The application of intelligent sensors in medical research: a review

The rapid advancement of biomedicine in the twenty-first century has been facilitated by the constant innovation in biomedical technology. The most crucial issue in the field of medicine is to use sensor technology to gather information from primitive organisms, particularly the human body. Design, development, and application of biomedical sensors in the study of clinical diseases’ diagnosis and therapy have all been significantly aided by the advancement of medicine. The interest in creating sensors significantly increased in the 1960s. Chemical and biological sensors have been swiftly created in response to an urgent practical necessity, enabling the creation of selective sensors for the direct detection of diverse ions and compounds. The traditional large-size sensors are quickly turning into miniature sensors and are rapidly applied in biological and medical fields. Currently, wearable electronic blood pressure monitors, home blood glucose meters, and quick body surface digital thermometers are commonly used. The advent of a wide variety of medical-grade wearable sensors that will enable real-time biometric data tracking of a large range of physiological characteristics will likely be one of the most revolutionary, exciting, and difficult changes to come to medicine over the next several years. For possible uses in the entertainment, health monitoring, and medical care industries, high-performance flexible strain sensors connected to clothing or human skin are necessary. The use of sensors in the development of biomedical diagnostic tools and medical equipment will enhance human quality of life in the twenty-first century. This article will introduce the current medical sensor field related to sensors for physical quantities, sensors for chemical quantities, sensors for biological quantities such as electronic nose


Sensor classification in the biomedical sector
In the field of biomedical sensing technology, sensors are categorized into the following three groups based on the substances being monitored: (1) Physical quantity sensors. Sensor systems that are usually run by converting a physical quantity (such as pressure, velocity, or flux) into electrical signaling and using signal conditioning [5].
(2) Chemical quantity sensors. Sensors are created to measure odor molecules, the amount of oxygen and carbon dioxide in the human body, the pH of bodily fluids like blood, sweat, and urine, as well as ions like Na + , K + , Ca 2+ , Cl − , and heavy metals.
(3) Biometric quantity sensors. A biomedical sensor, which is used to detect tissues, cells, enzymes, antigens, bodies, receptors, hormones, cholic acid, acetylcholine, serotonin, DNA and RNA, and protein biomass in organisms, detects a physical or chemical stimulus in a typical network and transmits that information into it as a biochemical signal [6].
The field of medical sensors includes several different classes in addition to the one described above based on the type of sensing substance.
(1) Classification based on size: Conventional sensors (millimeter and above) that can be used for tissue and in vitro detection, microsensors (at the micron level), that can be used for tissue and extracellular detection, and nanoscale sensors that can be used for both intracellular and extracellular detection are among these. For instance, in 2018, the first human trials for an image-guided cancer Figure 1 Schematic of common sensor principle surgery using a pH-transistor nanoscale sensor have already begun [7].
(2) Classification based on principles: Medical sensors can be divided into groups according to the conversion principles they employ. These include structural sensors that make use of electromagnetic laws or physical laws of motion, physical sensors that achieve non-electricity conversion by utilizing solid-state physical properties of materials and various physical and chemical effects, composite sensors that combine structural and physical sensing mechanisms, and biosensors that make use of active phenomena exhibited by microorganisms or biological tissues as integral components.
(3) Classification based on fundamental sensing functions: Sensors can also be categorized in accordance with their fundamental sensing capabilities. There are ten different categories in total, which include thermal sensitive elements, photosensitive elements, gas sensitive elements, force sensitive elements, magnetic sensitive elements, humidity sensitive elements, sound sensitive elements, radiation sensitive elements, color sensitive elements, taste sensitive elements, and others.
(4) Sensors can be divided into active and passive categories based on their operating traits. Active sensors can deliver a specific detection signal to the tested object, recognize changes brought about by the detection signal in the tested object, or create a signal by using the detection signal to cause an effect in the tested object. Radio frequency range detectors and gadgets based on photoacoustic effect analysis are two examples of active sensors. In contrast, passive sensors only pick up signals produced by the thing being monitored. Infrared thermometers and infrared camera systems are two examples of passive sensors. (5) According to the output: analog digital type and resistance type, inductor type and a capacitance type.
In many cases, the classification methods above are sometimes integrated, such as strain pressure sensors, capacitive heart sound sensors, thermoelectric glucose sensors, etc.

Development of biomedical sensor technology at home and abroad
Researchers in the medical field worldwide pay close attention to the advancement of biomedical sensing technology since it is essential to the growth of biomedicine. The United States is one of the industrialized nations that has identified biomedical sensing technology as a priority area of technological research dating back to the 1980s. China is spending a lot of money and talent developing sensors for the biomedical industry in an effort to keep up with the progress on developed countries. We are on the verge of entering the digital era of medicine, when vital information that was once hidden away in a handwritten medical record or paper logbook can now be gathered, compiled, and analyzed almost instantly, along with new, continuously gathered sensor data that can be tracked, compiled, summarized, and personalized. In an age when virtually limitless information is available, literally, at our fingertips, the information-intense field of health care should have been one of the first to take advantage of cognitive computing capabilities, rather than a laggard [8]. All of this is dependent on the creation of reliable and effective biological sensors. The result of combining biomedicine and electronic information technology is the biomedical sensor. The technology is itself very vibrant. Deep biological research, reliance on cutting-edge sensor technology for molecular recognition gene probing, and monitoring of neurotransmitters and neuromodulators create the necessary conditions for the development of biomedical sensor technology. The advancement of biomedical sensor technology is supported technically by the emergence of new fields and technologies, such as microelectronics, optoelectronics, molecular biology, and biochemistry. Under these circumstances, biomedical sensing technology has advanced significantly and developed quickly. The following bullet points sum up the evolution of biomedical sensing technologies.

Highlight
(1) Sensors are being used more and more frequently in the field of biomedicine, and nearly all medical devices now come with a variety of sensors. (2) Classifying the types of sensors used in conventional medical research according to various criteria is essential for developing a thorough understanding of sensors.
(3) Wearable sensors and biomolecular sensors, such as electronic tongues and electronic noses, have recently come to the forefront as important directions for the development of medical sensors in the future. Submit a manuscript: https://www.tmrjournals.com/bmec

Basic research and technological innovation
Technical science encompasses biomedical sensors. The development of new products was the primary emphasis of sensing technology research from the early 1970s to the mid-1980s, and it was gradually established in practice that the development of deep and high-level sensors must be based on broad, sound basic research. In order to improve the signal-to-noise ratio and shorten reaction times, it is fundamentally important to understand the molecular recognition mechanism and the specifics of the interface process for biomedical sensors. We need to pay attention to technological innovation in order to turn the outcomes of our study into commodities. A variety of processing technologies, including as biotechnology, semiconductor technology, precision machining, and chemical corrosion, are blooming and acting similarly. In addition, we should have a new understanding of the vital role played by artificial intelligence and big data in the development of medical sensors. Modern information technology makes it possible to collect, store and process a large amount of data, which also provides technical support for medical sensors to collect a large amount of human body information.

Technology for film-forming sensitive materials
The core element of a skin sensor is a combination of sensitive and matrix materials. Therefore, advancements in sensitive material film forming techniques are essential for the creation of more sophisticated sensors. Physical sensors often use electron beam epitaxy technology to create rapid film creation from their sensitive core material. Physical adsorption and chemical cross linking are frequently used in the molecular assembly of chemical sensors. Multi-enzyme system membrane, monoclonal antibody membrane, mediator membrane, and monomolecular membrane are some of the biosensors that have garnered a lot of interest.

Economies of scale and development center
There have long been economies of scale in the development of biomedical sensors and the numerous medical devices that are based on them. A vast wave of start-up businesses with the goal of creating, developing, and releasing innovative medical gadgets on the market is being created as a result of the development of technology and the numerous government programs advocated to improve citizens' access to healthcare. The stages of a medical product design and development strategy are shown in Figure 2 [9]. Industrialized nations have built numerous sensor technology institutes to hasten the advancement of biomedical sensing technology and position it as a leader in global competitiveness. In the US, Massachusetts Institute of Technology and Stanford University both have technology centers or sensor-focused labs. Japan has established a sensor-focused cutting-edge technology center at the University of Tokyo, Cranfield University in the United Kingdom has a biosensing technology center, and the University of Twente in the Netherlands has set up a European Sensor Technology center [10].

Medical sensor industry development characteristics
There is a rising need for very sensitive and selective analytical procedures in the disciplines of analytical chemistry, medical diagnostics, and biotechnology. These approaches should ideally enable in-real-time direct monitoring with dependable and miniaturized instruments. Biochemical sensors employ a variety of sensing technologies [11,12]. Medical sensors are currently thought of as a class of unique electronic devices. In actuality, it has many more qualities that make it far more complex than the typical device. The following factors can be used to describe its development features.

Knowledge-intensive
A wide spectrum of science and technology are used in the design, production, and application of sensors. For chemical sensors, for instance, the creation of sensitive materials necessitates the study of fields like quantum chemistry. The synthesis of these materials requires designers familiar with supramolecular chemistry, host-guest chemistry, and chemical biotechnology. Film forming techniques require technicians to understand surface chemistry interface physics and molecular assembly technology. In addition, they should be capable of using microelectronic technology, optoelectronic technology and precision machining technology to develop conversion devices.

High reliability
There is an increasing demand for highly sensitive and selective analytical methodologies in the fields of analytical chemistry, medical diagnostics, and biotechnology. These methodologies should ideally enable real-time direct monitoring using reliable and miniaturized instruments. Given that medical sensors are designed for use in humans, their proper functioning is of utmost importance. The medical device sector is subjected to oversight by various national regulatory entities. Notably, the European Commission Directorate and the US Food & Drug Administration (FDA) are prominent regulatory organizations worldwide. An overview of the top regulatory agencies globally is depicted in Figure 3 [9] of the article. These regulatory bodies require robust evidence that long-term usage of medical sensors is completely safe for the human body, devoid of any negative side effects, and capable of providing monitoring data with 100% accuracy. Fluid sensors should possess resistance to erosion by bodily fluids and facilitate easy cleaning. Implantable sensors should exhibit excellent compatibility with tissues to avoid rejection. In summary, medical sensors must be endowed with high stability and reliability to fulfill their intended purpose.

The precise and meticulous process
Special embedding technology is imperative for the integration of high-precision sensors with the matrix sensor, which is constructed using intricate process integration technology. The manufacturing process must effectively mitigate the risk of sensor leakage and deformation during extended immersion. Moreover, a precise technique is required for the coupling of the sensitive film to the end face of the optical fiber. The synergistic collaboration of chemical and mechanical techniques is crucial for achieving precise machining. A superior sensor simultaneously functions as an embodiment of scientific innovation and as an artifact of technological advancement.

Several major future development directions for biomedical sensors
Over the past few decades, the utilization of medical sensors has witnessed a remarkable surge in clinical surgical and disease diagnostic applications. The ideal smart biological sensors should possess cost-effectiveness, high sensitivity, precise selectivity, and comprehensive utility, while enabling real-time communication for remote monitoring, reporting, programming, and control by the operator. In fact, smart biomedical sensors are poised to accurately monitor and assess the concentration of vital biological or chemical substances, as well as physical parameters of the human body, transmitting the resultant data to users, scheduled individuals, physicians, and medical institutions as necessary. The ongoing development of medical sensors aims to further enhance the delivery of superior healthcare services to patients. In essence, the following characteristics are anticipated to shape the future advancements in medical sensor technology. Multi-parameter. When utilizing distinct sensors to monitor various features, serial operation is a challenging weakness to overcome. Such monitoring is not only wasteful, but it also makes synchronizing multi-parameter monitoring in time and space challenging. The integrated technology enables the development of multi-parameter sensors. The United Kingdom was the first to introduce an integrated blood electrolyte sensor capable of monitoring five parameters (Na + , K + , Ca 2+ , Cl − , and pH) in the early 1980s. The United States created a light-addressed potential sensor capable of simultaneously sensing 23 biochemical parameters in the late 1980s. Miniaturization technique. Biomedical science has transitioned into the molecular era, necessitating continuous evolution of sensor technologies to keep pace with advancements in life science research. Sensors play a pivotal role in detecting a diverse array of entities, ranging from complex systems, organs, and tissues, down to individual cells and chemical species. Ongoing developments encompass the design and fabrication of mechanical, physical, chemical, biological, and molecular sensors. Notably, customized nanoscale optical sensors exhibiting precise tunability offer the potential to dynamically monitor the local microenvironment with exceptional spatiotemporal resolution. This capability not only promises novel insights into the functional intricacies of various cell types but also facilitates comprehensive understanding of the spatial interactions between diverse neural signaling systems and neighboring cells. The miniaturization of sensors closely parallels the advancements in electronic device miniaturization, thereby enabling the detection of particles at the nuclear level. Given the multifaceted nature of biomedical sensing, the incorporation of sensors of varying sizes becomes essential, as research conducted at different levels within the human body provides complementary perspectives and insights. Optical sensor technology. Optical technology has emerged as a formidable contender to electrical technology in sensor development, challenging its established dominion. In the past two decades, light sensors have accounted for approximately 35% of biosensors employed for bacterial detection, with electrochemical sensors representing around 32% and piezoelectric sensors constituting roughly 16%. This shift can be attributed to two fundamental factors: Firstly, the integration of chemiluminescence, bioluminescence, and cell light communication has introduced novel concepts for constructing sensitive membranes. Secondly, advancements in photosensitive electronics and optical fiber technology have facilitated their application as versatile matrix sensors. Presently, fiber optics offer a potential pathway for implementing a diverse range of sensors. Light sensors have achieved remarkable maturity, expanding their application domains while continually enhancing their technical prowess. When compared to other sensor types, optical sensors exhibit notable attributes such as swift response times, heightened sensitivity, robust immunity against electromagnetic interference, and compact form factors. As a detection modality, sensors possess inherent characteristics that shape their evolution and development within the field.
At the same time, there are many challenges in the application of medical sensors. We identified and addressed a number of challenges in building medical sensor networks. The first challenge is to design a network rack for medical sensor networks. The second challenge is to perform publish/subscribe routing on low-power sensors. The third challenge concerns the monitoring of deployed medical sensor networks. Due to resource limitations, it is not practical to record network behavior at sensor nodes. However, the network dynamics need to be reconstructed for evaluation and debugging purposes [13].

Human physiological information and analysis
For a long time, optical technology has challenged electrical technology's established role in sensor development. In the last 20 years, light sensors have accounted for around 35% of bacterial detection biosensors, whereas electrochemical sensors have accounted for approximately 32% and piezoelectric sensors have accounted for approximately 16%. There are two basic causes behind this: First, chemiluminescence, bioluminescence, and cell light communication have provided new concepts for the construction of sensitive membranes; second, photosensitive electronics and optical fiber technology that can be employed as matrix sensors have been developed. According to the present trend, fiber optics can be used to implement practically all sensors. Physical quantities encompass bioelectrical information, magnetic information, pressure, vibration, velocity, flow rate, temperature, and the morphology of individual organs. Chemical quantities involve a diverse range of parameters such as O 2 , CO 2 , CO, H 2 O, NH 3 , K + , Na + , and Ca 2+ . Biomass comprises enzymes, antigens, antibodies, hormones, neurotransmitters, DNA and RNA, among others.

Basic molecular biomedicine
Biomedical sensing technology is currently progressing from the detection of macroscopic human information to the finding of human life events at the cellular and molecular levels. Cell chips, gene chips, protein chips, and molecular sensors are a few examples. At the cellular level, all of this is inextricably linked to the receptor-ligand recognition mechanism. Many factors influence receptor-ligand recognition, including bond type, group, molecule size, three-dimensional structure, and topological pattern. The following are the typical interactions between receptors and ligands: (1) Physical action. From a thermodynamic point of view, the entropy of the ligand or the receptor, or both configurations, is large before binding. The entropy of the complex will decrease after binding. The role of the coordination field should also be included here.
(2) Chemical interactions. It is usually achieved by hydrogen and coordination bonds and by the interaction between groups. This is by far the most studied and widely accepted explanation.
(3) Spatial effect. Usually the acceptor molecule has a hole in its configuration, and the hole is just big enough to embed the ion. The selectivity formed by the size of the hole is sometimes not ideal, and then we have to rely on the action of physical and chemical factors.

Biomedical sensor technology for physical quantities
The most common sort of sensor in the biomedical area is a sensor for physical values, sometimes known as a physical sensor. It employs certain physical phenomena to convert measurable physical quantities Submit a manuscript: https://www.tmrjournals.com/bmec into tractable signals. The rapid growth of Internet of Things (IoT) technology and artificial intelligence (AI) in recent decades has fueled the research boom in physical sensors. Material selection, structure design, and performance studies for physical sensors have piqued the interest of advanced manufacturing researchers around the world. Physical sensors have achieved significant technological advances for applications in domains such as electronic skin, biomedicine, and tissue engineering [14]. Physical sensors are widely used in business, agriculture, daily life, the military, and aerospace. Physical sensors have become increasingly used in medical diagnosis and therapy as biomedicines have advanced. Furthermore, as optical fiber, superconductivity, nanomaterials, and the rapid development and use of semiconductor technology emerge, physical sensors are gradually evolving toward multi-function, high accuracy, and integration. In the biomedical field, common physical sensors such as resistive sensors, inductive sensors, capacitive sensors, piezoelectric sensors, magnetoelectric sensors, photoelectric sensors, and thermoelectric sensors are often used [15].

Strain and piezoresistive sensors
The fundamental principle underlying resistive sensors involves the conversion of non-electric quantities into variations in resistance, which are subsequently measured. Based on their sensitive elements, resistive sensors can be categorized into three types: strain type, piezoresistive type, and potentiometer type. The resistance strain sensor utilizes a resistance strain gauge to convert strain into changes in resistance. It consists of a resistance strain-sensitive element affixed to an elastic component. The operation of piezoresistive sensors relies on the piezoresistive effect exhibited by specific classes of materials when subjected to elastic deformation. The growing interest in piezoelectric sensors has been validated by the escalating number of scientific publications documenting piezoelectric resistivity, as depicted in Figure 4 [16]. Piezoresistive sensors are predominantly fabricated using semiconductor materials and possess characteristics such as compact size, broad frequency response range, high sensitivity, and exceptional resolution capabilities. Resistive sensors are mainly used to measure pressure in medicine, such as common blood pressure (including invasive and non-invasive blood pressure measurement), intraocular pressure measurement. Owing to the outstanding sensing properties, especially high sensitivity and large stretchability, flexible piezoresistive strain sensors are advantageous for achieving intelligent sensing [17]. With the development of technology becoming more and more mature, the application of piezoresistive sensors is more and more extensive. Now we mainly introduce two examples of his application.
(1) Measurement of blood pressure. Blood pressure monitoring encompasses the evaluation of arterial pressure, venous pressure, and heart pressure. Each pressure signal comprises systolic blood pressure, diastolic blood pressure, and mean blood pressure. Blood pressure serves as a crucial physiological marker, fundamental in assessing cardiovascular function. Beyond assessing cardiac pumping efficiency, heart rate, peripheral vascular resistance, and arterial compliance, blood pressure measurement encompasses insights into overall blood volume and hemodynamic characteristics. The advent of long-term, implanted blood pressure monitoring systems is attributable to substantial advancements in electrical and biological engineering. Notably, breakthroughs in biomaterials and implant packaging have played a pivotal role in enabling the development of robust and enduring blood pressure monitoring systems suitable for long-term implantation. This progressive evolution, spanning several years, has significantly enhanced the biocompatibility, performance, and longevity of these implanted systems [18]. Fluid-coupled sensors present a straightforward approach to directly measure blood pressure. A fluid-filled catheter is inserted into the subject's body, facilitating the transfer of pressure data from the catheter's distal end to a pressure sensor located at the opposite end. This method, extensively utilized in clinical and scientific research, is widely accepted as a mature and reliable technique. The pressure sensor employed in the liquid coupling system is a commercially available instrument that utilizes resistive strain sensing technology to assess invasive blood pressure. Within this sensor, an elastic diaphragm translates pressure signals into diaphragm deformation, inducing strain on a strain gauge. This strain induces a change in resistance within the strain gauge, thereby converting the pressure into an electrical signal output. Noninvasive blood pressure measurement, on the other hand, employs air as a medium to examine blood pressure, mainly focusing on arterial pressure. Numerous measurement techniques and commercially available devices have been developed and extensively employed in clinical settings due to their user-friendly and noninvasive nature. The majority of electronic sphygmomanometers are adaptations of traditional sphygmomanometers, interconnected with electronic analytical control terminals. In response to the situation, the computer system automatically regulates the inflation and deflation of the cuff. Electronic sphygmomanometers utilize oscillometric wave technology to detect vibrations generated by blood flow against the vessel wall. The strongest vibrations manifest during cuff deflation when the cuff pressure aligns with the vascular pressure. By analyzing and calculating the vibration wave, it becomes possible to determine diastolic and systolic blood pressure values. The primary constituent of electronic sphygmomanometers is a pressure sensor, with a significant proportion of manufacturers utilizing solid-state piezoresistive sensors in their devices.
(2) Measurement of intraocular pressure. The measurement of intraocular pressure is another major use for tiny silicon-based force sensors. Many eye illnesses can be caused by a change in intraocular pressure. Intraocular pressure is an important parameter in the treatment of patients with glaucomatous optic neuropathy [19]. Glaucoma, the main cause of blindness, is caused by excessive intraocular pressure. The normal intraocular pressure varies from 1.33 to 2 kPa, whereas pressures above 2.8 kPa suggest eye illness. The applanation tonometer is one of the most widely used intraocular pressure measurement methods in the world, and it has long been considered the normative standard for intraocular pressure measurement. It is based on the Imber-Fick law. When the tonometer comes into contact with the cornea, the pressure drive rod shifts, causing an electrical signal to be generated in the pressure sensor. The Tono-Pen tonometer, designed on the basis of the Mackay-Mary tonometer, has been widely used due to its small size, lightweight, battery function, and ease of carrying.

Inductive sensor
Inductive sensors utilize electromagnetic induction to convert non-electric parameters such as displacement, pressure, flow, and vibration into changes in self-inductance or mutual inductance of a coil. These changes are then translated into voltage or current variations through the measuring circuit. Inductive sensors can be categorized into self-inductive, mutual inductive, and eddy current sensors. They offer advantages such as a straightforward design, reliable operation, high measurement precision, excellent zero stability, and significant output power. However, it's important to note that inductive sensors are not ideal for rapid dynamic measurements due to inherent limitations in sensitivity, linearity, and measurement range, as well as their low frequency response. Nevertheless, inductive sensors find valuable applications in the biomedical industry, particularly for monitoring subtle movements such as limb tremors and breathing patterns. They also facilitate long-distance transmission, recording, display, and control of information, making them extensively employed in industrial automatic control systems.

Capacitive sensor
The capacitive sensor is a sensor that converts the measured mechanical quantity, such as displacement, pressure, etc., into a change in capacitance, and its sensitive part is a capacitor with variable parameters. When the sensor is subjected to an external force, the distance between the two plates and the relative area of the plate or dielectric will change, which in turn will change the capacitance [20]. According to the varying parameters, capacitive sensors can be categorized into three types: variable polar distance, variable area, and variable dielectric constant. These sensors exhibit several advantages, including a simple structure, cost-effectiveness, high sensitivity, large overload capacity, excellent dynamic response characteristics, and robustness to harsh environmental conditions. Capacitive sensors have a wide range of applications and hold significant potential for further sensor development. In the medical industry, capacitive sensors play a crucial role in acoustic-electrical conversion and position feedback. They are frequently utilized to measure various physiological parameters such as intraocular pressure, blood pressure, respiration, among others. An example of a notable application is the Pyramid device, which utilizes capacitive proximity sensing technology exclusively licensed from Stanford University for continuous blood pressure monitoring. This device consists of multiple sensors integrated into a soft foam band, which can be comfortably worn around the wrist or on the foot ( Figure 5) [21]. The collected pulse waveform data is wirelessly transmitted to a mobile device, enabling real-time display of continuous blood pressure readings. This wearable device has been successfully employed in newborns to monitor and manage blood pressure, demonstrating its effectiveness and ease of use. Thus, capacitive sensors hold great potential for advancements in sensor technology and play a significant role in healthcare applications, particularly in continuous blood pressure monitoring using innovative wearable devices.

Piezoelectric sensor
The piezoelectric effect serves as the underlying principle for piezoelectric sensors, enabling them to measure forces and convert non-electrical energy, such as displacement and acceleration, using sensitive elements composed of piezoelectric materials. These materials can be broadly categorized into three groups: natural crystals, piezoelectric ceramics, and high molecular piezoelectric polymers. Piezoelectric sensors possess several advantages, including a wide frequency range, high sensitivity, simple structure, reliable operation, and lightweight design. In the field of biomedical applications, piezoelectric sensors play a fundamental role as the core component of ultrasonic transducers, which are extensively utilized to measure various pressures, including pulse and other physiological characteristics. Additionally, piezoelectric sensing has gained significant attention for high-temperature applications in sectors such as aerospace, automotive, power plants, and material processing. The appeal of piezoelectric sensing lies in its cost-effectiveness, compact sensor size, and simplified signal conditioning when compared to other high-temperature sensing techniques. Therefore, piezoelectric sensors offer great potential for advancements in measurement technology, with applications spanning a wide range of fields, including both medical and high-temperature industries [22,23].

Magnetoelectric sensor
A magnetoelectric sensor is a type of sensor that uses the magnetoelectric effect to convert measurable physical properties such as displacement, velocity, and acceleration into electrical signals. It is separated into two types: magnetoelectric induction sensors and Hall sensors. Magnetoelectric induction sensors are active sensors that can convert observed physical quantities into electrical signals without the use of an external power supply. The Hall sensor is a passive sensor that requires an external DC bias. Hall's preliminary experiments demonstrated that, in the presence of an external magnetic field perpendicular to the current running through the plate-like sample, charge carriers would be redirected to one side of the conductor, resulting in a transverse potential difference ( Figure 6) [24]. Based on the Hall effect, a number of Hall elements have been developed to aid in the control and reliability of small motors. Hall sensors are mostly used in medicine to regulate the motors of reverse pressure sensors in medical devices such as ventilators, infusion, insulin, syringe pumps, and renal dialysis machines . Magnetic field detection has advanced dramatically during the last few decades. Magnetoelectric sensors are widely utilized in medical treatment, automatic control, mechanical engineering, and other fields due to their high output power, consistent performance, and wide operating frequency band [25].

Photoelectric sensor
The photoelectric sensor consists of a fundamental component, namely a photoelectric sensitive device that efficiently converts light energy into electrical energy. The operation of optoelectronic devices relies on the fundamental principle of the photoelectric effect. Within this field, the internal photoelectric effect describes the phenomenon of altered conductivity in response to light, with photoresistors being a notable example of such photoelectric devices. Conversely, the external photoelectric effect refers to the liberation of electrons from stimulated object surfaces upon illumination, exemplified by photocells and photomultiplier tubes. Another intriguing phenomenon, the photogenerated volt effect, generates a specified electromotive force in a material under the influence of light, which is showcased by photocells. Optoelectronic devices hold immense significance due to their uncomplicated architectures, rapid responsiveness, compact dimensions, user-friendly nature, and exceptional reliability, finding extensive applications in medical systems, automated detection methodologies, and computer-based control systems. The utilization of optical sensors as a powerful analytical tool has gained substantial momentum, primarily in the detection of diverse analytes. In particular, optical sensors based on nanomaterials have demonstrated remarkable potential within the analytical domain, attributable to their cost-effectiveness, biocompatibility, and non-toxic properties. It has now been reported in Japan that doctors used an automated system with photoelectric sensors connected to a dual-channel recorder, tail cuff and sphygmomanometer to obtain indirect blood pressure measurements.

Thermoelectric sensor
The electrical resistivity of most substances is known to exhibit temperature dependence, commonly referred to as the thermal resistance effect. Based on this principle, thermal resistance involves the incorporation of both a conductor and a semiconductor material as temperature-sensitive components. Given the pervasive influence of temperature variations on numerous physical processes, a wide range of phenomena can be leveraged as the foundation for temperature sensors. Researchers endeavor to identify the most suitable approach among these phenomena that aligns with specific design requirements, including purpose, linearity, range, repeatability, and ease of conversion to electrical signals. Thermoelectric materials play a crucial role in the design and performance of various sensor types, thus experiencing a surge in popularity in recent years. Inorganic thermoelectric materials, on the whole, exhibit high electrical conductivity, thereby enhancing sensor sensitivity. Conversely, organic thermoelectric materials offer exceptional stability, enabling the sensors to maintain stable performance over time. Currently, there is a strong emphasis on sensor miniaturization in material research and device manufacturing. In this context, low-dimensional thermoelectric materials, particularly those at the nanoscale, hold immense promise for expediting sensor development. The design and fabrication of multifunctional and compact thermoelectric materials using well-aligned and efficiently assembled nanoparticles represent a viable and practical strategy for future sensors [26]. Furthermore, probe size and thermal mass are significant because they determine the interference and response rate induced by the measurement. Thermal expansion, which is basic to glass mercury thermometers and several other sensing elements in measurement and control, is perhaps the simplest and most commonly employed phenomenon in temperature detection. Transducers based on thermal resistance and thermoelectric effects are widely utilized in biomedicines for electronic temperature recording and display. An infrared sensor is also a thermal sensor that detects changes in temperature. According to the detecting technique, infrared sensors are classified as thermal infrared sensors or photonic infrared sensors. Infrared radiation is received and transformed into thermal energy in thermal infrared sensors, raising the temperature of the sensitive element. Figure 7 [27] shows the connection of the absorber to two different thermoelectric materials A and B, the other end of which is open. When the IR radiation flux is collected by the absorber, the thermocouple junction heats up due to the incident heat. The temperature difference ∆T is then stabilized to produce an output voltage Vout. This turns the heat signal into a voltage signal that can be observed.

Chemometric biomedical sensor technology
Chemical sensors have become indispensable in chemical analysis and detection. These sensors utilize specific physical characteristics of the system under investigation to convert chemical reactions of analytes into discernible signals. According to domestic scholars, chemical sensors are devices capable of qualitatively or quantitatively transforming the state or changes in diverse chemical substances, such as electrolytes, compounds, molecules, and ions, into electrical signals. In the national standard GB/T7665-1987, "General Terms for Sensors," a chemical sensor is defined as a sensor capable of perceiving a specified quantity of a particular chemical and converting it into a usable output signal. American scholar Catterall defines a chemical sensor as a device that selectively responds to a specific analyte through a chemical reaction, enabling qualitative or quantitative analysis of the analyte. These sensors are commonly employed for the detection of specific compounds or molecules. German academic Wolfbeis describes a chemical sensor as a compact device comprising a recognition element, an energy transducing element, and a signal processor. Such sensors enable constant and reversible detection of chemical substances. Emphasizing reversibility as a critical criterion, these sensors exhibit dynamic responsiveness to changes in chemical concentration during the detection process. It should be noted that irreversible devices capable of detecting a substance only once are referred to as probes. Chemical sensors are a highly valuable and cost-effective analytical tool for detecting target molecules, even in the presence of interfering substances. They consist of a general recognition element, a transducer, and an appropriate detection circuit. Interactions between the molecular recognition element and the target object lead to changes in various physical and chemical parameters such as ions, electrons, heat, mass, and light. These changes are then converted into qualitative or quantitative electrical or optical signals related to the analyzed substance through the transducer. Amplification, storage, and appropriate formatting determine the type and concentration of the displayed output. The quality of a chemical sensor relies on the suitability of the transducer and recognition element employed. Typically, the recognition element is immobilized on the transducer surface, often in the form of a membrane, to achieve maximum response and minimal interference or enable reusability. The key components of a chemical sensor device include the identification element, frequently referred to as the sensitive element. This element, typically measured non-electrically, provides output that is quantitatively related to other quantities. The sensor's selectivity allows it to specifically respond to a particular analyte or a specific class of analyte, thereby preventing interference from other chemicals. The transducer, or conversion element, is a real-world sensor capable of translating signals. It converts the non-electrical information generated by the recognition element into an electrical or optical signal that can be conveniently read.

Electrochemical sensor
Electrochemical sensors serve as invaluable tools for the qualitative or quantitative analysis of target substances. These sensors operate on the principle of converting the sensing signal, generated through the interaction of the tested substance with a specific sensing element, into an electrical signal that is directly proportional to the concentration of the target substance. The foundational principles of electrochemical sensors are illustrated in Figure 8 [28]. The working electrode, which fundamentally shapes the sensor's performance, stands as a crucial component. The entire electrochemical analysis system comprises an electrolyte, electrochemical detection equipment, and the sensing apparatus. Compared to other biosensors, electrochemical sensing technology offers distinctive advantages, including cost-effectiveness, simplicity of operation, high sensitivity, and specificity. Notably, electrochemical sensors have achieved remarkable advancements in the detection of antibiotics across various applications, assuring a promising future for this field.
Over time, electrochemical biosensor technology has undergone Submit a manuscript: https://www.tmrjournals.com/bmec (1) Utilization of nanomaterials: The application of nanomaterials has significantly contributed to the progress of electrochemical biosensor technology, owing to their favorable characteristics such as chemical stability, specificity, and biocompatibility. Among these nanomaterials, gold nanoparticles (AuNPs) have gained widespread usage. Modifying the electrode surface of the sensor with gold nanoparticles enhances the sensor's sensitivity and selectivity while improving the detection limit.
(2) Adoption of novel electrochemical detection technology: Ongoing advancements in electrochemical technology have led to recent breakthroughs in detection techniques. These novel electrochemical detection methods augment the sensor's sensitivity, selectivity, and stability. Additionally, real-time electrochemical detection technology has been investigated and applied, enabling the monitoring and control of sensor reactions in real-time.
This progress in the field of electrochemical biosensors underscores the transformative impact of nanomaterials and advanced electrochemical detection technologies. These developments have the potential to enhance biosensor performance, enabling more accurate and efficient detection in diverse applications such as healthcare, food safety, and environmental monitoring [29,30].

Ion sensor
Environmental science, biochemical research, food chemistry, and clinical diagnostics all have difficulties with the quantification and monitoring of ionic compounds. An ion sensor is a type of sensor that transforms the number of ions sensed into a useable output signal using ion selective electrodes. Ion sensors, which detect specific ion concentrations (activities) in solutions by choosing which compounds to monitor, play a crucial role in all branches of science. Ion sensors are progressing toward diversity and intelligent telemetry in recent years as a result of the advancement of semiconductor integration technology. Researchers have high hopes for the use of in vivo implanted sensors and ion sensors in clinical assessment. Although there have been basic investigations on ion sensors, the increase of sensitivity and selectivity as well as the enhancement of durability are essential in order to meet these objectives. Ion sensors have been the subject of a lot of studies, the creation of new sensors, and fundamental studies regarding the reaction mechanism of ion sensors [31,32]. The most commonly used ion sensors in medicine are ion selective optical sensors and metal example sensors. Ion-selective optical sensors based on ionophores, also known as ion-selective optoelectronic devices (sometimes referred to as optoelectronic devices), take advantage of the excellent selectivity of ionophores (ion acceptors) [33]. Another promising group of medical sensors is the metal ion sensor, which is also a significant and difficult area in analytical chemistry. Metal ions can have either positive or negative effects on biological things, which is why metal ion sensors have gained popularity in analytical chemistry. These sensors work by detecting small organic molecules like proteins, peptides, and organic polymers. In order to detect several metal ions at once, microarray metal ion sensors have been employed as a generic platform for protein and nucleic acid detection [34].

Gas sensor
The global air pollution problem has escalated due to rapid industrialization and urbanization, leading to the presence of harmful gases and volatile organic compounds (VOCs), which pose significant risks to human health. Research has demonstrated that air pollution-related complications, such as reduced lung function and damage, render individuals more susceptible to illnesses like MERS-CoV and COVID-19. To address these concerns, health organizations have recommended establishing short-term exposure limits for various hazardous gases and VOCs. Consequently, there is a pressing need for efficient monitoring techniques capable of accurately detecting these volatile compounds and dangerous gases. Gas detection devices have gained considerable attention due to their affordability, compactness, and user-friendliness. Additionally, gas sensing technologies based on microelectromechanical systems (MEMS) have emerged as promising solutions in this field. Compared to traditional gas sensors, MEMS-based gas sensing mechanisms offer notable advantages, including enhanced sensitivity, reduced power consumption, compact designs, and lower manufacturing costs. These advancements hold great potential for facilitating the diagnosis and management of human health-related problems [35]. A respiratory system biomarker for lung disorders, such as COVID-19, is methanol. It is also a typical chemical that, if ingested unintentionally, can be dangerous. In order to create core-shell CsPbBr3@ZnO nanocrystals, Xuan et al. suggested coating metal oxides on perovskite surfaces. These crystals were then used to create a gas sensor. The sensor displayed a strong capacity to distinguish methanol from unknown gas mixtures, obtaining an accuracy of 94% by utilizing machine learning methods [36]. This makes it possible to discriminate between mixed conditions and methanol. Gas sensor is a large category of chemical sensors. According to the gas-sensitive material of the sensor and the mechanism and effect of the interaction between the gas-sensitive material and the gas, it can be divided into electrochemical gas sensor, metal semiconductor gas sensor, solid electrolyte gas sensor, contact combustion gas sensor, optical gas sensor, surface acoustic wave gas sensor and other forms. The most widely used gas sensors are described below. Electrochemical gas sensor. Numerous combustible poisonous and dangerous gases are electrochemically active and can be oxidized or reduced in this way. These reactions allow for the detection of gas concentration and the differentiation of gas composition. The sensor signal is a diffusion limiting current, and the electrochemical sensor operates at a constant applied voltage. The working and counter electrodes, a reference electrode submerged in an electrolyte solution, and a potentiostat used to maintain a constant voltage are the typical components. Therefore, by measuring the amount of current generated at the working electrode, the concentration of the gas to be detected can be ascertained. A schematic of a common electrochemical gas sensor is shown in Figure 9 [37]. The transducer's shadow is also crucial with this type of sensor. A biological event is converted into an electrical signal by the transducer. Amperometric and potentiometric measurements are two of the most often utilized parameters in electrochemical sensing. While steady potential current associated with the reduction or oxidation of an electroactive species is observed in amperometric measurement, potentiometric analysis converts analytical data from the biorecognition process into potential. Because of this, they are widely used in illness diagnostics to find the right marker proteins, antibodies, DNA sequences, or cells. Semiconductor gas sensor. When gas is adsorbed on the surface of a metal oxide semiconductor element, its conductivity will change. Impedance devices based on such metal oxide semiconductor materials can be used as important components of sensors. The most popular and useful type of gas sensor at the moment is the semiconductor gas sensor. A typical semiconductor gas sensor's structure is shown schematically in Figure 10 [37]. After the sample gas is absorbed on the oxide surface, the oxide material's resistance changes as a result of catalytic oxidation. In this manner, the variation in semiconductor material resistance reflects the variation in sample gas concentration. A meter measuring the resistance was used to keep track of the concentration of the sample gas. To hasten the reaction, a base heater is utilized to heat the sensor to a steady temperature of between 200 and 250°C .
Compared with traditional metal oxide-based gas sensors, semiconductor gas sensors have faster responsiveness and more precise selectivity, which makes people have high expectations for its development and a lot of efforts have been made to improve the sensing performance. It is certain that gas sensors, mainly represented by the two types of gas sensors above, will play a greater role in the detection and treatment of respiratory diseases in the future [38].
Gas sensors have shown significant improvements in recent years across various key performance parameters. Emerging technologies such as Micro-Electro-Mechanical Systems (MEMS) have increasingly leveraged a combination of sensor hardware, integrated gas filters, and software technologies to enhance performance, achieving levels comparable to traditional analytical instrumentation solutions. Existing sensors in the market have demonstrated the ability to capture high-quality data while maintaining cost-effectiveness in their design, providing valuable insights for gas sensor manufacturers seeking to develop modern sensing solutions [39]. Early iterations of wearable physiological sensors, although initially overlooked due to the excitement surrounding their potential applications, suffered from low accuracy. However, the industry quickly recognized the need for substantial accuracy improvements in order to garner widespread acceptance and ensure sustainability. Consequently, the number of wearable sensors meeting the stringent accuracy requirements of medical devices or hospital equipment has been steadily increasing. Analogously, the mass adoption of physical sensors such as microphones, accelerometers, gyroscopes, and compact devices occurred when they achieved market-level accuracy. Presently, the sensors integrated into approximately 1 billion mobile devices shipped annually have seen their unit cost drop to less than $1. This trend highlights the opportunities for the gas sensor industry to enhance accuracy and drive broader acceptance. The advancement of gas sensors, alongside their increased accuracy, is facilitated by contemporary interdisciplinary approaches that contribute to the development of novel sensor capabilities and promote market growth. Progress in electronics, gas filters, packaging, and on-board data analysis techniques has the potential to significantly improve sensor stability and accuracy. Moreover, the application of artificial intelligence technologies and on-board data analysis algorithms, bolstered by powerful prediction models, holds immense promise for enhancing sensor performance. Overall, the steady advancements in gas sensor technologies are revolutionizing the field and paving the way for their widespread utilization. These improvements, driven by an array of factors such as MEMS integration, refined data analysis capability, and innovative interdisciplinary collaborations, bring us closer to highly accurate and reliable gas sensing solutions that can be seamlessly integrated into various applications [40].

Humidity sensor
Humidity is a fundamental parameter that represents the concentration of water vapor in the atmosphere, where water exists in a gaseous state. Within the realm of biological sciences, humidity plays a significant role, necessitating the control, perception, and monitoring of this environmental factor. The measurement of absolute humidity can be achieved through the utilization of a chemical hygrometer. By employing such devices, insights into common parameters can be gained based on the prevailing humidity levels in the surroundings. In the field of medicine, there is a critical need for rapid humidity sensors to facilitate the mapping of the human respiratory system and the diagnosis of lung disorders. These sensors are designed to detect and quantify the amount of water vapor exhaled by an individual. To address this demand, a variety of sensor types have been proposed for accurate humidity measurements. It is important to note that different humidity sensors exhibit varying response times to changes in humidity levels. A comparative analysis of the performance of several mainstream humidity sensors available on the market is presented in Figure 11 [41].
Acoustic sensors (red) are the slowest group. The second group (green) is experimental and commercial electronic sensors. The blue color represents the best performance of the optical humidity sensor.
In the choice of core sensing materials used in gas sensors, graphene is a good direction. Graphene has been shown to be sensitive to a number of different gasses. It has also provided the ultimate level of sensitivity by detection of single gas molecules. Several studies have previously reported on the influence of humidity on graphene-based devices. A common biomedical use for humidity sensors is the neonatal incubator ( Figure 12) [42], which has a significant impact on environmental control. Environmental factors play a significant role in the newborn survival rate. The infant body's surface heat rises rather quickly and has a limited capacity for self-regulation. Baby incubators,  which rely on humidity sensors, give babies an atmosphere with the right temperature that is good for their growth and development [43].

Biomass biomedical sensor technology
A biosensor also referred to as a biomass sensor is a sensor that uses bioactive chemicals to specifically recognize and quantify a variety of biological molecules. Biosensors often rely on biological detection of disease-specific biomarkers, with a transducer processing the data further. Small molecules, peptides, aptamers, antibodies, proteins, and other types of nanoparticles are among the diverse categories of biosensor designs. A schematic of the different parts of a biosensor is shown in Figure 13 [44]. Enzyme electrodes were originally used in biosensor research in the 1960s. Enzyme sensors, microbial sensors, immunosensors, and nucleic acid sensors (represented by DNA sensors) all first appeared in the middle of the 1970s. Cell sensors and tissue sensors have since developed. With the continuous development of biomedicine, microelectronics, optoelectronics, microelectromechanical processing and other engineering technologies, biosensors have achieved rapid development and shown considerable prospects.
In addition, numerous new nano biosensors composed of nanomaterials and the biochip, which is still a research hotspot, are significant components of biosensor research. Biosensors have been widely used in a variety of disciplines, including biomedical research, food safety, and environmental detection, due to their benefits of strong selectivity, quick determination speed, and high sensitivity. The 21st century is the era of information science and life science, respectively. Emerging between information and biotechnology is the biosensor. It has high application potential in clinical diagnosis, industrial control, food and drug analysis, environmental protection, and other research disciplines and will become a new development point for the country's economy. One of the crucial features of biosensors is their usage of biological components for molecular recognition. A biosensing component, such as an enzyme, antibody, or cell, is typically found in biosensors along with a physical and chemical sensor, also referred to as a transducer, such as an electrode or an optical fiber. The biomolecular recognition element uses the specific response between the treatment object and the test item to detect the treatment object. The related physical and chemical sensor is then transformed into an electrical or optical signal that can be quantitatively analyzed. The transducer, which among them can sense and measure the physical and chemical changes accompanying the aforementioned biological events, is the essential component of the biosensor.

Enzyme biosensor
Enzymes are able to catalyze biochemical specific biochemical reactions in the body of polypeptide protein or nucleic acid. They can speed up the rate at which a biological substance reacts, but they are not consumed during the reaction. Figure 14 [45] depicts the enzyme sensor's operating system. Various chemical modifications immobilize enzymes on the surface of chemical electrodes so they can function as catalysts. The enzyme-catalyzed reaction is single, allowing for a highly selective chemical analysis of particular elements in the substance being evaluated. In order to achieve the quantitative measurement of the concentration of specific components in the measured substance, researchers use the detection of the REDOX current of the electron transfer mediator to acquire the concentration of the product catalyzed by the enzyme. Numerous manufacturers sell sensors for the detection of typical blood biochemical components like glucose, urea, lactose, and creatinine in the field of medical laboratory and diagnostics. Typically, a semipermeable membrane on the enzyme sensor allows the chemical to be detected to diffuse onto the surface of the immobilized enzyme component. Since many enzymes are unstable, the main issue with using these sensors is that the performance of the sensors will degrade over time. Immobilization techniques like embedding can help to slow this down. One of the crucial processes that can be improved to increase the biosensor response, thermostability, and long-term stability is enzyme immobilization [46]. The existing immobilization methods include adsorption, chemical cross-linking, covalent conjugation, and physical embedding. By making reasonable use of the characteristics of enzyme, we can find the application of enzymes in Clark oxygen electrode, glucose enzymes electrode sensor measurement and lactic enzyme electrode sensor.

Immunosensor
In past decades, a great variety of immunosensors have been developed as alternatives to laborious and slow, classical culture-based methods for microorganism detection, including cantilever, conductometric, electrochemical, magnetic, optical and piezoelectric [47]. The fundamental idea behind an immunosensor is an immunological response, which modifies the properties of the sensor's biochemically sensitive membrane by specifically attaching an immobilized antibody to its corresponding antigen. Immune biosensors are thus a novel class of biosensors created by utilizing the properties of particular complementary structures between antigen and antibody [48]. Immune recognition (specific reaction to antigen and antibody on the sensor surface) and signal conversion (changes in optical or electrical characteristics brought on by particular binding in sensing) make up its two primary parts. Currently, researchers have made advancements in the development of a multi-channel electrochemical immunosensor array for the diagnosis of schistosomiasis japonica. The sensor array, as depicted in Figure 15 [49], utilizes an interface assembly approach. In this system, the antibodies present in serum samples interact with the signal probe HRP-Ab2, resulting in the formation of an immunocomplex with the capture antigen located on the working electrode. This interaction leads to the binding of the HRP enzyme to the electrode surface, enabling the subsequent transformation of this binding event into a catalytic amperometric readout.
Immunosensors offer a wide range of applications, including clinical diagnostics and microbiological detection. These applications benefit from their advantages of high analytical sensitivity, good specificity, and ease of use. In terms of regeneration and binding characteristics of the sensing surface, which are crucial for the successful reuse of the same sensor surface and the accuracy of detection results, immunosensors outperform alternative immunological approaches [50]. There are also technical challenges in the development of immunosensors, one of which is the immobilization of antigens or antibodies on the sensor surface. Orientation of the antibody plays a vital role in increasing the sensitivity and specificity of the biosensing platform. Sensitivity of an Immunosensor can be increased by controlling the orientation of the antibody on the sensor surface [51]. Immunoassay, as a promising approach for selective and sensitive analysis, has recently gained increasing attention in the quantitative detection of tumor markers and screening of cancers. As shown schematically in Figure 16 [52], immunosensors use strong Ab/Ag specificity to find the presence of their analyte. Immune sensor application basically has the following two or three aspects: (1) The electrochemical immunosensor. An electrochemical immunosensor combines an electrochemical converter with an immunological molecule identification mechanism. It is a type of biosensor that combines immunological and electrochemical analysis methods, uses antigens or antibodies as molecular recognition elements, comes into direct contact with electrochemical sensing elements, and transforms electrical signals into signals corresponding to certain chemical substance concentrations through sensing elements. Due to their special characteristics, which combine the high target affinity and specificity provided by antibody-mediated biometrics processes with the high sensitivity, low detection limit, affordability, small size, and portability of electrochemical sensors, this class of affinity biosensors has drawn significant research attention. These technique-based analytical tools are utilized for both  clinical and at-home disease diagnostics. Therefore, one of the most difficult topics in the creation of a sensitive electrochemical immunoassay is the development of unique, quick, label-free, simple, nontoxic, and reproducible test systems [53].
(2) Optical immunosensor sensor. In the past decades, the development of fiber optics and the improvement of instrumentation made an advance in optical immunosensors [54]. Optical immunosensors function by altering the sensor's response to light once it binds to a particular antigen. Similar to other immunosensors, label-free, easy to assemble and operate, low-cost, and highly sensitive devices are becoming more popular in optical sensor research. Surface plasmon resonance (SPR)-based sensors, fiber optic sensors, and different fluorescence-based sensors are examples of common optical immunosensors.
(3) Piezoelectric immunosensor. The piezoelectric immunosensor is a brand-new class of biosensor that makes use of the distinct binding properties of antigens and antibodies as well as the sensitivity of piezoelectric crystals to mass change. It can be used to quickly quantify different antigens or antibodies and examine their kinetics. The ability to observe immune responses directly is a benefit of piezoelectric immunosensors. Piezoelectric immunosensors can be used to quickly assess changes in crystal quality caused by the development of immunological complexes on their surface. However, this immunosensor has a poor level of assay repeatability and reliability, which is a drawback. A piezoelectric immunosensor for the detection of hepatitis A viral antigens has also been described in medicine [55].

DNA sensors
The human genome project was implemented and eventually finished, and genetic testing technology for the human body, such as viruses and bacteria specific sequences of bases in nucleic acids detection, has been gradually beginning to play a more and more important role in forensic clinical diagnosis and treatment areas. Large-scale genetic analysis necessitates the employment of detectors that are simpler, quicker, less expensive, and more compact. Through a number of pattern-recognition receptors that are positioned to detect viral entry into cells, mammalian cells have the capacity to detect viral nucleic acids. Viral RNA sensors are among these pattern-recognition receptors. Additionally, DNA sensors exist [56]. DNA sensors work by affinity interaction between a fixed probe and its complementary strand, the target, forming a stable double helix through base pairing. (Figure 17) [57] DNA sensors or sensor systems for the detection of small molecules, proteins, or even enzyme activities, based on optical or electrochemical readout, have been reported. The development of extremely sensitive and targeted genetic detection techniques has received a boost from the appearance of numerous new biological technologies. They include a number of DNA biosensing technologies Submit a manuscript: https://www.tmrjournals.com/bmec that have been created using the complementary base pairing of DNA double-strands, a concept that has recently received a lot of interest from biologists. To produce useful products, the subject of developing DNA sensors is very interdisciplinary and involves contributions from molecular biology, biotechnological microelectronics, bioelectrochemistry, and bioorganic chemistry [58]. It has become one of the important contents of biosensor research to develop DNA sensors that can provide real-time gene binding information without labeling by combining DNA molecules as sensitive elements with other sensing detection technologies. The immobilization methods of DNA probes include adsorption, self-assembly and covalent bonding.

Receptors and ion channel sensors
The receptor is a kind that can pass conformation change in chemistry and chemical molecular membrane specific binding protein. Chemical conformational changes, such as the opening of ion channels or the production of enzymes, can cause biological reactions. Research on receptors and ion channels has at least three possible benefits for the creation of biological sensors. (1) The entire process of biological evolution creates ion channels and receptors with extremely high affinities and specificities. (2) Many poisons and physiological processes of messenger substances are produced by natural targets and are thus present in receptors and ion channels, which can be exploited for clinical detection, environmental monitoring, drug research and development, and many other purposes. (3) A novel type of sensor that can combine the receptor and the ligand in real-time for real-time analysis is also required for the research of receptors and ion channels.

Electronic nose and tongue technology
A signal processing system and sensor array make up the electronic nose and tongue. A device called an "electronic nose" uses a variety of metal oxide sensors to identify complicated odors. The sensor array consists of generic sensors responsive to a range of scents composed of biological or chemical elements. The electronic tongue can be viewed as an analytical instrument for creating taste in a lab. In order to characterize complicated liquid samples, the device typically consists of a collection of extremely stable, cross-sensitive, and ion-selective sensors connected to stoichiometric treatments [59]. Electronic nose and electronic tongue technology have been explored and used in food, environment, and medical detection more widely with the development of smell and taste sensors. In order to achieve the objective and scientific sensory evaluation and eventually replace the artificial sensory evaluation, the electronic tongue and nose have good sensitivity and repeatability and can give quick, accurate, and real-time analysis [60,61].

Electronic nose
The electronic nose is an instrument that mimics the sense of smell in biological systems. Basically, the device is used to detect and discriminate complex odors at a low cost. E-Nose provides a low-cost, noninvasive, and rapid method for objective aroma determination. Odor is a sensation produced when compounds, called odorants, stimulate receptors located in the olfactory epithelium at the top of the nasal cavity. Odorants are hydrophobic, volatile compounds with molecular weight less than 300 Da. Humans can identify and distinguish up to 10,000 different substances based on their odors. Odor receptors in the nasal cavity can detect and discriminate these thousands of different chemical traces. The olfactory region of the two human nasal passages is a small region of about 2.5 cm 2 and contains about 50 million primary sensory receptor cells [62]. An electronic nose is a chemical sensor intelligent system developed by chemical sensor array and detection technology, computer information processing and other multidisciplinary technologies. Its basic structure includes the following three parts: gas sensor array, signal preprocessing unit and pattern recognition unit [63]. Figure 18 [64] shows the structure of a typical electronic nose. Array-based sensing systems imitate human taste and smell systems by generating specificity not from any one sensor, but rather as a distinctive composite response of each analyte. These systems are based on cross-responding sensor elements. Array-based sensing has become a potent method for finding analytes with a wide range of chemical compositions. The fundamental design principle of a set of sensors used in an electronic nose is that each sensor should maximize the instrument's overall sensitivity and offer a unique selectivity profile over the spectrum of target gas analytes to be detected or categorized for a given application. The sensor array should ideally consist of a single sensor that responds differently to a particular odor analyte, producing a distinctive fragrance pattern. If it is difficult to obtain unique aroma patterns for different gas analytes, the sensor selection must be modified or the number of sensors adjusted. These complex sensory interactions between smell and taste have been explored in electronic sensor studies by combining the use of electronic nose and electronic tongue technologies to assess the aroma and flavor of specific foods. The sensor selection of a single e-nose system is essential to achieve effective and efficient aroma identification or classification. It demands the sensors in the array to exhibit some cross-sensitivity in addition to their own sensitivity. The gas sensor should also adhere to the following specifications in real life. (1) High sensitivity and broad dynamic range of reaction. When the gas concentration being measured is low, the response signal is large enough. In addition, it has good response linearity at high measured gas concentrations. (2) The performance is consistent; the sensor's performance is unaffected by changes in the temperature and humidity of the surroundings; (3) The response time is quick and repeatable; (4) The maintenance is straightforward and affordable, etc. E-noses are a possible, simple, and inexpensive method for analyzing the patterns of inhaled volatile compounds.
The acceptance of things that are consumed plays a vital impact in taste perception. But a substantial number of medications on the market have a harsh taste. The electronic tongue is an innovative piece of testing technology and apparatus that uses a multi-sensor array to record the sensor response signal properties of liquid samples. This ground-breaking device makes it possible to analyze the sampled compounds qualitatively or quantitatively by using signal processing and pattern recognition techniques. The electronic taste biological sensor can recognize tastes because of its shape, which is modeled after the taste buds on the human tongue. A computer then compiles the data after converting the detected flavor information into the appropriate electrical signals [65,66]. Figure 19 [67] presents the basic principle of the electrochemical taste sensing system. This system mimics what happens when molecules with specific tastes  interact with taste buds on the human tongue. The taste buds are equipped with sensors that engage with surface molecules, inducing alterations in the electrical potential. These signals were juxtaposed with the recorded physiological action potentials, obtained by the computer, which correspond to the neural network at the physiological level. The collected data can be further assessed in relation to pre-existing sensor response matrices, which can be compared to human memory or associated with established taste patterns. With the former being made for liquid analysis and the latter for gaseous analysis, the fundamental difference between the electronic tongue and the electronic nose is in their respective target substrates. The research and development of the electronic tongue have advanced over the past few decades, despite the fact that it just recently became a reality. However, the field still needs time to develop. The electronic tongue market is currently dominated by the French business Alpha-MOS, which holds more than 99% of the global market share. They have had success using their electronic tongue systems in a variety of industries, including food, medicine, the environment, and the chemical sector.
In biological creatures, the taste buds on the tongue are what allow for the perception of flavor. When exposed to numerous taste stimuli in various conditions, each taste bud picks up unique information. These signals are sent from the nerves to the brain, where taste bud cells thoroughly analyze and interpret them. This helps the brain to distinguish between and classify various materials according to their sensory characteristics. This biological function served as the basis for the electronic tongue's initial design. Similar to the tongue in real beings, the sensor array in the electronic tongue system detects numerous chemicals and gathers diverse signals, which are subsequently processed and evaluated by a computer. Computers do analysis and processing through software to distinguish and identify various compounds, and then they provide sensory information about each item, replacing the activities of the brain in biological systems. A taste bud on the tongue has a cross-sensitive effect, and each independent sensor in the sensor array is similar. An independent sensor not only detects one chemical substance, but also detects a group of chemicals, and while it detects one type of specific chemicals, it also detects another part of other chemicals. In recent years, there have been increasing uses for the electronic tongue in the field of tea tasting. In the tea industry, training a tea taster can take years and is quite expensive. Utilizing some non-human technology would be more efficient. The electronic tongue sensor performs admirably in these applications in terms of temporal stability and sensitivity. Sensory analysis is typically used to differentiate the geographic origin of Longjing due to the resemblance of diverse origins (processing, appearance, and taste). Wine, milk, tea, beer, fruit juice, and other beverages have all been subjected to increased investigation using electronic tongue technology in recent years. Additionally, we found that human and electronic tongue evaluations correlated well, making it a promising substitute for human sensory analysis of tea.

Wearable sensors in medical research
The concept of wearable devices pertains to devices that remain consistently attached to an individual. The comfort, convenience of use, and discretion of these gadgets are akin to those of clothes. Wearable systems are discrete gadgets that help medical personnel get over the restrictions of outpatient technology and successfully monitor patients for weeks or even months at a time. These systems often rely on tiny wireless sensors housed inside bandages, patches, or other wearables like rings or shirts. Smartphones and wearable sensors are increasingly being used to facilitate disease and health monitoring. These sophisticated wearable sensors can take on various shapes, such as watches, caps, garments, shoes, patches, tattoos, and body jewelry. The physiological signals captured by these smart wearable sensors can be acquired, stored, and analyzed using smartphones and cloud computing, thereby opening avenues for diverse applications in disease management and healthcare [68,69]. Recent significant advancements in mobile computing and tiny technologies have increased interest in wearable technology. Wearable sensors and systems have made great strides, and are now at a point where their potential clinical uses can be evaluated. Not only has there been a significant growth in research efforts in this area in recent years, but there has also been a significant investment by many businesses that are actively working to produce wearable goods for clinical use. Wearable systems will soon be incorporated into standard clinical evaluations, according to the technology's persistently growing trajectory of adoption.
The need to continuously monitor patients for extended periods of time has generated interest in wearable systems. The typical clinical or hospital monitoring techniques used to keep track of physiological events, like blood pressure or ECG, only provide a cursory look into the patient's physiology. Clinical surveillance is subject to three main restrictions: (1) Inadequate sampling of rare events that might be useful for prognosis, diagnosis, or treatment.
(2) Unable to record physiological reactions during routine activities, downtime, and sleep. (3) Missing circadian variations in physiological signals, which seem to be correlated with the development of disease. As a result, wearable technology that can periodically or continuously record relevant physiological signals presents an essential remedy to the limitations imposed by conventional monitoring techniques [70,71]. The development of sensor and microelectronic technology has opened up new opportunities for the healthcare industry, notably in the area of clinical trials. For example, even though a patient's primary care doctor would not need daily heart rate and calorie output data, such information can be helpful to researchers looking into a novel medication that targets obesity. Consequently, despite some limitations, it is evident that as the potential for wearable electronics in capturing clinical trial data is recognized, their utilization will expand and become integrated into routine practice [72,73].
Currently, there is a great deal of interest in developing smart wearable devices. On smart wearables, the loop is closed and provides automatic feedback to the wearer to reduce risky behavior, enhance learning, or enable shared decision making. These systems will need to be highly sensitive and specific for each individual and will need to operate on models with high predictive power. These systems need to collect more information about the local environment, the psychological and physiological state of the user, and track potentially intrusive information such as geographic location and social interactions. The rapid development of integrated data infrastructure and complex algorithms running on cubes has made it possible to learn healthcare systems. For wider adoption of wearable devices for rehabilitation, understanding end uses must go hand in hand with technological developments. The routine and long-term use of wearable devices presents many challenges that short-term clinical studies have not addressed, such as durability, power consumption, comfort, and usability. Therefore, to drive the use of wearable systems for rehabilitation outside the clinic, a systematic and integrated approach is needed to develop user-centered systems for a wide range of rehabilitation applications. This approach will motivate and maintain engagement within the user community and demonstrate clear long-term health benefits [75].

Conclusion
Significant developments in sensor and sensor system technology over the previous ten years have driven the steady expansion of new medical monitoring technology. These developments have helped to sustain progress while also having the potential to significantly lower healthcare expenses. Concurrently, a review of present healthcare delivery strategies is necessary due to the growing worldwide population. Without significant adjustments, the rising expense of healthcare will be unaffordable. The development of wearable and mobile biomedical sensing technologies recently has made it possible to collect continuous, long-term biological signals, also known as biomedical intensive longitudinal data [76,77]. The flexibility of clinical diagnostics has been substantially improved thanks to technological developments, which have also aided in the formation of new consumer healthcare markets. Medical innovation has increasingly relied on sensors and sensor technologies, and their integration with other information and communication technologies (ICTs) is expected to lead to the introduction of some interesting new capabilities in the future. The advancement of healthcare has been considerably accelerated by the gradual introduction of novel sensing technologies, such as microelectromechanical systems (MEMS), biological chemical sensors, and others. These sensing technologies allow for routine or ongoing monitoring of people's health conditions, which promotes the adoption of cutting-edge proactive care methods. As a result, earlier and more affordable diagnoses and interventions result in better long-term patient outcomes. Continuous monitoring systems have been created in the field of telemedicine for the monitoring of chronic diseases, including diabetes, congestive heart failure, and chronic obstructive pulmonary disease (COPD) [78]. By anticipating and preventing acute disease exacerbations, this monitoring strategy enables continuous disease status observation, which reduces the physical and financial consequences of the condition. Commonly used in reaction to acute exacerbations, conventional emergency care models can have negative and lasting effects on patients' health, wellbeing, and physical degradation. Sensing technology has the potential to be used in the future to develop population-wide screening programs as part of long-term healthcare strategies with the purpose of enhancing public health. People actively use sensing technologies to monitor and proactively manage their own health, especially those who have a tendency toward hypochondria. It is anticipated that the broad use of sensing technologies in medical settings would considerably improve our knowledge of disease risks and the efficacy of intervention therapy. To better understand the aging process, several technical tools are available, such as early detection of cognitive decline, frailty, and motor and neurological diseases. With the use of sensor technology, it is possible to evaluate a person's health in its entirety and gain a thorough understanding of the changes taking place inside the human body. This plethora of information can be used as early warning signs for ailments that have the biggest effects on people's health [79,80].