textiles that are washable with lifetimes similar to conventional textiles. [9][10][11][12][13] Furthermore, our daily life cannot be without water, such as raining, bathing, swimming, and etc. With underwater wearable electronics, underwater activities can be effectively detected or analyzed, for example monitoring electrophysiological signals for athletes when they are training in the rain or water, detecting the moving, migration or feeling of the living creatures underwater. Therefore, to design stretchable conductive fibers capable of working in full water environment is fundamentally essential. Typical methods of developing stretchable conductive fibers include prestraining polymer fibers to induce the attached 1D conductive species (metal nanowires or carbon nanotubes (CNTs)) into wavy structure, [3,14] wrapping conductive species into spiral shape along an elastic polymer fiber, [3,10,[15][16][17][18][19] or using a conductive liquid or gel encapsulated in an elastomer. [20][21][22] However, liquid metal conductors are susceptible to leakage if the fibers are damaged, while hydrogel conductors dry out over time, and both exhibit changes in conductance of the fibers with strain. Carbon-based conductors have low conductivity with increasing length, [7,9,14,17,23,24] while metal composite conductors typically exhibit limited strain tolerance and poor cycle stability. [25,26] Conductive fibers with waterproof [27] or splash-resistance [28] have been studied, but stretchable conductive fibers that are capable of maintaining good conductivity at high strain as well as fully underwater long-time use have not been systematically reported yet.In this work, we presented a core-sheath stretchable conductive fiber (CSCF) which could be safely used in water and other harsh environments (such as sonication) for a long time. The ultrafine CSCF (≈30 µm in diameter) is composed of Lycra (polyurethane, PU) fiber, multiwall carbon nanotubes (MWCNTs), silver nanowires (AgNWs), and styrene-(ethylenebutylene)-styrene (SEBS) sequentially from inside to outside, which is defined as PU@CNTs@AgNWs@SEBS. Spray coating 1D conductive networks onto a prestraining Lycra fiber resulted in a highly stretchable conductive fiber (e.g., ΔR/R 0 ≈ 0.1 at 100% strain, cycled >100 000 times at 50% strain). Surface coating SEBS enabled a significantly reduced leakage both in current (<1 µA at 5 V) and element (Ag), thus safe to human

Graphene, with its properties of intrinsic flexibility, reliable electrical performance, and high chemical stability, is highly desirable as bioelectrodes for detecting electrophysiological signals. However, its mechanical properties limit its application to a great extentenergy dissipation mechanisms are not provided by the carbon network for external strain and it easily cracks. Herein, inspired by the very structure of the avian nest, we report a durable and nondisposable transparent graphene skin electrode for detecting electrophysiological signals, which was fabricated by semi-embedding highly graphitized electrospun fiber/monolayer graphene (GFG) into soft elastomer. Because of the semi-embedded structure and strong interaction between annealed electrospun fiber and graphene through graphitization, as-fabricated conductive film demonstrated high conductivity and transparency (∼150 Ω/□ at 83% transmittance), as well as a stable electrical performance under mechanical vibrations (strain, peel-off, stir, etc.). It can be used to reliably collect vital biometric signals, such as electrocardiogram (ECG), surface electromyogram (sEMG), and electroencephalogram (EEG). Furthermore, the semi-embedded GFG in the elastomer demonstrated excellent washability (rinsing/stirring in water) and repeatability (∼10 repeats) with high signal-to-noise ratio (up to 30 dB) while detecting sEMG. This is the first report of durable and transparent graphene skin electrode for biometric signals detection, revealing potential opportunities in wearable healthcare applications.

Epidermal electronics is regarded as the nextgeneration technology, and graphene is a promising electrode, which is a key building block of such devices. However, graphene has a tendency to crack at small strains with a rapidly increased resistance upon stretching. Here, to enable graphene applicable in epidermal electronics, we designed a novel graphene structure that is molybdenum chloride (MoCl 5 )-intercalated few-layer graphene (Mo-FLG) fabricated in a confined environment. In the case of bilayer graphene (BLG), MoCl 5 -intercalated bilayer graphene (Mo-BLG) exhibited a low sheet resistance of 40 Ω/square (sq) at a transmittance of 80%. Due to the self-barrier doping effect, the sheet resistance increased to only 60 Ω/sq after exposing to the atmosphere over 1 month. Transferred onto elastomer substrates, Mo-BLG can work as an electrode up to 80% strain and maintain a high conductivity that is durable over 2000 cycles at 30% strain. This mechano-electrostability is attributed to the special intercalated structure where the intercalated dopants act as lubricants to weaken the layer−layer interaction and allow a certain degree of sliding, as well as electrical crack-connectors to bridge the cracked domains at a high strain. Mo-BLG can be applied as epidermal electrodes to monitor electrophysiological signals such as electrocardiogram (ECG), electrooculogram (EOG), electroencephalography (EEG), and surface electromyogram (sEMG) with high signal-to-noise ratios (SNRs) comparable to commercial Ag/AgCl electrode. This is the first demonstration of epidermal electrodes based on intercalation-doped graphene applied in health monitoring, shedding light on the future development of graphene-based epidermal electronics.