Elastic microfiber for strain sensor, and manufacturing method therefor

Abstract

The present invention relates to an elastic microfiber for a strain sensor, and a manufacturing method therefor. According to exemplary embodiments of the present invention, a wearable sensor can be provided, the sensor being capable of resolving a mechanical hysteresis problem, which is a limitation of a conventional natural rubber-based strain sensor, and performing stable and reliable motion detection.

Description

Elastic microfibers for strain sensors and methods for manufacturing the same

[0001] The present specification discloses elastic microfibers for strain sensors and a method for manufacturing the same.

[0002] Cross-reference to related applications: This application claims priority to Korean Patent Application No. 10-2025-0022156, filed on February 20, 2025.

[0003] The present invention was carried out under the support of the Ministry of Science and ICT of the Republic of Korea under project number 2710011956 and project number 2022R1A2C200825613, the managing agency for the said project is the National Research Foundation of Korea, the research project name is “Individual Basic Research (MSIT)”, the research project name is “Ductural material with variable mechanical properties through control of intermolecular interactions”, and the research period is 2024.03.01 ~ 2025.02.28.

[0004] Generally, a strain sensor refers to a sensor that converts physical deformation, such as stretching or compression, into an electrical signal. Recently, flexible strain sensors have been widely applied in multifunctional wearable devices used for purposes such as detecting body movements and recording physical activity information. Given the high demand for wearable devices, extensive efforts have been invested in the development of efficient flexible strain sensors due to their resistance to deformations such as bending, twisting, and elongation. Numerous capacitive or resistive strain sensors have been developed to detect a subject's physiological state by recording resistance changes induced by joint movements or skin vibrations. While capacitive sensors offer excellent linearity but relatively low stretchability, resistive sensors enable simple signal detection at low cost due to their fast response and simple device structure. Generally, the two types of sensors used in flexible strain sensors are capacitive and resistive sensors; the capacitive sensor offers excellent linearity but relatively low stretchability, whereas the resistive sensor has the advantage of enabling simple signal detection at low cost due to its fast response and simple device structure.

[0005] The above resistive soft strain sensor is generally composed of a soft matrix and conductive particles that exhibit a change in resistance upon stretching or deformation induced by a change in the conduction path of a percolation network. The soft matrix generally utilizes various material systems such as hydrogels and elastomers. Among these, elastomers are receiving significant attention because they are easy to manufacture, allow for modulus adjustment, and do not cause evaporation or leakage of the medium. As the conductive particles, various conductive nanomaterials are generally used, such as carbon black (CB), multi-walled carbon nanotubes (CNT), silver nanowires (AgNW), graphene, and combinations thereof.

[0006] To create an efficient percolation network, conductive particles with high aspect ratios, such as CNTs and AgNWs, are widely used. The high aspect ratio of these conductive particles helps to effectively form continuous and uniform electronic paths within the elastomer of the sensor. However, when the strain sensor is stretched beyond the percolation threshold of the conductive particles, it causes a sharp increase in the sensor's resistance, which narrows the operating range and the linearity of the resistance change. To overcome these drawbacks of CNT and AgNW-based strain sensors, attempts are being made to improve network connectivity by using isolated structures or mixing them with other conductive particles. To this end, one-dimensional carbon nanotubes with a high aspect ratio were combined with two-dimensional graphene having an easily discontinuous network. This combination simultaneously improved stretchability and gauge ratio, thereby overcoming the trade-off between these two characteristics. In addition, it has been reported that hybrid conductive particles containing CNTs and CBs induce a wide strain sensing range through the movement of CB nanoparticles within the gaps of CNT disconnections to restore the percolation network of the sensor. Another report indicates that the sensitivity of the strain sensor is improved by the CNT / CB hybrid because the presence of CBs reduces the entanglement of the CNT conductive network structure, allowing it to be easily disconnected upon stretching.

[0007] As described above, while the performance of soft strain sensors has been improved to some extent by the aforementioned mixed conductive particles, a method for manufacturing high-performance soft strain sensors in large quantities—which exhibit a wide operating range, high linearity, fast response time, and long-term stability through a simple and easy process compared to conventional technologies—has not yet been implemented. Consequently, there is a need for a simple method to manufacture large-scale strain sensors.

[0008] [Prior Art Literature]

[0009] [Patent Literature]

[0010] Republic of Korea Registered Patent No. 10-2582412

[0011] In exemplary embodiments of the present invention, we aim to provide an elastic microfiber for a strain sensor that significantly improves mechanical hysteresis and realizes excellent electrical properties, and a method for manufacturing the same.

[0012] In one aspect, exemplary embodiments of the present invention provide an elastic microfiber for a strain sensor comprising cellulose nanofibers, an elastomer, conductive particles, and a conductive polymer.

[0013] In another aspect, exemplary embodiments of the present invention provide a method for manufacturing elastic microfibers for a strain sensor, comprising: a first solution comprising cellulose nanofibers and an elastomer; and a second solution comprising conductive particles and a conductive polymer, by mixing to obtain an elastomer solution; injecting the elastomer solution into an inner tube of an elastic microfiber manufacturing device and injecting a coagulant solution into an outer tube of the elastic microfiber manufacturing device to continuously coagulate the elastomer solution; and collecting elastic microfibers from an outlet of the elastic microfiber manufacturing device.

[0014] According to exemplary embodiments of the present invention, the mechanical hysteresis problem, which is a limitation of existing natural rubber-based deformation sensors, can be solved, and a wearable sensor capable of stable and reliable motion detection can be provided.

[0015] FIG. 1 is a schematic diagram showing the form of an apparatus for manufacturing elastic microfibers according to one embodiment of the present invention.

[0016] FIG. 2 is a schematic diagram showing the mechanism of a process for manufacturing elastic microfibers according to one embodiment of the present invention.

[0017] FIG. 3 is a graph showing the tensile stress and strain curves of an elastic microfiber according to one embodiment of the present invention.

[0018] Figure 4 is a graph showing the results of the analysis of the elastic modulus of elastic microfibers according to one embodiment of the present invention.

[0019] Figure 5 is a graph showing the mechanical hysteresis evaluation results of elastic microfibers according to one embodiment of the present invention.

[0020] FIG. 6a is a graph showing the results of the toughness evaluation of elastic microfibers according to one embodiment of the present invention.

[0021] FIG. 6b is a graph showing the results of evaluating the hysteresis loss of elastic microfibers according to one embodiment of the invention.

[0022] Figure 7 is a graph showing the X-ray diffraction (XRD) analysis results of an elastic microfiber according to one embodiment of the present invention.

[0023] FIGS. 8a to 8c are scanning electron microscope images showing the tensile fracture surface of an elastic microfiber according to one embodiment of the present invention and schematic diagrams showing the interaction mechanism between a cellulose nanofiber (CNF) filler and a natural rubber (NR) matrix.

[0024] FIG. 9 is a schematic diagram showing the manufacturing process of a strain sensor according to one embodiment of the present invention.

[0025] FIG. 10 is a graph showing the initial resistance measurement results of a strain sensor according to one embodiment of the present invention.

[0026] FIG. 11 is a graph showing the resistance measurement results according to the strain of a strain sensor according to one embodiment of the present invention.

[0027] FIG. 12 is a graph showing the change in relative resistance measured during the stretching-relaxation cycle of a strain sensor according to one embodiment of the present invention.

[0028] FIGS. 13a and FIGS. 13b are graphs showing the response time measurement results of a strain sensor according to one embodiment of the present invention.

[0029] FIGS. 14a and FIGS. 14b are graphs showing the mechanical hysteresis evaluation results of a strain sensor according to one embodiment of the present invention.

[0030] FIG. 15 is a graph showing the long-term stability evaluation results of a strain sensor according to one embodiment of the present invention.

[0031] FIG. 16 is an image showing a strain sensor according to one embodiment of the present invention attached to an index finger and a graph showing the results of measuring the change in relative resistance according to the operation.

[0032] FIG. 17 is an image showing a deformation sensor according to one embodiment of the present invention attached to the neck and a graph showing the results of measuring the change in relative resistance according to the operation.

[0033] FIG. 18a is an image showing a strain sensor according to one embodiment of the present invention attached to a wrist, and FIG. 18b is a graph showing the measurement results of the change in relative resistance according to operation.

[0034] FIG. 19a is an image showing a deformation sensor according to an embodiment of the present invention attached to an elbow, and FIG. 19b is a graph showing the results of measuring the change in relative resistance according to operation.

[0035] FIG. 20a is an image showing a deformation sensor according to one embodiment of the present invention attached to a knee, and FIG. 20b is a graph showing the results of measuring the change in relative resistance according to operation.

[0036] FIG. 21a is an image showing a deformation sensor according to an embodiment of the present invention attached to an ankle, and FIG. 21b is a graph showing the results of measuring the change in relative resistance according to operation.

[0037] FIG. 22 is an image showing a deformation sensor according to one embodiment of the present invention sewn onto a fabric knee pad.

[0038] FIG. 23 is a graph showing the measurement results of the change in relative resistance according to various movements of a strain sensor according to one embodiment of the present invention.

[0039] FIG. 24 is a schematic diagram showing an integrated system for wireless signal collection of a strain sensor according to one embodiment of the present invention.

[0040] FIG. 25 is a schematic diagram showing a circuit diagram of an integrated system for wireless signal collection of a strain sensor according to one embodiment of the present invention.

[0041] FIG. 26 is an image showing the collection of index finger movement signals through a deformation sensor according to one embodiment of the present invention and a graph showing the wireless signal measurement results according to the operation.

[0042] FIG. 27 is an image showing the collection of knee movement signals through a deformation sensor according to an embodiment of the present invention and a graph showing the wireless signal measurement results according to the operation.

[0043] FIG. 28 is a graph showing the results of wireless signal measurement according to walking motion through a deformation sensor according to one embodiment of the present invention.

[0044] FIG. 29 is a graph showing the results of wireless signal measurement according to the movement of going up and down stairs through a deformation sensor according to one embodiment of the present invention.

[0045] The terms used in this specification have been selected based on currently widely used general terms whenever possible, taking into account their functions in the present invention; however, these terms may vary depending on the intent of those skilled in the art, case law, or the emergence of new technologies. Additionally, in specific cases, terms have been arbitrarily selected by the applicant, and in such cases, their meanings will be described in detail in the relevant description of the invention. Therefore, terms used in this specification should be defined not merely by their names, but based on their meanings and the overall content of the present invention.

[0046] Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as generally understood by those skilled in the art to which this invention pertains. Terms generally understood should be interpreted as having the same meaning as they have in the context of the relevant technology, and should not be interpreted in an ideal or overly formal sense unless explicitly defined in this invention.

[0047] Numerical ranges include the numerical values ​​defined in the present invention. All maximum numerical limits given throughout this specification include all lower numerical limits as clearly written. All minimum numerical limits given throughout this specification include all higher numerical limits as clearly written. All numerical limits given throughout this specification will include all better numerical ranges within a wider numerical range, as clearly written.

[0048] As used herein, the words “comprising,” “having,” and “containing” are inclusive or open and do not exclude additional unmentioned elements or method steps. The term “or combinations thereof” as used herein refers to any permutation and combination of the items listed prior to the said term. For example, “A, B, C, or combinations thereof” is intended to include at least one of A, B, C, AB, AC, BC, or ABC, and, where the order is important in a particular context, BA, CA, CB, CBA, BCA, ACB, BAC, or CAB. Along with this example, combinations containing repetitions of one or more items or terms, such as BB, AAA, MB, BBC, AAABCCCC, CBBAAA, CABABB, etc. A person skilled in the art will understand that, typically, there is no limit to the number of items or terms in any combination unless otherwise obvious from the context.

[0049] Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. However, it is obvious that the present invention is not limited by the following embodiments.

[0050] terminology

[0051] A “deformation sensor” refers to a sensor that converts physical deformation, such as stretching or compression, into an electrical signal.

[0052] Elastic microfibers for strain sensors and sensors including the same

[0053] In one aspect, exemplary embodiments of the present invention provide an elastic microfiber for a strain sensor comprising cellulose nanofibers, an elastomer, conductive particles, and a conductive polymer.

[0054] The complex interaction between an efficient conductive network and a mechanically durable elastic matrix is ​​the core of piezoresistive soft strain sensors. These strain sensors consist of an elastic matrix infused with piezoresistive material, and the function of the strain sensor depends on the ability of conductive paths to reconfigure under mechanical deformation, resulting in a proportional resistance change. When the applied force is removed, the elastic matrix returns to its original dimensions and shape; within this elastic range, the internal structure of the piezoresistive material returns to its initial configuration, recovering the original resistance. An ideal strain sensor with reliable performance must possess high sensitivity, a wide operating range, and excellent mechanical durability while maintaining low electrical hysteresis under dynamic conditions.

[0055] Accordingly, the inventors completed the present invention by discovering that the viscoelasticity of an elastomer can be improved by introducing cellulose nanofibers (CNF) as a reinforcing material, and that conductive elastomer microfibers can be reinforced using cellulose nanofibers (CNF). To solve the limitations of conductive elastomers that increase the initial resistance of a strain sensor during repeated stretching cycles, the inventors incorporated cellulose nanofibers as a reinforcing material to form molecular bridges within an elastomer matrix containing conductive particles and a conductive polymer. As a result, the dynamic stability of an elastomer-based strain sensor was significantly improved.

[0056] The conductive particles and conductive polymers described above act as conductive fillers. When the conductive particles and conductive polymers are included together, they form an efficient percolation network within the elastomer matrix, which can significantly improve the conductivity, linearity, and hysteresis of the strain sensor. The conductive particles and conductive polymers serve as a percolation network for the elastomer-based strain sensor.

[0057] According to one embodiment of the present invention, the elastomer is natural rubber.

[0058] The above cellulose nanofibers are included in an amount of 1 to 10 parts by weight based on 100 parts by weight of the above elastomer. If the content of the above cellulose nanofibers exceeds the above upper limit, the dispersion of the cellulose nanofibers is poor, so the sample cannot be easily prepared.

[0059] According to one embodiment of the present invention, the elastomer is included in an amount of 40 to 80 weight percent based on the total weight of the elastic microfibers.

[0060] According to one embodiment of the present invention, the conductive particles are one or more selected from the group consisting of carbon black (CB), single-walled or multi-walled carbon nanotubes (CNT), metal nanowires (AgNW), metal nanoparticles (Nanoparticle), MXene, graphene, and graphene oxide.

[0061] According to one embodiment of the present invention, the conductive particles are included in an amount of 0.1 to 2.0 weight% based on the total weight of the elastic microfibers.

[0062] According to one embodiment of the present invention, the conductive polymer is one or more selected from the group consisting of polypyrrole, polythiophene, polyaniline, polyacetylene, polyphenylene sulfide, polyethylenedioxythiophene, polyphenylene vinylene, and PEDOT:PSS (Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)).

[0063] According to one embodiment of the present invention, the conductive polymer is included in an amount of 0.1 to 2.0 weight% based on the total weight of the elastic microfibers.

[0064] According to one embodiment of the present invention, the coagulant is one or more selected from the group consisting of calcium chloride and acetic acid solution.

[0065] In one aspect, exemplary embodiments of the present invention provide a strain sensor comprising an elastic microfiber for the strain sensor.

[0066] According to one embodiment of the present invention, the resistive flexible strain sensor may include a microfiber containing a flexible matrix and conductive particles.

[0067] According to one embodiment of the present invention, the flexible matrix can change the conduction path of the percolation network through stretching or deformation. As the flexible matrix, an elastomer, preferably a hydrogel and an elastomer, may be used; more preferably, an elastomer such as a silicone-based polymer (Ecoflex), polydimethylsiloxane (PDMS), and natural rubber (NR) may be used; and most preferably, natural rubber may be used. The elastomer has the advantages of being easy to manufacture, having adjustable modulus, and having no evaporation or leakage of the medium. While microfibers made of the silicone-based polymer and polydimethylsiloxane each had limitations in elasticity and processability, the excellent mechanical properties and wide operating range of the elastomer, particularly natural rubber, led to the development of strain sensors with superior characteristics. The excellent mechanical properties of natural rubber make it possible to manufacture microfibers and sew them onto fabrics for widespread application in smart wearable sensors. In addition, natural rubber has the advantage of being a low-cost, biocompatible green polymer.

[0068] According to one embodiment of the present invention, the cellulose nanofibers of the strain sensor promote the formation of a conductive network to improve electrical conductivity and maintain stable performance even under repeated deformation. In particular, by integrating an A / D converter that converts analog signals to digital for wireless communication and an MCU capable of Wi-Fi communication, sensor data can be transmitted and visualized in real time to an Android-based mobile app. Through the smartphone app, users can monitor and save operation data in real time, thereby enabling long-term operation analysis.

[0069] Method and apparatus for manufacturing elastic microfibers for strain sensors

[0070] In one aspect, exemplary embodiments of the present invention provide a method for manufacturing elastic microfibers for a strain sensor, comprising: a first solution comprising cellulose nanofibers and an elastomer; and a second solution comprising conductive particles and a conductive polymer, by mixing to obtain an elastomer solution; injecting the elastomer solution into an inner tube of an elastic microfiber manufacturing device and injecting a coagulant solution into an outer tube of the elastic microfiber manufacturing device to continuously coagulate the elastomer solution; and collecting elastic microfibers from an outlet of the elastic microfiber manufacturing device.

[0071] According to one embodiment of the present invention, the elastic microfiber manufacturing device comprises: an outer tube; and an inner tube formed within the outer tube, formed to be parallel and coaxial within the outer tube, and having a smaller diameter than the outer tube.

[0072] According to one embodiment of the present invention, the inner capillary comprises glass, and the outer capillary comprises Teflon.

[0073] In the first method for manufacturing the elastic microfibers described above, an elastomer solution injected into the inner capillary of the elastic microfiber manufacturing device is brought into contact with a coagulant solution injected into the outer capillary of the device, so that the elastomer solution is continuously coagulated by the outer coagulant solution through flow to form a thin, long, solid microfiber. The diameter of the solid elastic microfibers can be achieved by controlling the diameter of the inner capillary, wherein the inner capillary has a diameter of 0.8 mm to 1.5 mm, preferably 1.1 mm, and the outer capillary has a diameter of 1.6 mm to 3.0 mm, preferably 2.2 mm. At this time, the elastic microfiber formed through the inner capillary having a diameter of 1.1 mm may have a diameter of 830 µm due to the evaporation of moisture.

[0074] In addition, to control the diameter of the elastic microfiber, the injection rate of the elastomer solution into the inner capillary is 0.5 mL / min to 1.5 mL / min, preferably 1.0 mL / min, and the injection rate of the coagulant solution into the outer capillary is characterized by being a rate proportional to the diameter of the outer capillary and the diameter of the inner capillary. Preferably, when the outer capillary is 2.2 mm, since it has twice the diameter of the inner capillary, the coagulant solution can be injected at a faster rate into the outer capillary having a larger diameter in order to match the injection rate of the elastomer solution and the injection rate of the coagulant solution. At this time, the injection rate of the coagulant solution into the outer capillary may preferably be 2.0 mL / min, which is twice the injection rate of the elastomer solution into the inner capillary, which is 1.0 mL / min.

[0075] Preferably, when the internal elastomer solution comes into contact with the coagulant of the external capillary, the coagulation of the elastomer solution begins, and because fluid flow is present, the components of the coagulated elastomer solution can be obtained in the form of microfibers at the outlet of the microfluidic device. At this time, a container for collecting the microfibers may be located at the end of the microfluidic device, and subsequently, the collected microfibers can be dried.

[0076] In the second method for manufacturing the elastic microfibers described above, a device having the same structure as above is used, but unlike the first method, an elastomer solution injected into the outer capillary of the device and a coagulant solution injected into the inner capillary of the device come into contact, so that the elastomer solution is continuously coagulated by the inner coagulant solution through flow to form thin, long microfibers in the form of hollow fibers.

[0077] The diameter of the above-mentioned hollow fiber-shaped microfiber can be achieved by controlling the diameter of the above-mentioned outer capillary, wherein the above-mentioned outer capillary is characterized by having a diameter of 1.6 mm to 3.0 mm, preferably 2.2 mm. The diameter of the central empty region of the above-mentioned microfiber can be achieved by controlling the diameter of the above-mentioned inner capillary, wherein the above-mentioned inner capillary is characterized by having a diameter of 0.8 mm to 1.5 mm, preferably 1.1 mm.

[0078] In addition, to control the hollow diameter inside the microfiber, the injection rate of the coagulant solution into the inner capillary is 0.5 mL / min to 1.5 mL / min, preferably 1.0 mL / min, and the injection rate of the elastomer solution into the outer capillary is characterized by being a rate proportional to the diameter of the outer capillary and the diameter of the inner capillary. Preferably, when the outer capillary is 2.2 mm, since it has twice the diameter of the inner capillary, the elastomer solution can be injected at a faster rate into the outer capillary having a larger diameter in order to match the injection rate of the elastomer solution and the injection rate of the coagulant solution. At this time, the injection rate of the elastomer solution into the outer capillary may preferably be 2.0 mL / min, which is twice the injection rate of the coagulant solution into the inner capillary, which is 1.0 mL / min.

[0079] Preferably, when the elastomer solution of the outer capillary comes into contact with the coagulant of the inner capillary, coagulation of the components of the hollow fiber elastomer solution begins, except for the central portion where the coagulant flows, and because fluid flow is present, the coagulated components of the elastomer solution can be obtained at the outlet of the microfluidic device in the form of hollow fibers from which the coagulant solution has been removed. At this time, a container for collecting the microfibers may be positioned at the end of the microfluidic device, and subsequently, the collected microfibers can be dried.

[0080] In one embodiment, the coagulant is one or more selected from the group consisting of cationic solutions, preferably calcium chloride and acetic acid solutions. Preferably, the coagulant is a calcium chloride and acetic acid solution. Preferably, the coagulant comprises these in the form of an aqueous solution.

[0081] Additionally, the strain sensor can be easily sewn into fabric due to its mechanical properties and can be applied within smart clothing. The strain sensor can detect various human movements, such as active knee and elbow movements, fine pulsations, and vocalizations, with excellent sensitivity and can be applied to high-performance wearable sensors for monitoring human movements and physiological conditions.

[0082] The present invention aims to obtain a strain sensor combining high sensitivity, a wide operating range, and low hysteresis by synergistically combining the excellent electrical properties of conductive particles and conductive polymers with the mechanical reinforcement provided by cellulose nanofibers. By solving mechanical hysteresis, which is a critical challenge for elastomer-based strain sensors, the present invention provides a wearable strain sensor capable of reliable wireless monitoring in various applications.

[0083] {Example}

[0084] The present invention will be explained in detail below through examples. However, the following examples are merely illustrative to aid in the overall understanding of the present invention, and the content of the present invention is not limited to the following examples.

[0085] <Reference Example 1> Materials

[0086] Natural rubber (NR), an elastomer, and an aqueous solution of acetic acid (CH3COOH, purity > 99%, Mw approx. 60.05 g / mol), a coagulant component, were purchased from Duksan Chemical (Gyeonggi-do, Korea). PEDOT:PSS, a conductive polymer, and calcium chloride (CaCl2, purity ≥ 93.0%), a coagulant component, were purchased from Sigma Aldrich (St. Louis, Missouri, USA). Multiwalled carbon nanotubes (CNT; purity > 95%, outer diameter 20 to 30 nm, length 10 to 30 μm), conductive particles, were purchased from US Research Nanomaterials, Inc. (Houston, Texas, USA). Cellulose nanofibers (CNF), a reinforcing material (diameter 10 to 20 nm, length 2 to 3 μm), were purchased from Nanografi (Ankara, Turkey). These substances were used exactly as received, without further purification.

[0087] <Preparation Example 1> Preparation of elastic microfibers

[0088] 1. Preparation of elastomer solution and coagulant solution

[0089] An optimized amount of aqueous multi-walled carbon nanotube (CNT) dispersion (15 wt%, 0.16 ml) was mixed with an aqueous PEDOT:PSS solution (2 wt%, 0.03 g) under continuous stirring (60 rpm) for 24 hours to form a CNT / PEDOT:PSS conductive filler. Cellulose nanofibers (CNF) were mixed with 2.4 ml of an aqueous natural rubber (NR) solution (60 wt%) and stirred at 60 rpm for 12 hours to form a water-soluble mixture of natural rubber (NR) and cellulose nanofibers (CNF) (NR / CNF).

[0090] 0.46 wt% carbon nanotubes (CNT) and 0.5 wt% PEDOT:PSS were fixedly loaded into a natural rubber (NR) solution, while the composition of the cellulose nanofibers (CNF) was varied based on 100 parts by weight of the natural rubber (NR), denoted as NRCx (x = 1, 2, 3, 4, and 5).

[0091] Next, the CNT / PEDOT:PSS and NR / CNF solutions and deionized water were uniformly mixed under magnetic stirring to prepare a mixture with a total volume of 6 ml, thereby obtaining an elastomer solution. A water-soluble coagulant solution was prepared by dissolving 1 g of calcium chloride and 6 ml of acetic acid in 14 ml of deionized water.

[0092] 2. Preparation of an elastic microfiber manufacturing device

[0093] A custom-designed elastic microfiber manufacturing device was prepared. For device fabrication, glass tubes (Chase Scientific Glass, USA) with diameters of 1.5 mm and 5 mm, respectively, were used as the inner and outer capillaries. First, the glass tubes were washed using distilled water, ethanol, and acetone. To ensure smooth processing of the precursor solution, the washed capillaries were treated with an octadecyltrichlorosilane / toluene mixture (0.2% v / v) for 20 minutes, followed by drying at 80°C for 1 hour. This treatment modified the hydrophobicity of the capillary surfaces, enabling the fabrication of natural rubber-based microfibers with smooth surfaces. Finally, to assemble the device, the capillaries aligned along a common axis were attached to a glass substrate (76 × 25 mm) using epoxy adhesive. 2 It was attached to the capillary opening. A precursor solution was injected into the device using a Teflon tube (Sigma-Aldrich, USA).

[0094] FIG. 1 is a schematic diagram showing the form of an apparatus for manufacturing elastic microfibers according to one embodiment of the present invention.

[0095] 3. Manufacturing of elastic microfibers

[0096] The elastomer solution was injected into the inner capillary at a flow rate of 1 mL / min using a syringe pump (Legato 100, KD Scientific, USA), and the coagulant solution was injected into the outer capillary at a flow rate of 2 mL / min. Subsequently, the treated microfibers were collected in a clean Petri dish at the end of the microfluidic device and dried at room temperature for 24 hours to allow the solvent to completely evaporate, thereby producing natural rubber-based microfibers with uniform thickness and a diameter of 1.2 mm. The composition of the cellulose nanofiber (CNF) was varied based on 100 parts by weight of the natural rubber (NR) and denoted as CNT / PEDOT:PSS@NRCx (x = 1, 2, 3, 4, and 5). The prepared microfibers had a weight of 1.33 g cm⁻¹. -3 It showed a low density. It was cut to a specific length (1 cm) for physical property measurement.

[0097] <Reference Example 2> Manufacturing mechanism of elastic microfibers

[0098] FIG. 2 is a schematic diagram illustrating the mechanism of a process for manufacturing elastic microfibers according to an embodiment of the present invention. An elastomer solution and a coagulant solution injected into an elastic microfiber manufacturing device are combined at the junction of the outer tube and the inner tube, where negatively charged proteins surrounding natural rubber (NR) particles dispersed in the solution (step i) are neutralized by protons and calcium cations contained in the coagulant solution (step ii). The neutralized natural rubber (NR) particles coagulate by fusing with each other, and the polyisoprene chains trapped within the negatively charged protein membrane finally coagulate and gel (step iii).

[0099] <Experimental Example 1> Evaluation of Mechanical Properties of Elastic Microfibers

[0100] The mechanical properties of the elastic microfibers prepared in Preparation Example 1 were analyzed using a general-purpose testing machine (UTM, EZ-SM, Shimadzu, Japan). Figure 3 is a graph showing the tensile stress and strain curves of the elastic microfibers according to one embodiment of the present invention.

[0101] CNT / PEDOT:PSS@NRC“0” was found to have a maximum elongation of 1506% and a fracture stress of 1.73 MPa, while CNT / PEDOT:PSS@NRC“1” was found to have a maximum elongation of 1228% and a fracture stress of 1.90 MPa. The fracture stress and tensile modulus of CNT / PEDOT:PSS@NRCx microfibers increased as the cellulose nanofiber (CNF) content increased, whereas the maximum elongation decreased.

[0102] When 5 parts by weight of cellulose nanofibers (CNF) were added to microfibers based on 100 parts by weight of natural rubber (NR), the maximum elongation was 619% and the fracture stress was 2.49 MPa, which is a decrease of about 2.43 times compared to CNT / PEDOT:PSS@NRC“0” microfibers, but the fracture stress increased by about 1.43 times.

[0103] Since cellulose nanofibers (CNF) immobilize and reinforce the natural rubber (NR) matrix, making it stiffer, the maximum elongation decreases as the CNF content increases. The incorporation of CNF leads to the formation of physical crosslinking points, which restricts polymer chain mobility and consequently reduces the material's elasticity. Furthermore, at higher CNF content, aggregates form, further limiting the elongation.

[0104] Consequently, cellulose nanofibers (CNF) increase the strength and modulus of elasticity of the microfibers while simultaneously reducing stretchability and elasticity. This inverse relationship between stiffness and elongation is a characteristic trade-off observed in composites, where increased mechanical reinforcement leads to decreased elongation.

[0105] <Experimental Example 2> Evaluation of Elastic Modulus of Elastic Microfibers

[0106] The elastic modulus of the elastic microfiber prepared in Preparation Example 1 was analyzed from the tensile stress and strain curves. Figure 4 is a graph showing the results of the analysis of the elastic modulus of the elastic microfiber according to one embodiment of the present invention (Young's modulus).

[0107] Based on 100 parts by weight of natural rubber (NR), the elastic moduli of microfibers having compositions of 0 and 5 parts by weight of cellulose nanofibers (CNF), respectively, were measured to be 0.32 MPa and 5.76 MPa, respectively. This indicates that the elastic moduli increased 18-fold when 5 parts by weight of cellulose nanofibers (CNF) were added. In particular, the elastic moduli of the natural rubber (NR) microfibers were proportional to the composition of the cellulose nanofibers (CNF). This increase in elastic moduli according to the loading amount of the reinforcing material indicates the reinforcing effect of the cellulose nanofibers (CNF) exerted within the natural rubber (NR) matrix.

[0108] <Experimental Example 3> Evaluation of Mechanical Hysteresis of Elastic Microfibers

[0109] The mechanical hysteresis of the elastic microfibers prepared in Example 1 was evaluated using a general-purpose testing machine (UTM, EZ-SM, Shimadzu, Japan). A cyclic tensile test was performed at a speed of 50 mm / min with 100% strain without waiting time. Figure 5 is a graph showing the results of the mechanical hysteresis evaluation of elastic microfibers according to one embodiment of the present invention.

[0110] From Fig. 5, it can be seen that after the stretch-relax cycle test, the CNT / PEDOT:PSS@NRC“0” microfibers failed to fully recover their original dimensions due to hysteresis, revealing large loop regions. The observed residual strain indicates the viscoelasticity of the natural rubber (NR)-based microfibers, and natural rubber (NR) is a material characterized by viscoelasticity, which combines elastic and viscous behaviors during deformation. The phenomenon of mechanical hysteresis becomes evident when energy dissipated during loading and unloading cycles is not fully recovered. This viscoelasticity limits practical applications in detecting dynamic motion.

[0111] However, in the case of natural rubber (NR)-based microfibers reinforced with cellulose nanofibers (CNF), hysteresis was reduced, and the microfibers rapidly recovered to their initial dimensions after each stretch-relaxation cycle. This reduction in hysteresis became more pronounced as the CNF content within the microfibers increased. CNT / PEDOT:PSS@NRC“5” microfibers exhibited relatively low hysteresis after the stretch-relaxation cycle.

[0112] <Experimental Example 4> Evaluation of Toughness and Hysteresis Loss of Elastic Microfibers

[0113] The toughness of the elastic microfiber prepared in Example 1 was evaluated from the area under the mechanical hysteresis curve. In addition, the hysteresis loss was calculated by integrating the area enclosed by the loop (at 100% strain). FIG. 6a is a graph showing the results of the toughness evaluation of the elastic microfiber according to one embodiment of the present invention. FIG. 6b is a graph showing the results of the hysteresis loss evaluation of the elastic microfiber according to one embodiment of the invention.

[0114] From Fig. 6a, it can be confirmed that the toughness of the microfibers increased as the cellulose nanofiber (CNF) content within the microfibers increased. Additionally, from Fig. 6b, the contribution of cellulose nanofibers (CNF) to the viscoelastic properties of the natural rubber (NR) matrix was quantitatively analyzed. The hysteresis losses of CNT / PEDOT:PSS@NRC“0” and CNT / PEDOT:PSS@NRC“5” microfibers, having compositions of 0 and 5 parts by weight of cellulose nanofibers (CNF) respectively based on 100 parts by weight of natural rubber (NR), were 13.72 and 8.30 kJ / m, respectively. -3 It was found that the total loop area decreased due to the loading of cellulose nanofibers (CNF). This decrease in loop area indicates a reduction in energy dissipation due to the integration of the reinforcement and confirms improved elastic properties due to the interaction between the reinforcement and the natural rubber (NR) matrix.

[0115] <Experimental Example 5> Composition Analysis of Elastic Microfibers

[0116] The distribution of cellulose nanofiber (CNF) fillers within a natural rubber (NR) matrix was confirmed from X-ray diffraction (XRD) analysis using an Ultima IV diffractometer (Rigaku, Tokyo, Japan). Figure 7 is a graph showing the X-ray diffraction (XRD) analysis results of elastic microfibers according to one embodiment of the present invention.

[0117] It was confirmed that as the composition of cellulose nanofibers (CNF) increased, a peak at 22.7° associated with the cellulose I structure gradually appeared. When the composition of cellulose nanofibers (CNF) was low, the cellulose nanofibers (CNF) were well dispersed throughout the natural rubber (NR) matrix, but when the composition was high, cellulose nanofiber (CNF) aggregates began to form, resulting in a distinct cellulose nanofiber (CNF) peak.

[0118] <Experimental Example 6> Confirmation of Cellulose Nanofiber (CNF) Distribution within Elastic Microfibers

[0119] The distribution of cellulose nanofibers (CNF) within the elastic microfibers prepared in Preparation Example 1 was confirmed using a Regulus 8100 scanning electron microscope (Hitachi, Tokyo, Japan). FIGS. 8a to 8c are scanning electron microscope images showing the tensile fracture surface of elastic microfibers according to one embodiment of the present invention and schematic diagrams showing the interaction mechanism between the cellulose nanofiber (CNF) filler and the natural rubber (NR) matrix.

[0120] The formation of aggregated cellulose nanofiber (CNF) domains at high CNF compositions is clearly evident from cross-sectional SEM images of the tensile fractured surface of the microfibers. SEM images of CNT / PEDOT:PSS@NRC“1” microfibers show no aggregated CNF domains, suggesting good dispersion of the CNF filler within the natural rubber (NR) matrix. The presence of aggregated CNF domains was observed in CNT / PEDOT:PSS@NRC“3” microfibers, and the number of aggregated CNF domains increased further in CNT / PEDOT:PSS@NRC“5” microfibers. In addition, magnified cross-sectional SEM images of CNT / PEDOT:PSS@NRC“5” microfibers demonstrate the effective anchoring of natural rubber (NR) polymer chains by cellulose nanofiber (CNF) domains. Based on these observations, a mechanism for the reinforcing effect of cellulose nanofiber (CNF) fillers on a natural rubber (NR) matrix is ​​proposed. Incorporating rigid cellulose nanofibers (CNF) into a soft natural rubber (NR) matrix leads to the formation of CNF domain structures within the matrix at higher compositions. This is due to the presence of numerous hydroxyl groups (-OH) in the cellulose nanofibers (CNF). These CNF domains act as additional physical crosslinking points for the natural rubber (NR) macromolecular chains, efficiently redistributing and transferring applied external stress from the soft natural rubber (NR) matrix to the rigid CNF domains. This mechanism explains the observed increase in elastic modulus and the enhanced elastic properties of natural rubber (NR) microfibers reinforced with cellulose nanofibers (CNF).

[0121] <Manufacturing Example 2> Manufacture of a strain sensor

[0122] The elastic microfibers prepared in Preparation Example 1 were cut to a specific length (1 to 5 cm) and the two ends were twisted together with copper wire to firmly connect them. Subsequently, copper tape (electrode) was attached to the joint points of both ends to secure the connection between the microfibers and the copper wires. Next, the secured parts were encapsulated with insulating tape to protect them from damage caused by operation. FIG. 9 is a schematic diagram showing the manufacturing process of a strain sensor according to an embodiment of the present invention.

[0123] <Experimental Example 7> Evaluation of Electrical Characteristics of Strain Sensor

[0124] The electrical characteristics of the strain sensor prepared in Preparation Example 2 were evaluated using an LCR meter (4300, Wayne Kerr Electronics) at a frequency of 1 kHz. FIG. 10 is a graph showing the results of the initial resistance measurement of the strain sensor according to one embodiment of the present invention. From FIG. 10, it can be seen that the resistance of the 1 cm CNT / PEDOT:PSS@NRCx microfiber decreases as the cellulose nanofiber (CNF) content increases, reaching a minimum value of approximately 6.07 kΩ in the case of CNT / PEDOT:PSS@NRC “5”. This trend suggests that the cellulose nanofiber (CNF) plays a positive role in increasing the effective conductivity of the natural rubber (NR)-based microfiber.

[0125] The presence of cellulose nanofibers (CNF) modifies the linkage pathways within natural rubber (NR), where numerous hydroxyl groups present in the cellulose nanofibers (CNF) form hydrogen bonds between the cellulose nanofibers (CNF) and carbon nanotubes (CNT). These interactions improve the dispersion of carbon nanotubes (CNT) in the natural rubber (NR) matrix and, together with PEDOT:PSS, act as additional crosslinking agents to strengthen the linkage joints. Consequently, the effective penetration pathways increase, and the overall resistance decreases.

[0126] <Experimental Example 8> Measurement of Resistance According to Strain of Strain Sensor

[0127] The resistance of the strain sensor prepared in Preparation Example 2 was measured according to strain. Figure 11 is a graph showing the results of resistance measurement according to strain of the strain sensor according to one embodiment of the present invention. From Figure 11, it can be seen that for all types of microfibers, the resistance value increases in a linear trend as the applied strain increases. The high linearity of the strain sensor is attributed to the presence of the crosslinking agent PEDOT:PSS. This linearity helps maintain the penetration network even when the strain increases.

[0128] Integrating cellulose nanofibers (CNF) into a natural rubber (NR) matrix reduces the resistance of the strain sensor over the entire strain range, while simultaneously providing good linearity (R). 2 = 0.99) is maintained. From the change in resistance to the degree of strain of the sensor, the high linearity of the strain sensor ensures reliable conversion by the calibration curve. Therefore, the good linearity obtained for the CNT / PEDOT:PSS@NRC“5” microfiber indicates that it is suitable as an efficient strain sensor for monitoring physiological signals based on human motion.

[0129] <Experimental Example 9> Evaluation of Stability and Durability of Strain Sensor

[0130] The stability and durability of the strain sensor prepared in Preparation Example 2 were evaluated. Repetitive stretching-relaxation cycles were performed using an X-axis motorized system (SL2-15, ST1, Korea) at a speed of 1 mm / s for a holding time of 1 second. The long-term stability and durability of all CNT / PEDOT:PSS@NRCx strain sensors were investigated through repetitive stretching-relaxation cycles at 200% strain. FIG. 12 is a graph showing the change in relative resistance measured during the stretching-relaxation cycle of a strain sensor according to one embodiment of the present invention.

[0131] From Fig. 12, it can be seen that during operation for 2000 cycles, the relative resistance of the CNT / PEDOT:PSS@NRC“0” microfiber is small but continuously increases. This continuous increase may be attributed to the rearrangement of conductive fillers in the natural rubber (NR) matrix as it approaches a stable state. The viscoelastic properties of the CNT / PEDOT:PSS@NRC“0” microfiber hinder the rearrangement of conductive fillers, causing the relative resistance to continuously increase.

[0132] Meanwhile, it was observed that the change in relative resistance gradually improved as the composition of the cellulose nanofiber (CNF) filler increased. In particular, the relative resistance of the strain sensor assembled using CNT / PEDOT:PSS@NRC“5” microfibers changed negligibly even after 2,000 cycles of continuous operation. These results suggest effective fixation and interlocking of natural rubber (NR) chains by the cellulose nanofiber (CNF), which hinders the slippage of the natural rubber (NR) chains under higher stretching. Furthermore, the strong connections formed by the cellulose nanofiber (CNF) at the junctions between adjacent carbon nanotubes (CNT) facilitate the rearrangement of the conductive filler even under higher strain rates. Thus, the reinforcing effect of the cellulose nanofiber (CNF) helps maintain the elasticity of the natural rubber (NR)-based microfibers even at higher stretching levels. These results indicate that the fabricated natural rubber (NR)-based microfiber (CNT / PEDOT:PSS@NRC“5”) is a mechanically durable and reliable strain sensor for long-term real-world applications.

[0133] <Experimental Example 10> Evaluation of the Response Time of a Strain Sensor

[0134] For the strain sensor prepared in Preparation Example 2, the response time was measured during one cycle of stretching-relaxation at a strain of 5%. FIGS. 13a and FIGS. 13b are graphs showing the results of measuring the response time of a strain sensor according to an embodiment of the present invention.

[0135] From Fig. 13a, it can be seen that the CNT / PEDOT:PSS@NRC“5” strain sensor exhibits a remarkably fast response time of 43 ms. This is significantly better than the 65 ms of the CNT / PEDOT:PSS@NRC“0” strain sensor in Fig. 13b.

[0136] This enhanced responsiveness, attributed to improved elastic behavior due to cellulose nanofiber (CNF) reinforcement, demonstrates that the strain sensor is advantageous for real-time monitoring of high-frequency motion. The fast response time of the CNT / PEDOT:PSS@NRC“5” strain sensor helps detect high-frequency motion, making it useful for real-time applications.

[0137] <Experimental Example 11> Evaluation of Mechanical Hysteresis of Strain Sensor

[0138] Hysteresis of the change in resistance during the stretching-relaxation cycle is another important parameter for strain sensor performance. Cycle tensile tests were performed on the strain sensor prepared in Preparation Example 2 at strain rates of 100% and 200%. 5% s -1 The change in relative resistance was measured by applying strains of 100% and 200% at a speed. FIGS. 14a and FIGS. 14b are graphs showing the results of the mechanical hysteresis evaluation of a strain sensor according to one embodiment of the present invention.

[0139] From Fig. 14a, the hysteresis behavior of the CNT / PEDOT:PSS@NRC“5” strain sensor was found to be 2.7% at 100% strain and 7.4% at 200% strain, which is a significant improvement compared to the behavior of the CNT / PEDOT:PSS@NRC“0” strain sensor in Fig. 14, which was 9% at 100% strain and 30% at 200% strain. Additionally, the initial resistance of the CNT / PEDOT:PSS@NRC“5”-based strain sensor was fully recovered after releasing the strain, whereas the resistance of the CNT / PEDOT:PSS@NRC“0”-based strain sensor was not fully recovered.

[0140] <Experimental Example 12> Evaluation of Long-term Stability of Strain Sensor

[0141] The long-term stability of the strain sensor manufactured in Manufacturing Example 2 was evaluated in comparison with the prior art. FIG. 15 is a graph showing the results of the long-term stability evaluation of a strain sensor according to one embodiment of the present invention.

[0142] Conventional technology:

[0143]

[0037] TN Lam, GS Lee, B. Kim, H. Dinh Xuan, D. Kim, S. Il Yoo, J. Yoon, Microfluidic preparation of highly stretchable natural rubber microfiber containing CNT / PEDOT:PSS hybrid for fabric-sewable wearable strain sensor, Compos. Sci. Technol. 210 (2021) 108811.

[0144]

[0046] JW Han, B. Kim, J. Li, M. Meyyappan, A carbon nanotube based ammonia sensor on cellulose paper, RSC Adv. 4 (2014) 549-553.

[0145]

[0048] M. Bhattacharjee, M. Soni, P. Escobedo, R. Dahiya, PEDOT:PSS Microchannel-Based Highly Sensitive Stretchable Strain Sensor, Adv. Electron. Mater. 6 (2020).

[0146]

[0049] M. Amjadi, Y.J. Yoon, I. Park, Ultra-stretchable and skin-mountable strain sensors using carbon nanotubes-Ecoflex nanocomposites, Nanotechnology 26 (2015).

[0147]

[0050] Z. Tang, S. Jia, F. Wang, C. Bian, Y. Chen, Y. Wang, B. Li, Highly Stretchable Core-Sheath Fibers via Wet-Spinning for Wearable Strain Sensors, ACS Appl. Mater. Interfaces 10 (2018) 6624-6635.

[0148]

[0051] A. Krainoi, K. Boonkerd, Novel hybrid natural rubber nanocomposites with carbon nanotube and cellulose nanofiber for strain-sensitive sensor, Ind. Crops Prod. 187 (2022) 115455. https: / doi.org / 10.1016 / j.indcrop.2022.115455.

[0149]

[0053] J. Shintake, E. Piskarev, SH Jeong, D. Floreano, Ultrastretchable Strain Sensors Using Carbon Black-Filled Elastomer Composites and Comparison of Capacitive Versus Resistive Sensors, Adv. Mater. Technol. 3 (2018) 1-8.

[0150] From FIG. 15, the long-term stability of a strain sensor according to one embodiment of the present invention was verified, and the sensor maintained low hysteresis even after 500 cycles of operation. This excellent stability is attributed to the synergistic effect of a cellulose nanofiber (CNF) reinforcement that enhances the mutual locking of natural rubber (NR) chains and a microfluidic manufacturing process that promotes the alignment of carbon nanotubes (CNT) along the axis of the microfibers. The addition of PEDOT:PSS further strengthens the conductive network, thereby minimizing resistance fluctuations during repeated strain.

[0151] <Experimental Example 13> Evaluation of Various Human Motion Detection Capabilities of a Strain Sensor

[0152] The strain sensor manufactured in Manufacturing Example 2 was attached to various parts of the human body to evaluate various human motion detection capabilities.

[0153] FIG. 16 is an image showing a strain sensor according to an embodiment of the present invention attached to an index finger, and a graph showing the measurement results of the change in relative resistance according to the operation. The change in resistance of the sensor can be observed when the index finger is bent at various angles; when the finger is bent, deformation occurs in the microfibers, causing the resistance to increase, and when straightened, the resistance returns to the initial value. Because the strain sensor has good linearity, the relative resistance increases proportionally as the bending angle increases. These results indicate the excellent durability and stability of the device, which is an important characteristic for real-time applications.

[0154] FIG. 17 is an image showing a strain sensor attached to the neck according to an embodiment of the present invention and a graph showing the measurement results of changes in relative resistance according to movement. Changes in relative resistance according to minute head movements (left, right, up and down) indicate the ability of the strain sensor to distinguish subtle changes.

[0155] FIG. 18a is an image showing a strain sensor according to an embodiment of the present invention attached to a wrist, and FIG. 18b is a graph showing the measurement results of the change in relative resistance according to operation. When attached to the wrist, the strain sensor generated a continuous signal in response to wrist movement (bending and relaxing).

[0156] FIG. 19a is an image showing a strain sensor according to an embodiment of the present invention attached to an elbow, and FIG. 19b is a graph showing the measurement results of relative resistance change according to movement. FIG. 20a is an image showing a strain sensor according to an embodiment of the present invention attached to a knee, and FIG. 20b is a graph showing the measurement results of relative resistance change according to movement. FIG. 21a is an image showing a strain sensor according to an embodiment of the present invention attached to an ankle, and FIG. 21b is a graph showing the measurement results of relative resistance change according to movement. The excellent strain rate and linearity range of the natural rubber (NR)-based strain sensor are further utilized to record signals generated during large-scale joint movements, such as elbow and knee flexion. The relative resistance change of the strain sensor for elbow movement confirms adaptability to large-scale movements and stability during holding time. In addition, the strain sensor effectively captured distinct signals during knee and ankle flexion, respectively.

[0157] <Experimental Example 14> Evaluation of Motion Detection Capability According to Wearing a Deformation Sensor

[0158] To monitor physiological signals associated with human body movements such as walking, running, squatting, jumping, and stepping in real time, a comfortable and stable fit of the strain sensor is required. The strain sensor manufactured in Example 2 was worn on the human body to evaluate its motion detection capability. FIG. 22 is an image showing a strain sensor according to an embodiment of the present invention sewn onto a fabric knee pad. The sensor integrated into the knee pad was used to detect large-scale human body movements.

[0159] FIG. 23 is a graph showing the measurement results of changes in relative resistance according to various movements of a strain sensor according to an embodiment of the present invention. From FIG. 23, a distinct and unique output signal for the movement of a knee joint can be identified from the strain sensor according to an embodiment of the present invention. It shows the characteristic shape of. Since these natural rubber (NR)-based microfibers have high compatibility with fabrics and can be applied to capture minute physiological movements, potential applications in the field of next-generation smart wearable electronics can be anticipated.

[0160] <Manufacturing Example 3> Manufacturing of an integrated system for wireless transmission of signals

[0161] The strain sensor manufactured in Manufacturing Example 2 was applied to a smart wearable device to evaluate its wireless signal collection capability. An integrated system was designed for the simultaneous detection, processing, and wireless transmission of epithelial signals. The integrated system used an A / D converter to convert analog signals into signals that could be processed digitally. Subsequently, these signals were processed using a microcontroller unit (Microcontroller Unit, MCU, ESP8266), and the unit wirelessly transmitted the signals via Wi-Fi. FIG. 24 is a schematic diagram showing an integrated system for wireless signal collection of a strain sensor according to an embodiment of the present invention. In addition, real-time signals were monitored using a dedicated mobile application designed for Android. FIG. 25 is a schematic diagram showing a circuit of an integrated system for wireless signal collection of a strain sensor according to an embodiment of the present invention.

[0162] <Experimental Example 15> Evaluation of the Wireless Signal Collection Capability of a Strain Sensor

[0163] The ability to collect wireless signals was evaluated by applying the strain sensor to a smart wearable device.

[0164] FIG. 26 is an image showing the collection of index finger movement signals through a strain sensor according to an embodiment of the present invention, and a graph showing the results of wireless signal measurement according to the operation. FIG. 26 shows real-time signals wirelessly collected during the extension and relaxation of the index finger displayed on a mobile application interface. This integrated system enables the smooth and continuous collection, processing, and transmission of signals obtained during the bending of the index finger.

[0165] FIG. 27 is an image showing the collection of knee movement signals through a deformation sensor according to an embodiment of the present invention, and a graph showing the results of wireless signal measurement according to the movement. An integrated system including a power supply for wireless operation is attached to a knee pad to fit the entire system. This configuration enables the continuous collection of real-time wireless signals generated during daily activities.

[0166] FIG. 28 is a graph showing the results of wireless signal measurement according to a walking motion using a strain sensor according to an embodiment of the present invention. FIG. 29 is a graph showing the results of wireless signal measurement according to a stair climbing motion using a strain sensor according to an embodiment of the present invention. This seamless integration of the fabricated strain sensor and the wireless transmission system facilitates application in the fields of wearable smart technology and advanced healthcare monitoring.

[0167] Although exemplary embodiments of the present invention have been described above in relation to the preferred embodiments mentioned, various modifications and variations are possible without departing from the essence and scope of the invention. Accordingly, the appended claims will include such modifications and variations insofar as they fall within the essence of the invention.

Claims

1. Elastic microfibers for strain sensors comprising cellulose nanofibers, elastomers, conductive particles, and conductive polymers.

2. In Paragraph 1, The above elastomer is an elastic microfiber for a deformation sensor, which is natural rubber.

3. In Paragraph 1, Elastic microfibers for a strain sensor, wherein the cellulose nanofibers are included in an amount of 1 to 10 parts by weight based on 100 parts by weight of the elastomer.

4. In Paragraph 1, The elastic microfiber for a strain sensor, wherein the elastic body is included in an amount of 40 to 80 weight percent based on the total weight of the elastic microfiber.

5. In Paragraph 1, Elastic microfibers for strain sensors, wherein the conductive particles are one or more selected from the group consisting of carbon black (CB), single-walled or multi-walled carbon nanotubes (CNT), metal nanowires (AgNW), metal nanoparticles (Nanoparticle), MXene, graphene, and graphene oxide.

6. In Paragraph 1, Elastic microfibers for strain sensors, wherein the conductive particles are included in an amount of 0.1 to 2.0 weight% based on the total weight of the elastic microfibers.

7. In Paragraph 1, Elastic microfibers for strain sensors, wherein the conductive polymer is one or more selected from the group consisting of polypyrrole, polythiophene, polyaniline, polyacetylene, polyphenylene sulfide, polyethylenedioxythiophene, polyphenylene vinylene, and PEDOT:PSS (Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)).

8. In Paragraph 1, Elastic microfibers for strain sensors, wherein the conductive polymer is included in an amount of 0.1 to 2.0 weight% based on the total weight of the elastic microfibers.

9. A strain sensor comprising an elastic microfiber for a strain sensor according to any one of claims 1 to 8.

10. A first solution comprising cellulose nanofibers and an elastomer; and a second solution comprising conductive particles and a conductive polymer, mixed to obtain an elastomer solution; A step of injecting the elastomer solution into the inner tube of the elastic microfiber manufacturing device and injecting a coagulant solution into the outer tube of the elastic microfiber manufacturing device to continuously coagulate the elastomer solution; and A method for manufacturing elastic microfibers for a strain sensor according to one of claims 1 to 8, comprising the step of collecting elastic microfibers from the outlet of the elastic microfiber manufacturing device.

11. In Paragraph 10, The elastic microfiber for a strain sensor comprises: an outer tube; and an inner tube formed within the outer tube, formed to be parallel and coaxial within the outer tube, and having a smaller diameter than the outer tube.

12. In Paragraph 10, Elastic microfiber for strain sensors, wherein the above coagulant is one or more selected from the group consisting of calcium chloride and acetic acid solution.

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