Biomedical sensors are the vanguard of biomedical science and technology, and the correct conclusions of biomedical research depend on the correct measurement of biomedical sensors. The sensor is a very comprehensive science and technology.

The physical model of modern sensors is shown in the figure:

For traditional measurements, the sensitive membrane is equivalent to the interface between the sensor and the object being measured. Chemical sensors and biosensors can be represented by attaching a layer of sensitive membranes tailored to the needs of conventional sensors. The difference between the two depends on whether it is biologically active. Bioactive membrane materials are biosensors. There are 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 occur on the interface.

Medical requirements for sensors

1, high security (especially for the human body sensors and transducers), high sensitivity, high signal to noise ratio (high selectivity).

2. The measure to ensure physical security is the isolation and floating technology of electricity.

3. The requirement for high chemical safety is non-toxic and has no carcinogenic effects in the near and long term.

4. The requirement for high biosecurity is the absence of DNA and RNA mutations.

5. Measures to ensure high selectivity are the use of resonance effects, filtering techniques, adaptive techniques, molecular recognition and ion recognition techniques.

6. Measures to ensure high sensitivity are: physical, chemical and biological amplification techniques.

The main use of medical sensors

1. Detection of biological information: such as intracardiac pressure detection before cardiac surgery; basic research on cardiovascular disease needs to detect blood viscosity and blood lipid content.

2, clinical monitoring

For example, the patient needs to continuously measure physiological parameters such as body temperature, pulse, blood pressure, respiration, and electrocardiogram before and after the operation.

3, control

The physiological processes of the human body are controlled by the detected physiological parameters. Electronic prosthesis

The amount of measurement that needs to be measured in medicine

Classification of biomedical sensors

Classified by application: implantable sensors, temporary implanted body cavity (or incision) sensors, extracorporeal sensors, sensors for external devices

Implantable sensor

According to the working principle: physical sensors (displacement, force, temperature, humidity.), chemical sensors (various chemicals), biosensors (various enzymes, immunity, microorganisms, DNA...), bioelectric electrode 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 use of physical sensors for biomedical applications

The force sensor is used to measure the weight; the piezoelectric film sensor is used to measure the heart rate and the breathing mode; the thermopile sensor is used to measure the body temperature; the blood oxygen sensor is used to measure the blood oxygen content; the CO2 sensor is used to measure the metabolism; and the flow sensor is used to assist Breathing; the 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 components, concentrations, and the like into electrical quantities that have an exact relationship with them. Most of them use some functional membranes to select the specific chemical components to screen the measured components, and then use electrochemical devices to turn them into electrical quantities.

Generally, it is classified according to the response mechanism of the membrane electrode, the composition of the membrane, or the structure of the membrane. Such as ion selective electrode transducer, gas sensor electrode transducer, humidity sensor transducer, wire electrode transducer polymer matrix electrode transducer, ion sensitive field effect transistor transducer, ion selective microelectrode Energy, ion selective sheet transducer.

The chemical substances measured by various chemical transducers in biomedicine are: K+, Na+, Ca2+, Cl-, O2, CO2, NH3, H+, Li+, and the like.

biological sensor

Biosensors use bioactive substances to selectively identify and measure to achieve measurement. They are mainly composed of two parts: one is a functional recognition substance (molecular recognition element), which is specifically identified by the substance to be tested; the other is electrical and optical signals. A conversion device (transducer) that converts a chemical reaction generated by the analyte into an electrical or optical signal that is convenient for transmission.

The first biosensor was the enzyme electrode, and Clark and Lyons first proposed the idea of forming an enzyme electrode. In the mid-1970s, it was noticed that the lifespan of enzyme electrodes was generally short, and the price of purified enzymes was relatively expensive. Most of the enzymes were derived from microorganisms or animal and plant tissues, so naturally, people were inspired to study the derivative types of enzyme electrodes: New biosensors such as microbial electrodes, organelle electrodes, animal and plant tissue electrodes, and immune electrodes have greatly increased the variety of biosensors;

After entering the 1980s, with the continuous improvement of ion-sensitive field-effect transistors, in 1980 Caras and Janafa pioneered the development of enzyme FETs for the determination of penicillin.

Biosensor composition and basic principles

1. Molecular recognition component

2, the transducer

Transducer types include electrochemical electrodes, semiconductors, thermistors, surface plasmons, piezoelectric crystals, etc.

Biosensor classification

Molecular recognition component

Classified by device

Enzyme sensor

The catalysis of the enzyme is to decompose the substrate under certain conditions, so the catalytic action of the enzyme is essentially to accelerate the decomposition rate of the substrate.

The enzyme sensor is composed of a fixed enzyme and a base electrode. The design of the enzyme electrode mainly considers an electrode active material generated or consumed by an enzyme catalytic process. For example, an enzyme catalyzed reaction is an O 2 process, and an O 2 electrode or an H 2 O 2 electrode can be used; The process produces acid and the pH electrode can be used.

Enzyme sensor signal conversion method

1. Potentiometry

The potential method is to generate different susceptors by different ions, and calculate the concentration of various ions related to the enzyme reaction from the measured membrane potential. Generally, an ammonium ion electrode (ammonia electrode), a hydrogen ion electrode, a carbon oxide electrode, or the like is used;

2, current method

The current method is a method of calculating a substance to be measured from a current value obtained by an electrode reaction of a substance related to an enzyme reaction. The electrochemical device uses an oxygen electrode. a fuel cell type electrode and a hydrogen peroxide electrode;

Glucose sensor

working principle

Glucose sensor for measuring oxygen consumption + glucose sensor for measuring H2O2 production

The oxygen electrode is composed of: 1 immersed in an alkali solution by a Pb anode and a Pt cathode, and 2 a cathode surface covered with an oxygen penetrating glucose (matrix) film [Teflon, about 10 ¦Ìm thick].

The O2 principle of oxygen electrode measurement: the characteristic that oxygen is first reduced on the cathode. The O2 in the solution passes through the Teflon film to reach the Pt cathode. When a DC voltage is applied to the polarization voltage of oxygen (such as 0.7V), the oxygen molecules get electrons on the Pt cathode and are reduced: the current value and It is proportional to the concentration of O2.

O2+2H2O+4e=======4OH-

Glucose sensor for measuring H2O2 production

Glucose oxidase (GOD)

Glucose + H2O + O2 --- -- -- ¡ú gluconic acid + H2O2

Oxidation of glucose produces H2O2, which passes through a selective gas permeable membrane and oxidizes on the Pt electrode to produce an anodic current. The glucose content is proportional to the current, from which the glucose solution concentration can be measured.

When a voltage of 0.6V is applied to the Pt electrode, the resulting anode current is: H2O2¨D¡ª¡ª¡ª¡ª¡ª¡ª¡ú O2+2H++2e

Microbial sensor

Microbial sensors are classified into aerobic microbial sensors and anaerobic microbial sensors

The sensor is placed in a test solution containing an organic compound, and the organic substance is diffused into the microbial membrane and taken up by the microorganism (referred to as a chemical).

Aerobic microbial sensor

Microbial respiration can be determined by oxygen or carbon dioxide electrodes.

O2 electrode aerobic microbial sensor response curve

Anaerobic microbial sensor

Microbial metabolites can be determined and determined by ion selective electrodes

Formic acid sensor (anaerobic) principle:

Fixing hydrogen-producing Clostridium butyricum on a low-temperature jelly film and fixing it on the fuel cell Pt electrode;

When the sensor is immersed in a solution containing formic acid, the formic acid diffuses through the polytetrafluoroethylene membrane to the fusiform butyricum, and is hydrogenated to produce H2, which in turn passes through the polytetrafluoroethylene film and the Pt electrode on the surface of the Pt electrode. The redox reaction generates a current which is proportional to the H2 content produced by the microorganism, and the amount of H2 is related to the concentration of formic acid to be tested, so the sensor can determine the concentration of formic acid in the fermentation solution.

Immunosensor

The basic principle of an immunosensor is an immune response. The potential of the biosensing membrane is altered by the specific reaction of the immobilized antibody (or antigen) membrane with the corresponding antigen (or antibody).

Once the antigen or antibody is immobilized on the membrane, it forms a molecular functional membrane with a strong recognition of the immune response. For example, the antigen is immobilized on the acetylcellulose membrane, and since the protein is a bipolar electrolyte (the polarity of the positive and negative electrodes varies depending on the pH value), the antigen-immobilized membrane has a surface charge. Its membrane potential varies with membrane charge. Therefore, according to the change in the membrane potential of the antibody, the amount of the antibody can be measured.

Modern medical sensor technology has got rid of the shortcomings of traditional medical sensors such as large size and poor performance, and has formed new development directions such as intelligent, miniaturized, multi-parameter, remote control and non-invasive detection, and has achieved a series of technological breakthroughs. Other new types of sensors such as DNA sensors, fiber optic sensors, etc. are also in the ascendant. The innovation of medical sensor technology will certainly promote the rapid 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 also necessary to seize this opportunity, and strive to be intelligent, miniaturized, multi-parameter, remote control and non-invasive detection. Development provides an important impetus for the development of modern medicine. It is believed that while medical sensors continue to increase their technological content, the application of medical sensors in the medical field will become more and more extensive.