Conductive polymer composite for adhesion to flexible substrate and method for preparing same

Abstract

A conductive polymer composite for adhesion to a flexible substrate contains a polymer adhesive containing a curable polymer and a curing agent; and a conductive filler containing a metal and a carbonaceous material dispersed in the polymer adhesive. The conductive polymer composite is suitable for application to not only the human body but also other objects having irregular surface. In addition, due to enhanced adhesive strength of the conductive polymer composite to the flexible substrate, the reduction in conductivity or conductivity breakdown caused by external stress can be prevented and flexibility and stretchability can be improved.

Claims

1. A conductive polymer composite adhesive to a flexible substrate, comprising: a polymer adhesive comprising a curable polymer and a curing agent; and a conductive filler consisting of a metal and a carbonaceous material, wherein the metal and the carbonaceous material are dispersed in the polymer adhesive by using a chloroform, wherein the metal has a mixture of a first metal particle having a diameter of 100 to 200 nm, a second metal particle having a diameter of 1 to 4 m, and a third metal particle having a diameter of 5 to 10 m, wherein the carbonaceous material is a multi-walled nanotube, wherein the carbonaceous material is present in an amount of 0.1 to 2 parts by weight based on 100 parts by weight of the metal, wherein the conductive filler is present in an amount of 100 to 500 parts by weight based on 100 parts by weight of the polymer adhesive, wherein the flexible substrate comprises a polydimethylsiloxane (PDMS).

2. The conductive polymer composite for adhesion to a flexible substrate according to claim 1, wherein the polymer adhesive has a mixture viscosity of 3000 mPa.Math.s to 5000 mPa.Math.s before curing.

3. The conductive polymer composite for adhesion to a flexible substrate according to claim 1, wherein the polymer adhesive has a bulk tensile strength of 0.1 to 10 MPa after curing.

4. The conductive polymer composite for adhesion to a flexible substrate according to claim 1, wherein the polymer adhesive has a volume shrinkage ratio of 1 to 10% after curing.

5. The conductive polymer composite for adhesion to a flexible substrate according to claim 1, wherein the polymer adhesive is at least one selected from a silicone-based resin, a urethane-based resin, an acrylic resin, an isoprene-based resin, a chloroprene-based resin, a fluorine-based resin, butadiene rubber and styrene-butadiene rubber.

6. The conductive polymer composite for adhesion to a flexible substrate according to claim 1, wherein the polymer adhesive is a silicone-based resin.

7. The conductive polymer composite for adhesion to a flexible substrate according to claim 1, wherein the polymer adhesive comprises 10 to 120 parts by weight of the curing agent based on 100 parts by weight of the curable polymer.

8. The conductive polymer composite for adhesion to a flexible substrate according to claim 1, wherein the metal is at least one selected from gold (Au), silver (Ag), copper (Cu), nickel (Ni), aluminum (Al), platinum (Pt), ruthenium (Ru), rhodium (Rh), tungsten (W), cobalt (Co), palladium (Pd), titanium (Ti), tantalum (Ta), iron (Fe), molybdenum (Mo), hafnium (Hf), lanthanum (La) and iridium (Ir).

9. The conductive polymer composite for adhesion to a flexible substrate according to claim 1, wherein the multi-walled carbon nanotube has a diameter of 15 to 40 nm and a length of 10 to 50 m.

10. The conductive polymer composite for adhesion to a flexible substrate according to claim 1, wherein the conductive polymer adhesive provides electrical conductivity as an electrode on the flexible substrate.

11. A flexible electronic device comprising the conductive polymer composite for adhesion to a flexible substrate according to claim 1.

12. The flexible electronic device according to claim 11, wherein the flexible electronic device is any one selected from a flexible display, a flexible transistor, a flexible sensor and a solar cell.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a flow diagram sequentially illustrating a method for preparing a conductive polymer composite for adhesion to a flexible substrate of the present disclosure.

(2) FIG. 2 shows photographs showing the adhesive force measurement results of Test Example 2.

(3) FIG. 3 shows the stretch test result of Test Example 3.

DETAILED DESCRIPTION OF EMBODIMENTS

(4) Hereinafter, various aspects and exemplary embodiments of the present disclosure are described in more detail. Hereinafter, the exemplary embodiments of the present disclosure are described in detail with reference to accompanying drawings so that those of ordinary skill in the art to which the present disclosure belongs can easily carry out the present disclosure. However, the following description is not intended to limit the present disclosure to specific exemplary embodiments. When it is determined that there is a specific description of the related art in the following description of the present disclosure that may obscure the subject matter of the present disclosure, a detailed description thereof will be omitted.

(5) The terms used herein are merely used to describe particular exemplary embodiments, and are not intended to limit the present disclosure. A singular expression includes plural forms unless the context clearly indicates otherwise. In this application, the terms contain, include or have are used to specify that the features, numbers, steps, operations, elements or combinations thereof described in the specification are present, and the presence or addition of one or more other features, numbers, steps, operations, elements or combinations thereof are not intended to be precluded.

(6) Hereinafter, a conductive polymer composite for adhesion to a flexible substrate of the present disclosure is described.

(7) The conductive polymer composite for adhesion to a flexible substrate of the present disclosure contains: a polymer adhesive containing a curable polymer and a curing agent; and a conductive filler containing a metal and a carbonaceous material dispersed in the polymer adhesive.

(8) The curable polymer may be a thermosetting polymer or a photo-curable polymer.

(9) The polymer adhesive may contain specifically 10 to 120 parts by weight, more specifically 90 to 110 parts by weight, still more specifically 95 to 105 parts by weight, of the curing agent based on 100 parts by weight of the curable polymer.

(10) Specifically, the polymer adhesive may have a mixture viscosity of 3000 mPa.Math.s to 5000 mPa.Math.s before curing, a bulk tensile strength of 0.1 to 10 MPa after curing, and a volume shrinkage of less than or equal to about 1-10%.

(11) If the viscosity, bulk tensile strength and volume shrinkage ratio of the polymer adhesive are lower than the lower limit values or greater than the upper limit values, the adhesive strength, mechanical properties, electrical properties, etc. of the adhesive on the flexible substrate may be unsatisfactory.

(12) Further, the conductive polymer composite for adhesion to a flexible substrate of the present disclosure may contain specifically 100 to 500 parts by weight of the conductive filler, more specifically 150 to 470 parts by weight of the conductive filler, further more specifically 200 to 420 parts by weight of the conductive filler, based on 100 parts by weight of the polymer adhesive containing the curable polymer and the curing agent.

(13) When the content of the conductive filler is less than 100 parts by weight based on 100 parts by weight of the adhesive polymer, conductivity may not be achieved. And, when it exceeds 500 parts by weight, it is not preferable because the adhesive force on the flexible substrate may be reduced.

(14) The polymer adhesive may be specifically at least one selected from a silicone-based resin, a urethane-based resin, an acrylic resin, an isoprene-based resin, a chloroprene-based resin, a fluorine-based resin, butadiene rubber and styrene-butadiene rubber. More specifically, it may be a silicon-based resin.

(15) The metal may be at least one selected from gold (Au), silver (Ag), copper (Cu), nickel (Ni), aluminum (Al), platinum (Pt), ruthenium (Ru), rhodium (Rh), tungsten (W), cobalt (Co), palladium (Pd), titanium (Ti), tantalum (Ta), iron (Fe), molybdenum (Mo), hafnium (Hf), lanthanum (La) and iridium (Ir), Specifically, the metal may be at least one selected from gold (Au), silver (Ag) and copper (Cu). Even more specifically, it may be silver (Ag).

(16) The metal may be one selected from a metal particle having a diameter of 100 to 200 nm, a metal particle having a diameter of 1 to 4 m and a metal particle having a diameter of 5 to 10 m. More specifically, it may be a mixture of a metal particle having a diameter of 100 to 200 nm, a metal particle having a diameter of 1 to 4 m and a metal particle having a diameter of 5 to 10 m.

(17) The carbonaceous material may be specifically at least one selected from single-walled carbon nanotube, multi-walled carbon nanotube, graphene, graphite, carbon black, carbon fiber and fullerene. More specifically, the carbonaceous material may be at least one selected from single-walled carbon nanotube and multi-walled carbon nanotube. Even more specifically, the carbonaceous material may be multi-walled carbon nanotube having a diameter of 15 to 40 nm and a length of 10 to 50 m.

(18) The conductive filler may contain specifically 0.1 to 2 parts by weight, more specifically 0.15 to 1 part by weight, even more specifically 0.2 to 0.5 part by weight, of the carbonaceous material based on 100 parts by weight of the metal.

(19) The conductive polymer adhesive may be used as an electrode on a flexible substrate.

(20) The conductive polymer adhesive may be used as an electrode on a silicon substrate.

(21) The conductive polymer adhesive has flexibility and stretchability.

(22) FIG. 1 is a flow diagram sequentially illustrating a method for preparing a conductive polymer composite for adhesion to a flexible substrate of the present disclosure. Hereinafter, the method for preparing a conductive polymer composite for adhesion to a flexible substrate of the present disclosure will be described with reference to FIG. 1.

(23) First, a conductive filler dispersion is prepared by dispersing a metal and a carbonaceous material in an organic solvent (step a).

(24) The metal may be at least one selected from gold (Au), silver (Ag), copper (Cu), nickel (Ni), aluminum (Al), platinum (Pt), ruthenium (Ru), rhodium (Rh), tungsten (W), cobalt (Co), palladium (Pd), titanium (Ti), tantalum (Ta), iron (Fe), molybdenum (Mo), hafnium (Hf), lanthanum (La) and iridium (Ir), Specifically, the metal may be at least one selected from gold (Au), silver (Ag) and copper (Cu). Even more specifically, it may be silver (Ag).

(25) The metal may be one selected from a metal particle having a diameter of 100 to 200 nm, a metal particle having a diameter of 1 to 4 m and a metal particle having a diameter of 5 to 10 m. More specifically, it may be a mixture of a metal particle having a diameter of 100 to 200 nm, a metal particle having a diameter of 1 to 4 m and a metal particle having a diameter of 5 to 10 m.

(26) The carbonaceous material may be specifically at least one selected from single-walled carbon nanotube, multi-walled carbon nanotube, graphene, graphite, carbon black, carbon fiber and fullerene. More specifically, the carbonaceous material may be at least one selected from single-walled carbon nanotube and multi-wall carbon nanotube. Even more specifically, the carbonaceous material may be multi-walled carbon nanotube having a diameter of 15 to 40 nm and a length of 10 to 50 m.

(27) The conductive filler may contain specifically 0.1 to 2 parts by weight, more specifically 0.15 to 1 part by weight, even more specifically 0.2 to 0.5 part by weight, of the carbonaceous material based on 100 parts by weight of the metal.

(28) The organic solvent may be specifically at least one selected from chloroform, toluene, ethanol, methanol, dichloromethane and tetrahydrofuran. More specifically, it may be chloroform. Chloroform can facilitate the dispersion of the polymer adhesive and the conductive filler and, at the same time, can facilitate the drying of the organic solvent.

(29) The conductive filler may be dispersed easily by proceeding ultrasonic grinding and stirring processes simultaneously.

(30) Then, a mixture of a conductive filler and a polymer adhesive is prepared by mixing the conductive filler dispersion with a curable polymer and a curing agent and then removing the solvent (step b).

(31) The polymer adhesive may contain specifically 10 to 120 parts by weight, more specifically 90 to 110 parts by weight, still more specifically 95 to 105 parts by weight, of the curing agent based on 100 parts by weight of the curable polymer.

(32) Further, the content of the conductive filler may be specifically 100 to 500 parts by weight, more specifically 150 to 470 parts by weight, still more specifically 200 to 420 parts by weight, based on 100 parts by weight of the polymer adhesive containing the curable polymer and the curing agent.

(33) The mixing may be specifically carried out by simultaneously proceeding stirring and degassing processes.

(34) The drying may be performed by a magnetic stirrer.

(35) Next, an adhesive mixture coating layer is formed by coating the mixture of the conductive filler and the polymer adhesive on a flexible substrate (step c).

(36) The flexible substrate may be specifically a substrate containing any one selected from a silicone-based resin, a urethane-based resin, polyimide (PI), polyethylene terephthalate (PET), polyethylene naphthalate (PEN) and polytetrafluoroethylene (PTFE). More specifically, it may be a substrate containing a silicone-based resin containing polydimethylsiloxane (PDMS).

(37) The coating may be performed by a doctor blade method, a flat screen method, a spin coating method, roll coating, flow coating, gravure printing and flexographic printing. Specifically, it may be performed by a doctor blade method.

(38) The thickness of the adhesive mixture coating layer may be specifically 10 to 200 m, more specifically 50 to 150 m, even more specifically 90 to 110 m. When the thickness of the adhesive mixture coating layer is less than 10 m, the electrical conductivity of the finally formed conductive polymer adhesive film may be not sufficient. And, when it exceeds 200 m, processing time may increase during subsequent drying or curing treatment and the finally prepared conductive polymer adhesive film may have reduced flexibility.

(39) Finally, a conductive polymer composite is filmed on a flexible substrate by curing the adhesive mixture coating layer (step d).

(40) The curing treatment may be conducted specifically for 60 to 120 minutes at 150 to 200 C., more specifically for 80 to 100 minutes at 160 to 190 C.

(41) Specifically, the curing treatment may be performed after the adhesive mixture coating layer is formed on the flexible substrate and then dried. Accordingly, the finally prepared conductive polymer composite may have uniform electrical properties and improved adhesivity.

(42) The present disclosure provides a flexible electronic device containing the conductive polymer composite for adhesion to a flexible substrate.

(43) The flexible electronic device may be a flexible display, a flexible transistor, a flexible sensor, a solar cell, etc. However, the scope of the present disclosure is not limited thereto and it may be applied to a variety of electronic devices that require flexibility and stretchability.

(44) In particular, although not explicitly described in the following examples, the conductive polymer composite for adhesion to a flexible substrate according to the present disclosure was prepared by varying the kind and form of the metal and the carbonaceous material, the compositional ratio of the metal and the carbonaceous material and the kind of the organic solvent in the step (a), the compositional ratio of the conductive filler and the polymer adhesive in the step (b), the kind of the substrate, the method of coating the mixture of the conductive filler and the polymer adhesive and the thickness of the coated adhesive mixture coating layer in the step (c), and the curing temperature and time conditions in the step (d).

(45) The physical properties of the prepared conductive polymer composite for adhesion to a flexible substrate were confirmed by performing tests related to specific resistance, adhesive strength to a substrate and stretchability.

(46) As a result, only when all of the following conditions were satisfied, unlike the other conditions and other numerical ranges, the adhesion of the conductive polymer adhesive film to the substrate was superior and improved electrical conductivity together with superior flexibility and stretchability was achieved, such that remarkable electrical properties and mechanical properties could be exerted when applied to a variety of flexible electronic devices.

(47) In the step (a), the metal is a of mixture of (Ag) particles having a diameter of 100 to 200 nm, silver (Ag) particles having a diameter of 1 to 4 m and silver (Ag) particles having a diameter of 5 to 10 m, the carbonaceous material is multi-walled carbon nanotube having a diameter of 15 to 40 nm and a length of 10 to 50 m, and 0.1 to 2 parts by weight of the carbonaceous material based on 100 parts by weight of the metal is dispersed in chloroform, as one of a variety of organic solvents. In the step (b), 10 to 120 parts by weight of the curing agent is mixed based on 100 parts by weight of the curable polymer, 100 parts to 500 parts by weight of the conductive filler is mixed based on 100 parts by weight of the polymer adhesive containing the curable polymer and the curing agent, and the mixing is carried out by proceeding simultaneously stirring and degassing processes. In the step (c), the substrate is necessarily a flexible substrate, and the coating is performed by a doctor blade method to form the adhesive mixture coating layer with a thickness of 10 to 200 m. In the step (d), the curing treatment is performed for 60 to 120 minutes at 150 to 200 C.

(48) Hereinafter, the present disclosure is explained in more detail through examples, etc. In the followings, however, the examples, etc. should not be interpreted to reduce or limit the scope and content of the present disclosure. In addition, it is obvious that those of ordinary skill can easily carry out the invention based on the present disclosure including the following examples.

EXAMPLES

Example 1

(49) (1) Preparation of mixture of polymer adhesive and silver (Ag)/multi-walled carbon nanotube

(50) Silver (Ag)/multi-walled carbon nanotube (1.86/0.01 g) dispersed in chloroform was mixed with a transparent two-component curable polymer adhesive consisting of 500 mg of a base resin and 500 mg of a curing agent at a ratio of 1:1. After completion of the reaction, a mixture of the polymer adhesive and the silver/multi-walled carbon nanotube was prepared by removing the chloroform used in the dispersion of the filler at room temperature (25 C.). As the silver (Ag), a mixture of silver (Ag) particles having diameters of 100 to 200 nm, 1 to 4 m and 5 to 10 m, mixed at a weight ratio of 1:1:1 was used. As the multi-walled carbon nanotube, one having an average diameter of 30 nm and an average length of 30 m was used. The dispersion of the silver (Ag)/multi-walled carbon nanotube was carried out using an ultrasonic grinding device, and the mixing of the silver (Ag)/multi-walled carbon nanotube dispersed in chloroform and the matrix of the polymer adhesive was performed using a high-viscosity mixing/degassing device. The stirring and degassing processes were conducted at the same time.

(51) (2) Formation of conductive polymer composite film on polydimethylsiloxane (PDMS) substrate

(52) A film with a thickness of about 100 m was prepared on a polydimethylsiloxane (PDMS) substrate with a thickness of 1 mm by coating the composite of the polymer adhesive and the silver (Ag)/multi-walled carbon nanotube on the polydimethylsiloxane (PDMS) substrate. The prepared film was cured at 180 C. for 90 minutes.

Example 2

(53) A conductive polymer adhesive film was prepared in the same manner as in Example 1, except that 2.33 g, instead of 1.86 g, of silver (Ag) was used.

Example 3

(54) A conductive polymer adhesive film was prepared in the same manner as in Example 1, except that 3 g, instead of 1.86 g, of silver (Ag) was used.

Example 4

(55) A conductive polymer adhesive film was prepared in the same manner as in Example 1, except that 4 g, instead of 1.86 g, of silver (Ag) was used.

Example 5

(56) A conductive polymer adhesive film was prepared in the same manner as in Example 4, except that a polyimide (PI) film was used instead of the polydimethylsiloxane (PDMS) substrate.

Example 6

(57) A conductive polymer adhesive film was prepared in the same manner as in Example 4, except that a polyethylene naphthalate (PEN) film was used instead of the polydimethylsiloxane (PDMS) substrate.

Comparative Example 1

(58) (1) Preparation of mixture of polymer adhesive and silver (Ag)

(59) 1.86 g of silver (Ag) dispersed in chloroform was mixed with a transparent two-component thermosetting polymer adhesive consisting of 500 mg of a base resin and 500 mg of a curing agent at a ratio of 1:1. After completion of the reaction, a mixture of the polymer adhesive and silver (Ag) was prepared by removing the solvent used in the dispersion of the filler at room temperature (25 C.).

(60) (2) Preparation of polymer adhesive-silver composite film on polydimethylsiloxane (PDMS) substrate

(61) A film with a thickness of about 100 m was prepared on a polydimethylsiloxane (PDMS) substrate with a thickness of 1 mm by coating the composite of the polymer adhesive and silver on the polydimethylsiloxane (PDMS) substrate. The prepared film was cured at 180 C. for 90 minutes.

Comparative Example 2

(62) A conductive polymer adhesive film was prepared in the same manner as in Comparative Example 1, except that 2.33 g, instead of 1.86 g, of silver (Ag) was used.

Comparative Example 3

(63) A conductive polymer adhesive film was prepared in the same manner as in Comparative Example 1, except that 3 g, instead of 1.86 g, of silver (Ag) was used.

Comparative Example 4

(64) A conductive polymer adhesive film was prepared in the same manner as in Comparative Example 1, except that 4 g, instead of 1.86 g, of silver (Ag) was used.

Comparative Example 5

(65) (1) Preparation of conductive epoxy adhesive mixture

(66) A commercially available two-component thermosetting conductive epoxy adhesive (ELCOAT A-200) consisting of a base resin and a curing agent at a ratio of 3:2, were mixed under stirring by proceeding stirring and degassing processes at the same time using a high-viscosity mixing/degassing device.

(67) (2) Preparation of conductive epoxy adhesive film on polydimethylsiloxane (PDMS) substrate

(68) A film with a thickness of about 100 m was prepared on a polydimethylsiloxane (PDMS) substrate with a thickness of 1 mm by coating the conductive epoxy adhesive mixture on the polydimethylsiloxane (PDMS) substrate. The prepared film was cured at 150 C. for 20 minutes.

Comparative Example 6

(69) A conductive polymer adhesive film was prepared in the same manner as in Comparative Example 5, except that a polyimide (PI) film was used instead of the polydimethylsiloxane (PDMS) substrate.

Comparative Example 7

(70) A conductive polymer adhesive film was prepared in the same manner as in Comparative Example 5, except that a polyethylene naphthalate (PEN) film was used instead of the polydimethylsiloxane (PDMS) substrate.

(71) The components of the conductive polymer composites of Examples 1-6 and Comparative Examples 1-7 are compared and summarized in Table 1 below.

(72) TABLE-US-00001 TABLE 1 Conductive polymer adhesive Base Curing Base Curing resin agent Ag MWNT resin agent MWN (parts by (parts by (parts by (parts by Substrate (mg) (mg) Ag (g) T (g) weight) weight) weight) weight) Example 1 PDMS 500 500 1.86 0.01 50 50 186 1 Example 2 PDMS 500 500 2.33 0.01 50 50 233 1 Example 3 PDMS 500 500 3 0.01 50 50 300 1 Example 4 PDMS 500 500 4 0.01 50 50 400 1 Example 5 PI 500 500 4 0.01 50 50 400 1 Example 6 PEN 500 500 4 0.01 50 50 400 1 Comp. Ex. 1 PDMS 500 500 1.86 0 50 50 186 0 Comp. Ex. 2 PDMS 500 500 2.33 0 50 50 233 0 Comp. Ex. 3 PDMS 500 500 3 0 50 50 300 0 Comp. Ex. 4 PDMS 500 500 4 0 50 50 400 0 Comp. Ex. 5 PDMS ELCOAT A-200 Comp. Ex. 6 PI ELCOAT A-200 Comp. Ex. 7 PEN ELCOAT A-200

TEST EXAMPLES

Test Example 1: Measurement of Specific Resistance

(73) The specific resistance of the conductive polymer composites attached onto the polydimethylsiloxane (PDMS) substrate according to Examples 1-4 and Comparative Examples 1-5 was measured using a surface resistance meter (Napson, CRESBOX). The result is shown in Table 2 below.

(74) TABLE-US-00002 TABLE 2 Polymer Multi-walled adhesive Silver (Ag) carbon nano- Specific content content tube content resistance Untreated 100 parts 0 part 0 part group by weight by weight by weight Example 1 100 parts 186 parts 1 part by weight by weight by weight Comp. Ex. 1 100 parts 186 parts 0 part by weight by weight by weight Example 2 100 parts 233 parts 1 part 1.14 10.sup.3 by weight by weight by weight Comp. Ex. 2 100 parts 233 parts 0 part 5500 by weight by weight by weight Example 3 100 parts 300 parts 1 part 1.55 10.sup.4 by weight by weight by weight Comp. Ex. 3 100 parts 300 parts 0 part 5.4 10.sup.3 by weight by weight by weight Example 4 100 parts 400 parts 1 part 1.22 10.sup.4 by weight by weight by weight Comp. Ex. 4 100 parts 400 parts 0 part 9.8 10.sup.3 by weight by weight by weight Comp. Ex. 5 ELCOAT A-200 3.4 10.sup.3

(75) It can be seen that the conductive polymer composites of Examples 1-4, which contain 1 part by weight of multi-walled carbon nanotube, exhibit significantly lower specific resistance values as compared to those prepared under the same conditions except for the carbon nanotube (Comparative Examples 1-4).

Test Example 2: Measurement of Adhesive Strength

(76) The adhesive strength of the conductive polymer composites prepared in Examples 4-6 and Comparative Examples 5-7 was tested using a Scotch tape (Cat. #583D, 3M). The result is shown in FIG. 2.

(77) It can be seen that the conductive polymer composite of Example 4 using a silicone-based resin exhibits remarkably superior adhesive strength as compared to the epoxy resin-based conductive adhesive of Comparative Example 5.

Test Example 3: Stretch Test

(78) The conductive polymer composite of Example 3 prepared on the polydimethylsiloxane (PDMS) substrate was connected to a multimeter (6512, Keithley) and change in resistance was measured while repeatedly stretching by 30% of the initial length. The result is shown in FIG. 3.

(79) It can be seen that the adhesive prepared using silver and multi-walled carbon nanotube and then cured on the polydimethylsiloxane (PDMS) substrate is stable without change in electrical properties even after being stretched.

(80) Although the exemplary embodiments of the present disclosure have been described, those of ordinary skill in the art will be able to make various modifications and variations to the present disclosure through addition, changing, deletion, etc. within a range not departing from the spirit of the disclosure defined in the claims, and such modifications and variations are also included within the scope of the present disclosure.