied wether the augmented model could account for different current waveforms yielding different nerve activation patterns, however this could not be replicated. The conclusion from this work is thus that ion drift on the macroscopic scale as modelled here only gives small, almost non-significant results.