Superhydrophobic and Highly Elastic Strain‐Sensing Fiber Embedded with Carbon Nanotubes and Aerogels Based on the Dipping and Drying Method

For a fiber‐based strain sensor to be used as a wearable device, its conductivity and sensing characteristics should be stably maintained even during repeated mechanical movements. Additionally, the sensing characteristics should remain unaffected by external contaminants, such as water or sweat, as the sensor is expected to be in contact with the human body. In this study, a superhydrophobic and highly elastic strain‐sensing fiber with durability against continuous tension and contraction while maintaining a stable sensing performance even when in contact with water and sweat is developed. A carbon nanotube is embedded, which is a highly conductive material, inside a spandex fiber with high elasticity and shape recovery rate, enabling the stable measurement of repetitive joint movements under various strain conditions. Furthermore, a superhydrophobic silica aerogel is embedded inside the spandex fiber to facilitate stable sensing without malfunction even when exposed to external contaminants. The proposed strain‐sensing fiber can monitor joints of the human body during various movements, such as dumbbell pressing, squatting, walking, and running. Therefore, the study findings can contribute to the development of wearable healthcare devices that warrant reliable sensing.


Introduction
[3][4][5][6][7] Wearable strain sensors are highly useful as healthcare devices because they can easily track the physical movements DOI: 10.1002/admi.202300820 of the human body and can be particularly useful as diagnostic tools for monitoring a patient's rehabilitation treatment and recovery status.
[15] Researchers have explored a 3D wrinkled or wavy structure for electrodes to absorb stress using nonstretchable organic or inorganic metallic materials; however, certain limitations exist in the formation of the 3D structure, rendering it difficult to ensure stability when stretching in multiple directions. [16,17]Therefore, mogul-patterned structures, biomimetic 3D structures, and pop-up structures that form 3D microstructures on substrates, capable of omnidirectional stretching, are being investigated.[28][29][30][31] For instance, a slight increase in the strain of the fiber can rapidly change the resistance and rate of change of resistance of the conductive fiber, facilitating the recognition of movement at each bending angle of a joint.[34][35][36] Carbon nanotubes (CNTs)/Polyurethane helical yarn was produced using electrospinning, spray coating, and twisting processes.This yarn exhibited excellent recoverability of less than 900% and tensile elongation of up to 1700%. [37]urther research has been conducted to create hierarchical microsized hairy architectures by nanoimprinting on fibers coated with polyurethane/Ag nanoparticles.The goal was to increase sensitivity to various stimuli, such as pressure, stretching, and bending, by contact resistance around the hairy architectures.The results showed less than 200% elasticity and stable performance for 2200 cycles. [38]However, the coated conductive material in these fiber-based strain sensors may not uniformly adhere to the rough surfaces of the fibers, and the conductivity may decrease when the conductive material peels off during mechanical movement. [39,40]Additionally, because these sensors remain in direct contact with the user's skin, a highly durable technology should be developed to protect the sensor from sweat and other contaminants.
Therefore, in this study, we fabricated a superhydrophobic and highly elastic strain-sensing fiber by embedding a CNT and an aerogel within a highly elastic spandex fiber via a simple dipping and drying method.The conductive CNTs and hydrophobic silica aerogel microspheres were sequentially embedded both inside and outside the highly elastic spandex fiber; this endowed the strain sensor with a contact angle of ≈153°, resulting in superhydrophobicity.The fabricated strain-sensing fiber exhibited a stable resistance change rate even at over 1000 stretch-release cycles, and the sensor operated stably without malfunction even when exposed to liquids, such as water or sweat.This sensor can effectively detect the bending angles of arms and legs during walking or running, rendering it useful in wearable electronic devices and for personalized health monitoring.

Results and Discussion
Strain sensors can identify the movement of human joints through electrical signal analysis based on the changes in electrical properties, particularly electrical resistance, caused by external force-induced length deformations.Typically, the changes in resistance are caused not only by the movement of the human joint to be measured but also by environmental factors, such as temperature and humidity; therefore, the strain sensor should be designed and analyzed considering the environmental changes affecting the resistance.In the case of wearable applications, the attached strain sensor is easily exposed to sweat or rainwater; therefore, the reliability of its sensitivity to such environmental changes must be ensured. [41]In this study, a superhydrophobic and highly elastic strain-sensing fiber that can repel external water or sweat was fabricated by embedding a conductive CNT and a hydrophobic silica aerogel into a highly elastic spandex fiber (Figure 1a,b).The spandex fiber is composed of dozens of strands bundled together, and CNTs and aerogel are embedded inside the spandex fiber using a dipping and drying process.The addition of CNT and aerogels to the spandex fiber endows the fiber with excellent conductivity and hydrophobicity through aerogel, facilitating a stable sensing operation with superhydrophobic properties even when exposed to rain or sweat while maintaining the flexibility of the fabricated fiber.[44][45][46] However, nonconductive binders can prevent the formation of CNT-based conductive networks on the fiber surface. [47,48]There are several studies to increase the durability of the conductive network of CNTs by using conductive polypyrrole or iron ions.However, the coating is unstable and easily peels off due to poor bonding with the matrix. [47,49]Figure 1c depicts a schematic of commercial spandex fabric, stitched with the superhydrophobic and highly elastic strain-sensing fiber, placed on the elbow; the corresponding dynamic response characteristics with flexion and extension repeated five times at an angle of ≈30°are depicted in Figure 1d.Sensing stability was compared by creating a sweat-exposed environment that occurs during actual exercises.The joint movement was measured after the CNT-aerogelembedded spandex fiber, which was made hydrophobic using CNT-embedded spandex fiber and aerogel, was exposed to artificial sweat for 5 min.The CNT-embedded spandex fiber exhibited highly unstable resistance changes during joint movement, whereas the CNT-aerogel-embedded spandex fiber exhibited a stable resistance change characteristic of ≈60%.
Figure 2a illustrates a schematic of the fabrication process for obtaining the superhydrophobic and highly elastic strain-sensing fiber.Polymer swelling is a process in which solvent molecules penetrate the polymer matrix and change the volume.When a spandex fiber, which is a bundle of multiple strands in close contact, is dipped into tetrahydrofuran (THF), the THF molecules penetrate the polymer chains.The interactions between the infiltrated THF molecules and polymer chains result in the expansion of the volume of the bundle-shaped strands, decreasing the adhesion between the strands and widening them.When the spandex fiber is dried, the THF molecules volatilize, shrinking the polymer chains back to their initial state. [50]We used this simple method of dipping and drying to sequentially embed the CNTs and aerogel inside and outside the spandex fiber, which served as the matrix.The solvent-driven expansion-contraction of soft and flexible spandex fibers induced evenly dispersed CNTs or aerogels in the THF solvent to penetrate and embed well inside and outside the spandex fiber.Subsequently, as the swollen strands shrank during the drying process, the CNT and aerogel came close.Therefore, the CNT and aerogel embedded inside the spandex fiber were prevented from separating despite external shock or continuous stretching of the fiber.As shown in Figure 2b, the pristine spandex fiber exhibited a smooth surface, and Si elements, generated by the aerogel, were not detected by energy-dispersive X-ray spectroscopy (EDS) elemental mapping.Only networked CNTs were observed on the surfaces of the CNTembedded spandex fibers, and Si was not detected in the EDS elemental mapping (Figure 2c).
Aerogels are mesoporous materials with low density, low thermal conductivity, high porosity, hydrophobicity, and high surface area. [51]An aerogel is a 3D network of silica nanoparticles, wherein the hydrophilic -OH groups are replaced by hydrophobic alkoxide groups on the surface of the silica nanoparticles via surface modification during fabrication.THF is a moderately polar solvent that can dissolve several polar and nonpolar chemical compounds.Therefore, the aerogel does not disperse in water, yet disperses efficiently in THF.An aerogel-embedded spandex fiber was produced by dipping and drying a pristine spandex fiber in an aerogel solution (aerogel dispersed in THF).
Figure 2d shows a photograph and field-emission scanning electron microscopy (FE-SEM) images of the aerogel-embedded spandex fiber, whose diameter is ≈450 μm.The FE-SEM image of the surface confirms the presence of aerogel on the surface of the spandex.Additionally, the EDS elemental mapping image of the cross-section confirms the even distribution of Si elements, generated by the aerogel, throughout the cross-section.The embedded aerogel was observed using FE-SEM and EDS elemental mapping images of the surface of the inner strands by removing the outer strands of the aerogel-embedded spandex fiber (Figure S1, Supporting Information), which indicates that the aerogel was embedded both outside and inside the spandex fiber.The Water contact angle (WCA) of pristine spandex fiber without aerogel was 119.4 ± 4.2°, whereas that of the aerogel-embedded spandex fiber increased to 138.3 ± 2.2°-152.7 ± 3.2°as the concentration of aerogel in THF increased to 0.03-1.64wt% (Figure 2e).
The CNT surfaces are inherently hydrophobic, self-associated, and aggregate through hydrophobic and van der Waals interactions, rendering it difficult to disperse CNTs in water or organic solvents.[54][55][56][57][58] In this study, all three methods were used to maximize CNT dispersion in the THF solution.Before dispersing the CNTs in a THF solution, they were initially treated with ultraviolet (UV)-ozone to generate oxygen-containing groups, such as hydroxyl (─OH) and carboxylic (─COOH) groups, on the surface of the nonpolar CNTs, thereby making them polar.Subsequently, when mixing the surface-modified CNTs with the THF solution, trifluoroacetic acid (TFA), which comprised both hydrophobic trifluorocarbon and polar carboxylic groups similar to those in a surfactant, was used as a cosolvent to facilitate the dispersion of the CNTs.This CNT-THF-TFA solution was then mechanically mixed via sonication.The amount of CNT in the CNT-THF-TFA solution was adjusted to 0.01-0.2and 1.64 wt% aerogel was additionally embedded in the CNT-embedded spandex fiber to ensure superhydrophobicity.The results in Figure 2f confirm the presence of CNT and aerogel on the surface of the CNT-aerogelembedded spandex fiber, and the EDS elemental mapping image of the cross-section indicates that the Si element generated by the aerogel is evenly distributed throughout the fiber (Figure S2, Supporting Information).The diameter of the CNT-aerogelembedded spandex fiber was ≈450 μm.
Figure 2g depicts the electrical resistance and WCA results for the CNT-aerogel-embedded spandex fibers according to the CNT concentration.When the amount of CNTs in the CNT-THF-TFA solution exceeded 0.2 wt%, the phenomenon of CNT agglomeration was confirmed (Figure S3, Supporting Information).Accordingly, the electrical resistance of CNT-aerogel-embedded spandex fiber was lowered by repeatedly coating 0.2 wt% of CNT-THF-TFA solution up to six times.As the concentration of CNT (the number of coatings) increased to 0.01 (1), 0.05 (1), 0.1 (1), 0.2 (1), 0.2 (2), 0.2 (3), and 0.2 (4) wt%, the electrical resistance of the CNT-aerogel-embedded spandex fiber continued to decrease to 45.6 ± 27.5, 16.9 ± 4.4, 12.2 ± 2.9, 10.2 ± 2.0, 9.6 ± 2.2, 7.6 ± 2.2, and 6.7 ± 1.4 kΩ, respectively; however, the resistance remained constant at ≈5.5 kΩ after 0.2 (5) wt%.In the case of the WCA, superhydrophobicity of more than 150°was maintained despite the increase in the CNT concentration and number of coatings.Based on the aforementioned experimental results, five times coating of 0.2 wt% of CNT and 1.64 wt% of aerogel, which exhibited low electrical resistance and high WCA results, were considered optimal conditions.Figure 3a depicts the thermal degradation characteristics of the pristine and CNT-aerogel-embedded spandex fibers.Significant mass loss began at temperatures exceeding ≈250 °C and the mass continued to decrease.At ≈650 °C, all polymer chains of spandex fibers were degraded.The weight loss curve of the fibers exhibited a similar trend to that of the embedded CNT-aerogel.This indicates that the embedding process did not cause physical or chemical damage to the polymer chains of the spandex fibers.The percentages of residue after degradation were ≈0.1% and 3.8% (pristine and CNT-aerogel-embedded spandex fibers) at 850 °C, which increased slightly depending on the embedding of CNT and aerogels.
Figure 3b illustrates a strain-stress graph of the pristine and CNT-aerogel-embedded spandex fibers measured using thermomechanical analyzer (TMA) equipment.Typically, spandex fibers are composed of a copolymer of polyglycol (soft-segment block) and polyurethane (hard-segment block); therefore, they exhibit excellent elongation ability and a rapid recovery rate, extending up to five to eight times their original length. [59]In this study, the pristine and CNT-aerogel-embedded spandex fibers did not break even when stretched 600% from the initial length of 0.7-5 mm, which was the maximum length allowed by the TMA equipment, and exhibited a tensile strength of ≈2.5 MPa.Furthermore, the trend of the stress-strain curve was similar, regardless of the embedding of the CNT-aerogel.Therefore, our analysis confirmed that the spandex fiber does not undergo physical and chemical deformation despite the repeated expansioncontraction processes performed by solvent swelling and that its mechanical properties and flexibility are maintained.
Figure 3c depicts the relative resistances of the CNT-aerogelembedded spandex fiber under strain conditions of 0-300%; the resistance increased from 1.0 ± 0.2% to 638.5 ± 32.3% depending on the elongation of 1-250%.However, at 300% stretching, the standard deviation was significant at 2751.6 ± 1274.8%, re-ducing the reliability of the value.At 300% stretching, the relative resistance significantly increased to 2751.6 ± 1274.8%, but the significant standard deviation reduced its reliability.This was attributed to the formation of cracks in the current path as the gap between the networked CNTs widened significantly. [60]Note that measuring changes in electrical properties beyond the 300% stretch was impossible.
Figure 3d illustrates the reliability of the electrical properties of the CNT-aerogel-embedded spandex fiber according to exposure to water and artificial sweat.As a reference, a CNTembedded spandex fiber manufactured without the aerogel was exposed to water and artificial sweat.The dynamic response results indicated that the current of the CNT-embedded spandex fiber changed immediately when the water or artificial sweat was dropped on the fiber.This implies that the overall resistance of the sensing fiber increases because of the water or artificial sweat absorbed by the CNT networks.In the case of the CNT-aerogelembedded spandex fiber, the superhydrophobic properties of the aerogel repelled water or artificial sweat, maintaining the current constant.Therefore, when using superhydrophobic and highly elastic strain-sensing fibers obtained by embedding CNT and aerogels as wearable sensors that detect the movement of human joints, the sensor can operate stably without malfunction even when exposed to water or sweat.
Figure 4a depicts the dynamic response of the changes in the relative resistance owing to repeated stretching and releasing of the superhydrophobic and highly elastic strain-sensing fiber under various strain conditions of 1-100%.As the strain increases, Relative resistance changes under various strain conditions.a) Dynamic responses when stretching and releasing are repeated ten times under strain conditions of 1%, 5%, 10%, 30%, 50%, 70%, and 100%.b) Cycling stability characteristics of the dynamic response confirmed through 1000 repetitions of stretch-release cycles under the strain condition of 30%.c) Maximum relative resistance changes repeated 1000 times under strain conditions of 1%, 5%, 10%, 30%, 50%, 70%, and 100%.
the changes in the relative resistance also increase.Figure 4b illustrates a representative dynamic response result of repeating the stretch-release cycles 1000 times under a strain condition of 30%.During the measurement process, the strain in the range of 0-30% could not be accurately measured due to mechanical errors, and slight deviations in the values were observed because of the measurement of slightly larger or smaller positions; however, the overall change in the relative resistance remained constant.Figure 4c indicates that when stretching is repeated 1000 times under various strain conditions, the maximum relative resistance changes are 0.9 ± 0.2%, 11.9 ± 0.3%, 26.4 ± 2.7%, 59.2 ± 1.0%, 75.1 ± 1.4%, 94.1 ± 1.6%, and 114.8 ± 5.2% for strain conditions of 1%, 5%, 10%, 30%, 50%, 70%, and 100%, respectively (Figure S4, Supporting Information).In addition, typical elastic behavior was confirmed through the tensile hysteresis curve obtained by performing a total of six loading-unloading cycles of 100% tension (Figure S5, Supporting Information).This confirms that the proposed superhydrophobic and highly elastic strain-sensing fiber exhibits excellent resistance recoverability and reproducibility, even after repeated stretching and releasing from small to large deformations.Therefore, the developed strain-sensing fiber can be suitable for measuring resistance changes according to the movement of the joint where various types of deformations occur.Moreover, small or large movement changes in human joints that occur in daily life and during exercise activities can be reliably observed.
We attached the superhydrophobic and highly elastic strainsensing fiber on the elbow and knee of a subject to measure the dynamic response based on various motions.Typically, sensing fibers placed on joints undergo resistance changes as stretching occurs owing to the bending of the joint.Figure 5a,b depict the results of the repeated flexion-extension movement of the el-bow and knee, respectively.As the elbow was flexed to 30°, 60°, and 90°in a straightened state, the sensing fibers were stretched by ≈20%, 30%, and 55%, with relative resistance changes of 46.0 ± 0.9, 60.6 ± 1.1, and 55%, respectively; the highest value observed was 82.6 ± 1.9%.The sensing fiber placed on the knee experienced strains of ≈25%, 46%, and 81% depending on the flexion angle, increasing the relative resistance changes to 49.5 ± 1.7%, 67.2 ± 1.6%, and 105.7 ± 3.6% at 30°, 60°, and 90°, respectively.As the flexion angles of the elbow and knee increased from 30°to 90°, the changes in relative resistance increased, and the sensing ability remained stable even after eight repetitions.
Furthermore, the proposed superhydrophobic and highly elastic strain-sensing fiber was attached to the left elbow and knee of the human body to monitor various exercise states, including dumbbell pressing, squatting, walking, and running.During dumbbell pressing, the flexion-extension occurred only in the elbows while sitting on a chair (Figure 5c).The elbow exhibited a relative resistance change of 0-80% when the bending angle repeatedly ranged from 0°to 90°; however, the relative resistance change of the knee remained constant at ≈100% as no movement occurred when bent at an angle of 0°to 90°.In general, squats involve repeated up and down movements with arms extended forward; therefore, a bending angle of ≈90°occurred only at the knees (Figure 5d), and only the sensor attached to the knee exhibited a relative resistance change of ≈100%.During walking and running, bending occurred in both the elbow and knee, with the left and right arms moving with the right and left legs, respectively (Figure 5e,f).Therefore, the sensing fibers attached to the left elbow and knee indicated alternating resistance changes.During walking, relative resistance changes of 0-40% and 0-60% were observed for the sensors on the elbow and knee, respectively.During running, relative resistance changes of 10-80% and 15-100% were repeatedly observed in the sensors attached to the elbow and knee, respectively.As the elbow and knee were bent and not completely straightened when running, the minimum values of the relative change in the sensing graph were ≈10% and 15% for the sensors on the elbow and knee, respectively.

Conclusion
In this study, we developed a fiber-based strain sensor that operates stably despite repeated mechanical movements and exposure to external contaminants, such as water and sweat.The objective of our analysis was to increase the performance reliability of a wearable strain sensor when monitoring the movement of human joints.A simple dipping and drying process was used to embed highly conductive CNTs and superhydrophobic silica aerogels inside multiple strands that formed highly elastic spandex.This produced a superhydrophobic and highly elastic strainsensing fiber with a low resistance of ≈5.5 kΩ and a contact angle of 153°.The fabricated strain-sensing fiber maintained a stable sensing performance even after 1000 repetitions of stretching and releasing and despite exposure to water and sweat solutions under strain conditions of up to 100%.The developed superhydrophobic and highly elastic strain-sensing fibers were attached to the elbow and knee to measure various real-time movements, such as dumbbell pressing, squatting, walking, and running.The results indicate that the proposed strain-sensing fibers can increase the operational reliability of wearable strain sensors.

Experimental Section
Fabrication and Analysis of Physical Properties of the Developed Strain-Sensing Fiber: The superhydrophobic and highly elastic strain-sensing fiber was fabricated in two steps: a CNT embedding process to ensure conductivity and an aerogel embedding process to ensure hydrophobicity.The CNT embedding process involves an UV-ozone treatment for 1 h to polarize the surface of a multi-walled CNT with a diameter of ≈20 nm, length of ≈5 μm, and purity >99 wt% (Carbon Nano-material Technology Co.).Four different CNT amounts of 0.002, 0.01, 0.02, and 0.04 g were added into the mixed solution of 18 g of THF (anhydrous, 99.9%, Sigma-Aldrich) and 2 g of TFA (99.0%,Tokyo Chemical Industry Co. Ltd.) to obtain the CNT-THF-TFA solutions of four concentrations, namely, 0.01, 0.05, 0.1, and 0.2 wt%, respectively, prepared via sonication for 1 h at 25 °C.A CNT-THF-TFA solution was prepared and used each time during the repetitive CNT embedding process.A 150-mm-long spandex fiber (Hyosung TNC Co., Ltd.) was immersed in the CNT-THF-TFA solution and sonicated for 10 min.Subsequently, a CNT-embedded spandex fiber was produced by drying the fiber at 25 °C for 10 min and by further drying in a forced convection oven (C-DF2, CHANG SHIN SCIENCE) at 90 °C for 10 min.For aerogel embedding, 0-0.5 g of silica aerogel microsphere with a diameter of ≈5 μm and a pore size of ≈30 nm (JIOS AeroVa) was added to 30 g of THF and sonicated for 10 min to achieve 0-1.64 wt% aerogel dispersion.The CNT-embedded spandex fiber was dipped in the aerogel dispersion for 10 min, taken out, and dried at 25 °C for 10 min to produce the CNTaerogel-embedded spandex fiber.The surface morphology of this fiber was measured using FE-SEM (S-4800, Hitachi).The elemental distributions inside and outside the CNT-aerogel-embedded spandex fiber were analyzed using EDS (EDS-7557, Oxford Instruments).Furthermore, the mechanical and thermal properties were evaluated using a TMA (TMA7100, Hitachi) and TGA; TGA7300, Hitachi), respectively.The electrical resistance was measured using a digital multimeter (FLUKE-175 EJKCT, Fluke) after fabricating the electrodes using silver paste (ELCOAT P-100, CANS) on both ends of a 12-mm-long CNT-aerogel-embedded spandex fiber.The hydrophobicity was evaluated by measuring the WCA using a contact angle analyzer (Phoenix 300, SEO Co.).Artificial sweat (BZ155, Biochemazone) has a pH of 5.5 and consists of amino acids (glycine, L-Alanine, L-Arginine, L-Asparagine, etc.), minerals (sodium, calcium, magnesium, zinc, etc.), and various metabolites (uric acid, lactic acid, urea, and ammonia).
Fabrication and Operation of Joint Sensors Using the Proposed Strain-Sensing Fiber: Spandex-based elbow and knee warmers with adequate elasticity and fit were used to measure the movements of elbow and knee joints.A 150-mm-long superhydrophobic and highly elastic strain-sensing fiber was stitched into the warmers, with stitches at 3 mm intervals, starting 6 mm from the center of the sensing fiber.The total length of the fibers sewn into the warmer was 66 mm.The sensing fiber was placed in the warmer such that it was located at the center of the elbow and knee joints.Additionally, the sensing fibers were placed parallel to the arm and leg skeleton to maximize the change in tension of the sensing fibers according to the joint movement.Copper wires were connected to both ends of the sensing fiber using a silver paste, which prevented the electrode connections from detaching owing to the repetitive stretching and bending conditions.The changes in current caused by the stretching of the sensing fiber were measured using a semiconductor parameter analyzer (B1500A, Agilent); a voltage of 250 mV was applied to both ends of the fiber.The current was converted into a resistance (R fiber ) using R fiber = 0.25/I fiber , where I fiber denotes the current through the fiber measured by the probe station.Furthermore, the relative resistance was calculated as ΔR/R 0 = (R−R 0 )/R 0 , where R 0 and R correspond to the original resistance and resistance after stretching, respectively.J.Y. consents to the use of the picture in connection with the photographs.
Statistical Analysis: Measurements for water contact angle, resistance, and relative resistance change were performed at least five times to derive the results.All data were expressed as mean ± standard deviation.

Figure 1 .
Figure 1.Superhydrophobic and highly elastic strain-sensing fiber.a) Schematic of the superhydrophobic and highly elastic strain-sensing fiber, fabricated by embedding a carbon nanotube (CNT) and an aerogel, stitched onto a commercial spandex fabric.b) Photograph of the developed strain-sensing fiber.Aerogel is embedded to endow the fiber with superhydrophobicity. c) Schematic of detecting elbow movements using a commercial spandex fabric stitched with the proposed strain-sensing fiber.d) Dynamic response characteristics detecting an elbow flexion-extension movement of ≈30°after exposing the CNT-embedded and CNT-aerogel-embedded spandex fibers to artificial sweat for ≈5 min.

Figure 2 .
Figure 2. Fabrication of the superhydrophobic and highly elastic strain-sensing fiber.a) Schematic of manufacturing a superhydrophobic and highly elastic strain-sensing fiber.Field-emission scanning electron microscopy (FE-SEM) images and energy-dispersive X-ray spectroscopy (EDS) elemental mapping image of b) the pristine spandex fiber and c) the CNT-embedded spandex fiber.d) Photograph, FE-SEM images, and EDS elemental mapping image of the aerogel-embedded spandex fiber.e) Water contact angle (WCA) of the aerogel-embedded spandex fiber according to the concentration of aerogel.f) Photograph, FE-SEM images, and EDS elemental mapping image of the CNT-aerogel-embedded spandex fiber.g) Resistance and WCA of the CNT-aerogel-embedded spandex fiber according to the concentration of CNT.

Figure 3 .
Figure 3. Mechanical and electrical properties of the CNT-aerogel-embedded spandex fiber.a) Thermomechanical analyzer (TMA) graphs and b) strain-stress graphs of pristine and CNT-aerogel-embedded spandex fibers.c) Relative resistance changes according to the strain of the CNTaerogel-embedded spandex fiber.d) Changes in the current of CNT-embedded and CNT-aerogel-embedded spandex fibers according to exposure to water or artificial sweat.

Figure 5 .
Figure 5. Monitoring various human motions using the superhydrophobic and highly elastic strain-sensing fiber.Dynamic response characteristics of the a) elbow and b) knee flexion-extension movements repeated eight times at angles of 30°, 60°, and 90°.Dynamic response characteristics were obtained by monitoring c) dumbbell pressing, d) squatting, e) walking, and f) running movements in real time using the hydrophobic strain-sensing fiber attached to the elbow and knee.