WO2018226162A1 - Core-shell nanofibers for capacitive sensing and triboelectric applications - Google Patents

Abstract

A core-shell nanofiber having a core and a shell surrounding the core is provided, wherein the core comprises an ionogel and the shell comprises polyvinylidene difluoride (PVDF) or a copolymer thereof. In a preferred embodiment, the ionogel comprises 1 -butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][TFSI] and poly(dimethylsiloxane) (PMDS). A method of preparing the core-shell nanofiber and a device comprising the core-shell nanofiber for use in capacitive sensing and triboelectric applications are also provided.

Description

CORE-SHELL NANOFIBERS FOR CAPACITIVE SENSING AND

TRIBOELECTRIC APPLICATIONS

Cross-Reference To Related Application

[0001] This application claims the benefit of priority of Singapore Patent Application No. 10201704637P filed on 7 June 2017, the content of it being hereby incorporated by reference in its entirety for all purposes.

Technical Field

[0002] Various embodiments relate to a core-shell nanofiber and a method of preparing a core-shell nanofiber from an ionogel and a polymer. Various embodiments also relate to a device for capacitive pressure sensing and a triboelectric nanogenerator.

Background

[0003] Ionic liquids have attracted considerable attention for their use in applications such as lithium metal batteries, supercapacitors, light emitting electrochemical cell and electrochromic applications, due to their tunable fluidic viscosity, high electrical conductivity, and excellent thermal stability. High capacitance of ionic liquids due to formation of electrical double layers (EDLs), for example, have seen their use as high capacitance "dielectric" for electronic devices.

[0004] Polymeric electrolytes based on ionic liquids have been developed by incorporating an ionic liquid into a crosslinkable gel matrix to form an ionogel, otherwise termed as an ion gel matrix or an ionic gel matrix. Advantageously, the polymeric electrolytes may have high ionic conductivity from the ionic liquid, along with improved mechanical properties (e.g. improved stretchability) from the gel matrix.

[0005] Even though ionogels have been used for capacitive sensing, they are typically constructed as thin films for interfacial capacitive sensors. Such thin films, however, suffer from relatively small surface area thereby limiting the device's functionality, e.g. sensitivity and response time. To further increase tactile sensitivity and reduce response time, microstructures such as pyramidal structures have been introduced on the thin films' surfaces. However, fabrication of the surface microstructures is largely carried out using traditional lithography, which is time consuming, costly and has limited scalability.

[0006] Ionogels have also been used in triboelectric nanogenerators (TENGs), which are flexible nanogenerators that convert mechanical energy into electrical energy. Various types of energy generated by, e.g. human motion, vibration, wind, and water, may be harvested by TENGs by making use of the triboelectric effect. The working principle of a TENG is based on contact electrification and electrostatic induction. For instance, when two films are contacted, friction may occur between the two films due to their natural surface roughness, and this tends to lead to an equal amount of charge generated at the surfaces of the two films, wherein one film is oppositely charged to the other. The friction may occur when surfaces of the two films are pressed against each other or when the contacted surfaces are separated (i.e. rough surfaces rubbing into or against each other, respectively). An electric potential is thus formed at the interface region between the two films. When the two films contact and separate, an alternating potential set up by such a motion drives electrons in an external load to flow back and forth.

[0007] To enhance the triboelectric effect, studies have been pursued to increase friction by fabricating microstructures on the surfaces of such films, so as to result in a higher output performance. With such microstructured films, the triboelectric charges may be more easily separated and a larger electric dipole moment tends to form between electrodes. Nevertheless, the ionogels remain highly susceptible to dehydration, causing deterioration in the electrical performance of flexible nanogenerators over time.

[0008] There is thus a need to provide for an improved ionic gel material, and methods of preparing the ionic gel material that address or at least alleviate one or more of the above-mentioned problems.

Summary

[0009] In a first aspect, a core-shell nanofiber having a core and a shell surrounding the core is provided. The core comprises an ionogel and the shell comprises polyvinylidene difluoride or a copolymer thereof. [0010] In a second aspect, a method of preparing a core-shell nanofiber having a core and a shell surrounding the core, wherein the core comprises an ionogel and the shell comprises polyvinylidene difluoride or a copolymer thereof, is provided. The method comprises

a) providing an ionogel,

b) providing a shell solution comprising polyvinylidene difluoride or a copolymer thereof, and

c) electrospinning the ionogel and the shell solution to form the core-shell nanofiber.

[0011] In a third aspect, there is provided for a device comprising a first electrode and an opposing second electrode, and a nanofiber mat comprising a core-shell nanofiber according to the first aspect or prepared by a method according to the second aspect disposed between the first electrode and the second electrode. Brief Description of the Drawings

[0012] The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present disclosure are described with reference to the following drawings, in which:

[0013] FIG. la shows a schematic diagram for preparation of the poly(dimethylsiloxane) (PDMS) ionic gel.

[0014] FIG. lb illustrates the process for preparing a core-shell PDMS ionic gel/PVDF-HFP nanofiber mat by electrospinning. PVDF-HFP refers to poly(vinylidene fluoride-co-hexafluoropropene).

[0015] FIG. lc shows a field emission scanning electron microscopy (FESEM) image of a core-shell PDMS ionic gel/PVDF-HFP nanofiber mat. The scale bar represents 1 μπι.

[0016] FIG. Id shows the transmission electron microscopy (TEM) image of a core- shell PDMS ionic gel/PVDF-HFP nanofiber mat. The scale bar represents 200 nm.

[0017] FIG. le shows the TEM image of pristine PVDF-HFP nanofiber mat. The nanofiber mat does not have the core-shell configuration shown in FIG. Id. The scale bar represents 100 nm. [0018] FIG. If shows a cross-sectional FESEM image of a core-shell PDMS ionic gel/PVDF-HFP nano fiber mat. The scale bar represents 100 μηι.

[0019] FIG. l g shows the Fourier-transform infrared (FT-IR) spectrum of the core- shell PDMS ionic gel/PVDF-HFP nano fiber mat (I) and the FT-IR spectrum of the pristine PVDF-HFP nano fiber mat (II).

[0020] FIG. 2a shows the device configuration of a pressure sensor based on the core- shell PDMS ionic gel/PVDF-HFP nanofiber mat.

[0021] FIG. 2b shows the repeated real time responses to a load of 200 mg (at 0.01 kPa) from the pressure sensor having the configuration shown in FIG. 2a.

[0022] FIG. 2c depicts the mechanism for change in capacitance of the core-shell PDMS ionic gel/PVDF-HFP nanofiber mat when the pressure is applied. Specifically, scheme (I) shows the dielectric constant is changed when the nanofiber mat is compressed. Scheme (II) shows the reduction in distance between the top and bottom electrodes, and scheme (III) shows the fiber contact area is increased.

[0023] FIG. 3a illustrates the maximum slope of the relative capacitance changes of core-shell PDMS ionic gel/PVDF-HFP nanofiber mats having different amounts of ionic liquid loading in the pressure range of 0.01 kPa to 1.5 kPa. The amount of PDMS is fixed for all the samples.

[0024] FIG. 3b shows the pressure-response curves of core-shell PDMS ionic gel/PVDF-HFP nanofiber mats prepared with different amounts of ionic liquid. The amount of PDMS is fixed for all the samples.

[0025] FIG. 3 c shows the stress-strain curves of pristine PVDF-HFP nano fibers and PDMS ionic gel/PVDF-HFP nanofiber mats prepared with different amounts of ionic liquid to form the PDMS ionic gel. The amount of PDMS is fixed for all the samples.

[0026] FIG. 3d shows the real time pressure waveforms of the measured heart rate. The left inset shows that the device has been mounted onto the wrist and the right inset shows a magnified view of a pulse having the characteristic peak typically measured at the radial artery.

[0027] FIG. 3e shows the Young's Modulus of PVDF-HFP nanofiber mat with PDMS ionic gel prepared using different amounts of ionic liquid. The amount of PDMS is fixed for all the samples. [0028] FIG. 3f shows a typical Nyquist plot of impedance analysis on PDMS ionic gels prepared with different amounts of ionic liquid. The amount of PDMS is fixed for all the samples.

[0029] FIG. 3g shows a magnified view of the high frequency region of the impedance spectra in FIG. 3f.

[0030] FIG. 3h shows the ionic conductivity for PDMS ionic gels prepared from different amounts of ionic liquid. The amount of PDMS is fixed for all the samples.

[0031] FIG. 4a shows the device configuration of a triboelectric nanogenerator (TENG) for the core-shell PDMS ionic gel/PVDF-HFP nanofiber mats and the mechanism of the core-shell PDMS ionic gel/PVDF-HFP nanofibers mats for electric power generation.

[0032] FIG. 4b shows the output voltage of the TENG for the core-shell PDMS ionic gel/PVDF-HFP nanofiber mats under a pressure of 700 kPa at 5 Hz.

[0033] FIG. 4c shows the current density of the TENG for the core-shell PDMS ionic gel/PVDF-HFP nanofiber mats under the pressure of 700 kPa at 5 Hz.

[0034] FIG. 5 shows the output voltage and the current signal of a TENG under a forward connection (as represented by the top row of drawings) and under a reverse connection (as represented by the bottom row of drawings).

[0035] FIG. 6a shows the output voltage of TENG in the pressure range of 100 kPa to 700 kPa.

[0036] FIG. 6b shows the voltage-response curves of 40 wt% ionic liquid loaded core-shell PDMS ionic gel/PVF-HFP nanofiber mats. Specifically, FIG. 6b shows the output voltage in the high pressure range of 100 kPa to 700 kPa.

[0037] FIG. 6c shows the output power density of the TENG with respect to resistance of an external load.

[0038] FIG. 6d shows images (I) and (II). In image (I) on the left, a schematic diagram of LED bulbs operation circuit with a full-wave bridge rectifier is depicted.

In image (II) on the right, a photograph of the serial connections of 300 LED bulbs driven by the TENG (device size: 5 cm x 5 cm) is shown.

[0039] FIG. 6e shows the voltage-response curves of the 40 wt% ionic liquid loaded core-shell PDMS ionic gel/PVF-HFP nanofibers, wherein the output voltage is shown for a pressure range of 40 kPa to 100 kPa. [0040] FIG. 6f shows the output voltage and current density of a TENG based on the core-shell PDMS ionic gel/PVDF-HFP nanofibers with respect to different external loads of varying resistance.

[0041] FIG. 6g shows the output voltage of a TENG with respect to time for a pressure range of 40 kPa to 100 kPa.

[0042] FIG. 6h shows the output current signal of TENG under high pressure range of 100 to 700 kPa.

[0043] FIG. 7 shows the output voltage of a TENG based on the core-shell PDMS ionic gel/PVDF-HFP nanofiber mats before and after six months of storage.

[0044] FIG. 8a shows the output voltage of a TENG based on the core-shell PDMS ionic gel/PVDF-HFP nanofiber mats under different magnitudes of force.

[0045] FIG. 8b shows the current signal of a TENG based on the core-shell PDMS ionic gel/PVDF-HFP nanofiber mats under different magnitudes of force. Detailed Description

[0046] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced.

[0047] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and/or combinations and/or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0048] Various embodiments disclosed herein refer to a core-shell nanofiber having a core and a shell surrounding the core, wherein the core comprises an ionogel and the shell comprises polyvinylidene difluoride or a copolymer thereof.

[0049] Advantageously, the core-shell nanofibers disclosed herein may be used to form nanofiber mats for use in capacitive pressure sensing and triboelectric applications. Capacitance of the nanofiber mats disclosed herein has been shown to dramatically increase when external pressure was applied to the nanofiber mats. This may be due to increase in dielectric constant ε_Γ as a result of reduction in air content, as the dielectric constant of the core-shell nanofibers is higher than that of the displaced air. The increase in capacitance of the nanofiber mats may also result from reduction in thickness of the ionogel in the core-shell nanofiber upon compression. Due to the reduction in thickness of the ionogel, contact area of the electrical double layers formed in the ionogel may also increase to result in enhanced interfacial capacitance. The increase in dielectric constant and the fiber contact area as well as the reduced distance between the separated electrodes may translate into an improved device for capacitive pressure sensing, whereby high pressure sensitivity in the low pressure range may be achieved due to significant increase in the capacitance of the pressure sensor upon compression. In application as a triboelectric nanogenerator, the ionogel in the core-shell nanofiber may be prevented from dehydrating, for example, thereby maintaining performance of the triboelectric nanogenerator.

[0050] With the above in mind, various embodiments refer in a first aspect to a core- shell nanofiber having a core and a shell surrounding the core, wherein the core comprises an ionogel and the shell comprises polyvinylidene difluoride or a copolymer thereof.

[0051] The term "core-shell" as used herein refers to an arrangement of materials in which one of the materials envelops the other material(s), while the term "nanofiber" refers to an elongated or threadlike filament having a diameter in the order of nanometers. Accordingly, the term "core-shell nanofiber" refers to a structural configuration of a nanofiber in which an external layer formed of a second material surrounds at least an outer lateral surface of the core formed of a first material, thereby forming the core-shell nanofiber.

[0052] In various embodiments, the core comprises an ionogel. As used herein, the term "ionogel", otherwise termed an ionic gel or an ion gel, refers to a nanocomposite material formed of an ionic liquid (IL) which is entrapped within and/or dispersed in a polymeric matrix. Advantageously, the ionic liquid may act in tandem with the polymeric matrix to provide high ionic conductivity with improved mechanical properties to the ionogel. In various embodiments, the ionic liquid is physically mixed with polymer(s) making up the polymeric matrix and does not form a chemical bond with the polymer. The ionic liquid may, for example, be miscible with the polymer(s) making up the polymeric matrix, such that the ionic liquid is dispersed at least substantially uniformly in the polymeric matrix.

[0053] The ionogel according to embodiments disclosed herein is obtainable by crosslinking a polymer with a crosslinking agent in the presence of an ionic liquid. In so doing, the ionic liquid may be entrapped within and/or dispersed in a polymeric matrix formed by the crosslinked polymer.

[0054] In various embodiments, the polymer may be an elastomeric polymer. By incorporating an elastomeric polymer into the ionogel, the resultant ionogel may be flexible. The term "flexible" as used herein refers to materials which are compliant and respond in the presence of external forces by deforming readily. For example, the ionogel may flex or bend readily upon application of a force, and is able to return at least substantially to its original non-extended configuration after removal of the force.

[0055] The elastomeric polymer may, for example, be formed by polymerizing monomers or prepolymers selected from the group consisting of (poly)siloxanes, (poly)epoxides, (poly)urethanes, and the like, and combinations thereof. The term "(poly)" as used herein means that siloxanes, epoxides, and/or urethanes may also be used to form the elastomeric polymer, apart from or in addition to polysiloxanes, polyepoxides, and/or polyurethanes. In some embodiments, the elastomeric polymer comprise or consist of polymers formed from polymerizing monomers or prepolymers of poly(dimethylsiloxane), silicone rubber, poly(glycerol sebacate), epoxy resins, polysulfide rubber, urethane rubber, urethane plastic, polyacrylic rubber, butyl rubber, ethylene-vinyl acetate (EVA), and combinations thereof.

[0056] In some embodiments, the polymer or the elastomeric polymer is selected from the group consisting of poly(dimethylsiloxane), silicone rubber, poly(glycerol sebacate), epoxy resins, polysulfide rubber, urethane rubber, urethane plastic, polyacrylic rubber, butyl rubber, ethylene-vinyl acetate, a copolymer thereof, and a combination thereof. In specific embodiments, the polymer or the elastomeric polymer is selected from the group consisting of poly(dimethylsiloxane), silicone rubber, poly(glycerol sebacate), a copolymer thereof, and a combination thereof.

[0057] In specific embodiments, the polymer or the elastomeric polymer is poly(dimethylsiloxane) or a copolymer thereof. Advantageously, poly(dimethylsiloxane) may provide good elastic properties as well as biomedical compliance with human tissue and living cells. The poly(dimethylsiloxane) according to embodiments disclosed herein is obtainable by polymerizing a prepolymer comprising dimethylsiloxane with vinyl groups in the presence of a curing agent comprising dimethylsiloxane with vinyl groups and dimethylsiloxane with Si-H groups. Other polymers, such as silicone rubber or poly(glycerol sebacate), may be obtainable in the same manner as described above for poly(dimethylsiloxane). For instance, the silicone rubber or poly(glycerol sebacate) may be obtainable by polymerizing a prepolymer of silicone rubber with vinyl groups or a prepolymer of poly(glycerol sebacate) with vinyl groups, respectively, in the presence of a curing agent. In the case of silicone rubber, the curing agent may comprise a prepolymer of silicone rubber with vinyl groups and a prepolymer of silicone rubber with Si-H groups. In the case of poly(glycerol sebacate), the curing agent may comprise a prepolymer of poly(glycerol sebacate) with vinyl groups and a prepolymer of poly(glycerol sebacate) with Si-H groups.

[0058] The prepolymer and the curing agent may be present in a weight ratio of about 10: 1 to about 10:3, such as about 10: 1 to about 10:2, about 10:2 to about 10:3, about 10: 1, about 10:2, or about 10:3.

[0059] The polymer may be crosslinked with a crosslinking agent in the presence of an ionic liquid to form the ionogel, wherein the crosslinking may be carried out to enhance stability of the polymer prior to use in forming nanofibers.

[0060] The term "crosslink" as used herein refers to an interconnection between polymer chains via chemical bonding, such as, but not limited to, covalent bonding, ionic bonding, or affinity interactions (e.g. ligand/receptor interactions, antibody/antigen interactions, etc.). The chemical crosslinking may be carried out by reactions, such as any one of free radical polymerization, condensation polymerization, anionic or cationic polymerization, or step growth polymerization.

[0061] Accordingly, the term "crosslinking agent" refers to an agent which induces crosslinking. The crosslinking agent may be any agent that is capable of inducing a chemical bond between adjacent polymeric chains.

[0062] In various embodiments, the crosslinking agent comprises a polymer having a terminal hydroxyl group and an organosilicate. Generally, crosslinking agents that possess good compatibility with the ionic liquid may be used. In various embodiments, the polymer having a terminal hydroxyl group comprised in the crosslinking agent is selected from the group consisting of poly(dimethylsiloxane)- OH, silicone rubber-OH, poly(glycerol sebacate)-OH, epoxy resins-OH, polysulfide rubber-OH, urethane rubber-OH, urethane plastic-OH, polyacrylic rubber-OH, butyl rubber-OH, ethylene-vinyl acetate-OH (EVA-OH), a copolymer thereof, and a combination thereof. In specific embodiments, the polymer having a terminal hydroxyl group comprised in the crosslinking agent is selected from the group consisting of poly(dimethylsiloxane)-OH, silicone rubber-OH, poly(glycerol sebacate)-OH, a copolymer thereof, and a combination thereof.

[0063] The polymer having a terminal hydroxyl group comprised in the crosslinking agent used for crosslinking may depend on the polymer used. For example, in embodiments wherein the polymer comprises or consists of poly(dimethylsiloxane), the polymer having a terminal hydroxyl group comprised in the crosslinking agent may be poly(dimethylsiloxane)-OH. Likewise, in embodiments wherein the polymer comprises or consists of silicone rubber or poly(glycerol sebacate), the polymer having a terminal hydroxyl group comprised in the crosslinking agent may be silicone rubber-OH or poly(glycerol sebacate)-OH, respectively. Advantageously, this allows formation of a more uniform polymeric matrix in the ionogel.

[0064] In some embodiments, the polymer having a terminal hydroxyl group comprised in the crosslinking agent is poly(dimethylsiloxane)-OH or a copolymer thereof.

[0065] In addition to the polymer having a terminal hydroxyl group, an organosilicate may be comprised in the crosslinking agent to increase level of crosslinking between the polymeric chains. Examples of organosilicate may include, but are not limited to, silsesquioxane, and tetraalkyl orthosilicate, wherein the alkyl group is two to four carbon atoms. In various embodiments, the organosilicate may comprise or consist of tetraethylorthosilicate (TEOS).

[0066] In various embodiments, the polymer having a terminal hydroxyl group comprised in the crosslinking agent, the organosilicate, and the ionic liquid are present in a weight ratio of about 1 :1 :1 to about 2:2: 1 to form the ionogel. Lower amounts of ionic liquid used may lead to lower conductivity of the ionogel, rendering the performance of the ionogel insufficient. Meanwhile, lower amounts of crosslinking agent and organosilicate used may render formation of the ionogel difficult.

[0067] As mentioned above, the ionogel is obtainable by crosslinking a polymer with a crosslinking agent in the presence of an ionic liquid. The term "ionic liquid" as used herein refers to an ionic salt which is a liquid at room temperature, defined herein as a temperature in the range of about 20 °C to about 40 °C.

[0068] Generally, any ionic liquid may be used. The ionic liquid may, for example, be selected from the group consisting of l-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][TFSI], l-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM] [TFSI], 1 -hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [HMIM] [TFSI] , 1 -butyl-3 -methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [BMP] [TFSI], 1 -ethyl-3-methylimidazolium tetracyanoborate, l-ethyl-3-methylimidazolium tris(per fluoroethyl)trifluorophosphate, and a combination thereof.

[0069] In some embodiments, the ionic liquid is 1 -butyl-3 -methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM] [TFSI] .

[0070] Amount of the ionic liquid in the core-shell nanofiber may be 40 wt% or less of the total weight of the core-shell nanofiber. For example, amount of the ionic liquid in the core-shell nanofiber may be in the range of about 1 wt% to about 40 wt%, about 5 wt% to about 40 wt%, about 10 wt% to about 40 wt%, about 20 wt% to about 40 wt%, about 30 wt% to about 40 wt%, about 10 wt% to about 30 wt%, about 10 wt% to about 20 wt%, about 15 wt% to about 35 wt%, or about 20 wt% to about 30 wt%, of the total weight of the core-shell nanofiber.

[0071] Diameter of the core in the core-shell nanofiber is not particularly limited and may, for example, be in the range of about 5 nm to about 50 nm, about 10 nm to about 50 nm, about 20 nm to about 50 nm, about 30 nm to about 50 nm, about 40 nm to about 50 nm, about 5 nm to about 40 nm, about 10 nm to about 40 nm, about 20 nm to about 40 nm, about 30 nm to about 40 nm, about 5 nm to about 30 nm, about 10 nm to about 30 nm, about 20 nm to about 30 nm, about 5 nm to about 20 nm, about 10 nm to about 20 nm, about 5 nm to about 10 nm. [0072] A shell surrounds the core of the core-shell nanofiber, and may be disposed directly on the core of the core-shell nanofiber. Advantageously, the shell may help to protect the ionogel core by keeping it intact, hydrated, and stable from outer environment. In so doing, electrical performance of the core-shell nanofiber may be maintained over time.

[0073] The shell may comprise polyvinylidene difluoride (PVDF) or a copolymer thereof. In various embodiments, the shell comprises a PVDF homopolymer, or a PVDF copolymer or terpolymer such as poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE), poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP), poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE), and/or poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (PVDF-TrFE-CFE) . PVDF and PVDF based polymers, including PVDF copolymers and PVDF terpolymers, may be used in applications such as capacitors, piezoelectric nanogenerator and triboelectric nanogenerator due to their excellent flexibility, mechanical strength and chemical resistance.

[0074] In specific embodiments, the shell comprises poly(vinylidene fluoride-co- hexafluoropropene) (PVDF-HFP). Advantageously, the PVDF-HFP copolymer is more miscible, as compared to PVDF homopolymer, with ionic liquid in which the - HFP group may act as a crosslinking site for the gelation process and eases the formation of nanofibers through electrospinning. In addition, the dielectric constant of PVDF-HFP is higher than PVDF which enhances the performance of a triboelectric nanogenerator.

[0075] The shell comprising polyvinylidene difluoride (PVDF) or a copolymer thereof may have a thickness of at least 10 nm, such as about 10 nm to about 500 nm, about 50 nm to about 500 nm, about 100 nm to about 500 nm, about 150 nm to about 500 nm, about 200 nm to about 500 nm, about 300 nm to about 500 nm, about 400 nm to about 500 nm, about 10 nm to about 50 nm, about 10 nm to about 100 nm, about 10 nm to about 150 nm, about 10 nm to about 200 nm, about 10 nm to about 300 nm, about 10 nm to about 400 nm, or about 10 nm to about 20 nm.

[0076] Various embodiments refer in a second aspect to a method of preparing a core- shell nanofiber having a core and a shell surrounding the core, wherein the core comprises an ionogel and the shell comprises polyvinylidene difluoride or a copolymer thereof.

[0077] The method comprises providing an ionogel, providing a shell solution comprising polyvinylidene difluoride or a copolymer thereof, and electrospim ing the ionogel and the shell solution to form the core-shell nanofiber.

[0078] Providing the ionogel may include crosslinking a polymer with a crosslinking agent in the presence of an ionic liquid. Examples of suitable polymer, crosslinking agent and ionic liquid have already been mentioned above.

[0079] In various embodiments, the polymer may be an elastomeric polymer. Examples of suitable elastomeric polymer have already been described above. In various embodiments, the polymer or the elastomeric polymer is selected from the group consisting of poly(dimethylsiloxane), silicone rubber, poly(glycerol sebacate), epoxy resins, polysulfide rubber, urethane rubber, urethane plastic, polyacrylic rubber, butyl rubber, ethylene-vinyl acetate, a copolymer thereof, and a combination thereof. In some embodiments, the polymer or the elastomeric polymer is selected from the group consisting of poly(dimethylsiloxane), silicone rubber, poly(glycerol sebacate), a copolymer thereof, and a combination thereof. In specific embodiments, the polymer or the elastomeric polymer is poly(dimethylsiloxane) or a copolymer thereof.

[0080] In various embodiments, crosslinking the polymer with the crosslinking agent in the presence of the ionic liquid comprises providing a mixture comprising the crosslinking agent and the ionic liquid, and adding a prepolymer of the polymer and a curing agent to the mixture. Examples of suitable prepolymers and curing agent have already been mentioned above.

[0081] Advantageously, crosslinking the polymer with the crosslinking agent in the presence of the ionic liquid may be carried out at room temperature without application of an external heat.

[0082] In embodiments wherein the polymer is poly(dimethylsiloxane) or a copolymer thereof, for example, the prepolymer of the polymer may comprise dimethylsiloxane with vinyl groups, and the curing agent may comprise dimethylsiloxane with vinyl groups and dimethylsiloxane with Si-H groups. As mentioned above, the prepolymer and the curing agent may be present in a weight ratio of about 10: 1 to about 10:3. [0083] The method disclosed herein may further comprise adding a solvent to the mixture after adding the prepolymer of the polymer and the curing agent. This may be carried out to control viscosity and surface tension of the ionogel for electrospinning. In various embodiments, the solvent helps in obtaining an ionogel having a viscosity of about 1 poise to about 20 poise, about 5 poise to about 20 poise, about 10 poise to about 20 poise, about 1 poise to about 10 poise, about 5 poise to about 10 poise, or about 1 poise to about 5 poise. In various embodiments, the solvent also helps in obtaining an ionogel having a surface tension of about 35 dyne/cm to about 55 dyne/cm, about 40 dyne/cm to about 55 dyne/cm, about 45 dyne/cm to about 55 dyne/cm, about 50 dyne/cm to about 55 dyne/cm, about 35 dyne/cm to about 50 dyne/cm, about 40 dyne/cm to about 50 dyne/cm, about 45 dyne/cm to about 50 dyne/cm, about 35 dyne/cm to about 45 dyne/cm, about 40 dyne/cm to about 45 dyne/cm, or about 35 dyne/cm to about 40 dyne/cm.

[0084] In various embodiments, the solvent may be an aprotic solvent. Non-limiting examples of aprotic solvent may include tetrahydrofuran, dichloromethane, ethyl acetate, and a combination thereof. The phrase "aprotic solvent" as used herein refers to a solvent that does not contain acidic hydrogen and does not act as a hydrogen bond donor. In various embodiments, the solvent is tetrahydrofuran.

[0085] The polymer or the polymer thus formed may be crosslinked with a crosslinking agent in the presence of an ionic liquid. As mentioned above, the crosslinking agent may comprise a polymer having a terminal hydroxyl group and an organosilicate. Generally, the crosslinking agents may possess good compatibility with the ionic liquid. For example, the polymer having a terminal hydroxyl group comprised in the crosslinking agent may be selected from the group consisting of poly(dimethylsiloxane)-OH, silicone rubber-OH, poly(glycerol sebacate)-OH, epoxy resins-OH, polysulfide rubber-OH, urethane rubber-OH, urethane plastic-OH, polyacrylic rubber-OH, butyl rubber-OH, ethylene-vinyl acetate-OH (EVA-OH), a copolymer thereof, and a combination thereof. In specific embodiments, the polymer having a terminal hydroxyl group comprised in the crosslinking agent may be poly(dimethylsiloxane)-OH or a copolymer thereof, while the organosilicate may be tetraethylorthosilicate (TEOS). [0086] Amount of the polymer having a terminal hydroxyl group comprised in the crosslinking agent and the organosilicate that is used may vary according to the amount of ionic liquid present. In various embodiments, the polymer having a terminal hydroxyl group comprised in the crosslinking agent, the organosilicate, and the ionic liquid are present in a weight ratio of about 1 :1 : 1 to about 2:2: 1. Lower amounts of ionic liquid used may lead to lower conductivity of the ionogel, rendering the performance of the ionogel insufficient. Meanwhile, lower amounts of crosslinking agent and organosilicate used may render formation of the ionogel difficult.

[0087] In addition to providing the ionogel, the method disclosed herein may include providing a shell solution comprising polyvinylidene difluoride or a copolymer thereof. Examples of suitable copolymers of polyvinylidene difluoride have already been discussed above. Providing the shell solution may, for example, comprise dissolving the polyvinylidene difluoride or a copolymer thereof in an organic solvent. Generally, any organic solvent which is able to dissolve polyvinylidene difluoride or a copolymer comprising polyvinylidene difluoride may be used. In various embodiments, the organic solvent is selected from the group consisting of tetrahydrofuran, Ν,Ν-dimethylformamide, butanone, acetone, and a combination thereof.

[0088] In specific embodiments, the organic solvent comprises N,N- dimethylformamide and tetrahydrofuran. The Ν,Ν-dimethylformamide and tetrahydrofuran may be present in a volume ratio in the range of about 1 : 1 to about 1 :3, such as about 1 :1 to about 1 :2, 1 :2 to about 1 :3, or about 1 : 1, about 1 :2, or about 1 :3.

[0089] Concentration of the polyvinylidene difluoride or a copolymer thereof in the shell solution may be in the range of about 80 mg/mL to about 120 mg/mL, such as about 90 mg/mL to about 120 mg/mL, about 100 mg/mL to about 120 mg/mL, about 110 mg/mL to about 120 mg/mL, about 80 mg/mL to about 1 10 mg/mL, about 80 mg/mL to about 100 mg/mL, about 80 mg/mL to about 90 mg/mL, or about 90 mg/mL to about 1 10 mg/mL.

[0090] The ionogel and the shell solution may be subjected to electrospinning to form the core-shell nanofiber. In electrospinning, fibers may be formed by application of an electrical charge on a liquid to draw micro- or nano-fibers from the liquid. The process may comprise the use of a spinneret with a dispensing needle, a syringe pump, a power supply and a grounded collection device.

[0091] Material to form the fibers may be present as a melt or a spinning solution in the syringe, and driven to the needle tip by the syringe pump where they form a droplet. To form a core-shell nanofiber according to embodiments disclosed herein, the spinneret may comprise an inner channel and an outer channel for forming the core and the shell. The ionogel and the shell solution may be fed respectively into the inner channel and the outer channel via a respective syringe.

[0092] When voltage is applied to the needle, a droplet is stretched to an electrified liquid jet. The jet is elongated continuously until it is deposited on the collector as a mat of fine fibers of micrometer or nanometer sized dimensions. Electrospinning is able to afford control of the length of the resultant electrospun fibers by, for example, varying the voltage applied to the needle or by varying the composition of the melt or solution in the syringe.

[0093] Various embodiments refer in a third aspect to a device comprising a first electrode and an opposing second electrode, and a nanofiber mat comprising a core- shell nanofiber according to the first aspect or prepared by a method according to the second aspect disposed between the first electrode and the second electrode.

[0094] Generally, any electrically conducting material may be used to form the first electrode and the second electrode. In various embodiments, the first electrode and the opposing second electrode independently comprise a metal selected from the group consisting of copper, gold, silver, platinum, aluminum, nickel, an alloy thereof, and a combination thereof.

[0095] In various embodiments, the device is a capacitive pressure sensor, which may be used in a myriad of applications, such as consumer electronics in the form of touchpads, touchscreens, and biometric identification devices. Capacitive pressure sensors may use the property of capacitance to measure pressure. Typically, a compressible, dielectric material is disposed directly between a first electrode and an opposing second electrode to form a capacitive element. Each of the first electrode and the second electrode may store an electrical charge at the surface of the electrode at the boundary with the dielectric, whereby the electrical charge at each electrode is opposite to that stored on the other electrode.

[0096] When pressure is applied to the capacitive element, distance between the first electrode and the second electrode may be reduced to result in a change in capacitance. The detected capacitance may be correlated to a pressure value. By forming a matrix of the capacitive elements, individual pressure values for each capacitive element may be processed to create a two dimensional map of the pressure distribution.

[0097] Advantageously, the core-shell nanofiber disclosed herein may be used to form a nanofiber mat for use in devices to provide high electrical sensitivity, fast response time, low power consumption, compact circuit layout, and which allows for a relatively simple device construction compared to devices that work based on resistance. The core-shell nanofiber also provides for a larger surface area as compared to thin films thereby expanding the capacitive pressure sensor's functionality, e.g. sensitivity and response time. Use of traditional lithography, which is time consuming, costly and has limited scalability, is also avoided. The core-shell nanofiber disclosed herein also allows use in applications where higher pressures, such as pressures greater than 10 kPa are used.

[0098] The nanofiber mat is in electrical contact with the first electrode and the opposing second electrode, and may be disposed directly between the first electrode and the opposing second electrode, meaning that the nanofiber mat may be in direct contact with the first electrode and the opposing second electrode. The first electrode and the second electrode may independently comprise a metal selected from the group consisting of copper, gold, silver, platinum, aluminum, nickel, an alloy thereof, and a combination thereof.

[0099] Thickness of the nanofiber mat is not particularly limited. It may, for example, be in the range of about 0.05 mm to about 10 mm, such as about 0.05 mm to about 8 mm, about 0.05 mm to about 5 mm, about 1 mm to about 10 mm, about 5 mm to about 10 mm, about 3 mm to about 7 mm, or about 2 mm to about 6 mm.

[00100] In various embodiments, the device is a triboelectric nanogenerator. As used herein, the term "triboelectric nanogenerator" refers to a device which is able to convert mechanical energy into electrical energy. By making use of the triboelectric effect in the device, various types of energy generated by, e.g. human motion, vibration, wind, and water may be harvested. Advantageously, by using a core-shell nanofiber disclosed herein whereby the ionogel is encapsulated within a shell formed of PVDF or a copolymer thereof, dehydration of the ionogel which causes deterioration in electrical performance of flexible nanogenerators over time may be prevented or at least alleviated.

[00101] In various embodiments, the device further comprises a triboelectric material, wherein the triboelectric material has a triboelectric polarity different from that of the nanofiber mat. Generally, materials making use of the triboelectric effect may be ordered in a sequence based on levels of their attractions to electric charges, otherwise termed as "triboelectric polarity sequence". When two materials are in frictional contact with each other, negative charges may be transferred from surface of a material having a relative positive polarity in the triboelectric polarity sequence to surface of a material having a relative negative polarity in the triboelectric polarity sequence.

[00102] In various embodiments, the triboelectric material has a negative triboelectric polarity or a relatively negative polarity to that of the nanofiber mat. The triboelectric material may, for example, be selected from the group consisting of polyimide, silicone, silicone rubber, Teflon, and a combination thereof.

[00103] The triboelectric material may be disposed on an electrode different from that of the nanofiber mat. For example, the nanofiber mat may be disposed on the first electrode, while the triboelectric material is disposed on the opposing second electrode. Alternatively, the triboelectric material is disposed on the first electrode, while the nanofiber mat is disposed on the opposing second electrode. Either one or both the nanofiber mat and the triboelectric material may be disposed directly on a respective electrode, meaning that the nanofiber mat and/or the triboelectric material may be in direct contact with the respective electrode.

[00104] The first electrode and the second electrode having the nanofiber mat and the triboelectric material disposed thereon may be movable between a first configuration in which the nanofiber mat and the triboelectric material are in contact with each other, and a second configuration in which the nanofiber mat and the triboelectric material are spaced apart from each other. This includes situations in which the first electrode and the second electrode are arranged so that the nanofiber mat and the triboelectric material are brought into contact with each other from a spaced apart arrangement, or are placed such that they are able to slide against each other.

[00105] In various embodiments, the first electrode and the second electrode are arranged so that the nanofiber mat and the triboelectric material are brought into contact with each other from a spaced apart arrangement. The first electrode and the second electrode may be arranged on a respective substrate, wherein the first electrode and the second electrode are being spaced apart by a compressible spacer disposed between the substrates. The compressible spacer, for example, may be selected from the group consisting of an acrylic foam tape, silicone rubber, poly(glycerol sebacate), and a combination thereof.

[00106] The invention has been described broadly and generically herein. Each of the narrower species and subgeneric groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

[00107] Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.

[00108] In the context of various embodiments, the term "about" or "approximately" as applied to a numeric value encompasses the exact value and a reasonable variance.

[00109] As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

[00110] The word "substantially" does not exclude "completely" e.g. a composition which is "substantially free" from Y may be completely free from Y. Where necessary, the word "substantially" may be omitted from the definition of the invention.

[00111] In the context of various embodiments, the articles "a", "an" and "the" as used with regard to a feature or element include a reference to one or more of the features or elements. [00112] Advantages associated with various embodiments of the present device as described above may be applicable to the present method, and vice versa.

[00113] Other embodiments are within the following claims and non-limiting examples. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.

Examples

[00114] Various embodiments disclosed herein refer to a method of producing core- shell nanofibers from an ionic gel and a polymer. Various embodiments disclosed herein also relate to a device for capacitive pressure sensing and a triboelectric device, both of which are based on the core-shell nanofibers comprising the ionic gel and polymer. The ionic gel and polymer may form the core and shell of the nanofibers, respectively.

[00115] One example of the ionic gel may be a PDMS ionic gel while one example of the polymer may be poly(vinylidene fluoride-co-hexafluoropropene) (i.e. PVDF-HFP copolymer). The ionic gel may be called an ion gel in the present disclosure. In the present disclosure, the layer of nanofibers that is formed may be refen-ed to as a nanofiber mat. The present method and devices, and their uses, are described by way of non-limiting examples, as set forth below.

[00116] Example 1A: Materials

[00117] Poly(dimethylsiloxane) (PDMS) is used as one of the examples in the present disclosure because it offers good elastic properties as well as biomedical compliance with human tissue and living cells. In the present method, PDMS is crosslinked to enhance its stability prior to use for forming stable PDMS nanofibers through electrospinning.

[00118] PVDF and PVDF based polymers, including PVDF copolymers and PVDF terpolymers, may be used in applications such as capacitors, piezoelectric nanogenerator and triboelectric nanogenerator due to their excellent flexibility, mechanical strength and chemical resistance. Compared to PVDF homopolymer, the PVDF-HFP copolymer is more miscible with ionic liquid in which the -HFP group acts as a crosslinking site for the gelation process and eases the formation of nanofibers through electrospinning. In addition, the dielectric constant of PVDF-HFP is higher than PVDF which enhances the performance of a triboelectric nanogenerator.

[00119] Example IB: General Description of Present Method and Devices

[00120] The general procedure for preparing each component of the present method and devices, using certain materials as a non-limiting example, is described below.

[00121] The PDMS ionic gels were generally prepared by using crosslinking via sol- gel synthesis via the following steps:

[00122] (a) Poly(dimethylsiloxane) with hydroxy terminated end group (PDMS-OH, molecular weight (MW) of about 500) was mixed with tetraethylorthosilicate (TEOS) solution and l-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][TFSI] in weight ratio of 1 : 1 : 1 to 2:2:1 to form an ionic liquid solution. 1- ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM] [TFSI] may be used instead of [BMIM] [TFSI] .

[00123] (b) The ionic liquid solution was stirred for 30 mins at room temperature.

[00124] (c) Subsequently, PDMS (Sylgard 184) which comprises liquid components (a mixture of platinum (Pt) catalyst and a prepolymer (i.e. a polymer precursor of dimethylsiloxane with vinyl groups)) and curing agent (dimethylsiloxane with vinyl groups and Si-H groups) (weight ratio of 10:1 to 10:3) were added into the ionic liquid solution. The PDMS in this step may be a silicone rubber. Alternatively, the PDMS may be poly(glycerol sebacate) (PGS).

[00125] (d) Finally, tetrahydrofuran (THF) solvent was added to the ionic liquid solution to form the PDMS ion gel.

[00126] The core-shell PDMS ion gel/PVDF-HFP nanofibers were generally prepared by crosslinking via electrospinning. The preparation procedure according to an embodiment is as follows:

[00127] (a) PVDF-HFP was dissolved in a mixed solvent of DMF/THF (volume ratio of 1 : 1 to 1 :3). Instead of PVDF-HFP, a homopolymer PVDF or a PVDF based copolymer or a PVDF based terpolymer may be used. Some examples of PVDF based copolymer or PVDF based terpolymer may include poly(vinylidene fluoride-co- trifluoroethylene) (PVDF-TrFE), poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE), poly(vinylidene fluoride-trifluoroethylene-chloiOfluoiOethylene) (PVDF-TrFE-CFE), etc. Alternative mixed solvents may include a mixture of or any two solvents from Ν,Ν-dimethylformamide (DMF), butanone (MEK), acetone and THF.

[00128] (b) The concentration of PVDF-HFP was about 80 to 120 mg/mL. The PVDF-HFP solution was stirred at a temperature of 40°C to 80°C for about 4 to 6 hours to obtain a transparent shell solution.

[00129] (c) FIG. lb illustrates the setup for core-shell electrospinning. The PVDF- HFP shell solution and the PDMS ionic gel core solution were fed into the outer channel (15 G, OD 1.8 mm, ID 1.4 mm) and inner channel (21 G, OD 0.8 mm, ID 1.4 mm) of the spinneret, respectively.

[00130] (d) At the nozzle tip, the inner chamiel protruded out of the outer channel by about 0.5 mm. The flow rate of the PDMS ionic gel and PVDF-HFP solutions were controlled by using syringe pumps at rates of 0.5 to 0.7 mL/hr and 1 to 1.5 mL hr, respectively. An aluminum foil, used as the collector, was placed on the ground.

[00131] (e) The distance between the tip and collector was fixed at about 8 cm to 15 cm, and the applied voltage was about 20 kV to 25 kV. The core-shell PDMS ionic gel/PVDF-HFP nanofiber mat was collected on the aluminum foil for 1 hour at room temperature.

[00132] (f) After collection, the core-shell PDMS ionic gel/PVDF-HFP nanofiber mat was dried at 80°C to 100°C for 3 hours to 6 hours in air.

[00133] The general manufacturing procedure to fabricate the flexible capacitive pressure sensor device may depend on the configuration designed for the sensor device. The configuration of the capacitive pressure sensor is illustrated in FIG. 2a. The device is constructed based on a metal-insulator-metal (MIM) structure where the metal is copper electrode and the insulator is the core-shell PDMS ionic gel/PVDF- HFP nanofibers. Gold, silver, platinum, aluminum or nickel may be used instead of copper.

[00134] The method to fabricate a flexible triboelectric device may depend on the configuration designed for the triboelectric device. One example configuration is illustrated in FIG. 4a. As illustrated in FIG. 4a, the core-shell PDMS ionic gel/PVDF- HFP nanofibers with the copper electrode were placed on the polyethylene terephthalate (PET). VHB tape (3 M) was used as a spacer. Kapton film was used as a strong negative triboelectric material. Instead of Kapton, alternatives like silicone, silicone rubber and Teflon may be used. The spacer material can also be a silicone rubber (e.g. PDMS, Ecoflex or poly(glycerol sebacate)).

[00135] Example 1C: Present Method based on Poly(dimethylsiloxane) Ionic Gel

[00136] Preparation of PDMS ion gels: Poly(dimethylsiloxane) with hydroxyl terminated end group (PDMS-OH, molecular weight (MW) of about 500) was mixed with TEOS solution and l-butyl-3-methylimidazolium bis(tiifluoromethylsulfonyl)imide [BMIM][TFSI] in a weight ratio of 1 :1 : 1. The solution was stirred for 30 mins at room temperature. Subsequently, the PDMS (Sylgard 184) which consisted of liquid components (a mixture of Pt catalyst and prepolymer dimethyl siloxane with vinyl groups) and curing agent (prepolymer dimethylsiloxane with vinyl groups and Si-H groups) (in a weight ratio of 10: 1) were added into the ionic liquid solution. In the present disclosure, the term prepolymer may be used interchangeably with the term "polymer precursor". Finally, tetrahydrofuran (THF) solvent was added to the ionic liquid solution to form the PDMS ionic gel.

[00137] Core-shell electrospun PDMS ionic gel/PVDF-HFP nanofiber mats:

PVDF-HFP (Sigma-Aldrich, MW of about 455,000) was dissolved in a mix solvent of DMF/THF (volume ratio = 1 : 1). The concentration of PVDF-HFP was 100 mg/mL. The PVDF-HFP solution was stirred at 60°C for 6 hours to obtain a transparent solution. FIG. lb illustrates the setup for core-shell electrospinning. PVDF-HFP and PDMS ion gel solutions were fed into the outer channel (15 G, OD 1.8 mm, ID 1.4 mm) and inner channel (21 G, OD 0.8 mm, ID 1.4 mm) of the spinneret, respectively. At the nozzle tip, the inner channel protruded out of the outer channel by about 0.5 mm. The flow rates of the PDMS ionogel and PVDF-HFP solutions were controlled using syringe pumps at 0.7 mL/hr and 1 mL/hr, respectively. An aluminum foil placed on the ground was used as the collector. The tip to collector distance was fixed at 15 cm, and the applied voltage was 25 kV.

[00138] Fabrication of the flexible capacitive pressure sensor: The device configuration of a capacitive pressure sensor is illustrated in FIG. 2a. The device was constructed based on a metal-insulator-metal (MIM) capacitor structure where the metal was Cu electrode and the capacitive layer was the core-shell PDMS ion gel/PVDF-HFP nanofiber mat. The device size was 1 cm ^χ 1 cm.

[00139] Fabrication of the flexible triboelectric device: As illustrated in FIG. 4a, the core-shell PDMS ionic gel/PVDF-HFP nanofiber mat with Cu electrode was placed on the polyethylene terephthalate (PET). Kapton film was used as the strong negative triboelectric material. VHB tape (3 M) was used as a separation material to keep the Kapton film and the nanofiber mat apart.

[00140] Example ID: Characterization

[00141] The crystallinity of the core-shell PDMS ion gel/PVDF-HFP nanofiber mat was identified by a XRD (X-ray diffraction) with Cu Ka radiation (λ = 1.5418 A) at 30 kV and 20 mA and at a scan rate of 2° min from 10° to 80° (2Θ). Field emission scanning electron microscopy (FESEM, JEOL 7600F) operating at 5 kV was employed to determine the core-shell PDMS ionic gel/PVDF-HFP nanofibers morphology. Detailed structure analyses were performed using transmission electron microscopy (TEM, JEOL 2010) operated at 200 kV. The compression test was performed on the core-shell PDMS ionic gel/PVDF-HFP nanofiber mat by using Instron 5567. The capacitance measurement from the capacitive pressure sensor was obtained using the LCR (Inductance, Capacitance, Resistance) meter (Agilent E4980A). To analyze the device sensitivity, the LCR meter was used to detect capacitive changes under various mechanical loads on the device. The voltage outputs from the triboelectric pressure sensor were measured by an oscilloscope (Trektronix, MDO 3024, input resistance = 10 ΜΩ), the current output was measured by a low- noise current pre-amplifier (Stanford Research System, Model SR570, input resistance = 4 Ω). The dynamic mechanical pressure was applied by a magnetic shaker (Sinocera, Model JZK-20).

[00142] In the present disclosure, the PDMS ionic gel has been prepared by sol-gel reaction to create the crosslinked PDMS using TEOS and PDMS-OH. The TEOS and PDMS-OH were employed to be a crosslinking agent between the ionic liquid and PDMS polymer. The schematic diagram of the PDMS ionic gel preparation is shown in FIG. la. The electrospinning process of the core-shell PDMS ionic gel/PVDF-HFP nanofiber mat is shown in FIG. lb. The PVDF-HFP was used for the shell solution for electrospinning as it easily forms nanofiber due to its high molecular weight. The electrospinning of PDMS ionic gel nanofibers was realized through a PDMS prepolymer (PDMS solely with vinyl groups) and a curing agent to form a stable 3D network via covalent bonds. The morphology of the electrospun PDMS ionic gel core- shell nanofiber is shown in FIG. lc. A TEM image depicting the core-shell nanofiber structure is shown in FIG. Id. The diameters of the core-shell nanofiber and the PDMS ionic gel core in the nanofiber were about 235 nm and 35 nm, respectively.

[00143] On the other hand, pristine PVDF-HFP nanofibers do not form a core-shell structure, which is shown in FIG. le. The thickness of the core-shell PDMS ionic gel/PVDF-HFP nanofiber mat (the layer of nanofibers) was around 100 μιη as shown in FIG. If. Infrared spectroscopy was used to characterize the core-shell PDMS ionic gel/PVDF-HFP nanofiber mat as shown in FIG. lg. In the region between 500 and 750 cm^"1, two strong bending bands were observed, which correspond to a S0₂ asymmetric bending (610 cm^"1) and another S0₂ asymmetric bending (612 cm^"1) from the [EMIM][TFSI]. The peaks at 1090 and 1022 cm^"1 were attributed to the Si-O-Si of siloxane, which showed evident vibration peaks for the PDMS fiber. The -CH₃ rocking and Si-C vibrations appeared at around 802 cm^"1. The electrospun core-shell PDMS ionic gel/PVDF-HFP nanofiber mat had a crystalline structure of a and β phases. The vibrational bands at 1264 and 842 cm^"1 were attributed to the β phases of PVDF-HFP. Several peaks in the region between 1 197 and 762 cm^"1 were attributed to a phases of PVDF-HFP .

[00144] Example 2: Capacitive Pressure Sensing Device

[00145] The PDMS ionic gel/PVDF-HFP core-shell nanofiber mats were used for constructing a capacitive pressure sensor based on a metal-insulator-metal (MIM) configuration with Cu as the top and bottom electrode, as shown in FIG. 2a. FIG. 2b shows the capacitance response of the nanofiber mats, which are able to sense very small pressure of 0.01 kPa. The capacitance was altered by placing and removing a load cell of 0.01 kPa in FIG. 2b. The mechanism for the nanofiber mats' capacitance change when pressure was applied, is shown in FIG. 2c, schemes (I) to (III). The capacitance of a parallel plate capacitor with area A and thickness d can be written as:

[00146] where C is the capacitance, in Farads; A is the area of overlap of the two plates, in square meters; ε₀ is the vacuum dielectric constant (ε₀ is about 8.54 ^χ 1( ¹² F/m); ε_Γ is the dielectric constant of the material between the plates; and d is the separation between the plates. The change in capacitance for a constant applied pressure allows for the static pressure to be measured.

[00147] The dramatic increase in capacitance of the core-shell nanofiber mats when the external pressure was applied could be attributed to three main factors: (1) the dielectric constant ε_Γ was changed when the PDMS ionic gel/PVDF-HFP nanofiber mats were compressed since the displaced air has a lower dielectric constant (ε_Γ= 1.0) than PDMS (ε_Γ = 3.0) and PVDF-HFP (ε_Γ = 11.38) as shown in FIG. 2c (scheme (I)), (2) when the distance between the top and bottom electrodes was reduced (d to d'), the thickness of the elastomeric PDMS inside the core-shell PDMS ionic gel/PVDF-HFP nanofibers could be reduced as well (see FIG. 2c (scheme (II)) since PDMS possesses good elastic properties in the regime that is less than 100 kPa, and (3) the contact area of the formed electrical double layer is increased (A to A') due to reduced spacings as a result of the highly deformable properties of PDMS, which results in an enhanced interfacial capacitance as shown in FIG. 2c (scheme (III)). The increase of the dielectric constant and the fiber contact area as well as the reduced distance between the separated electrodes led to a significant increase in the capacitance of the pressure sensor upon compression and therefore a high pressure sensitivity in the low pressure range can be achieved for the present device having the core-shell nanofiber derived from the present method.

[00148] Example 3: Effects of the Amounts of Ionic liquid

[00149] The pressure response curves for different amounts of ionic liquid loaded in the core-shell PDMS ionic gel/PVDF-HFP nanofibers are presented in FIG. 3a and FIG. 3b. The pressure sensitivity S can be defined as the slope of each of the curves in FIG. 3a and FIG. 3b, based on the equation where S = δ (AC/C₀)/8p = (1/C₀)*8C , where p denotes the applied pressure, and C and C₀ denote the capacitance with and without applied pressure, respectively. The 40 wt% ionic liquid loaded core-shell PDMS ionic gel/PVDF-HFP nanofiber mat exhibited much higher pressure sensitivity than others. In the pressure range of less than 1.5 kPa, sensitivity of the sample with 40 wt% ionic liquid was 71 times higher than pristine PVDF-HFP nanofiber mat due to a larger interfacial capacitance produced at a higher amount of PDMS ionic gel as shown in FIG. 3a. Compared with conventional fiber-based pressure sensor, the present pressure sensor exhibited better sensitivity and wider detection limits. The lower detection limit of the present device was improved by 12.5 times compared to conventional PDMS-coated conductive fibers. It is noteworthy that sensitivity of the core-shell PDMS ion gel/PVDF-HFP nanofiber mat in the low pressure regime was 50 times higher than the reported value for a ZnO-PMMA composite. In addition, the present device was able to extend the range of possible measurements up to 10 kPa as shown in FIG. 3b. However, there was a change in sensitivity when the pressures were higher than 1.5 kPa. The sensitivity changed due to the increasing elastic resistance with increasing compression. For real world applications, this progressive damping is desirable, since it increases the range of detectable pressure for cases with high loading pressures where high sensitivity is not required for the capacitive pressure sensor, and therefore results in a more versatile pressure sensor. The mechanical properties of pristine PVDF-HFP and different weight percentages of ionic liquid loading are shown in FIG. 3c.

[00150] Compared to pristine PVDF-HFP nanofiber mat, the 40 wt% ionic liquid loaded nanofiber mat exhibited a higher compressive strain at the same compressive stress. This indicates that higher amount of ionic liquid used in preparing the core- shell nanofiber leads to a larger deformation upon applied pressure. The nanofibers with a higher amount of ionic liquid loaded PDMS ionic gel could have a lower elastic modulus as shown in FIG. 3e. It is noted that the elastic modulus of 10 wt% ionic liquid loaded core-shell PDMS ionic gel/PVF-HFP nanofiber mat is dramatically reduced compared to PVF-HFP nanofiber mat, which results in 6 times higher pressure sensitivity (0.04 kPa^"1) compared to pristine PVDF-HFP as shown in FIG. 3a. However, nanofibers with PDMS ionic gel prepared with more than 40 wt% ionic liquid had difficulty forming due to high ionic conductivity. The high ionic conductivity of the solution may cause large instabilities during the electrospinning as a high voltage operation (25 kV) is required to fabricate the core-shell nanofiber mat.

[00151] The ionic conductivity of different weight percentages of PDMS ionic gel was measured by electrochemical impedance spectroscopy (EIS) with a frequency range of 0.01 Hz to 100 kHz. FIG. 3f shows a typical Nyquist plot of the impedance analysis on PDMS ionic gel prepared with different weight percentages of ionic liquid. At high frequency (about 100 kHz), the corresponding value of the intercept on the real axis (horizontal axis) represents the instrinsic resistance of the ionic gel as the ohmic resistance of the testing device is negligible as shown in FIG. 3g. Therefore, the ionic conductivity can be calculated according to the formula:

_ _L_

σ ^~~ RS

[00152] where σ is the ionic conductivity, L is the distance between the two electrodes, R is the resistance of ion gel, and S is the geometric area of the electrode interface.

[00153] The ionic conductivity was increased by increasing amount of the ionic liquid as shown in FIG. 3h. The sensitivity, flexibility, and robustness of the present device allows it to be utilized as a wrist-based heart-rate monitor. The device was attached directly to the wrist of a living subject to measure the radial arterial pulse wave as shown in FIG. 3d. It was demonstrated that the device can track the number of pulses through the relative capacitance change. The test subject had a stable heart rate of 75 times per minute, which was consistent with the counting results and this showed that the subject was healthy and relax. A higher, arrhythmic or otherwise altered heart rate may indicate states of stress and physical exercise or detect early signs of potentially lethal heart defects and diseases. The inset in FIG. 3d shows the close up view on a pulse having the characteristic peak typically measured at the radial artery. There are two peaks per pulse. The first peak corresponds to shutting of valves that allows blood into the heart and the second peak is shutting of valves that allows blood out flow from the heart. The radial pulse wave could be a useful index of the arterial stiffness. Therefore, the present device has potential for applications in health diagnostics e.g. hypertension, atherosclerosis, heart failure, etc.

[00154] Example 4: Triboelectric and Self-Powered Device

[00155] The PDMS core-shell PDMS ionic gel/PVDF-HFP nanofiber mats were configured into a triboelectric sensor and a self-powered device by utilizing the triboelectric effect at high dynamic mechanical pressure. The triboelectric nanogenerator (TENG) device is composed of polyethylene terephthalate (PET) sheets as the substrates, a spacer, a Cu electrode, a Kapton layer and the core-shell PDMS ion gel/PVDF-HFP nanofiber mats as shown in FIG. 4a. The operation principle of a TENG is based on coupling of the electrostatic induction and triboelectric effect.

[00156] The mechanism for electric power generation using the core-shell PDMS ionic gel/PVDF-HFP nanofiber mats is shown in FIG. 4a. Initially, there was no charge induced by the electric potential difference between two electrodes. When external compressive force was applied, the core-shell PDMS ionic gel/PVDF-HFP nanofibers and Kapton were brought into contact with each other in the pushed state. Surface charge transfer occured at the interface as a result of the triboelectric effect. The Kapton film possesses strong negative triboelectric polarity based on the triboelectric series. The positive and negative charges were induced at the core-shell PDMS ionic gel/PVDF-HFP nanofiber mats and the Kapton surface, respectively. When the pressure was released, the Kapton and the core-shell PDMS ionic gel/PVDF-HFP nanofiber mats surface separated from each other. The dipole moment became stronger at this stage. Thus, a strong electric potential difference was generated between the electrodes. The connection to the device (switching polarity) with the measurement equipment was reversed in order to confirm that the measured output performance originated from the TENG and to eliminate the influence of the noise caused by the measurement system as shown in FIG. 5. The connecting configuration where the PDMS core-shell PDMS ionic gel/PVDF-HFP nanofiber mats is connected to a positive probe and the Kapton film is connected to a negative probe, is defined as a forward connection. On the other hand, the inverted connection is defined as the device measured with the reverse connection. The output signals were reversed when the device is reversely connected, which implied that the output signals were generated by a triboelectric effect. FIG. 4b and FIG. 4c show the generated output voltage and current density for different amounts of ionic liquid loaded in the core-shell nanofiber of the nanofiber mat under a dynamic mechanical pressure up to 700 kPa (a compressive force of 70 N) at an applied frequency of 5 Hz. The output voltage and current density increase with an increasing amount of ionic liquid loading in the PDMS ion gel, which implied that the electrical double layer formed by electrochemical effect in the ion gel nanofibers played an important role in increasing the amount of induced charge and the capacitance of the triboelectric layers, resulting in an improvement of the output performance of TENG. At higher amounts of ionic gel, the two interfaces may stick together, causing the charge to rebalance during the releasing state. Therefore, nanofibers with 40 wt% of ionic liquid were used for further study.

[00157] While conventional TENGs may be utilized for dynamic pressure sensing, the tribo-based sensors tend to encounter the limit of saturated output voltage at high pressure range. Since self-polarized polyvinydifluoride-trifluoroethylene (PVDF- TrFE) sponge could be utilized for ultra large range pressure detection, it is worthwhile to investigate the performance of triboelectric pressure sensor at higher pressure range. The output voltage signals of TENG under higher pressure range are shown in FIG. 6a. The output voltage was increased by increasing the dynamic mechanical pressure. The pressure sensitivity of the triboelectric nanogenerator-based pressure sensor was calculated from the slope of the voltage response curve (S = d(AVWs)/dP, where AV is the relative change in the output voltage, V_s is the final saturation voltage, and P denotes the applied pressure. The sensitivity of the 40 wt% ionic liquid loaded core-shell PDMS ionic gel/PVDF-HFP nanofiber mats was 0.068 V kPa from 100 kPa to 700 kPa as shown in FIG. 6b. The sensitivity of the core- shell PDMS ionic gel/PVDF-HFP nanofiber mats was at least comparable to the polarized PVDF-TrFE sponge. The higher sensitivity of the core-shell PDMS ionic gel/PVDF-HFP nanofiber mat was attributed to the increase of inductive charges, capacitance and surface area. In the pressure range from 40 kPa to 100 kPa, the sensitivity of the 40 wt% ionic liquid loaded nanofiber mat was 0.102 V kPa^~' as shown in FIG. 6e.

[00158] In the present example, it is demonstrated that the core-shell PDMS ionic gel/PVDF-HFP nanofiber mat could be used not only as a triboelectric sensor but also as a self-powered device. External loads with varying resistance can be connected to TENGs for different applications. The systematical study of output performance with different external loads of varying resistance is shown in FIG. 6f. The output voltage increases from 0.1 V to 31 V by increasing the resistance of the load from 1 kO to 100 ΜΩ. The output current decreases slightly from 3.92 μΑ to 3.06 μΑ. The instantaneous power density can be calculated by P = V²/R. The maximum output power obtained at different resistance is shown in FIG. 6c. The maximum power density can reach up to 0.9 W/m² at a load of 10 kQ. The power density of the core- shell PDMS ionic gel/PVDF-HFP nanofiber mat was 3 times higher than those reported for conventional silicone rubber based TENGs. The core-shell PDMS ionic gel/PVDF-HFP nanofiber mat allows a scalable design of the device sample. An array of 300-LED can be powered instantaneously by the output generated from the TENG as shown in FIG. 6d (both images (I) and (II)). The present device operates in a stable output performance even after a shelf life of six months, indicating the robustness of the materials and device configuration as shown in FIG. 7.

[00159] Example 5: Summary of Results and Advantages of the Present Method and Devices

[00160] As demonstrated above, the present core-shell nanofiber of PDMS ionic gel/PVDF-HFP was successfully prepared by using crosslinking agent(s) for electrospinning. PDMS is an inorganic polymer which has been used in several applications due to its favourable properties, such as optical transparency, elasticity and chemical stability. PDMS, however, is immiscible in many ionic liquids. By using tetraethylorthosilicate (TEOS) as a crosslinking agent, a hybrid ionic gel can be formed from various polymers, such as PDMS, poly(methyl methacrylate) (PMMA), through sol-gel reaction.

[00161] Advantages of the present method and devices comprising the present core- shell nanofiber, e.g. PDMS ion gel/PVDF-HFP nanofibers, are briefly discussed.

[00162] The present method is a sol-gel synthesis method that circumvents the use of polymerization initiators by using crosslinking agent(s) to form covalent bonds between the plurality of polymer precursors (i.e. prepolymer), e.g. dimethylsiloxane with vinyl groups, which in turn results in a three dimensional network polymeric matrix.

[00163] The PDMS ionic gel/PVDF-HFP core-shell nanofiber mat is scalable. The mat can be used not only for capacitive pressure sensor at low pressure but also as a self-powered device at high pressure. Utilizing the core-shell PDMS ionic gel/PVDF- HFP nanofiber mat as a capacitive sensor offers higher sensitivity of 0.43 kPa^"1 in the 1.5 kPa range. The present core-shell nanofiber mat can also be used as a pressure sensor in the high pressure region up to 70 N as shown in FIG. 8a and FIG. 8b, demonstrating the useability of the present core-shell nanofiber as a wide range pressure sensor.

[00164] The fabrication of self-powered devices using the core-shell PDMS ionic gel/PVDF-HFP nanofiber mat is also advantageous. This is because the maximum power density of the core-shell PDMS ionic gel/PVDF-HFP nanofiber mat could be brought up to 0.9 W/m², which is sufficient to light up 300 light emitting diodes (LEDs).

[00165] Example 6: Commercial and Potential Applications

[00166] In the present disclosure, the present method provides for making core-shell PDMS ionic gel/ PVDF-HFP nanofiber mats via electrospinning, which can be used in various pressure ranges for the dual functional applications of tactile sensing with respect to static and dynamic pressure, and power generation. Utilizing the core-shell PDMS ionic gel/PVDF-HFP nanofiber mats as a capacitive sensor offers high sensitivity of 0.43 kPa in the pressure range from 0.01 kPa up to 1.5 kPa. Compared to conventional zinc oxide nanowire/poly(methylmethacrylate) film and a PDMS- coated conductive fiber, the sensitivity of the present core-shell PDMS ionic gel/PVDF-HFP nanofiber mats is at least 50 and 2 times higher, respectively.

[00167] Furthermore, the present capacitive sensor has been demonstrated as a wearable pulse rate detector that serves as a heart-rate indicator. In addition, the core- shell PDMS ionic gel/PVDF-HFP nanofiber mats could be used as a self-powered device and as a triboelectric sensor with sensitivities of 0.068 V kPa and 0.102 V kPa at higher pressures ranging from 100 kPa to 700 kPa and 40 kPa to 100 kPa, respectively. The maximum power density of the core-shell PDMS ionic gel/PVDF- HFP nanofiber mats could be brought up to 0.9 W/m², which is sufficient to light up three hundred light emitting diodes (LEDs). The improvement in electrical power generated by the core-shell PDMS ionic gel/PVDF-HFP nanofiber mats can be attributed to the increased amount of inductive charges and capacitance in the triboelectric layer.

[00168] The present method and devices described herein may be useful for industries in new or advanced electronic materials, which has a global market totaled nearly $4.9 billion in 2016. This market is expected to be worth $10.3 billion in 2021 and $24.1 billion by 2026 at a five-year compound annual growth rate (CAGR) of 18.5%, for the period of 2021 to 2026. The present method and devices may also possess utility as energy harvesters, where the global market may reach $3.3 billion by 2020, with a compound annual growth rate (CAGR) of 23.9%. The present method and device disclosed herein has been demonstrated as a capacitive pressure sensor, which can be used for in medical applications.

[00169] While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.

Claims

1. A core-shell nanofiber having a core and a shell surrounding the core, wherein the core comprises an ionogel and the shell comprises polyvinylidene difluoride or a copolymer thereof.

2. The core-shell nanofiber according to claim 1, wherein the ionogel is obtainable by crosslinking a polymer with a crosslinking agent in the presence of an ionic liquid.

3. The core-shell nanofiber according to claim 1 or 2, wherein the polymer is an elastomeric polymer.

4. The core-shell nanofiber according to claim 3, wherein the elastomeric polymer is selected from the group consisting of poly(dimethylsiloxane), silicone rubber, poly(glycerol sebacate), epoxy resins, polysulfide rubber, urethane rubber, urethane plastic, polyacrylic rubber, butyl rubber, ethylene- vinyl acetate, a copolymer thereof, and a combination thereof.

5. The core-shell nanofiber according to claim 3 or 4, wherein the elastomeric polymer is poly(dimethylsiloxane) or a copolymer thereof.

6. The core-shell nanofiber according to claim 5, wherein the poly(dimethylsiloxane) is obtainable by polymerizing a prepolymer comprising dimethylsiloxane with vinyl groups in the presence of a curing agent comprising dimethylsiloxane with vinyl groups and dimethylsiloxane with Si-H groups.

7. The core-shell nanofiber according to claim 6, wherein the prepolymer and the curing agent are present in a weight ratio of about 10:1 to about 10:3.

8. The core-shell nanofiber according to any one of claims 2 to 7, wherein the crosslinking agent comprises a polymer having a terminal hydroxyl group and an organosilicate.

9. The core-shell nanofiber according to claim 8, wherein the polymer having a terminal hydroxyl group comprised in the crosslinking agent is selected from the group consisting of poly(dimethylsiloxane)-OH, silicone rubber-OH, poly(glycerol sebacate)-OH, epoxy resins-OH, polysulfide rubber-OH, urethane rubber-OH, urethane plastic-OH, polyacrylic rubber-OH, butyl rubber-OH, ethylene-vinyl acetate-OH (EVA-OH), a copolymer thereof, and a combination thereof.

10. The core-shell nanofiber according to claim 8 or 9, wherein the polymer having a terminal hydroxyl group comprised in the crosslinking agent is poly(dimethylsiloxane)-OH or a copolymer thereof.

11. The core-shell nanofiber according to any one of claims 8 to 10, wherein the organosilicate comprises tetraethylorthosilicate (TEOS).

12. The core-shell nanofiber according to any one of claims 8 to 11, wherein the polymer having a terminal hydroxyl group comprised in the crosslinking agent, the organosilicate, and the ionic liquid are present in a weight ratio of about 1 : 1 : 1 to about 2:2: 1.

13. The core-shell nanofiber according to any one of claims 2 to 12, wherein the ionic liquid is selected from the group consisting of l-butyl-3- methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][TFSI], 1- ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM] [TFSI], l-hexyl-3-methylimidazolium bis(trifluoiOmethylsulfonyl)imide [HMIM] [TFSI], 1 -butyl-3-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [BMP][TFSI], l-ethyl-3-methylimidazolium tetracyanoborate, l-ethyl-3-methylimidazolium tris(per fluoroethyl)trifluorophosphate, and a combination thereof.

14. The core-shell nanofiber according to any one of claims 2 to 13, wherein the ionic liquid is l-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][TFSI].

15. The core-shell nanofiber according to any one of claims 1 to 14, wherein amount of the ionic liquid in the core-shell nanofiber is 40 wt% or less of the total weight of the core-shell nanofiber.

16. The core-shell nanofiber according to any one of claims 1 to 15, wherein the shell comprises poly(vinylidene fluoride-co-hexafluoropropene).

17. A method of preparing a core-shell nanofiber having a core and a shell surrounding the core, wherein the core comprises an ionogel and the shell comprises polyvinylidene difluoride or a copolymer thereof, the method comprising

a) providing an ionogel,

b) providing a shell solution comprising polyvinylidene difluoride or a copolymer thereof, and

c) electrospinning the ionogel and the shell solution to form the core-shell nanofiber.

18. The method according to claim 17, wherein providing the ionogel comprises crosslinking a polymer with a crosslinking agent in the presence of an ionic liquid.

19. The method according to claim 18, wherein crosslinking the polymer with the crosslinking agent in the presence of the ionic liquid comprises a) providing a mixture comprising the crosslinking agent and the ionic liquid, and

b) adding a prepolymer of the polymer and a curing agent to the mixture.

20. The method according to claim 19, further comprising adding a solvent to the mixture after adding the prepolymer of the polymer and the curing agent.

21. The method according to claim 20, wherein the solvent is tetrahydrofuran.

22. The method according to any one of claims 18 to 21 , wherein the polymer is an elastomeric polymer.

23. The method according to claim 22, wherein the elastomeric polymer is selected from the group consisting of poly(dimethylsiloxane), silicone rubber, poly(glycerol sebacate), epoxy resins, polysulfide rubber, urethane rubber, urethane plastic, polyacrylic rubber, butyl rubber, ethylene-vinyl acetate, a copolymer thereof, and a combination thereof.

24. The method according to claim 22 or 23, wherein the elastomeric polymer is poly(dimethylsiloxane) or a copolymer thereof.

25. The method according to claim 24, wherein the prepolymer of the polymer comprising dimethylsiloxane with vinyl groups, and the curing agent comprises dimethylsiloxane with vinyl groups and dimethylsiloxane with Si-H groups.

26. The method according to claim 25, wherein the prepolymer and the curing agent are present in a weight ratio of about 10: 1 to about 10:3.

27. The method according to any one of claims 18 to 26, wherein the crosslinking agent comprises a polymer having a terminal hydroxyl group and an organosilicate.

28. The method according to claim 27, wherein the polymer having a terminal hydroxyl group comprised in the crosslinking agent is selected from the group consisting of poly(dimethylsiloxane)-OH, silicone rubber-OH, poly(glycerol sebacate)-OH, epoxy resins-OH, polysulfide rubber-OH, urethane rubber-OH, urethane plastic-OH, polyacrylic rubber-OH, butyl rubber-OH, ethylene-vinyl acetate-OH (EVA-OH), a copolymer thereof, and a combination thereof.

29. The method according to claim 27 or 28, wherein the polymer having a terminal hydroxyl group comprised in the crosslinking agent is poly(dimethylsiloxane)-OH or a copolymer thereof.

30. The method according to any one of claims 27 to 29, wherein the organosilicate comprises tetraethylorthosilicate (TEOS).

31. The method according to any one of claims 27 to 30, wherein the polymer having a terminal hydroxyl group comprised in the crosslinking agent, the organosilicate, and the ionic liquid are present in a weight ratio of about 1 :1 : 1 to about 2:2: 1.

32. The method according to any one of claims 17 to 31, wherein providing a shell solution comprising polyvinylidene difluoride or a copolymer thereof comprises dissolving the polyvinylidene difluoride or a copolymer thereof in an organic solvent.

33. The method according to claim 32, wherein the organic solvent is selected from the group consisting of tetrahydrofuran, N,N-dimethylformamide, butanone, acetone, and a combination thereof.

34. The method according to claim 32 or 33, wherein the organic solvent comprises Ν,Ν-dimethylformamide and tetrahydrofuran at a volume ratio in the range of about 1 : 1 to about 1 :3.

35. The method according to any one of claims 17 to 34, wherein concentration of the polyvinylidene difluoride or a copolymer thereof in the shell solution is in the range of about 80 mg/niL to about 120 mg/niL.

36. A device comprising

a) a first electrode and an opposing second electrode, and

b) a nanofiber mat comprising a core-shell nanofiber according to any one of claims 1 to 16 or prepared by a method according to any one of claims 17 to 35 disposed between the first electrode and the second electrode.

37. The device according to claim 36, wherein the nanofiber mat is disposed directly between the first electrode and the opposing second electrode.

38. The device according to claim 37, wherein the device is a capacitive sensor.

39. The device according to claim 36, wherein the nanofiber mat is disposed directly on the first electrode, the device further comprising a triboelectric material disposed on the opposing second electrode, wherein the triboelectric material has a triboelectric polarity different from that of the nanofiber mat, and

wherein the first electrode and the second electrode are movable between a first configuration in which the nanofiber mat and the triboelectric material are in contact with each other, and a second configuration in which the nanofiber mat and the triboelectric material are spaced apart from each other.

40. The device according to claim 39, wherein the triboelectric material has negative triboelectric polarity.

41. The device according to claim 39 or 40, wherein the triboelectric material is selected from the group consisting of polyimide, silicone, silicone rubber, Teflon, and a combination thereof.

42. The device according to any one of claims 39 to 41, wherein the first electrode and the second electrode are arranged on a respective substrate, and wherein the first electrode and the second electrode are being spaced apart by a compressible spacer disposed between the substrates.

43. The device according to claim 42, wherein the compressible spacer is selected from the group consisting of an acrylic foam tape, silicone rubber, poly(glycerol sebacate), and a combination thereof.

44. The device according to any one of claims 39 to 43, wherein the device is a triboelectric nanogenerator.

45. The device according to any one of claims 36 to 44, wherein the first electrode and the opposing second electrode independently comprises a metal selected from the group consisting of copper, gold, silver, platinum, aluminum, nickel, an alloy thereof, and a combination thereof.