The invisible electrode – zero prep time, ultra low capacitive sensing

The principle technical difficulty in measuring bioelectric signals from the body, such as electroencephalogram (EEG) and electrocardiogram (ECG), lies in establishing good, stable electrical contact to the skin. Traditionally, measurements of human bioelectric activity use resistive contact electrodes, the most widely used of which are ‘paste-on’ (or wet) electrodes. However, the use of wet electrodes is a highly invasive process as some preparation of the skin is necessary in order for the electrode either to adhere to the skin for any length of time or to make adequate electrical contact to the skin. This is uncomfortable for the subject and can lead to considerable irritation of the skin over time, an issue of particular concern in measurements of EEG signals, which typically require an array of electrodes positioned about the head. Despite over 40 years of investigation, including the development of several alternative electrode technologies, no reliable method for making electrical contact to the skin that does not require some modification of its outer layer has been developed. For example, Ag-AgCl dry electrodes, NASICON ceramic electrodes, and saline solution electrodes do not require any skin preparation, but for each electrode the subject experiences skin irritation over extended periods, and there are various issues that cause the performance of the sensors to degrade over time. Alternatively, insulated electrodes that use capacitive coupling to measure the potential changes on the skin have in the past, for noise considerations, used exotic materials to generate a high capacitive coupling (~1 nF) to the skin. The intrinsic noise of insulated electrodes is adequate for bioelectric measurements, but these high capacitance sensors also exhibit long-term compatibility issues with the skin and are sensitive to motion artifact signals due to the electrode’s high sensitivity to relative motion between the skin and the electrode itself. As a result of advances in semiconductor processing techniques and through the use of innovative circuit designs, QUASAR has developed a new class of insulated bioelectrodes (IBEs) that can measure the electric potential at a point in free space. This has made it possible to make measurements of human bioelectric signals without a resistive connection and with modest capacitive coupling to the source of interest. These electrodes are genuinely noninvasive in that they require no skin preparation, have no long-term compatibility issues with the skin, and can measure human bioelectric activity at the microvolt level through clothing while remaining largely immune to motion artifact signals. This paper will present measurements of bioelectric activity made using QUASAR’s IBEs, and corresponding data measured using conventional wet electrodes will also be presented for comparison. The presentation will include through-clothing measurements of bioelectric signals, the rejection of motion artifact signals, non-invasive (i.e. no skin preparation) EEG measurements of alpha-rhythm signals, and the noise levels observed using both types of sensors. In a series of tests conducted on unprepared skin, it was observed that both the IBEs and conventional wet electrodes had similar noise levels. This noise level was higher than the expected noise level, which had been predicted based upon the intrinsic noise characteristics of the sensors. The fact that both sensors suffered from this higher noise level suggested a common mechanism, which was later identified as skin noise. It has been reported in the literature that one of the fundamental noise sources for any bioelectric measurement made on unprepared skin is epidermal artifact noise. This noise is due to potentials developed in the skin itself that are indistinguishable from the bioelectric signal of interest. There exist techniques that can reduce this noise level by as much as a factor of 5, but they involve modification of the skin’s outer layer either by abrasion or chemical absorption of conducting fluid. These methods are not comfortable for the subject and may be difficult to perform on subjects with especially sensitive skin, such as neonates, burn victims, or the elderly. In addition to QUASAR’s IBE sensors, this paper will also discuss a new free-space electrode that is designed to be insensitive to epidermal artifact noise, and thus is capable of bioelectric measurements at the microvolt level in the absence of any skin preparation. The new device exhibits significantly less capacitive coupling to the source of interest than the current generation of QUASAR IBEs, without the increase in intrinsic sensor noise that would accompany a reduction in electrode capacitance.