Dry electrode bio-potential recordings

32nd Annual International Conference of the IEEE EMBS Buenos Aires, Argentina, August 31 - September 4, 2010

Dry electrode Bio-potential Recordings Gaetano Gargiulo 1,2, Paolo Bifulco2, Alistair McEwan1*, Joubin Nasehi Tehrani1, Rafael A. Calvo1*, Maria Romano2, Mariano Ruffo2, Richard Shephard1, Mario Cesarelli2, Craig Jin1*, Armin Mohamed3 and André van Schaik 1* 1 School of Electrical and Information Engineering, The University of Sydney, NSW Australia. 2 Dipartimento di Ingegneria Elettronica e delle Telecomunicazioni (D.I.E.T.) “Federico II” University of Naples, Campania, Italia. 3 Royal Prince Alfred Hospital, Sydney, NSW, Australia. * Senior Members, IEEE Abstract— As wireless bio-medical long term monitoring moves towards personal monitoring it demands very high input impedance systems capable to extend the reading of bio-signal during the daily activities offering a kind of “stress free”, convenient connection, with no need for skin preparation. In particular we highlight the development and broad applications of our own circuits for wearable bio-potential sensor systems enabled by the use of an FET based amplifier circuit with sufficiently high impedance to allow the use of passive dry electrodes which overcome the significant barrier of gel based contacts. In this paper we present the ability of dry electrodes in long term monitoring of ECG, EEG and fetal ECG.

I. INTRODUCTION

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or standard ECG measurements electrodes are usually attached to the patient’s skin after full skin preparation, which includes cleaning, shaving, mechanical abrasion to remove dead skin, and moisturizing. Moreover, a layer of electrically conductive gel is applied in between the skin and the electrodes to reduce the contact impedance [1]. However, the electrolytic gel in wet electrodes dehydrates over time, which reduces the quality of the recorded signals; this is the major problem which can affect long term recording of ECG signals, e.g. Holter recording. In addition, the gel might leak, particularly when monitoring an athlete during training or where the performance causes sweating, which could short circuit the recording sites. Short circuits between recording sites are even a bigger problem for monitoring athletes who are immersed in water. Securing the wet electrodes in place is also complicated, since the electrodes cannot be glued directly to the skin due to the presence of the gel. The use of dry or insulating electrodes avoids these problems [2]. Long-term EEG recordings pose their own unique set of challenges that include the desiccation of the conductive gel or paste. The drying decreases the signal-to-noise ratio and increases the contact impedance, and when added with sweat and gel leakage, electrical short circuits between adjacent sites can result. The use of dry electrodes may be a possible solution for stable long-term EEG recordings, and they are easier to use. There are a number of advantages that our dry electrode system offers. Firstly, it does not require conductive gel or paste between the skin and the electrode surface to record a signal.

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Furthermore, skin/scalp preparation may be avoided when the system it is used in combination with our flexible passive electrodes. We have also developed a simple methodology to apply our flexible, dry and flat electrode to the scalp without the need to shave hair. Pregnancy diagnosis is one of the most frequently performed diagnostic procedures undertaken on cattle [3-7]. Timely testing of individual cows for pregnancy supports optimal management of individual animals and the maximization of farm profit for both the dairy and beef production systems [8, 9]. The current recommendations from the major beef and dairy industry research and development organizations in Australia is for each mated female to be pregnancy tested at least once per year [8, 9]. In many countries and jurisdictions invasive pregnancy testing remains an act of veterinary science and non-veterinarians are unable to operate. Thus, a system based upon the non-invasive detection of the foetal heart beat may allow the development of a real-time, non-invasive and rapid pregnancy diagnostic system for cattle. II. MATERIALS AND METHODS The high input impedance bio-potential recording system used here has been described in references [10-13]. Its performance is attributed to the optimized INA116 amplifier from TI and careful attention to preservation of input impedance, CMRR; furthermore guarding on the PCB and use of, shielded cables and electrodes reduce noise allowing the system to record weak bio-signal like the EEG with measured contact impedance greater than 100 kΩ. A. ECG In order to confirm the ECG recording capability of the new system, it was firstly tested with a suitable signal simulator (Philips Fluke Medsim 300B). This kind of simulator is designed to ensure that both the ECG and patient monitor are in working order. It is capable of simulating ECG signal on all the 12 lead simultaneously with standard level outputs (up to 5 mVpp). The new system was then tested using

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standard ‘wet’ AgAgCl electrodes and dry electrodes on a volunteer subject in parallel with a standard medical approved device over 48 hours. The reference device was g.Bsamp which is used in the biomedical engineering department at “Federico II” the University of Naples. Parameters including signal bandwidth, and the gain of the new system were adjusted as much as possible to match the reference machine. In this study dry electrode where held in place at the sides of the chest with a comfortable and easy to wear elastic band on unprepared skin. Reference electrodes where placed closest as possible on prepared (cleaned and shaved) skin patches. B. EEG Evaluation of EEG systems can be difficult, since many parameters are involved, and EEGs cannot be reproduced in different recording sessions. The evaluation of new EEG recording systems is normally conducted by comparison with a reference system using two methods. In one method, referred to as the parallel method, electrodes for the test and reference EEG system are placed at neighboring locations on the scalp and the signals are examined simultaneously in the time and frequency domains [14]. In another method, referred to as the serial method, the comparison between the reference and test system is made serially by first connecting the reference system to a subject who performs a particular task and then repeating this measurement with the test system [15]. We used the parallel method to record EEG signals, particularly we evaluated the alpha rhythm [16]. The alpha rhythm is easily recognizable and is the strongest brain signals of the EEG. They usually can be detected with the naked eye and can be used as a preliminary and cursory evaluation in EEG recordings [1]. The test EEG system was configured with dry electrodes placed in the standard positions C3, C4 and Cz. The reference system (Compumedics ® Abbotsford, Victoria used at Royal Prince Alfred Hospital, Sydney, NSW) recorded simultaneously from standard locations Fc3, Fc4, Fcz, C1, C2, C5, C6, PC3, PC4 and PCz. The reference EEG system used standard golden brass cup electrodes with conductive gel (wet electrode) held in position using with collodion glue. Each dry electrode was surrounded by four wet electrodes. These experiments were performed on eight healthy volunteers (seven male one female, age range between 25 – 45 years old). [12] It is important to underling that, an experienced electroencephalographer examined the recorded EEG signals for the three presented experiments; thus we can say that the EEG signals recorded from all the subjects, from both the test and reference EEG systems demonstrated normal brain activity in awake subjects. In this study our dry electrodes where held in place using a drop of collodion directly applied on the surface of the electrode. Our dry electrode is made with silicon rubber and can be directly glued on the skin/scalp. Skin contact impedance was measured for both electrodes. While for the reference system it was compulsory to keep this below 5 kΩ, with our system it is possible to take measurement with contact impedance greater than 100 kΩ [12]

C. Foetal ECG As in adults, the well-being of a foetus can be evaluated from its own ECG signal. The signal recordable from a maternal abdominal ECG lead can be regarded as a summation of two different contributes: the maternal ECG (mECG) signal and the fECG signal. Thus a system of abdominal lead placed to cover the entire area, may show a measurable foetal ECG trace. The system has to be multichannel because in some cases the orientation of the foetus inside the uterus is not known. [17, 18]The system proposed was also successfully employed for human and farm animal foetal recording (the latter results are made accessible as courtesy from HEARD systems).

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(b) Figure 1: (a) Example of application of dry electrodes to the abdomen of a pregnant volunteer (b) HEARD systems hand held prototype (courtesy of HEARD systems)

In human recordings passive dry electrodes were sewed inside an elastic garment that could be comfortably worn by the expectant mother without skin preparation. In cattle, dry electrode recordings were performed on the flank using a prototype device developed with Heard Systems. The primary aim here is to record Fetal ECG for pregnancy detection. Circular arrays of multiple dry electrodes rest against the flank of the cow for rapid recordings without the need for skin preparation. III. RESULTS A. ECG Using standard ECG electrodes, correlation between the recordings made with our amplifier and those made with the g.Bsamp was larger than 0.96. [10, 19] Visual inspection of the data (depicted in Figure 2), shows no evident differences

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between the signals confirmed by analytical analysis. Point to point correlation between the two recordings was scoring always larger than 0.96. Data recorded after the electrodes were left in place on the subject for 48 hours, showed evident differences between the signals with an evident information loss for the system using the wet electrodes (see Figure 3).

is more apparent in the frequency domain as shown in Figure 5. Observe the difference in the spectrum around 9 Hz between the eyes open (bold) and eyes closed cases as recorded by a dry electrode (solid line) and a standard wet electrode (dashed line) recorded in standard position C4 and Cp4, respectively. Visually, both the dry and wet electrodes yield similar results and the data shown are representative of the recordings across the eight subjects. Moreover, analytical comparisons, after appropriate time alignment of the signals, showed that the average correlation coefficient between the dry electrode signal and the average of the four surrounding standard electrode signals was 0.94 (across all the subjects). [12]

Figure 4: Alpha wave replacement, dry electrodes are in bold (source ref. [12])

Figure 2: (top) Direct comparison of ECG signals recorded from freshly installed dry electrodes in the proposed system, (bottom) g.Bsamp using Holter Ag/AgCl electrodes

Figure 5: Power spectral density showing alpha wave replacement (solid line C3 dry electrode, dashed Cp3 wet electrode; bold eyes open)

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Figure 3: Direct comparison of ECG signals recorded after 48 hours (top) proposed system with dry electrodes, (bottom) g.Bsamp using Holter Ag/AgCl electrodes

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Figure 6: magnification of one dry electrode abdominal lead. (top) Raw voltage data. (bottom) extracted foetal ECG.

Figure 6 shows an excerpt of data recorded from one out of several abdominal leads from a volunteer subject at the 28th week of gestation. In the picture, foetal QRS complexes are boxed. The top panel represents the raw data as they are recorded. Here foetal ECG is barely observable from the recording. Signal processing to reduce the maternal ECG (bottom panel) made the foetal ECG more visible. The system to be used on cattle is currently under development indeed is has been shown during several farm field trials with HEARD systems that it is capable of detection of foetal signals down to 11 weeks of pregnancy. A single lead excerpt of a combined recording of foetal and maternal ECG in cattle is depicted in Figure 7. 1

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Figure 7: combined recording of foetal (top) and maternal (bottom) ECG from a 16 weeks pregnant cow (Courtesy of HEARD systems) IV. DISCUSSION AND CONCLUSION Giga-ohm input impedance FET amplifiers enable the recording of bio-potentials in a range of important applications: ECG in athlete monitoring, EEG in long term recordings such as the brain computer interface and fetal ECG detection in non-compliant subjects such as cattle. Important considerations must be made when designing the leads, electrodes and printed circuit boards to maintain noise immunity, input impedance and high common mode rejection ratio. Safety is an important consideration where the increasing availability of wireless and low power instrumentation enables ideal isolation from high voltage supplies but at the same time introduces a further concern due to the RF radiation. The availability of optimized instrumentation amplifiers such as the INA116 allows dry recordings without the need for active electrodes that are associated with increased costs and mechanical instability. These amplifiers also conveniently allow single referenced setups to avoid the use of common mode feedback - difficult to provide with dry electrodes in the past due to increased contact impedance. The contact impedance has been a concern in the past for dry electrode recordings; however the high input impedance of the bio-amplifier can now overcome this limitation. For safety it is also important to preserve contact impedance as it occurs as part of the body’s natural protection against electric currents.

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