applications, and smart sportswear. [13][14][15][16][17] In this regard, stretchable and wearable electronic devices in the 1D form, which can be directly integrated into daily clothes without any inconsistency, are greatly promising for future wearable electronics. [18][19][20][21][22][23] In addition, the hierarchical property of the fibrous structures (fiber: a small and short piece of a strand, filament: a long strand, yarn: an intertwined 1D structure of fibers or filaments, and fabric: a flexible substance consisting of a network of yarns) makes 1D electronic devices and systems remarkably suitable for advanced wearable electronics. The 1D assemblies including the 1D electronic devices also have unique characteristics appropriate to wearable electronics such as softness, stretchability, breathability, and high tolerance to damage. [3] Stretchability, in particular, is one of the most important properties for practical wearable applications because smart clothes or textiles including such 1D electronic devices should be covered on soft and curved human body. [24] Furthermore, some parts of clothes are frequently stretched and deformed during natural movements in daily life, thereby increasing the importance of stretchability of 1D electronic devices. Although many of existing clothes have achieved certain stretchability with only rigid yarns through specific textile structures such as woven or knitted structures, the stretchability resulting from such textile structures is insufficient to cover high stretchability desired in specific applications. For example, high stretchability of textiles is highly required for sportswear in order to achieve a form-fitting property, high comfortability, and elasticity during exercise. For such purpose, various stretchable yarns such as spandex have been widely used in textile industry. These properties of textiles are also essential for various sensing applications of wearable and textile electronic, resulting that high stretchability should be achieved for the 1D electronic devices. [25,26] In addition, the high stretchability resulting from the use of stretchable conductive yarns can successfully prevent a bagging issue of smart textiles which degrades stability and reproducibility of the smart textiles. For achieving the 1D stretchable electronic devices and systems, the development of 1D stretchable electrodes such as conductive yarns or filaments with high electrical conductivity and stretchability is basically essential above other things. In this regard, recent advances toward developing various high-performance Research on wearable electronic devices that can be directly integrated into daily textiles or clothes has been explosively grown holding great potential for various practical wearable applications. These wearable electronic devices strongly demand 1D electronic devices that are light-weight, weavable, highly flexible, stretchable, and adaptable to comport to frequent deformations during usage in daily life. To this end, the development of 1D electrodes wit...

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Sensing mechanical tissue deformation in vivo can provide detailed information on organ functionality and tissue states. To bridge the huge mechanical mismatch between conventional electronics and biological tissues, stretchable electronic systems have recently been developed for interfacing tissues in healthcare applications. A major challenge for wireless electronic implants is that they typically require microchips, which adds complexity and may compromise long‐term stability. Here, a chipless wireless strain sensor technology based on a novel soft conductor with high cyclic stability is reported. The composite material consists of gold‐coated titanium dioxide nanowires embedded in a soft silicone elastomer. The implantable strain sensor is based on an resonant circuit which consists of a stretchable plate capacitor and a coil for inductive readout of its resonance frequency. Successful continuous wireless readout during 50% strain cycles is demonstrated. The sensor element has a Young's modulus of 260 kPa, similar to that of the bladder in order to not impair physiological bladder expansion. A proof‐of‐principle measurement on an ex vivo porcine bladder is presented, which shows the feasibility of the presented materials and devices for continuous, wireless strain monitoring of various tissues and organs in vivo.

Research on the field of implantable electronic devices that can be directly applied in the body with various functionalities is increasingly intensifying due to its great potential for various therapeutic applications. While conventional implantable electronics generally include rigid and hard conductive materials, their surrounding biological objects are soft and dynamic. The mechanical mismatch between implanted devices and biological environments induces damages in the body especially for long‐term applications. Stretchable electronics with outstanding mechanical compliance with biological objects effectively improve such limitations of existing rigid implantable electronics. In this article, the recent progress of implantable soft electronics based on various conductive nanocomposites is systematically described. In particular, representative fabrication approaches of conductive and stretchable nanocomposites for implantable soft electronics and various in vivo applications of implantable soft electronics are focused on. To conclude, challenges and perspectives of current implantable soft electronics that should be considered for further advances are discussed.

Cardiothoracic open-heart surgery has revolutionized the treatment of cardiovascular disease, the leading cause of death worldwide. After the surgery, hemodynamic and volume management can be complicated, for example in case of vasoplegia after endocarditis. Timely treatment is crucial for outcomes. Currently, treatment decisions are made based on heart volume, which needs to be measured manually by the clinician each time using ultrasound. Alternatively, implantable sensors offer a real-time window into the dynamic function of our body. Here it is shown that a soft flexible sensor, made with biocompatible materials, implanted on the surface of the heart, can provide continuous information of the heart volume after surgery. The sensor works robustly for a period of two days on a tensile machine. The accuracy of measuring heart volume is improved compared to the clinical gold standard in vivo, with an error of 7.1 mL for the strain sensor versus impedance and 14.0 mL versus ultrasound. Implanting such a sensor would provide essential, continuous information on heart volume in the critical time following the surgery, allowing early identification of complications, facilitating treatment, and hence potentially improving patient outcome.

In article number 1902532, Taeyoon Lee and co‐workers review the recent progress of stretchable fiber‐based electronic devices. They consider not only representative conductive materials and fabrication techniques for stretchable conductive fibers, but also designs and applications of various stretchable fiber‐based electronic devices, including mechanical sensors and energy devices.

Nanowires are often key ingredients of high-tech composite materials. The properties and performance of devices created using these, depend heavily on the structure and density of the embedded nanowires. Despite significant efforts, a process that can be adapted to different materials, compatible with current nanowire deposition methods, and that is able to control both variables simultaneously has not been achieved yet. In this work, we show that we can use low magnetic fields (80 mT) to manipulate nanowires by electrostatically coating them with superparamagnetic iron oxide nanoparticles in an aqueous solution. Monolayers, multilayers, and hierarchical structures of oriented nanowires were achieved in a highly ordered manner using vacuum filtration for two types of nanowires: silver and gold-coated titanium dioxide nanowires. The produced films were embedded in an elastomer, and the strain-dependent electrical properties of the resulting composites were investigated. The orientation of the assembly with respect to the tensile strain heavily impacts the performance of the composites. Composites containing nanowires perpendicular to the strain direction exhibit an extremely low gauge factor. On the other hand, when nanowires are arranged parallel to the strain direction, the composites have a high gauge factor. The possibility to orient nanowires during the processing steps is not only interesting for the shown strain sensing application but also expected to be useful in many other areas of material science.

Measurement of in vivo strain patterns of musculoskeletal soft tissues (MSTs) during functional activities reveals their biomechanical function, supports the identification and understanding of pathologies, and quantifies tissue adaptation during healing. These scientific and clinical insights have motivated the development and application of various strain sensors to quantify MST strains in either intraoperative or dynamic in vivo conditions. In this study, a strain sensor system is developed based on stretchable electronics and radio frequency identification technologies. In this system, a flexible inductor‐capacitor‐resistor sensor is fabricated such that it can be wirelessly excited by a custom‐designed readout box through electronic resonance. The resonant frequency of the sensor changes when the capacitor is stretched, which is then also recorded by the readout box at a sampling rate of 1024 Hz. Suturing the stretchable capacitor onto the MST allows it to be stretched in line with musculoskeletal deformations, hence providing an indirect method to assess strain patterns in vivo. Application of the system ex vivo indicates that the signal remains linear between 0 and 25% strain and is electronically stable in a simulated in vivo environment for one week and over 100 000 cycles of fatigue loadings. The strain sensor exhibits excellent resolution (0.1% strain, ≈9 µm) during wireless strain measurement. Finally, sensor implantation and strain measurement onto the medial gastrocnemius tendon of a sheep indicate that the sensor is able to record repetitive strain patterns in vivo during dynamic movements. This study indicates the potential scientific and clinical applicability in vivo.