Flexible Sensor Complete Guide

Numerous advanced detection devices now employ a vast array of sensors, finding applications in diverse industries such as manufacturing, maritime exploration, environmental protection, medical diagnostics, biotechnology, space development, and smart homes. As the demands of the information age continue to grow, there is an escalating need for enhanced performance criteria, including extended range, precision, and stability in measured information. Traditional sensors face challenges when measuring gas, pressure, and humidity in unique conditions with distinctive signals.

In response to the increasing complexity of signals and environments, the evolution of new sensor technology follows several trends: the development of innovative materials, processes, and sensors; the realization of sensor integration and intelligence; and the implementation of hardware systems and elements in sensing technology. These sensors are designed for cross-integration with various fields and feature microminiaturization of devices. Concurrently, flexible sensors are required to meet diverse trends, necessitating transparency, flexibility, extensibility, free bendability, foldability, lightness, and wearability. The emergence of flexible sensors aligns with the evolving needs of the times and the demand for the development of flexible matrix materials.

I. Characteristics and Categorization of Bendable Sensors

Attributes of Bendable Sensors

The terms "flexible materials" and "rigid materials" are interchangeable, with flexibility, low modulus, and deformability being key characteristics of flexible materials. Common flexible materials include polyvinyl alcohol (PVA), polyester (PET), polyimide (PI), polyethylene naphthalene (PEN), paper, and textile materials.

A "flexible sensor" denotes a sensor crafted from flexible materials, offering adaptability, ductility, and the ability to be freely bent or folded. Its construction is versatile and diverse, allowing for customization to meet specific measurement conditions. Applications of these flexible sensors span electronic skin, healthcare, electronics, electrical engineering, sports equipment, textiles, aerospace, environmental monitoring, and other domains.

Classification of Flexible Sensors

Flexible sensors exist in various forms and sizes, with classification methods varying. Examples include flexible pressure sensors, gas sensors, humidity sensors, temperature sensors, strain sensors, magneto-impedance sensors, and thermal flow sensors.

Depending on the detection mechanism, flexible sensors are categorized as flexible resistive sensors, flexible capacitive sensors, flexible piezomagnetic sensors, and flexible inductive sensors.

II. Common Materials for Flexible Sensors

Flexible Substrate

Properties such as thinness, transparency, flexibility, and stretchability, along with insulation and corrosion resistance, are essential for flexible substrates to meet the demands of flexible electronic devices. Polydimethylsiloxane (PDMS) is a leading flexible substrate due to its easy availability, chemical stability, clarity, and thermal stability. Its unique adhesive and non-adhesive zones enable easy adhesion to electronic components, especially under ultraviolet light. Achieving stretchability in wearable sensors involves bonding thin conductive materials with low Young's modulus directly to flexible substrates or utilizing inherently stretchy conductors.

Metal Materials

Commonly used for electrodes and wires, metals like gold, silver, and copper exhibit strong electrical conductivity. Conductive nano-inks, comprising nanoparticles and nanowires, are prevalent in current printing methods, allowing metal nanoparticles to be sintered into thin films or wires.

Inorganic Semiconductor Materials

Inorganic semiconductor materials such as ZnO and ZnS, known for their robust piezoelectric capabilities, find application in wearable flexible electronic sensors. A notable example is a flexible pressure sensor based on the mechanoluminescence characteristics of ZnS: Mn particles, enabling quick response and high spatial resolution.

Organic Materials

Large-scale pressure sensor arrays are crucial for future wearable sensors. Signal crosstalk issues in piezoresistive and capacitive pressure sensors are addressed by employing large-scale flexible piezotransistors, focusing on transistor properties such as carrier mobility, operating voltage, and on/off current ratio. Thiophene polymers and naphthalene/perylene tetramine are common materials for p-type and n-type field-effect transistors, respectively.

Carbon Materials

Carbon nanotubes and graphene are widely used carbon compounds for flexible wearable electrical sensors, offering excellent crystallinity, electrical conductivity, and specific surface area control. Carbon nanotubes, in particular, possess a high specific surface utilization rate of 100%.

Graphene's Attributes and Wide-ranging Applications

Graphene boasts characteristics such as lightness, thinness, transparency, and impressive electrical and thermal conductivity. Its applications span sensor technology, mobile communication, information technology, and electric cars, presenting significant and extensive prospects.

In the realm of carbon nanotubes, the conductive polymer sensor, formed by the amalgamation of multi-arm carbon nanotubes and silver with a printing technique, maintains a substantial conductivity of 20Scm1 even under a 140% stretching condition.

A remarkable achievement is the development of highly stretchable transparent field-effect transistors utilizing graphene/single-wall carbon nanotube electrodes and a wrinkled inorganic dielectric layer, forming a single-wall carbon nanotube network grid channel. The collaborative utilization of carbon nanotubes and graphene ensures resilience, as evidenced by no drain current change after undergoing one thousand stretch-relaxation cycles with a 20% amplitude, attributed to the presence of a wrinkled alumina dielectric layer. This underscores the robust sustainability of the system.

III. Widely Utilized Flexible Sensors

Flexible Gas Sensor

The flexible gas sensor incorporates a gas-sensitive film material on its electrode surface. The substrate is both lightweight and flexible, enabling large-scale manufacturing. The film material, with its heightened sensitivity and straightforward production method, garners considerable attention. Addressing the requirements for portability and low power consumption in unique scenarios, this gas sensor overcomes drawbacks observed in previous counterparts, such as incomplete measuring ranges, limited scope, and high costs. Its detection capability is both simple and accurate, earning widespread recognition.

Flexible Pressure Sensor

Found in applications like smart clothing, smart sports gear, and robotic "skin," flexible pressure sensors utilize basic materials such as polyvinylidene fluoride, silicone rubber, and polyimide. Diverging from load cells with metal strain gauges and regular pressure diffusion sensors featuring n-type semiconductor chips, these sensors excel in flexibility, conductivity, and piezoresistive characteristics.

Flexible Humidity Sensor

Humidity sensors fall into two categories: resistive and capacitive. In resistive humidity sensors, the substrate is coated with a moisture-sensitive layer, altering the resistivity and resistance values when exposed to water vapor. Capacitive sensors commonly employ polymer films like polystyrene, polyimide, and butyric acetate. The evolution of humidity sensors extends beyond simplicity to integrated, intelligent, multi-parameter detection. Flexible humidity sensors gain prominence due to their cost-effectiveness, low energy consumption, ease of manufacturing, and integration into smart systems. Substrate materials and humidity-sensitive films undergo various processes like dip coating, spin coating, screen printing, and inkjet printing.

The flexible sensor structure is versatile, allowing arbitrary arrangements based on measurement conditions, facilitating accurate measurements in special environments and with special signals. These bendable sensors revolutionize electronic skin, biomedicine, wearable electronics, and aerospace applications. However, challenges persist, including the technical maturity of materials like carbon nanotubes and graphene, along with considerations of cost, application scope, and service life. Common flexible substrates face drawbacks related to high temperatures, resulting in stress and weak adhesion. Further advancements are needed in assembly, arrangement, integration, and packaging technologies for flexible sensors.

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