A surface electrode array-based system for functional electrical stimulation

Sha, N 2008, A surface electrode array-based system for functional electrical stimulation , PhD thesis, University of Salford.

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The goal of this thesis is to develop an electrode array-based functional electrical stimulation (FES) system for foot drop correction. The thesis reports a series of studies on the design and development of an electrode array, describes how these results informed the design of a practical array and demonstrates methods for using this array in combination with a multi-channel stimulator to appropriately steer the foot. In the first study a finite element model was used to predict the effects of electrode design on the spatial spread of stimulation selectivity in tissue underlying the cathode. The model suggested that the Imm thick hydrogel of 5000m or higher resistivity placed between the electrode array and skin can effectively maintain selectivity, when compared with the no hydrogel case. In recognition of the potential discomfort problems associated with array-based FES, two studies were carried out to investigate the effect of hydrogel properties on discomfort. Finite element modelling was used to predict the effect of hydrogel properties on current density distribution in the skin, a parameter associated with discomfort. Increasing hydrogel resistivity led to an increase in the homogeneity of current density distribution in relevant areas of the skin and a likely decrease in peak current density. A single-blinded randomised study of the effects of hydrogel resistivity on discomfort was also conducted in which the discomfort associated with stimulation through a low impedance electrode was compared with that experienced during stimulation with a high impedance electrode. The high impedance electrode allowed 9% more current to be passed for an equivalent sensation to that experienced with the low impedance electrode. A 28% decrease in discomfort with the use of the high impedance electrode was also reported. The results of the studies on electrode design informed the design of a practical electrode array, consisting of 64 small electrodes, each of which was independently controllable via a purpose-built stimulator. interfaced with the skin through a thin layer of high resistivity hydrogel. Groups of 2x2 adjacent actiYe electrodes were termed Virtual Electrodes (VEs). Two approaches were presented to find the best VE(s) based on foot response to stimulation. In the first approach foot responses during a slowly ramped increase in stimulus current to each of 49 YEs were measured to directly identify suitable VE(s) (slowly ramped stimulation). Secondly, foot responses to various short bursts of stimulation (twitch stimulation) were then used to predict the subsets containing the best VEe s) identified using the first method. 9 of the 10 subj ects required a single VE and one subj ect needed 2 YEs to steer the foot to the target orientation. It was identified that the lowest number of incorrectly identified YEs using twitch stimulation was found with a four pulse stimuli train at maximal current. As the comprehensive slowly ramped stimulation search is too slow to be used in clinical practice, a two stage process, based on an initial search using twitch stimulation, followed by a more focused ramped stimulation search, is suggested as a future approach. The work presented in the thesis was demonstrated on healthy seated subjects and hence further work is required to investigate its application for people with foot drop.

Item Type: Thesis (PhD)
Depositing User: A Johnson
Date Deposited: 13 Jul 2017 10:39
Last Modified: 13 Jul 2017 10:39
URI: http://usir.salford.ac.uk/id/eprint/42964

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