Stretchable yarn/fiber electronics with conductive features are optimal components for different wearable devices. This paper presents the construction of coil structure−based carbon nanotube (CNT)/polymer fibers with adjustable piezoresistivity. The composite unit fiber is prepared by wrapping a conductive carbon CNT sheath onto an elastic spandex core. Owing to the helical coil structure, the resultant CNT/polymer composite fibers are highly stretchable (up to approximately 300%) without a noticeable electrical breakdown. More specifically, based on the difference in the coil index (which is the ratio of the coil diameter to the diameter of the fiber within the coil) according to the polymeric core fiber (spandex or nylon), the composite fiber can be used for two different applications (i.e., as strain sensors or supercapacitors), which are presented in this paper. The coiled CNT/spandex composite fiber sensor responds sensitively to tensile strain. The coiled CNT/nylon composite fiber can be employed as an elastic supercapacitor with excellent capacitance retention at 300% strain.

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Significant progress in healthcare fields around the world has inspired us to develop a wearable strain–temperature sensor that can monitor biomedical signals in daily life. This novel self-powered temperature–strain dual-parameter sensor comprises a mechano-electrochemical harvester (MEH) and a thermally responsive artificial muscle (TAM). The MEHTAM system generates electricity from strain and thermal fluctuations. In addition, the sensor is comfortable to wear, owing to its stretchability (>100%), softness (<3 MPa), and one-dimensional fibers (diameter 230 μm). The MEH induces a change in the electrochemical capacitance, resulting in an electrical signal under applied strain (34 μA/m) and stress (20 μA/(m·MPa)). The TAM can be used as a mechanical temperature sensor, because the tensile stroke responds linearly to changes in temperature. As the harvester and artificial muscle are combined, the MEHTAM system generates electricity, owing to external and internal mechanical stimuli caused by muscle contractions as a response to temperature changes. The MEHTAM system that we have developed—a self-powered, strain–temperature dual-parameter sensor that is soft, stretchable, and fiber-shaped—is an interesting candidate for the production of comfortable, wearable, dual-parameter sensors.

Recently, several attempts have been made to activate or functionalize macroscopic carbon nanotube (CNT) yarns to enhance their innate abilities. However, a more homogeneous and holistic activation approach that reflects the individual nanotubes constituting the yarns is crucial. Herein, a facile strategy is reported to maximize the intrinsic properties of CNTs assembled in yarns through an electrochemical inner-bundle activation (EIBA) process. The as-prepared neat CNT yarns are two-end tethered and subjected to an electrochemical voltage (vs Ag/AgCl) in aqueous electrolyte systems. Massive electrolyte infiltration during the EIBA causes swelling of the CNT interlayers owing to the tethering and subsequent yarn shrinkage after drying, suggesting activation of the entire yarn. The EIBA-treated CNT yarns functionalized with oxygen-containing groups exhibit enhanced wettability without significant loss of their physical properties. The EIBA effect of the CNTs is experimentally demonstrated by hydration-driven torsional actuation (∼986 revolutions/m) and a drastic capacitance improvement (approximately 25-fold).

Batteries are used in all types of electronic devices from conventional to advanced devices. Currently, batteries are evolving in the direction of extremely personalized yarn− or textile−structured textronic systems. However, the absence of a protective layer on such batteries is a critical limitation to their practical use. In this study, we developed a wearable and washable MnO2−Zn textile battery that maintains its electrochemical capacity under various external environmental conditions through a vacuum−sealed packaging. The packaged textile battery was fabricated by vacuuming a polymer envelope containing the battery, followed by heat sealing with a vacuum packaging machine. The interior and exterior regions of the textile battery are completely separated by the packaging sheath to preclude leakage and intrusion of substances. The resulting packaged textile battery exhibits stable capacity retention performance under varying temperature and humidity; mechanical deformations due to bending, twisting, rubbing, and pressing; and several mechanical, chemical, and their combined washing cycles. On the basis of these demonstrations, we expect that our vacuum−packaged textile battery will offer new possibilities for practical and convenient use of textronics.