TRANSDUCERS AND BIOELECTRODES

CHAPTER 2 TRANSDUCERS AND BIOELECTRODES

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Examples: Biopotential recording system

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ECG recording system - an example

Consists of: • • • • •

lead system electrodes connection to the amplifier, cables, amplifier, filters recorder computer analysis

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Input Human Body

Output Electrodes

Amplifier

Display/ recorder

Figure 2.1 Block diagram of a system for measuring bioelectric signals

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2.1 Biopotential electrodes 1.

Provide an interface between living tissue and electronic device • One of the most common biomedical sensors is simply an electrode to measure and record potentials in the body • This seems to be a very simple function, but in fact an electrode recording biopotentials is actually a transducer, converting ionic currents in the body into electronic currents in the electrode • This transduction function greatly complicates electrode design

2.

Basic element in the recording • important to understand • requirement for biocompatibility • how an electrode behaves as a transducer • how its behavior affects the recorded signal

• Bioelectricity is a naturally phenomenon that arises because living organism are composed of positive and negative ions in various quantity and concentration • The migration of ions through a region is called ionic conduction • Bioelectrodes are class of sensors that transduce ionic conduction in the body or tissue into electronic conduction so that the signal can be processes in electronic circuits • In other words, a bioelectrode transducer couples the voltage on the surface of the body to an electronic instrument The main applications of bioelectric transducer Juliana Johari

Electrocardiogram (ECG)

Electroencephalogram (EEG)

(EMG)

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• The bioelectrodes are considered as a passive sensor – one that provides its own energy or derives it from the phenomenon being measured.

• The electrodes are either invasive or non-invasive types. For the invasive types, the electrodes penetrate the skin such as needle electrode.

• Metal electrodes commonly consist of a metallic material placed in an electrolyte containing its ions (example: silver-silver chloride)

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Silver-silver chloride electrode • When the metal electrode is in equilibrium with the electrolyte there is no net current flow.

• However, contact with skin or some other tissue causes a net current flow across the surface of the electrode.

Figure 2.2 Ag/AgCl electrode, cross section

• The electrode is now polarized and no longer in equilibrium with the surrounding electrolyte or tissue.

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• The current density is kept to minimum in sensing electrodes to minimize any effects on surrounding tissues.

• Electrode skin impedance ranges from a 103 to 105 Ω.

• Conductive paste is used to the electrode skin interface to reduce the impedance and improve the contact surfaces.

• Impedance depends on the design of the electrode, the properties of the conductive paste and the frequency of the signal generated.

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• Drift and noise are common problems associated with electrodes. Noise is generated as a result of motion artifact between the electrode and skin/tissue. • Baseline drift of the electrode up to magnitude of 100 mV/min has been reported. • Preparation of the skin surfaces, and the use of specially designed electrodes and materials have positive effects in reducing both noise and drift.

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2.2 Electrode Potentials • Biopotentials exist in organisms because the body is made up of cells resting in an electrolytic fluid consisting mostly of sodium, potassium and chloride ions • The differential concentrations of theses ions inside and outside each cell create a potential gradient across the membrane that makes up the cell wall. • For typical human cell, the trans-membrane potential is between 70 and 90 mV, with the inside of the cell being negative with respect to the outside environment.

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• Human skin and other tissues of higher order organisms such as humans are electrolytic and so can be modeled as electrolytic solutions

• Consider a metallic electrode immersed in an electrolytic solution. • Almost immediately after immersion, the electrode begin to discharge some of the ions in the solution, while some of the ions in the solution start combining with the metallic electrodes. After a short while, a charge gradient builds up creating a potential difference or electrode potential, Vc known as a half-cell potential (HCP).

metal

Figure 2.3 metallic electrode immersed in an electrolytic solutions

redistribution of ion concentration on the metal surface

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• Similarly, when an electrode is placed on skin surface, the electrode will begin to discharge some of metallic ions into the electrolyte, while some of the ions in the electrolyte start combining with the metallic electrodes. • After a while, a charge gradient builds up, refer to Figure 2.4. The charge distribution is similar to that of capacitor, being positive over one surface and negative over another and produce an electric potential, Vc .

Figure 2.4 Surface electrode charge distribution

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The electrical equivalent circuit of the impedance model of the interface between skin surface and the electrode is shown in figure 2.5 below

Vh = electrode potential Cd = electrode capacitance Rd = leakage resistance Rel = series resistance (metal, skin, electrolyte)

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Half-cell potentials (a=1, T=25o C) – With respect to standard hydrogen-hydrogen electrode – No current flowing between electrode and electrolyte

Table 2.1 Half-cell Potentials of Common Elements Material

HCP (Volts)

Zn → Zn2+ + 2e-

-0.763 V

Pb → Pb2+ + 2e-

-0.126 V

H2 → 2H- + e-

0.000 (definition)

Ag+Cl-→ AgCl + e-

0.223 V

Ag → Ag+ + e-

0.799 V

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• Each electrode will exhibit its own HCP (Va and Vb) since the two electrodes are dissimilar, the two potentials will be different.

• The difference (Vd) between them causes an electronic current flow through an external circuit. The differential potential, sometimes called an electrode offset potential and is defined as Vd = Va - Vb • The electrode offset potential will be zero when the two electrodes are made of identical materials, which is the usual case in bioelectric sensing

Figure 2.6 shows two different electrodes are immersed in the same electrolytic solution.

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2.3 Types of Bioelectrode Multi-use electrodes • metal plate electrodes • suction electrodes • needle electrodes • micro electrodes Single-use (disposal) electrodes • without electrolyte paste • pre-gelled electrodes • dry electrodes Active (integrated) electrodes

Metal plates

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• The oldest • Also known as disc or strap on electrode

Metallic suction electrode

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• It is used as chest electrode for short term ECG measurement. • It is held in place against the skin by suction

Floating electrodes

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Cross-sectional view of a disposable electrode

Flexible electrodes

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• Modern demands for increased efficiency in neurology monitoring, make metal cups and adhesive tapes are thing of the past. • Many modern electrodes are selfadhesive or stick-on-tapes.

Needle electrodes

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• Indwelling electrodes – intended to be inserted into the body.

• One examle of application is threading through the veins usually in the right arm to the right side of the heart in prder to measure the intracardiac ECG waveform.

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Micro-fabricated electrodes

• They are ultrafine devices that are used to measure the potentials at the cellular level.

• In practice, the microelectrode penetrates a cell that is immersed in an ‘ifinite’ fluid (such as physiological saline) which is in turn connected to a reference electrode.

Micro electrodes

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Supported metal microelectrodes

A glass micropipet electrode filled with an electrolytic solution

(a) Metal-filled glass micropipet. (b) Glass micropipet or probe, coated with metal film.

(a) Section of fine-bore glass capillary. (b) Capillary narrowed through heating and stretching. (c) Final structure of glass-pipet microelectrode.

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2.4 Materials of the Electrodes • The choice of electrode material will affect the half-cell and offset potential.

• Some materials look good initially but have such a large change with time and chemical environment that they are almost rendered useless in practical application.

• There are two general categories of material combination: • A perfectly polarized or perfectly nonreversible electrode -in which there is no net transfer of charge across the metal/electrolyte interface • A perfectly non-polarized or perfectly reversible electrode - in which there is an unhindered transfer of charge between the metal of the electrode and the electrolyte; for example: silver-silver chloride (Ag-AgCl)

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1.

Find the most commonly used materials in bioelectrodes and state their benefits. Give four examples.

2. Draw block diagram that represents a therapeutic medical instrument 3. In carrying out their various functions, the body will generate their own monitoring signals and which is known as bioelectric potentials. Describe with the help of diagrams the phenomenon (origin) of theses bioelectric potentials, that is how the potential is said to be depolarized and repolarized.

‘Be thankful when you don’t know something for it gives you the opportunity to learn’

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2.5 Problems Associated to the Electrodes Two main problems related to the electrodes are: 1. Electrode Contact Impedance (ECI) 2. Half-Cell Potential (HCP)

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Electrode Contact Impedance (ECI) • To measure any physiological variable, the electrode contact impedance varies from 10 kΩ for moisture/damp skin to about 500 kΩ for dry skin.

• In order to measure the physiological variable input impedance of the amplifier should be very large compared to ECI to produce the correct signal.

• Other methods to reduce the ECI effect are: 1. use some conductive liquid such as gel or paste saline (salt + water) 2. use buffer (isolation amplifier) with unity gain

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Half-Cell Potential (HCP) • This problem arises due to the interfacing between electrode on skin surface which produces a small potential difference (voltage) known as HCP.

• Electrode HCP become a significant problem in bioelectric signal acquisition and recording because of the great difference between theses dc potentials and normal biopotentials.

• For example, a typical half-cell potential for a biomedical electrode is 1 V or so whereas biopotentials are less than 1/1000 the half potential !!!

• The surface manifestation of the ECG signal is 1 to 2 mV while EEG scalp potentials are on the order of 50 V. Thus, the half-cell potential is 1000 times greater than the peak ECG potential and 20, 000 times greater than the EEG signal.

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To overcome the HCP, we can either use: i. Using differential amplifier ii. DC offset voltage known as zero suppression method iii. AC coupled

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i. Using differential amplifier

• Figure above shows a circuit model of a biomedical surface electrodes using two identical electrodes. • In this circuit, the differential amplifier will cancel the effects of electrode half-cell potentials Vea and Veb (since the two half-cell potentials should be the same). The biopotential signal is represented as a differential voltage, Vd .

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ii. DC offset voltage known as zero suppression method The signal acquisition circuit may be designed to provide a counter offset to cancel the half-cell potential of the electrode. Although this approach has certain initial appeal, it is limited by the fact that the halfcell potential changes with time and with the relative motion between skin and electrode. Electrode motion can cause a wildly varying dc baseline.

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iii. AC coupled • The input amplifier can be ac-coupled. This approach permits removal of the signal component from the dc offset. This approach is perhaps the most appealing especially where variations of the dc offset are of substantially lower frequency than the signal frequency components. • AC coupled differential input amplifier is required for signal acquisition.

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Figure 2.6 Typical for the recording of bioelectric signals - ECG

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Types of noise DC-offset voltage • electrode potential Artifacts • any unwanted signals that can heavily distort the ECG • myoelectric activity • electrode or cable motion • large transients Interference • the effect of coupling external energy into the measurement circuit • capacitive or inductive coupling

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Sources of noise Noise generated by the patient • myoelectric activity

Noise generated by the electrodes • electrode potential • electrode motion artifact External noise coupled • interference Noise generated by the instrumentation • noise of the amplifier and resistors • imbalances • unidealities

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Common mode voltage VCM Major problem in recording of biopotentials Noise voltage • in a patient • with respect to the ground • common for all electrodes (input) • caused by coupling of external energy

Output of an amplifier

common mode rejection ratio

Common mode voltage VCM

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Common mode rejection ratio, CMRR • High CMRR effectively eliminates the effect of VCM in the output of the system • In real systems CMRR is affected (reduced) by • unidealities of the amplifier • imbalance of electrode impedances • input stray capacitances

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Nine ways to get better adhesion and higher trace quality of ECG 1.

Start by choosing a high quality electrode. Electrodes are not all the same. They vary widely in the quality of the conductive adhesive and the quality of the backing. Be sure to choose an electrode with the best characteristics of both.

2.

Make sure the site is clean and dry. Uickly wipe the area with an alcohol pad if you need to remove oil, moisture or dirt. Then wait for the skin to dry before you apply the electrode.

3.

Reduce electrical resistance on high impedance skin. Tough, leathery or suntanned skin may need to be abraded before electrodes are applied.

Expose skin on hairy areas. Part smooth hair aside with your fingers.

Clip or shave heavy or dense, curly hair.

4.

5.

Increase electrode contact on difficult aites. On hairy or contoured sites, apply greater downward pressure on the electrode backing.

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6.

Allow ‘time on the skin’. Make sure that you apply all the electrodes before you connect the lead wires. Time on the skin will build adhesion and lower impedance dramatically.

7.

Position the electrodes to minimize lead wire stress. Usually this means pointing the electrodes toward the waist but always angle them in the direction of lead wire pull.

8.

Make sure the lead wire connection is clean and secure. Clip the connector securely onto the electrode tab. Take extra care to make sure you do not make contact with the adhesive. This can cause problems with the trace quality.

9.

Maximize the time between placing the last electrode connection and recording the trace. Perform all routine tasks such as entering the patient data and assessing the computer after you apply the electrodes and connect the lead wires. This will help relax the patient and subdue any electrical noise. You may need the ask the patient not to talk and to lie completely still.

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Find a way to be thankful for your troubles and they can become your blessings……….

Thank You