1 College of Printing and Packaging Engineering, Beijing Institute of Graphic Communication,Beijing 102600, China
2 Beijing Engineering Research Center of Printed Electronics, Beijing Institute of Graphic Communication,Beijing 102600, China
3 Center for Stretchable Electronics and Nano Sensors, Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education, College of Physics and Optoelectronic Engineering, Shenzhen University,Shenzhen 518060, China
The flexible pressure sensor with high flexibility, easy conformality, high sensitivity and fast response is a novel flexible electronic device. It is also the critical device for the development of tactile artificial intelligence, internet of things, wearable electronics and relative technologies. The strategies based on development of sensitive functional materials, device structure design and construction, and optimization of fabrication methods have been widely used to improve the comprehensive performance of flexible pressure sensors. Among them, utilizing the microstructure of functional layer of flexible pressure sensor to enhance its performance is generally considered to be one of the most effective ways. In this paper, the latest research progress of microstructured flexible pressure sensors in recent years is summarized. It mainly focuses on the performance enhancement mechanism of microstructured flexible pressure sensor, microstructure construction and fabrication methods, new sensitive functional materials, as well as its applications in human-machine interaction, medical and health and other relative fields. Finally, the future development of microstructured flexible pressure sensor is prospected.
Zhao Jing, Wang Ziya, Mo Lixin, Meng Xiangyou, Li Luhai, Peng Zhengchun.
Performance Enhancing Mechanism,Implementation and Practical Advantages of Microstructured Flexible Pressure Sensors.
Progress in Chemistry, 2022, 34(10): 2202-2221 DOI:10.7536/PC220117
2 Flexible pressure sensor types and performance parameters
2.1 Sensor types
2.2 Main performance parameters
3 Performance enhancing mechanism of microstructured sensors
3.1 Capacitive flexible pressure sensor
3.2 Resistive flexible pressure sensor
3.3 Piezoelectric flexible pressure sensor
4 Fabrication methods for microstructured flexible sensors
4.1 Lithography template
4.2 Natural template
4.3 Sacrificial template
4.4 Vacuum freeze-drying
4.5 3D printing
4.6 Other methods
5 Materials design for flexible pressure sensor
5.1 Flexible substrate
5.2 Microstructured functional materials
6 Practical advantage of microstructured pressure sensors
6.1 Medical health and exercise monitoring
6.2 Human-machine interaction
7 Conclusion and outlook
1 Introduction
With the rapid development of artificial intelligence, Internet of Things and wearable electronics, flexible pressure sensors, as one of their key basic components, have attracted much attention because of their broad application prospects in human-computer interaction, medical health, electronic skin and perception reconstruction. Taking wearable electronic technology as an example, according to the survey data released by IDTechEx, the global wearable device users are close to 900 million by 2021, and its market share will grow to more than 75 billion US dollars in 2025[1]. Despite the rapid development of wearable devices, they still face problems such as rigidity, difficulty in attaching to the human body, and poor implantable compatibility. Flexible pressure sensor has the characteristics of lightness, flexibility, stretchability and strong biocompatibility, which can well solve the problem of elastic modulus adaptation between traditional rigid devices and human skin and organs. It is regarded as the key core device for the transformation of wearable products from 1.0 to 2.0, and has great market application prospects. So far, a variety of strategies have been developed to improve the overall performance of flexible pressure sensors, including the development of sensitive functional materials[2⇓~4]; Device structure design and construction[5⇓~7]; Optimization of manufacturing methods, etc[4,8].
Among them, it is generally considered to be the most effective way to improve the performance of flexible pressure sensors by microstructuring the functional layer of the device. The elastic modulus of the flexible pressure sensor can be effectively reduced by constructing the microstructure on the functional layer of the flexible pressure sensor, and the deformation rate of the functional layer under stress is increased, so that greater electrical response is obtained; At the same time, the stress concentration caused by the microstructure is also beneficial to the faster response time of the device. On the other hand, combined with the design and optimization of flexible pressure sensor sensitive functional materials, the sensitivity, pressure response range, response time, detection limit and other comprehensive properties of the device can be further improved. Figure 1 lists several commonly used microstructures, including pyramidal structure, hemispherical structure, columnar structure, internal porous structure, random structure, and multi-level layered three-dimensional structure[9].
In recent years, although a number of review papers on flexible pressure sensors have been reported, there are still very few papers specifically on micro-structured devices, which comprehensively summarize and analyze the mechanism, implementation methods, functional materials and application research[10⇓~12]. Therefore, this paper will focus on the mechanism of microstructure enhancing the performance of flexible pressure sensors, the construction and implementation methods of microstructure, the commonly used flexible pressure sensor materials, and the typical applications of devices in human-computer interaction, health care and other fields.On this basis, the future development direction of microstructured flexible pressure sensors is prospected.
2 Types and Performance Parameters of Flexible Pressure Sensors
2.1 Type
According to the different sensing mechanisms, the current mainstream flexible pressure sensors can be divided into four types: piezoelectric, resistive, capacitive and triboelectric. Figure 2 shows the working mechanism of the above four flexible pressure sensors[13]. In addition, new types of sensors such as off-electrode, field effect transistor and photoelectric effect have also attracted wide attention[14⇓⇓⇓⇓~19][20⇓⇓~23][24⇓~26].
The piezoelectric flexible pressure sensor (Figure 2a) converts the pressure signal into a voltage signal through piezoelectric materials (such as polyvinylidene fluoride-trifluoroethylene P (VDF-TrFE), zinc oxide ZnO, and polypropylene PP). It has the characteristics of high sensitivity and fast response, and can realize self-power supply of the device. It is suitable for real-time monitoring of dynamic mechanical changes. The resistive flexible pressure sensor (Figure 2b) converts pressure changes into changes in resistance or current. Its device structure and preparation process are simple, easy to read, high sensitivity, and not easily disturbed by external fields, but easily affected by temperature. The capacitive flexible pressure sensor (Figure 2C) senses pressure through the change of capacitance when it is pressed. Its advantages are low energy consumption, simple structure, no temperature dependence, and sensitivity to small forces, but it is vulnerable to external field interference, and its circuit is complex[6,7]. The flexible pressure sensor based on triboelectricity (Figure 2D) uses the principle of triboelectricity, which has the advantages of simple preparation process and high output voltage, but its test stability needs to be strengthened, and the phenomenon of measurement signal drift may occur in the process of pressure monitoring.
2.2 Main performance parameters
In order to obtain a flexible pressure sensor with excellent comprehensive performance, it is necessary to quantitatively evaluate its performance with some parameters, including sensitivity, response time, pressure response range, detection limit, cycle stability, signal-to-noise ratio, pressure resolution, etc.
2.2.1 Sensitivity
Sensitivity refers to the rate of change () of the output electrical signal of the flexible pressure sensor under unit pressure, which is usually calculated by using the slope of the "electrical signal rate-pressure" curve of the device, and the unit is “kPa-1”. As one of the most important performance parameters of flexible pressure sensor, sensitivity has been paid much attention by researchers. Sensitivity is also related to other performance parameters such as pressure response range, response time, minimum detection limit, signal-to-noise ratio, and pressure resolution.
The ideal flexible pressure sensor should have high sensitivity in a wide pressure response range, but the sensitivity of most devices decreases rapidly with the increase of pressure, and its high sensitivity can only be maintained in a small pressure response range. The relationship between sensitivity and pressure range of some microstructured devices reported in the literature is shown in Figure 3. Most devices have high sensitivity (>1 kPa-1, some devices exceed 10 kPa-1) in a small pressure range (< 2 kPa); However, the sensitivity decays rapidly by 1 to 2 orders of magnitude when the pressure is greater than 2 kPa, and continues to decay by 1 to 2 orders of magnitude in the range of 10 to 50 kPa, and finally reaches saturation in the range of 100 kPa. This is due to the fact that the deformation caused by the microstructure of the device decreases with the increase of pressure, and the electrical response tends to be saturated. At present, the Young's modulus of elastic polymers (such as PDMS) commonly used in flexible pressure sensors is in the order of MPa, and the deformation of PDMS is generally saturated at about 100 kPa after the introduction of microstructure. Therefore, it is difficult for a single microstructure to achieve the unification of high sensitivity and wide pressure response range. The second mechanism can continue to maintain the high sensitivity of the device in a wide pressure response range when the electrical response brought by the microstructure tends to be saturated by using a hierarchical composite microstructure or introducing another sensitivity enhancement mechanism to cooperate with it. Guo et al. Prepared a capacitive pressure sensor with a "multi-stage unstable self-compensation" structure. The unstable protruding microstructure is easy to bend when compressed, while the grooves on the surface and bottom can accommodate the deforming surrounding structure.Thus allowing the structure to be further compressed under high pressure.Coupled with the electric double layer formed at the interface between the ionic gel and the electrode, the sensor has extremely high sensitivity (Smin>220 kPa-1) over an extremely wide pressure response range (0.08 Pa to 360 kPa)[14]. While focusing on the high sensitivity and wide pressure response range of the device, it should also be noted that high sensitivity will inevitably lead to an increase in pressure noise, and how to effectively distinguish and filter the pressure noise unrelated to the test to obtain a flexible pressure sensor with a high signal-to-noise ratio is also worthy of attention.
2.2.2 Response time
Response time includes two aspects, one is the time required for the sensor to change from the electrical signal to the stable state, and the other is the time required for the electrical signal to change from the stable output value to the initial value after the applied pressure is removed, which is also called relaxation time. These two time parameters are important parameters to evaluate the real-time dynamic pressure sensing performance of the sensor. On the one hand, the response time is related to the viscoelasticity of the functional material used in the sensor and the microstructure design of the device; On the other hand, it is also related to the resolution of the test equipment. Baig et al. Used a gradient interlocking hemispherical structure to coat PEDOT: PSS/PUD (poly (3,4-ethylenedioxythiophene): polystyrene sulfonate/polyurethane) dispersions with different conductivities on the surface of a hemispherical PDMS film.The sensor was interlocked with another hemispherical TPU (thermoplastic polyurethane) film coated with Ag NWs/Pt (silver nanowires/platinum) conductive layer, and the response time of the sensor was 10-2ms, and the sensitivity was 3.8×105kPa-1 in the range of 0. 025 Pa ~ 100 kPa[27].
2.2.3 Pressure Response Range and LOD
The pressure response range refers to the pressure range in which the flexible pressure sensor can operate stably, that is, within this pressure range, the sensor can correctly convert the pressure signal into an electrical signal. According to different response ranges, the sensor can be applied to different fields, as shown in Figure 4. Low pressure sensing is mainly used for sound monitoring, vital signs monitoring, etc. Medium voltage sensing can be used in bionic electronic skin, robot hand grasping, sports health monitoring and other fields. High pressure sensing has broad application prospects in intelligent devices, automobile tires, underwater detection, aerospace and so on.
The lowest detection limit is the minimum pressure response within the sensor's operating range. As mentioned above, the lowest detection limit, sensitivity and pressure response range are interrelated, and the above properties including the lowest detection limit can be effectively improved by introducing microstructures into the device and cooperating with sensitive materials. In 2021, Gao et al. Designed a material with a micropillar array on the surface and a porous interior as the sensing layer, whose sensor has a minimum detection limit of 0.25 Pa, a sensitivity up to 10 000 kPa-1, and a pressure range of 500 kPa[28].
2.2.4 Cyclic stability
Stability refers to the change of the sensing ability of the flexible pressure sensor under the condition of multi-cycle use. It is an important parameter to evaluate whether the device can be used in practice for a long time and multiple cycles. Generally, continuous loading-unloading test cycles are used to evaluate the stable output of the electrical signal of the sensor. The higher the number of loading-unloading test cycles, the smaller the change of the output signal of the sensor, which means that the stability of the pressure sensor is better. Zou et al. Fabricated an ionically flexible pressure sensor by using a polycarbonate membrane as a template to fabricate a dielectric layer with a surface micropillar structure. The sensor can withstand at least 20,000 repeated pressure cycles without fatigue, and the sensitivity can reach 35.96 kPa-1 in the low pressure range of 0 ~ 0.1 kPa, and still has 0.06 kPa-1 in the range of 20 ~ 100 kPa[38].
2.2.5 Other performance parameters
In addition to the above parameters, other parameters are used to describe the performance of the flexible pressure sensor, such as stretchability and pressure resolution. When the flexible pressure sensor is attached to the human epidermis, the sensor faces bending, stretching, compression and other actions with the movement of the joint, which requires the device to have a certain degree of stretchability, so as to maintain good performance stability in the process of movement. Pressure resolution is more intuitive to reflect the response of pressure to stimulation. High pressure resolution can still detect the signal change of small pressure under high pressure, which can make the sensor have more application scenarios[39]. In addition, biocompatibility, self-healing, environmental stability, self-functionality, degradability, pressure resolution and other performance parameters can be used to characterize the sensor.
3 Performance Enhancement Mechanism of Microstructured Sensor
3.1 Capacitive flexible pressure transducer
According to the sensitivity calculation formula (1) of the capacitive flexible pressure sensor, there are two ways to improve its sensitivity: increasing the rate of change of electrode spacing under pressure (improving the d0/d) and increasing the rate of change of dielectric relative permittivity (improving the εr/εr0). Fig. 5 takes the porous microstructure of the dielectric layer as an example to reveal the enhancement mechanism of the functional layer microstructure on the sensitivity of the capacitive flexible pressure sensor[40]. Compared with the dielectric layer without microstructure (fig. 5A), the dielectric layer with internal porous microstructure (fig. 5B) compresses more under the same pressure, resulting in a greater rate of change in the electrode spacing of the flexible pressure sensor; At the same time, in the process of pressing the dielectric layer with porous microstructure, the volume ratio of air to elastic resin in the composite dielectric layer is changed because the compression ratio of air is larger than that of elastic resin,This in turn causes a change in the dielectric constant of the composite dielectric, which further increases the rate of change of the capacitance of the device during compression (Figure 5C).
In formula (1), d0, εr0, A0 are the initial electrode spacing, dielectric relative permittivity and electrode area, respectively, and d, εr, A are the above parameters under pressure.
Guo et al. Reported a capacitive flexible pressure sensor based on PDMS/conductive carbon porous composite dielectrics, and used COMSOL software to simulate the relationship between Young's modulus of composite dielectrics and their porosity.The results are compared with the experimental results, which show that the reciprocal of Young's modulus of the composite dielectric is positively correlated with its porosity, and the strain of the porous composite dielectric is 6. 5 times that of the amorphous PDMS when the pressure is 100 kPa[41]. If the micro-nano structure is introduced on the surface of the functional layer of the device to form the local stress concentration of the microstructure, it is also beneficial to obtain a larger change rate of the electrode spacing. Guo et al. used ionic gel to replicate the micro-nano structure on the surface of a leaf and applied it to the dielectric layer of a capacitive flexible pressure sensor. According to the force analysis, when an external pressure of 100 kPa is applied to the whole sensor, the force on the top of the conical microstructure formed by the ionic gel can reach 1 MPa, while the force on the bottom is only 0.18 MPa (Fig. 6)[42]. It can be seen that the existence of the microstructure can form local stress concentration, and the microstructure itself is easy to deform because of its fine structure, so that the sensor has a larger change in electrode spacing under the same force, and the sensitivity is improved. Bao et al. Prepared a dielectric film with a pyramidal microstructure. While changing the elastic modulus of the dielectric layer, the air between the pyramids was gradually discharged during the compression process, and the dielectric constant of the dielectric layer changed. Compared with the device without a microstructure, the sensitivity was increased by about 30 times, and the response speed and detection limit of the device were also significantly improved[43].
3.2 Resistive flexible pressure transducer
The resistive flexible pressure sensor is formed by packaging an elastic functional layer with certain conductivity in contact with a flexible electrode, and converts an applied pressure stimulus into a variable resistance signal. Generally speaking, the change of resistance of resistive flexible pressure sensor under stress is mainly caused by two factors: the change of resistance of the functional layer itself and the change of contact resistance between the functional layer and the electrode. The introduction of microstructure will help to enhance the resistance change rate of the device under stress. The mechanism of the performance enhancement of the resistive flexible pressure sensor by the microstructure will be described from the above two aspects.
(1) The change of the resistance of the functional layer itself. The functional layer of the resistive flexible pressure sensor is generally composed of an elastic polymer with low Young's modulus and a filler with high electrical conductivity, and its conductive mechanism is based on the percolation theory of composite materials[44⇓~46]. According to the percolation theory, when the volume fraction of conductive filler is low, the average distance between conductive particles in the polymer matrix is large, the conductive path can not be formed, and the resistivity of the composite is high. With the increase of the content of conductive filler, the average distance between conductive particles decreases, and some conductive paths begin to form, and the resistivity of the composite decreases. When the volume fraction of the conductive filler reaches a certain threshold, the conductive filler particles in the polymer matrix are close to each other and connected to form a complete three-dimensional conductive network throughout the polymer matrix.The resistivity of the composite decreases dramatically, up to 10 orders of magnitude, from the initial insulating polymer matrix to a composite with good conductivity. Based on the percolation theory, the content, morphology and distribution of the conductive filler can be optimized to make the composite system close to but not reach the percolation state[47]. When the external pressure is applied, the spacing of the conductive fillers in the composite is compressed in the direction of the force and reaches an interconnected seepage state, and the resistance of the system decreases rapidly, thus showing the sensitivity to pressure, as shown in Figure 7A[48]. The introduction of the microstructure will effectively adjust the elastic modulus of the composite, so that the composite can obtain a larger deformation under a smaller pressure and quickly recover its shape when the external force is withdrawn, thus enabling the device to have higher sensitivity, response speed and wide range.
When there is a conductive fibrous structure inside the functional layer, as shown in Figure 7B, only part of the fiber intersections are in contact before the force is applied. After the force is applied, due to the compression deformation of the material, the contact points between the conductive fibers increase or even become linear contact. After the pressure is released, the fiber contact returns to the initial state, so that the contact resistance changes continuously. Salim et al. Reported a resistive sensor based on porous polyurethane and graphene composite. The porous functional layer endows the sensor with super-compressive elasticity, and the adjacent graphene sheets are connected when compressed, which leads to a rapid decrease in resistance. The sensor prepared by this strategy has a sensitivity of 7.62 kPa-1 in the pressure range of 0 – 50 kPa[49].
(2) The change of interfacial contact resistance. One of the important ways to enhance the performance of resistive flexible pressure sensors is to amplify the change rate of interfacial contact resistance under stress by constructing microstructures between the functional layer and the electrodes of the device[50]. According to the microstructure morphology, it can be divided into primary structure and hierarchical structure. The primary structure is shown in Figure 7c, and the height, size, shape, and spatial distribution frequency of the microstructure are basically the same, showing a high degree of regularity on the whole. In the process of stress, the number of contact points between the electrode and the functional layer basically does not change with the pressure, while the area of the contact points increases with the increase of the pressure, which shows that the overall resistance of the device decreases macroscopically. Park et al. Introduced a regular pyramidal microstructure into the conductive functional layer, and with the increase of pressure, the pyramidal structure deformed, and the structure of the tip and the electrode changed from point contact to rectangular contact.Therefore, the contact area between the functional layer and the electrode is increased, and the resistive flexible pressure sensor obtained by this strategy has a sensitivity of 4.88 kPa-1 in the pressure range of 0 – 6 kPa[51]. The advantage of the primary structure is that it is conducive to large-scale manufacturing, and it is easy to establish the relationship between the microstructure and the device performance, so as to regulate the device performance. The disadvantage of the primary structure is that with the gradual increase of the loading pressure, the microstructure will be difficult to further compress after being compressed to a certain extent. Because all the microstructures of the same size are in contact with the electrode at the initial stage, their electrical response tends to reach saturation during the gradual compression process. To sum up, the overall device response sensitivity also undergoes a transition from high to low to saturation, and the pressure response range is generally small. The introduction of hierarchical structure provides a feasible solution to the above problems.
Hierarchical structure means that the size, morphology, height, size and other geometric parameters of the microstructure between interfaces are different, and the overall microstructure shows a high degree of random characteristics. As shown in Fig. 7d, due to the different sizes of the microstructure units, new microstructures are always in contact with the electrode surface during the whole pressure loading process, that is, the number of conductive channels is constantly changing; On the other hand, in different stages of pressure loading, there are always microstructures in different compression States, that is, the area of interface contact points is different, and the corresponding conductivity is also at different levels. The mechanical state of the device is analyzed, and it is found that the compression state of the structure with different sizes is different. Macroscopically, there is a large change in the resistance value during the whole compression process, and the device can maintain high sensitivity in a wide pressure range[52]. Yang et al. Used an unpolished silicon wafer as a template to conformally form carbon nanowalls on PDMS with an irregular microstructure to form electrodes, and the resulting resistive flexible pressure sensor had a sensitivity of up to 6.64 kPa-1 within 0 – 1 kPa and a sensitivity of 0.15 kPa-1 within 1 – 10 kPa[53].
3.3 Piezoelectric flexible pressure transducer
The working principle of piezoelectric flexible pressure sensor is based on the piezoelectric effect of the material itself. When the crystal is subjected to an external force in a certain direction, an electric polarization phenomenon occurs in the material, and a large number of opposite charges are generated at the same time; When the external force is removed, the material returns to its uncharged state.
The existence of microstructure can not only improve its piezoelectric properties, but also reduce the interference between materials themselves. When dynamic pressure is applied to the piezoelectric film, the accumulation of dynamic charge will occur on both sides of the film, as shown in Figure 8. Piezoelectric materials without microstructure have limited compression and limited piezoelectricity, while piezoelectric materials with microstructure have high compressibility and strong strain, which will increase the piezoelectric effect and improve the sensitivity of the sensor. In addition, there is complete solid contact between the piezoelectric material without microstructure and the substrate, and there is crosstalk between the sensing points. However, the contact surface between the piezoelectric material with microstructure and the substrate is small and not completely continuous, and the microstructure contact can reduce the mutual interference between the pressure sensing points. Shao et al. Of Xi'an Jiaotong University used nanoimprinting technology to uniformly imprint P (VDF-TrFE) thin films into vertically aligned micropillar arrays, and prepared a flexible sensor array based on piezoelectric-enhanced vertically aligned P (VDF-TrFE) micropillars. Compared with the planar thin film sensor, the sensitivity of the imprinted micropillars was increased by about 3.3 times[55].
4 Micro-structure manufacturing method
At present, the fabrication methods of flexible pressure sensor microstructure mainly include photolithography template method, natural template method, salting-out method, vacuum freeze-drying method, 3D printing method, printing manufacturing method and magnetic field induced microstructure formation method. Different microstructure manufacturing methods have their own characteristics in terms of process complexity, manufacturing cost, microstructure controllability, production efficiency, and suitability for mass production. This section will introduce the above methods in detail and compare their advantages and disadvantages.
4.1 Photolithographic template method
Photolithographic template method refers to the use of photolithographic technology to process specific micro-nano structures on silicon wafers and other substrates, and then use this as a template to replicate the microstructure with polymers (such as PDMS, PMMA, etc.), so as to obtain a polymer matrix with surface microstructure[56⇓⇓~59]. In 2010, Bao Zhenan of Stanford University in the United States first prepared a micron pyramid array on the surface of PDMS by photolithography template method, and used it in the dielectric layer of capacitive flexible pressure sensor[43]. On this basis, in 2014, they further established the relationship between microstructure shape, size, distribution and other factors and device sensitivity, and achieved the regulation of device performance to a certain extent[56]. In 2014, Chen Xiaodong et al. From Nanyang Technological University in Singapore used PDMS to replicate the silicon wafer microstructure prepared by photolithography, and deposited graphene oxide on the surface of PDMS. After reduction, the microstructured electrode was obtained, and the sensitivity of the packaged resistive flexible pressure sensor was 5.53 kPa-1 at low pressure. The preparation process of the microstructured electrode is shown in Fig. 9[60]. The method has the advantages that the controllable preparation of the device microstructure can be realized, and the method is beneficial to the research and establishment of the corresponding relationship between the device microstructure and the performance. The disadvantages are that the cost of lithography is high, the process is complex, and the compatibility with flexible devices needs to be improved.
4.2 Natural template method
In order to avoid the problems of high cost and complex processing technology caused by photolithographic templates, people use leaves, sandpaper, silk, petals and other micro-nano structures that can be seen everywhere in daily life as natural templates to obtain micro-structured functional layers for flexible pressure sensors by polymer turnover[32,61⇓⇓ ~64]. In 2019, Huang et al. Used sandpaper as a natural template to obtain a graphene electrode with a multi-level micro-nano structure after two times of PDMS turnover. The test results showed that the graphene was well conformal on the surface of the microstructure, and the obtained microstructure electrode was packaged with an interdigital electrode.The obtained flexible pressure sensor has a high sensitivity (2.5~1051 kPa-1) in the range of 0.01 – 400 kPa, and Fig. 10 is a schematic diagram of the micro-structured surface conformal electrode obtained by this method, the characterization results, and its device packaging structure[65]. Zhang et al. Used leaves as templates to uniformly coat carbon nanotubes and graphene on a PDMS flexible substrate with microstructures by chemical vapor deposition, and the sensitivity of the resistive sensor obtained by face-to-face packaging could reach 19.8 kPa-1 in the pressure range of less than 0.3 kPa[66]. Although the preparation cost of this method is very low, the random micro-nano structure of the natural template makes the consistency and controllability of the device poor, which can not meet the batch preparation of the device.
4.3 Salting-out method
Salting-out method means that solid particles (such as salts, sugars, etc.) Are mixed into the polymer precursor in advance, and after the polymer is solidified, it is soaked in a specific solvent to dissolve the solid particles, thus forming a porous structure inside the polymer[67⇓⇓~70]. In 2019, Zhang et al. Obtained a polymer-based conductive composite with an internal porous structure with the help of white granulated sugar. Combined with the surface microstructure prepared by the natural template method, the flexible pressure sensor has a sensitivity of 83.9 kPa-1 in the pressure range of less than 140 Pa, and the detection limit is 0.5 Pa. The schematic diagram of the microstructure preparation process is shown in Figure 11[71]. Cho et al. Used sugar and microspheres to make elastomer composite layers with pores of different sizes, and the resulting sensor could operate over a wider pressure range (~ 150 kPa)[72]. Guo et al. Fabricated a porous structure by dissolving NaCl, and the resulting sensor had a good linear response (1.1 kPa-1) in the pressure range within 10 kPa[41]. This method also has a certain randomness in the formation of microstructure, and is not suitable for batch preparation.
4.4 Vacuum freeze drying
Vacuum freeze-drying technology is a technology that uses the vacuum sublimation phenomenon of ice, that is, water is frozen into ice by cooling, and then water molecules are sublimated under vacuum conditions to obtain porous materials[73⇓~75]. In 2018, Crispin et al. Prepared an organic ion/electron hybrid conducting aerogel based on PEDOT by freeze-drying. Fig. 12 is a schematic diagram of the morphology of the aerogel and the sensor after freeze-drying.The internal porous microstructure of the aerogel improves the mechanical elasticity and the specific surface area, and finally realizes the sensing of three parameters of pressure, temperature and humidity without crosstalk[74]. In 2019, Sun et al. Controlled the freezing process to crystallize water directionally, and in the vacuum drying process, the nano-lignocellulose formed a bridge-like support structure for carbon sponge along the direction of crystallization water.Aerogel dielectrics with excellent compressibility and mechanical strength (2 to 5 times higher than traditional metal, carbon and organic materials) are obtained, and the performance of flexible pressure sensors is improved[75]. This technology can regulate the internal pore structure to a certain extent by controlling the freeze-drying conditions, but its cost is relatively high and the process is complex.
4.5 3D printing method
3D printing is a rapid prototyping technology, which can be better compatible with polymers, nanocomposites, hydrogels and other types of materials, and its application in flexible pressure sensing is gradually increasing in recent years[76⇓⇓⇓⇓⇓~82]. The advantage of 3D printing technology is that it can realize the controllable fabrication of different morphologies and microstructures, thus providing a powerful means for the establishment of structure-performance relationship and performance regulation of devices. Wang et al. Prepared the ink suitable for 3D printing by depositing and coating carbon nanotubes on the surface of PDMS microspheres, and prepared the spiral microstructure electrode of bionic human fingerprint by 3D printing. The flexible resistive pressure sensor obtained by packaging with the interdigital electrode has a sensitivity of 2.08 kPa-1 within a pressure range of less than 0.12 kPa, and its preparation process is shown in Fig. 13 (a, B)[83]. Our research group prepared a linear microstructure elastic dielectric layer based on PDMS and conductive carbon black composite by 3D ink direct writing method (Fig. 13C), and controlled the rheological properties of the ink by optimizing the components of the composite. From the rheological curve (Fig. 13C), it can be seen that the ink has obvious shear-thinning characteristics. By adjusting the printing conditions, the microstructure morphology can be controlled (Figure 13C), which has been initially used in flexible pressure sensors, and its performance has been significantly enhanced, and related papers are being written. While 3D printing technology brings convenience and good controllability to the preparation of microstructures, it also faces the shortcomings of relatively low processing efficiency, high cost, and generally requires additional processes for subsequent treatment.
4.6 Other methods
In addition to the above microstructure manufacturing methods, in recent years, three-dimensional multi-level composite micro-nano structures formed by nanomaterials growth, magnetic field induced microstructure formation, printing manufacturing and other micro-structuring methods have been used to improve the performance of flexible pressure sensors[30][84][85,86]. Our research group has previously prepared a composite functional ink based on PDMS, thermal expansion microcapsules, and low-dimensional nano-conductive fillers, and prepared a capacitive flexible pressure sensor array by using full-printing technology (Figure 14 a)[85]. The results show that the thermally expanded microcapsules can form a controllable porous microstructure inside the functional layer of the flexible pressure sensor (Fig. 14b), and the sensitivity of the device in different pressure ranges can be significantly improved by adding silver nanowires to the porous polymer matrix to form a synergistic mechanism with the porous microstructure (Fig. 14C). Compared with the pure PDMS dielectric layer, the sensitivity of the device can be improved by up to 1229 times in the pressure range of 0 ~ 5 kPa and by 60 times in the middle and high pressure range of 10 ~ 700 kPa after adding the thermal expansion microcapsules and silver nanowires. The response time of the capacitive flexible pressure sensor array prepared based on this method is 50 ms (Fig. 14d), the capacitance change of the device is less than 5% after 3600 cycles of impact under 100 kPa pressure (Fig. 14e), the detection limit is 1.3 Pa, and the dispersion of the array performance is less than 5%, which can be used to monitor human movement (Fig. 14 f), health and pressure distribution (Fig. 14 G).
Table 1 systematically compares the above microstructure manufacturing methods in terms of process complexity, cost, suitability for batch production, microstructure controllability, and production efficiency. It can be seen that the above microstructuring methods still have room for further improvement in several indicators. To develop a new method for manufacturing microstructures with simple process, low cost and suitable for mass production, which requires effective control and efficient manufacturing of microstructures, is very necessary for the improvement of manufacturing technology in the field of advanced sensors.
5 Flexible pressure sensor material
In addition to good sensing characteristics, flexible pressure sensors should also have excellent flexibility, stretchability, biocompatibility, self-healing, self-power supply and other characteristics according to the actual application scenarios. As mentioned earlier, microstructuring is an important way to improve the performance of flexible pressure sensors. At the same time, the selection and optimization of the materials used in the device will form a synergistic effect with its microstructure, which will determine the final application performance of the flexible pressure sensor. Generally speaking, the flexible pressure sensor consists of three parts: the flexible substrate, the electrode layer and the sensing function layer. The electrode layer based on gold, silver, indium tin oxide (ITO) and other materials has been widely studied. This paper will mainly introduce the flexible substrate and the sensing function layer.
5.1 Flexible substrate
5.1.1 Non-functional support substrate
In view of the market demand for large area, low cost and flexibility, the current flexible substrates for flexible pressure sensors mainly focus on organic polymer materials, such as polydimethylsiloxane (PDMS), polyurethane (PU), polyimide (PI), polyethylene terephthalate (PET) and so on. PDMS is a linear polysiloxane with an alternating backbone of — Si — O — Si —, which has excellent light transmittance, biocompatibility, and thermal stability, and is often used in stretchable flexible pressure sensors[87]. For example, Shiratori et al. Prepared a PDMS film with a bionic fish scale microstructure using an air/water interface, which has a tensile elongation of 102.86%, and when it is used in a sensor, its sensitivity can reach 70.86 kPa-1 in the pressure range of less than 1 kPa, and its performance is significantly enhanced[32]. PU is a kind of polymer containing repeating carbamate groups (— NH — COO —) in the macromolecular chain, which can be plasticized and dissolved in many solvents, and has good wear resistance, elasticity, easy processing and other characteristics. PI has excellent stability, insulation and mechanical properties, and can withstand temperature changes from -40 to 400 ℃. PET is a high temperature resistant polyester film with excellent optical and mechanical properties. In addition, organic polymers such as polyethylene naphthalate (PEN), polycarbonate (PC), polyethersulfone (PES), and the fully biodegradable material poly (butylene adipate/terephthalate) (PBAT) produced by BASF in Germany can also well meet the flexibility and strength requirements of flexible pressure sensors. The flexible polymer substrate provides a carrier support for the microstructured flexible pressure sensor, so that the microstructured flexible pressure sensor has the characteristics of good flexibility, mechanical strength, processability and the like, and is widely applied.
5.1.2 Self-healing substrate
Flexible sensors are prone to aging, damage, failure and other problems with the extension of use time, thus affecting the accuracy of data collection and transmission. Therefore, endowing materials with rapid self-healing response plays an important role in improving their sensing stability and prolonging their service life. Self-healing materials are capable of "automatically" repairing damage or cracks through the reconstruction of reversible chemical bonds or the use of healing agents under external stimuli (such as light, heat, etc.).
Inspired by the self-repair mechanism of multiple hydrogen bonds, Mi et al. used the reversible hydrogen bonds formed between PDMS and toluene 2,4-diisocyanate (TDI), and then introduced the carboxylated carbon nanotube (CNT — COOH) conductive filler into the PDMS-TDI elastomer matrix. The repair mechanism is shown in Figure 15[88]. Carboxylated carbon nanotubes form a conductive network in the elastomer matrix, which enriches the hydrogen bonds in the composite, and helps to improve the stretchability (more than 1000%), self-healing ability (98.1%) and sensitivity (2.43) of the elastomer. Li et al. Developed a healable polyurethane (HPU) substrate based on Diels-Alder chemical synthesis, and used it in the dielectric layer and electrode of the capacitive flexible pressure sensor to achieve the overall healability of the device, and the sensitivity could be restored to 85% of the original level after the sensor was cut and healed[89]. Compared with traditional flexible electronic devices, flexible substrates with self-repairing properties can enhance the service life of electronic devices and improve the reliability and stability of data information.
5.2 Microstructured functional layer
The functional layer material is the "core part" of the flexible pressure sensor. According to different sensing mechanisms, it is necessary to consider the selection of different types of functional materials and the design of diverse microstructures. Generally speaking, the functional layer is mostly composite material, which is composed of elastic polymer matrix and functional filler. The elastic polymer matrix is similar to the polymers of the above flexible substrates. In recent years, the commonly used functional fillers include carbon nanomaterials, nanometals, conductive polymers, two-dimensional materials MXene and conductive hydrogels.
5.2.1 Carbon nanomaterials
Carbon-based nanomaterials, such as Graphene, carbon nanotubes (CNTs) and carbon black (CB), are suitable materials for flexible pressure sensors due to their unique chemical stability and biocompatibility. Graphene is a two-dimensional (2D) carbon-based material composed of hexagonal carbon atoms bonded by sp2 bonds, which has large specific surface area and high carrier mobility, as well as excellent mechanical properties and thermal stability[90]. Carbon nanotubes (CNTs) have excellent anisotropy and good conductivity. When CNTs are added to nanocellulose and freeze-dried, a porous structure can be formed inside the aerogel. When 25% CNTs are added, the conductivity of the aerogel is 0. 029 S/cm, which is beneficial to the improvement of the sensor sensitivity[91]. Nano-carbon black is a kind of amorphous carbon with a large surface area. Jiang et al. Dispersed spherical nano-carbon black into polyurethane to prepare TPU/CB slurry, and formed a convex structure by screen printing. The percolation effect caused by deformation under pressure improved the sensitivity of the sensor (5.205 kPa-1 in the range of 0 ~ 100 kPa)[92].
5.2.2 Nanometal
Nanometals mainly include metal nanoparticles, nanosheets and nanowires with different morphologies. These nanometals have good mechanical flexibility and electrical properties, and are widely used in the preparation of flexible pressure sensors[93]. Li et al. Of Fudan University formed a composite material by doping PDMS with urchin-like nano-metal nickel, which concentrated the charge on the tip of the nickel particle, thus producing a local enhancement effect of the electric field, so that the flexible pressure sensor obtained had excellent linearity in the pressure range of up to 1.7 MPa[94]. Metal nanowires are often used as electrodes and functional layer additives for flexible pressure sensors because of their good flexibility and light transmittance after forming thin films. For example, Lou et al. Control the dielectric constant of the dielectric layer by replicating the photolithographic template to form a pyramid structure and doping Ag NWs, thereby improving the sensitivity and response range of the sensor[95].
5.2.3 Two-dimensional material MXene
MXene is a two-dimensional transition metal carbide, nitride or carbonitride, which not only has the high specific surface area of two-dimensional materials, but also has metal conductivity, and the surface groups make it have good hydrophilicity and rich physical and chemical properties. In addition, due to the wide distance between layers, MXene has high sensitivity and flexibility to pressure, so it has become a popular material for the preparation of flexible pressure sensors. For example, Park et al. prepared the filamentous network structure dielectric layer of MXene/PVDF-TrFE composite by electrospinning. After adding MXene, a micro-dipole and micro-capacitance network was formed inside the composite dielectric layer (Fig. 16), which increased the dielectric constant by about 100%, reduced the compressive modulus by about 58%, and significantly improved the sensitivity of the sensor[96]. Li et al. fabricated a micro-spinous structure through a sand paper template and deposited highly conductive MXene nanosheets on the microstructure PDMS by thermal spraying. With the increase of pressure, the contact area between MXene and the electrode changed continuously, thus improving the sensing performance of the pressure sensor[97].
5.2.4 Conductive polymer
Sensors based on conductive polymers, such as PEDOT, PPy, PANI, etc., have attracted much attention because of their good flexibility, ductility and high conductivity. PEDOT and its polymers have been widely used in the preparation of flexible pressure sensors in recent years because of their high conductivity (550 ~ 1000 S/cm) and good light transmittance. For example, Crispin et al. Prepared a new type of organic mixed ionic-electronic conducting aerogel with PEDOT: PSS, nanocellulose (NFC) and GOP aqueous dispersions, and made a porous structure functional layer by freeze-drying. The aerogel can sense pressure, temperature and humidity with minimal parameter crosstalk[74,98]. Baig et al. Used the active layer of the flexible pressure sensor by coating multiple layers of (PEDOT: PSS)/PUD solutions with different conductivities on the PDMS hemisphere, as shown in Fig. 17, the current path from the outer layer to the inner layer was gradually activated with higher to lower resistance respectively when pressed, resulting in a delayed saturation trend of the piezoresistance change of the sensor, thus generating a linear pressure sensing capability in a large range[27].
5.2.5 Conductive hydrogel
Hydrogels are three-dimensional elastic crosslinked hydrated polymer networks with high water content, and conductive hydrogels can be divided into two types: electronically conductive (ECHs) and ionically conductive (ICHs)[99]. The electronic conductive hydrogel is generally obtained by directly crosslinking a conductive material or polymerizing and gelating a hydrogel precursor solution and a conductive filler; Ionic conductive hydrogel polymer networks contain a large number of free ions, which are usually obtained by dissolving ionic salts into the hydrogel. Conductive hydrogels combine electrical conductivity and elasticity, and have good flexibility, ductility and biocompatibility, which are widely used in the preparation of sensors.
Guo et al. Prepared a dielectric layer with a self-filling structure by using PVA/H3PO4 ionic gel (as shown in Fig. 18). When compressed by force, the ionic gel and the electrode layer form an electric double layer, and the ions on the electrode and the opposite ions on the ionic gel gather at a distance of nanometers, which significantly improves the capacitance, thereby improving the sensitivity and expanding the pressure response range[14]. Zhang et al. Used GelMA hydrogel replica pyramid microstructure as the dielectric layer of the sensor. The Young's modulus (1.2 ~ 15.9 kPa) and relative dielectric constant (16 ~ 39) of GelMA hydrogel can be customized in a wide range, and the sensitivity of the sensor is one order of magnitude higher than that of polymer PDMS in the range of 0 ~ 17 kPa[100]. However, due to the high water content of conductive hydrogel, easy to lose water, low Young's modulus and natural flexibility, the hydrogel sensor is easy to be damaged and not durable, so it should be considered to improve its water retention and mechanical support performance when used.
6 Application
Sensors are a key technology in the era of Internet of Everything, and have broad application prospects in the fields of human health and motion monitoring, human-computer interaction, and smart medicine (Fig. 19). In the past few years, great progress has been made in the research of new microstructured flexible pressure sensors, and their comprehensive sensing performance has been significantly improved. In the field of smart medicine, micro-structured flexible sensors are used to monitor key parameters of human health, such as pulse, heart rate, blood pressure and gait. In the field of human-computer interaction, micro-structured flexible pressure sensors are used to capture various pressure stimulation signals in real time and quickly, and then output them to the actuator after being processed by the system to complete human-computer interaction.
6.1 Medical Health and Exercise Monitoring
With the development of big data and artificial intelligence, people are more inclined to use data to manage their health. Accurate and rapid acquisition of various pressure signals (such as pulse, muscle stress, plantar pressure, etc.) generated by human activities is of great significance to the development of smart medicine (Fig. 20).
Inspired by the microstructure of human skin, Bao et al. Designed and prepared a flexible sensor array with a scalable matrix network structure. Each sensing unit is connected by a serpentine structure, which can simultaneously monitor the changes of pressure, strain, temperature, humidity, proximity, thermal, magnetic, optical and other properties without crosstalk (Fig. 21)[103]. Bao Zhenan et al. Designed and fabricated an ultra-thin flexible pressure sensor by combining the pyramid with the micro-pillar structure, which significantly enhanced the ability of the sensor to obtain pressure signals from the human body by using the amplification effect of the micro-pillar interface[105]. The device can accurately detect the weak pulse signal of human neck vein and identify patients with heart disease. Hu et al. Used graphene aerogel microspheres to prepare a temperature and pressure dual-parameter flexible sensor. The good compressibility of aerogel makes the device extremely sensitive to small pressures, which can be used to assess the severity of Parkinson's disease[106].
People's walking posture and gait can reflect their physical health to a certain extent. By designing intelligent insoles with plantar pressure sensing function, people's movement information and gait patterns can be monitored in real time. Xiao et al. Fabricated a microstructured flexible pressure sensor using screen printing technology, and integrated it into a smart insole to diagnose high arch foot and flat foot and identify walking gait[86]. Zhuang et al. Fabricated a flexible pressure sensor with a conductive functional layer with an internal three-dimensional porous structure by using selective laser sintering technology, and placed it at four pressure sensing positions on the sole to accurately detect the pressure distribution and monitor sports health[34].
6.2 Human-computer interaction
In the human-computer interaction system based on flexible pressure sensor, firstly, the flexible sensor captures the human motion state and real-time sign signals, then the system processes and outputs the signals, and then the machine performs specific functions. In 2021, Shen Guozhen et al., Institute of Semiconductors, Chinese Academy of Sciences, summarized the research progress of human-computer interaction system based on flexible sensors, and divided the human-computer interaction system into three parts: input, communication and signal processing, and output (Figure 22). The signal input end mainly obtains signals through wearable flexible sensors[108].
Zhu et al. Fabricated a high-performance sensor using vanadium nitride nanosheets with vertically aligned carbon nanotube arrays. The sandwich structure of vanadium nitride nanosheets and carbon nanotube arrays arranged layer by layer formed a highly conductive 3D network structure, which provided high sensitivity for the strain response of the sensor (10% strain range, strain coefficient of 386)[109]. The sensor is integrated with the robot system, and the sensor transmits the sensed finger bending movement signal to the control system of the robot hand, thus realizing the real-time control of the robot hand (Figure 23 a).
Park et al. Fabricated a capacitive flexible pressure sensor with an internal porous microstructure and integrated it on the surface of a manipulator for object grasping tests (Fig. 23b)[110]. The results show that the microstructure can effectively improve the sensing performance of the sensor, so that the manipulator can grasp stably, continuously and accurately in the grasping experiment.
By studying the influence of different sandpaper structures on the performance of the sensor, Wang et al. Obtained a highly sensitive microstructured sensor, which was made into a wearable smart glove and connected to a single-chip computer system to replace the function of a mouse[61]. High-performance micro-structured sensors endow smart gloves with excellent sensitivity and stability, making them excellent in human-computer interaction. Lou et al. Fabricated a micro-structured transparent flexible pressure sensor array based on silver nanowires and made it into a wearable touch keyboard, which can be used for human-computer interaction input interface[95].
7 Conclusion and prospect
Flexible pressure sensor is a key basic component for the development of artificial intelligence and Internet of Things technology, and has shown broad application prospects in human-computer interaction, intelligent medical treatment, electronic skin, perception reconstruction and so on. In recent years, the use of microstructures to enhance the performance of flexible pressure sensors has been a hot research topic in this field, and significant progress has been made. On this basis, this paper elaborates and summarizes the mechanism of microstructure enhancing the performance of flexible pressure sensors, the construction and implementation methods of microstructure, the commonly used flexible pressure sensor materials, and the typical applications of devices in human-computer interaction, health care and other fields.It is expected to provide a reference for the design and construction of microstructures in the field of advanced sensing, the consolidation of manufacturing technology foundation and the innovation of sensing mechanism.
Although great progress has been made in the research of microstructured flexible pressure sensors, there are still some deficiencies in the aspects of microstructure manufacturing methods, sensing mechanisms, and device performance. The existing microstructured manufacturing methods still have shortcomings in several indicators, such as process complexity, cost, suitability for mass production, microstructure controllability and production efficiency. It is urgent to develop a new method of microstructure manufacturing with simple process, low cost and suitable for mass production for the improvement of manufacturing technology in the field of advanced sensors. In terms of sensing mechanism, a single microstructure strengthening mechanism is difficult to meet the requirements of flexible pressure sensors for further improvement of comprehensive performance, and the research on multi-modal sensing mechanism and device implementation methods is still relatively lacking, so it is urgent to explore new sensing performance enhancement synergistic mechanism. By using multi-level three-dimensional micro-nano structure, combined with the design and optimization of flexible pressure sensor sensitive functional materials, it is expected to form a "microstructure-material" synergistic performance enhancement mechanism to further improve the sensitivity, pressure response range, response time, detection limit and other comprehensive performance of the device. In terms of device performance, there are still some consensus problems, such as difficulty in achieving both high sensitivity and wide pressure response range, poor biocompatibility, multi-mode signal crosstalk, device interface mismatch in working environment, insufficient spatial resolution and pressure resolution of devices, and poor environmental stability, which are being studied in depth by researchers.
With the deepening of research, flexible pressure sensors are gradually endowed with more functions, including self-healing, self-energy supply, degradability, stretchability, etc. The device integration has also been greatly improved. Therefore, it is the development trend in this field to integrate multi-modal flexible sensors with flexible power supplies, energy storage devices, flexible circuits, etc., and use integrated additive manufacturing technology to obtain low-cost, large-area, flexible sensing systems, and then apply them in electronic skin, implantable smart medicine and other scenarios. Flexible pressure sensor has become a typical interdisciplinary and multi-technology field, which requires researchers from different disciplines to work together to promote its development. I sincerely hope that this paper can bring readers some thoughts and technical references about the development of flexible pressure sensor.
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