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The ability to integrate complex electronic and optoelectronic functionalities within soft and thin fibers is one of today's key advanced manufacturing challenges. Multifunctional and connected fiber devices will be at the heart of the development of smart textiles and wearable devices. These devices also offer novel opportunities for surgical probes and tools, robotics and prostheses, communication systems, and portable energy harvesters. Among the various fiber‐processing methods, the preform‐to‐fiber thermal drawing technique is a very promising process that is used to fabricate multimaterial fibers with complex architectures at micro‐ and nanoscale feature sizes. Recently, a series of scientific and technological breakthroughs have significantly advanced the field of multimaterial fibers, allowing a wider range of functionalities, better performance, and novel applications. Here, these breakthroughs, in the fundamental understanding of the fluid dynamics, rheology, and tailoring of materials microstructures at play in the thermal drawing process, are presented and critically discussed. The impact of these advances on the research landscape in this field and how they offer significant new opportunities for this rapidly growing scientific and technological platform are also discussed.

Electronic and photonic fiber devices that can sustain large elastic deformation are becoming key components in a variety of fields ranging from healthcare to robotics and wearable devices. The fabrication of highly elastic and functional fibers remains however challenging, which is limiting their technological developments. Simple and scalable fiber-processing techniques to continuously codraw different materials within a polymeric structure constitute an ideal platform to realize functional fibers and devices. Despite decades of research however, elastomeric materials with the proper rheological attributes for multimaterial fiber processing cannot be identified. Here, the thermal drawing of hundreds-of-meters long multimaterial optical and electronic fibers and devices that can sustain up to 500% elastic deformation is demonstrated. From a rheological and microstructure analysis, thermoplastic elastomers that can be thermally drawn at high viscosities (above 10 Pa s), allowing the encapsulation of a variety of microstructured, soft, and rigid materials are identified. Using this scalable approach, fiber devices combining high performance, extreme elasticity, and unprecedented functionalities, allowing novel applications in smart textiles, robotics, or medical implants, are demonstrated.

Fibers are the building blocks of a broad spectrum of products from textiles to composites, and waveguides to wound dressings. While ubiquitous, the capabilities of fibers have not rapidly increased compared to semiconductor chip technology, for example. Recognizing that fibers lack the composition, geometry, and feature sizes for more functions, we set out years ago to explore the boundaries of fiber functionality. Our approach focused on a particular form of fiber production, thermal-drawing from a preform. This process has been used for producing single material fibers, but by combining metals, insulators, and semiconductors all within a single strand of fiber, an entire world of functionality in fibers has emerged. Fibers with optical, electrical, acoustic, or optoelectronic functionalities can be produced at scale from relatively easy-to-assemble macroscopic preforms. Two significant opportunities now present themselves. First, can we expect that fiber functions escalate in a predictable manner, creating the context for a "Moore's Law" analogue in fibers? Second, as fabrics occupy an enormous surface around our bodies, could fabrics offer valuable service to augment the human body? Towards answering these questions, we detail the materials, performance, and limitations of thermally-drawn fibers in different electronic applications and envision their potential in new fields.

imaging systems, [3][4][5][6][7] nanophotonics, [8] light generation and communication, [1] energy harvesting, [9][10][11] and advanced textiles. [12,13] To functionalize optical fibers, a first strategy relies on wafer-based techniques [8] and modified deposition [14,15] to integrate functional materials at the tip or within a few tens of centimeters of microstructured silica fibers. An alternative approach exploits the well-established thermal drawing of macroscopic preforms that integrate the desired multimaterial architecture. [1] This has the advantage of simplicity and scalability, since the fiber pulling step results in tens of kilometers of fibers that have the same crosssectional structure as the initial preform. Thus far however, realizing high-quality semiconducting materials that can act as high-performance photodetectors using the thermal drawing process remains a challenge. A recent strategy consists in the thermal drawing within silica fibers of high melting point materials such as silicon, [16] germanium, [17] or various compounds. [18] The melting and solidification during drawing result however in semiconductors with a highly polycrystalline microstructure, requiring local postdrawing annealing or laserbased steps to engineer a desired microstructure. [15,18,19] It is also difficult to integrate electrodes in contact with the semiconducting domains and despite an ingenious and promising method, no device with good optoelectronic properties has been shown. [17] The alternative strategy relies on exploiting the polymer fiber platform. It has several advantages compared to its silica counterpart, including low-temperature processing, robust mechanical properties, simple integration of electrodes, and the ability to impart fibers with complex architectures and multiple functionalities. [1] Thus far however, the postdrawing crystallization schemes applied to semiconducting chalcogenide glasses have resulted in poor control over the phase, grain size, and orientation, impairing device performance. [5,20] A powerful approach to control the microstructure and enhance optoelectronic performance is via the growth of welloriented semiconducting nanowires. This approach has not been exploited in the frame of fiber-integrated devices since conventional fabrication procedures of semiconducting nanowires are complex and only adapted to specific substrates seemingly incompatible with the thermal drawing process and the fiber materials and geometry. [8,21] Here, we demonstrate for the first time the robust and scalable integration of high-qualityThe recent ability to integrate semiconductor-based optoelectronic functionalities within thin fibers is opening intriguing opportunities for flexible electronics and advanced textiles. The scalable integration of high-quality semiconducting devices within functional fibers however remains a challenge. It is difficult with current strategies to combine high light absorption, good microstructure and efficient electrical contact. The growth of semiconducting nanowires is ...

Fibers that harvest mechanical energy via the triboelectric effect are excellent candidates as power sources for wearable electronics and functional textiles. Thus far however, their fabrication remains complex, and exhibited performances are below the state-of-the-art of 2D planar configurations, making them impractical. Here, we demonstrate the scalable fabrication of micro-structured stretchable triboelectric fibers with efficiencies on par with planar systems. We use the thermal drawing process to fabricate advanced elastomer fibers that combine a micro-textured surface with the integration of several liquid metal electrodes. Such fibers exhibit high electrical outputs regardless of repeated large deformations, and can sustain strains up to 560%. They can also be woven into deformable machine-washable textiles with high electrical outputs up to 490 V, 175 nC. In addition to energy harvesting, we demonstrate self-powered breathing monitoring and gesture sensing capabilities, making this triboelectric fiber platform an exciting avenue for multi-functional wearable systems and smart textiles.

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