Biomedical sensors are a major application field of sensors, and there are not a few manufacturers of medical sensors. In many cases, biomedical sensors are sensors with a high technical threshold and are the vanguard of biomedical science and technology. Modern biomedical research depends on the correct measurement of biomedical sensors. Through this article, you will have a comprehensive understanding of the classification and basic principles of biomedical sensors.
The physical model of a modern sensor is shown in the figure:
For traditional measurands, the sensitive membrane is equivalent to the interface between the sensor and the measured object. Adding a layer of sensitive film specially made according to different needs in front of the traditional sensor can represent chemical sensor and biological sensor. The difference between the two depends on whether they have biological activity. Bioactive membrane materials are biosensors. There may be two interfaces in the sensor, one is the interface between the measured medium and the sensitive film, and the other is the interface between the sensitive film and the sensor. Complex physical, chemical or biological processes take place at the interface.
Medical requirements for sensors
1. High safety (especially for sensors and transducers used in the human body), high sensitivity, and high signal-to-noise ratio (high selectivity).
2. The measures to ensure physical safety are electrical isolation and floating technology.
3. The requirement to ensure high chemical safety is non-toxicity and no short-term and long-term carcinogenic effects.
4. The requirement to ensure high biological safety is no DNA and RNA mutations.
5. The measure to ensure high selectivity is to use resonance effect, filtering technology, adaptive technology, molecular recognition and ion recognition technology.
6. The measures to ensure high sensitivity are: physical, chemical and biological amplification techniques.
Main uses of medical sensors
1. Detection of biological information: such as detection of intracardiac pressure before heart surgery; detection of blood viscosity and blood lipid content in basic research on cardiovascular diseases.
2. Clinical monitoring
For example, patients need to continuously detect physiological parameters such as body temperature, pulse, blood pressure, respiration, and ECG before and after surgery.
3. Control
Use the detected physiological parameters to control the physiological processes of the human body. electronic prosthetics
Quantities that need to be measured in medicine
Classification of Biomedical Sensors
Classified by application form: implantable sensors, temporarily implanted body cavity (or incision) sensors, in vitro sensors, sensors for external devices
implantable sensor
According to the working principle, there are: physical sensors (displacement, force, temperature, humidity...), chemical sensors (various chemical substances), biological sensors (various enzymes, immunity, microorganisms, DNA...), bioelectric electrodes Sensors (ECG, EEG, EMG, neuron discharge...)
physical sensor
A sensor made of physical properties or physical effects is called a physical sensor, or a device that converts a physical quantity into an electrical quantity that can be recognized by a computer is called a sensor.
Classification and Application of Biomedical Physical Sensors
Force sensors are used to measure weight; piezoelectric film sensors are used to measure heart rate and breathing patterns; thermopile sensors are used to measure body temperature; blood oxygen sensors are used to measure blood oxygen levels; CO2 sensors are used to measure metabolism; flow sensors are used to assist Breathing; a force sensor is used to measure the amount of oxygen remaining in the oxygen cylinder.
chemical sensor
A chemical sensor is a device that converts chemical composition, concentration, etc. into an electrical quantity that has an exact relationship with it. It mostly uses the selective effect of certain functional membranes on specific chemical components to screen out the measured components, and then uses electrochemical devices to convert them into electrical quantities.
Generally, the classification depends on the response mechanism of the membrane electrode, the composition of the membrane or the structure of the membrane. Such as ion-selective electrode transducers, gas-sensitive electrode transducers, humidity-sensitive electrode transducers, wire-coated electrode transducers, polymer matrix electrode transducers, ion-sensitive field effect tube transducers, ion-selective microelectrode transducers Transducers, ion selective sheet transducers.
The chemical substances measured by various chemical transducers for biomedicine include: K+, Na+, Ca2+, Cl-, O2, CO2, NH3, H+, Li+, etc.
biological sensor
Biosensors use the selective identification and determination of biologically active substances to achieve measurement. They are mainly composed of two parts: one is the functional recognition substance (molecular recognition element), which specifically recognizes the measured substance; the other is the electrical and optical signal. Conversion device (transducer), which converts the chemical reaction generated by the measured object into an electrical or optical signal that is convenient for transmission.
The first biosensor to come out was an enzyme electrode, and Clark and Lyons first proposed the idea of forming an enzyme electrode. In the mid-1970s, people noticed that the lifespan of enzyme electrodes is generally relatively short, and the price of purified enzymes is also relatively expensive, and most of various enzymes come from microorganisms or animal and plant tissues, so people are naturally inspired to study the derivatives of enzyme electrodes: New biosensors such as microbial electrodes, organelle electrodes, animal and plant tissue electrodes, and immune electrodes have greatly increased the types of biosensors;
After entering the 1980s, with the continuous improvement of ion-sensitive field-effect transistors, in 1980 Caras and Janafa took the lead in developing an enzyme FET that can detect penicillin.
Composition and basic principle of biosensor
1. Molecular recognition element
2. Transducer
The types of transducers include electrochemical electrodes, semiconductors, thermistors, surface plasmons, piezoelectric crystals, etc.
Classification of Biosensors
According to molecular recognition elements
Classified by device
enzyme sensor
The catalysis of enzymes is to decompose substrates under certain conditions, so the catalysis of enzymes is essentially to accelerate the decomposition rate of substrates.
The enzyme sensor is composed of immobilized enzyme and basic electrode. The design of the enzyme electrode mainly considers the electrode active material produced or consumed by the enzyme catalysis process. If an enzyme catalyzed reaction consumes O2, an O2 electrode or H2O2 electrode can be used; if the enzyme catalyzes the reaction The process produces acid and a pH electrode can be used.
Enzyme sensor signal conversion method
1. Potential method
The potential method is to calculate the concentration of various ions related to the enzyme reaction from the measured membrane potential through the generation of different ions in different receptors. Generally, ammonium ion electrode (ammonia gas electrode), hydrogen ion electrode, carbon oxide electrode, etc. are used;
2. Current method
The current method is a method of calculating the measured substance from the current value obtained from the electrode reaction of the substance related to the enzyme reaction. The electrochemical device uses an oxygen electrode. Fuel cell type electrodes and hydrogen peroxide electrodes, etc.;
glucose sensor
working principle
Glucose sensor to measure oxygen consumption + glucose sensor to measure H2O2 production
Oxidase (GOD): glucose+H2O+O2―――――→gluconic acid+H2O2
Therefore, there are three methods for testing glucose concentration: 1. Measure the consumption of O2; 2. Measure the amount of H2O2 produced; 3. Measure the pH change caused by gluconic acid.
Glucose sensor to measure oxygen consumption
Oxygen electrode composition: ① Pb anode and Pt cathode immersed in alkaline solution, ② The surface of the cathode is covered with an oxygen-penetrating glucose (matrix) membrane [Teflon, about 10 μm thick].
The principle of O2 measurement by oxygen electrode: use the characteristic that oxygen is first reduced on the cathode. The O2 in the solution passes through the Teflon membrane and reaches the Pt cathode. When a DC voltage is applied as the polarization voltage (such as 0.7V) of oxygen, the oxygen molecule gets electrons on the Pt cathode and is reduced: its current value is the same as It is proportional to the concentration of O2.
O2+2H2O+4e=======4OH-
Glucose Sensor for Measuring H2O2 Production
Glucose oxidase (GOD)
Glucose+H2O+O2――――――→gluconic acid+H2O2
Glucose oxidation produces H2O2, and H2O2 passes through the selectively gas-permeable membrane and oxidizes on the Pt electrode to generate anodic current. The glucose content is proportional to the current, from which the concentration of the glucose solution can be measured.
When a voltage of 0.6V is applied to the Pt electrode, the anode current generated is: H2O2―――――→ O2+2H++2e
microbial sensor
Microbial sensors are divided into aerobic microbial sensors and anaerobic microbial sensors
The sensor is placed in the measured solution containing organic compounds, and the organic matter diffuses to the microbial film and is ingested by the microorganisms (called incorporation).
Aerobic microbial sensor
The respiration of microorganisms can be used to determine the structure of oxygen electrodes or carbon dioxide electrodes
O2 electrode aerobic microbial sensor response curve
Anaerobic Microbial Sensor
Microbial metabolites can be determined, which can be determined by ion-selective electrodes
Formic acid sensor (anaerobic) principle:
Immobilize the hydrogen-producing Clostridium butyricum on the low-temperature jelly film, and fix it on the Pt electrode of the fuel cell;
When the sensor is immersed in a solution containing formic acid, formic acid diffuses to Clostridium butyricum through the polytetrafluoroethylene membrane, and after being catalyzed, H2 is produced, and H2 passes through the polytetrafluoroethylene membrane on the surface of the Pt electrode and the Pt electrode to generate The oxidation-reduction reaction generates a current, which is directly proportional to the H2 content produced by microorganisms, and the H2 content is related to the concentration of formic acid to be measured, so the sensor can measure the concentration of formic acid in the fermentation solution.
Immunosensor
The basic principle of the immunosensor is the immune response. The specific reaction between the immobilized antibody (or antigen) membrane and the corresponding antigen (or antibody) is used to change the potential of the biosensitive membrane.
Once the antigen or antibody is immobilized on the membrane, it will form a molecular functional membrane with a strong recognition immune response. For example, the antigen is immobilized on the acetyl cellulose membrane, because the protein is a bipolar electrolyte (the polarity of the positive and negative electrodes changes with the pH value), so the antigen immobilized membrane has a surface charge. The membrane potential varies with the membrane charge. Therefore, according to the change of antibody membrane potential, the amount of antibody attached can be measured.
Modern medical sensor technology has got rid of the technical shortcomings of traditional medical sensors such as large size and poor performance, and has formed a new development direction such as intelligence, miniaturization, multi-parameter, remote control and non-invasive detection, and has achieved a series of technological breakthroughs. Some other new sensors such as DNA sensors, optical fiber sensors, etc. are also in the ascendant. The innovation of medical sensor technology will surely promote the faster development of modern clinical medicine.
With the advent of the information age, sensor technology has become an important technical foundation of the information society, and medical sensors are bound to seize this opportunity and strive towards intelligent, miniaturized, multi-parameter, remote control and non-invasive testing. development, providing an important impetus for promoting the development of modern medicine. It is believed that while medical sensors continue to improve their technological content, the application of medical sensors in the medical field will become more and more extensive.