Characterisation of wearable and implantable physiological measurement devices

Abstract

Electrodes are an important part of any biopotential measurement application. The electrodes will be in direct galvanic contact with the skin or tissue of the measurement subject. The interface between the electrode and electrolyte has a complicated structure involving both physical and chemical processes and reactions. The thesis revises the complex interface in terms of reactions occurring at the interface, the formation and structure of the double layer at the interface, electrode potential, and various electrical equivalent interface models. The artefacts occurring at the interface are also introduced and possibilities to reduce them are discussed.

A relation between the artefacts and electrode materials is established through electrochemical noise measurements performed with several metallic electrodes as well as with textile electrodes. The electrochemical noise arising from the interface as a function of time reflects the stabilisation time of the current electrode--electrolyte interface. The electrochemical noise will reduce as time from the application of the electrode on the subject elapses. The time the interface needs to reach its steady state is called the stabilisation time of the electrode. Electrode materials that possess a short stabilisation time are the most suitable ones for applications where artefacts are probable. Such applications are the ones that involve e.g. motion or deformation of the skin of the subject.

Noise measurements were conducted with gold (Au), silver (Ag), silver-silver-chloride (Ag/ AgCl), platinum (Pt), stainless steel (AISI 316L), and textile (silver and copper yarns as conductive material) electrodes. The results show that Ag/AgCl electrodes have the shortest stabilisation time or, alternatively, least noise in the biopotential measurement applications. Stainless steel electrodes also showed good performance in terms of low electrochemical interface noise. It was also verified that all the electrodes will exhibit an equivalent noise level despite of the material as time elapses 10 minutes or more from the application of the electrodes on the electrolyte. Based on the measurement results, optimal materials to be used as electrodes can be determined.

The complex electrode-electrolyte interface can also be expressed as an electrical equivalent circuit model known as the lumped-element model. The model component values were determined from the measurements of some of the electrodes under research in the electrochemical noise measurements. The knowledge about the component values provides means to calculate the impedance of the electrode which has to be taken into account in designing the measurement amplifier. The interface components also form a natural RC-filter which has to be taken into account when determining the measurement signal bandwidth.

The measurements of the model components were performed with a square wave method, a novel and relatively simple measurement technique. The measurements done in the thesis applied the technique successfully to the component measurements although originally the technique was used for other purposes. The measurement results obeyed the frequency vs. impedance curves widely accepted among scientists.

Some implantable biopotential measurement devices have been designed and realised and the results are reported in the thesis. An inductively powered implantable electrocardiogram (ECG) measurement device is presented and both in vitro and in vivo measurement results are reported. A resonance-based biopotential measurement device is also introduced. The measurement device has an extremely simple construction and is basically a resonating LC-tank whose impedance is modified by a varactor. The reflected impedance of the LC-tank can be measured at the detector device from which the biopotential can be derived. Measurement results of the human ECG measured from the skin surface with the device are reported in the thesis.