Controlled fragmentation of multimaterial fibres and films via polymer cold-drawing

Biomimetic Mechanically Robust Chiroptical Hydrogel Enabled by Hierarchical Bouligand Structure Engineering

Natural bouligand structures enable crustacean exoskeletons and fruits to strike a combination of exceptional mechanical robustness and brilliant chiroptical properties owing to multiscale structural hierarchy. However, integrating such a high strength-stiffness-toughness combination and photonic functionalities into synthetic hydrogels still remains a grand challenge. In this work, we report a simple yet general biomimetic strategy to construct an ultrarobust chiroptical hydrogel by closely mimicking the natural bouligand structure at multilength scale. The hierarchical structural engineering of long-range ordered cellulose nanocrystals' bouligand structure, well-defined poly(vinyl alcohol) nanocrystalline domains, and dynamic interfacial interaction synergistically contributes to the integration of high strength (23.3 MPa), superior modulus (264 MPa), and high toughness (54.7 MJ m-3), as well as extraordinary impact resistance, which far exceed their natural counterparts and synthetic photonic hydrogels. More importantly, seamless chiroptical and solvent-responsive patterns with high resolution can also be scalably integrated into the hydrogel by localized manipulation of the photonic band, while maintaining good ionic conductivity. Such exceptional mechanical-photonic combination holds tremendous potential for applications in wearable sensors, encryption, displays, and soft robotics.

Fibration of powdery materials

Non-destructive processing of powders into macroscopic materials with a wealth of structural and functional possibilities has immeasurable scientific significance and application value, yet remains a challenge using conventional processing techniques. Here we developed a universal fibration method, using two-dimensional cellulose as a mediator, to process diverse powdered materials into micro-/nanofibres, which provides structural support to the particles and preserves their own specialties and architectures. It is found that the self-shrinking force drives the two-dimensional cellulose and supported particles to pucker and roll into fibres, a gentle process that prevents agglomeration and structural damage of the powder particles. We demonstrate over 120 fibre samples involving various powder guests, including elements, compounds, organics and hybrids in different morphologies, densities and particle sizes. Customized fibres with an adjustable diameter and guest content can be easily constructed into high-performance macromaterials with various geometries, creating a library of building blocks for different fields of applications. Our fibration strategy provides a universal, powerful and non-destructive pathway bridging primary particles and macroapplications.

Capping layer enabled controlled fragmentation of two-dimensional materials by cold drawing

Cold drawing, a well-established processing technique in the polymer industry, was recently revisited and discovered as an efficient material structuring method to create ordered patterns in composites consisting of both cold-drawable polymers and brittle target materials. Such a high-yield and low-cost manufacturing technique enables the large-scale fabrication of micro-ribbon structures for a wide range of functional materials, including two-dimensional (2D) layered materials. Compared to the abundant phenomenological results from experiments, however, the underlying mechanisms of this technique are not fully explored. Here, supported by experimental investigation, finite element calculations, and theoretical modeling, we systematically study the effect of a capping layer on the controlled fragmentation of 2D materials deposited on polymer substrates during the cold drawing. The capping layer is found to prevent the premature fracture of the 2D thin films during elastic deformation of the substrate, when a specific requirement proposed by the theoretical model is satisfied. Controlled fragmentation is enabled in the necking stage due to the protective effect of the capping layer, which also influences the size of the resulting fragments. Flexible and stretchable electrodes based on 2D material ribbons are fabricated to demonstrate the effectiveness of the proposed roadmap. This study gives an accurate understanding of interactions between 2D materials, polymer substrates, and capping layers during cold drawing, and offers guidance for potential applications such as flexible electronics.

Cold-programmed shape-morphing structures based on grayscale digital light processing 4D printing

Shape-morphing structures that can reconfigure their shape to adapt to diverse tasks are highly desirable for intelligent machines in many interdisciplinary fields. Shape memory polymers are one of the most widely used stimuli-responsive materials, especially in 3D/4D printing, for fabricating shape-morphing systems. They typically go through a hot-programming step to obtain the shape-morphing capability, which possesses limited freedom of reconfigurability. Cold-programming, which directly deforms the structure into a temporary shape without increasing the temperature, is simple and more versatile but has stringent requirements on material properties. Here, we introduce grayscale digital light processing (g-DLP) based 3D printing as a simple and effective platform for fabricating shape-morphing structures with cold-programming capabilities. With the multimaterial-like printing capability of g-DLP, we develop heterogeneous hinge modules that can be cold-programmed by simply stretching at room temperature. Different configurations can be encoded during 3D printing with the variable distribution and direction of the modular-designed hinges. The hinge module allows controllable independent morphing enabled by cold programming. By leveraging the multimaterial-like printing capability, multi-shape morphing structures are presented. The g-DLP printing with cold-programming morphing strategy demonstrates enormous potential in the design and fabrication of shape-morphing structures.

Optimization of thermoelectric properties of carbon nanotube veils by defect engineering

Carbon nanotubes (CNTs), with their combination of excellent electrical conductivity, Seebeck coefficient, mechanical robustness and environmental stability are highly desired as thermoelectric (TE) materials for a wide range of fields including Internet of Things, health monitoring and environmental remediation solutions. However, their high thermal conductivity (κ) is an obstacle to practical TE applications. Herein, we present a novel method to reduce the κ of CNT veils, by introducing defects, while preserving their Seebeck coefficient and electrical conductivity. Solid-state drawing of a CNT veil embedded within two polycarbonate films generates CNT veil fragments of reducing size with increasing draw ratio. A successive heat treatment, at above the polycarbonate glass-to-rubber transition temperature, spontaneously reconnects the CNT veils fragments electrically but not thermally. Stretching to a draw ratio of 1.5 and heat repairing at 170 °C leads to a dramatic 3.5-fold decrease in κ (from 46 to 13 W m-1 K-1), in contrast with a decrease in electrical conductivity of only 26% and an increase in Seebeck coefficient of 10%. To clarify the mechanism of reduction in thermal conductivity, a large-scale mesoscopic simulation of CNT veils under uniaxial stretching has also been used. This work shows that defect engineering can be a valuable strategy to optimize TE properties of CNT veils and, potentially, other thermoelectric materials.

Surface-Embedding of Mo Microparticles for Robust and Conductive Biodegradable Fiber Electrodes: Toward 1D Flexible Transient Electronics

Fiber-based implantable electronics are one of promising candidates for in vivo biomedical applications thanks to their unique structural advantages. However, development of fiber-based implantable electronic devices with biodegradable capability remains a challenge due to the lack of biodegradable fiber electrodes with high electrical and mechanical properties. Here, a biocompatible and biodegradable fiber electrode which simultaneously exhibits high electrical conductivity and mechanical robustness is presented. The fiber electrode is fabricated through a facile approach that incorporates a large amount of Mo microparticles into outermost volume of a biodegradable polycaprolactone (PCL) fiber scaffold in a concentrated manner. The biodegradable fiber electrode simultaneously exhibits a remarkable electrical performance (≈43.5 Ω cm^-1 ), mechanical robustness, bending stability, and durability for more than 4000 bending cycles based on the Mo/PCL conductive layer and intact PCL core in the fiber electrode. The electrical behavior of the biodegradable fiber electrode under the bending deformation is analyzed by an analytical prediction and a numerical simulation. In addition, the biocompatible properties and degradation behavior of the fiber electrode are systematically investigated. The potential of biodegradable fiber electrode is demonstrated in various applications such as an interconnect, a suturable temperature sensor, and an in vivo electrical stimulator.

Self-shrinking soft demoulding for complex high-aspect-ratio microchannels

Microchannels are the essential elements in animals, plants, and various artificial devices such as soft robotics, wearable sensors, and organs-on-a-chip. However, three-dimensional (3D) microchannels with complex geometry and a high aspect ratio remain challenging to generate by conventional methods such as soft lithography, template dissolution, and matrix swollen processes, although they are widespread in nature. Here, we propose a simple and solvent-free fabrication method capable of producing monolithic microchannels with complex 3D structures, long length, and small diameter. A soft template and a peeling-dominant template removal process are introduced to the demoulding process, which is referred to as soft demoulding here. In combination with thermal drawing technology, microchannels with a small diameter (10 µm), a high aspect ratio (6000, length-to-diameter), and intricate 3D geometries are generated. We demonstrate the vast applicability and significant impact of this technology in multiple scenarios, including soft robotics, wearable sensors, soft antennas, and artificial vessels.

Microchannels are the essential elements for the design of artificial devices but the fabrication of three dimensional (3D) microchannels with complex geometry and a high aspect ratio remains challenging. Here, the authors demonstrate a simple and solvent-free fabrication method capable of producing monolithic microchannels with complex 3D structures, long length, and small diameter.

Experimental and Numerical Study on the Protective Behavior of Weldox 900 E Steel Plates Impacted by Blunt-Nosed Projectiles

To demonstrate the importance of incorporating Lode angle into fracture criterion in predicting the penetration resistance of high-strength steel plates, ballistic tests of blunt-nosed projectiles with a diameter of 5.95 mm impacted 4 mm thick Weldox 900 E steel plates were conducted. Impacting velocity range was 136.63~381.42 m/s. The fracture behavior and the ballistic limit velocities (BLVs) were obtained by fitting the initial-residual velocities of the projectiles. Subsequently, axisymmetric finite element (FE) models parallel to the tests were built by using Abaqus/Explicit software, and the Lode-independent Johnson–Cook (JC) and the Lode-dependent ASCE fracture criterion were incorporated into the finite element model for numerical simulation. Meanwhile, to verify the sensitivity of the mesh size in the numerical simulation, different mesh sizes were used in the shear plug area of the target. It can be found that Weldox 900 E steel has obvious mesh size sensitivity by comparing the experimental results and numerical simulation, and the JC fracture criterion and the ASCE fracture criterion predicted similar BLV for the same mesh size.

Phototherapy and optical waveguides for the treatment of infection

With rapid emergence of multi-drug resistant microbes, it is imperative to seek alternative means for infection control. Optical waveguides are an auspicious delivery method for precise administration of phototherapy. Studies have shown that phototherapy is promising in fighting against a myriad of infectious pathogens (i.e. viruses, bacteria, fungi, and protozoa) including biofilm-forming species and drug-resistant strains while evading treatment resistance. When administered via optical waveguides, phototherapy can treat both superficial and deep-tissue infections while minimizing off-site effects that afflict conventional phototherapy and pharmacotherapy. Despite great therapeutic potential, exact mechanisms, materials, and fabrication designs to optimize this promising treatment option are underexplored. This review outlines principles and applications of phototherapy and optical waveguides for infection control. Research advances, challenges, and outlook regarding this delivery system are rigorously discussed in a hope to inspire future developments of optical waveguide-mediated phototherapy for the management of infection and beyond.

Surface Structured Polymer Blend Fibers and Their Application in Fiber Reinforced Composite

Melt-spun surface structured fiber could be a large-scale versatile platform for materials with advanced surface function and local properties. Fibers with distinct surface and bulk structures are developed by tailoring the viscosity ratio and blend ratio of polymer component using the melt-spinning method. Spherical bulge and fibril groove structured fibers are obtained in different viscosity ratio and blend ratio systems. The interfacial bonding between fiber and matrix is improved due to the mechanical interlocking between the structured surface and matrix. The low-viscosity second phase stays as a spherical droplet even in high content. The second phase in matched- and high-viscosity ratio cases is deformed into fibril like droplet which causes an in-situ fibration of the second phase in polymer blend fiber with an enhanced mechanical property. This method provides a simple route to developing polymer materials with surface structure and appropriate mechanical properties to apply in textile and polymer fiber-reinforced composite materials.

Hybrid Plasmonic Fiber-Optic Sensors

With the increasing demand of achieving comprehensive perception in every aspect of life, optical fibers have shown great potential in various applications due to their highly-sensitive, highly-integrated, flexible and real-time sensing capabilities. Among various sensing mechanisms, plasmonics based fiber-optic sensors provide remarkable sensitivity benefiting from their outstanding plasmon-matter interaction. Therefore, surface plasmon resonance (SPR) and localized SPR (LSPR)-based hybrid fiber-optic sensors have captured intensive research attention. Conventionally, SPR- or LSPR-based hybrid fiber-optic sensors rely on the resonant electron oscillations of thin metallic films or metallic nanoparticles functionalized on fiber surfaces. Coupled with the new advances in functional nanomaterials as well as fiber structure design and fabrication in recent years, new solutions continue to emerge to further improve the fiber-optic plasmonic sensors' performances in terms of sensitivity, specificity and biocompatibility. For instance, 2D materials like graphene can enhance the surface plasmon intensity at the metallic film surface due to the plasmon-matter interaction. Two-dimensional (2D) morphology of transition metal oxides can be doped with abundant free electrons to facilitate intrinsic plasmonics in visible or near-infrared frequencies, realizing exceptional field confinement and high sensitivity detection of analyte molecules. Gold nanoparticles capped with macrocyclic supramolecules show excellent selectivity to target biomolecules and ultralow limits of detection. Moreover, specially designed microstructured optical fibers are able to achieve high birefringence that can suppress the output inaccuracy induced by polarization crosstalk and meanwhile deliver promising sensitivity. This review aims to reveal and explore the frontiers of such hybrid plasmonic fiber-optic platforms in various sensing applications.

Robust DNA-Bridged Memristor for Textile Chips

Electronic textiles may revolutionize many fields, such as communication, health care and artificial intelligence. To date, unfortunately, computing with them is not yet possible. Memristors are compatible with the interwoven structure and manufacturing process in textiles because of its two-terminal crossbar configuration. However, it remains a challenge to realize textile memristors owing to the difficulties in designing advanced memristive materials and achieving high-quality active layers on fiber electrodes. Herein we report a robust textile memristor based on an electrophoretic-deposited active layer of deoxyribonucleic acid (DNA) on fiber electrodes. The unique architecture and orientation of DNA molecules with the incorporation of Ag nanoparticles offer the best-in-class performances, e.g., both ultra-low operation voltage of 0.3 V and power consumption of 100 pW and high switching speed of 20 ns. Fundamental logic calculations such as implication and NAND are demonstrated as functions of textile chips, and it has been thus integrated with power-supplying and light emitting modules to demonstrate an all-fabric information processing system.

Recent Progress and Perspectives of Thermally Drawn Multimaterial Fiber Electronics

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, exploration of the boundaries of fiber functionality began some years ago. The approach focuses 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 one expect that fiber functions escalate in a predictable manner, creating the context for a "Moore's Law" analog in fibers? Second, as fabrics occupy an enormous surface around the body, could fabrics offer a valuable service to augment the human body? Toward answering these questions, the materials, performance, and limitations of thermally drawn fibers in different electronic applications are detailed and their potential in new fields is envisioned.

In-fibre particle manipulation and device assembly via laser induced thermocapillary convection

The ability to manipulate in-fibre particles is of technological and scientific significance, yet particle manipulation inside solid environment remains fundamentally challenging. Here we show an accurately controlled, non-contact, size- and material-independent method for manipulating in-fibre particles based on laser-induced thermocapillary convection. The laser liquefaction process transforms the fibre from a solid media into an ideal fluid environment and triggers the in-fibre thermocapillary convection. In-fibre particles, with diameter from submicron to hundreds of microns, can be migrated toward the designated position. The number of particles being migrated, the particle migration velocity and direction can be precisely controlled. As a proof-of-concept, the laser-induced flow currents lead to the migration-to-contact of dislocated in-fibre p- and n-type semiconductor particles and the forming of dual-particle p-n homo- and heterojunction directly in a fibre. This approach not only enables in-fibre device assembly to achieve multi-component fibre devices, but also provide fundamental insight for in-solid particle manipulation.

Particle manipulation inside a fibre is challenging yet meaningful to achieve for fibre devices. Here, the authors demonstrate a method to manipulate in-fibre particles via laser induced thermocapillary convection, enabling the formation of semiconductor junctions directly in a fibre.

Optofluidic in-fiber integrated surface-enhanced Raman spectroscopy detection based on a hollow optical fiber with a suspended core

In this Letter, we propose, to the best of our knowledge, the first in-fiber optofluidic Raman surface-enhanced Raman spectroscopy (SERS) sensor based on a microstructured hollow fiber (MHF) with a suspended core. Taking advantage of the unique internal structure, we immobilize silver nanoparticles with an SERS effect in the MHF by chemical bonding. The Raman signal of the microfluidic sample is excited by the excitation light in the suspended core through an evanescent field. Then the online SERS signal can be coupled back into the core and detected. To demonstrate the feasibility of the device, rhodamine 6G is chosen as the analyte, and high-quality SERS spectra are detected with the limit of detection of 1×10^-14  M. Furthermore, an online optofluidic test is conducted on ceftriaxone (C₁₈H₁₈N₈O₇S₃) to examine its capabilities in antibiotic sensing. The results show that the LOD of the samples is 10^-6  M. Significantly, this Letter provides an integrated optofluidic in-fiber SERS sensor with a microchannel that can be integrated with chip devices without spatial optical coupling, which has a broad application in medicine and food safety, as well as various aspects of biological in-fiber sensing.

Macroscale Double Networks: Design Criteria for Optimizing Strength and Toughness

The double network concept, based on the fracture of sacrificial bonds, has been revolutionary toward the creation of robust soft materials. Based on the essence of double network hydrogels, macroscale, three-dimensional printed rigid sacrificial networks are embedded within silicone rubber stretchable matrices. Preferential fracture of the sacrificial network results in a ∼60 time increase in stiffness and a ∼50% increase in the work of extension compared with the neat matrix. Maximizing yield strength while maintaining multistep internal fracture occurs when the strength of the sacrificial network approaches the strength of the matrix. Upon determining the optimal sacrificial network strength, the sacrificial bond section density can be increased to maximize energy dissipation and toughening efficiencies up to ∼70% of the maximum theoretical toughness are achieved. High toughness and dissipation are achieved because topological interlocking enables significant force transmission to the sacrificial network at smaller length scales than interfacial adhesion, allowing much higher sacrificial bond density. This method is general and can be used with a variety of materials systems, without requiring strong interfacial adhesion, contrasting traditional composite systems. Demonstrating that the double network concept can be used at length scales far beyond the molecular scale will have important implications toward the development of future structural materials.

Strain-programmable fiber-based artificial muscle

Artificial muscles may accelerate the development of robotics, haptics, and prosthetics. Although advances in polymer-based actuators have delivered unprecedented strengths, producing these devices at scale with tunable dimensions remains a challenge. We applied a high-throughput iterative fiber-drawing technique to create strain-programmable artificial muscles with dimensions spanning three orders of magnitude. These fiber-based actuators are thermally and optically controllable, can lift more than 650 times their own weight, and withstand strains of >1000%. Integration of conductive nanowire meshes within these fiber-based muscles offers piezoresistive strain feedback and demonstrates long-term resilience across >105 deformation cycles. The scalable dimensions of these fiber-based actuators and their strength and responsiveness may extend their impact from engineering fields to biomedical applications.

Spinning and Applications of Bioinspired Fiber Systems

Natural fiber systems provide inspirations for artificial fiber spinning and applications. Through a long process of trial and error, great progress has been made in recent years. The natural fiber itself, especially silks, and the formation mechanism are better understood, and some of the essential factors are implemented in artificial spinning methods, benefiting from advanced manufacturing technologies. In addition, fiber-based materials produced via bioinspired spinning methods find an increasingly wide range of biomedical, optoelectronic, and environmental engineering applications. This paper reviews recent developments in the spinning and application of bioinspired fiber systems, introduces natural fiber and spinning processes and artificial spinning methods, and discusses applications of artificial fiber materials. Views on remaining challenges and the perspective on future trends are also proposed.

Polyethersulfone optical fibers with thermally induced microbubbles for custom side-scattering profiles

Polyethersulfone (PES) optical fibers are drawn and thermally processed in order to generate variable side-illumination profiles. The thermal treatment allows microbubbles to be formed in an outer layer of the PES fiber, providing light scattering with controllable amplitudes (0.25-2.5 cm^-1). Several fibers with different scattering profiles, such as uniform axial irradiation and multiple irradiation spots, are demonstrated. A small microbubble-induced scattering spot on the surface may be used for side-coupling of ambient light into the fiber. These mechanically flexible all-PES fibers with custom-designable scattering profiles may be useful for spatially tuned delivery of light for various applications, including phototherapy.

Toughening stretchable fibers via serial fracturing of a metallic core

Tough, biological materials (e.g., collagen or titin) protect tissues from irreversible damage caused by external loads. Mimicking these protective properties is important in packaging and in emerging applications such as durable electronic skins and soft robotics. This paper reports the formation of tough, metamaterial-like core-shell fibers that maintain stress at the fracture strength of a metal throughout the strain of an elastomer. The shell experiences localized strain enhancements that cause the higher modulus core to fracture repeatedly, increasing the energy dissipated during extension. Normally, fractures are catastrophic. However, in this architecture, the fractures are localized to the core. In addition to dissipating energy, the metallic core provides electrical conductivity and enables repair of the fractured core for repeated use. The fibers are 2.5 times tougher than titin and hold more than 15,000 times their own weight for a period 100 times longer than a hollow elastomeric fiber.

Touchpoint-Tailored Ultrasensitive Piezoresistive Pressure Sensors with a Broad Dynamic Response Range and Low Detection Limit

Wearable pressure sensors with high sensitivity, broad dynamic response range, and low detection limit are highly desirable to enable the applications in electronic skins and soft robotics. In this work, we report a high-performance wearable pressure sensor based on microstructured polydimethylsiloxane (PDMS)/Ag and rough polyimide/Au interdigital electrodes. By tailoring the touchpoints, the resulting pressure sensors show ultrahigh sensitivity (259.32 kPa^-1 in the range of 0-2.5 kPa), broad dynamic response range (0-54 kPa), fast response (∼200 μs), and low detection limit (0.36 Pa). Furthermore, the effect of different sensor structural configurations, PDMS geometrical feature, and Ag thickness on the performance of the pressure sensors are systematically investigated. Thanks to these merits, the fabricated pressure sensor is capable of real-time monitoring pulse wave and can act as artificial skin for robot hand to detect weak pressure changes, leading to the great application promise in the fields of biomedical, real-time health monitoring, and intelligent robot.

Ultraflexible Glassy Semiconductor Fibers for Thermal Sensing and Positioning

Flexible, large-area, and low-cost thermal sensing networks with high spatial and temporal resolution are of profound importance in addressing the increasing needs for industrial processing, medical diagnosis, and military defense. Here, a thermoelectric (TE) fiber is fabricated by thermally codrawing a macroscopic preform containing a semiconducting glass core and a polymer cladding to deliver thermal sensor functionalities at fiber-optic length scales, flexibility, and uniformity. The resulting TE fiber sensor operates in a wide temperature range with high thermal detection sensitivity and accuracy, while offering ultraflexibility with the bending curvature radius below 2.5 mm. Additionally, a single TE fiber can either sense the spot temperature variation or locate the heat/cold spot on the fiber. As a proof of concept, a two-dimensional 3 × 3 fiber array is woven into a textile to simultaneously detect the temperature distribution and the position of heat/cold source with the spatial resolution of millimeter. Achieving this may lead to the realization of large-area, flexible, and wearable temperature sensing fabrics for wearable electronics and advanced artificial intelligence applications.

Fusion and clustering of spherical micelles by extruding through a cylindrical channel

Experiments have shown that worm-like cylindrical micelles can be obtained by extruding spherical micelles through a cylindrical channel.

Advanced Multimaterial Electronic and Optoelectronic Fibers and Textiles

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.

Highly sensitive gas refractometers based on optical microfiber modal interferometers operating at dispersion turning point

In most fiber-optic gas sensing applications where the interested refractive index (RI) is ~1.0, the sensitivities are greatly constrained by the large mismatch between the effective RI of the guided mode and the RI of the surrounding gaseous medium. This fundamental challenge necessitates the development of a promising fiber-optic sensing mechanism with the outstanding RI sensitivity to achieve reliable remote gas sensors. In this work, we report a highly sensitive gas refractometer based on a tapered optical microfiber modal interferometer working at the dispersion turning point (DTP). First, we theoretically analyze the essential conditions to achieve the DTP, the spectral characteristics, and the sensing performance at the DTP. Results show that nonadiabatic tapered optical microfibers with diameters of 1.8-2.4 µm possess the DTPs in the near-infrared range and the RI sensitivities can be improved significantly around the DTPs. Second, we experimentally verify the ultrahigh RI sensitivity around the DTP using a nonadiabatic tapered optical microfiber with a waist diameter of ~2 μm. The experimental observations match well with the simulation results and our proposed gas refractometer provides an exceptional sensitivity as high as -69984.3 ± 2363.3 nm/RIU.

Light-Guiding Biomaterials for Biomedical Applications

Optical techniques used in medical diagnosis, surgery, and therapy require efficient and flexible delivery of light from light sources to target tissues. While this need is currently fulfilled by glass and plastic optical fibers, recent emergence of biointegrated approaches, such as optogenetics and implanted devices, call for novel waveguides with certain biophysical and biocompatible properties and desirable shapes beyond what the conventional optical fibers can offer. To this end, exploratory efforts have begun to harness various transparent biomaterials to develop waveguides that can serve existing applications better and enable new applications in future photomedicine. Here, we review the recent progress in this new area of research for developing biomaterial-based optical waveguides. We begin with a survey of biological light-guiding structures found in plants and animals, a source of inspiration for biomaterial photonics engineering. We describe natural and synthetic polymers and hydrogels that offer appropriate optical properties, biocompatibility, biodegradability, and mechanical flexibility have been exploited for light-guiding applications. Finally, we briefly discuss perspectives on biomedical applications that may benefit from the unique properties and functionalities of light-guiding biomaterials.

In situ TEM observation of rebonding on fractured silicon carbide

Silicon carbide (SiC) is widely used in harsh environments and under extreme conditions, including at high-power, high-temperature, high-current, high-voltage and high-frequency. The rebonding and self-matching of stack faults (SFs) is highly desirable to avoid catastrophic failure for SiC devices, especially for specific applications in the aerospace and nuclear power industries. In this study, a novel approach was developed using an eyebrow hair to pick up and transfer nanowires (NWs), in order to obtain in situ transmission electron microscope (TEM) images of the rebonding and self-matching of SFs at atomic resolution. During rebonding and healing, the electron beam was shut off. Rebonding on the fractured surfaces of monocrystalline and amorphous SiC NWs was observed by in situ TEM at room temperature. The fracture strength was 1.7 GPa after crack-healing, restoring 12.9% of that of a single crystal NW. Partial recrystallization along the <111> orientation and the self-matching of SFs are responsible for the rebonding of the monocrystalline NW. In comparison, the fracture strengths were 6.7 and 5.5 GPa for the first and second rebonding, respectively recovering 67% and 55% of that of an amorphous NW. Atomic diffusion contributed enormously to the rebonding on fractured surfaces of an amorphous NW, resulting in a healed surface consisting of an amorphous phase and crystallites. This rebonding function provides new insight into the fabrication of high-performance SiC devices for the aerospace, optoelectronic and semiconductor industries.

Highly Stretchable and Wearable Strain Sensor Based on Printable Carbon Nanotube Layers/Polydimethylsiloxane Composites with Adjustable Sensitivity

Strain sensors that are capable of monitoring complex human motions with high accuracy are highly desirable for developing wearable electronics. This paper reports the fabrication of highly stretchable and sensitive multidirectional strain sensors with tunable strain gauge factors by employing a digitally controlled printer to incorporate carbon nanotube (CNT) layers into polydimethylsiloxane (PDMS) substrates. The fabricated sensors exhibit a high stretchability (up to 45%) and sensitivity with a gauge factor of 35.75. The gauge factors could be easily modulated by tuning the number of CNT printing cycles to accommodate diverse requirements. The cyclic loading-unloading test results revealed that the composite strain sensors exhibited excellent long-term durability. Particularly, in this work, for the first time, human-motion-induced strain was measured by a motion capture system and compared with the strain data obtained from the fabricated strain sensors. The deviation of strains measured by composite sensors is less than 20%, indicating the great accuracy of CNT/PDMS sensors to quantify the amount of motion-induced strain. Of significant importance is that due to the flexibility of the printing technique used, rosette-type sensors were fabricated to simultaneously measure strains along multiple axes. These superior sensing capabilities of the fabricated CNT/PDMS strain sensors give them great application potential in motion-detecting systems.

Robust microfluidic construction of hybrid microfibers based on konjac glucomannan and their drug release performance

The exploration of methods to produce a novel wound dressing with sustained drug release properties in ultrasmall scales is of great scientific and technological interest. Herein, we propose konjac glucomannan/polyvinylidene fluoride (KGM/PVDF) hybrid microfibers having hydrophilic and hydrophobic segments based on microfluidic-oriented core–sheath composite microfibers, where the KGM/PVDF hybrid microfibers are wrapped in situ in CH₃OH. The morphology of KGM/PVDF microfibers is uniform, smooth, and crack-free. Enrofloxacin (Enro) is loaded onto the microfibers as a representative cargo to test their release performance. The KGM/PVDF/Enro microfibers show sustained drug release performance (13 days), excellent heat resistance, antibacterial activity and promotion of wound healing. This study is an avenue toward the microfluidic design of hydrophilic/hydrophobic hybrid microfibers as wound dressings, and it will guide the development of next-generation wound dressing.

The exploration of methods to produce a novel wound dressing with sustained drug release properties in ultrasmall scales is of great scientific and technological interest.

Uniaxially Stretched Flexible Surface Plasmon Resonance Film for Versatile Surface Enhanced Raman Scattering Diagnostics

Surface-enhanced Raman scattering (SERS) spectroscopy affords a rapid, highly sensitive, and nondestructive approach for label-free and fingerprint diagnosis of a wide range of chemicals. It is of great significance to develop large-area, uniform, and environmentally friendly SERS substrates for in situ identification of analytes on complex topological surfaces. In this work, we demonstrate a biodegradable flexible SERS film via irreversibly and longitudinally stretching metal deposited biocompatible poly(ε-caprolactone) film. This composite film after stretching shows surprising phenomena: three-dimensional and periodic wave-shaped microribbons array embedded with a high density of nanogaps functioning as hot-spots at an average gap size of 20 nm and nanogrooves array along the stretching direction. The stretched polymer surface plasmon resonance film gives rise to more than 10 times signal enhancement in comparison with that of the unstretched composite film. Furthermore, the SERS signals with high uniformity exhibit good temperature stability. The polymer SPR film with excellent flexibility and transparency can be conformally attached onto arbitrary nonplanar surfaces for in situ detection of various chemicals. Our results pave a new way for next-generation flexible SERS detection means, as well as enabling its huge potentials toward green wearable devices for point-of-care diagnostics.

Tapered nanoscale chalcogenide fibers directly drawn from bulk glasses as optical couplers for high-index resonators

We report production of air-clad tapered chalcogenide fibers by directly drawing bulk glasses between cleaved tips of tapered silica fibers. Exploiting these tapered fibers with nanoscale waists as evanescent optical couplers, we demonstrate phase-matched coupling of light into on-chip whispering gallery mode chalcogenide microresonators with coupling efficiencies as high as 95%. To the best of our knowledge, this is the first-time demonstration of critical coupling into high-index microresonators by using high-index tapered fibers. The tapered chalcogenide fibers can also be utilized as optical couplers for microresonators made of various high-index materials, as well as for nonlinear optical applications.

Lithography Assisted Fiber-Drawing Nanomanufacturing

We present a high-throughput and scalable technique for the production of metal nanowires embedded in glass fibres by taking advantage of thin film properties and patterning techniques commonly used in planar microfabrication. This hybrid process enables the fabrication of single nanowires and nanowire arrays encased in a preform material within a single fibre draw, providing an alternative to costly and time-consuming iterative fibre drawing. This method allows the combination of materials with different thermal properties to create functional optoelectronic nanostructures. As a proof of principle of the potential of this technique, centimetre long gold nanowires (bulk T_m = 1064 °C) embedded in silicate glass fibres (T_g = 567 °C) were drawn in a single step with high aspect ratios (>10⁴); such nanowires can be released from the glass matrix and show relatively high electrical conductivity. Overall, this fabrication method could enable mass manufacturing of metallic nanowires for plasmonics and nonlinear optics applications, as well as the integration of functional multimaterial structures for completely fiberised optoelectronic devices.