Today, high-performance electronic materials, smart homes and internet of things (IoT) technology are rapidly developing. Sensor technology, particularly flexible pressure sensors, has now become an integral part of human lives. The invention of pressure sensors dates back to 1954 and is centered upon Smith's discovery of silicone and germanium's compressive resistance effects. The sensing methods of flexible pressure sensors can be categorized into piezoelectricity, capacitance and piezoresistivity.
Identifying the efficiency of these sensors plays an essential role in various fields of application to attain the optimum practical match. Experiments have shown that certain conductive polymeric compounds have a piezoresistive effect, but the influence of piezoresistivity on polymeric insulation materials is hard to reach. However, new research in recent years has shown that conducting fillers can make insulating polymers conductive. The selection of substrate is thus constantly a crucial problem for professionals to examine, particularly conductive fillers.
Piezoresistive flexible pressure sensors
The piezoresistive sensors deform the compound by applying an outside force. They implicitly change the touch position and distribution of the conductive fillers within the composite and then allow the resistance of the composite to frequently change. Thus, they do not involve a complex sensor structure. A wide variety of pressure tests, low power consumption and easy production processes contribute to extensive analysis of these sensors as opposed to piezoelectric and capacitive pressure sensors. They are perfect for physical exercise, sealing inspection and medical examination. Currently, several commercial products are on the market such as the smart bra, which can track the heart pulse by varying resistance, thereby preventing diseases.
Capacitive flexible pressure sensors
A capacitive flexible pressure sensor offers quick response, a broad dynamic range and high sensitivity, and is built on the parallel plate capacitor concept. It works by adjusting the sensor's capacitance by changing the distance between plates of the capacitors as an external force is applied. This capacitance can be found using C = εA/d, where d is the separation among the electrodes, A is the effective plate area and ε is the dielectric constant. Exterior pressure may adjust the elastic material displacement to respond to forward force by changing d, the shear force by changing A, and tension by collectively changing these two parameters. For the dielectric layer, the high sensitivity of sensors is difficult to attain due to the high Young modulus of elastomer content, for example, certain dielectric elastomers, like PDMS, with a small Young modulus of 5 kPa.
Piezoelectric flexible pressure sensors
Researchers have given a lot more attention to compact piezoelectric pressure sensors, thanks to their simple material preparation, easy electrical signal attainment and low cost. They consist primarily of piezoelectric sensitive compounds capable of transforming mechanical energy into electrical energy. Its process of transduction can be interpreted as follows: if the compound is disfigured due to the outside pressure, negative and positive charges segregation happens in the working compound or material. The negative and positive charges appear in opposite directions on the two opposite sides of the material, and there will be a potential difference developed inside the material. The effect of external factors is tested by examining these potential differences.
Flexible pressure sensors for electronic skin
Research on flexible pressure sensing technologies, gaining from their outstanding mechanical and electronic outputs, have made excellent strides in recent years, and show promise for real implementation. These technologies enhance and encourage the formation of many other sensing technologies, including wearable pressure sensors and e-skin tactile sensors, relevant in the areas of medical diagnostics, artificial intelligence (AI) and physical health monitoring.
E-skin flexible tactile sensors are broadly used for the sensing of the human skin tactician and thermal sensing. E-skin may be applied as a fabric to the exterior of a robot or a human body and can be handled as different figures to mimic the sensory functionality of the human skin due to its soft and light characteristics. After that, the physiological status can be identified and the robot's intelligence can be improved. In the 1970s, studies of e-skin tactile sensors started for the first time. Early on, the tactile sensor technologies were matured with a focus on achieving extensibility, flexibility, multifunctionality and light weight. Many tactile sensors were silicon microsensors, which were rendered by the use of MEMS technology. However, at that time, extensibility and flexibility criteria could not be fulfilled and have inspired researchers to search for new materials. New materials of note include a "sandwich" pressure sensor array developed to enable e-skin tactile sensors with high extensibility, stability and flexibility, as well as a broad range of accurate readings. This provided a strong base for the production of AI robots and extremely soft elastic e-skin.
Challenges for electronic skin
Improvement of environmental agility and establishment of a new human-machine deployment mode employing multiple sensors is the focus for the present science and technology development. Many challenges related to e-skin still need to be resolved. First, designing optimal bionic skin tactile sensors to maintain a high tensile rate need a high degree of elasticity and flexibility, which demands a high level of preparation techniques, materials and structure designs. Therefore, the massive prices of producing tactile sensors restrict the manufacturing of e-skin sensors on a large scale.
Secondly, it is particularly important to resolve the issue of signal crosstalk between the neighboring sensor modules of the sensor array. Different categories of lightweight multi-dimensional or 3D tactile sensors with various structures are designed to prevent the impact. The e-skin lightweight tactile sensor properties also differ greatly from the human skin's detailed sensations. Moreover, many of the current touch sensors have sole roles, primarily focused on pressure measurement, and only a small fraction feature measuring temperature, pressure, roughness, humidity and other parameters. Thus, the development of a new kind of multipurpose e-skin tactile sensors must reach human skin efficiency.
