Wireless Dry-Contact Biopotential Electrode

Proceedings of the 3rd International Conference on E-Health and Bioengineering - EHB 2011, 24th-26th November, 2011, Iaşi, Romania ___________________________________________________________________________________________________________________

Wireless Dry-Contact Biopotential Electrode Emil Valchinov, Aleksejs Rutkovskis and Nicolas Pallikarakis University of Patras, Department of Medical Physics, Biomedical Technology Unit, Patras, Greece E-mail: [email protected] Abstract-A wireless ECG/EEG system using dry-contact electrodes is presented. The system consists of a pair of coin sized dry-contact electrodes manufactured on a standard printed circuit board that can operate both on top of the skin and through clothing. A wearable wireless base unit assures the transmission of the ECG/EEG signals to a computer for storage and processing. Each electrode provides a differential gain of 60dB or 80dB over a 0.7-150Hz bandwidth with a noise level of 0.8μV RMS and can be embedded within comfortable layers of fabric. Signals are digitized at the base unit by a 13-bit differential input ADC. The system is ideally suited for integration in wearable ECG chest harness or an EEG headband and can be used to monitor patient cardiac and neural signals in real time at home or in health care facilities. The preliminary results show that the proposed dry-contact electrode performs comparable to Ag/AgCl electrodes using either double sided adhesive disks, chest harness or headband for movement-free attachment. Keywords: ECG, EEG, Dry-Contact Electrode, Body Sensor.

I.

INTRODUCTION

Electrocardiogram (ECG) and Electroencephalogram (EEG) recordings offer a non-invasive means to quantify the heart and brain activity in either medical diagnostic or long term monitoring. Nowadays conventional ECG/EEG recording technologies exploit ubiquitously disposable Ag/AgCl gel based electrodes and the quality of the measurement largely rely on direct, low ohmic contact between the sensor and the subject’s skin. However the widely used clinical grade disposable Ag/AgCl pre-gelled self adhesive electrodes are often cited as irritating [1] and the gel may dry over time making the long-term recordings very difficult. Moreover, the disposable Ag/AgCl electrodes are not environmentally friendly and cost-effective. As an alternative, gel-free dry electrodes [2-3] are recently becoming much more common. However, this approach still require direct electrical contact with the skin and is limited to body areas with no hair. The main drawbacks are their high sensitivity to the condition of the skin and high susceptibility to motion artifacts. In contrast to wet and dry-contact electrodes, non-contact capacitive electrodes do not require ohmic connection to the body and thus are insensitive to skin condition. Since they are capable of measuring biopotentials through hair or clothing, they appear highly suitable for embedment inside garment for a completely unobtrusive patient-friendly monitoring. Although the first working non-contact biopotential sensors were reported decades ago [4] and despite the plethora of works [5-7] that recently appeared in the literature, a device

ISBN: 978-606-544-078-4

for medical applications is not yet on the market. The main obstacle for the non-contact electrodes that still remains and gives origin to all technical difficulties, is the large and highly varying skin-electrode coupling impedance, mainly capacitive. That is what also defines their main drawbacks; high inherent susceptibility to motion/friction artifacts; poor settling times and excessive low frequency noise often exceeding the interface thermal noise. The large skinelectrode coupling impedance puts requirements for ultrahigh input impedance amplifier with low noise levels and thus necessitates an amplifier with zero inputs currents, very low input current noise and minimum input and guard capacitances. These requirements make the design of the front-end op-amp biasing a real challenge since it must not significantly degrade the high input impedance of the amplifier, contribute to the equivalent input capacitance or add excessive noise. One successful solution is a bias network built around two bootstrapped low-leakage anti-parallel diodes [8-9]. Alternatively, it was proposed the so called “bias-free” technique, where no external biasing network is implemented [10] and the op-amp inputs are left floating, thus achieving optimal noise performance. In that case, although the op-amp inputs self-bias through the internal op-amp ESD protection structure and other parasitic leakages, the DC operating point remains undefined within the rail-to-rail input range. Thus the luck of a bias network often results in high unpredictable output offset and low frequency drift. In this paper, we attempt to address these shortcomings by presenting an active dry-contact biopotential electrode that can sense local biopotentials both on top of the skin and through clothes and hair. This was achieved by a careful proprietary electrode design combined with a high-gain active driven grounding, providing sufficient path for the input bias currents and a stable DC operating point. II.

SYSTEM DESIGN

A simplified schematic diagram of the wireless, dry contact sensor system is shown in Figure 1. It consists of two units; wearable on-body electrodes that amplify the local biopotentials and a wireless base unit that filters, samples and transmits the ECG/EEG signals to a computer for storage and processing. The two system parts are connected via a flat 1.25mm ribbon cable. Each electrode contains an onboard buffer, high pass filter and amplifier. The overall electrode design is based on the works previously presented by Chi et al. [10-12]. Each electrode consists of a small round standard Printed Circuit

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Proceedings of the 3rd International Conference on E-Health and Bioengineering - EHB 2011, 24th-26th November, 2011, Iaşi, Romania ___________________________________________________________________________________________________________________ C1

Electrode 1

A2 A1

R1

LMP7702

220nF R3

Wireless Base Unit R6 R7

A3

LMP7702

2.7k 1M

R5 1M

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13-Bit Differential ADC

TLV2462

C4 68nF

[MCP3302] SPI Interface

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common mode line

iSense Core 2 Module

Vcm

[JN5148 module] IEEE 802.15.4

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1nF R9 1M R10

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Fig. 1. Schematic of the implemented wireless ECG/EEG system with dry-contact electrodes.

Board (PCB) of the size of a 2 Euro coin with a 26mm diameter which acts as a physical substrate. The biopotentials are sensed through a 250 mm2 metal plate consisting of solid cooper fill on the PCB’s bottom layer covered with a proprietary solid substance. The active shielding from external electric field pickup is implemented with an outer ring around the sensing plate and with an inner PCB plane just above the sensing plate and the outer ring. The front-end amplifier circuit is built on the PCB’s top layer with the precision CMOS operational amplifier LMP7702 which is an optimal choice with respect to input bias and noise current, voltage noise, supply and input voltage range and unity gain bandwidth. The first front-end op-amp (A1) is configured as a unity gain voltage buffer and provides only a signal conversion and no gain. The active shield is driven through R2 by a buffered version of the input signal. This solution have been shown to be effective [7] in guarding the amplifier input without introducing additional loading to the input. The R1 resistor is used to protect the input of the amplifier. To remove the DC offset as well as low frequency noise, a passive RC filter (R3, C1), with a corner frequency of 0.7 Hz, is used. The filter is followed by a non-inverting amplifier which amplifies the difference between the local biopotential and the common mode voltage Vcm. The two non-inverting amplifiers (A2) at Electrode 1 and 2, form a distributed fully differential stage connected through the common mode line, Vcm. The mid-band differential mode gain for the ECG and EEG pair of electrodes is set to 60dB and 80dB by using respectively 1k and 100ohm resistor values for R4. For true common-mode input signals, the current through R4 is zero,

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so that common-mode signals are absent in the amplified differential signal at the ADC input. Thus, the Common Mode Rejection Ratio (CMRR) of the system is relatively insensitive to component matching and amplifier’s loop gain. Although the use of a buffer at the amplifier front-end is theoretically disadvantageous from a noise perspective, in practice the noise from the skin-electrode interface dominates

Fig. 2. Picture of the dry-contact electrode (left) and the wireless base unit.

the subsequent stages. Moreover, having a unity gain buffer eliminates the need for precisely matched passive components needed to achieve a high CMRR. The connection between the system’s signal ground and the subject, also known as subject grounding, is implemented by an actively driven dummy ground electrode, in order to reduce the common-mode interference. The ground electrode consists of a round PCB of the same size and contact plate covering like in Electrode 1 and 2, but without electronic components. On the base unit, the common-mode signal, Vcm is buffered (A5) and then connected to an inverting amplifier (A6) with a gain of 60dB. The op-amp output is then fed back

Proceedings of the 3rd International Conference on E-Health and Bioengineering - EHB 2011, 24th-26th November, 2011, Iaşi, Romania ___________________________________________________________________________________________________________________ to the subject’s body through resistor R10 and the ground electrode. This circuit theoretically provides an additional 60dB of CMRR for the system. The necessary power and signal ground for the electrodes are provided by the wireless battery-powered base unit (Fig. 2). The effectiveness of the driven dry-contact ground is illustrated in Figure 3 and 4 and is comparable to the classical driven-right-leg circuit design [13]. The combination of active grounding and fully isolated battery-powered system results in a very clean signal free from 50Hz noise. The system can work also with passive or floating grounding [10] but suffers from large 50Hz noise and other low frequency artifacts. The wireless base unit contains anti-aliasing filters, ADC, microcontroller and 2.4GHz radio. The last two were implemented with the iSense Core Module 2 [14], based on the ultra-low power wireless module JN5148 (IEEE 802.15.4). This module provides a comprehensive solution with 32-bit RISC CPU, large memory, and a high performance radio with included RF components. The electrodes output signals are filtered by 2nd order 150Hz low pass filter to prevent aliasing. Bessel filter type in a Salen-Key topology is preferred for its excellent transient response and linear phase. The filtered signals are digitized by a 13-bit differential input conventional SAR ADC (MPC3302), resulting in a LSB of 0.8μV over an input range of 3.3mV and RMS quantization noise of 0.24μV referred to the amplifier input. The measured RMS voltage noise as referred to the amplifier input in the -3dB frequency range 0.7-150Hz was 0.8μV. The system is powered by a 3.7V, 400mAh rechargeable lithium-ion polymer battery providing about 24 hours of continuous monitoring. For the purpose of the measurements presented in this paper, we used a simplified ZigBee communication protocol based on the IEEE 802.15.4 standard, targeting mainly long battery life and error-free data transfer. The amplified biopotential was sampled at 1kHz, transmitted to a PC and handled by logging application written in LabView development environment. III.

lower left side (bellow the heart apex) of the ribcage, corresponding to a standard Lead II torso placement. The ground electrode was placed on the lower right side of the ribcage. Figure 3 shows 1.5 second plots of ECG samples taken with electrodes placed on top of the skin while the subject was standing, walking, jumping and running. Figure 4 shows a 1.5 second plot of ECG sample obtained with electrodes placed over a cotton T-shirt underneath an elastic compression vest while the subject was standing. All relevant features are clearly visible and no 50Hz noise is viewed in the signal. As expected, the signal remains mostly undisturbed

Fig. 3. ECG records obtained from electrodes placed on top of the skin over the chest, during various activities of the subject.

RESULTS

Double sided adhesive disks, custom made compression ECG vest and EEG headband were used, to perform preliminary test measurements with the proposed dry-contact electrode system. The subject was a healthy 25 years old male. Experiments were performed in a standard electrical engineering lab full of electric equipment and intentionally placed desktop PC, LCD monitor and a power line cord at approximately half a meter from the subject. For ECG measurements on top of the skin the electrodes were attached only through double-sided adhesive disks [15]. For ECG tests over a T-shirt a compression vest was used to ensure optimal electrode fixation to the body by providing firm thoracic elastic enclosure. In both cases Electrode 1 and 2 were positioned on the upper right (right shoulder) and the

Fig. 4. Sample ECG trace measured through a cotton T-shirt over the subject’s chest (Lead II torso placement).

while the subject is standing or walking. In the case of running or jumping there are some artifacts but the relevant ECG features are still clearly visible and the signal baseline is stable. No difference of the signal amplitude is observed despite the extremely high source impedances when recording through clothing which theoretically might be expected to cause signal attenuation. Figure 5 shows a 70

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Proceedings of the 3rd International Conference on E-Health and Bioengineering - EHB 2011, 24th-26th November, 2011, Iaşi, Romania ___________________________________________________________________________________________________________________ seconds long ECG record obtained from an actively moving and jumping subject with electrodes placed on top of the skin pressed against the body by the compression vest. The trace remains stable and without significant baseline fluctuations during all the record.

combining an innovative mechanical construction, proper PCB, circuit and system design, it is possible to obtain signals on top of the skin and through clothing or hair, suitable for medical-grade ECG/EEG applications. The preliminary results showed that the proposed dry-contact electrode can tolerate coupling impedance up to several hundred megaohms relatively insusceptible to its variation thus allowing sensing of local biopotentials through clothing or hair. The system presented showed reduced susceptibility to motion artifacts especially with electrodes placed on top of the skin. Optionally when placed over clothes with high electrical isolation, the electrode can operate like a “bias-free” fully non-contact sensor with pure capacitive coupling. The combination of low profile dry-contact sensors, active driven ground and a wireless data transfer resulted in excellent interference rejection, motion artifact reduction and comfortable wear, making the proposed system a potential solution for future mobile health applications. However further studies are needed to evaluate the influence of skin perspiration, fabric material and electrode-skin separation distance over the signal quality.

Fig. 5. A 70 seconds long ECG record obtained from electrodes placed on top of the skin over the chest of an actively moving subject.

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[2] [3] [4] [5] [6] [7] Fig. 6. Spectrogram of a closed eyes EEG recorded bipolarly with electrodes placed on the forehead (Fp2) and on the back (Oz2) over the hair.

A simple tight elastic headband was used to provide firm electrode fixation to the subject’s head. Electrode 1 was placed on the forehead (Fp2) and Electrode 2 on the back (Oz2) over the hair. Figure 6 shows a spectrogram of the 250 seconds EEG record. The subject was sitting and asked to close his eyes and relax. Alpha wave activity is seen (the red horizontal line at about 10Hz) in the time-frequency plot, as expected for awake, relaxed subject. The time-domain EEG record used to calculate the spectrogram was at full bandwidth (0.7-150Hz), without additional digital filtering. IV.

[9]

[10] [11] [12] [13]

CONCLUSION

We preset the design of a wireless biopotential monitoring system using dry-contact electrodes. It was shown that by

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