Recent Advances in Wearable Tactile Sensors Based on Electrospun Nanofiber Platform

Wearable electronics have triggered the great development of flexible tactile sensors for promising applications such as healthcare monitoring, motion detection, and human‐machine interaction. However, most of the flexible sensors are constructed on compact and airtight polymer films with inferior flexibility and no breathability, hindering the wearability and comfortability for long‐time continuous operation usage. To address such challenges, flexible sensors based on the electrospun polymer platform with comprehensive advantages of ultrathin thickness, superior flexibility, excellent stretchability, high porosity, low density, and surface functionality are emerging. It has become a hot research direction, and considerable progress has been made. Therefore, it is necessary to timely review the latest findings on the rapidly developing electrospun nanofiber‐based tactile sensors. Firstly, the principle of the electrospinning technique, the factors affecting the nanofiber morphology, and the engineering of the nanofibers are briefly introduced, and the key material and structural factors affecting the sensing performance are analyzed. Secondly, representative work on the electrospun nanofiber‐based tactile sensors is discussed in detail according to the sensing mechanism. Thirdly, unique properties of electrospun nanofibers, such as superior flexibility, breathability, hydrophobicity, anti‐bacterial, and self‐cleaning, are highlighted. Finally, the remaining challenges and future development trends are outlined.


Introduction
Flexible tactile sensor as a transducer can generate measurable electric signals under the specific stimuli of pressure or force, and DOI: 10.1002/adsr.202200047 have potential in the frontier field of wearable sensing for physiological signal monitoring, human motion capture, electronic skin, human-machine interaction, etc. [1][2][3][4] After years of extensive research, great achievements have been made in the design and fabrication of flexible sensors based on a variety of advanced nanomaterials as the conducting fillers and elastic polymers as the flexible substrates. [2,[5][6][7][8] In order to improve the key sensing parameters of sensitivity, detection range, response time, stability, and reliability, a lot of attempts have been tried by means of utilizing different sensing mechanisms, compositing multiple materials, and constructing various microstructures.
However, in addition to the high sensing performance, the wearability and comfortability of the developed flexible sensors for a long time of continuous sensing operation should also be well considered in practical wearable sensing applications. It is worth noting that the flexible sensors were constructed on either flexible polymer substrates such as polyethylene terephthalate (PET) and polyimide (PI) or stretchable polymer substrates such as polydimethylsiloxane (PDMS), Ecoflex, and thermoplastic polyurethane (TPU), [9][10][11][12][13] but most of them were fabricated in the form of compact films. Such airtight films are not air or moisture permeable, so wearing the sensors contacting the skin after a long time would cause uncomfortable consequences such as redness, allergies, and even inflammation. To address such challenges, flexible sensors based on textile materials with internal porous and channel structures for air and moisture passing through are emerging and have become a promising research direction. [14][15][16] However, conventional natural and synthetic textiles are made of thick fibers with a large diameter in the range of tens to even hundreds of micrometers and the overall thickness of the textile substrate is usually large, which cannot achieve an ideal conformal attachment on the soft skin. In comparison, the textile made of interwined nanofibers by the facile electrospinning technique possesses comprehensive characteristics such as ultrathin thickness, superior flexibility, excellent stretchability, high porosity, low density, and surface functionality, and is an excellent substrate for developing flexible and breathable sensors with much better wearability and comfortability. [17][18][19][20] In addition, the intrinsic micro-porous structure also endows the sensor with improved compressibility, which benefits in achieving both high sensitivity and wide detection range. Furthermore, unique properties such as sweat inertness ability, bacteria prevention and self-cleaning ability are also achieved. [21][22][23][24] Table 1 provides a comparison between the flexible substrates made of electrospun nanofibers and conventional polymer films and textiles.
Up to date, sufficient attention has been attracted on the development of flexible electrospun nanofiber-based tactile sensors with progressive achievements being made (Figure 1), therefore it is necessary to have a comprehensive review to timely summarize the latest findings in this rapidly developing field.
The research content of wearable tactile sensors based on the electrospun nanofiber platform is depicted in Figure 2. In the following sections, firstly, the electrospinning technique of fabricating elastomeric nanofibers is briefly introduced, including the working principle of electrospinning and the manufacturing factors, materials selection and structure engineering of the electrospun nanofibers as well as their influences on the sensing performance. Secondly, by classification of the pressure sensing principle, recent representative work on the piezoresistive, capacitive, piezoelectric, and triboelectric nanofiber-based sensors is comprehensively reviewed. Thirdly, unique properties of nanofiberbased sensors such as superior flexibility, air permeability, sweat inertness ability, bacteria prevention, and self-cleaning ability are highlighted. Finally, critical challenges that still exist and perspectives on the future development are outlined.

Electrospinning of Nanofibers
Since the beginning of civilization, many routes have been attempted to produce synthetic polymer-based fibers derived from a gel, dry, wet, or melt spinning. However, these methods are limited to the jets stretchable to a limited extent, with a high diameter range. [25] William Gilbert was the first person to observe the electrostatic movement of liquid in 1600. After a considerable time gap, Charles Vernon and the team reported drawing fibers from the liquid under the external electric condition in 1887. The first patent was filed in electrospinning by John Francis in 1900. [26,27] The extensive research has continued till now, and more researches on electrospun nanofibers has enhanced year by year with the addition of various new applications like biosensing, biomedical device, water treatment, energy harvesting, conversion and storage, catalysis, etc. [28,29] Recently, the electrospinning technique has been widely utilized as a facile and effective approach to fabricate ultrathin fibers with a small diameter ranging from nanometers to micrometers. Researchers have begun trying to use the electrospun nanofiber membrane as a platform to develop the advanced flexible sensors, due to its intrinsic fabric network structure with excellent mechanical property and air permeability. The amount of research on this emerging field has been increasing year by year. [28,29]

Principle of Electrospinning
Nanofibers can be produced through the electrospinning process by the uniaxial electrostatic stretching and elongating of liquid droplets of polymer solution or pristine polymer that melts when electrified. [30,31] A typical basic electrospinning setup consists of a high voltage power supply, a syringe pump, a hypodermic needle spinneret with a blunt tip, and a grounded conductive collector, [32,33] which is quite simple and easily accessible to almost every laboratory. As illustrated in Figure 3a, polymer precursors are extruded from the spinneret to produce a pendant droplet as a result of surface tension. Upon electrification, the electrostatic repulsion among the surface charges that feature the same sign deforms the droplet into a Taylor cone, from which a charged jet is ejected. [34] These strong electrostatic forces initiating the electrospinning process. Under the strong electric filed, as the jet is stretched into thiner diameters, it solidifies quickly, leading to the deposition of solid fibers on the grounded collector.

Factors Affecting Nanofiber Morphology
The diameter morphology plays an important role in the performance of the electrospun fibers, which are mainly determined by several specific electrospinning parameters, including applied voltage, distance between the tip and collector, flow rate of the precursor solution, and environmental conditions of humidity and temperature (Figure 3b). [35] The applied voltage is the primary factor for the formation of electrospun fibers. The successful electrospinning process happens only when the electric potential energy is higher than the surface energy of droplets of the chosen polymer solution. Firstly, the polymer liquid droplet is extruded at the needle tip by electrostatic charges and deforms into a cone shape known as a Taylor cone. Secondly, thinner and smaller fiber is drawn due to the strong stretching effect applied on the polymer liquid droplet by a higher electric voltage. Thirdly, further increase in electric voltage is recognized to decrease the size of the Taylor cone and increase the velocity of the jet. Finally, the ejected fibers would deposit the grounded collector across a certain distance for solvent to evaporate. [36,37] During the electrospinning process, the critical value of the electric voltage that is needed to generate the conical shape for the polymer liquid droplet is also influenced by the property of the polymer mixtures. [38] The molecular weight of the polymer makes a property variation of the electrospun nanofibers. The viscosity of the polymer solution increases when the molecular weight is large. [39,40] For a more viscous polymer liquid, the electric voltage needs to be increased correspondingly to reach a higher critical value that is capable of generating an electrostatic repulsion strong enough to overcome the sum of the surface tension and the viscoelastic force of the liquid.
The flow rate of the polymer solution can be precisely controlled by the injection speed of the syringe pump, which can also influence the formation of nanofibers. In general, the increase of the flow rate would result in thicker fibers with enlarged diameters because larger amount of polymer liquid is extruded upon the same time.
A suitable distance between the tip and collector is also required to ensure a full extension and solidification of the jet, and thereby the formation of the solid-state fibers. A typical distance of ≈15 cm is most commonly adopted in literature. [41] When the jet is flying from the spinneret to the collector, the fibers are in a transitional unstable stage. The electric fields should be large enough to accelerate the polymer solution jet across the whole distance. [42] Thinner fibers will be formed as the distance is increased due to the longer stretching distance. On the other hand, a distance that is increased beyond a certain level would increase the diameter of the fibers because of the significantly weakened electric field strength.
By taking all the above experimental actors into account, it is an intricate interplay of multiple processing parameters determining the overall morphology of the electrospun fibers. For example, with the increase of the flow rate, the critical electric voltage needs to be increased and the distance between the tip and collector needs to be shortened to ensure a full extension and solidification of the jet. [43] In the first place, the fibers must be successfully electrospun and then the morphology of the fibers can be tuned by changing the electrospinning parameters within an appropriate workable range.
In addition, the humidity and temperature of the environment also affect the morphology of the electrospun fibers. [44] The relative humidity influences the evaporation rate of the solvent and thus the solidification rate of the jet. Both high and low humidity would yield thicker fibers with a larger diameter. Under a high humidity, the solidification process slows down because the solvent tends to reserve, leading to a thicker fiber. In comparison, under a low humidity, the solvent tends to evaporate quickly, hindering the extension of the jet, also leading to an increased diameter of the fiber. [45] The temperature acts by influencing the thermal dynamics of the polymer solution. At an elevated temperature, both the surface tension and viscosity of the polymer solution would be reduced, favoring the formation of thinner fibers. However, the evaporation rate of the solvent would be accelerated, limiting the extension rate of the jet. Therefore, a competing balance of both aspects should be reached to obtain an optimized fiber. [46]

Polymer Materials for Electrospinning
A variety of polymer materials in the form of either solvent solution or melting liquid can be used to electrospin fibers. When combined with the sol-gel chemistry, composite materials can also be electrospun into fibers. By introducing the foreign components with different dimensions and morphologies into the polymer precursor solution, the electrospun fibers can be endowed with more functions. For each kind of polymer, the molecular weight plays a key role in the morphology of the electrospun fibers, with high molecular weight to obtain thicker fibers due to the increased solution viscosity. [39,40] The commonly used polymer materials for electrospinning are TPU, polyvinylidene fluoride (PVDF), poly(vinyl alcohol) (PVA) and polyacrylonitrile (PAN), the molecular formula of which are shown in Figure 4. The following section will give a brief introduction on the feasible solvent and content for electrospinning as well as the characteristics of the electrospun nanofibers made of diversified polymer materials, which is also summarized in Table 2.
TPU is a polymer material composed of alternate rigid blocks and soft segments. A large number of carbamate groups in the soft segments endows TPU with excellent elasticity, while the  [58] urea groups in the rigid blocks give TPU the high mechanical strength and wear resistance. In addition, the microphase separation structure makes TPU have a good biocompatibility, [47] which enables TPU to be used in the biomedical devices, elastic clothing, and other human-related fields. [48] By nature, TPU is hydrophobic so it can be used as waterproof material. [49] By using the polar organic solvents of dimethylformamide (DMF) and tetrahydrofuran (THF), TPU nanofibers can be electrospun with well mechanical property of superior flexibility and stretchability as well as air and moisture permeability. PVDF is a special plastic material in fluoroplastics, which possesses strong mechanical strength, hydrophobicity, chemical stability, biocompatibility as well as high dielectric strength and UV radiation resistance. PVDF can be solved in polar organic solvents such as DMF, N,N dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), THF, and N-methylpyrrolidone (NMP). PVDF contains four kinds of crystalline phases named as , , , and phase, and the piezoelectric effect can arise upon the phase. Therefore, among other polymer materials, PVDF is also a promising candidate for mechanical energy harvesting applications. [50,51] Research has proven that the phase existing in the electrospun PVDF nanofibers is much higher than that of the conventional PVDF cast film, so the electrospinning technique is an effective way to improve the piezoelectric performance of PVDF. [52,53] In addition, the high flexibility of PVDF nanofibers also promotes the application of piezoelectric devices in wearable electronics and tissue engineering. [54] PVA is a polymer material that is easily soluble in water and thus can be directly electrospun from its aqueous solution without using the toxic organic solvents. By its intrinsic nature of the hydrophilic hydroxyl (─OH) in its molecule, PVA is a green material that can be completely degraded by microorganisms in soil. Due to its excellent wound exudate absorption capacity, excellent anti-bacterial property, long-term slow-release property and good biocompatibility, PVA can be directly used as wound dressing, cell scaffold, and tissue reconstruction in vitro. Meanwhile, PVA has a high mechanical property that can support the mechanical strain such as tension, compression, and distortion for the development of mechanical sensors. [55] The electrospun PVA nanofibers maintain the characteristic of hydrophilicity and water solubility but also leads to a challenge of poor water resistance during long time usage. During fabrication, the water solubility and moisture absorption of the electrospun PVA nanofibers can be controlled by adjusting the raw materials, content formula, and procedure parameters to meet the needs of different applications.
PAN is a polymer material obtained by free radical polymerization of the acrylonitrile monomer. It is soluble in polar organic solvents such as DMF, DMSO, sulfolane, and ethyl nitrate and also in concentrated aqueous solutions such as nitric acid, thiocyanate, perchlorate, zinc chloride, and lithium bromide. Among the solvents, DMF is widely used due to its strong solubility and high dielectric constant. Different from other polymer materials, PAN is the precursor accounting for about 90% of carbon fibers manufactured today. [56] Electrospinning of PAN followed by stabilization and carbonization has become a straight forward and convenient way to fabricate continuous carbon nanofibers. Due to the high specific surface area, mats, felts or membranes made of electrospun PAN nanofibers have seen their extensive uses in the fields of sensors and catalysis.

Regulation of Nanofiber Structure
The structure and composition of the electrospun nanofibers can be controlled by using different electrospinning needles and adding multiple component materials in the precursor solution, to meet requirements by different applications. In a typical electrospinning setup, one hollow needle as the spinneret for the continuous production of single individual nanofiber (Figure 5a). At the single needle mode, the nanofiber throughput is low, typically 1-5 mL h −1 by flow rate or 0.1-1.0 g h −1 by fiber mass. As the electrospinning process progresses, a large number of individual nanofibers randomly distribute on the collector to form a network mat (Figure 5b), [59] which can be carefully peeled off and directly used as fabric membrane for further treatment.
Besides, the multi-needle electrospinning offers a straightforward route to effectively increase the number of fiber connections at the same time by using multiple needles arranged by a certain layout on the same disc ( Figure 5c). [64] Geometry parameter of the spacing distance between adjacent needles in the layout of the needle array must be well designed to enable the success of multi-needle electrospinning. The spacing distance is mainly determined by the diameter of individual needle and the property of the precursor solution for electrospinning. The smallest spacing is a distance at which the polymer liquid droplet suspended at the tips of neighboring needles do not fuse together among each other. [65] The number of the needles that can be arranged over a certain area is dependent on the spacing and layout, and Adapted with permission. [60] Copyright 2022, Nano Energy. c) Schematic diagram of needle array and d,e) SEM images of electrospun nanofibers ejected from multiple needles. Adapted with permission. [61] Copyright 2013, Journal of Applied Polymer Science; Adapted with permission. [62] Copyright 2008, Polymer. f) Schematic diagram of co-axial spinneret with an inner and outer structure for different fluids passing through and g) SEM image of electrospun nanofibers with a hollow structure. Adapted with permission. [63] Copyright 2004, Nano Letters.
it ultimately determines the throughput of fiber production. The layout of the needle array controls the actual distribution of the applied electric field. In order to obtain a uniform ejection of jets for all needles (Figure 5d,e), the electric fields around the needles should be homogenized. Otherwise, parts of needles may not be able to reach the critical voltage for jetting, causing the clogging of needles. On the other hand, the same flow rate should also be maintained among different needles. By separately placing needles with different inner diameters in the central and peripheral locations, the same flow rate can be obtained for all needles. Therefore, the applied voltage and the flow rate among different needles must be well optimized. [66] Furthermore, an elaborately designed coaxial needle consisting of two concentric hollow needles ( Figure 5f) has also been developed to generate a coaxially electrified jet for electrospinning of nanofibers with a unique core-sheath structure ( Figure 5g). [63] Two syringe pumps are used to drive two types of fluids into the inner and outer needles, respectively, at separately addressable flow rates. When the core and shell fluids meet at the exit end of the coaxial needle, the shell fluid will wrap around the core fluid to form a compound Taylor cone in the presence of an external electric field, followed by the ejection of a coaxial jet. Finally the core-sheath nanofibers with distinct compositions for the core and sheath can be obtained. The high co-axiality level of the coaxial needle is the preliminary request to ensure the robust and reproducible generation of the core-sheath nanofibers, [67] and the processing key to the success of the coaxial electrospinning is to ensure that the inner and outer fluids form a compound jet and stay together in a concentric manner. [68]

Regulation of Nanofiber Composition
In order to add other functions like conductivity to fibers, foreign component materials, such as metals, metal oxides, carbons, conducting polymers and even ionic liquid, can be incorporated into the precursor solution as fillers to initiate a co-electrospinning process for the fabrication of composite nanofibers. Alternatively, the fillers can be electrosprayed directly onto the surface of the electrospun nanofibers during the electrospinning process. [69,70] Other than that, the fillers can be decorated onto the surface of the electrospun nanofibers through post-treatment of the nanofiber membrane such as dip-coating and hydrothermal heating, in which the fillers attach the fibers by means of hydrogen bonding, chemical bonding or electrostatic force. [71,72] For example, the electrospun crosslinked PVA nanofibers were first functionalized with 3-mercaptopropyltrimethoxysilane and then immersed in an aqueous solution containing Au nanoparticles. As a result, Au nanoparticles were tightly attached onto the surface of the PVA nanofibers due to the strong affinity between the thiol group and the Au surface. [71] Besides composition, the intrinsic solid structure of the electrospun nanofibers can be modified with internal pores to drastically increase the specific surface area of the resultant nanofiber mat.

Sensing Principles of Nanofiber-Based Sensor
In principle, a tactile or pressure sensor is a transducer that can transform external force or pressure exerting on it as the input stimuli into a measurable electric signal (e.g., current, voltage, resistance, capacitance, etc.) that can be read by electric equipment. Currently, flexible tactile sensors have been thoroughly researched based on four types of pressure sensing mechanisms, that is, piezoresistive, capacitive, piezoelectric, and triboelectric sensors. [73] By combining the advanced conductive nanomaterials (e.g., silver nanowires (AgNW), carbon nanotubes (CNT), graphene, MXene, conducting polymers, etc.) as sensing fillers and the elastic polymer films (e.g., PET, PI, PDMS, TPU, etc.) as the flexible and even stretchable substrates, a variety kinds of flexible tactile sensors have been developed. [74][75][76][77] As for the fabric sensors, the selection of functional materials and design of device configurations must be re-considered based on the unique nanofiber network structure according to the feature of each sensing mechanism. First of all, the self-supporting flexible electrospun nanofiber membrane plays a key role in serving as the building platform of such fabric sensor. Then the various kinds of functional fillers can be composited on the nanofiber membrane to form the fabric sensing layers such as electrodes, dielectrics, and electrolytes.
The pressure sensing mechanism of the four types of nanofiber-based sensors is illustrated in Figure 6. The principle of piezoresistive tactile sensors is based on piezoresistive effect, which occurs when the electrical resistance of the conductive material changes in response to applied stimuli. Piezoresistive tactile sensors have been widely investigated owing to their simple device structure, low energy consumption, easy read-out mechanism, and broad range of detection. Electronic or ionic conducting fillers can be added into the nanofiber mat to form the sole piezoresistive sensing layer for the fabric piezoresistive sensors ( Figure 6a). The principle of capacitive tactile sensors is based on capacitance change when the pressure applied to the surface of the sensing device. Capacitive devices for tactile sensing have demonstrated high sensitivity, compatibility with static force measurement, and low power consumption. Sandwiched by two conductive nanofiber electrodes, the pristine nanofiber mat with polymer as fabric dielectric or the nanofiber mat composting ionic conducting fillers as fabric electrolyte form the fabric capacitive sensors ( Figure 6b). The piezoelectric tactile sensors produced voltage in response to applied mechanical stresses derived from oriented, permanent dipoles in the material and have been widely used in the detection of dynamic pressures such as vibrations of sound and slip for its high sensitivity and fast response time. Sandwiched by two conductive nanofiber electrodes, the nanofiber mat compositing the piezoelectric PVDF copolymers, or the electrospun PVDF-based nanofiber mat itself forms the fabric piezoelectric sensors (Figure 6c). The general principle of triboelectric sensors is to convert mechanical energy into electrical energy. The triboelectric effect is a common phenomenon in our daily life, which appears when contacting materials are under friction caused by normal touch, shear friction from sliding motion, or torsions. This mechanism enables the triboelectric devices to generate electrical signals in response to various mechanical stimuli and thus can be used as self-powered tactile sensors. Two nanofiber mats compositing two kinds of triboelectric fillers with largely different polarities rub each other to form the fabric triboelectric sensors (Figure 6d).

Sensing Materials and Its Influence on Sensing Performance
The sensing materials play the most important role in the sensing performance, and types of the functional materials should be well selected before the design of the sensor. In a sensor device, the electrodes exist for all types of sensors, and various kinds of conductive nanomaterials can be incorporated into electrospun nanofibers. The compositing approach can be generally divided into two kinds, that is, direct depositing of the conductive fillers on the electrospun nanofibers or co-electrospinning of the mixed precursor solution containing both polymers and conductive fillers. The most common conductive material is the metal materials such as gold and silver. They have the most advantage of superior high conductivity, which is required for obtaining highly conductive electrodes. However, conventional metals in the bulk formation are rigid and cannot be used for fabricating flexible electrodes. They need to be made into the unique form of nano-metals such as nanoparticles and nanowires before being used as conductive fillers to form the conductive network on the substrate of electrospun nanofibers. [78] Carbon-based materials are another mostly used conductive material, which have high conductivity as well as high mechanical strength and good heat resistance. Besides, they exhibit the unique property of acid and alkali resistance, and thus are solely chosen for making the conductive electrodes that interact with electrolyte. Typical carbon-based materials include the amorphous carbon black, one-dimensional (1D) CNT with a large length-to-diameter ratio and graphene in the two-dimensional (2D) nanosheet. [79] Besides, conductive polymers such as polyaniline (PANi), polypyrrole (PPy), polythiophene (PTh), and its derivates can also be used to prepare the flexible electrodes. [80] In addition to electrode materials, the dielectric or iontronic materials in the capacitive or supercapacitive sensor, the piezoelectric materials in the piezoelectric sensor and the triboelectric materials in the triboelectric sensor also play an important role in the sensing function. Typical polymer materials such as polyethylene oxide (PEO) [81] can be directly used for electrospinning dielectric nanofibers, and ionic liquids such as 1-hexyl-3methylimidazolium tetrafluoroborate ([HMIM][BF 4 ]) [82] need to be co-electrospun into polymer nanofibers as the nanofiber electrolyte. In addition to the PVDF and its copolymers with intrinsic piezoelectric property, BaTiO 3 [83] can also be composited into the common polymer nanofibers to obtain the piezoelectric effect. Usually polytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP), perpluoroalkoxy alkane (PFA) are incorporated in the polymer nanofibers to achieve the triboelectric effect. [84] Then the concentration of the functional materials should be optimized to achieve the best sensing performance. The obtained functional device components largely depend on the type and size of the functional nano fillers. It is known that nanomaterials tend to aggregate and thus appropriate methods such as modification with functional groups and ultrasonic treatment need to be applied to obtain a stable and uniform dispersion. During the fabrication process of the sensing composite materials, the amount of the functional fillers can also be adjusted, and the concentration of the fillers has a significant impact on the sensing performance of the sensor. In general, the more the conductive fillers, the higher the conductivity of the nanofiber electrodes, which benefits in achieving higher sensing performance. [85] This behavior also applies to the dielectric, electrolyte, piezoelectric, and triboelectric nanofiber materials. [86,87] However, when the concentration of the functional fillers is too high, the agglomeration of nanomaterials is inevitable, which would deteriorate the network structure of the electrospun nanofibers and even block the injector spinneret for failure of electrospinning. Therefore, the concentration of the functional fillers must be reasonably optimized to balance the sensing performance and the structural morphology of the nanofiber sensors.

Influence of Nanofiber Structure on the Sensing Performance
In addition to sensing materials, the morphological structure of the nanofiber substrate also has a crucial impact on the sensing performance. The fiber at nanoscale enables the fabric network to have a high porosity. [88] The remarkably high porosity increases the overall volume of internal voids, which would improve the compressibility of the sensing components and result in an enhanced sensitivity upon the micro-structuration strategy. [89] Because more deformation is obtained upon identical pressing or stretching. Besides, the thickness of the electrospun nanofiber layer can be increased by increasing the electrospinning duration, which would lead to wider detection range due to the total deformation of the sensing components. In addition to sensing performance, the intrinsic porous structure of the electrospun nanofiber as substrate offers numerous internal channels for air and moisture to pass through the whole sensor device for unique property of permeability, [90] which has never been achieved by conventional sensors based on compact and airtight polymer films as substrate. Furthermore, the high porosity also provides enormous adsorption sites for the effective capture or release of corresponding particles, molecules and functional groups, which make the sensor interact close with surrounding materials and sense external environment more effectively. [91] The arrangement of the nanofibers can also be adjusted in either the randomly oriented or the well aligned way. [92] When the collector is a flat metal plate, the electrospun nanofibers would randomly drop on the substrate. The obtained nanofiber network would exhibit an isotropic porous structure, which is used for making common sensors with isotropic press or strain detection capability. When the collector is a metal roller, the electrospun nanofibers would be drawn and stretched in a parallel arrangement to some extent towards the rolling direction depending on the rotation speed of the roller and the flow rate of the spinning solution. In general, the faster the rotation speed and the slower the flow rate, the better the parallelism of the electrospun nanofibers. Compared with the randomly distributed nanofibers, the ones with good parallel arrangement exhibit loose interactions and thus higher strain capacity in the direction perpendicular to the fiber alignment direction. [93]

Recent Research and Developments on Wearable Nanofiber-Based Sensors
The numerous representative research work recently reported in literature are summarized in details according to the four types of sensing principles as follows, which is also briefly listed in Table 3.

Piezoresistive Nanofiber-Based Sensors
In the piezoresistive sensors, the applied pressure causes the dimensional deformation of the fabric piezoresistive layer and thus results in the change of resistance (Figure 6a), which is governed by the equation of R = lS −1 , where R, , l, and S represents the overall resistance, resistance coefficient, length and cross-sectional area of the conductor. As for the piezoresistive nanofiber-based sensors, the change of the overall resistance mostly arises from the increasing connection sites inside the conductive nanofiber network.
Flexible strain sensor based on the micro-cracking mechanism shows high sensitivity due to the effective change of the conductive path upon stretching, which has been widely studied in the past. However, the detection range of such sensor based on conventional conducting materials is relatively narrow and unable to be applied on motion monitoring with larger strain, which needs to be improved. By spray-coating the ink of CNTs onto a pre-stretched electrospun TPU nanofiber mat, Zhou et al. [94] developed a crack-based strain sensor (Figure 7a), which had high sensitivity in a greatly widened workable sensing range (a gauge factor (GF) of 428.5 within 100% strain, 9268.8 for a strain of 100-220%, and larger than 83 982.8 for a strain of 220-300%), a fast response time (about 70 ms), superior durability (>10 000 stretching-releasing cycles), and excellent response toward bending. The remarkable sensing characteristics benefit from the microstructure of the advanced CNT branch at the crack and the excellent stretchability of the TPU nanofiber mat.
Vacuum filtrating the functional materials in the nanoscale through the electrospun nanofiber mat is another facial way to form the effective conductive network on the nanofiber-based substrate. By using vacuum filtrating MXene/CNTs on TPU nanofiber mat, Dong et al. [95] developed a strain sensor with both a broad working range (up to 330%) and high sensitivity (maximum GF of 2911) as well as superb long-term durability (2600 Figure 7. Summary of the research on piezoresistive nanofiber-based sensors. a) A crack-based strain sensor. Adapted with permission. [94] Copyright 2019, ACS Applied Materials & Interfaces. b) A bilayer-conductive structure strain sensor. Adapted with permission. [95] Copyright 2022, ACS Applied Materials & Interfaces. c) Anisotropic strain sensor made of asymmetric nanofiber composites. Adapted with permission. [96] Copyright 2022, Chemical Engineering Journal. d) A stretchable strain sensor decorating the novel two-dimensional (2D) and graphene-like material Ti 3 C 2 . Adapted with permission. [97] Copyright 2021, Smart Materials and Structures. e) A flexible piezoresistive sensor by depositing a silver conductive layer on the surface of an electrospun PU film. Adapted with permission. [98] Copyright 2022, Nanomaterials. f) An ultrathin flexible piezoresistive sensor with the layered nano-network structure. Adapted with permission. [22] Copyright 2020, ACS Nano. g) A strain sensor based on the carbon black (CB)/polyaniline (PANi) nanoparticles/TPU composite film. Adapted with permission. [99] Copyright 2022, ACS Applied Materials & Interfaces. h) A flexible pressure sensor composed of iron oxide (Fe 3 O 4 ) and caron nanofiber (CNF). Adapted with permission. [100] Copyright 2021, RSC Advances. Ref www.advancedsciencenews.com www.advsensorres.com cycles under the strain of 50%) (Figure 7b). The excellent sensing characteristic benefits from the synergy of the two parts and the hydrogen bond interaction between the porous TPU nanofiber mat and the MXene sheet. In another work, Wang et al. [96] prepared an asymmetric nanofiber composite for the anisotropic strain sensing (Figure 7c) by the similar facile vacuum filtration method. The carbon nanofibers (CNF)/PDMS were deposited onto an aligned TPU nanofiber mat surface, forming a two layered structure. Due to the hierarchical structure constructed by the randomly distributed CNF and PDMS with a low surface energy, the CNF/PDMS layer demonstrated super-hydrophobicity. The nanofiber composite was also cytotoxicity-free and exhibited excellent biocompatibility and biosafety. The asymmetric nanofiber composite showed anisotropy in the mechanical property and sensing behavior.
In addition to the 1D CNTs, 2D nanosheets are also employed to prepare the conductive nanofiber-based substrate. By decorating the novel graphene-like material Ti 3 C 2 with excellent electron and ion transmission rates on the electrospun TPU nanofiber mat, Zhao et al. [97] developed a stretchable strain sensor (Figure 7d). The sensor exhibited a high GF (2500 kPa −1 ) in a high strain range (250-300%). When the strain was below 50%, a good durability and stability (stretching/releasing of 1000 cycles) was observed.
Besides carbon-based functional materials, metals are also used as the conductive fillers to fabricate the nanofiber-based electrode. By depositing a silver conductive layer on the surface of an electrospun PU film, Xue et al. [98] developed a flexible piezoresistive sensor based on the electrospun nanofibers (Figure 7e). With this unique microstructure, piezoresistive pressure sensor delivered a high sensitivity (10.53 kPa −1 in the range of 0-5 kPa and 0.97 kPa −1 in the range of 6-15 kPa), fast response time (60 ms) and recovery time (30 ms), and a long-time stability (over 10 000 cycles). Combined with the high performance and stable structure, the developed sensor could be attached on skin or cloths to monitor both subtle and large-scale human motions, such as joint bending and pronunciation.
Multiple types of conductive materials are also used to achieve a synergistic effect for enhanced sensing performance. By compositing AgNWs and graphene with polyamide (PA) nanofibers to form the layered nano-network structure, Li et al. [22] developed an ultrathin flexible piezoresistive sensor (Figure 7f). In the hierarchical nano-network, AgNWs were evenly interspersed in the PA nanofiber network, forming the conductive pathways. In addition, graphene acted as bridges of crossed AgNWs. The PA nanofibers served as a backbone, providing an effective protection for AgNWs and graphene as pressure was applied. The sensor exhibited a high sensitivity (134 kPa −1 in the range of 0-1.5 kPa), a low detection (3.7 Pa), a wide detection range (>75 kPa), and an excellent durability (>8000 cycles). Furthermore, the ultrathin property (7 μm) provided high skin conformability during wearing. These superior performances lay a foundation for the application of pressure sensors in physiological signal monitoring and pressure spatial distribution detection. By combining the methods of electrospinning, in-situ polymerization and ultrasonication treatment, Zhai et al. [99] proposed a "point-topoint" conductive network and developed a strain sensor based on the carbon black (CB)/PANi nanoparticles/TPU composite film (Figure 7g). The sensor showed a wide sensitive range (up to 680% strain), highly sensitive response with a low detection limit (0.03% strain) and a high GF (3030.8), together with good sensing stability, fast response and recovery time (80 and 95 ms) and good durability (10 000 stretching-releasing cycles). These merits endow the pressure sensor with the ability to precisely detect wrist pulse, phonation, breathing, and finger bending in real-time.
Conductive nanofiber-based electrodes can also be obtained by directly carbonizing the electrospun polymer nanofibers. By carbonized electrospun nanofibers into fabric conductive materials, Cai et al. [100] developed a flexible pressure sensor composed of iron oxide (Fe 3 O 4 ) and CNF (Figure 7h). The electrospun nanofibers could self-assemble into a 3D network structure, which benefited in improving the sensing performance. The pressure sensor demonstrated a wide working range (0-4.9 kPa) and a high sensitivity (0.545 kPa −1 ) as well as an ultralow detection limit (6 Pa). Additionally, a rapid response time, good stability, high hydrophobicity, and excellent flexibility were also achieved. Such assembled strain sensors with the ability of precisely detecting full-range human motions and organic solvents, showing a potential application in human-machine interaction and environmental monitoring.

Capacitive Nanofiber-Based Sensors
In the capacitive sensors, the applied pressure causes the dimensional deformation of the sandwich-typed device structure, where the distance between the top and bottom two electrodes is decreased and thus the capacitance would change (Figure 6b), according to the equation of C = 0 r Sd −1 , where C, 0 , r , S, and d represents the overall capacitance, vacuum dielectric constant, relative dielectric constant of the dielectric material, overlapping area of and distance between the two electrodes. As for the capacitive nanofiber-based sensors based on the electrostatic charges, the porous network structure of the fabric dielectric further improves the compressibility, leading to an enhanced sensitivity and broadened detection range. As for the supercapacitive nanofiberbased sensors based on the electric double-layered capacitors (EDLC) at the top and bottom electrode/electrolyte interfaces, the huge supercapacitance arises due to the change of effective contacting area between the fabric electrode and electrolyte, [16,[101][102][103] which is far higher than the electrostatic capacitance with much improved sensitivity and avoids possible interference by environmental noises.
The electrospun polymer nanofiber mat can be directly used as the fabric dielectric layer in an electrostatic capacitive sensor. By electrospinning P(VDF-TrFE) nanofibers as dielectric layer, Kim et al. [104] developed a highly sensitive capacitive pressure sensor (Figure 8a). Due to the nanofiber microstructure, the sensor showed a high sensitivity (2.81 kPa −1 when pressure < 0.12 kPa), a fast response time (42 ms), and a small hysteresis. It is expected that these capacitive pressure sensors could be utilized for application in skin-like wearable electronics. In another work, by electrospinning TPU solution as dielectric with both external microstructure and internal pores, Li et al. [105] developed a capacitive pressure sensor based on the electrospun dielectric with a dual structure of both external microstructure and internal pores (Figure 8d). By employing the 300-mesh screen, the sensor manifested a high sensitivity (0.28 kPa −1 ), a fast response time (65 ms), Figure 8. Summary of the research on capacitive nanofiber-based sensors. a) A highly sensitive capacitive pressure sensor. Adapted with permission. [104] Copyright 2017, Applied Physics Letters. b) A capacitive pressure sensor based on the electrospun dielectric with a dual structure of both external microstructure and internal pores. Adapted with permission. [105] Copyright 2022, ACS Applied Electronic Materials. c) A capacitive torsion sensor. Adapted with permission. [106] Copyright 2019, Polymer International. d) A flexible and breathable iontronic pressure sensor based on the all-nanofiber platform. Adapted with permission. [107] Copyright 2022, Nano Energy. a remarkable pressure resolution, and a reliable durability (over 1000 cycles). Furthermore, a 4 × 4 multi-pixel array was also prepared with the dual-structured pressure sensor to observe the external pressure spatial distribution. The fabricated pressure sensor is employed not only for detecting human limb movements and object grasping but also for mapping the pressure distribution in an array.
In addition to the planar type of tactile sensor, the yarn type of torsion sensor is also developed. By electrospinning PVDF nanofibers on the Cu wire as the dielectric, Choi et al. [106] developed a capacitive torsion sensor (Figure 8b). The novel yarntyped sensor was constructed by twisting two nanofiber-coated Cu wires with different coating thickness. It was proposed that the response of the capacitance to the twist level was affected not only by the change in the distance between the two Cu wired electrodes but also by the change in the dielectric constant resulted from the porous 3D nonwoven structure of the nanoweb and the molecular mobility of the PVDF nanofibers. This simple coating method for nanofiber-based yarns provides useful approach to develop new type of flexible sensors.
Liquid containing ions can also be incorporated into the electrospun polymer nanofibers to form the ionic nanofibers as the fabric electrolyte to develop the iontronic supercapacitive sensors. By sandwiching one electrospun ionic liquid-based TPU nanofiber mat by two AgNW-based electrospun TPU nanofiber mats, Cui et al. [107] developed a flexible and breathable iontronic pressure sensor based on the all-nanofiber platform (Figure 8c). The sensor exhibited a high sensitivity (6.21 kPa −1 ), a wide working range (23 Pa to 120 kPa), a fast response and recovery time (170 and 135 ms), and an excellent stability (over 6000 loading/unloading cycles). Benefiting from the porous structure formed by the interconnecting nanofibers, water vapors could penetrate through both the fabric electrode and electrolyte layers, endowing the sensor with unique merit of moisture breathability. Figure 9. Summary of the research on piezoelectric nanofiber-based sensors. a) A flexible pressure sensor based on piezoelectric PVDF hybrid film using MXene nanosheet reinforcement. Adapted with permission. [108] Copyright 2021, Journal of Alloys and Compounds. b) A composite piezoelectric nanofiber material. Adapted with permission. [109] Copyright 2021, Coatings. c) Piezoelectric sensor with graded nanocomposite film. Adapted with permission. [110] Copyright 2020, Nano Energy. d) Piezoelectric thin films with topological nanofibers. Adapted with permission. [111] Copyright 2022, Advanced Functional Materials.
By virtue of the remarkable performance, the developed sensor could be employed in human psychological signal monitoring and wearable encrypted information transmission system.

Piezoelectric Nanofiber-Based Sensors
In the piezoelectric sensors, the piezoelectric sensing layer responds to the mechanical stress by the applied pressure with the output of voltage generation, which is caused by the alignment of electric dipole moments in the unique anisotropic crystalline materials (Figure 6c). Due to the intrinsic dynamic response behavior, piezoelectric sensors are widely used in the dynamic pressure detection. As for the piezoelectric nanofiber-based sensors, the typical polymeric piezoelectric material of PVDF and its copolymers can be directly electrospun into nanofibers.
Conductive nanomaterials can be composited in the PVDF nanofibers to further enhance the piezoelectric property. By incorporating MXene into the electrospun PVDF nanofiber mat as the piezoelectric material, Zhao et al. [108] developed a flexible piezoelectric sensor (Figure 9a). The MXene nanosheets were used as a reinforcement material, which efficiently promoted the piezoelectric output with improved sensitivity. The sensor based on the MXene/PVDF hybrid film showed a superior voltage sensitivity (up to 0.0480 vN −1 ), fast recovery time of 3.1 ms and stable operation capability under cyclic force. In another work, multiple functional materials can be used to achieve the synergistic effect on performance improvement. By using three kinds of inorganic doping materials of AgNO 3 , FeCl 3 •6H 2 O and nano graphene to modify the electrospun PVDF nanofiber mat, Li et al. [109] prepared a composite piezoelectric nanofiber material (Figure 9b) and the voltage peak reached 1.8 V. It was tested that the dopant could effectively promote the formation of phase, thereby improving the piezoelectric properties. In addition, the static bending strength and elastic modulus were also enhanced.
Piezoelectric sensors can also be developed as the piezoelectric nanogenerators for the additional application in harvesting mechanical energy. By electrospinning P(VDF-TrFE), polydopamine (Pdop), and barium titanate (BaTiO 3 ) composite mat, Xu et al. [110] developed a hierarchical structure-based flexible piezoelectric nanogenerators (FPENGs) with high output (6 V, 1.5 μA) (Figure 9c). The improved output of the PENG is attributed to the high density of interfaces in the hierarchical microstructure and the corresponding enhancement of dielectric response. The hierarchical nanocomposite membrane designed in this study provides an effective approach for developing mechanical energy harvesters, wearable sensor network and self-powered devices.
The sensing performance can also be enhanced by modifying the structure and mechanical property of the electrospun piezoelectric nanofibers. By modulating the orientation and mechanical vibration of the electrospun piezoelectric membrane, Chen et al. [111] successfully enhanced the electromechanical conversion performance (Figure 9d). The piezoelectric output of devices with an optimal topological pattern exhibited a 300% increase as well as a 478% expansion of the frequency response range compared to the conventional piezoelectric devices. This new concept provides a new approach to construct high-performance soft bioelectronics.

Triboelectric Nanofiber-Based Sensors
In the triboelectric sensors, the sensing mechanism relies on the triboelectrification and electrostatic induction effects, that is, two electrode layers made of different triboelectric polarity properties rub against each other with opposite electrical charges induced on both sides of the contact surfaces, and the generated charges could be maintained at the surface even after the two electrode layers separate from each other ( Figure 6d). As for the triboelectric nanofiber-based sensors, two kinds of triboelectric fillers are usually compositing with the electrospun nanofiber mats to form the device.
Conductive metals can be employed with the electrospun polymer nanofibers as the triboelectric electrode materials. By utilizing the electrospinning and screen printing technique, Cao et al. [112] developed a self-powered friction electrical sensor based on nanofiber membrane and an Ag electrode (Figure 10a). The pile of nanofibers and the conductive network of Ag nanoparticles ensured a gas channel across the whole device, yielding a high gas permeability (6.16 mm s −1 ) compared with conventional flexible devices composed of airtight films. It shows potential in application of medical monitoring and multi-function intelligent system. In another work, by coating thin Al foil electrodes on both sides of an electrospun poly(vinyl chloride) (PVC) nanofiber mat to increase the effective friction area of tribo-surface, Phan et al. [113] developed a novel triboelectric nanogenerator (TENG) based on the aero-elastic flutter-membrane as energy-harvester (Figure 10c). The airflow-induced triboelectric power generation from a single unit of the flutter-membrane was up to 0.33 μW. The aerodynamic and aeroelastic TENG have great potential to be used in numerous areas of self-powered electronic systems and in situ wireless sensor applications for automobiles or aircraft.
Functional materials with micro-/nano-structures can also be composited with the polymer nanofibers for triboelectric sensors. By co-electrospinning of silica aerogel and PI nano-covered layer to make multiply and stabilized yarns, Xing et al. [114] developed an all-yarn-based TENG to harvest energy and sense biological motion (Figure 10b). The device was capable of outputting a large transferred charge density (30 nC cm −2 ), a high peak power (0.17 mW with external load of 180 MΩ), and a fast average response time (<15 ms). Unlike ordinary fibers and yarns, the coreshell structured design showed a high sensitivity to all kinds of mechanical triggering sources. This aerogel nano-covered triboelectric yarn has great application prospects for energy generation and motion detection in high-temperature and many other high-risk environments.
Multiple types of polymers are also used at the same time for a synergistic effect to improve the triboelectric performance. By the unique electrospinning self-assembly of wet electrified jets made from polymers of PVDF, TPU, and PVA accumulating into structurally designable hetero-structures with unit size from micron to millimeter scale, Zhang et al. [115] developed a versatile electrospun micro-pyramid arrays that combine gradient micropyramid geometry (Figure 10d). Multi-functions of daytime radiative cooling, pressure sensing and bio-energy harvesting were achieved. The capacitive-triboelectric hybrid sensor shows a high sensitivity (19 kPa −1 ), ultralow detection limit (0.05 Pa), and ultrafast response time (0.8 ms) for motion detection as well as a large triboelectric and piezoelectric output (105.1 μCm −2 ) for reliable biomechanical energy harvesting ( Table 3).

Multi-Functionalities of Nanofiber-Based Sensors
Besides the high sensing performance, other aspects involving the wearability and comfortability after a long time of skinattaching wearing should also be taken into account. Figure 11 illustrates the sensor-skin system built by the top sensor sheet and the bottom covered skin. In a normal uncovered condition, the skin would keep a fluent micro-exchange of fresh air and moisture between the external environment and the human body for a healthy state of the skin. [116] In addition, the skin would secret sweat for the human body to cool down during motions through the water evaporation effect, along which the heat of the human body dissipates into the external environment. However, both of the above processes are blocked when the skin is tightly covered by a conventional sensor sheet that is constructed on an airtight film. In principle, from the sensing accuracy point of view, the flexible sensor is required to be attached on the covered skin in a tight and conformal way in order to acquire an accurate sensing signal with no interference by partial delamination. In addition, from the practical wearable sensing application point of view, the wearable sensor is designed to monitor the sensing signals in a continuous way, so the wearing of the sensor takes a long time. Therefore, the overall bad effects of the air and moisture blocking, sweat accumulating and temperature increasing would inevitably lead to the redness, allergies and even inflammation of the covered skin, which is harmful to the skin health and would arise concerns by wearers. Therefore, an ideal wearable sensor by Figure 10. Summary of the research on triboelectric nanofiber-based sensors. a) A self-powered friction electrical sensor based on nanofiber membrane and an Ag electrode. Adapted with permission. [112] Copyright 2018, Nano Research. b) A novel TENG based on aeroelastic film. Adapted with permission. [113] Copyright 2017, Nano Energy. c) An all-yarn-based triboelectric nanogenerator (TENG). Adapted with permission. [114] Copyright 2022, Advanced Functional Materials. d) Electrospun micro-pyramid arrays combined with ultrathin, ultralight, gas-permeable structures. Adapted with permission. [115] Copyright 2022, Nature Communications. design should possess the multi-functions of superior flexibility, air and moisture permeability and sweat inertness for a comfortable wearing. The electrospun nanofiber-based sensors do have such unique merits. In addition, the abilities of anti-bacteria and self-cleaning are also reported for a healthy wearable application.

Superior Flexibility
In general, flexible electronic are envisioned to possess the functional operations of sensing, processing, communication, power-ing, and controlling on a flexible substrate, which can seamlessly attached or worn on skin. [117] The components of data processing and communication, energy delivering and powering, and system controlling can be manufactured through the advanced semiconductor technique in a small form factor, and then assembled with integration on a flexible substrate, which would not hinder the flexibility of the whole system much. However, the sensors, as the key component that has to directly contact the skin with sufficiently large enough size to fulfill the function of accurate sensing, must be designed and fabricated from both material and device points of view in order to possess the

Flexibility
Healthcare behavior monitoring and individual protection self-powered electronics [115] Adv. Sensor Res. 2023, 2, 2200047 Figure 11. Schematic diagram showing a nanofiber-based sensor with multi-functionalities being attached on skin during a wearable sensing application.
unique properties of high flexibility and even certain stretchability that conventional rigid sensors do not have. [118,119] Compared with the conventional polymer films that have a compact internal structure, the mats interwined by numerous electrospun polymer nanofibers show superior flexibility due to the intrinsic topological internal structure of fabric network with redundant voids for deforming, compressing and stretching. Such superior flexibility makes the electrospun nanofiber mat an ideal elastic platform for constructing flexible sensors, which can accommodate to the curved and soft skin for a conformal attachment to accurately detection various physiological signals and body motions with different intensity of amplitudes. [120,121] In a specific work, Wang et al. [122] developed a strain sensor with an excellent network structure based on the reduced graphene Oxide (rGO)-decorated electrospun TPU nanofiber mats. The electrospun TPU nanofiber mat served as an ideal flexible and stretchable substrate, and the uniformly dispersed rGO on TPU nanofiber surface connected with each other and formed good conducting paths (Figure 12a). The maximum stress it can bears up to 5.3 MPa when its strain 700%. As a result, a high sensitivity of 11 within 10% strain and 79 within 100% strain in reversible strain regime) was achieved. Full-range human motion detections, including both subtle motions such as cough, phonation, and subtle muscle movements of cheek and large motions such as finger bending, walking, and jumping, were comprehensively demonstrated.

Breathability
The local skin on human body keeps a continuous and fluent micro-exchange of fresh air with the open external environment to maintain a healthy state. Clothes worn in our daily life is made of fabric materials with large internal pores and channels and thus would not hinder such air exchange between the skin and the external environment. However, when covered by airtight films for a long time, the air exchange can be completely blocked, which would result in bad biological effects to skin such as redness, swelling and even inflammation. It is also the case for wearing a Band-Aid on the wound of skin for days even though the tape is made of fabric textiles. As for the wearable sensors that are supposed to be tightly attached on skin for a long-time continuous detection of sensing signals, the breathability becomes especially important for a comfortable and healthy monitoring. However, conventional flexible sensors built on flexible polymer films with compact internal structures are airtight, which can be used an instant proof-of-concept demonstration of wearable sensing functions but fails to be practically used for long-time monitoring applications. In comparison, the electrospun nanofiber mats made of different polymer materials have the unique internal structure of numerous micro-pores and micro-channels circled by the randomly distributed nanofibers. The size of the micro-pores and micro-channels is in the micrometer range of larger than 1 μm, which can let the air and moisture vapor with a size in the range of nanometers freely pass through. [123,124] Thus the breathability can be well achieved for the sensors constructed on the electrospun nanofiber platform, [127,128] which has an air permeability similar to that of Nylon and medical tape and far higher than that of the airtight PDMS films (Figure 12b). Many parameters such as fiber diameter, porosity, thickness, and surface wettability can affect the air permeability of the electrospun nanofiber mat. [129,130] In addition, the measured air permeability is also determined by the rate of applied air flow through the fabric sample. [131] Meanwhile, these material and structure features also determine the functional sensing performance such as sensitivity, detection range, and response time. Therefore, a well balance between the sensing performance and the breathability should be obtained according to requirements in specific applications. [60]

Hydrophobicity
In addition to various stimuli perception, the skin as the boundary organ between the inside of human and the external environment also performs the key function of stabilizing the temperature of human body. At elevated temperature, the skin would secret sweat to cool down human body due to the evaporation of water, which dissipates heat into the external environment. A large part of the wearable sensor's applications is to continuously monitor the physiological signals and body motions when the wearers do a variety kind of activities such as walking, running, jumping, climbing, weight lifting, etc. In addition to exercisers who want to keep a healthy body through exercising and fitness, athletes who compete to win by the tiny difference in the physiological status and the skilled movements after numerous practices might be the first practical consumers for wearable sensors. Therefore, toward practical wearable sensing applications, flexible sensors would inevitably encounter the issues imposed by sweat. Profuse sweating would accumulate sweat moisture together into tiny droplets and even a small pool of liquid, and such process would accelerate when the skin is covered. The sweat solution would gradually soak the wearable sensors and inevitably disturb the sensing functions. Therefore, flexible sensors with certain degree of hydrophobicity are highly required. Development of hydrophobic sensors is dependent on the material selection and structure engineering of appropriate polymers, that is, the polymer should have a low surface energy and the surface should be roughened for a large contact angle between the liquid droplet and the surface of the material. [132,133] The above two requirements can be simultaneously met by electrospinning of hydrophobic polymer precursors into polymer nanofiber mats with resultant uniquely formed surface roughness of fabric texture. The diameter of the fibers varies from nanometers to micrometers, and can be well controlled by the polymer characteristics and the electrospinning parameters. [134] In a specific work, the hydrophobicity of the electrospun TPU nanofiber-based sensor sheet was thoroughly evaluated by dropping different kinds of liquids including water, artificial sweat and NaOH solution (Figure 12c). [60] Large contact angles greater than 130°were observed for all kinds of droplets, confirming hydrophobic nature of the fabric sensor, and the hydrophobicity can be maintained even when the fabric sensor sheet is stretched. It is worth noting that the hydrophobicity of the fabric sensor does not contradict the breathability because the size of micro-pores and microchannels lies in the range of micrometer that is smaller than that of liquid droplets (millimeter scale) and far larger than that of air and moisture molecules (nanometer scale). Besides, the posttreatment of incorporating specific materials is beneficial in generating additional roughness in order to increase the hydrophobicity. Furthermore, by engineering the nanofibers with a second scale of roughness through integration of nanoparticles or pores, the trapped air between the liquid droplet and the nanofiberbased mat can be further improved, yielding a superhydrophobic sensor. [135]

Anti-Bacterial Ability
Besides that the electrospun-nanofiber-based sensors are biofriendly to the covered skin due to breathability and hydrophobicity, the electrospun nanofiber materials have also been verified to be especially suitable for the help of wound healing because the network structure containing internal nano-/micropores could prevent the passage of bacteria and other pathogens through the nanofiber membrane and detain them on the outside surface. [136] Furthermore, it is feasible to load therapeutic and antibacterial agents onto the nanofibers to enhance the wound healing effect. For example, Henke et al. fabricated a stable photoactive polystyrene (PS) nanoparticle from the electrospun sulfonated PS nanofibers (Figure 12d). The antibacterial property of the nanoparticles could be tuned by the selection of encapsulated porphyrin derivatives. The nanoparticles with encapsulated 5,10,15,20-tetraphenylporphyrin photosensitizer had strong antibacterial and antiviral properties due to the formation of highly reactive singlet oxygen, whereas the nanoparticles with encapsulated platinum octaethylporphyrin (Pt-OEP) were applied for oxygen sensing. [125] For such kind of sensor, the antibacterial efficiency and the biocompatibility should be researched for enhancement. [137]

Self-Cleaning Ability
Since the electrospun nanofibers with low surface energy and hierarchical roughness on the micrometer and even nanometer scale can be fabricated, [138] a self-cleaning property can also be produced due to the hydrophobic surface. [139] Due to the large water contact angles, nearly spherical water droplets would form, which can freely roll across the surface by carrying away dust and dirt. [140] In a specific work, through the titanium carboxylate coordination bonding of gallic acid (GA) and tetrabutyl titanate (Ti(OBu) 4 ) in aqueous solution, Zhang et al. [126] fabricated a biodegradable electrospun stereo-complex polylactide (PLA) membrane with super-hydrophilicity (Figure 12e). The hydrophobic nanofiber membrane showed an effective and efficient separation property for a wide range of oil and water mixtures and oil-in-water emulsions. In addition, the nanofiber membrane also possessed splendid antifouling and self-cleaning performance resulting from the photo-catalytic property of TiO 2 under ultraviolet irradiation.

Conclusions and Perspectives
In summary, the electrospinning technique provides a facile way to fabricate nanofibers with a broad selection of polymer materials and an engineering design of fabric structures that can be well controlled. The electrospun nanofibers with comprehensive characteristics of ultrathin thickness, superior flexibility, excellent stretchability, high porosity, low density, and surface functionality can be used as an ideal substrate for construction of flexible tactile sensors based on four pressure sensing mechanisms with corresponding device structures. The synthetic effect between the porous nanofiber network and various active sensing materials endows the developed sensors with versatile sensing functionalities with high sensing performance. Other unique merits of air permeability, hydrophobicity, anti-bacterial ability, and self-cleaning ability can be also achieved. Up to date, a considerable amount of work has been done with progressive achievements in this emerging field of flexible tactile sensors based on electrospun nanofibers, which will open a new window for the development of new type of sensing devices for next-generation wearable electronics. Toward practical wearable sensing applications, technological challenges still exist, which requires further perspective research in the following directions.
Regarding the material of the electrospun nanofibers, the main factors affecting the internal structure and morphology of nanofibers (fiber thickness, porosity, orientation, etc.) are identified to be the spinning solution parameters (types of polymers, concentration of functional fillers, viscosity of solutions, etc.), electrospinning parameters (applied voltage, distance between the needle tip and collector, solution flow rate, collector formation, etc.), and environmental parameters (temperature and humidity), but it remains the qualitative understanding. For the accurate design of new sensors and mass production with uniform quality, quantified fundamental rules governing the influencing behavior should be studied in depth to better guide the experimental development.
Regarding the structure of the sensor device, the stability and durability of the fabricated fabric sensing components is still inferior toward practical long-time usage, because the nanofiber substrate has less effective internal contact area for the functional nano fillers to deposit on and the nano fillers would tend to fall off upon frequently repeat pressing, bending and stretching. It is required to explore effective ways to further strengthen the bonding between the functional fillers and the nanofiber substrate for a prolonged life time. Besides, several nanofiber layers with different functions such as electrodes, dielectric, electrolyte, piezoelectric, triboelectric, substrate, and package are assembled together into one sensor unit. The bonding between adjacent layers is also usually weak, which would lead to possible delamination after long-time usage. It is of significant to develop nanofiberbased sensors with an integrated structure.
Regarding the sensor device toward practical wearable sensing applications, in addition to the senser unit that detects the sensing signal at one single point, the sensing array consisting of multiple sensing units aligned in the two-dimensional arrangement is deserved to research for the spatial pressure distribution mapping. Besides, multi-functional sensors with extra sensing capabilities of temperature, humidity, sweat and gas may be also needed for a versatile sensing functionality. Based on the fabric sensor device, corresponding electronic components such as the signal processing, chip controlling and energy supplying could also be developed for a well integration on the electrospun nanofiber platform.
www.advancedsciencenews.com www.advsensorres.com Shijie Guo is a full professor at the Hebei University of Technology and an adjunct professor at the Fudan University, China. He received his Master and Ph.D. degrees in Mechanical Engineering from the Tokyo Institute of Technology in 1989 and 1992, respectively. He is the Head of the Engineering Research Center of Intelligent Rehabilitation Device and Detection Technology at the Ministry of Education and Hebei Key Laboratory of Robot Sensing and Human-Robot Interaction, and the Principal Scientist of the State Key Laboratory of Reliability and Intelligence of Electrical Equipment. His research interests include electronic skin, intelligent nursing beds, exoskeleton robots, healthcare robots, and human-robot interaction.