Download Biomedical Instrumentation Electrodes

University of Zagreb
Faculty of Electrical Engineering and
Computing
Biomedical Instrumentation
Transducers and sensors
prof.dr.sc. Ratko Magjarević
November 2015
Biomedical transducers
A transducer is a device that converts a quantity from the measured object into an
electrical signal.
Biomedical transducers are transducers with specific uses in biomedical applications:
physiological measurement,
patient monitoring,
health care.
Measurement quantities: physical and chemical quantities that reflect the
physiological functions in a living body.
Examples:
blood composition - determined from a sample extracted from the body
real-time and continuous measurements - transducer is attached to the body
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Electrodes
• Bioelectrical potentials - recall:
bioelectrical potentials occur at the cell
membrane due to difference in
ions concentration (mostly Na +, K + and Cl-) in
intracellular fluid and in the extracellullar space
• Potential difference at the cellular membrane
may be in the range within 5mV and 100mV
• This potential difference is called the resting
potential
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Electrodes
• Bioelectrical resting potentials:
– resting potential inside the cell is negative
comparing to the environment
– resting potential of nerve and muscle cells is
typically -70mV to -85mV
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Electrodes
• Action potentials:
when the cell membrane is stimulated, there is
a sudden change in membrane conductance, first
for sodium ions (cell depolarization), and
then to potassium ions (repolarization)
• Negative potential inside the cell
reduces, such that short-term potential may become
positive
• Such a potential difference is called the action
potential
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Electrodes
• How to access the cell and measure
bioelectrical potentials?
– individual cells
• thickness of the semi-permeable membrane approx.
10nm
• measurement of in vivo or in vitro
– groups of cells - tissue or organ
• access to tissue or organ - a non-invasive (bloodless) or
invasive measurements
• mutual influence of different tissues /organs
(potentials, impedance)
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Electrodes
Electrode is an interface
• to connect the measurement devices and
measure bioelectrical potentials, electrode is
used as an interface, however..
The electrode is also a transducer
• exchange charge carriers :
– in electrical circuits, electrons are charge carriers
– in the body, ions are charge carriers
• connects to the surface of the body (skin, mucous
membranes) or on/in the organ inside the body
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Electrodes
• Most of bioelectric potentials strive to measure
noninvasively, e.g. from the surface of the body, by
placing electrodes on the skin
• Electrical characteristics of different tissues
– specific conductivity (specific resistance)
– specific dielectric constant
• Characteristics of biological tissue are:
– nonlinearity (dependence on frequency and current density),
– inhomogenity (unequal material properties of the body)
– anisotropy (different properties in different dirrections, typically along
the fiber-cells)
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Electrodes
• Using a model of the interface for better understanding
of the interface electrode -tissue
• Passive electrical characteristics of the skin - electrode
interface strive to express by ideal electric components
with intent parameters
– Resistance
– Capacity
• This model can be used for measurement electrodes in
limited frequency range
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Equivalent circuit of the skinelectrode
Electrode
Skin
Virtual electrode
Biological
issue
Electrode – skin intarface and its simplified electrical
circuit
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Equivalent circuit of the skinelectrode
d
R 
P
A
1
 qn
A
CP  
d

μ - charge mobility
q - charge
n - number of electrons in volume unit
Rp - resistance between the electrode and the well-conductive layer of tissue
(virtual electrode)
d – skin thickness
A - electrode surface
ρ - specific resistance
Cp - capacity between the electrode and virtual electrode
ε - dielectric constant of the skin
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Equivalent circuit of the skinelectrode
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Equivalent circuit of the skinelectrode
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Nonlinearity of the electrode
interface
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Electrodes
Electrode-electrolyte interface
The current crosses it from left to right. The electrode consists of metallic atoms C.
The electrolyte is an aqueous solution containing cations of the electrode metal C + and anions A-.
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Metal-electrolyte potential
Potential
double-layer
Metal
Electrolyte
Electrolyte
Potential
Charge
Dissociation of water to H+
and OH- ions
Potential double-layer at the interface metal-electrolyte
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Metal-electrolyte potential
Standard electrode potential relative to standard hydrogen electrode at 20°C
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Metal-electrolyte potential
• If you plunge a metal in a solution of its salt, the
half cell potential E0 M appears, also the voltage
dependent on the concentration of metal ions in
solution:
RT
E0.5 M  E0 M 1 
ln cM 1
nF
• If there is some other metal also immersed in a
solution of its own ions, its potential will be
E0.5 M
RT
 E0 M 2 
ln cM 2
nF
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Polarization voltage
• If these two solutions are separated with semi-permeable
membrane to allow passage of ions, and to avoid the
original combination of solutions, the potential difference between
the solutions can be measured according to the formula
E  E0.5 M 1  E0 M 2
RT [cM 1 ]
 E0 M 1  E0 M 2 
ln
nF [cM 2 ]
• Each electrode that comes in contact with the electrolyte will
have the potential of the expression above. This potential
is undesirable in the measurement of biological voltage
because when using high gain dc amplifier, it causes saturation
of the amplifier. To avoid saturation, amplifier with less gain in the
input is used and the next stages of amplification are
separated with condenser.
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Dry electrodes
• Used to avoid the
appearance of polarization
voltage.
• The problem is in large input
impedance, which makes
them susceptible to
interference.
• Therefore, the electrode
itself incorporates an
amplifier designed to reduce
the high input resistance to
a small value and thus
reduce the impact of
interference.
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Dry electrodes with integrated amplifier
Microelectrodes
• They are used to measure the biological
potential of the cells
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Microelectrodes
• cell attached recording:
– pipette touching the
membrane and forming
a high-ohmic junction
(~ 1GOhm)
• whole cell recording:
– by suction through a
pipette the membrane
breaks - solution in the
pipette and inside of the
cells become uniform
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Microelectrodes
Extracellular recording
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Microelectrodes
Action potentials recorded extracellularly
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Subcutaneous electrodes
• They are used, for example, to measure the voltage
on the individual muscle fibers or groups of neurons in
the brain
Subcutaneous needle electrodes a) monopolar,
b) bipolar c) wire d) electrode array d) cortical
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Subcutaneous electrodes
• Example of
subcutaneous
electrodes used for
deep brain stimulation
• Example of electrode
implantation for deep
brain stimulation
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Subcutaneous electrodes
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Surface EEG electrodes
• EEG electrodes (passive, active)
• Conductive paste
and gel
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Surface EEG electrodes
• 10-20 system - standards for placing EEG
electrodes
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EEG recording
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Surface ECG electrodes
• Adhesive:
• Suction (pump to
suck air)
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Surface ECG electrodes
• Electrodes for extremities (hands and feet):
• EKG recodring
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EMG electrodes
• Surface
• Subcutaneous
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EMG recording
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Surface electrodes - more examples
• EOG electrodes:
• Electrodes for
electrostimulation
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Literature
• John G. Webster: Medical Instrumentation,
Chapter 5, Biopotential Electrodes
• A. Šantić, Biomedicinska elektronika, 3.
poglavlje, Elektrode za mjerenje biopotencijala
i električne smetnje
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