Flexible Pressure/Strain Sensors Based on 3D Conductive Materials

Lujie Fan; Li Chen; Yin He; Hao Liu

doi:10.7536/PC200616

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (5) : 767-778 DOI: 10.7536/PC200616

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Flexible Pressure/Strain Sensors Based on 3D Conductive Materials

Lujie Fan ¹
, Li Chen ¹
, Yin He ¹
, Hao Liu ¹^,²^,^*

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Abstract

As an important part of intelligent wearable device, flexible pressure/strain sensors are widely used in human motion detection, health monitoring, robot and electronic skin. Among them, 3D conductive materials(such as aerogels, sponges and foams) have 3D interconnection microstructures and have excellent compressibility and conductivity, which bring a breakthrough that enables sensors to have high sensitivity and wide range. According to the raw materials for the preparation of 3D conductive materials, this article divides them into five types: biomass materials, carbon materials, polymer materials, metal nanomaterials, and MXene materials. Then, the preparation methods of 3D conductive materials and their innovation and development in the field of sensor applications are summarized. Finally, the prospect of flexible strain/pressure sensors based on 3D materials is discussed.

Contents

1 Introduction

2 Basic characteristics of flexible strain/pressure sensors

3 Preparation of 3D material and its application in flexible strain/pressure sensors

3.1 3D biomass materials

3.2 3D carbon materials

3.3 3D polymer materials

3.4 3D metal nanomaterials

3.5 3D Mexene materials

4 Application

5 Conclusion and prospect

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Keywords

3D conductive materials / flexible pressure/strain sensors / human monitoring / electronic skin and robots

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Lujie Fan, Li Chen, Yin He, Hao Liu. Flexible Pressure/Strain Sensors Based on 3D Conductive Materials. Progress in Chemistry, 2021, 33(5): 767-778 DOI:10.7536/PC200616

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1 Introduction

Flexible stress/strain sensors have many potential applications, such as personalized health monitoring, human motion detection, electronic skin and robotics, and their demand is growing^{[1⇓⇓⇓~5]}. These flexible sensors can be integrated into electronic devices or attached to clothing and skin to monitor the health of the human body by detecting pulse pressure, facial expressions and other subtle pressures related to human activities^{[6⇓⇓⇓~10]}. Flexible stress/strain sensors convert mechanical deformation (such as tension or compression) into electrical signals, and most have been developed based on three conversion mechanisms: resistive, capacitive, and piezoelectric^[11⇓~13]^[14,15]^[16,17]. When the pressure changes, the resistance, capacitance and voltage of the above sensors will change accordingly. The flexible stress/strain sensors introduced in this paper are mainly capacitive and resistive.

In practical applications, flexible stress/strain sensors are required to have high sensitivity, high stability and wide detection limits. In order to further improve the performance of the sensor, researchers have made efforts from the aspects of materials and structures. A variety of geometrically structured sensors have been reported, such as pyramidal structure, interlocking structure, hemispherical structure, wrinkled structure and irregular template structure, which improve the sensitivity and detection limit of the sensor to a certain extent, but have the disadvantages of high price and complex preparation process^[18]^[19]^[20]^[21,22]^[23,24]. 3D conductive materials, such as aerogel, sponge, foam and so on, due to their conductive interconnection network structure and excellent compressibility, can sense less pressure and produce more contact or separation when they are stressed, thus obtaining higher sensitivity sensors. At the same time, 3D conductive materials can withstand large deformation and generate continuous electrical signals accordingly, thus maintaining good sensitivity in a wide range of pressures, and their development prospects in the field of sensors have attracted much attention^{[25⇓⇓~28]}. The sensing scheme of the flexible stress/strain sensor based on three-dimensional materials is shown in Figure 1.

2 Basic characteristics of flexible stress/strain sensor

The structure of a flexible stress/strain sensor usually consists of a sensing element, electrodes and wires, and the performance of the sensing element plays a decisive role in the sensor. Excellent mechanical elasticity, compressibility and good electrical conductivity are the basic requirements for sensitive elements used in flexible stress/strain sensors.

3D conductive materials have high porosity, low density, large specific surface area and excellent compressibility and conductivity. They are one of the research hotspots in the field of materials science because of their wide variety, simple preparation methods and excellent properties. In recent years, the raw materials for the preparation of 3D conductive materials mainly focus on carbonizable conductive biomass materials, carbon materials, polymer materials, metal nanomaterials and MXene^[29⇓~31]^[32⇓~34]^[35⇓~37]^[38⇓~40]^[41⇓~43]. The preparation method mainly comprises two types: (1) one or more materials are mixed and assembled into the aerogel by a sol-gel method combined with a freeze-drying method. Conductive electrical gel can be obtained directly by doping conductive materials in the gel, or it can be endowed with conductivity by carbonization or coating conductive materials. When aerogels are prepared by freeze-drying, the liquid in the aerogels is replaced by air, and the microstructure is preserved. (2) The conductive material is directly coated on the 3D sponge, for example, a commercially available polyurethane (PU) sponge is coated with a cellulose nanofiber @ carbon black (CNF @ CB) conductive layer to obtain a 3D porous structure pressure sensor with high sensitivity and multiple functions^[44]. As a flexible sensing material, 3D conductive material has solved the problem of high sensitivity and wide detection limit to a certain extent^[45].

Flexible stress/strain sensors have several key performance parameters, including sensitivity, detection limit, stability, and response time. Sensitivity is one of the important indexes to evaluate the sensor, which determines the measurement accuracy of the sensor and can be expressed by the specification factor GF and the sensitivity S. The sensitivity is calculated as follows:

(1)

GF = Δ X - X 0 Δε

(2)

S = Δ X - X 0 Δ P

Where ΔX is the change in resistance or capacitance,

X 0

is the initial resistance or initial capacitance, Δε is the change in strain, and ΔP is the pressure change. The detection limit of pressure sensor refers to the pressure range that the sensor can stably detect, including the lowest detection limit and the highest detection limit. According to the needs of practical applications, such as the detection of human pulse pressure and foot pressure, the sensor needs to have a wide range of detection. Stability includes repeatability and repeatability error. The higher the repeatability of the sensor is, the smaller the repeatability error is, and the higher the stability is. Response time refers to the shortest time consumed by the sensor from force to response. The shorter the response time is, the more sensitive the sensor is.

In this paper, the flexible stress/strain sensors based on 3D conductive materials reported at home and abroad in recent years are reviewed. According to the raw materials, the sensors can be divided into five types: biomass materials, carbon materials, polymer materials, metal nanomaterials and MXene materials.The preparation methods, properties and applications of the five types of sensors are summarized, and the challenges and future application prospects in this field are discussed.

3 Fabrication of 3D Materials and Their Applications in Flexible Stress/Strain Sensors

3.1 3D biomass material

With the depletion of energy on the earth, renewable biomass materials (such as nano-cellulose, wood, bacterial cellulose, etc.) Have become a key part of the research of new functional materials because of their unique biocompatibility, sustainability, low toxicity and renewability. Biomass aerogels have been widely used in adsorption, supercapacitors, energy storage, sensors and other fields due to their simple preparation process, low cost and excellent mechanical properties^[46⇓~48]. Table 1 summarizes the flexible stress/strain sensors based on 3D biomass aerogel materials in recent years, including preparation methods, sensing methods, sensitivity, minimum detection limit and detection range, repeatability and other indicators.

3.1.1 Nano cellulose fiber aerogel

There are a large number of hydroxyl groups on the surface of nanocellulose fibers, and stable 3D network structure can be obtained through intramolecular and intermolecular physical crosslinking of hydrogen bonds. However, due to the existence of hydrogen bonds, it is difficult to dissolve in common solvents, and can be dispersed uniformly by mechanical methods such as high-pressure homogenization, high-power ultrasonic dispersion and other physical methods. TEMPO oxidation method can also be used to carboxylate hydroxyl molecules, weaken hydrogen bond force and promote mechanical decomposition^[49]. Then, aerogels with 3D network structure were prepared by sol-gel method combined with supercritical drying or freeze-drying.

In The past few decades, many researchers have adopted a bottom-up approach to extract cellulose nanofibers, such as Zhou et al., who separated CNF from pure rice straw by TEMPO oxidation and mechanical blending.4-ethylenedioxythiophene)/poly (styrenesulfonate) (PEDOT/PSS), in which the SO₃H groups of PSS are hydrogen bonded and protonated to form a PEDOT/PSS/CNF aerogel with a highly conductive network structure. Fig. 2 (a) shows the preparation process of the PEDOT-PSS/CNF aerogel^[50]. Experiments show that the conductivity of PEDOT/PSS/CNF aerogel is 10 times higher than that of pure PEDOT/PSS aerogel. According to the different dissolution systems of cellulose, Luo et al dissolved cellulose in sodium hydroxide/urea solution, added 4% combed cotton and 80% Na₂SO₄ solution as reinforcing fiber and pore forming agent, and freeze-dried cellulose aerogel under the best conditions^[51]. Subsequently, pyrrole was in-situ polymerized on the cellulose aerogel to obtain a conductive polypyrrole/cellulose aerogel pressure sensor with a maximum sensitivity of 58.9 kpa^-1 during compression.

3.1.2 Wood Carbon Sponge

Unlike the above methods, in 2018, Chen et al., through a scalable and sustainable top-down approach, directly processed natural balsa wood into highly lightweight, compressible and conductive wood carbon sponge through simple chemical treatment and high temperature carbonization for the first time and applied it to the sensor field, but the strain sensor failed to detect human pulses^[53]. Taking advantage of the different structures of wood, Huang et al. Also used simple chemical methods and carbonization processes to treat balsa wood, and pumped PDMS into the channels of wood sponge to convert rigid non-conductive bulk wood into flexible and conductive carbon/silicon composites.A flexible pressure sensor with high sensitivity (10.74 kpa^-1), wide linear range (up to 100 kpa), fast response time (20 ms) and good working stability (less than 2.89% variation in 13 000 cycles) is realized, which can accurately and continuously measure pulse and respiration^[45].

3.1.3 Bacterial cellulose aerogel

Bacterial cellulose (BC) is a natural biomass with ultra-fine nanostructure, high crystallinity, high purity, good flexibility and high tensile strength. Compared with natural cellulose, BC does not contain hemicellulose, lignin and other impurities, and its purification is simple. Hosseini et al. Converted in-situ biosynthesized BC/multi-walled carbon nanotube (MWCNT) nanocomposite hydrogel into conductive nanocomposite aerogel by supercritical osmium method, and prepared a strain sensor for monitoring human movement^[54]. The strain sensor has a GF of 21 and a response time of 390 ms. Although BC has excellent performance, its preparation process is too cumbersome and time-consuming.

3.1.4 Polysaccharide aerogel

Chitosan (CS) and konjac glucomannan (KGM) were used to prepare aerogels and applied to flexible stress/strain sensors. For example, Liu et al. prepared an ultra-light, porous and highly conductive composite sponge from CS/carbon black by solution mixing and freeze-drying, which can be used to detect different sounds^[55]. The results showed that the compressive modulus and compressive strength of the composite sponge increased with the increase of carbon black content, but the viscosity of the solution increased with the increase of carbon black content, which made it difficult to pour into the mold during the preparation of the sponge. In order to improve the compressibility and elasticity of the biomass aerogel, inorganic nano-materials can also be added as the aerogel skeleton. Ding et al. Obtained flexible SiO₂ nanofibers by electrospinning technology, and mixed them with KGM to prepare super-elastic carbon nanofiber aerogel with ordered honeycomb structure. The SEM image is shown in Figure 2 (B)^[52]. The aerogel has a very low density (0.14 mg/cm³) and has been prepared into pressure sensors with detection limits as low as 10 pa.

3.2 3D carbon material

Carbon materials have always been an indispensable resource in the progress of human civilization. The efficient synthesis and application of carbon materials and their derivatives are also promoting the progress of science and technology and improving the quality of human life. In order to overcome the shortcomings of powder carbon materials in practical applications, 3D bulk graphene aerogel (GA), carbon nanotube aerogel (CA) and their composite aerogels have emerged. However, due to the brittleness of carbon, its resistance to fatigue is generally limited when high compressive strains are applied. In order to realize the compressibility of carbon aerogels, various methods have been developed, such as chemical vapor deposition, freeze-drying, template removal, 3D printing and chemical bonding, to prepare compressible carbon aerogels and apply them to the field of sensors^[56]^[57⇓~59]^[60]^[61]^[62]. Table 2 summarizes the flexible stress/strain sensors based on 3D carbon materials.

3.2.1 Graphene aerogel

Graphene aerogels have attracted much attention in the field of sensing due to their excellent electrical conductivity and mechanical flexibility. However, graphene aerogels have poor resilience due to the aggregation of graphene sheets caused by π-π interaction and van der Waals force between graphene sheets in aerogels.

In order to improve the flexibility of graphene aerogels, some flexible polymer materials can be combined with graphene sheets to form 3D porous structures. Huang et al improved the compression properties of graphene aerogels by adding carboxymethyl cellulose. There is a strong hydrogen bond interaction between graphene sheets and carboxymethyl cellulose chains. After high temperature annealing, a stable conductive network is formed, showing stable high elasticity^[57]. Zhou et al. Developed a graphene aerogel combined with low molecular weight carbon precursors such as cellulose nanocrystals and urea^[58]. As shown in Fig. 3 (a), the large space between the wavy rGO layers ensures high compressibility, and the strong interaction (carbon welding) between the low molecular weight carbon precursor and rGO forms a flexible layer capable of reversible compression, with a compressive strain as high as 99%, and a small change of 0.25 Pa can be detected. Zhang et al. Used a simple and rapid method to remove the PU template to obtain redox graphene aerogel, which completely replicated the internal structure of PU sponge^[60]. In addition, Zu et al. Of Tongji University reported an unprecedented super-hydrophobic redox graphene (rGO)/polyorganosiloxane aerogel^[63]. The aerogel has a coral-like three-dimensional network structure, and the preparation of a flexible multifunctional sensor for simultaneously detecting temperature, strain and pressure by using a single material is realized.

3.2.2 Carbon nanotube aerogel

Initially, carbon nanotube aerogels were prepared by chemical vapor deposition, but the high cost and toxicity of by-products limited their application. Due to the high surface energy of carbon nanotubes, carbon nanotubes are easy to agglomerate when carbon nanotubes aerogel is prepared by sol-gel method. Carbon nanotubes can be dispersed into the solution containing surfactant (sodium dodecyl benzene sulfonate) by ultrasonic wave to achieve uniform dispersion^[64]. Chen et al. Prepared polyimide (PI)/CNT aerogel by chemical bonding method. The precursor of PI (PAA) has abundant functional groups, which is conducive to strong molecular interface interaction with carbon nanotubes, and achieves good dispersion of carbon nanotubes in water^[62]. The aerogel has an ultra-high sensitivity (11.28 kPa^-1), a detection limit as low as 10 Pa, a fast response time of 50 ms, and a recovery time of 70 ms. PI/CNT aerogel sensors can be used to monitor various human movements and make electronic skin.

3.2.3 Graphene and carbon nanotube composite aerogel

When carbon nanotubes and graphene are used as aerogels alone, their elasticity is relatively poor. The combination of carbon nanotubes and graphene will produce a synergistic effect, which not only maintains the pure carbon component of sp² hybridization, but also has a very low density. Peng et al. Developed a CNT/rGO/CNF carbon aerogel with multi-layer arch structure and excellent mechanical properties by bidirectional freezing method^[59]. The results show that the existence of CNF is helpful to obtain continuous and ordered wavy carbon layers, and the connection effect of CNT can significantly improve the mechanical strength of carbon layers. Gao Chao of Zhejiang University and others used 3D printing method to extrude uniform GO-Mwcnt gel ink through a movable nozzle (diameter of about 250 μm) and deposit it into the program-controlled 3D structure, thus realizing the preparation of pure carbon porous material stretchable aerogel, as shown in Fig. 3 (B)^[61]. The prepared carbon aerogel has strong fatigue resistance, the tensile rate can reach 200%, and the use cycle times can be as high as 10⁶.

3.3 3D polymer material

Compared with natural polymer materials, synthetic polymer materials can meet various functional needs according to demand. In the field of flexible sensors, flexible polymers are usually used as flexible substrates of sensors. Common flexible polymers include polydimethylsiloxane (PDMS), polyvinyl alcohol (PVA), PU, thermoplastic polyurethane (TPU) and so on^[65]^[66,67]^[68]. Polymer materials can be converted into 3D materials by freeze-drying, electrospinning, template and 3D printing. Table 3 summarizes the flexible stress/strain sensors based on 3D polymer materials.

Among many polymers, PDMS is widely used in the field of sensors because of its excellent properties. Fig. 4 shows SEM images of PDMS sponges prepared using sugar cubes, polystyrene spheres (PS), and 3D printing. It can be seen from the image that PDMS has strong plasticity and can be prepared into sponges with various pore structures.

In addition, Kou et al. Doped NH₄HCO₃ powder as a porogen into PDMS/GR composites (NH₄HCO₃ decomposes into gas when heated at high temperature), and prepared a conductive PDMS sponge capacitive pressure sensor with uniform pores^[14]. Huang et al. Prepared PVA/copper nanowire (CuNW) composite aerogel by freeze-drying method, with compressive strain up to 60% and adjustable sensitivity (0.267~0.04 kpa^-1)^[69]. In order to study the bonding mechanism between copper nanowires and polymers and better understand why CuNW-PVA composites have good mechanical properties, they used molecular dynamics to simulate the stress-strain behavior of the composite model. Sang et al. mixed carbon nanostructures (CNS) and TPU powder with commercially available popcorn salt by a simple thermoforming method, and then prepared a composite porous sponge by a water-assisted desalination method, as shown in Figure 5^[68]. The obtained TPU/CNS sponge sensor has a linear strain range of up to 60% and a GF of 1.5. Due to the support of the TPU porous sponge, a stable conductive path is formed, thereby having reversible piezoresistive properties.

The 3D flexible polymer material can be endowed with excellent conductivity by coating, ion sputtering, carbonization and other methods. Zhang et al. Obtained conductive PU sponges coated with CNF/AgNW by a simple impregnation process^[66]. The conductivity and the mechanical properties of the conductive sponge can be effectively adjusted by changing the dipping times, and the sponge has the capability of detecting the size of motion in a compression strain range of 0 to 80%. Wu et al. Used ion sputtering technology to prepare conductive gold-plated PU sponge, and obtained pressure sensors with adjustable crack density through different pre-compression strains^[67]. Commercial melamine foam (MF) is an environmentally friendly material with a density in the 9.87±0.1 mg·cm^-3 range and a porosity greater than 99%^[74]. After carbonization, MF is composed of a unique 3D interconnected triangular carbon network and a small part of broken carbon fibers, which has a rich microporous and mesoporous structure, and can be used as a flexible pressure sensor to detect human movement in real time and reliably^[70].

3.4 3D metal nanomaterial

Metal nanowires and nanoparticles are used as conductive materials in the field of flexible sensors due to their good conductivity. In recent years, CuNWs, AgNWs and gold nanowires (AuNWs) have been successfully synthesized and applied in different fields, among which the raw materials for the preparation of CuNWs are relatively widely distributed and low-cost^[75]. Metal nanomaterials can not only be coated on 3D materials as conductive materials, but also form 3D porous materials through self-assembly. Table 4 summarizes the flexible stress/strain sensors based on 3D metallic nanomaterials.

Weng et al. Used a simple coating process to coat AgNW on a commercially available PU sponge to prepare a flexible sensor with good signal response under tensile, compressive, bending and other deformations^[76]. Before coating with AgNWs, the buckled structure was introduced by thermal compression and the sponge scaffold was modified with polydopamine (PDA) to improve the adhesion between the nanowires and the scaffold. The obtained conductive sponge has remarkable deformability, conductivity and stability, also shows interesting negative Poisson's ratio, and can meet the multi-scale mechanical requirements of the soft curve of the human body. Zhao et al. proposed a novel thermal sensing based scalable pressure sensor using a multilayer porous PDMS/silver nanosheet (AgNP) sponge as a piezoelectrically conductive medium^[77]. The sensor has a high sensitivity with a detection limit as low as 4.1 Pa and a wide detection range up to 200 kPa. The pressure sensor and the temperature sensor are integrated to realize temperature compensation.

Xu et al. Successfully prepared CuNW aerogels by one-step method, and first proposed the "bubble controlled assembly" mechanism, which realized the adjustable pore structure and density (4.3~7.5 mg·cm^-3) of copper nanowire aerogels^[78]. In the process of forming hydrogel, one-dimensional nanowires with large aspect ratio accumulate and link with each other, forming a strong self-assembled sponge after freeze-drying. Subsequently, ultra-light flexible pressure sensors with tunable sensitivity (0.02~0.7 kPa^-1) were fabricated using CuNWs aerogels. The experimental results show that the pore structure and density have a great influence on the performance of the sensor. However, the elastic modulus of the sensor is small, and it can only detect the micro-pressure within 210 pa, so its application range is limited. In addition, although CuNWs have the same photoelectric properties as AgNWs, they are easily oxidized in air. To this end, Wu et al. mixed the synthesized CuNW with water-soluble polyvinylpyrrolidone (PVP) and then freeze-dried to assemble a 3D aerogel. After in-situ annealing, the PVP attached to a single CuNW was converted into a thin graphene protective layer, thus forming a unique CuNW @ graphene core-shell nanoskeleton to construct a three-dimensional porous aerogel^[79]. Because the graphene shell is hydrophobic, it can be used as an effective barrier for oxygen and water molecules, which is conducive to the stability of aerogel performance. At the same time, Meng et al. Obtained core-shell Cu-AgNWs with the best oxidation resistance through a simple electrochemical displacement reaction without any heating, stirring and reducing agents^[80]. Inspired by biomaterials, they proposed a highly sensitive, scalable strain sensor based on a pearl-like "brick and paste" structure, and successfully prepared a pearl-like conductive composite with rGO and Cu-AgNWs as "bricks" and scalable polystyrene-butadiene-styrene-butadiene block (SBS) as "paste". The composite material is applied to a tensile sensor and has wide sensing range (374%) and high sensitivity (GF is up to 87%).

3.5 3D Mxene material

MXene is a novel two-dimensional (2D) material with a large hydrophilic surface, excellent thermal stability, ease of handling, and high electrical conductivity. MXene can be prepared by etching the MAX phase, and the morphology of the crystal can be effectively controlled by adjusting the etching method to form nanoparticles, monolayers or multilayer nanosheets. Research shows that MXene and its composites show excellent flexibility and ductility, which makes them widely used in wearable sensors, energy storage, electrochemical catalysis and electromagnetic shielding^{[81⇓⇓⇓~85]}. Table 5 summarizes the flexible stress/strain sensors based on 3D MXene materials.

Ma et al. Found that the conductivity of Ti₃C₂ varies with the distance between layers, which makes it a promising material for piezoresistive sensors^[86]. However, due to the low aspect ratio and the weak interaction between MXene sheets, it is difficult to assemble MXene nanosheets into continuous macrostructures with ideal mechanical and sensing properties. In order to solve this problem, Zhuo et al. Used cellulose nanocrystals (CNCs) as nano-supports to connect MXene nanosheets into sheet-like carbon aerogels with super mechanical properties and ultra-high linear sensitivity^[87]. The interaction between MXene and CNCs produces a hyperstable wavy lamellar structure. It is capable of withstanding extremely high compressive strains of 95% and long-term compression (10,000 cycles, 50% strain), demonstrating extremely high sensitivity (114.6 kPa^-1) and a wide linear range (50 Pa to 10 kPa). They also used BC as a nano-binder to connect MXene nanosheets into a layered structure to produce carbon aerogel with extremely high mechanical and sensing properties, and its ultra-high structural stability can withstand 99% strain and more than 100 cycles^[88].

Yue et al. Used PU sponge as a flexible substrate to prepare MXene sponge by coating process, which has high sensitivity of 147 kpa^-1, low detection limit of 9 pa, fast response time of 138 ms and durability of 10 000 cycles^[89]. MXene sponge sensors can monitor human physiological signals (such as breathing, joint movement and pulse) in real time. In order to enhance the interfacial interaction between MXene sheet and sponge skeleton, Li et al. Treated PU sponge with CS to obtain CS @ PU sponge with positive charges^[90]. The negatively charged MXene flakes were then coated. The PU sponge after CS treatment can cover more MXene sheets and has higher conductivity. MXene @ CS @ PU sponge sensors have good reversibility, can withstand 5000 compression cycles, and have good sensing performance even after washing in water, indicating good mechanical and environmental stability. Zhu et al. Spontaneously wrapped negatively charged MXene nanosheets on the surface of positively charged PS spheres through electrostatic self-assembly^[91]. MXene/PS spheres were added to the graphene oxide (GO) solution with vigorous stirring. A novel piezoresistive sensor based on hollow MXene sphere/rGO composite aerogel was prepared by freeze-drying the MXene/PS/GO mixture and annealing at 450 ℃ in argon atmosphere to remove the PS sphere template.

4 Application

In recent years, the problem of population aging in China has become increasingly serious, and the health monitoring of the elderly has aroused great social concern. The application of sensors in the medical industry is a simple and effective way to reduce the pressure of medical treatment while preventing and monitoring the health problems of the elderly in real time. In order to explore the application of sensors in medical devices, Huang et al attached the sensors to the wrist of the human body to collect arterial pulse signals, and integrated them into bandages to monitor the respiratory signals of the chest^[45]. As shown in Fig. 6 (a, B), the sensor can distinguish three different peak values of pulse pressure (P₁, P₂, P₃). Physiologically, the augmentation index (AI_r=P₂/P₁) generally increases with age due to the degree of arterial stiffness. According to the calculated enhancement index, a person's physiological age can be inferred. In addition, the respiratory signals captured by the sensors applied to the bandage are consistent with those measured by standard commercial physiological monitoring systems, which can be used for early diagnosis of pneumothorax. As shown in figs. 6 (e-f), the flexible stress/strain sensor can also be used to detect human motion, recognize voice, and facial expressions. Wearable sensors collect physiological data from the human body, such as breathing rate, heartbeat, movement, etc., and transmit the data to medical centers, doctors and family members through wireless networks, which can provide medical assistance in emergencies^[92].

Another important application direction of flexible stress/strain sensors is electronic skin and robots^[94,95]. In order to simulate the multiple tactile receptors of human skin, the sensors can be assembled into a sensing array in combination with a flexible substrate. Flexible multi-functional pressure sensors with different sensing modules are studied, which can identify multiple stimuli from complex environments. As shown in Figure 7 (a), Zu et al. attached the sensor array to the surface of a transparent glass filled with hot water, and could intuitively see the temperature distribution of the measuring cup^[63]. Insects can be placed on a planar sensor array to clearly monitor their crawling trajectories. However, the current flexible stress/strain sensors based on 3D conductive materials are too single and have a certain thickness, and there are still many challenges in the actual use process, which need our continuous innovation and research. Flexible stress/strain sensors can be integrated in different robotic systems, such as sensing systems for prosthetic limbs, hand grips, mobile robots, and robotic tentacles. As shown in Figure 7 (B), Sencadas et al. placed the prepared sensor on a robot hand. When the robot hand grabbed an object, the resistance of the sensor decreased under stress, and after the object was released, the resistance of the sensor returned to its original value^[95]. The robot hand can grasp objects of different shapes and textures. Zhang et al. Made a sensor sample according to the shape of the robot hand, and sewed a "coat" for it with black Lycra fabric with good insulation as a protective layer^[96]. It is worth mentioning that the shape of the sensor can be made into any shape as required. The capacitance value of the sensor can be obtained through experiments, and the value is used as the threshold value for stopping the action. According to the program, the robot can do the experiment automatically. 3D smart sensing materials combine sensing and actuation, providing an opportunity to design new robots to challenge the limitations of traditional electromechanical systems.

5 Conclusion and prospect

In recent years, researchers have made great progress in the research of flexible stress/strain sensors based on 3D conductive materials. In this paper, the latest research progress of flexible stress/strain sensors based on 3D conductive materials in recent years is reviewed, which are divided into five types: biomass materials, carbon materials, polymer materials, metal nano-materials and MXene materials, and the related preparation methods are introduced. A large number of experiments show that 3D conductive materials can significantly improve the sensitivity and detection limit of flexible stress/strain sensors. In addition, the applications of sensors in human health monitoring, motion detection, electronic skin and robotics are also introduced.

Flexible stress/strain sensors based on 3D conductive materials have shown many advantages in the field of flexible wearable due to their special 3D interconnection microstructures, but it is still a great challenge to achieve flexible stress/strain sensors with high sensitivity and wide detection limits. According to the application requirements of artificial intelligence, biomedicine and other fields, the future development trend of flexible stress/strain sensors based on 3D conductive materials is mainly manifested in the following aspects. First, it is still necessary to prepare a highly sensitive sensor with a wide detection limit. Secondly, in order to realize the wide application of flexible stress/strain sensors in normal life, we need to strive to develop flexible stress/strain sensors with low cost, high performance and mass production. It solves the problems of power supply and signal transmission while realizing the portability, aesthetics and practicability of the product. Finally, in order to meet the application requirements in the field of electronic skin, it is particularly important to develop sensor arrays that can detect various physical quantities at the same time, and it is an urgent problem to endow flexible stress/strain sensors with versatility.

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