BME 313 Bioinstrumentation Instructor: Wei Lin
BiopotentialBiopotentials are the results of electrochemical activities of excitable cells, which are the components of nervous, muscular or glandular tissue.
Na+: 10 times higher outside than insideK+: 40 times greater inside than outsideCl-: 30 times higher outside than inside
Biopotential
BME 313 Bioinstrumentation Instructor: Wei Lin
Cell membrane potential is the difference between intercellular potential and extracellular potential.
BME 313 Bioinstrumentation Instructor: Wei Lin
Biopotential
i
ok K
KnFRTE
][][
ln
Nernst Equation:
Goldman Equation:
oCliNaik
iCloNaok
ClPNaPkPClPNaPkP
FRTE
][][][][][][
ln
BME 313 Bioinstrumentation Instructor: Wei Lin
State of Excitable Cell
There are two states of the excitable cell:
Images from http://www.gregalo.com/action_potential.jpg
BME 313 Bioinstrumentation Instructor: Wei Lin
Propagation of Action Potential
BME 313 Bioinstrumentation Instructor: Wei Lin
Propagation of Action Potential
Myelinated transduction
BME 313 Bioinstrumentation Instructor: Wei Lin
Volume Conductor
http://www.cvphysiology.com/Arrhythmias/A014.htm
The three dimensional conducting medium composed of tissue is referred as volume conductor. The source of electrical signals are the excitable cells serving the current sources.
BME 313 Bioinstrumentation Instructor: Wei Lin
Electrocardiogram (ECG)
Sinoatrial node (SA)
Atrioventrical Node (AV)
Bundle of His
Rest of the heart
Common Bundle
Purkinje fiber
BME 313 Bioinstrumentation Instructor: Wei Lin
Electrocardiogram (ECG)
BME 313 Bioinstrumentation Instructor: Wei Lin
Electroencephalogram (EEG)
EEG is the electrical activity of the brain. EEG can be recorded using three
different types of electrodes Scalp electrodes Cortical electrodes (electrocorticogram) Depth electrodes
BME 313 Bioinstrumentation Instructor: Wei Lin
Types of EEG (Alpha waves)
Alpha waves are rhythmic waves in the frequency range of 8-12 Hz arising from synchronous and coherent (in phase / constructive) electrical activity of large groups of neurons in the human brain. They are found in EEGs when a normal person is in a quiet, resting state and are most intense in the occipital area.
BME 313 Bioinstrumentation Instructor: Wei Lin
Types of EEG (beta waves)
Beta waves normally occur in the frequency range of 14 to 30 Hz and could be as high as 50Hz during intense mental activity.
BME 313 Bioinstrumentation Instructor: Wei Lin
Types of EEG (theta waves)
Theta waves have frequency range of 4 to 7 Hz. They mainly occur in the parietal and temporal regions in children. They can also occur in adults during emotional stress.
BME 313 Bioinstrumentation Instructor: Wei Lin
Types of EEG (delta waves)Delta waves include all the waves in EEG below 3.5 Hz. They occur in deep sleep, in infancy and in serious organic brain disease.
BME 313 Bioinstrumentation Instructor: Wei Lin
Other Biopotentials
Electromyogram (EMG): electrical activities of skeletal muscle.
Electroretinogram (ERG): electrical activities of retina stimulated with light.
BME 313 Bioinstrumentation Instructor: Wei Lin
Biopotential Electrode
Electrode/Electrolyte interface Half cell potential Polarization of electrode Electrode circuit model.
BME 313 Bioinstrumentation Instructor: Wei Lin
Biopotential Electrodes
The electrode consists of metallic atoms C. The electrolyte is an aqueous solution containing cations of the electrode metal C+ and anions A-.
C
CC A-
A-C+
C+
e-
e-
Electrode Electrolyte
BME 313 Bioinstrumentation Instructor: Wei Lin
Electrode Electrolyte Interface
Assuming the electrode has same material as cation in the electrolyte, the electrode material becomes oxidized to form a cation and one or more electrons. The electrons remain as the charge carrier in the electrode. The anion can also be oxidized at the electrode to form a neutral atom and release one or two electrons to the electrode. The reverse of the above reaction is reduction. It controls the movement of electrons in the opposite direction.
C Cn+ + ne-
Am- A + me-
BME 313 Bioinstrumentation Instructor: Wei Lin
Half-cell Potential
When the electrode is inserted into the electrolyte, the concentration of cations and anions at the interface changes. As the result, there is an electric potential difference between the electrolyte surrounding the electrode and the electrolyte in other places. The difference is called half-cell potential, which cannot be measured.
+
+
+
+
+
+
+
Electrode
Electrolyte
Half-cell potential
BME 313 Bioinstrumentation Instructor: Wei Lin
Polarization
When current passes through the electrode-electrolyte interface, it changes the half-cell potential. The change is called overpotential and it has three components
Ohmic overpotential
Concentration overpotential
Activation overpotential
+ Total overpotential
BME 313 Bioinstrumentation Instructor: Wei Lin
Nernst Equation
BA
DC
aaaa
nFRTEE ln0
When two aqueous ionic solutions of different concentration are separated by an ion-selective semi-permeable membrane, an electric potential exists across the membrane and is given by Nernst equation.
For the general oxidation-reduction reaction: neDCBA
Where E : Half Cell PotentialE0 : Standard Half Cell Potential a : activity of cationsn : Number of electrons
BME 313 Bioinstrumentation Instructor: Wei Lin
Polarizable and Non-Polarizable Electrodes
Perfectly Polarizable Electrodes No actual charge crosses the electrode-electrolyte
interface when a current is applied. The current across the interface is a displacement current and the electrode behaves like a capacitor.
Perfectly Nonpolarizable Electrode Current passes freely across the electrode-
electrolyte interface, requiring no energy to make the transition. These electrodes see no overpotentials.
BME 313 Bioinstrumentation Instructor: Wei Lin
Silver/Silver Chloride Electrode
eAgAg AgClClAg
Cl
sAg a
KnFRTEE ln0
BME 313 Bioinstrumentation Instructor: Wei Lin
Equivalent Circuit Model for Electrode
Ehc
Rd
Cd
Rs
Ehc: half cell potentialRd and Cd: the impedance associated with the electrode-electrolyte interfaceRs: the series resistance associated with interface effects and the resistance in the electorlyte.
BME 313 Bioinstrumentation Instructor: Wei Lin
Electrode Skin Interface and Motion Artifact
Surface electrode
BME 313 Bioinstrumentation Instructor: Wei Lin
Micro electrode
BME 313 Bioinstrumentation Instructor: Wei Lin
Electrode Arrays
BME 313 Bioinstrumentation Instructor: Wei Lin
Electric stimulation of Tissue
BME 313 Bioinstrumentation Instructor: Wei Lin