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BG3105 > Bioelectrodes > Flashcards

Bioelectrodes Flashcards

1
Q

Functions of bioelectrodes

A

Provides electrical interface between the body (or tissues/cells) and electronic (measuring or stimulating) apparatus, by converting ionic current to electronic current or vice versa

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2
Q

Types of bioelectrodes

A
  1. Biopotential recording electrodes
    - measurement of bioelectric events
    - Ionic current -> electronic current
    - Low current
  2. Stimulation electrodes
    - Delivery of current to living tissue for functional stimulation
    - Electronic current -> ionic current
    - High current
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3
Q

Explain the electrode- electrolyte interface

A 
For charge (or current) across the EEI interface, electrochemical reactions must occur at the EEI. 
When a metallic electrode is immersed in an electrolytic solution, some metallic ions dissociate into the solution, creating an excess negative charge on the electrode surface and excess positive charges in the solution near the electrode. Thus an electric double layer forms and a potential builds up which is called as half cell potential. Different materials exhibit different hall cell potentials
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4
Q

Describe the electrical model of an electrode: ideal polarizable electrode

A
  • No net transfer of charge occurs across the EEI -> no electrode reactions can occur to convert ionic current to electronic current
  • Electrode behaves like a capacitor, and only capacitive current is flowing upon a change of potential
  • The potential at EEI is altered by charge accumulation
  • Suitable for stimulation
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5
Q

Describe the electrical model of an electrode: ideal non-polarizable electrode

A
  • Unhindered exchange of charge occurs as a result of oxidation-reduction reactions to convert ionic current to electronic current or vice versa
  • The potential of at the EEI does not change from its eqm potential with the application of even a large current density; and because electrode reaction is extremely fast, the electrical resistance at the interface is small
  • Suitable for biopotential recording
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6
Q

Example of noble metals that is close to behaving as perfectly polarizable electrodes

A

Platinum

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7
Q

Example of electrode that is close to behaving as perfectly nonpolarizable electrodes

A

Ag/AgCl electrode - low half cell potential and very stable

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8
Q

Electrical model of an electrode: Faradic resistance (Rd)

A

Reflects the rate of charge movement (from electrode to electrolyte or vice versa) resulting from the electrochemical reaction at EEI
For ideal non-polarizable electrodes, Rd -> 0

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9
Q

Electrical model of an electrode: Double layer capacitance (Cd)

A

magnitude depends inversely in the separation of the charged surfaces. In this case, the thickness (d) is molecular -> capacitance is remarkably high. Typical, Cd = 10-20uF/cm^2

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10
Q

Function of biopotential electrodes

A

They serve as transducers to convert ionic currents into electronic currents via electrochemical reactions (oxidation/reduction reactions at the EEI)

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11
Q

What are the most commonly-used biopotential electrodes

A

Ag/AgCl electrodes

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12
Q

Advantages of Ag/AgCl electrodes as biopotential electrodes

A
  • Fee 2-way exchange of Ag+ and Cl- ions
  • Minimal change in charge distribution near electrode
  • Very stable half-potential (insensitive to current)
  • Easily produced
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13
Q

Problems of Ag/AgCl electrodes as biopotential electrodes

A
  • Silver may be toxic when used inside the human body

- Mechanically vulnerable

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14
Q

What is overpotential

A

It is the difference between the observed half cell potential and the equilibrium zero-current half cell potential

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15
Q

Why do overpotentials exist

A

polarization of the electrode

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16
Q

What contribute to overpotentials

A

Ohmic, concentration and activation overpotentials

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17
Q

What is ohmic overpotential

A

It is the direct result of the resistance across the double-layer (Rd)
According to Ohm’s law, the voltage across the double layer = Rd*I (the sign depends on the current direction)
Note that Rd often also varies with the current

18
Q

What is concentration overpotential

A

It results from changes in the distribution of ions, in the electrolyte, in the vicinity of the electrode-electrolyte interface. In the presence of current, oxidation reaction or reduction reaction at the interface dominates over the other, and that changes ion concentrations.

19
Q

What is activation overpotential

A

The activation energy of the oxidation reaction differs from that of the reduction reaction -> overpotential resulted from the activation energy depends on the direction of the current

20
Q

What is the recording problem associated with half cell potential

A

The movement of the electrode disturbs the EEI and results in a momentary change of the half cell potential. If a pair of electrodes are used, movement of 1 electrode -> potential diff between 2 electrodes -> motion artifact -> can be a serious cause of interference in the measurement of biopotentials

21
Q

How to solve the problems associated with half cell potential

A
  1. Use electrodes that have low and stable half cell potentials
  2. To use differential amplifier to acquire the signal, so large DC half-potential is not amplified. But note that the gains required to process low-level biosignals also act on the difference between 2 half cell potentials
  3. To provide counter-ofset voltage in the pre-amplifier design to cancel the half cell potential of the electrode. Limitation: half cell potential changes with time and the relative motion between skin and electrode
  4. Electrode motion can cause a widely varying baseline (motion artifact). Using nonpolarizable electrode minimizes motion artifact. Using gel, suction, adhesive or floating electrode to ensure electrode remains stable in position.
  5. Use a high-pass filter at the input of amplifier. This approach permits removal of the DC and low frequency motion-artifact components from the recording. Esp appealing when variations of the DC offset are of substantially lower frequency than the signal frequency components (e.g. EMG). But signal may be distorted if it contains major low-frequency components (ECG)
22
Q

What is the high impedance issue about

A

Surface electrodes are those that are placed in contact with the skin of the subject. Human skin tends to have high impedance compared with other voltage sources.

23
Q

What are the recording problems associated with high impedance

A
  1. Amplitude reduction of recorded signal

2. High thermal voltage noise

24
Q

Solutions for recording problems associated with high impedance

A
  1. Cleaning skin, skin abrasion, and applying conductive gel -> reduce the impedance between the skin and electrode
  2. Using an amplifier with high input resistance (impedance) - at least 10 times more than the source impedance (5Mohm >) - minimizes signal loss
25
Q

Examples of body-surface electrodes

A
  • Metal plate ECG electrodes
  • Ag/AgCl ECG electrodes
  • Suction ECG electrodes
  • Floating ECG electrodes - for long-term recording, the actual electrode element is recessed in a cavity so that it does not come in contact with the skin, helps to reduce the motion artifact
  • Flexible body-surface electrodes - carbon-filled silicone rubber electrode,, flexible thin-film neonatal electrode
26
Q

Describe how needle electrodes for ECG and EMG can be used

A
  • The electrode is inserted into the tissue immediately beneath the skin by puncturing the skin at a large oblique angle
  • The ECG needle is only used for exceptionally poor skin, especially on anesthetized patients, and in animal experiments
  • Issue of infection - so needle electrodes are either disposable or re-sterilized
27
Q

Needle and wire electrodes for percutaneous measurement of biopotentials

A
  1. Insulated needle electrode
  2. Coaxial needle electrode
  3. Bipolar coaxial electrode
  4. Fine wire electrode connected to hypodermic needle
28
Q

Electrodes for detecting fetal ECG during labor, by means by intracutaneous needles

A
  1. Suction electrode

2. Helical electrode

29
Q

What are indwelling electrodes

A
  • Intended to be inserted into the body
  • Not to be confused with needle electrodes, which are intended for insertion into the layers beneath the skin
  • Typically a tiny, exposed metallic contact at the end of a long insulated catheter
  • In one application, the electrode is threaded through the patient’s veins (usually in the right arm) to the right side of the heart to measure the intracardiac ECG waveform
  • Certain low-amplitude, high-frequency features become visible only when an indwelling electrode is used
30
Q

Implantable electrodes for detecting biopotentials

A
  1. Wire-loop electrode
  2. Silver-sphere cortical-surface potential electrode
  3. Multi-element depth electrode
31
Q

Microfabricated electrode array

A
  1. 1D plunge electrode array
  2. 2D array
  3. 3D array
32
Q

Design of functional electrical stimulation

A
  • Nerve stimulation is achieved by passing (usually large) current between 2 or more electrodes implanted in the body
  • In order to produce functional nerve activation, the appropriate spatial and temporal patterns of stimulation must be determined for the desired stimulus resposne
  • > requires an understanding of both the stimulus properties and resulting nerve response properties
33
Q

Design considerations of electrode properties

A
  1. Number and positions of electrodes
  2. Material
  3. Size
  4. Shape
34
Q

Stimulating current properties

A
  1. Strength
  2. Waveform - shapes - monophasic, biphase, chopped, triphasic, asymmetric; parameters - pulse amplitude, pulse width, interphase gap, pulse rate
35
Q

The operating characteristics of an electrode depend on

A
  • Effective capacitance per unit area

- The reversible or irreversible electrochemical reaction between the electrode and electrolyte

36
Q

Avoiding electrochemical reactions

A
  • For monophasic stimulation, the charge continually builds up at the electrode interface -> rarely used.
  • The build up of charge is usually avoided by using charge-balanced biphasic current pulse (no net accumulation of charge due to stimulation -> remain in capacitive region)
37
Q

Anodic irreversible reaction leads to

A

Irreparable electrode damage

38
Q

How can the capacitive region be expanded

A

Increasing electrode capacitance e.g. via roughening the electrode surface to increase its effective surface area

39
Q

Factors to consider when choosing electrode material

A
  • The best stimulating electrodes are made from noble metals (or at least stainless steel), which undergo only minimal chemical reactions. The most widely used electrode materials are platinum, platinum-iridium and stainless steel
  • Extent of reversible behavior ( capacitive region + region of reversible electrochemical reactions )
  • Biocompatibility with the tissue
  • Mechanical compatibility with the tissue
40
Q

Examples of practical stimulation electrodes

A
  • Brain stimulation (surface electrodes): usually platinum is used
  • Brain stimulation (penetrating electrodes): usually micro-machined silicon needle-electrodes are used
  • Pacemakers: first major application of electrical stimulation of excitable cells
  • Nerve stimulation (cuff electrodes): surround nerve bundle for confined stimulation, reducing the required current
  • Intramuscular (coiled-wire electrodes): actually stimulate motor axons, not muscle fibers
  • Retinal (electrode arrays): stimulate different populations of retinal cells coding for different spatial positions
  • Cochlear implants: electrode array to stimulate different populations of auditory nerve fibers coding for different pitches
  • Upper limb stimulation: stimulates peripheral nerve fibers of motor neurons; used in spinal cord injury or stroke patients
  • Lower limb stimulation: footdrop control, standing control
  • Bladder control: intradural or extradural electrodes
  • Phrenic nerve stimulator: provides diaphragm pacing to aid respiration

BG3105 (6 decks)

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