Recent Advances of Capacitive Sensors: Materials, Microstructure Designs, Applications, and Opportunities

Capacitive sensors have advanced rapidly to create new applications including wearable sensors for human health monitoring, integrated sensors for intelligent surgical devices, tactile interfaces for robots. Compared to other types of pressure or strain sensors, capacitive sensors require low power consumption and offer excellent linearity and fast response time. Herein, this review concentrates on the recent advancements and developments of high‐performance capacitive sensors with new materials and microstructures, which significantly enhance their sensitivity, accuracy, linearity, and response time. This work also provides a review of recent applications, from which current challenges and future opportunities are proposed and discussed.


Introduction
Wearable, flexible, and stretchable sensors based on piezoresistive, triboelectric, piezoelectric, and capacitive mechanisms have experienced rapid advancement to meet sensing demands in a diverse range of applications (e.g., soft prosthetic hand for amputees [1][2][3][4] and flexible, human activity recognition and body signal monitoring, [5,6] artificial intelligence, [7,8] and large area skin-interfaced sensing [9] ). Piezoelectric and triboelectric sensors have the merits of self-power property and fast response time. However, piezoelectric sensors are not able to perform the static sensing, [10] and the output of triboelectric sensors is susceptible to variations of ambient environment such as temperature, humidity, and electromagnetic interference. Resistive sensors have been widely investigated, due to their simple structure and working mechanism, [10] but their applications were significantly DOI: 10.1002/admt.202201959 hindered by hysteresis, temperature and humidity dependence, and relative higher power consumption. [11] Thus, among these sensors, capacitive sensors have relatively simple structures, excellent linearity, low hysteresis, and low power consumption, [12,13] making them highly versatile in a wide range of applications such as human health monitoring, body motion detection, and robotics. Capacitive sensors can be readily integrated with other types of sensors (e.g., accelerometers, resistance temperature sensors, and impedance-type humidity sensors) to accomplish multi-functional sensing tasks. [14] Capacitive sensors can be categorized as pressure, temperature, and strain sensors, although they are all based on a change in capacitance as the sensing mechanism.
For capacitive sensors, both material selection and structure design are significant to ensure a high sensing performance of the sensor. In this work, the materials for electrode are classified as metallic and nonmetallic conductive materials while the materials for the core layer are classified as nonionic materials and ionic materials, respectively. For capacitive type pressure sensors, to improve their pressure sensing performance, microstructured designs such as air-gap design, [15] porous structures, [16][17][18][19][20][21][22][23][24][25][26][27][28][29] wrinkled structure, [30][31][32][33][34][35][36] and pyramids structures [19,[37][38][39][40][41][42][43][44][45][46][47] have been introduced. Recently, to further improve the sensitivity of capacitive pressure sensors, the dielectric material in middle layer of the sensor have been replaced by ionic gel-based materials to form electric double layer (EDL) capacitance, becoming supercapacitive sensors with ultrahigh sensitivity. [48,49] Capacitive temperature sensors [50][51][52][53][54] exhibit great potential to be applied in anti-icing systems, [55] body temperature measurement, and hightemperature chemical reaction monitoring. In recent, to improve the sensing performance of temperature sensor, ionic material is used to improve the performance of temperature sensor, [56,57] and the structure like kirigami was introduced, [58] contributing toward the enhancement of sensitivity and stretchability. For capacitive strain sensors, constrained by capacitance variation determined by their geometric effect, it is challenging to achieve a sensitivity greater than 1. [59] To ensure the stretchability and repeatability of capacitive strain sensors, the stretchable performance of both dielectric layer and stretchable substrate are of vital importance. Polymer-based materials such as PDMS and hydrogelbased materials are widely used to secure the roles of core layers, because of their preferable stretchability. Table 1 summarizes the merits and challenges for abovementioned three types of capacitive sensors.
Enormous amount of the research works had been published aiming to improve the sensitivity of capacitive sensors with various design strategies. Although the sensitivity is of vital significance for sensors to ensure the basic sensing function, it is not the only critical parameter which needs to be concentrated on. Another important parameter is the signal to noise ratio (SNR). A high SNR is crucial to ensure the sensing accuracy and reliability of capacitive sensors. Moreover, linearity in a wide sensing range is also important to linearly correlate the sensing input with the output of capacitance changes. For industrial and com-mercial applications, other factors like costs, large-production capability, and robustness should be taken into account as well. All these mentioned parameters and factors should be comprehensively considered for sensor design, rather than being considered separately. Herein, different from previously published review papers, [10,11,[60][61][62][63][64] this work predominantly focuses on the recent developments, applications, and the underlying challenges of capacitive sensors from new perspectives (Figure 1). Specifically, Section 2 provides the basic functioning mechanism of capacitive sensors. Then in Section 3, we present existing and state-of-theart designs of capacitive sensors with respect to electrodes and core layers. Both their advances and limitations are summarized. Sensitivity definition and mechanism of electric double layer (EDL) mechanism. A) Schematic expression of sensitivity for capacitive pressure sensors or strain sensors. B) The schematic illustration of EDL mechanism. Reproduced with permission. [82] Copyright 2021, American Chemical Society. Section 4 pays attention to the potential applications of capacitive sensors in electronic skin, biomimicking, medical, and many other aspects. Finally, Section 5 formulates an outline regarding the existing challenges and further opportunities for capacitive sensors.

Capacitive Pressure Sensor
Capacitive sensing mechanism based on the capacitance variation when the sensor is under different stimuli such as compression, stretching, and temperature. For traditional parallel-plate capacitors, the capacitance C can be expressed as [81] C = 0 r A d (1) where k 0 , k r , A 0, and d 0 are the vacuum permittivity, relative permittivity, initial surface area of electrodes, and initial distance between two paralleled electrodes. For pressure sensors, the sensitivity can be characterized by a pressure gauge factor, K p (Figure 2A) where ΔC and Δp denote capacitance variation and pressure variation, respectively, ΔA and Δd are the changes in surface area of electrodes and the changes of dielectric layer thickness, respectively. For capacitive sensors with dielectric layers fabricated by nonionic elastomers, a lower elastic modulus of dielectric material can enhance the compressibility of the sensor and increase the variation of dielectric thickness under applied pressure, thus improving the sensitivity of the sensor. In addition, structured dielectric layer reduces its equivalent elastic modulus as well, eventually enhancing the sensitivity of the sensor via larger changes in dielectric thickness under the same level of pressure. However, these designs are still unable to significantly enhance the sensitivity of the sensor. With the application of ionic hydrogel, supercapacitive sensor facilitates the improvement of sensitivity.

Supercapacitive Pressure Sensor
With the application of ionic gel materials, supercapacitive pressure sensors with EDL capacitance demonstrate significantly enhanced sensitivity under pressure by introducing changes in the EDL area at the contact interface between electrode and core layer. Specifically, the EDL contact area increases with applied pressure, eventually producing a large capacitance change of the supercapacitive sensor ( Figure 2B), which can be expressed via C EDL with the following equation where A EDL1 and A EDL2 denote contact area between interfaces between electrolyte core layer and two electrodes, and UAC is the unit area capacitance. Briefly, the significant factor for EDL mechanism is the contact area change between electrode and core layer, in this case, capacitive pressure sensors designed with EDL mechanism always be integrated with microstructures of electrolyte core layer. The equivalent circuit of supercapacitive sensor is shown in Figure 2B. EDL capacitance is generated at the interface between the electrolyte core layer and both top and bottom electrodes, denoting as C EDL , and the parallel-plate capacitance is denoted as C R . For EDL-based sensors, the value of EDL capacitance is 5-6 orders of magnitude higher than parallel-plate capacitance. [49] Thus, the parallel-plate capacitance can be neglected for supercapacitive pressure sensors.

Capacitive Strain Sensor
Capacitive sensors can be used for in-plain strain detection as well. In this case, the strain sensitivity K can be expressed as ( Figure 2A) where Δ represents the variation of applied strain. Based on the linear elasticity, under the uniaxial strain, the length of the dielectric layer and stretchable substrate experience an elongation in the stretching direction, but contraction occurs in width and thickness direction. It is noted that we assume the stretchable substrate and the dielectric layer have the same Poisson's ratio . In this case, the capacitance C s of strain sensor under uniaxial stretch condition becomes C s = C 0 (1 + ). Therefore eventually, the gauge factor is equivalent to 1 theoretically for the parallel-plate capacitor strain sensors with geometric effect.
To overcome the geometric effect, a few publications recently reported that the GFs of capacitive strain sensors were more than 1 by creating new shapes and manipulating the mechanical properties of materials to increase the sensitivity of capacitive strain sensors. More details will be given in Section 3.

Capacitive Temperature Sensor
Temperature detection also can be performed with capacitive sensor, and the sensitivity K T is expressed as [58] K T = ΔC ΔTC 0 (6) where C 0 is the capacitance at a reference temperature T 0 , [58] k T represent the sensitivity to temperature which can be expressed as [58,62] K T = + + where and are the thermal expansion coefficient, and permittivity coefficient of core layer material, respectively.

Materials and Design for Capacitive Sensors
Capacitive sensors are commonly designed with a sandwich structure which consists of two parallel-plate electrodes and one core layer in the middle. Both the materials selection and the structure design of electrodes and core layers are critical for capacitive sensors to achieve high sensing performance in terms of sensitivity, linear range, SNR ratio, and stability.

Capacitive and Supercapacitive Pressure Sensors
Both electrode and core layer can be designed with microstructures to improve the sensing performance of capacitive pressure sensors. For example, the ionic hydrogel with microstructures of core layers greatly improves the performance of the sensor, especially for sensitivity (more than 1000 times), which is named supercapacitive pressure sensor. With the development of advance manufacturing technology, the fabrication and manufacturing methods for microstructured sensors are fast changing with higher precision and accuracy. As a cutting-edge advanced manufacturing method, 3D printing technology has been employed to fabricate electrodes [83] and core layers [16,84,85] with microstructures for capacitive pressure sensors.

Electrodes
As an indispensable conductive element for capacitive sensors, electrodes in a capacitive sensor carry charges to ensure the electrical function and operation of the capacitors and capacitive sensors. As conductive materials, both metallic and nonmetallic materials have been applied to fabricate electrodes for capacitive sensors to fulfill the specific sensing tasks. Electrically Conductive Materials: Metals: As common conductive materials, metallic materials such as gold, [82,[86][87][88][89][90] copper, [68,91] and silver [17,33,34,48,[92][93][94][95][96] thin films have been used as electrodes for capacitive pressure sensors. For example, Bai et al. use evaporating [90] and ion sputtering [89] methods to deposit conductive gold thin films on PI and PET substrates to serve as the electrodes. Apart from thin metallic films as electrodes, metal nanowires especially silver nanowires (AgNWs) have also been widely applied as electrodes for capacitive sensors. [94,97] For example, Kulkarni et al. [97] fabricated electrodes with AgNW through spray deposition of AgNWs on poly(ethylene terephthalate) (PET) substrate. The capacitive sensor based on AgNWs showed good transparency, which is an ideal for wearable electronics, defence, and military applications. [97] As metallic materials such as silver, gold, and platinum are intrinsically conductive, and can be easily deposited on polymer thin films, they are ideal and preferable options for electrodes of capacitive sensors. Moreover, materials like gold, platinum, and silver are noble metals showing outstanding resistance to chemicals and moisture. Even under high temperature and humidity, the electrical properties of metallic elements remain very stable, thus ensuring the constant function of electrodes for capacitive sensors.
Carbon and Graphene Materials: Besides traditional metal materials, nonmetallic conductive materials like graphene, carbon nanotube (CNT), carbon nanofiber (CNF), conductive fabric, reduced graphene oxide (rGO), and conductive hydrogels have also been employed as electrodes of capacitive sensors. For capacitive pressure sensors, electrodes made of nonmetallic conductive materials can also be designed with microstructures to improve the sensing performance (e.g., sensitivity and stretchability). For example, to improve the stretchability, Peng et al. [17] took the advantage of stretchable porous PDMS/CNF structure to fabricate multilayered sensors combining resistive strain sensor and capacitive pressure sensor to measure directional multiple forces.
Structures: 2D Thin Films with Wrinkled Structures: Wrinkled structured electrodes were adopted in capacitive sensors because of their better responsiveness to pressure. Wrinkled structured electrodes are able to produce more significant deformation under the same pressure in comparison to unstructured electrodes. For example, by fabricating wrinkled electrodes ( Figure 3A) with rGO-based material, Nie et al. [31] successfully fabricated a sensor with wide-range pressure sensing capability (0.1-1.3 MPa). To fabricate wrinkled rGO electrode, the rGO film was prepared via Hummers method, [98][99][100] and then the wrinkled structure was formed by pasting the rGO film onto a prestretched VHB tape followed by relaxing. [31] Figure 3B shows microscopy image of the wrinkled rGO electrode. Compared to flat electrode, the wrinkled electrode can increase the effective contact area with hydrogel core layer under pressure, resulting in a significantly improved sensitivity. As pressure applied, the area between two wrinkled Reproduced with permission. [31] B) Microscopy image of wrinkled rGO film. [31] Copyright 2020, American Chemical Society. C) Schematic illustration of skin-like sensors with spring-like CNT electrode arrays. Reproduced with permission. [109] Copyright 2013, John Wiley and Sons. D,E) Schematic demonstration of capacitive pressure sensor with graphene-based electrode array. F) 3D mapping of capacitance variation when objects approaching the sensor. Reproduced with permission. [65] Copyright 2017, American Chemical Society. electrodes and hydrogel dielectric layer increased, leading to an increase of sensor's capacitance. [31] Mesh-Structured Electrode Arrays: Arrays of electrodes, [65,66,72] unlike single pair of electrodes, are designed for application of capacitive touch sensors. To design a skin-like sensor for pressure and strain detection, Bao and co-workers [72] used CNT-based electrode arrays to fabricate a capacitive sensor ( Figure 3C) via spray coating of conductive CNT inks. The electrode arrays structure is able to accommodate strains of ≈150% and show a high conductivity value of ≈2200 S cm −1 . [72] The capacitive pressure sensors are potentially for human interactions and feedback of bio-signals, such as medical devices and robotic systems. [72] Capacitive pressure sensor with electrode arrays can simultaneously perform sensing capability for tactile and biological stimuli, providing a potential for e-skin applications. Kim et al. [66] used CNT electrodes to fabricate the skin sensors, and the CNT microyarns were prepared via chemical vapor deposition (CVD) method before direct spinning. [66] However, this design suffers from low  [11] Copyright 2021, Wiley-VCH GmbH. B) Schematic illustration for stress direction detection (relative capacitance change) with biomimetic e-skin. Reproduced with permission. [61] Copyright 2021, Wiley-VCH GmbH.
sensitivity and narrow linear sensing range. Graphene is also a preferable material for flexible electronic devices, [101,102] due to its good mechanical flexibility and optical transmittance. [103][104][105][106][107][108] Kang et al. [65] fabricated a capacitive touch sensor with graphene based electrode array on PET films ( Figure 3D,E). In this study, the graphene was synthesized via chemical vapor deposition on a Cu foil. Cu was subsequently etched away in ammonium persulfate after coating the graphene layer with poly(methyl methacrylate). Finally, conductive graphene was transferred onto a PET substrate and the patterned graphene was carried out by photolithography and oxygen plasma etching. [65] By detecting the capacitance variation when the object approaches the sensor, this electrode array design makes it possible to conduct contactless sensing of the shape and location of external object (Figure 3F). [65] 3D Microstructures Including Pyramids, Pillars, and Domes: In order to further mimic human skin for robotic applications, for example, to distinguish and measure the normal and shear force with hierarchical design, Bao's group [41] introduced a microstructured (pyramid and dome) electrode into the capacitive e-skin sensors (Figure 4A), profiting from the biomimetic hierarchical structure of dermis-epidermis human skin ( Figure 4A). The CNT electrodes were fabricated through the spray coating of conductive CNT inks onto the photolithography patterned PU substrate. In this design, owing to the geometry of the dome and the anisotropic deformation, forces in different directions applied on the top side of the sensor, will lead to capacitance changes around the dome mapped in differential pixels ( Figure 4B). [41] Therefore, this sensor can be used to detect the direction of the applied force. Their investigation also optimized the sensor with respect to pyramids width and separation distance among pyramids. However, the fabrication process such as photolithography is relatively complicated, possibly increasing the cost and limiting the commercialization of the sensors.
To improve the sensitivity of the sensor by changing the distance between two electrodes under pressure, Zhang et al. [110] Figure 5. Microstructure of pyramid, dome, and pillar structure of electrodes for capacitive pressure sensors. A) CNT electrodes on micropyramid structure film of capacitive pressure sensor. Reproduced with permission. [110] Copyright 2021, Wiley-VCH GmbH. B) Capacitive pressure sensor using gold electrodes on PDMS membranes with randomly distributed hemisphere dome-like microstructure. C) Various pressure condition of microdome structure of electrodes. D) Capacitance change with pressure of the sensor with randomly distributed hemisphere dome-like microstructure. Reproduced with permission. [88] Copyright 2021, American Chemical Society. E) Schematic illustration of ultrasensitive capacitive sensor using gold electrodes on PDMS substrates with hemisphere arrays and sensitivity of sensor. Reproduced with permission. [87] Copyright 2020, Elsevier. F) Schematic illustration of pressure sensor using hybrid PVA/PANI electrodes with reliefs. G) Sensitivity illustration for pressure sensor using hybrid PVA/PANI electrodes with reliefs. Reproduced with permission. [111] Copyright 2021, American Chemical Society. H) Schematic illustration of capacitive pressure sensor with graphene microconformal pillar-like electrodes. Reproduced with permission. [112] Copyright 2019, American Chemical Society.
used CNT film as electrodes on Ecoflex film with micropyramid structure to fabricate a pressure sensor. As shown in Figure 5A, the pyramid micorstructured substrate was fabricated by replicating a template mold and subsequently a conductive electrode layer of CNT was deposited on the micropyramid via a stamp-ing method. As the pressure applied, the pyramid microstructure was compressed and the distance between electrodes decreased and hence the capacitance increased. Their research also discovered that the sensitivity of the pressure sensor can be improved by increasing either the thickness of pyramids or the separation distance between them. [110] Although the linear sensing range of the pressure sensor can be extended by increasing the thickness of pyramid, [110] the nonlinearity issue still existed in the higher pressure range. Moreover, as the stress concentrated on the tip of the pyramids, the high sensitivity was only observed within the lower pressure range.
Apart from pyramids shaped structure, dome microstructures were also designed to improve the performance of capacitive sensors. Recently, Sharma et al. [88] used gold electrode on PDMS membranes to form randomly distributed hemisphere dome-like microstructure ( Figure 5B,C). This microstructure was fabricated via a templating method with sandpaper. Specifically, precursor of PDMS was spined onto commercially purchased silicon carbide sandpaper. After curing, solid PDMS was peeled off from the sandpaper template, followed by the deposition of Ti/Au layer onto the structured PDMS substrate via electron beam evaporation. As results, this microstructure contributes toward the contact area change between electrode and core layer, under EDL mechanism. This design exhibited a wide sensing range (0-30 and 30-250 kPa, Figure 5D). Xiong et al. [87] sputtered conductive gold thin films onto PDMS substrate with dome structure as electrodes ( Figure 5E) to build an ultrasensitive capacitive pressure sensor. As pressure applied, the change of contact area between electrode and dielectric layer increased. In this design, although the linear range was greatly improved (≈130 kPa), the nonlinear problem still existed ( Figure 5E) in high pressure region. [87] Similarly, based on the contact area induced capacitance change, Zhou et al. [111] designed the hybrid PVA/PANI hydrogels electrodes with domes ( Figure 5F) to improve the sensitivity. In this work, they addressed the freezing and dehydration issues of hydrogels by adding binary solvents. The resulting hydrogels exhibited splendid flexibility and the sensor with hydrogel electrodes showed a high sensitivity of 7.70 kPa −1 with the sensing range of 0 to 7.4 kPa. [111] Further, their results revealed that electrodes with larger diameters or thinner thickness could trigger larger capacitance changes, resulting in higher values in sensitivity and SNR. [111] However, because of the nonlinear deformation and the contact area change between electrodes and dielectric layer, nonlinearity issue still persisted in this design ( Figure 5G).
Yang et al. [112] designed tunable microconformal pillar-like graphene electrodes ( Figure 5H) to improve the sensitivity of the capacitive pressure sensor. In their work, the size of micropillars was in the range of a few nanometers to tens of micrometers, and the highest sensitivity was 7.68 kPa −1 with symmetric double microconformal design within a low pressure range (≈0.5 kPa). [112] However, due to the nonlinear deformation of microconformal structure under compression, this sensor has a quite small sensing range, which limited it application.
For capacitive pressure sensors, electrodes with 3D structures significantly contribute to the enhancement of sensitivity. As for micropyramids, the stress concentration on the pyramid tops significantly facilitates the sensitivity at the low-pressure range. However, the compressibility of pyramid deteriorates with increasing applied pressure, causing a decrease in sensitivity in high pressure region. Pillar electrodes have been designed with more uniform compressibility. Nevertheless, the pillar structure still makes the sensors suffer from an obvious decrease of sensitivity in high pressure region. To a certain extent, dome mi-crostructures can alleviate this problem by gradually increasing the contact area as the applied pressure increases, which is ideal for EDL-based supercapacitive pressure sensors.

Core Layers
For capacitive pressure sensors, properties of the core layers are critical to sensor performance in sensitivity. Soft materials like PDMS, PVDF, Ecoflex, sponge, and ionic hydrogel have been widely employed as the core layer of the capacitive sensors to endow them flexibility and stretchability. Microstructure design of core layers can improve the sensitivity of capacitive pressure sensors and extend their linear sensing range. In recent years, to significantly improve the sensitivity, ionic hydrogels have been used as the electrolyte core layers of the supercapacitive sensor for pressure sensing. The microstructured core layers can be fabricated via mould templating, photolithographic and 3D printing techniques.
Elastic Dielectric Materials: Compared with solid core layer, the presence of various microstructures within dielectric layer can help reduce its effective modulus, contributing to more significant capacitance changes under pressure and hence higher pressure-sensitivity.
1) Porous structure Compared with their solid counterparts, capacitive pressure sensors with porous dielectric layer show improved compressibility and flexibility under the same pressure loadings, and also make change in dielectric constant under applied pressure, therefore producing more significant changes in capacitance and a higher sensitivity. Besides, porous dielectric layer renders the sensors with lightweight property. [24] As pointed out by another review paper, [11] the pore size of porous dielectric layer is difficult to be controlled by methods such as sugar particle templating and expandable forming agent. To address this issue, Shao et al. [25] used the expandable microsphere consisting of hydrocarbon core and thermoplastic shell ( Figure 6A) to control the size and distribution of pores. Specifically, the hydrocarbon shell vaporized during the expansion caused by heating, and the intrinsic thermoplastic shell remained and floated toward the upper PDMS surface, forming a uniformly distributed porous structure on the upper side of PDMS layer. [25] Compared with pure solid PDMS dielectrics, their design significantly strengthened the sensitivity of the capacitive sensor ( Figure 6B).
In another work, Kang et al. [29] fabricated a porous structure by using polystyrene (PS) beads. Specifically, PS beads were first stacked on silicon substrate and then PDMS was coated on the substrate with stacked microbeads. [29] Lastly, the PS beads were dissolved and the resulting porous PDMS layer was used as the dielectric layer of capacitive pressure sensor. [29] The pore size of the porous dielectric layer and the corresponding sensitivity of the pressure sensors can be controlled by changing the size of PS beads. PS beads with three different diameters (2, 4, and 6 μm) were selected to fabricate dielectric layer with various pore sizes. The sensitivity of the sensor with solid PDMS dielectric layer (without pores) was only 0.08 kPa −1 , whereas the ones with porous dielectric with the pore diameters of 2, 4, and 6 μm improved the sensitivity to 0.41, 0.48, and 0.63 kPa −1 , respectively. Therefore, this work not only demonstrated porous dielectric . Dielectric layer with porous, pyramid, cone, and wrinkled structure, for capacitive pressure sensors. A) Schematic demonstration of expandable microsphere. B) Sensitivity comparison between capacitive pressure sensors with and without porous PDMS dielectric. Reproduced with permission. [25] Copyright 2020, Wiley-VCH GmbH. C) Capacitive pressure sensor with porous pyramid dielectric. D) Sensitivity comparison between pure solid pyramid and porous pyramid dielectric. Reproduced with permission. [19] Copyright 2019, American Chemical Society. E) Capacitive pressure sensor with coneshape porous dielectric layer. Reproduced with permission. [21] Copyright 1969, Elsevier. F) Schematic and SEM illustration of capacitive pressure sensor with wrinkled dielectric layer, and functioning conditions under both low and high pressure. G) Influences on sensor sensitivity caused by various exposure time under fixed 40% prestrain. H) Strengthened sensitivity by introducing hollows into wrinkled PDMS dielectric layer. Reproduced with permission. [32] Copyright 2019, American Chemical Society. www.advancedsciencenews.com www.advmattechnol.de contributed to the sensitivity enhancement in comparison with pure PDMS dielectric, but also elucidated that the increased pore size of porous dielectric further improved the sensitivity of capacitive pressure sensor. For the sensor with porous dielectric layer of larger pores, the higher sensitivity was attributed to better compressibility of the dielectric layer and larger variation of dielectric constant by changing the volume proportion of the air under applied pressure.
2) Pyramid structure Pyramid microstructures are also widely applied for dielectric layer to improve the sensitivity. Compared with other microstructures, the pyramids tend to have a reduced equivalent elastic modulus and are easier to be compressed under the same level of pressure, resulting in a larger reduced distance between two electrodes. Bao's and co-workers [40,44,47] performed the experimental and modeling works on optimization pyramid structures with respect to separation distance, base length, and height of pyramids, indicating that these factors influencing both initial capacitance and capacitance change. Their work provided a general guide for the microstructure design of dielectric layer for capacitive sensors, to meet the requirement for diverse applications. By combing the merits of both porous microstructures and pyramids, Yang et al. [19] used silicon mold method [46,47,113,114] to introduce pores into pyramid dielectric structure ( Figure 6C). Like the porous structure mentioned above, this work also used PS beads to produce pores in pyramids. First, the blade-coated PS beads were stacked onto the pyramid patterned silicon mold, followed by adding the PDMS precursor (with cure agent at weight ratio of 10:1) onto the mold. [19] Then, the mold was transferred onto the top of a flexible substrate which has a thin spin-cast layer of precured PDMS. [19] By applying a certain pressure, the space between PS beads was filled with liquid PDMS precursor, which would be cured eventually. [19] Lastly, toluene was used to dissolve the PS beads after peeling pyramid structured PDMS off from the mold. [19] As results, The lower compressive modulus of PDMS and larger change in relative permittivity under pressure resulted in a higher sensitivity of the sensor. [19] Specifically, with the presence of porous structure, the sensitivity of sensor is approximately twice larger than that of pure solid pyramid one ( Figure 6D). Micropyramid improves the sensitivity of capacitive pressure sensor; however, it should be noted that nonlinearity in high pressure sensing is observed due to the stress concentration on the tip of the pyramid. Another structure similar to pyramid one is the cone-shaped dielectric layer fabricated with PDMS ( Figure 6E), and the sensor performance such as sensitivity can be improved through combining with the porous element. [21] 3) Wrinkled structure Wrinkled microstructure [30,32] can also be introduced into the dielectric layer of the capacitive pressure sensors to improve their performance. For example, by introducing wrinkled microstructure into an elastic Ecoflex dielectric layer, Baek et al. [30] demonstrated that the sensitivity of the capacitive pressure sensor can be significantly improved. Similarly, Zeng et al. [32] designed a tunable and flexible capacitive pressure sensor with wrinkled PDMS dielectric layer ( Figure 6F) via ultraviolet ozone method. The wrinkled microstructures can be controlled by the UV exposure time and prestrain condition, and they affected the sensitivity of the sensor especially in the high-pressure range ( Figure 6G). By introducing pores into PDMS wrinkled dielec-tric layer, the elastic resistance was further reduced, and the dielectric layer becomes more compressible. Thus, the sensitivity was further enhanced under applied pressure ( Figure 6H). Further, the sensor in this design showed a fast response time (<50 ms) and low detectable pressure limit (1.5 Pa). [32] This work demonstrated promising applications in disease diagnosis, continuous real time health monitoring, and other advanced clinical/biological wearable electronics. [32] 4) Hemisphere structure As discussed above, it is challenging to achieve a wide linear sensing for capacitive pressure sensors by simply optimizing microstructure alone. To extend the linear sensing range, Ji et al. [115] adopted the gradient microdome architecture (GDA) (Figure 7A) combining addition of conductive CNTs to profoundly extend the linear sensing range of capacitive pressure sensor ( Figure 7B). The GDA structure was obtained through a template, which was manufactured via microengraving technology. For results, unlike other microdome structures with uniform heights, this design used the graded and standardized microdomes to make the contact between electrode and domes in sequence when the sensor was compressed. In this work, the authors unveiled that microdome height, numbers of microdomes for each grade and substrate thickness could largely affect the sensing performance in linearity. The GDA design allowed sequential contact between electrode and microdomes from taller to shorter pixels ( Figure 7B), also sequentially incurred the variation in dielectric constant of the dielectric layer of PDMS/CNFs. [115] Consequently, their results showed that the linear sensing range can be extended up to 1700 kPa ( Figure 7C), and high pressure resolution under the load of 10, 100, 500, and 1000 kPa ( Figure 7D). According to Figure 7C, compared to the dielectric layer with 3.5 wt% CNT, the microstructure without CNT (pure PDMS) exerted an adverse effect on linear range. In addition, uniform microdome architecture (UDA) was ineffective to result in a wide linear sensing range. However, it is worth noting that some other factors like diameter, separation distance, and distribution of graded microdomes may also affect the linear range, which was not explored in their work.
Ionically Conductive Hydrogel Materials: Ionic hydrogel contributes toward supercapacitive sensors. With the application of ionically conductive materials as the electrolyte core layers, EDL [116][117][118][119][120] capacitive theory has been applied to improve the sensitivity of the supercapacitive sensor by forming EDLs between electrically conductive electrodes and ionically conductive core layer. Supercapacitive pressure sensors based on EDL usually require the microstructures in core layer to induce the contact area change between electrode and electrolyte (core layer).
1) Pyramid structure To eliminate the interference from other stimuli, Su et al. [121] designed a strain-unperturbed supercapacitive pressure sensor based on EDL mechanism with ionic elastomer. The elastomer consisted of 1-ethyl-3-methylimidazalium bis(trifluoromethylsulfonyl)imide (EMIM:TFSI) and poly(vinylidene fluoride-cohexafluoropropylene) (PVDF-HFP), and hexamethylenediamine serves as the crosslinker. The electrolyte core layer was designed as pyramid structure. This work added a spacer to make the sensor insensitive to applied strain, decoupling the strain effect of pressure sensing. [121] As pressure applied, the ionic elastomer deformed, and the contact area . Gradient dome structure of dielectric layer for capacitive pressure sensor. A) Schematic illustration of capacitive pressure sensor with GDA dielectric. B) Demonstration of operating principle under pressure of capacitive pressure sensor with GDA dielectric. C) Sensitivity comparison of capacitive pressure sensor between UDA and GDA design with 3.5 wt% CNT and without CNT. D) High-pressure resolution with the preload of 10, 100, 500, and 1000 kPa. Reproduced with permission. [115] Copyright 2021, Wiley-VCH GmbH.
change between electrode and electrolyte core layer incurs a higher sensitivity of the sensor. This sensor was equipped a low pressure detection limit of 0.2 Pa, and had been applied for the physical interactions on human skin or soft robotic skin. [121] 2) Hemisphere structure Under compression, dome-like structure of electrolyte core layer with ionically conductive materials can facilitate the capacitance changes through the contact area change between electrode and electrolyte core layer. Yin et al. [120] developed an organic thin-film transistor [9] pressure sensor based on ionic hydrogels with microdome structures. [120] In their design, elastic ionic polyacrylamide hydrogel was used to fabricate the electrolyte core layer with microdome structure ( Figure 8A). The authors used the microdomes with the base length of 80, 40, 20, and 10 μm to denote sensor 1, 2, 3, and 4, respectively. The experimental results revealed that the microdomes with smaller size triggered higher sensitivity ( Figure 8B). Similarly, Chhetry et al. [48] developed a pressure sensor with randomly distributed dome-like microstructured ionic film electrolyte core layer ( Figure 8C). Figure 8D shows the FEA simulation under various pressure levels. [48] The ionic core was fabricated with 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) and poly(vinylidene fluoride-co-hexafluoropropylene) (P(VdF-HFP)) via a sandpaper template. By adjusting the composition of P(VdF-HFP)/[EMIM][TFSI], the areal capacitance of the sensor as well as its response to pressure will change accordingly. [48] Specifically, a higher concentration of [EMIM][TFSI] produced a larger value in relative capacitance change, [48] indicating that the practical design can be customized to meet the specific requirements. To tackle the problem of nonlinearity, recently, Bai et al. [89] designed a hierarchical structured electrolyte core layer to form interlocks with bottom electrode (Figure 8E), and fabricated with the 3D printed patterned template. The hierarchical Figure 8. Dome structures of electrolyte core layer for supercapacitive pressure sensor. A) Schematic illustration of capacitive pressure sensor with EIPFbased microstructured core layer and its functioning mechanism. B) Sensitivity of supercapacitive pressure sensors 1-4 with EIPF-based microstructured core layer. Reproduced with permission. [120] Copyright 2019, Elsevier. C) Schematic illustration of randomly distributed microstructure, strengthened effect on capacitance change triggered by microstructure and equivalent circuit of EDL-based capacitive pressure sensor. D) FEA simulation of various pressure levels for randomly distributed microstructured core layer with 9 × 9 arrays. Reproduced with permission. [48] Copyright 2019, American Chemical Society. E) Schematic illustration of graded interlock dome design with micropillars based on EDL mechanism. F) Relative capacitance change of supercapacitive pressure sensor based on graded interlock dome design with micropillars. Reproduced with permission. [89] Copyright 2022, American Chemical Society. electrolyte core layer consists of micropillars on domes with PVA/H 3 PO 4 ionic hydrogel. In this design, the micropillars reduced the initial contact area between electrolyte core layer and electrode to incur a higher sensitivity ( Figure 8F), and the graded structure with interlocks contributed toward the linearity within the pressure range to 500 kPa.
3) Porous structure Porous structures also improved the performance of supercapacitive pressure sensor with EDL mechanism. Yang et al. [91] made use of EDL mechanism to design a sensor with porous ionic electrolyte core layer of polyvinyl alcohol/potassium hydroxide (PVA/KOH). The porous microstructure significantly improved the compressive deformability of the sensor, while also enhancing the sensitivity of the pressure sensor with a fast response of 50 ms. [91] Further investigation was conducted by fabricating sensors with three different KOH/PVA weight ratio, which were 1:5, 1:2, and 1:1. The results indicated that variation of weight fraction between KOH and PVA significantly affected the sensor performance in terms of sensitivity and linear sensing range, as the concentration of KOH played an important role in determining the conductivity and elastic modulus of the electrolyte core layer. 4) Wrinkled structure Wrinkled structure is also a promising design to improve the sensitivity of sensor via inducing changes of the contact area under EDL mechanism. For example, to enlarge the change of EDL area under applied pressure, Qin et al. [49] designed a supercapacitive pressure sensor based on EDL mechanism using a wrinkled-like microstructured (Figure 9A,B) electrolyte core layer with PVA/H 3 PO 4 via molding method. In this design, the whole structure had two parts including hill-like and ridge-like parts. The hill-like part was highly sensitive to small loads, resulting in a high sensitivity value of 37.7 kPa −1 . [49] The ridge-like part resulted in a better linear range in higher pressure range (100-350 kPa), due to a gradually decreasing slope. [49] 5) Surface texture structures Apart from the above-mentioned microstructures, attempts have been made to replica different surface textures. To effectively strengthen the sensitivity of the sensor with EDL mechanism, Bai et al. [90] used PVA/H 3 PO 4 film to fabricate the electrolyte core layer with graded intrafillable architecture ( Figure 9C-E) by templating the surface structure of sandpaper. The capacitive pressure sensor showed significantly improved sensitivity (>220 kPa −1 , Figure 9F) by imposing large EDL area change between electrode and electrolyte core layer. Moreover, a high pressure resolution of 18 Pa was also achieved in this work, making it possible to conduct highly accurate sensing in applications such as e-skin and weight measurement. [90] Similarly, Xiao et al. [82] designed the multilayer double-sided protrusive microstructures of electrolyte core layer ( Figure 9G) with P(VDF-HFP)-based ion gel to extend the linear sensing range (0.13-2063 kPa, Figure 9H). The best result was achieved by 12 layers of the double-sided microstructure via a simple sand-paper mould. For these two designs, both intrafillable and protrusive structures may incur problem in quality control, although some studies with these designs achieved good sensing performance.
In brief, under EDL mechanism, all these structures contribute toward the improvement of the sensitivity through the variation of contact area between electrodes and electrolyte core layer under pressure. Specifically, the well-defined interlocked hierarchical dome structure not only improved sensitivity, but also helped to extend the linearity of the sensor, due to the application of ionic material and constant increase of contact area under pressure.

Discussion
Sensitivity of capacitive type sensors is of vital significance as it is a key parameter for high SNR ratio and high-pressure resolution. [115] In recent years, enormous numbers of publications about capacitive type pressure sensors are related to sensitivity improvement with various materials and structure design strategies. Specifically, microstructures design and application of ionic hydrogel materials greatly improve the sensitivity of capacitive type pressure sensor. For microstructured designs, pyramid, dome, porous microstructures are the most common options to fabricate capacitive type pressure sensors to improve sensing performance. Ionic hydrogel materials, making a huge space for the application of EDL mechanism, further and significantly enhance the sensitivity and push the supercapacitive sensor to the front. As summarized in Table 2, as time progresses, sensitivity for pressure sensing has been improved gradually. For sensitivity, the value has increased to over 3000 kPa −1 , producing by the ionic hydrogel based supercapacitive sensor with intrafillable design. [90] The linear sensing capability of capacitive type pressure sensors is also important to directly correlate the input pressure with the output capacitance by a simple linear fitting. However, it is challenging to achieve wide range of linear sensing capability for microstructures like cone-shape and pyramid because of the nonlinear deformation originating from the structural stiffening with increasing pressure. Although some designs showed piecewise linearity, it is not the real and intrinsic linearity phase, and it produces an unclear region in transition area between two adjacent linear ranges. In brief, the sensor with better linear output simplifies the calibration and minimize the uncertainty in its output scaling. [115] For example, sensors with good sensing linearity can still output the result by measuring the relative change in capacitance without zero resetting, which is difficult for nonlinear sensors. Compared to nonlinear ones, linear sensors can work without nonlinear compensation sectors.
Within existing works summarized in Table 2, the realization of both high sensitivity and wide linear sensing range has been barely reported. It is anticipated that state-of-the-art technologies and methods such as machine learning [42] and artificial intelligence may be helpful to find optimum microstructure designs to extend the linear pressure sensing range for capacitive sensors. For example, use the graded/gradient design of microdome structure or semiellipse structure, and the optimization works compute the parameters such as size, separation distance and geometry of the graded microstructure to widen linear sensing range. For materials, ionic hydrogels are potential to be used to fabricate these microstructures to improve the sensitivity and simultaneously extend the linear sensing range. For industrial applications, high pressure resolution is crucial to obtain an accurate pressure mapping, which requires higher sensitivity and wider linear range as prerequisites. . Electrolyte core layer of with various structures, for capacitive pressure sensors. A) Schematic illustration of supercapacitive pressure sensor based on EDL mechanism using hill-ridge architecture microstructured core layer. B) Fabrication process of supercapacitive pressure sensor based on EDL mechanism using hill-ridge architecture microstructured core layer. Reproduced with permission. [49] Copyright 2021, Wiley-VCH GmbH. C) Schematic illustration of EDL-based supercapacitive pressure sensor with graded intrafillable architecture core layer. D) Fabrication process of EDLbased supercapacitive pressure sensor with graded intrafillable architecture core layer. E) SEM image of graded intrafillable architecture. F) Capacitance change with pressure and sensitivity of EDL-based supercapacitive pressure sensor with graded intrafillable architecture core layer. Reproduced with permission. [90] Copyright 2020, the Authors, published by Springer Nature. G) Schematic illustration of supercapacitive pressure sensor with multilayer double-sided protrusive microstructure. H) Sensing performance of supercapacitive pressure sensor with multilayer double-sided protrusive microstructure. Reproduced with permission. [82] Copyright 2021, American Chemical Society. www.advancedsciencenews.com www.advmattechnol.de  [ 122] Another point is that existing works barely mentioned the SNR values for their sensor performance. For wearable applications, a higher value of SNR improves the accuracy of detection of lower pressure like pulse or heartbeat, ensuring the reliability of the sensors. Table 2 makes a summarization of SNR (if applicable) for capacitive pressure sensors reported in the last decades.
Apart from these challenges, the classification of low pressure and high pressure are not well-defined. For example, both Ji et al. [115] and Xiao et al. [82] mentioned broad pressure sensing range, nonetheless, the concept of broad or wide pressure sensing range is not defined and specified. For wearable devices, biocompatibility is essential to ensure the safe close contact between device and human skin. Non-biocompatible materials like KOH or H 3 PO 4 have been widely used as ionic hydrogels for capacitive pressure sensors, which may not be suitable for the fabrication of wearable devices. Biocompatible ionic hydrogel such as PVA/NaCl/glycerol [123,124] is potentially applied to involve the fabrication of EDL-based sensors. Innovatively, the solid-state conductive ionoelastomer is also a promising selection for EDLbased sensors and this material can be 3D printed. [125] www.advancedsciencenews.com www.advmattechnol.de

Capacitive Strain Sensors
Capacitive strain sensors show the potential applications in biomedical fields such as electronic skin and noninvasive diagnosis, due to their preferable stretchability. The mechanical stimuli of strains are transformed into electrical signal, making the stimuli understandable and readable by humans. [126] Conventionally, sensitivity is a major problem for capacitive strain sensors due to limited capacitance changes constrained by geometric effect as demonstrated in Section 2. Nonetheless, material and structure design can improve the sensing performance of capacitive strain sensors.

Electrodes
For capacitive strain sensors, it is critical to retain electric conductivity of electrodes under stretching. [67] Both metallic and nonmetallic (e.g., carbon based materials) conductive materials have been used as electrodes of capacitive strain sensors.
A typical microstructure design to improve the performance of capacitive strain sensor is wrinkled electrode structure (Figure 10A). [59] In this design, the wrinkled structure was fabricated with gold film on parylene via CVD method. Compared to traditional parallel plate capacitive sensor, this Poisson's effect successfully exceeded the theoretical limitation and improved the sensitivity of the sensor ( Figure 10B). Thickness of parylene films varied from 500 nm to 2.5 μm, and the results showed that sensor fabricated with 500 nm thickness parylene films gave the best performance in reliability, stability, and sensitivity. [59] Sensors fabricated with thicker parylene film were observed with partly irreversible deformation of the wrinkled structures, which may led to the degradation of the sensing performance. [59] Apart from this, negative Poisson's ratio mechanism [127][128][129] has been employed to improve the sensitivity of capacitive strain sensors. Similarly, Shintake et al. [130] used liquid metal (eutectic gallium-indium) as electrodes and silicon elastomer as auxetic structure. The hierarchically auxetic structure design improved the sensitivity and also ensured the stretchability of the sensor.
The interdigitated structure contributed toward sensitivity improvement of capacitive strain sensors as well. [76,[131][132][133] Particularly, Kim et al. [131] introduced the interdigitated electrode (Figure 10C) to design a wearable and transparent capacitive strain sensor with high sensitivity of −1.57 in the strain range of 30% ( Figure 10D,E). The interdigitated AgNWs electrode was fabricated through a simple capillary force lithography method. [131] Different from conventional parallel capacitors, the structure is single membrane design which reduced the thickness and weight of the sensor, making it more stretchable and meanwhile easy to be applied onto human skin. [131] However, nonlinearity occurred beyond the strain range of 30% ( Figure 10E).
Recently, in order to further improve the sensitivity of capacitive strain sensor, Nesser et al. [134] designed fragmented resistive paper-like electrodes with CNT ( Figure 10F). In this work, the homogeneous single-walled CNT was obtained via vacuum filtration method, and then laser was applied to fabricate the cracked electrodes. Their design was able to get a large capacitance change within small strain. [134] Under certain measuring frequency of capacitance, the propagating cracks of the frag-mented electrodes under stretching lead to rapid decrease of the effective capacitance. This design resulted in a high GF value of 37 at 3% strain. [134] Apart from the above microstructures, twistable and helical electrodes [135,136] (Figure 10G,H) were developed for wearable and implantable capacitive strain sensors. Specifically, Lee et al. [136] used the highly conductive and stretchable fibers based on silver nanoparticles as electrode, and finally successfully improved the sensitivity of the sensor to ≈12. In this study, the stretchable conductive fibers were fabricated by direct chemical reduction process. [137][138][139] Specifically, PU-based stretchable spandex fibers were put into deionized water for sonification before immersion in a solution of AgCF 3 COO in ethanol to absorb Ag + ions. [136] After the evaporation of ethanol in the fibers, chemical reducing agent was added carefully onto the fibers in order to convert the Ag + to Ag nanoparticles. [136] Innovatively, compared to other implantable sensors, this suturable fiber sensor can be used for wireless monitoring of biomechanical tissue strain. [136] Moreover, this design also solved the issue about the structure mismatch between the sensors and tissues/organs. [136] In their design, the working mechanism of the sensor can be divided into two modes: the straightening of two double helical electrodes and further stretched condition after the contact of two electrodes. [136] The capacitance change of the sensor can be varied by adjusting turn density (cm −1 ) of the helical structure. Compared to traditional parallel capacitive sensors, the helical design improved the sensitivity and stretchability of the strain sensor. Apart from metal and carbon nanomaterials, conductive hydrogels with high stretchability and conductivity have also been applied to fabricate capacitive strain sensors. [140]

Core Layers
Apart from electrodes, the material and structure innovations of core layers have also been reported to improve the sensing performance of capacitive strain sensors. By modulating the dielectric property of capacitive sensor, Liu et al. [142] introduced poly(dopamine) (PDA)-modified CNTs into the dielectric layer consisting of the polar-nonpolar fluoro-silicone multiblock copolymers ( Figure 11A). Compared to conventional silicon rubber materials, this design was able to improve the strain sensing performance by improvement of dielectric constant via synergistic polarization effect. [142] To resolve the contradiction between high sensitivity and high stretchability, EDL mechanism was applied to fabricate the supercapacitive strain sensor. Xu et al. [77] used nanocomposites containing ionic hydrogel and silver nanofibers AgNF to fabricate the capacitive strain sensor ( Figure 11B), which is different from traditional capacitive strain sensors. Profoundly, this EDL-based design greatly improve the strain sensing performance with a stretchability of 1000% and a GF value of 165. [77] In this design, the EDL capacitance of the sensor was considerable large as the sensor was under static state. [77] However, the contact area between electrode and core decreased as the sensor was stretched, eventually resulting in a decrement of the EDL capacitance (Figure 11C). [77] Apart from high sensitivity and wide linear sensing range, other desirable properties such as self-healing property have also  [59] Copyright 2018, American Chemical Society. C) Interdigitated electrode design and functioning principle for capacitive strain sensor. D,E) Sensitivity of capacitive strain sensor with interdigitated electrode design. Reproduced with permission. [131] Copyright 2017, American Chemical Society. F) Electrode fragmentation design for capacitive strain sensor. Reproduced with permission. [134] Copyright 2021, American Chemical Society. G) Schematic illustration of twistable design of capacitive strain sensor. Reproduced with permission. [135] Copyright 2016, American Chemical Society. H) Schematic illustration of helical capacitive strain sensor. Reproduced with permission. [141] Copyright 2022, Wiley-VCH GmbH. Figure 11. Elastomer and ionic hydrogel-based core layer for capacitive strain sensors. A) Functioning mechanism of dielectric elastomer sensor (a) initial state (b) stretching state. Reproduced with permission. [142] Copyright 2021, Elsevier. B) Schematic illustration of capacitive strain sensor based on nanocomposites containing ionic hydrogel and AgNF. C) EDL-based supercapacitive strain sensor under static state and stretched state. Reproduced with permission. [77] Copyright 2019, Royal Society of Chemistry. D) Steps for Synthesis of PDA-PAA-CR 3+ based dielectric layer for capacitive strain sensor. Reproduced with permission. [78] Copyright 2020, Royal Society of Chemistry. E) Fabrication process illustration for (a) GO-CNTs hybrid and (b) self-healing PU-RGC nanocomposites. F) Fabrication process of self-healable capacitive strain sensor. G) Capacitance change with applied strain before and after self-healing of capacitive strain sensor. Reproduced with permission. [79] Copyright 2018, Wiley-VCH GmbH.
been introduced into stretchable capacitive sensors. Rao et al. [78] used polydiacetylene (PDA)-PAA-CR 3+ as dielectric layer to fabricate self-healing capacitive strain sensor ( Figure 11D). The self-healing mechanism is based on the intrinsic dynamic cross-linking property of electrostatic interactions between carboxylate groups pf PDA/PAA and Cr 3+ , and interactions of hydrogen bonding between carboxylic groups of PAA and PDA. [78] They also successfully extended the stretchability to 500% and achieved high sensitivity with the gauge factor of 160. Apart from hydrogels, another capacitive strain sensor with www.advancedsciencenews.com www.advmattechnol.de  [ 131] wrinkled Au on parylene -VBH tape 3.05 140% 2018 [59] -Hydrogel-based -VHB film 0.4% 200% 2019 [ 143] anisotropic conductive film -Hydrogel-based (EDL) 165 1000% 2019 [77] -Ag -Polydiacetylenehydrogel 160 500% 2020 [78] Fragmentated CNT on PDMS -PDMS 37 3% 2021 [ 134] helical Ag nanoparticles on elastomeric fiber -Hollow 12 ≈30% 2021 [ 136] self-healing capability was designed based on polyurethane/reduced graphene oxide/carbon nanotubes (PU-RGC) nanocomposites ( Figure 11E,F). [79] The self-healing ability of dielectric was achieved by introducing the disulfide bonds into the main chain of PU as chain-extender, and reactions of disulfide exchange and weak hydrogen-bond played significant role in the self-healing property of dielectric. [79] The capacitance changes with applied strain before and after self-healing are almost identical ( Figure 11G), indicating an excellent effectiveness of self-healing nanocomposites. [79] Overall, capacitive strain sensor is experiencing rapid development recently. Nevertheless, further advancements are still needed to improve their sensing performance. First, there is still a challenge to endow capacitive strain sensors with high sensitivity. Second, linear sensing capability is disrupted when structure engineering is applied to improve the sensitivity of capacitive strain sensors. In other words, it is challenging to simultaneously achieve both high sensitivity and wide linear range for capacitive strain sensors.

Discussion
It is challenging to significantly improve the sensitivity of capacitive strain sensors. To increase their sensitivity, electrodes of capacitive strain sensors were designed with microstructures. Complex 3D architectures, origami structures and 3D mesostructures showed the potential to improve the flexibility of sensors. Although microstructures like auxetic and wrinkled designs are developed, there still comes an intrinsic challenge of high sensitivity with simultaneous high stretchability.
For strain sensors, stretchability is an indispensable element to guarantee the normal function of the sensor. In recent years, with the development of materials synthesis, hydrogels are being applied to fabricate capacitive type sensors, to avoid materials failure of dielectric layer, and many designs endow the sensors with self-healing capability. However, most of hydrogel-based materials are temperature or humidity sensitive. In other words, calibration is required if the sensor is used under fluctuated temperature or humidity conditions, affecting the accuracy of the sensing tasks. Table 3 summarizes the typical development in sensitivity and stretchability of capacitive strain sensors in recent years. Table 3, both stretchability and sensitivity of capacitive strain sensors have been significantly improved in recent years. Hydrogel-type materials and EDL mechanism provides new insights to further improve the performance of the sensors.

Capacitive Temperature Sensors
Body temperature is an essential index for human health monitoring and capacitive type sensor also plays significant roles in temperature monitoring. Compared to resistive based temperature sensors, capacitive sensor does not have the problem of selfheating at work. [144] To better perform temperature monitoring, wearable sensors need to be endowed with good linearity, high resolution, and minimized interference from other mechanical stimuli.
Ionic materials and nanocomposites are becoming popular to construct capacitive sensors for temperature monitoring. Yoon and Chang [57] used PDMS/CNT and ionic fluid as dielectric later and electrodes, respectively, to fabricate the temperature sensor ( Figure 12A,B). In this design, the concentration of CNT in dielectric layer incurs a change in sensitivity because of the change of dielectric property. As a result ( Figure 12C), with the increasing concentration (1-6 wt%) of CNT, the sensitivity increased, and the sensor showed a good linearity within the temperature range from 23 to 63°C. This design provided promising applications in human body temperature detection and wearable electronics.
For wearable temperature sensing, it is essential to eradicate or minimize the cross-sensitivity from mechanical stimuli. Recently, Yu et al. [58] used thermoplastic polyurethane (TPU) and AgNWs as core layer and electrodes, respectively, to fabricate the capacitive temperature sensor with kirigami structure (Figure 12D), which can undermine the cross-interference of mechanical deformations. Compared to some commonly used materials for core layers (e.g., PDMS), TPU showed higher temperature coefficient of permittivity and coefficient of thermal expansion, which improves the temperature sensitivity of the capacitive sensor. [58] This design achieved a high sensing resolution of temperature (0.14°C) ( Figure 12E). [58] Their results indicated that the optimized kirigami structure of capacitive temperature sensor can effectively reduce the cross-sensitivity of strain, Figure 12. Capacitive temperature sensors. A,B) Schematic illustration of capacitive temperature sensor fabricated with ionic liquid electrodes and CNT/PDMS nanocomposites. C) Sensitivity of capacitive temperature sensor with different concentration of CNT in dielectric layer. Reproduced with permission. [57] Copyright 2017, Royal Society of Chemistry. D) Schematic demonstration of capacitive temperature sensor with kirigami design. E) Experimental and computational results for relative capacitance change of kirigami-structured capacitive temperature sensor under strain at 0%, 50%, and 100% (from left to right). Reproduced with permission. [58] Copyright 2021, Royal Society of Chemistry.
endowing the sensor with high sensitivity, linearity, and stretchability, and greatly improving the resolution of temperature sensing for point-of-care medical and wearable applications. [58] In recent years, ionic hydrogel is also used for temperature sensors. Exactly, Wu et al. [56] designed a temperature sensor ( Figure 13A) can be operated under capacitance mode, and the sensor was endowed good thermal sensitivity (24.25% per°C, Figure 13B), fast response (0.19 s) and recovery (0.08 s) time, a wide detection range (−28 to 95.3°C), and high-resolution (0.08°C). In this design, polyacrylamide (PAAm)/Carrageenan Figure 13. Capacitive ionic temperature sensors. A) Schematic illustration of ionic hydrogel based capacitive temperature sensor. B) Sensitivity of ionic hydrogel based capacitive temperature sensor. C) Schematic illustration of addition of LiBr into hydrogel for promoting stability. Reproduced with permission. [56] Copyright 2021, American Chemical Society.
double network (DN) hydrogel containing KCl was used as the middle layer of the sensor, and PDMS was adopted to encapsule the sensor. [56] Through the addition of lithium bromide (LiBr) into DN hydrogel, the dehydration of hydrogel was avoided, because of the dissociation of LiBr into Li + and Br − into water and then prevent the evaporation of water molecules via the strong interactions with water molecules ( Figure 13C). [56] At subzero temperature, the hydrogen bonds between water molecules are broken by strong hydrated LiBr, restraining the formation of ice crystals. [56] In this design, apart from the thermal expansion of the core layer in thickness, the dissociation of -carrageenan network released the K + ions and formed the double later at the interface with the increasing temperature. [56] Meanwhile, higher temperature improved the absorption of Li + and Br − ions at the interface, strengthening the double later effect and enhancing the sensitivity of the sensor. This work contributed toward the wearable applications of the capacitive temperature sensor with higher sensitivity, good stability, and biocompatibility. Recently, Yu et al. [145] applied temperature responsive ionic conductive organogel (PVA/NaCl/glycerol) as the core material to achieve a high temperature sensitivity of 0.095°C −1 . The ion mobility of the organogel which determined the capacitance of sensor was highly dependent on temperature and hence the capacitance changes were closely correlated with temperature changes. Meanwhile, by cutting the top and bottom electrode layers into kirigami pattern, the unfavorable interference from stretching and compression associated with human motions can be significantly suppressed and as a result a medical-grade temperature resolution of 0.2°C was obtained in this work, which is promising for wearable health monitoring devices. Table 4 summarizes the key sensing parameters for capacitive temperature sensors including sensitivity and temperature resolution from some recent published works. Year PAAm/Carrageenan DN hydrogel 0.2454 0.8 2021 [56] TPU 0.007 0.14 2021 [58] PVA/NaCl/glycerol 0.095 ±0.2 2022 [ 145]

Applications of Capacitive Type Sensors
In preceding years, there comes rapid progress and development in medical, sports, soft robots, and touch screen industries, which results in keen demands for sensing and monitoring devices. Undoubtedly, this situation provides an excellent opportunity for the promotion of capacitive type sensors, due to its advanced performances such as sensitivity, in recent years.

Medical and Health Monitoring Applications
Capacitive sensors can find wide applications in point-of-care (POC) monitoring, which is an effective way to detect individual health conditions. In recent, Sharma et al. [146] investigated and fabricated a reusable graphene oxide modified double interdigitated capacitive (DIDC) sensing platform ( Figure 14A) to detect SARS-CoV-2. In this design, the authors used a labelfree hybridization approach, monoclonal IgG antibodies (Abs) to target SARS-CoV-2 S1 protein. [146] In this sensor, 1-ethyl-3(3-dimethylaminopropyl)carbodiimide-N-hydroxy succinimide (EDC-NHS) served as crosslinker, which was attached by covalent bonds onto rGO substrate, for the functionalization of Reproduced with permission. [146] Copyright 2021, American Chemical Society. C) Schematic demonstration of clinical based and POC electromechanical diagnosis processes. Reproduced with permission. [148] Copyright 2021, Elsevier.
anti-SARS-CoV-2 Abs. [146] The capacitance response can be reflected by the concentration of SARS-CoV-2 S1-His protein. Consequently, the DIDC biosensor was granted with low limit of detection (1 fg mL −1 ) within a fast response of 3s. [146] Moreover, wide detection range (1.0 mg mL −1 to 1.0 fg mL −1 ), good linearity (18.56 nF g −1 , Figure 14B) and high sensitivity (1.0 fg mL −1 ) were achieved for SARS-CoV-2 detection. [146] Similarly, Hwang et al. [147] designed a capacitive type sensor to detect COVID-19 as a POC device. In this work, the DNA biosensor was fabricated with Pt/Ti interdigitated electrodes on glass substrates. Figure 14C shows the illustration of clinical based and POC electromechanical diagnosis processes. Compared to clinical based diagnosis, the POC diagnosis are portable, wearable, mass-productive, and low cost, showing a good application prospect on health monitoring. [148] Capacitive type sensors can be designed as preferable devices for noninvasive health diagnosis and monitoring such as intraocular pressure. For example, within the last decade, capacitive type sensors were designed as the continuous intraocular pressure (IOP) monitoring devices. [149][150][151] Representatively, Chen et al. [151] Adv. Mater. Technol. 2023, 8, 2201959 www.advancedsciencenews.com www.advmattechnol.de developed a capacitive contact lens sensor (Figure 15A,B) for persistent noninvasive intraocular pressure detection wirelessly. The sensor was embedded into a silicon rubber. This design allowed patients to conduct the IOP monitoring at home, which is beneficial and convenient for glaucoma monitoring and treatment. [151] As shown in Figure 15B, the sensor was composed by two layers: reference layer and sensing layer. The upper capacitor electrode and inductive coil were embedded into reference layer of the sensor and the lower capacitor electrode was implanted inside the sensing layer. [151] Any change in the corneal curvature resulted in a change of gap between electrodes, eventually causing the capacitance changes. [151] In recent years, miniaturized and portable health monitoring devices are becoming more and more popular. For example, glucose, a stimulus causing the diabetes, can be detected with capacitive type sensors [152,153] in real-time and continuous monitoring in the future. Recently, Choi et al. [154] developed a capacitive sweat rate sensor ( Figure 15C,D) to measure the sweat loss which is a significant parameter in evaluating the physical performance and healthy condition for individuals, subsequently diagnosing the ailments like hyperthermia or hyponatremia. In this design, the sweat passed through the inlet hole and gradually filled the microfluidic channel during perspiration ( Figure 15E), eventually resulting in the capacitance change due to an increase in relative permittivity value caused by inlet of sweat. [154] For senior citizens, hypertension is one of the health problems which may cause some cardiovascular diseases. In this case, blood pressure is an important parameter in monitoring these diseases. More recently, capacitive type sensing devices have been employed to perform the blood pressure monitoring. [19,70,112,155] Kim et al. [70] developed a wearable capacitive pressure sensor ( Figure 15F) for beat-to-beat blood pressure monitoring. Their testing results indicated a good consistency with the arterial pulse waveforms measured by Clearsight device ( Figure 15G). Furthermore, capacitive type sensors can also be used to monitor human respiratory rate. [26,32,115,156] Sharma et al. [156] designed a physical signal acquisition capacitive pressure sensor ( Figure 15H) for both respiration and radial artery pulse wave monitoring ( Figure 15I). Choi et al. [28] used porous elastomer and percolation CNT to develop a capacitive pressure sensor which can be used as wristband to conduct epidermal pulse rate monitoring.
Capacitive sensors also can be applied in surgical aspect. In recent, Kalidasan et al. [157] designed a bioelectronic sensing device for deep surgical wounds and postsurgical monitoring to avoid bleeding, infection, dehiscence, and other complications. Compared to traditional clinic-based surgical site monitoring devices and existing bioelectronic sensors, this device has the capability to provide the accurate and continuous wound monitoring with the wireless data transmission. [157] In this work, a conductive polymer and incorporating pledges with capacitive sensors worked through the radiofrequency identification, monitoring the physicochemical states of the deep surgical sites. [157] This work leads to a new sight for wireless sensing in medical and surgical applications.
Capacitive type sensors exhibit a robust capability in rehabilitation for a very long time. Researchers have developed various physical rehabilitation devices based on capacitive sensors. Chen et al. [158] applied the topological modification to design a capacitive sensor (Figure 16A,B) for physical rehabilitation. This textile-based capacitive sensor was endowed with excellent flexibility and comfortable properties, making the sensor compatible to be attached on human skin. [158] This device was capable of assisting the rehabilitation of chronic obstructive pulmonary disease (COPD), which can be alleviated via fast deep breathing. [158] Therefore, the device was used to perform the sensing of exhale and inhale ( Figure 16C,D). In this work, this rehabilitation training can be conducted in three states: standing, sitting, and lying. In addition, this device was also able to complete the voiceprint information monitoring, helping people to undergo the rehabilitation from language developmental delay or dysplasia (Figure 16E). [158] In recent, Zhu et al. [159] designed a skin-electrode mechanosensing structure (SEMS) sensor ( Figure 17A) to detect physical biosignals. Innovatively, this design distinguished from traditional wearable sensors and employed the human skin interface as natural ionic material to measure physiological signals and external mechanical stimuli. [159] Human skin is naturally humid because the sweat was pumped to skin interface. As perspiration contains Na + and Cl − , free electrons work as the carriers in the electrodes as ionic fluxes facilitate the conduction in human tissue and perform the exchange of electronic and ionic signals as the metal electrode was attached on skin interface. [159,160] In the SEMS design, Au-coated PDMS micropillars ( Figure 17B) were able to stimulate the capacitance variation to change the sensitivity via the change in contact area between the micropillars and human skin interface based electrode ( Figure 17C,D). [159] However, the metabolism situation of each person is not identical, probably affecting the sensing results and accuracy. The SEMS sensor was effective to perform the fast-walking detection ( Figure 17E) and smart protective gloves for pulse wave and touch sensing can be fabricated based on this sensor ( Figure 17F).
Nowadays, except the application for human health monitoring, wearable devices are also widely employed to detect body motion [18,27,49,77,87,90,115,[161][162][163] or gesture recognition. [87,140,164,165] For public, these devices can provide users more information regarding their working-out condition during and after exercise. For athletes, wearable devices can indicate whether their gestures are accurate and normative enough, which is able to assist the training for athletes. These sensors can be attached to human body such as wrists and fingers, put into the socks, gloves, or embedded into clothes and insoles of shoes. In recent, Li et al. [27] used 3D printing method with carbon fibers to endow the capacitive pressure sensor with body motion detection capability. The sensor can be attached to human joints including fingers, knees, and elbows to detect the limb motions and covert the motion to readable electric signals. [27] However, the existing designs for gesture recognition lacked high accuracy. For example, a hill-ridge architecture-based iontronic design [49] was used for hand-writing identification, where the low recognition rate was not able to differentiate high signal similarity of some letters.
As for sensors used for health industries, both high sensitivity and high SNR are required to maintain the function and ensure the sensing accuracy of these devices. For example, generally, human pulse waveforms can help understand the age and cardiovascular health conditions of individuals, [166] sensors with high resolution undoubtedly enhance the accuracy of diagnosis for related diseases, in a noninvasive manner. Moreover, Figure 15. Medical applications of capacitive sensors. A) Schematic demonstration of sensing mechanism of capacitive contact lens sensor for intraocular pressure monitoring. B) Schematic illustration of capacitive contact lens sensor in cross-sectional view and breakdown view. Reproduced with permission. [151] Copyright 2013, Elsevier. C) Schematic illustration of capacitive sweat rate sensor. D) Breakdown demonstration of capacitive sweat rate sensor. E) Schematic demonstration of equivalent circuits of capacitive sweat rate sensor. Reproduced with permission. [154] Copyright 2020, American Chemical Society. F) Schematic illustration of capacitive pressure sensor for beat-to-beat blood pressure monitoring. G) Arterial pulse waveforms measure by capacitive sensor (top) and Clearsight device (bottom). Reproduced with permission. [70] Copyright 2019, Wiley-VCH GmbH. H) Schematic illustration of physical signal acquisition capacitive pressure sensor. I) Radial arterial pulse wave (left) and respiration before/after exercise monitoring (right) using physical signal acquisition capacitive pressure sensor. Reproduced with permission. [156] Copyright 2020, American Chemical Society.  [158] Copyright 2020, American Chemical Society. D) Capacitance change of exhale and inhale during fast deep breathing practice for COPD rehabilitation. E) Capacitance change as people speak different phrases, for rehabilitation for language developmental delay or dysplasia. Reproduced with permission. [158] Copyright 2020, American Chemical Society.
biocompatibility and comfort need more attention in wearable designs to protect human skin. For example, for ionic materials, corrosive and toxic ingredients such as H 3 PO 4 should be replaced by other biocompatible ionic materials.

Robotic Applications
Besides human body, capacitive type sensors also show a broad application prospect in robotics. With the progress of machine Figure 17. Wearable applications of capacitive sensors. A) Schematic illustration of equivalent circuits for SEMS sensing device. B) Schematic of Aucoated PDMS micropillars used for SEMS sensing device. C) Schematic of SEMS sensing devices. D) Au-coated PDMS micropillars deformation and changes of contact area between micropillars and human skin interface based electrode under pressure. E) The signal feedback for fast walking detection with SEMS sensing device. F) Schematic demonstration of SEMS-based smart protective gloves for pulse wave and contact sensing. Reproduced with permission. [159] Copyright 2021, the Authors, published by Springer Nature.
learning and artificial intelligence, sensors are required to complete more accurate sensing tasks to ensure the normal function of robotic and other related electronic devices under harsh conditions. It is well-known that many special missions can be implemented by robots. For example, robots were recruited to clear nuclear waste after the Fukushuma Daiichi nuclear disaster in Japan. Therefore, sensing performance is critical to effectively execute the tasks and simultaneously provide feedback to terminals.
For biomimetic e-skins, Boutry et al. [41] designed a bioinspired e-skin ( Figure 4A) through capacitive sensing mechanism, aiming to transmit the feedback to control a robot arm. The capacitive sensor was effective to provide the feedback for normal and shear force and the e-skin was able to interact with deformable and delicate objects (e.g., raspberry). [41] Zheng et al. [167] adopted the contact area controlling strategy to design a linear response capaci-tive e-skin with high sensitivity. This device was able to detect a low pressure of 2.88 Pa and also perform the shape and hardness sensing. [167] In order to further mimic diversified sensations of nature skin, recently, Zhang et al. [168] introduced a robust proton-conductive ionic skin. Within this e-skin design, the capacitive type of sensor ( Figure 18A) was responsible for the pressure sensing with an ultrahigh sensitivity in low pressure (Figure 18B), a fast response time ( Figure 18C) and a preferable stability/repeatability ( Figure 18D). To perform multisensing tasks, Zou et al. [169] developed dual-mode sensor which can potentially be applied to e-skin. In this design, capacitive sensing was combined with resistive sensing mechanism to independently perform the pressure and temperature sensing, respectively. The dual-mode sensor was designed with a sandwich structure with two electrodes on the top and bottom of the structure, and the top electrodes was endowed with extra ability to generate resistive Figure 18. Robotic applications of capacitive sensors. A) Schematic illustration of capacitive pressure sensor used in a robust proton-conductive ionic skin. B) Sensitivity of capacitive pressure sensor used in a robust proton-conductive ionic skin. C) Demonstration of fast response time for capacitive pressure sensor used in a robust proton-conductive ionic skin. D) Illustration of excellent stability and repeatability of capacitive pressure sensor used in a robust proton-conductive ionic skin. Reproduced with permission. [168] Copyright 2021, the Authors, published by Springer Nature. E) Schematic illustration of human dermal sensory system. [170] F) Schematic illustration of operating of soft robotic prostheses. G) Sensitivity characterization of iontronic capacitive pressure sensor for potential robotic prostheses applications. Reproduced with permission. [172] Copyright 2021, Wiley-VCH GmbH. H) Schematic demonstration of SCMN multisensory e-skin for potential amputee prostheses applications. Reproduced with permission. [14] Copyright 2018, the Authors, published by Springer Nature. I) Schematic illustration of AI-motivated e-skin. J) Demonstration of robot interaction for different sign language gestures. Reproduced with permission. [122] Copyright 2022, Wiley-VCH GmbH.
response. [169] Navaraj and Dahiya [170] designed a fingerprint-like capacitive-piezoelectric sensor to mimic the human skin. For this e-skin design, the intrinsic limit of piezoelectric sensors makes them difficult to detect sustained static pressures. The integration with capacitive pressure sensor can compensate the weakness of piezoelectric sensor. [170] Finally, spatiotemporal tactile stimuli including static and dynamic pressures and textures could be detected and discriminated by this e-skin. [170] However, human dermal sensory ( Figure 18E) is one of the most sophisticated systems to perform the detection of various forces, temperature, pain, and humidity. In this case, further development and work are still indispensable to fully mimic the skin sensory system for robotic applications.
For both medical and robotic applications, capacitive type sensor plays a significant role to provide feedbacks for sensation of limb protheses. [171,172] Shen et al. [172] developed a sensing device based on cutaneous ionogel mechanoreceptors to detect physiological signals and potentially be utilized for prostheses and soft machines ( Figure 18F). EDL mechanism was employed in this design to improve the sensitivity of the device. Moreover, compared to traditional capacitive sensors, this design was able to perform the pressure sensing for protheses with a fast response and broader sensing range. However, this design was not equipped with a wide linear sensing range ( Figure 18G), which is important for robotic applications, as abovementioned, a less linear sensor always requires calibration and increases the difficulty in feedback data processing. Limb prostheses are desired to improve the living quality of amputees. However, in human body, the process to transmit the sensory stimuli to our brain neuro is rather complicated. In other words, several steps are required to help amputees to establish the natural-like sensations with prostheses. [64] Hua et al. [14] developed multifunctional sensing system which potentially can be utilized for protheses. Notably, this stretchable and conformable matrix network (SCMN) skin-like sensing system ( Figure 18H) was comparatively robust, having the capability to detect temperature, in-plain strain, humidity, light, pressure, proximity, etc. [14] The SCMN design was endowed the capabilities for simultaneous multistimuli sensing, an adjustable sensing range and large-area expandability, establishing a personalized intelligent prosthetic hand for amputees. [14] Except for noncontact sensing, capacitive proximity sensors had been discovered to be suitable for displacement sensing, [173] which may facilitate the progress of robot development. For surgical robots, it is indispensable to be endowed with displacement sensing, to perform the surgery accurately for patients. For surgical robots, capacitive pressure sensors are expected to conduct the stiffness sensing for human organs, and the feedback is essential for artificial neural system of robots to analyze and make decision about the strength of required applied force for surgical operations to avoid injuries to human organs.
Capacitive sensing technology contributes toward artificial intelligence (AI) as well. Recently, Niu et al. [122] designed an AImotivated human full-skin bionic e-skin ( Figure 18I). In this work, they combined the capacitive sensor with triboelectric sensor to perform the gesture cognition ( Figure 18J) and material cognition. Specifically, the triboelectric sensor and EDLbased supercapacitive sensor were developed to mimic the vellus hair layer and epidermis-dermis interlocked layer and dermis-hypodermis interlocked layer, respectively. The cone-shaped interlocked electrolyte core layer enhanced the sensitivity (8053.1 kPa −1 ) because of the stress concentration at the contact point. [122] This design builds a new avenue for artificial intelligence in tactile perception and communications between signers and nonsigners. [122] Generally, as for the sensors for robotics applications, both sensitivity and linearity are important to ensure the normal function and simplify the calibration of the device, as abovementioned. However, other aspects should also be considered as well, according to different sensing tasks. For example, fire-fighting robots need to work under high temperature conditions, in this case, how to ensure the consistency of sensitivity and linearity of the sensors under large temperature variations ought to be considered.

Touch Screen Applications
Apart from medical and robotic applications, capacitive sensor can be employed for touch screens due to its simple structure and low power consumption, where transparent materials are required to fabricate the capacitive touch screen. Kim et al. [174] developed a low cost capacitive touch screen sensor ( Figure 19A) based on silver nanopaste ink. Specifically, the electrode arrays for capacitive sensors were printed on indium tin oxide (ITO)patterned PET thin film, which endows the touch screen with thinner configuration and potential to replace traditional input devices including keyboards and mice [174] with better humancomputer interaction. Similarly, Franco et al. [175] used the screenprinting technology to fabricate a transparent and flexible capacitive multitouch sensing surface with graphene-based conductive inks as electrodes. As shown in Figure 19B, there was a noticeable capacitance change with touch between finger and screen. In their work, the biodegradable carboxymethyl cellulose served as blinder, contributing toward the environmental conservation. [175] Ideally, touch screen sensor arrays are self-healable, as replacing a cracked screen with a new one is always costly. Over the past a few years, Li et al. [176] designed a transparent capacitive touch screen based on a healable silver nanowire-polymer composite electrode ( Figure 19C). In this design, the damaged electrode could be healed via heating, and the performance of the touch screen would be restored under heating at 80°C for 30 s. [176] Thus, it is promising to improve and ensure the durability and reliability of electronic devices by introducing self-healing properties.
Overall, apart from sensitivity and durability, other properties such as optical transparency are essential for touch screen sensors. For future designs, self-healable touch screen is highly anticipated for widespread application in smart devices, which can be commercialized in an affordable price. Moreover, some touch screens of smart devices have a lower sensitivity in winter at cold regions of the world, causing the inconvenience for users under emergency, which requires future work design of a touch screen with consistent sensitivity under variable temperature conditions.

Challenges and Future Opportunities
In the last decade, with the aid of material design, novel microstructures and EDL mechanism, the performance of Figure 19. Applications of capacitive sensors for touch screens. A) Schematic illustration low-cost, thin, and transparent touch screen sensor arrays. Reproduced with permission. [174] Copyright 2020, Wiley-VCH GmbH. B) Capacitance change of eco-friendly graphene-based capacitive sensing surface under touch. Reproduced with permission. [175] Copyright 2021, Wiley-VCH GmbH. C) Fabrication process of patterned composite film for healable capacitive touch screen. Reproduce from permission. [176] Copyright 2014, American Chemistry Society.
capacitive type sensors has been significantly improved with respect to sensitivity, sensing range, SNR, and stretchability. At the same time, these sensors show tremendous application prospects in the areas of medical, sports, robotics, and electronic devices. However, challenges are still distinctly remaining, leaving further opportunities and new directions for the extensive and widespread application of capacitive type sensors. 1) Biocompatibility. For wearable devices, biocompatibility is of vital significance, and traditional materials like some ionic hydrogels with corrosive liquid acids does not fulfill this requirement. Prototypically, as an ionic material, PVA/H 3 PO 4 hydrogel is commonly employed to get involved in the fabrication of electrolyte core layers for supercapacitive sensors. This ionic hydrogel profoundly enhances the sensitivity of the capacitive [EMIM][TFSI] 1 V 2020 [ 179] sensor, due to the EDL mechanism. Nevertheless, H 3 PO 4 is not an ideal substance for wearable electronics, because of its acidic property, which may cause damage or be allergic to human skin. 2) Materials failure. Material failure under fatigue, especially for dielectric or electrolyte layer, is a significant problem to assure the normal function of capacitive sensors. For wearable electronics, it is essential to endow the materials with better durability and repeatability. Even though most existing publications showed good repeatability under cyclic loading and stretching, material failure such as plastic deformation or cracking is barely considered, quantified, and reported. For sensor design, mechanical property ought to be considered coupling with electric property. 3) Well-defined structure. A well-defined structure for both electrodes or dielectric/electrolyte core layer is crucial and critical for reproducible sensing performance. Although some designs achieved better sensing performance in either sensitivity or linearity, their design suffered from the scarcity of well-defined structure, affecting the consistency and quality control for industrial manufacture. 4) Wireless sensing and self-power. For wireless sensing devices, it is indispensable to be equipped with self-powered capability. Table 5 summarizes the details for these capacitive sensors with low operating voltages, although capacitive type sensor has low power consumption in comparison to resistive sensor, external power is still needed to maintain the normal function, limiting the progress of wireless sensing for capacitive sensor.
For wearable devices, opportunities are furnished to design and fabricate the capacitive type sensor with biocompatibility, durability, and wireless sensing capability, which are of vital importance for wearable electronics. For example, in a medical application, human body biosignals are detected and measured by wearable capacitive sensors, and then wirelessly transmitted to the computer terminal for analysis, facilitating the progress of real-time monitoring and diagnosis and improving medical efficiency.

Conclusion
This review comprehensively discusses recent developments and advancements including materials, structures, and applications of capacitive type sensors. Particularly, state-of-the-art structure designs and recent EDL-based hydrogel materials are summarized. All these advancements contribute toward the perfor-mance improvement of capacitive type sensors, facilitating the wider applications of these sensors. Despite the existing challenges and difficulties, capacitive type sensors show a tremendous potential to be applied in medical, robotics, and electronics fields such as human health monitoring, e-skin, and touch screen sensor, respectively. Shuhua Peng obtained his Ph.D. degree in polymer science and engineering from Deakin University. He is currently a Lecturer and ARC Future Fellow in School of Mechanical and Manufacturing Engineering at University of New South Wales Sydney. His research covers soft nanocomposite materials for applications such as wearable electronics and sensors, energy harvesting and storage, and soft robotics.