Currently, many intelligent detection devices have extensively adopted various types of sensors, and their applications have long since permeated various fields such as industrial production, marine exploration, environmental protection, medical diagnosis, bioengineering, space exploration, and smart homes. As the application demands of the information age increase, the expected values and ideal requirements for various performance parameters such as the range, accuracy, and stability of the measured information are gradually rising. The measurement needs of gases, pressures, and humidity under special environments and with special signals present new challenges to ordinary sensors .
Faced with increasingly diverse and challenging signals and environments, novel sensor technologies are evolving in the following directions: developing new materials, processes, and sensors; achieving sensor integration and intelligence; miniaturizing sensor hardware systems and components; and integrating sensors with other disciplines. Simultaneously, there is a desire for sensors to possess characteristics such as transparency, flexibility, extensibility, bendability (even foldability), portability, and wearability. With the development of flexible matrix materials, flexible sensors that meet these trends have emerged.
Thin-film flexible pressure sensor
Characteristics and Classification of Flexible Sensors
1. Characteristics of flexible sensors
Flexible materials are a concept contrasted with rigid materials. Generally, flexible materials possess properties such as softness, low modulus, and easy deformation. Common flexible materials include: polyvinyl alcohol (PVA), polyester (PET), polyimide (PI), polyethylene naphthalate (PEN), paper, and textile materials.
Flexible sensors, on the other hand, are sensors made of flexible materials. They possess excellent flexibility, ductility, and can even be bent or folded freely. Furthermore, their structures are flexible and diverse, allowing for arbitrary arrangement according to measurement requirements, and enabling convenient detection of complex measurements. New flexible sensors are widely used in fields such as electronic skin, healthcare, electronics, electrical engineering, sports equipment, textiles, aerospace, and environmental monitoring.
2. Classification of Flexible Sensors
There are many types of flexible sensors, and they can be classified in various ways.
According to their applications, flexible sensors include flexible pressure sensors, flexible gas sensors, flexible humidity sensors, flexible temperature sensors, flexible strain sensors, flexible magnetoresistive sensors, and flexible heat flow sensors, etc.
According to their sensing mechanisms, flexible sensors include flexible resistive sensors, flexible capacitive sensors, flexible piezomagnetic sensors, and flexible inductive sensors.
Common materials for flexible sensors
1. Flexible substrate
To meet the requirements of flexible electronic devices, properties such as thinness, transparency, flexibility, good stretchability, insulation, and corrosion resistance have become key indicators for flexible substrates.
Among the many flexible substrate options, polydimethylsiloxane (PDMS) has become the preferred choice. Its advantages include easy availability, chemical stability, transparency, and good thermal stability. In particular, its distinct adhesion and non-adhesion regions under ultraviolet light allow for easy adhesion of electronic materials to its surface. Many flexible electronic devices achieve significant flexibility by reducing the substrate thickness; however, this method is limited to near-flat substrate surfaces. In contrast, stretchable electronic devices can adhere completely to complex and uneven surfaces. Currently, there are generally two strategies to achieve stretchability in wearable sensors. The first method is to directly bond thin conductive materials with low Young's modulus to a flexible substrate. The second method is to assemble devices using conductors that are themselves stretchable. This is typically done by incorporating conductive materials into an elastic matrix.
2. Metallic materials
Metallic materials, typically conductors such as gold, silver, and copper, are primarily used for electrodes and wires. For modern printing processes, conductive nano-inks, including nanoparticles and nanowires, are often chosen as conductive materials. Besides their excellent conductivity, metallic nanoparticles can also be sintered into thin films or wires.
3. Inorganic semiconductor materials
Inorganic semiconductor materials, represented by ZnO and ZnS, have shown broad application prospects in the field of wearable flexible electronic sensors due to their excellent piezoelectric properties.
A flexible pressure sensor based on the direct conversion of mechanical energy into optical signals has been developed. This matrix utilizes the mechanoluminescence properties of ZnS:Mn particles. The core of mechanoluminescence is photon emission induced by the piezoelectric effect. Under pressure, the electronic bands of piezoelectric ZnS exhibit a voltage effect, causing them to tilt. This promotes the excitation of Mn²⁺, and the subsequent de-excitation process emits yellow light (around 580 nm).
A fast-response sensor (response time less than 10 ms) is obtained through this mechanoluminescence conversion process, achieving a spatial resolution of 100 μm using a top-down photolithography technique. This sensor can record dynamic pressure from a single point of slippage, and can be used to identify handwriting and scan two-dimensional planar pressure distribution by obtaining emission intensity curves in real time. All these characteristics make inorganic semiconductor materials one of the most promising candidates for future fast-response and high-resolution pressure sensor materials.
4. Organic materials
Large-scale pressure sensor arrays are crucial for the future development of wearable sensors. Pressure sensors based on piezoresistive and capacitive signal mechanisms suffer from signal crosstalk, leading to measurement inaccuracies. This problem has become one of the biggest challenges in the development of wearable sensors.
Due to their excellent signal conversion and amplification performance, transistors offer the possibility of reducing signal crosstalk. Therefore, much research in wearable sensors and artificial intelligence revolves around how to obtain large-scale flexible varistor transistors.
A typical field-effect transistor (FET) consists of five parts: source, drain, gate, dielectric layer, and semiconductor layer. Based on the type of majority carriers, they can be classified into p-type (hole) FETs and n-type (electron) FETs.
Traditionally, p-type polymer materials used in field-effect transistor research are mainly thiophene polymers, with poly(3-hexylthiophene) (P3HT) being the most successful example. Naphthalenetetramethylenediamine (NDI) and perylenetetramethylenediamine (PDI) have shown good n-type field-effect properties and are the most widely studied n-type semiconductor materials, widely used in small-molecule n-type field-effect transistors.
Typical transistor parameters include carrier mobility, operating voltage, and on/off current ratio. Compared to inorganic semiconductor structures, organic field-effect transistors (OFETs) have the advantages of high flexibility and low manufacturing cost, but also have the disadvantages of low carrier mobility and high operating voltage.
5. Carbon materials
Carbon materials commonly used in flexible wearable electronic sensors include carbon nanotubes and graphene. Carbon nanotubes are characterized by high crystallinity, good conductivity, large specific surface area, and the ability to control the size of micropores through the synthesis process, achieving a specific surface utilization rate of up to 100%.
Graphene is lightweight, thin, transparent, and has excellent electrical and thermal conductivity. It holds immense and broad application potential in sensing technology, mobile communications, information technology, and electric vehicles.
In the application of carbon nanotubes, a conductive polymer sensor obtained by printing multi-arm carbon nanotubes and silver composites still exhibits a conductivity as high as 20 S•cm−1 under 140% stretching.
In the combined application of carbon nanotubes and graphene, a highly stretchable transparent field-effect transistor was fabricated, which integrates graphene/single-walled carbon nanotube electrodes and a wrinkled inorganic dielectric layer of single-walled carbon nanotube mesh channels. Due to the presence of the wrinkled alumina dielectric layer, no drain current change was observed under more than 1,000 stretch-dry cycles with a 20% amplitude, demonstrating excellent sustainability.
Common flexible sensors
1. Flexible gas sensor
Flexible gas sensors employ a thin-film material sensitive to gases deposited on the electrode surface. This flexible substrate is lightweight, flexible, and easily bendable, allowing for large-area fabrication. The thin-film material itself also attracts significant attention due to its higher sensitivity and relatively simple manufacturing process. This effectively meets the portability and low power consumption requirements of gas sensors in special environments, overcoming the disadvantages of traditional gas sensors such as poor portability, incomplete measurement range, small measurement range, and high cost. It enables simple and accurate detection of NH, NO, and ethanol gases, thus drawing widespread attention.
2. Flexible pressure sensor
Flexible pressure sensors have wide applications in smart clothing, smart sports, and robot "skin." Polyvinylidene fluoride, silicone rubber, and polyimide are widely used as base materials in the fabrication of flexible pressure sensors. They differ from force sensors that use metal strain gauges and ordinary diffused pressure sensors that use n-type semiconductor chips, and possess better flexibility, conductivity, and piezoresistive characteristics.
3. Flexible humidity sensor
Humidity sensors are mainly classified into two categories: resistive and capacitive. Resistive humidity sensors are characterized by a film made of a moisture-sensitive material coated on a substrate. When water vapor in the air is adsorbed onto the moisture-sensitive film, the resistivity and resistance of the element change, and this characteristic can be used to measure humidity. Capacitive humidity sensors are generally made of polymer films; commonly used polymer materials include polystyrene, polyimide, and cellulose acetate butyrate.
Humidity sensors are rapidly evolving from simple humidity-sensitive elements towards integrated, intelligent, and multi-parameter detection. Traditional wet-bulb and dry-bulb hygrometers or hair hygrometers can no longer meet the needs of modern scientific development. Flexible humidity sensors have been widely studied due to their advantages such as low cost, low energy consumption, ease of manufacturing, and easy integration into intelligent systems. The substrate materials for these flexible humidity sensors are similar to those for other flexible sensors, and there are many methods for manufacturing humidity-sensitive films, including dip coating, spin coating, screen printing, and inkjet printing.
Flexible sensors offer diverse and flexible structural forms, allowing for arbitrary arrangement to meet measurement requirements. They enable convenient and rapid measurement of special environments and signals, addressing the challenges of miniaturization, integration, and intelligentization in sensor development. These novel flexible sensors play a crucial role in electronic skin, biomedicine, wearable electronics, and aerospace. However, the fabrication technologies for materials like carbon nanotubes and graphene used in flexible sensors are still immature, presenting challenges related to cost, applicability, and lifespan. Commonly used flexible substrates suffer from poor high-temperature resistance, leading to high stress and weak adhesion between the substrate and the thin film material. Furthermore, the assembly, arrangement, integration, and packaging technologies for flexible sensors require further improvement.
