Bioamplifier

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A Bioamplifier is an electrophysiological device, a variation of the instrumentation amplifier, used to gather and increase the signal integrity of physiologic electrical activity for output to various sources. It may be an independent unit, or integrated into the electrodes.

Contents

History

Efforts to amplify biosignals started with the development of electrocardiography. In 1887, Augustus Waller, a British physiologist, successfully measured the electrocardiograph of his dog using two buckets of saline, in which he submerged each of the front and the hind paws. [1] A few months later, Waller successfully recorded the first human electrocardiography using the capillary electrometer. [1] However, at the time of invention, Waller did not envision that electrocardiography would be used extensively in healthcare. The electrocardiograph was impractical to use until Willem Einthoven, a Dutch physiologist, innovated the use of the string galvanometer for cardiac signal amplification. [2] Significant improvements in amplifier technologies led to the usage of smaller electrodes that were more easily attached to body parts. [1] In the 1920s, a way to electrically amplify the cardiac signals using vacuum tubes was introduced, which quickly replaced the string galvanometer that amplified the signal mechanically. Vacuum tubes have a larger impedance, so the amplification was more robust. Also, its relatively small size compared to the string galvanometer contributed the widespread use of the vacuum tubes. Furthermore, the large metal buckets were no longer needed, as much smaller metal-plate electrodes were introduced. By the 1930s, electrocardiograph devices could be carried to the patient's home for the purpose of bedside monitoring. [3] With the emergence of electronic amplification, it was quickly discovered that many features of the electrocardiography were revealed with various electrode placement. [4]

Variations

Electrocardiography

Electrocardiography (ECG or EKG) records the electrical activity of the heart, across the surface of the thorax skin. The signals are detected by electrodes attached to the surface of the skin and recorded by a device external to the body. [5]

The amplitude of ECG ranges from 0.3 to 2 mV for the QRS complex, which is used to determine the interbeat interval from which the frequency is derived. The typical requirements for the amplifiers to be used in ECG include: [1]

Electromyography

Electromyography (EMG) records the electrical activity produced by skeletal muscles. It records various types of muscle signals from simple relaxation by using placing electrodes on the subject's forehead, to complex neuromuscular feedback during stroke rehabilitation. The EMG signals are acquired from the electrodes applied over or nearby the muscles to be monitored. The electrodes delegates signals to the amplifier unit, usually consisting of high performance differential amplifiers. The usual types of the signal of the interest are in the range of 0.1–2000 mV amplitude, over a bandwidth of about 25–500 Hz. [1]

Although many electrodes still connect to an amplifier using wires, some amplifiers are small enough to mount directly on the electrode. Some minimal specifications for a modern EMG amplifier includes: [1]

Electroencephalography

Electroencephalography (EEG) instrumentation is similar to EMG instrumentation in terms of involving the placement of many surface electrodes on the patient's skin, specifically, on scalp. While EMG acquires the signals from muscles below the skin, EEG attempts to acquire signals on the patient's scalp, generated by brain cells. Simultaneously, EEG records the summed activity of tens of thousands to millions of neurons. As the amplifiers became small enough to integrate with the electrodes, EEG has become to have the potential for long term use as a brain–computer interface, because the electrodes can be kept on the scalp indefinitely. The temporal and spatial resolutions and signal to noise ratios of EEG have always lagged behind those of comparable intracortical devices, but it has the advantage of not requiring surgery. [6]

High performance differential amplifiers are used for amplification. Signals of interest are in the range of 10 µV to 100 µV, over the frequency range of 1–50 Hz. Similar to EMG amplifiers, [1] EEG benefits from the usage of integrated circuit. The chances of EEG is also mainly from the asymmetrical placement of electrodes, which leads to increased noise or offset. [7] Some minimal specifications for a modern EEG amplifier includes: [1]

Galvanic skin response

Galvanic skin response is a measurement of the electrical conductance of the skin, which is directly influenced by how much the skin is moist. Since the sweat glands are controlled by the sympathetic nervous system, the skin conductance is crucial in measuring the psychological or physiological arousal. [8] The arousal and the eccrine sweat gland activity are clinically found to have direct relation. High skin conductance due to sweating can be used to predict that the subject is in a highly aroused state, either psychologically or physiologically, or both. [9]

Galvanic skin response can be measured either as resistance, called skin resistance activity (SRA) or skin conductance activity (SCA), which is a reciprocal of SRA. Both SRA and SCA include two types of responses: the average level and the short-term phasic response. Most modern instruments measure conductance, although they both can be displayed with the conversion made in circuitry or software. [1]

Other

Electrocorticography (ECoG) records the cumulative activity of hundreds to thousands of neurons with a sheet of electrodes placed directly on the surface of the brain. In addition to requiring surgery and having low resolution, the ECoG device is wired, meaning the scalp cannot be completely closed, increasing the risk of infection. However, researchers investigating ECoG claim that the grid "possesses characteristics suitable for long term implantation". [6]

The neurotrophic electrode is a wireless device that transmits its signals transcutaneously. In addition, it has demonstrated longevity of over four years in a human patient, because every component is completely biocompatible. It is limited in the amount of information it can provide, however, because the electronics it uses to transmit its signal (based around differential amplifiers) require so much space on the scalp that only four can fit on a human skull. [10]

In one experiment, Dr. Kennedy adapted the neurotrophic electrode to read local field potentials (LFPs). He demonstrated that they are capable of controlling assistive technology devices, suggesting that less invasive techniques can be used to restore functionality to locked-in patients. However, the study did not address the degree of control possible with LFPs or make a formal comparison between LFPs and single unit activity. [11]

Alternatively, the Utah array is currently a wired device, but transmits more information. It has been implanted in a human for over two years and consists of 100 conductive silicon needle-like electrodes, so it has high resolution and can record from many individual neurons. [6]

Design

Acquiring signals

Nowadays, mostly digital amplifiers are used to record biosignals. The amplification process does not only depend on the performance and specifications of the amplifier device, but also closely binds to the types of electrodes to attach on the subject's body. Types of electrode materials and the mount position of electrodes affect the acquirement of the signals. [12] Multielectrode arrays are also used, in which multiple electrodes are arranged in an array.

Electrodes made with certain materials tend to perform better by increasing surface area of the electrodes. For instance, Indium tin oxide (ITO) electrodes have less surface area than those made with other materials, like titanium nitride. More surface area results in reducing impedance of electrode, then neurons signals are obtained easier. ITO electrodes tend to be flat with a relatively small surface area, and are often electroplated with platinum to increase surface area and improve signal-to-area ratio. [13]

Digital amplifiers and filters are produced small enough nowadays to be combined with electrodes, serving as preamplifiers. The need for preamplifiers is clear in that the signals that neurons (or any other organs) produce are weak. Therefore, preamplifiers preferably are to be placed near the source of the signals, where the electrodes are adjacent to. Another advantage for having preamplifiers close to the signal source is that the long wires lead to significant interference or noise. Therefore, it is best to have the wires as short as possible. [13]

However, when wider bands are needed, for instance a very high (action potentials) or a low frequency (local field potentials), they could be filtered digitally, perhaps with second-stage analog amplifier before being digitized. There may be some drawbacks when several amplifiers in cascade. It depends on the type, analog or digital. However, in general, filters cause time-delay and amendments are needed to have signals in sync. Also, as extra complexity is added, it costs more money. In terms of digital amplifiers, a lot of works that the laboratories do are feeding back signals to the networks in closed loop, real-time. As a result, more time is needed to apply on signals when there are more digital amplifiers on the way. One solution is using field-programmable gate array (FPGA), the “blank slate” integrated circuit that is written whatever on it. Using FPGA sometimes reduces a need to use computers, resulting in a speed-up of filtering. Another problem with cascaded filters occurs when the maximum output of the first filter is smaller than the raw signals, and the second filter has a higher maximum output that the first filter. In that case, it is impossible to recognize if the signals have reached the maximum output or not. [13]

Design challenges

The trend with the development in electrodes and amplifiers has been reducing its size for better transportability, as well as making them implantable on the skin for prolonged recording of the signals. Preamplifiers, head-stage amplifiers stay the same except that they should have different form-factors. They should be lightweight, waterproof, not scratching skin or scalp from parts that they need to mount themselves, and they should dissipate heat well. Heat dissipation is a big issue, because extra heat may cause in the temperature of nearby tissue to rise, potentially causing a change in the physiology of the tissue. One of the solutions to dissipate heat is the usage of the Peltier device. [13] Peltier device, uses Peltier effect or thermoelectric effect to create a heat flux between the two different types of materials. A Peltier device actively pumps the heat from one side to the other side of the device, consuming electrical energy. Conventional cooling using compressed gases would not be a feasible option for cooling down an individual integrated circuit, because it needs many other devices to operate such as evaporator, compressor and condenser. Overall, a compressor-based system is more for a large-scale cooling jobs, and is not viable for small-scale system like bioamplifiers. The passive cooling, like heat sink and fan, only limits the rise of temperature above the ambient condition, while Peltier devices can actively pull heat right out of a thermal load, just like compressor-based cooling systems. Also, Peltier devices can be manufactured at sizes well below 8 mm square, therefore they can be integrated to the bioamplifiers without making them to lose mobility. [14]

See also

Related Research Articles

In electronics, the figures of merit of an amplifier are numerical measures that characterize its properties and performance. Figures of merit can be given as a list of specifications that include properties such as gain, bandwidth, noise and linearity, among others listed in this article. Figures of merit are important for determining the suitability of a particular amplifier for an intended use.

<span class="mw-page-title-main">Amplifier</span> Electronic device/component that increases the strength of a signal

An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal. It may increase the power significantly, or its main effect may be to boost the voltage or current. It is a two-port electronic circuit that uses electric power from a power supply to increase the amplitude of a signal applied to its input terminals, producing a greater amplitude signal at its output. The ratio of output to input voltage, current, or power is termed gain. An amplifier, by definition has gain greater than unity.

<span class="mw-page-title-main">Electrocardiography</span> Examination of the hearts electrical activity

Electrocardiography is the process of producing an electrocardiogram, a recording of the heart's electrical activity. It is an electrogram of the heart which is a graph of voltage versus time of the electrical activity of the heart using electrodes placed on the skin. These electrodes detect the small electrical changes that are a consequence of cardiac muscle depolarization followed by repolarization during each cardiac cycle (heartbeat). Changes in the normal ECG pattern occur in numerous cardiac abnormalities, including cardiac rhythm disturbances, inadequate coronary artery blood flow, and electrolyte disturbances.

A low-noise amplifier (LNA) is an electronic amplifier that amplifies a very low-power signal without significantly degrading its signal-to-noise ratio. An amplifier will increase the power of both the signal and the noise present at its input, but the amplifier will also introduce some additional noise. LNAs are designed to minimize that additional noise. Designers can minimize additional noise by choosing low-noise components, operating points, and circuit topologies. Minimizing additional noise must balance with other design goals such as power gain and impedance matching.

<span class="mw-page-title-main">Biofeedback</span> Gaining awareness of biological processes

Biofeedback is the process of gaining greater awareness of many physiological functions of one's own body by using electronic or other instruments, and with a goal of being able to manipulate the body's systems at will. Humans conduct biofeedback naturally all the time, at varied levels of consciousness and intentionality. Biofeedback and the biofeedback loop can also be thought of as self-regulation. Some of the processes that can be controlled include brainwaves, muscle tone, skin conductance, heart rate and pain perception.

<span class="mw-page-title-main">Preamplifier</span> Electronic amplifier that converts weak signal into strong signal

A preamplifier, also known as a preamp, is an electronic amplifier that converts a weak electrical signal into an output signal strong enough to be noise-tolerant and strong enough for further processing, or for sending to a power amplifier and a loudspeaker. Without this, the final signal would be noisy or distorted. They are typically used to amplify signals from analog sensors such as microphones and pickups. Because of this, the preamplifier is often placed close to the sensor to reduce the effects of noise and interference.

<span class="mw-page-title-main">Willem Einthoven</span> Dutch physiologist

Willem Einthoven was a Dutch doctor and physiologist. He invented the first practical electrocardiograph in 1895 and received the Nobel Prize in Physiology or Medicine in 1924 for it.

<span class="mw-page-title-main">Active filter</span>

An active filter is a type of analog circuit implementing an electronic filter using active components, typically an amplifier. Amplifiers included in a filter design can be used to improve the cost, performance and predictability of a filter.

<span class="mw-page-title-main">Holter monitor</span> Portable device for cardiac monitoring

In medicine, a Holter monitor is a type of ambulatory electrocardiography device, a portable device for cardiac monitoring for at least 24 hours.

<span class="mw-page-title-main">Electromyography</span> Electrodiagnostic medicine technique

Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an instrument called an electromyograph to produce a record called an electromyogram. An electromyograph detects the electric potential generated by muscle cells when these cells are electrically or neurologically activated. The signals can be analyzed to detect abnormalities, activation level, or recruitment order, or to analyze the biomechanics of human or animal movement. Needle EMG is an electrodiagnostic medicine technique commonly used by neurologists. Surface EMG is a non-medical procedure used to assess muscle activation by several professionals, including physiotherapists, kinesiologists and biomedical engineers. In Computer Science, EMG is also used as middleware in gesture recognition towards allowing the input of physical action to a computer as a form of human-computer interaction.

In neuroscience, single-unit recordings provide a method of measuring the electro-physiological responses of a single neuron using a microelectrode system. When a neuron generates an action potential, the signal propagates down the neuron as a current which flows in and out of the cell through excitable membrane regions in the soma and axon. A microelectrode is inserted into the brain, where it can record the rate of change in voltage with respect to time. These microelectrodes must be fine-tipped, high-impedance conductors; they are primarily glass micro-pipettes, metal microelectrodes made of platinum, tungsten, iridium or even iridium oxide. Microelectrodes can be carefully placed close to the cell membrane, allowing the ability to record extracellularly.

<span class="mw-page-title-main">Einthoven's triangle</span> Concept in electrocardiography

Einthoven's triangle is an imaginary formation of three limb leads in a triangle used in electrocardiography, formed by the two shoulders and the pubis. The shape forms an inverted equilateral triangle with the heart at the center. It is named after Willem Einthoven, who theorized its existence.

<span class="mw-page-title-main">String galvanometer</span>

A string galvanometer is a sensitive fast-responding measuring instrument that uses a single fine filament of wire suspended in a strong magnetic field to measure small currents. In use, a strong light source is used to illuminate the fine filament, and the optical system magnifies the movement of the filament allowing it to be observed or recorded by photography. The principle of the string galvanometer remained in use for electrocardiograms until the advent of electronic vacuum-tube amplifiers in the 1920s.

<span class="mw-page-title-main">Biosignal</span> Any signal in living beings that can be continually measured and monitored

A biosignal is any signal in living beings that can be continually measured and monitored. The term biosignal is often used to refer to bioelectrical signals, but it may refer to both electrical and non-electrical signals. The usual understanding is to refer only to time-varying signals, although spatial parameter variations are sometimes subsumed as well.

<span class="mw-page-title-main">Valve RF amplifier</span> Device for electrically amplifying the power of an electrical radio frequency signal

A valve RF amplifier or tube amplifier (U.S.) is a device for electrically amplifying the power of an electrical radio frequency signal.

<span class="mw-page-title-main">Preselector</span> Radio signal filtering device

A preselector is a name for an electronic device that connects between a radio antenna and a radio receiver. The preselector is a band-pass filter that blocks troublesome out-of-tune frequencies from passing through from the antenna into the radio receiver that otherwise would be directly connected to the antenna.

Targeted reinnervation enables amputees to control motorized prosthetic devices and to regain sensory feedback. The method was developed by Dr. Todd Kuiken at Northwestern University and Rehabilitation Institute of Chicago and Dr. Gregory Dumanian at Northwestern University Division of Plastic Surgery.

<span class="mw-page-title-main">Electroencephalography</span> Electrophysiological monitoring method to record electrical activity of the brain

Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocortex and allocortex. It is typically non-invasive, with the EEG electrodes placed along the scalp using the International 10-20 system, or variations of it. Electrocorticography, involving surgical placement of electrodes, is sometimes called "intracranial EEG". Clinical interpretation of EEG recordings is most often performed by visual inspection of the tracing or quantitative EEG analysis.

<span class="mw-page-title-main">Neurotrophic electrode</span> Intracortical device designed to read the electrical signals of the brain

The neurotrophic electrode is an intracortical device designed to read the electrical signals that the brain uses to process information. It consists of a small, hollow glass cone attached to several electrically conductive gold wires. The term neurotrophic means "relating to the nutrition and maintenance of nerve tissue" and the device gets its name from the fact that it is coated with Matrigel and nerve growth factor to encourage the expansion of neurites through its tip. It was invented by neurologist Dr. Philip Kennedy and was successfully implanted for the first time in a human patient in 1996 by neurosurgeon Roy Bakay.

An electrogram (EGM) is a recording of electrical activity of organs such as the brain and heart, measured by monitoring changes in electric potential.

References

  1. 1 2 3 4 5 6 7 8 9 Webster, John G. (2006) Encyclopedia of Medical Devices and Instrumentation Volume I. New Jersey: Wiley-Interscience. ISBN   978-0-471-26358-6.
  2. ↑ Rivera-Ruiz, M; Cajavilca, C; Varon, J (1927). "Einthoven's String Galvanometer: The First Electrocardiograph". Texas Heart Institute Journal. 35 (2): 174–8. PMC   2435435 . PMID   18612490.
  3. ↑ Rowbottom ME, Susskind C. In: Electricity and Medicine: History of their Interaction. San Francisco (CA): San Francisco Press; 1984.
  4. ↑ Ernstine and Levine report the use of vacuum-tubes to amplify the electrocardiogram instead of the mechanical amplification of the string galvanometer. Ernstine, AC; Levine, SA (1928). "A comparison of records taken with the Einthoven string galvanometer and the amplifier-type electrocardiograph". American Heart Journal. 4 (6): 725–731. doi:10.1016/S0002-8703(29)90554-8.
  5. ↑ "ECG- simplified. Aswini Kumar M.D.". LifeHugger.
  6. 1 2 3 Brumberg, J. S.; Nieto-Castanon, A.; Kennedy, P. R.; Guenther, F. H. (2010). "Brain–computer interfaces for speech communication". Speech Communication. 52 (4): 367–379. doi:10.1016/j.specom.2010.01.001. PMC   2829990 . PMID   20204164.
  7. ↑ Northrop, R. B. (2012). Analysis and Application of Analog Electronic Circuits to Biomedical Instrumentation. CRC Press.
  8. ↑ Martini, Frederic; Bartholomew, Edwin (2003). Essentials of Anatomy & Physiology. San Francisco: Benjamin Cummings. p. 267. ISBN   0-13-061567-6.
  9. ↑ Carlson, Neil (2013). Physiology of Behavior. New Jersey: Pearson Education, Inc. ISBN   978-0-205-23939-9.
  10. ↑ Interview with Dr. Kennedy, Senior Research Scientist, Neural Signals, Inc., 9/30/2010
  11. ↑ Kennedy, P. R.; Kirby, M. T.; Moore, M. M.; King, B.; Mallory, A. (2004). "Computer control using human intracortical local field potentials". IEEE Transactions on Neural Systems and Rehabilitation Engineering. 12 (3): 339–344. doi:10.1109/TNSRE.2004.834629. PMID   15473196. S2CID   8760734.
  12. ↑ Bronzino, Joseph D. (2006). The Biomedical Engineering Handbook, Third Edition. Florida: CRC Press.
  13. 1 2 3 4 Interview with Dr. Potter, Associate Professor, Georgia Institute of Technology, 10/16/2013
  14. ↑ "Tellurex - Peltier FAQ". Tellurex. Retrieved 27 November 2013
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