Many intelligent detection devices now use a huge number of different sensors

Many intelligent detection devices now use a huge number of different sensors, and their applications have already spread to industries like manufacturing, maritime exploration, environmental protection, medical diagnostics, biotechnology, space development, and smart homes. The expectations and idealization needs for numerous performance criteria such as the range, precision, and stability of the measured information are gradually increasing as the information age's application requirements grow. Ordinary sensors have additional hurdles when it comes to measuring gas, pressure, and humidity in unique conditions and with unique signals.

Faced with an increasing number of unique signals and environments, new sensor technology has evolved to include the following trends: developing novel materials, processes, and sensors; realizing sensor integration and intelligence; and realizing the hardware system and element of sensing technology. Sensors for cross-integration with different fields; device microminiaturization. Simultaneously, the sensor should be transparent, flexible, extendable, freely bendable or even folding, lightweight, and wearable. Flexible sensors that fulfill the features of the above-mentioned diverse tendencies emerge as the times necessitate the development of flexible matrix materials.

Figure. 1 Thin Film Flexible Pressure Sensor

Ⅰ.  Characteristics And Classification Of Flexible Sensors

 1. Features of flexible sensors

The terms "flexible materials" and "stiff materials" are interchangeable. Softness, low modulus, and ease of deformation are all characteristics of flexible materials. Polyvinyl alcohol (PVA), polyester (PET), polyimide (PI), polyethylene naphthalene (PEN), paper, textile materials, and other flexible materials are common.

The term "flexible sensor" refers to a sensor constructed of flexible materials that is flexible, ductile, and can be bent or folded freely. The construction is adaptable and diversified, and it may be arranged in any way to meet the needs of the measuring conditions. There are complex measurands found. Electronic skin, health care, electronics, electrical engineering, sports equipment, textiles, aerospace, environmental monitoring, and other areas all use new flexible sensors.

 2. Classification of flexible sensors

Flexible sensors come in a variety of shapes and sizes, and categorization methods vary as well.

Flexible pressure sensors, flexible gas sensors, flexible humidity sensors, flexible temperature sensors, flexible strain sensors, flexible magneto-impedance sensors, and flexible thermal flow sensors are all examples of flexible sensors.

Flexible sensors are classified as flexible resistive sensors, flexible capacitive sensors, flexible piezomagnetic sensors, and flexible inductive sensors, depending on the detecting mechanism.

Ⅱ.  Common Materials For Flexible Sensors 

1. Flexible substrate

Properties including thinness, transparency, flexibility, and stretchability, as well as insulation and corrosion resistance, have become crucial markers of flexible substrates in order to meet the demands of flexible electronic devices.

Polydimethylsiloxane (PDMS) has risen to the top of the list of flexible substrates. Easy availability, chemical stability, clarity, and thermal stability are only a few of its benefits. The unique properties of the adhesive zone and the non-adhesive zone make the surface easily adhere to electronic components, especially under ultraviolet light. Many flexible electronic devices gain great bendability by reducing substrate thickness; nevertheless, this strategy is limited to substrate surfaces that are virtually flat. Stretchable electronics, on the other hand, can fully attach to complicated and uneven surfaces. There are currently two approaches to achieving stretchability in wearable sensors. The first method involves bonding thin conductive materials with low Young's modulus directly to flexible substrates. The gadget can also be assembled utilizing conductors that are stretchy by nature. It's commonly made by combining conductive materials with an elastic matrix.

2. Metal materials

Metals are generally conductivity materials like gold, silver, and copper that are mostly utilized for electrodes and wires. Conductive nano-inks, which include nanoparticles and nanowires, are the most common conductive materials used in current printing methods. Metal nanoparticles can be sintered into thin films or wires, in addition to having strong electrical conductivity.

3. Inorganic semiconductor materials

Because of their strong piezoelectric capabilities, inorganic such as have demonstrated significant application potential in the realm of wearable flexible electronic sensors.

A flexible pressure sensor has been developed based on the direct conversion of mechanical energy into optical signals. The mechanoluminescence characteristics of ZnS: Mn particles are used in this matrix. The emission of photons induced by the piezoelectric effect is at the heart of electroluminescence. The piezoelectric effect tilts the electronic energy band of piezoelectric under pressure, which can increase Mn2+ excitation and the following de-excitation process, which generates yellow light (about 580 nm).

This mechanoluminescence conversion process produces a sensor with a quick response (less than 10ms) and a spatial resolution of 100m thanks to the top-down photolithography procedure. This sensor can scan the two-dimensional planar pressure distribution by getting the emission intensity curve in real time and record the dynamic pressure of a single point slip, which may be utilized to discern the signer's handwriting. All of these properties make inorganic semiconductor materials one of the most promising future candidates in the field of high-resolution and fast-responding pressure sensor materials.

4. Organic materials

Large-scale pressure sensor arrays are critical for the development of wearable sensors in the future. Signal crosstalk occurs in pressure sensors based on piezoresistive and capacitive signal mechanisms, resulting in inaccurate measurements. This has become one of the most difficult difficulties in the development of wearable sensors.

The usage of allows for the reduction of signal crosstalk due to their flawless signal conversion and amplification performance. As a result, large-scale flexible piezotransistors are the focus of many studies in the fields of wearable sensors and artificial intelligence.

The source, drain, gate, dielectric layer, and semiconductor layer make up a conventional field-effect transistor. It can be separated into p-type (hole) n-type (electron) field-effect transistors based on the kind of majority carriers.

Thiophene polymers are the most common p-type polymer materials utilized in field-effectresearch, with the poly(3-hexylthiophene) (P3HT) system being the most effective example. The most commonly investigated n-type semiconductor materials, naphthalene tetramine (NDI) and perylene tetramine (PDI), display good n-type field effect capabilities and are widely used in small molecule n-type field-effect transistors.

Carrier mobility, operating voltage, and on/off current ratio are all properties. Organic field-effect transistors (OFETs) have the advantages of high flexibility and low fabrication cost over inorganic semiconductor architectures, but they also have low carrier mobility and a high working voltage.

5. Carbon material

Carbon nanotubes and graphene are two commonly utilized carbon compounds for flexible wearable electrical sensors. Carbon nanotubes have great crystallinity, good electrical conductivity, a large specific surface area, and the size of micropores may be regulated by the synthesis process, with a specific surface utilization rate of 100%.

Figure. 2

Lightness, thinness, transparency, and high electrical and thermal conductivity are all features of graphene. Sensor technology, mobile communication, information technology, and electric cars, it has tremendously essential and broad application prospects.

The conductivity of the conductive polymer sensor generated by combining multi-arm carbon nanotubes and silver and printing is still as high as 20Scm1 under 140 percent stretching in the application of carbon nanotubes.

Highly stretchable transparent field-effect transistors with graphene/single-wall carbon nanotube electrodes and a wrinkled inorganic dielectric layer single-wall carbon nanotube network grid channel are created using the combined application of carbon nanotubes and graphene. There is no drain current change after one thousand stretch-relaxation cycles of 20% amplitude due to the presence of a wrinkled alumina dielectric layer, indicating strong sustainability.

Ⅲ.  Common Flexible Sensors 

1. Flexible gas sensor

On the electrode surface of the flexible gas sensor is gas-sensitive film material. The substrate is lightweight, flexible, and maybe manufactured over a vast area. Additionally, the film material has a higher sensitivity and a very simple manufacturing method. There is a lot of focus. This gas sensor fits the requirements for portability and low power consumption in unique situations and overcomes the disadvantages of earlier gas sensors, such as incompletion of measuring range, small range, and high cost. The detection is simple and accurate, and it has gotten a lot of attention.

2. Flexible pressure sensor

Figure. 3

Smart clothing, smart sports, and robot "skin" all use flexible pressure sensors. As basic materials for flexible pressure sensors, polyvinylidene fluoride, silicone rubber, polyimide, and other polymers have been widely used. They differ from load cells that use metal strain gauges and regular pressure diffusion sensors that use n-type semiconductor chips. The sensor's flexibility, conductivity, and piezoresistive characteristics are all excellent.

3. Flexible humidity sensor

Humidity sensors are divided into two categories: resistive and capacitive. The humidity-sensitive is distinguished by the fact that the substrate is covered with a moisture-sensitive layer. The resistivity and of the element alters when water vapor in the air is adsorbed on the moisture-sensitive film. Humidity is something that can be measured. Polymer films are commonly used in moisture-sensitive capacitors. Polystyrene, polyimide, butyric acetate, and other polymer compounds are commonly employed.

Humidity sensors are fast evolving from simple humidity sensors to integrated, intelligent, multi-parameter detection. Traditional wet and dry bulb hygrometers, as well as hair hygrometers, are no longer able to keep up with the demands of modern technology. Flexible humidity sensors have received a lot of attention because of their low cost, low energy consumption, ease of manufacturing, and incorporation into smart system fabrication. The substrate materials used to make this sort of flexible humidity sensor are similar to those used to make other flexible sensors, and humidity-sensitive films can be made using a variety of processes, including dip coating, spin coating, screen printing, and inkjet printing.

The flexible sensor structure is flexible and diverse, and it can be arbitrarily arranged according to the measurement conditions, allowing it to easily and accurately measure special environments and special signals, as well as solve miniaturization, integration, and intelligent sensor development problems. These new bendable sensors are a game-changer. Electronic skin, biomedicine, wearable electronics, and aircraft all benefit from it. However, the technical level of materials preparation for flexible sensors like carbon nanotubes and graphene is still immature, and there are still issues like cost, application scope, and service life to consider. Commonly used flexible substrates have the disadvantage of not being resistant to high temperatures, resulting in high stress and weak adhesion between the flexible substrate and the film material. The assembly, arrangement, integration, and packaging technology of flexible sensors also need to be further improved.