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Preparation and Application of Micro-Structured Elastomer Dielectric Layer

Jinhua Liao , Jiajun Gao , Yuchao Wang , Wei Sun

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (6) : 975 -987.

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Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (6) : 975-987 DOI: 10.7536/PC201114
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Preparation and Application of Micro-Structured Elastomer Dielectric Layer
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Abstract

Micro-structured elastomer films are elastomer films endowed with pores or patterned arrays of specific structures on the top layer or in the bulk. Such films have been extensively used as dielectric layers for flexible electronics. In this review, both the fabrication techniques and the applications of the micro-structured elastomer dielectric layers are introduced. Different types of elastomers used as dielectric layers are firstly introduced. Fabrication methods of micro-structured elastomer dielectric layers with porous and non-porous arrays are summarized, including sodium chloride template method, sugar template method, bicarbonate template method, microsphere template method and silicon template method. The applications of the micro-structured elastomer dielectric layers in stress-strain sensors and nano-generators are also illustrated.

Contents

1 Introduction

2 Types of elastomer dielectric layers

3 Preparation of micro-structured elastomer dielectric layers

3.1 Porous elastomer dielectric layers

3.2 Elastomer dielectric layers with non-porous arrays

4 Application of micro-structured elastomer dielectric layers

4.1 Sensing applications of micro-structured elastomer dielectric layers

4.2 Applications of micro-structured elastomer dielectric layers in nano-generators

5 Conclusion and outlook

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Keywords

micro-structured elastomer / dielectric layer / sensor / nano-generator

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Jinhua Liao, Jiajun Gao, Yuchao Wang, Wei Sun. Preparation and Application of Micro-Structured Elastomer Dielectric Layer. Progress in Chemistry, 2021, 33(6): 975-987 DOI:10.7536/PC201114

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1 Introduction

In recent years, with the explosive growth of smart wearable devices, flexible electronic devices have attracted more and more attention. At the same time, elastomer materials such as polydimethylsiloxane (PDMS), polyurethane (PU) and styrene-butadiene-styrene block copolymer (SBS) have gradually become the key dielectric materials for the construction of flexible electronic devices due to their lightness, transparency, flexibility, insulation and corrosion resistance[1⇓ ~ 3]. Taking the flexible pressure sensor as an example, the elastomer material can be compounded with conductive fillers such as carbon nanotubes (CNT), metal particles, metal nanowires and graphene (GR) to prepare a dielectric layer film, and the sensor device constructed thereby can be widely used in the fields of flexible electronic skin, mobile medical assistance, energy harvesting, flexible touch screens, artificial limbs and the like[4⇓⇓ ~ 7][8,9][10⇓ ~ 12][10,13][14][15].
With the development of flexible electronic devices, there are more and more application scenarios, and higher requirements for device performance are also put forward. For example, for flexible pressure sensors, how to obtain as high sensitivity as possible under very low pressure and in a wide range of pressure measurement has become the direction of researchers. However, the Triboelectric Nano-generator (TENG), which uses triboelectricity and electrostatic induction to collect energy from the surrounding environment, is still limited to a low output power. How to improve its output performance through structural design has become an important issue in this application direction. In order to improve the performance of these applications, more and more researchers have adopted the method of fabricating porous or specially shaped arrays of micro-nano-scale structures on the surface or inside of elastomers to regulate the physical properties of elastomers, thereby improving the performance parameters of devices[16,17][1,18,19]. For example, in pressure sensor applications, a microstructured elastomeric film can achieve greater compressibility and dielectric constant adjustability than an unstructured film, thereby achieving an increase in sensing sensitivity[20,21]. The micro-structured elastomer dielectric layer can also provide a larger friction area for TENG, reduce the effective thickness, and ultimately achieve higher output power[22,23].
At the beginning, the research on microstructured elastomeric dielectric layers was mainly represented by some simple array structured designs proposed by Bao et al., but in recent years, more and more experimental research work has been done in this field[24]. At present, in addition to pyramidal, cylindrical, linear and regular array types, porous, nanoneedles, microvilli, interlocking micro-dome arrays, micro-domes and cross-reticulated structures, as well as rough structures replicated by natural biological templates such as lotus leaves and rose petals have also been reported[16][25][26][27][18,28,29][30][31][32]. With the development of preparation technology of micro-structured elastomer dielectric layer, its applications in sensor, nano-generator and other fields are developing. However, there are few systematic reviews on the preparation methods of microstructured elastomeric dielectric layers and their different applications. In this paper, the recent progress in this field is reviewed from three aspects: the types of elastomeric dielectric materials, the preparation methods of micro-structured elastomeric dielectric layers, and the applications of micro-structured elastomeric dielectric layers in the fields of sensing and nano-power generation.

2 Types of elastomeric dielectric layer material

Because the elastomer has higher compressibility and lower Young's modulus, it can obtain greater deformation under the action of stress, which can lead to greater capacitance change in electronic devices. Therefore, efforts have been made to use elastomers with low Young's modulus as dielectric layer materials for microelectronic devices such as sensors, nanogenerators, actuators, and the like. The main types of elastomeric materials include polydimethylsiloxane, polyurethane, and acrylate elastomers.
(1) polydimethylsiloxane
A main chain of that polydimethylsiloxane is an Si-O bond, molecular chain are relatively flexible, and the interaction force between the molecular chains is weak, so that the polydimethylsiloxane has the advantage of low elastic modulus, high thermal stability, good elasticity, high deformation response speed, excellent electrical insulation performance, long service life and the like,And are suitable for compounding with a variety of fillers to achieve optimization of their electrical and mechanical properties, attributes that make polydimethylsiloxane the material of choice for elastomeric dielectric layers[33]. However, the dielectric constant of polydimethylsiloxane is relatively low, ranging from 2 to 3, which leads to its low energy density and requires a higher driving voltage. Therefore, it is necessary to modify polydimethylsiloxane or add conductive fillers to increase the dielectric constant to improve its application performance[34⇓ ~ 36].
(2) Polyurethane
Polyurethane, also known as polyurethane, is a polymer whose backbone contains repeating structural units of carbamate (— NHCOO —). Because it contains a large number of polar groups, its dielectric constant is higher than 7, so it is an ideal dielectric material[37]. However, the electrical breakdown strength of polyurethane is too low and the insulation is poor. In addition, the polyurethane molecule has not only the chemical crosslinking between the chain segments of the long molecular chain, but also the physical crosslinking of a large number of internal crystalline regions, which leads to its high Young's modulus, so that it can not be greatly pre-stretched like silicone rubber and acrylate materials, so it is difficult to solve the problem of electromechanical instability that may occur during work[38].
(3) acrylate elastomer
The dielectric property of the acrylate material is very excellent, the dielectric constant is about 4.7, the breakdown strength is high, the adhesion of the acrylate elastomer and the flexible electrode is good, the price is low, and the acrylate material can be used without processing[39].
Although the dielectric properties of acrylate elastomers are excellent, they can only be achieved under the condition of large pre-strain, which often causes mechanical and electrical instability, stress relaxation and other application problems at the junction of elastomers and rigid bodies.Moreover, due to the viscoelasticity of the acrylate elastomer, the electromechanical response of the actuator prepared by using the acrylate elastomer as a dielectric layer is poor, the efficiency is low, and the response time is long[33,40].

3 Preparation of Microstructured Elastomeric Dielectric Layer

By introduce a micro-size physical rough structure into that compact elastomer film by a physical or chemical means, the prepare micro-structured elastomer can obtain higher compressibility and larger stress deformation than a compact elastomer dielectric lay[26,27]. Taking the application of capacitive pressure sensor as an example, under the action of external pressure, the air in the rough structure of the microstructure elastomer dielectric layer is slowly squeezed out.O that the effective dielectric constant of the dielectric layer is significantly increased, and the capacitance of the sensor is further improved, and therefore, the sensing sensitivity is greatly improved compared with the conventional dense dielectric layer sensor. In the application of the triboelectric nano generator, the advantages of large specific surface area and good compressibility of the microstructure elastomer dielectric layer are utilized, so that the prepared generator has large friction area, small effective thickness and high capacitance value, and the triboelectric power output is greatly improved compared with that of a traditional elastomer dielectric layer.
Microstructured elastomeric dielectric layers can be divided into two types: porous and non-porous arrays. Wherein the porous elastomer dielectric layer is an ordered or disordered porous structure constructed on the surface or the bulk layer, and the preparation method comprises a soluble micro-nano particle template method, a liquid phase separation method, a vapor sealing casting method, a microwave treatment method and the like[41]. The non-porous array elastomer dielectric layer is an elastomer film with a raised array structure on the surface, and is an elastomer dielectric layer with a raised structure, such as a pyramid shape, a line shape, a column shape and the like, which is reproduced on the surface of a silicon chip with a rough array structure[42][31]. Methods for preparing the porous and non-porous array type elastomeric dielectric layers will be described in detail below.

3.1 Method for preparing porous elastomer dielectric layer

3.1.1 Solid template method

(1) NaCl template method
The NaCl template method is a method for preparing porous elastomer materials by blending NaCl particles with the elastomer or extruding the NaCl particles into cubes and immersing the cubes in the elastomer, and dissolving the NaCl particles or the cubes after the elastomer is cured to introduce a pore structure[43,44]. Du Qing prepared porous PDMS dielectric layer by NaCl template method and used it as a capacitive flexible pressure sensor.The effects of filling amount of NaCl particles and grinding time on the sensitivity of the sensor were studied, and the increase of grinding time resulted in the decrease of NaCl particle size[45]. Under the conditions of 20 wt% loading of PDMS particles and grinding time of 45 min, the highest sensitivity of the capacitive sensor can be obtained, which is up to 0.76 kPa-1 and more than three times higher than that of the non-porous PDMS film sensor. Zhu et al. Used NaCl particles and regularly arranged metal needles as templates to prepare PDMS dielectric layers with both through channels and micropores[46]. The sensor composed of the highly compressible dielectric layer and the filter paper coated with highly conductive polypyrrole (PPy) has high sensitivity (1.15 kPa-1) and fast response/recovery behavior (response time < 125 s) in a wide pressure range (5 Pa ~ 1 MPa)[17]. Tian et al. Extruded salt particles into salt cubes, immersed them in PDMS prepolymer, and cured them in a vacuum environment (the elastomer penetrated into the gaps of the salt cubes under the action of capillary force), and then dissolved the salt cubes to obtain a porous PDMS dielectric layer and fabricated a wearable pressure sensor (Fig. 1)[47,48]. The pore size obtained by pore-forming of the salt cube is increased by an order of magnitude compared with that of the common salt particle condition, and thus a sensing sensitivity of up to 1.096 kPa-1 is obtained.
(2) sugar template method
The sugar template method is to drip the elastomer into sugar cubes or blend the elastomer with sugar particles, and after solidification, dissolve the sugar cubes or sugar particles with deionized water to finally obtain the elastomer porous film[49⇓~51]. Park et al. Prepared platinum catalyzed silicone rubber/CNT porous elastomer composite films using commercially available sugar cubes as templates[52]. By repeating the preparation of the composite film ten times and making capacitive pressure sensors from it, the researchers compared the capacitive response behavior of capacitive pressure sensors prepared using different batches of sugar cube templates.The results show that the electrical properties are highly consistent in repeated tests, which reflects that the macroscopic sensing properties established by the microscopic disordered porous structure of the dielectric layer have good correlation in this preparation method. The pressure sensor can realize highly sensitive and stable pressure sensing in a large tactile pressure range (0 to 130 kPa) and can identify pressure information spatially distributed on a plane. Choi et al. Prepared porous PDMS sponges using sugar particles of different sizes (300, 500, and 1500.mu.m) and by 3D soft lithography (Fig. 2)[53].
The TENG made of sponge with a pore size of 300 μm has a power up to 44.1 W/m2 at a resistance of 10 MΩ, and can simultaneously light 80 white LED lamps at 85% RH. Wang et al. Filled PDMS with sugar particles and silver nanoparticles (Ag NPs) and constructed porous Variable Micro-capacitors (VMC) films and further prepared TENG[54]. The filling of Ag NPs increases the dielectric constant and energy density of the elastomer, thus increasing the output power of TENG. The porosity of the VMC thin film can be controlled by changing the addition of the sugar particle template.When the addition of the sugar particle template is 20% and the filling content of Ag NPs is 0. 5 wt.%, the maximum output power per unit area of TENG reaches 6 W/m2, and about 100 green light-emitting diodes can be continuously illuminated. Shiratori et al. First mixed sugar and salt particles, then spin-coated PDMS on the solidified sugar salt particles, covered with a layer of solidified sugar salt template, and finally removed the sugar salt particles with water to obtain a composite elastomer dielectric layer with both rough surface structure and porous internal structure[44]. A flexible capacitive pressure sensor was fabricated by superimposing the dielectric layer on the transparent Cu/Ni nano-electrode layer. The dielectric layer structure prepared by the method is provided with a relatively large protruding structure and internal micropores, and the inward curling of the protruding structure and the effective closing of the micropores under the action of a compressive force increase the effective dielectric constant of the system and improve the sensitivity of the sensor. The response and relaxation time (162 ms) of the sensor is 250 times less than that of the sensor prepared by planar PDMS.
The implementation principle of the sugar template method is similar to that of the NaCl template method, and the method has the advantages of simple preparation process, low cost, no toxicity and environmental protection, and the prepared dielectric layer has high compressibility. The disadvantage is that if the sugar template is not dissolved completely, the viscosity of the film will increase, and if it is subjected to continuous pressure in the use process, it will not be able to recover to its original deformation, resulting in hysteresis.
(3) Bicarbonate Template Method
The principle of the bicarbonate template method is that after the bicarbonate particles are blended with the elastomer, the salt particles are removed by thermal decomposition or reaction with hydrochloric acid to obtain the elastomer porous film[55]. Guo et al. firstly reported the method of preparing porous PDMS film by using ammonium bicarbonate (NH4HCO3) as porogen, and realized the preparation of the film embedded with large-area porous PDMS dielectric layer by one-step method (Figure 3). The prepared capacitive pressure sensor can show fast response and high sensitivity under extremely low (1 Pa) and high (1 MPa) pressure conditions[56]. Tan et al. prepared high-performance GR/PDMS composite sponge by adjusting the concentration of GR and NH4HCO3, and further fabricated a flexible wireless pressure sensor using porous GR/PDMS composite sponge and copper as electrodes and antennas[57]. The pressure sensor has the advantages of low cost, simple test, high stability, no battery and the like. Chen Tong et al. Prepared a large area and high porosity PDMS sponge dielectric layer using NH4HCO3 particles as a pore-forming template, and thus constructed a flexible capacitive pressure sensor[58]. The sensor has an extremely wide pressure detection range (0 ~ 50 kPa), fast response time (< 150 ms), stable repeatability (> 1000 cycles) and high sensitivity (0.23 kPa-1).
(4) Microsphere template method
Baik et al. stacked multiple layers of PS microspheres with different diameters on a planar silicon wafer, and then poured the elastomer into the PS microspheres array. After heating and curing, the PS microspheres were dissolved with acetone, and finally the porous PDMS film was obtained and used to make a new Sponge Structure-based Triboelectric Nano-generator (STNG) (Fig. 4)[59]. The STNG has very stable high-yield performance (output voltage and current up to 130 V and 0.10 mA/cm2) over a wide range of humidity conditions (20% to 80% RH) and can power 25 LEDs. Hu et al. Blended the elastomer with ZnO microspheres, cured to obtain ZnO/PDMS composite film, removed the ZnO microspheres by hydrochloric acid etching to obtain a porous elastomer film with highly ordered pore structure, and further successfully prepared a Flexible and Transparent Tribo electric Nanogenerator (FT-TENG)[60]. The porosity of the PDMS film with a honeycomb structure prepared by using ZnO microspheres as a template is 12%, and the maximum output voltage is 271 V, which is 3. 7 times of the output voltage of a planar FT-TENG.

3.1.2 Other template method

In addition to the use of solid templates, other template methods, such as water templates, Vapor Encapsulation Casting (VEC) and microwave processing, have also been used to prepare porous elastomeric dielectric layers[61]. Among them, the water template method is a method in which water is mixed with an elastomer in a certain proportion and then the water is evaporated to obtain a porous elastomer film. Lee et al. Prepared the dielectric layer of porous elastomer film by water template method, and then prepared the capacitive pressure sensor[62]. This pressure sensor with a porous elastomeric dielectric layer shows a sufficiently high pressure-dependent sensitivity at relatively low pressures (5 kPa), up to 1.18 kPa-1. Baik et al. introduced Ag NPs into deionized water and mixed with PDMS. After the water evaporated, Ag NPs/PDMS porous composite film was obtained, and TENG was prepared[23]. When the water content of the composite membrane increases to 50% and the porosity increases to about 59%, the TENG output power reaches 13 mW. Based on this material system, they further prepared a self-powered sensor to monitor the local touch action of the human hand. When the hand touches the sensor, 16 white LED lights can be lit at the same time.
The implementation process of the VEC method can be summarized as follows: in a heated high-pressure closed environment containing water, the steam generated by heating has high kinetic energy under high pressure, so that the steam penetrates into the PDMS during collision with the surface of the uncured elastomer, combines with the steam inside the PDMS, and finally forms pores on the surface and inside of the cured elastomer. Song et al. Spin-coated the elastomer in a sealed autoclave containing water, and obtained a PDMS Double-structure Dielectric Layer with a porous surface layer and a dense bottom layer through the water vapor permeation process, and thus prepared a Double Dielectric Layer Triboelectric Generators (DTEG) with a double-structure dielectric layer (Fig. 5)[63]. In that implementation of the VEC method, the thickness ratio of the obtain dual-structure dielectric layer and the output performance of the DTEG can be precisely control by changing the thickness and vapor pressure of uncured PDMS. The maximum output power of the DTEG is 875 μW when the thickness ratio of the PDMS dual-structure dielectric layer is 0. 95, which is 330% higher than that of the DTEG with dense PDMS dielectric layer. Kumar et al. Used a thin layer of cleaning cloth with rough structure as a structural template, poured PDMS directly on the thin layer for curing and completed the combination of the two.The PDMS/cleaning cloth composite porous film with high porosity was prepared, which was used as a dielectric layer and ITO/polyethylene terephthalate (PET) electrode to form a flexible capacitive pressure sensor[64]. The effective area of the stress sensing process is changed and the sensitivity of the sensor is adjusted by placing glass sheets of different sizes on the sensor to conduct pressure. The results show that when the effective area is increased from the 36 mm2 to the 324 mm2, the sensing sensitivity is increased by 10 times. Oh et al. Solidified PDMS by heating the mixture of PDMS and organic solvent with microwave to obtain a porous elastomer film, which was used to make a capacitive pressure sensor[41]. The process can adjust the pore size and porosity of the porous film by controlling the mixing ratio of the sacrificial solvent. When the proportion of the sacrificial solvent is 40 wt%, the prepared porous PDMS film has the highest pressure sensing sensitivity of 0.813 kPa-1. By using commercial opaque glass as the casting substrate and further introducing the micro-structure on the surface of the PDMS film, the micro-structure dielectric layer can obtain stronger deformation ability, and the sensing sensitivity can be further improved to a 1.43 kPa-1.

3.2 Method for preparing non-porous array type elastomer dielectric layer

3.2.1 Ilicon template method

The silicon template method comprises the following steps of: firstly, preparing array structures with different shapes on a silicon wafer by using a photolithography process to serve as templates, pouring an elastomer solution on the array templates, and then solidifying to obtain a corresponding array-type rough structure[65]. Bao et al. Fabricated pyramidal array structured PDMS thin films with extremely high replication accuracy by silicon template method, and thus fabricated flexible capacitive pressure sensors[24]. The pressure sensing sensitivity and response speed of the obtained sensor are far superior to those of an unstructured elastic film sensor with the same thickness, and the sensing effect can be adjusted by controlling the size of an array unit. Chen et al. Prepared RGO/PDMS dielectric layers with pyramidal and pleated array structures and fabricated flexible tactile sensors from them[66]. This retractable tactile sensor based on the microstructured GR array, with its raised array structure and sensitive graphene layer, has a detection limit as low as 1.5 Pa and a response time of only 0.2 ms (Figure 6). Choi et al. fabricated PDMS thin films with a disk-like protrusion array structure, from which an integrated piezoelectric pressure sensor was fabricated[67]. The sensitivity of the microstructured piezoelectric pressure sensor is three times higher than that of a common piezoelectric pressure sensor, and the microstructured piezoelectric pressure sensor also has fast response capability and an extremely wide pressure response range (0.23 to 10 kPa),To a large extent, it simulates the function of human skin and can realize the sensing identification of fast and slow sweeping of a feather on the surface of the sensor. Lee et al. Coated a highly conductive poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) film on the surface of the prepared PDMS pyramid array as a dielectric layer, and prepared a resistive pressure sensor with a PET film deposited with Au as an electrode[68]. The pressure sensor exhibits high sensitivity (21 kPa-1) at a low pressure of < 100 Pa. In addition, the resistive pressure sensor has an ultra-high mechanical stability (> 10,000 cycles without any failure or crack) and a fast response time (90 ms). The pressure sensor has a high sensing accuracy for human blood pulse (the measured pulse frequency is 78 times per minute, which is equivalent to the pulse frequency of healthy adults). Liu et al. Fabricated flexible PDMS films with a unique pyramid-wall-grid (PWGM) structure using a silicon mold[69]. Two PDMS films with PWGM are stacked to form a piezoresistive pressure sensor.Due to the unique pyramidal structure and the layered deformation structure with different heights, the sensitivity of the device is as high as 383 665.9 kPa-1 in the pressure range of 0 to 1. 6 kPa. In addition, a 4 × 4 array pressure mapping panel is constructed with PWGM flexible PDMS film, which makes the piezoresistive pressure sensing display have spatial resolution. Park et al. combined silicon template method with PS microsphere template method to prepare pyramidal array films with porous structure on the surface, which were used to fabricate capacitive pressure sensors[70]. The sensor can detect the pulse and gentle airflow on the wrist, and can distinguish the number of fruit flies with high sensitivity (each fruit fly is equivalent to 0.14 Pa of pressure).

3.2.2 Other template method

Wang et al. sprayed the mixed solution of PDMS prepolymer and curing agent on a piece of sandpaper with a rough surface through a spray gun, and then covered it with another piece of sandpaper. After PDMS was cured, a PDMS film with a double-sided rough structure was obtained[71]. The conductive PDMS/carbon fiber composite film with a single-sided rough structure was prepared by spraying the carbon fiber-filled PDMS on the surface of a single piece of sandpaper, and the capacitive pressure sensor was composed of the conductive PDMS/carbon fiber composite film as an electrode and the PDMS dielectric film with a double-sided rough structure. The sensitivity of the PDMS film sensor prepared by the sandpaper with the surface roughness of 74. 4 μm was up to 0.82 kPa-1, which was 40 times higher than that of the sensor prepared by the PET film template. Shim et al. fabricated a high performance pressure sensor using paper with a rough surface as a template[72]. They coated graphite in 8 B pencil on the surface of paper as an electrode by writing, and then spin-coated a layer of heptane solution of PDMS on the surface of graphite to prepare a PDMS dielectric layer, thus building a higher roughness on the PDMS layer caused by the rough graphite substrate. Two sheets of coated paper covered with a PDMS coating and a graphite electrode were superposed to obtain a double dielectric layer capacitive pressure sensor (fig. 7). Guo et al. prepared a PDMS dielectric layer with a double-sided groove structure. They first obtained an acrylonitrile-butadiene-styrene block copolymer (ABS) mold with a periodic groove structure on the surface by 3D printing.A PDMS dielectric layer with a groove array structure was obtained by multi-step mold replication, and then a double PDMS dielectric layer was prepared by superimposing two structured PDMS films, thus a capacitive pressure sensor was fabricated[73].
The advantages and disadvantages of the various templating methods and their representative microstructured elastomer morphologies are summarized in Table 1.

4 Application of microstructured elastomeric dielectric layer

With the introduction of the strategy of microstructured elastomeric dielectric layers and the reports of various preparation methods mentioned above, microstructured elastomeric dielectric layers are playing an increasingly important role in the field of flexible electronic devices. In this paper, the recent research work on the applications of microstructured elastomeric dielectric layers in the fields of sensors and nanogenerators is reviewed.

4.1 Sensing Applications of Microstructured Elastomeric Dielectric Layer

4.1.1 Sensing Applications of Porous Elastomeric Dielectric Layer

Kim et al. fabricated a stretchable energy-harvesting capacitive pressure sensor using a porous elastomer dielectric layer, which consists of a porous PDMS dielectric layer and a single-walled carbon nanotube (SWNTs) film electrode[13]. The SWNTs thin film is obtained by spraying on a preheated porous PDMS film. The SWNTs film has high stretchability and good triboelectric property with the surface of PDMS. Capacitive pressure sensors are capable of detecting, differentiating, and capturing a variety of mechanical stimuli, such as normal pressure, lateral strain, bending, and sound-driven vibrations (Figure 8). Oh et al. Made a capacitive pressure sensor with porous elastomer film prepared by sugar template method. The pressure sensor can maintain good sensitivity (0.01~0.02 kPa-1) in the working range of 10 Pa ~ 1.2 MPa[49]. A flexible piano keyboard is made by forming an array of multiple sensors, which can realize the working principle of a real acoustic piano, and the sound level can be controlled by the pressing speed of the keyboard. Lacour et al. deposited three gold film electrodes with parallel stripes on the surface of commercial PU foam by mask deposition method, thus making a capacitive pressure sensor[74]. The density of PU foam can be controlled by changing the conditions in the preparation process of PU foam, which further affects the sensitivity of the prepared sensor. The sensor can detect the pressure in the range of 1 ~ 100 kPa. The sensor is strong and deformable and is not affected by distortion and sharp indentations. Gao et al. Used citric acid particles as structural templates to prepare porous CNT/PDMS composite sponge dielectric layers and thus constructed strain sensors[75]. The addition of CNTs increases the dielectric constant of the elastomer, so that the sensor has a higher capacitive response, and the response increases with the addition of CNTs, thereby improving the sensitivity of the sensor. The porosity of the composite sponge can be controlled by adjusting the mass ratio of PDMS to citric acid particles. The sensor can clearly track the movement of skin/muscle when volunteers drink alcohol and judge different breathing patterns, such as deep breathing and normal breathing, through relative resistance changes. Park et al. prepared a capacitive pressure sensor with CCTO @ PU hybrid sponge material as the dielectric layer and flexible CNT/PDMS composite material as the electrode[76]. The surface of the CCTO is modified by using 3-aminopropyltriethoxysilane, so that amino groups on the surface of the CCTO are combined with cyanate ester groups of PU, and the degree of interface polarization is improved, thereby obtaining a high dielectric constant. The CCTO @ PU hybrid sponge sensor exhibits flexibility and high sensitivity. The sensor can detect the pressure changes of human fingertips during grasping, and can also monitor various human activities such as voice, swallowing, joint bending and so on in real time. Lu et al prepared Ag NPs/PDMS composite porous elastomer dielectric layer by NH4HCO3 template method and fabricated capacitive pressure sensor[56]. The pressure sensor has good durability, and the capacitance response of the sensor is still stable after 5000 times of tests of applying 50 kPa load and releasing 100 kPa load respectively. By introducing Ag NPs into the porous elastomer, the sensitivity of the porous composite capacitive pressure sensor is improved by 101. 5% in the pressure range from 0 to 100 kPa. The capacitive pressure sensor embedded in each phalanx of the artificial hand can detect the small pressure change caused by the pulse of the artificial wrist and the pressure distribution on different phalanxes of the wrist.

4.1.2 Sensing Application of Non-porous Array Elastomeric Dielectric Layer

Bao et al. prepared a PDMS dielectric layer with a pyramidal array structure on the surface by a silicon template method, and further assembled it with a conjugated semiconductor polymer layer and a gold electrode to form a flexible transistor pressure sensor.The sensor has a sensitivity of up to 8.4 kPa-1, a sensing response time of less than 10 ms, and excellent sensing performance after more than 15,000 use cycles[77]. The sensor is used for monitoring the pulse of the radial artery of a human body and can realize high-fidelity sensing performance in a non-invasive use mode. They further prepared a bandage pressure sensor with microvillus interface structure based on bionic design, and the upper part of the sensor is a polyethylene naphthalate/chromium/gold electrode layer.The dielectric layer is a pyramidal PDMS layer, the lower part of the sensor is a columnar microstructure, and different structural layers are laminated together and sealed with commercial adhesive tape to form a single sensor device[26]. The flexible pressure sensor can measure a weak signal from a neck vein pulse, and can screen out patients suffering from heart disease by detecting different pulse waveforms, thereby being beneficial to rapid diagnosis of cardiovascular diseases and the like. Huang et al. fabricated a passive wireless sensor embedded in a commercial bandage, using an inverted pyramid elastomer film as the dielectric layer and a polyimide film as the electrode[78]. The flexible wireless pressure sensor, located between the wound dressing and the medical sponge on the bandage, can assess neovascularization and cell proliferation under pressure by continuously monitoring the mechanical pressure at the wound site within the wound. Shen et al. fabricated a flexible and transparent capacitive pressure sensor based on Ag NWs/PDMS composite dielectric layer with pyramidal array structure by silicon template method[79]. The sensor can achieve sensitive sensing down to 1.4 Pa with a response time of less than 30 ms. The researchers further prepared a flexible wearable keyboard composed of a 5 × 5 sensor array, which realized the inductive input function similar to the traditional keyboard. Li et al. Used the rose petal surface as a structural template to obtain a rough structure on the PDMS surface by casting replication, and then dropped the dispersion of iridium nanoparticles (Ir NPs) coated CNTs (Ir NPs @ CNTs) onto the PDMS surface to prepare a biomimetic patterned PDMS film compounded with Ir NPs @ CNTs, and thus prepared a strain sensor[80]. The sensor has a sensitivity of 20.33 and a response time of more than 242 ms. The device can be used to monitor human pulse, wrist movement and voice.

4.2 Nanogenerator applications of microstructured elastomeric dielectric layer

4.2.1 Nanogenerator applications of porous elastomeric dielectric layer

Yoo et al. prepared porous Au NPs/PDMS sponge with high dielectric constant assisted by polydopamine (PDA), and further used it to prepare TENG[81]. PDA is applied on the surface of PDMS as an adhesive layer to promote the uniform adsorption of Au NPs, and the effective doping of Au NPs improves the dielectric constant of the dielectric layer, while the porous structure improves the compressibility of the dielectric layer, which together improve the output performance of the generator. With an external resistor of 10 MΩ, the TENG has a maximum output power of 115 mW. Guo et al. Prepared PDMS porous dielectric layer by NaCl template method, and filled SiO2, TiO2, BaTiO3 and SrTiO3 particles with different volume ratios at the same time, and thus prepared TENG with different output properties[82]. Compared with the SiO2, the TiO2 and the BaTiO3 particles, the filling of the SrTiO3 particles enables the dielectric layer to have higher relative dielectric constant and transferred charge quantity,Therefore, the TENG prepared from the SrTiO3/PDMS porous elastomer composite film has the highest output power of 6.47 W/m2, and can light at least 44 green light-emitting diodes in series.

4.2.2 Nano-generator application of non-porous array type elastomer dielectric layer

Wang et al. proposed a novel Flexible Nano-generator (FTNG) based on the triboelectric effect, which is fabricated from elastomeric dielectric layers with different array structures in order to increase the capacitance change of the device[14]. The peak voltage and current of the FTNG with pyramidal array dielectric layer structure can reach 18 V and 0. 7 μA, respectively. Their FTNGs can be used as a self-powered pressure sensing device with a sensing limit of 13 MPa, which can sense the pressure changes of a single water droplet (8 mg) and a single feather (20 mg). Choi et al. prepared a micro-nano composite multi-level array structure on the surface of PDMS by femtosecond laser maskless irradiation etching method, and used it as the polymer friction layer of TENG device. Compared with the unstructured PDMS layer, it can achieve a larger contact area between the metal electrode and the friction layer, thus obtaining a larger output power[83]. This TENG with a structured friction layer can generate voltages up to 42.5 V and currents up to 10.1 μa, with a maximum output power of 107.3μW/m2 (Figure 9).

5 Conclusion and prospect

In this paper, the preparation methods and applications of microstructured elastomeric dielectric layers are reviewed. According to the two types of microstructures, porous and non-porous arrays, the preparation methods of microstructured elastomeric dielectric layers are introduced, including NaCl template method, sugar template method, bicarbonate template method, microsphere template method and silicon template method. In the introduction of the preparation methods, the influence of the microstructure dielectric layers prepared by different methods on the performance of the constructed flexible devices is described, including sensing sensitivity, detection limit, detection response time, output power and so on. The latest applications of microstructured elastomeric dielectric layers in the fields of sensors and nanogenerators are also introduced. In the part of sensor applications, we mainly introduce the important work of microstructured elastomeric dielectric layers in wearable detection and input devices in recent years.
In the past decade, the field of micro-structured elastomeric dielectric layers has developed rapidly from scratch, and its application scope is also expanding. But at the same time, there are many problems to be solved in the development of this field. First, the existing preparation methods are still lack of applicability for industrial scale production.There is still a lack of discussion on how the experimental factors in the preparation method affect the rapid preparation in large quantities, which will also be the direction to be explored in the future industrial application of this field. In addition, in the actual use scenarios of flexible electronic devices with the participation of microstructured elastomeric dielectric layers, there is a structural compression effect caused by the packaging process of devices, for example, in sensor applications, this effect will reduce the sensitivity of devices, and more research is needed to find an optimized preparation scheme in this regard. Finally, there is still a lack of systematic theoretical models to predict the quantitative relationship between microstructure parameters and the performance of the fabricated flexible devices in order to achieve the highest design efficiency of the fabrication method.How to establish a calculation model to correlate the definite relationship between the arrayed microstructure and the specific application performance parameters will be the key to improve the repeatability of the preparation method. It is believed that with the deepening of the research on micro-structured elastomeric dielectric layers, the field of flexible electronic devices will gain a strong boost.

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Funding

General Research Foundation of Department of Education of Zhejiang Province(Y202043655)

Natural Science Foundation of Ningbo(2018A610113)

Natural Science Foundation of Ningbo(2019A610187)

K.C. Wong Magna Fund in Ningbo University

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