Textile electrode design and simulation for bio-signal recording

Abstract

This thesis describes a skin and skin-electrode model to simulate bio-signal measurements under different conditions targeting human bio-potential measurements. From reviewing different electrode designs, it is clear that the impedance between the skin and an electrode has a significant effect. The skin-electrode impedance is frequently used as a standard to estimate the performance of an electrode in electrode research. During measurements, the impedance of the skin-electrode interface forms a potential divider which reduces the input signal. Hence it is necessary to identify the effect of the skin-electrode impedance. In this thesis, a skin and electrode model was built using COMSOL simulation; and the effect of the skin and external conditions was analysed.

The skin-electrode model here was different from traditional skin electrode circuit models in the way that the different skin layer properties were included as parameters. With these skin parameters, the key factors in the skin-electrode model can be established and optimised. In addition, the relationship between different electrode sizes and the skin-electrode impedance was quantified using this skin-electrode model.

In this skin-electrode model, the skin and electrode have their own significant parameters that affect the total skin-electrode impedance. For the skin, it was found that the dielectric constant of the skin layer is the most significant parameter in determining the total skin electrode impedance; the skin-electrode impedance difference resulting from the highest and lowest dielectric constant from the literature of the stratum corneum in the skin is over 90 %. For the electrode, when the electrode size was increased from 0.5 cm2 to 2 cm2 , the skin electrode impedance was reduced by about 60 %; and when the electrode size was increased from 2 cm2 to 3 cm2 , the reduction was approximately 13 %. Hence the optimized or efficient electrode size is located between 1.5 cm2 to 2 cm2.

Furthermore, a textile electrode implemented by screen printing was selected to test the performance of the skin-electrode model. The electronic textile is a technology which provides a flexible and comfortable platform for sensors and other electronics. If textile electrodes can be shown to provide satisfactory performance, then they may provide an attractive alternative to other more conventional electrodes. For the textile electrode, the electrode is made using two different materials - silver and silicone carbon rubber. Both electrodes have shown that the most efficient electrode size is around 2 cm2 . Moreover, the textile silicone carbon electrode provides an adhesive and soft electrode surface without the need for gel, this reduces the movement between the adhesive and soft surface during measurements and hence results in less impedance noise than a silver textile electrode. Meanwhile, both electrode materials were simulated in the skin-electrode model with the limitations of the skin-electrode model having been identified.

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Identifiers

ORCID for Michael Tudor: orcid.org/0000-0003-1179-9455

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