Biopotential signals are electric potential signals that is measured between points in living cells, tissues, and organisms, and which accompanies all biochemical processes.These cells generate an potential when they are stimulated and they present a resting potential electrically. The membrane potential of an inactive cell is called the resting potential. The potentials are generated by the exchange of ions between inside and outside cell. Intracellular concentration of potassium (K+) ions is 30-50 times higher than extracellular it. Sodium ion (Na+) concentration is 10 times higher outside the membrane than inside. In resting state, the membrane is permeable only for potassium ions. K+ concentration of the interior of the cell is much higher than the exterior. Therefore, a diffusion gradient of K+ occurs towards the exterior of the cell making the interior more negative relative to the exterior, which results in an electrical field direct inward cell being built up. In steady state, the diffusion gradient of the K+ ions is balanced by the electrical field and the equilibrium is reached with a polarization voltage of nearly −70 mV. When membrane stimulation exceeds a threshold level of about 20 mV, so called action potential occurs. Sodium (Na+) and potassium (K+) ionic permeability of the membrane is changed. Sodium ion permeability increases very rapidly, allowing sodium ions to flow from outside to inside, result in the polarity of cell being more positive. Oppositely, the more slowly increasing K+ ion permeability permits K+ ions to flow from inside to outside, thus
returning membrane potential to its resting value. As the potential reaches to 40 mV, the permeability of the membrane to Na+ ions decreases and to K+ increases, resulting in the membrane potential swiftly decrease towards its rest state. The biopotential signals, such as EEG, ECG, EMG, are the result of several action potentials produced by a combination of different cells [1].
Many organs in the human body, such as the heart, brain, and muscles, display their function through electric activity [2]. The heart, for example, produces a signal called the electrocardiogram (ECG). The brain produces a signal called an electroencephalogram (EEG) or electrocorticogram (ECoG). The activity of muscles, such as contraction and relaxation, produces an electromyogram (EMG).
Measurement of these biopotential signals can help doctor to diagnose what happened with these organs. Therefore, more and more studies are about the biopotential signals readout circuit. Biopotential signals have the characteristics of small amplitude, low frequency and variability. Table I shows the frequency and amplitude characteristics of EEG, ECoG, ECG and EMG. The following articles briefly introduce about these biopotential signals individually:
Table I The electrical characteristics and applications of these biopotential signals [3].
Source Amplitude (mV) Bandwidth (Hz) Clinical and Research Use
EEG 0.001-0.01 0.5-40 Seizure detection, the diagnosis
of encephalopathy
ECoG 0.1-1 0.5-40* Sleep studies, seizure detection,
cortical mapping
ECG 1-5 0.05-100 Diagnosis of ischemia,
arrhythmia, conduction defects
EMG 1-10 20-2K Muscle function, neuromuscular
disease, prosthesis
*Note: In order to observe the relations between KHz signals with seizure detection, the bandwidth is extended to 7KHz in this work.
EEG & ECoG:
The brain transmit electrical signals when subject‟s intention to do some action.
These signals are generated by neurons in the cortical layers. There are several modalities to capture the electrical activities of brain in clinical practice. The methods are classified by the locations the electrodes are placed on, and by the spatial and spectral frequency of their captured signals. Electroencephalogram (EEG) is measured by electrodes are placed on scalp which is 2-3 cm away from the surface of the cortex.
This is safest way to record brain activities. In contrast, electrocorticogram (ECoG) which need to use invasive electrodes is more unsafe. The recording electrodes are approximated on the cortical surface, and therefore ECoG has larger amplitude than EEG [4].
The EEG is typically described in terms of rhythmic activity and transients. By means of Fourier transform power spectrum from the raw EEG signal is derived. In power spectrum contribution of sine waves with different frequencies are visible.
Although the spectrum is continuous, ranging from 0 Hz up to one half of sampling
frequency, the brain state of the individual may make certain frequencies more dominant [5].
The normal EEG wave is classified five wave groups. Delta waves arise in the frequency below 3.5 Hz. They happen in deep sleep, in children, and in serious organic brain disease. Theta waves include all the waves between 4 and 7 Hz. These occur mainly in parietal and temporal regions in children, but they also occur during mental stress in some adult, particular in feeling of disappointment and frustration.
The best-known and most extensively studied rhythm of the human brain is the normal alpha rhythm. The frequency range is between 8 and 13 Hz. They are found in normal persons when they are awake in a quiet, resting state of cerebration. When the awake subject is asleep, the alpha waves disappear completely. Beta wave normally occur in the frequency range of 14 to 30 Hz, and sometime as high as 50 Hz. They can be divided into Beta I and Beta II. The frequency of about 14 to 26 Hz of they is called Beta I, affected by mental activity as alpha waves. The rest waves of Beta is Beta II, on the other hand, appear during intense activation of the central nervous system and during tension [6]. Gamma waves are patterns of brain waves in humans with a frequency >30 Hz. Recent reseach have pointed out GAMMA waves observation can help to comprehend epilepsy [7][8]
The intensities of brain waves on the surface of the brain may be as large as 10mV [6]. The amplitude of EEG measured on the scalp is only few micro-volts. The electrodes of ECoG are placed more close to cortical layer. Therefore, the amplitude is larger 100 times than EEG. It‟s important about noise issue at the level of amplitude.
The noises come from physiological, environmental, and electronic sources.
Physiological sources of interference are motion artifact, muscle noise, eye motion or blink artifact, and sometimes even heartbeat signals. Electrical interference arises from the usual sources: 60 Hz power lines, radio frequencies (RF), and electrically or
magnetically induced interference. Moreover, the electronic components in the amplifier also contribute noise. Good design and measuring techniques can mitigate the effects of such noise and interference. Electrode locations and names are specified by the International 10-20 system [9] for most clinical and research applications. This system ensures that the naming of electrodes is consistent across laboratories. In most clinical applications, 19 recording electrodes (plus ground and system reference) are used. A smaller number of electrodes are typically used when recording EEG from neonates. Additional electrodes can be added to the standard set-up when a clinical or research application demands increased spatial resolution for a particular area of the brain. High-density arrays (typically via cap or net) can contain up to 256 electrodes more-or-less evenly spaced around the scalp.
ECG:
Electrocardiogram (ECG) records the activities of heart by placing the electrodes on the skin. It is a noninvasive recording produced by an electrocardiographic device.
In clinical applications, a lead system called 12-lead system has been used usually.
Ten electrodes placed on the torso, arms, and legs are used for a 12-lead ECG. And these electrodes can be combined into a number of pairs. (For example: Left arm (LA), right arm (RA) and left leg (LL) electrodes form the pairs: LA+RA, LA+LL, RA+LL) The output from each pair is known as a lead. ECGs from these different leads help define the nature of the activity on a specific part of the heart muscle. Different types of ECGs can be referred to by the number of leads that are recorded, for example 3-lead, 5-lead or 12-lead ECGs. A 12-lead ECG is one in which 12 different electrical signals are recorded at approximately the same time and are common in research and clinical application.
The ECG signals at the surface of the body are small in amplitude, which make
the measurements be interfered with noise. An important consideration in good ECG signal acquisition is the use of high-quality electrodes. Electrodes made out of silver coated with silver chloride or of sintered Ag-AgCl material, are recommended. An electrolytic gel is used to enhance conduction between the skin and the electrode metal. Artifacts at the electrode-skin contact as well as electromagnetic interference from all sources must be minimized. Since ECG instruments are often used in critical-care environments, they must be electrically isolated for safety and protected from the high voltages generated by defibrillators. [3]
EMG
Muscle fibers generate electric activity whenever muscles are active. EMG signals are recorded by placing electrodes close to the muscle group. For example, a pair of electrodes placed on the biceps and another pair placed on the triceps can capture the EMG signals generated when these muscles contract. EMG signals recorded in this manner have been shown to give a rough indication of the force generated by the muscle group. Electrodes used for such applications should be small, securely attached and should provide recordings free of artifacts. Either silver-silver chloride or gold plated electrodes perform quite well, although inexpensive stainless steel electrodes may also suffice.
Since the frequency range of EMG signals is higher than that of ECG and EEG signals, and since the signals are of comparable or larger amplitudes, the problem of motion artifact and other interference is relatively less severe. Filtering can reduce the artifact and interference: for example, setting the bandwidth to above 20 Hz can greatly reduce the skin potentials and motion artifacts.
Recording activity directly from the muscle fibers themselves can be clinically valuable in identifying neuromuscular disorders. Therefore, invasive electrodes are needed to access the muscle fibers or the neuromuscular junction. Fine-needle
electrodes or thin stainless steel wires are inserted or implanted to obtain local recording from the fibers or neuromuscular junctions. [3]
Other biopotential acquisition systems follow similar principles of measurement.
In conclusion, the most of the biopotential signals have small amplitude, low frequency, and narrow bandwidth. Therefore, a circuit compatible all classifications of biopotential signals is difficult to design. The effort is required to minimize the noise and interference by improving electrode design and placement and optimizing the amplifier circuit.