Microfluidic Fabrication of Bio-Inspired Microfibers with Controllable Magnetic Spindle-Knots for 3D Assembly and Water Collection

Bio-inspired hierarchical wearable patch for fast sweat collection

Sweat contains abundant physiological and metabolic data to evaluate an individual's physical health. Since the non-exercise sweat secretion rate is low, with an average value of 1-10 μl h-1 cm-2, sweat is generally collected during exercise for existing wearable sweat sensors. To expand their applications to include daily scenarios, these sensors developed for sports and fitness are challenged by the difficulty of collecting trace amounts of sweat. This study proposes a wearable patch inspired by the hierarchical structure of Sarracenia trichomes, allowing for the spontaneous and fast collection of a small amount of secreted sweat. The patch contains microfluidic channels featuring a 20 μm-wide rib structure, fully utilizing the capillary force, thereby eliminating the issue of sweat hysteresis. Furthermore, with only 0.5 μl of the sweat secreted at the collection site, it can converge on the detection medium located within the center reservoir. Volunteer verification demonstrated a twofold increase in sweat collection efficiency compared to traditional wearable patches. This patch serves as an efficient sweat-collection configuration, promising potential for diverse in situ sweat colorimetric analyses.

Three-Dimensional Printable Magnetic Microfibers: Development and Characterization for Four-Dimensional Printing

This study proposes a novel and simple fabrication method of magnetic microfibers, employing filament stretching three-dimensional (3D) printing, and demonstrates the capacity of four-dimensional (4D) printing of the proposed magnetic microfibers. A ferromagnetic 3D printing filament is prepared by the mixture of neodymium-iron-boron (NdFeB) and polylactic acid (PLA), and we investigate the characteristics of the ferromagnetic filament by mixing ratio, magnetic properties, mechanical properties, and rheological properties through experiments. By thermal extrusion of the ferromagnetic filament through a 3D printer nozzle, various thicknesses (80-500 μm) and lengths (less than ∼5 cm) of ferromagnetic microfibers are achieved with different printing setups, such as filament extrusion amount and printing speed. The printed ferromagnetic microfibers are magnetized to maintain a permanent magnetic dipole moment, and 4D printing can be achieved by the deformations of the permanently magnetized microfibers under magnetic fields. We observe that the mixing ratio, the thickness, and the length of the magnetized microfibers provide distinct deformation of the microfiber for customization of 4D printings. This study exhibits that the permanently magnetized microfibers have a great potential for smart sensors and actuators. Furthermore, we briefly present an application of our proposed magnetic microfibers for bionic motion actuators with various unique undulating and oscillating motions.

Designing of anisotropic gradient surfaces for directional liquid transport: Fundamentals, construction, and applications

Many biological surfaces are capable of transporting liquids in a directional manner without energy consumption. Inspired by nature, constructing asymmetric gradient surfaces to achieve desired droplet transport, such as a liquid diode, brings an incredibly valuable and promising area of research with a wide range of applications. Enabled by advances in nanotechnology and manufacturing techniques, biomimetics has emerged as a promising avenue for engineering various types of anisotropic material system. Over the past few decades, this approach has yielded significant progress in both fundamental understanding and practical applications. Theoretical studies revealed that the heterogeneous composition and topography mainly govern the wetting mechanisms and dynamics behavior of droplets, including the interdisciplinary aspects of materials, chemistry, and physics. In this review, we provide a concise overview of various biological surfaces that exhibit anisotropic droplet transport. We discussed the theoretical foundations and mechanisms of droplet motion on designed surfaces and reviewed recent research advances in droplet directional transport on designed plane surfaces and Janus membranes. Such liquid-diode materials yield diverse promising applications, involving droplet collection, liquid separation and delivery, functional textiles, and biomedical applications. We also discuss the recent challenges and ongoing approaches to enhance the functionality and application performance of anisotropic materials.

Microfluidic Approaches for Microactuators: From Fabrication, Actuation, to Functionalization

Microactuators can autonomously convert external energy into specific mechanical motions. With the feature sizes varying from the micrometer to millimeter scale, microactuators offer many operation and control possibilities for miniaturized devices. In recent years, advanced microfluidic techniques have revolutionized the fabrication, actuation, and functionalization of microactuators. Microfluidics can not only facilitate fabrication with continuously changing materials but also deliver various signals to stimulate the microactuators as desired, and consequently improve microfluidic chips with multiple functions. Herein, this cross-field that systematically correlates microactuator properties and microfluidic functions is comprehensively reviewed. The fabrication strategies are classified into two types according to the flow state of the microfluids: stop-flow and continuous-flow prototyping. The working mechanism of microactuators in microfluidic chips is discussed in detail. Finally, the applications of microactuator-enriched functional chips, which include tunable imaging devices, micromanipulation tools, micromotors, and microsensors, are summarized. The existing challenges and future perspectives are also discussed. It is believed that with the rapid progress of this cutting-edge field, intelligent microsystems may realize high-throughput manipulation, characterization, and analysis of tiny objects and find broad applications in various fields, such as tissue engineering, micro/nanorobotics, and analytical devices.

Microfiber Fabricated via Microfluidic Spinning toward Tissue Engineering Applications

Microfibers, a type of long, thin, and flexible material, can be assembled into functional 3D structures by folding, binding, and weaving. As a novel spinning method, combining microfluidic technology and wet spinning, microfluidic spinning technology can precisely control the size, morphology, structure, and composition of the microfibers. Particularly, the process is mild and rapid, which is suitable for preparing microfibers using biocompatible materials and without affecting the viability of cells encapsulated. Furthermore, owing to the controllability of microfluidic spinning, microfibers with well-defined structures (such as hollow structures) will contribute to the exchange of nutrients or guide cell orientation. Thus, this method is often used to fabricate microfibers as cell scaffolds for cell encapsulation or adhesion and can be further applied to biomimetic fibrous tissues. In this review, the focus is on different fiber structures prepared by microfluidic spinning technology, including solid, hollow, and heterogeneous structures, generated from three essential elements: spinning platform, fiber composition, and solidification methods. Furthermore, the application of microfibers is described with different structures in tissue engineering, such as blood vessels, skeletal muscle, bone, nerves, and lung bronchi. Finally, the challenges and future development prospects of microfluidic spinning technology in tissue engineering applications are discussed.

Recent Trends of Microfluidics in Food Science and Technology: Fabrications and Applications

The development of novel materials with microstructures is now a trend in food science and technology. These microscale materials may be applied across all steps in food manufacturing, from raw materials to the final food products, as well as in the packaging, transport, and storage processes. Microfluidics is an advanced technology for controlling fluids in a microscale channel (1~100 μm), which integrates engineering, physics, chemistry, nanotechnology, etc. This technology allows unit operations to occur in devices that are closer in size to the expected structural elements. Therefore, microfluidics is considered a promising technology to develop micro/nanostructures for delivery purposes to improve the quality and safety of foods. This review concentrates on the recent developments of microfluidic systems and their novel applications in food science and technology, including microfibers/films via microfluidic spinning technology for food packaging, droplet microfluidics for food micro-/nanoemulsifications and encapsulations, etc.

Adhesion behaviors of water droplets on bioinspired superhydrophobic surfaces

The adhesion behaviors of droplets on surfaces are attracting increasing attention due to their various applications. Many bioinspired superhydrophobic surfaces with different adhesion states have been constructed in order to mimic the functions of natural surfaces such as a lotus leaf, a rose petal, butterfly wings, etc. In this review, we first present a brief introduction to the fundamental theories of the adhesion behaviors of droplets on various surfaces, including low adhesion, high adhesion and anisotropic adhesion states. Then, different techniques to characterize droplet adhesion on these surfaces, including the rotating disk technique, the atomic force microscope cantilever technique, and capillary sensor-based techniques, are described. Wetting behaviors, and the switching between different adhesion states on bioinspired surfaces, are also summarized and discussed. Subsequently, the diverse applications of bioinspired surfaces, including water collection, liquid transport, drag reduction, and oil/water separation, are discussed. Finally, the challenges of using liquid adhesion behaviors on various surfaces, and future applications of these surfaces, are discussed.

Spider Silk-Inspired Artificial Fibers

Spider silk is a natural polymeric fiber with high tensile strength, toughness, and has distinct thermal, optical, and biocompatible properties. The mechanical properties of spider silk are ascribed to its hierarchical structure, including primary and secondary structures of the spidroins (spider silk proteins), the nanofibril, the "core-shell", and the "nano-fishnet" structures. In addition, spider silk also exhibits remarkable properties regarding humidity/water response, water collection, light transmission, thermal conductance, and shape-memory effect. This motivates researchers to prepare artificial functional fibers mimicking spider silk. In this review, the authors summarize the study of the structure and properties of natural spider silk, and the biomimetic preparation of artificial fibers from different types of molecules and polymers by taking some examples of artificial fibers exhibiting these interesting properties. In conclusion, biomimetic studies have yielded several noteworthy findings in artificial fibers with different functions, and this review aims to provide indications for biomimetic studies of functional fibers that approach and exceed the properties of natural spider silk.

Microfluidics-Enabled Soft Manufacture of Materials with Tailorable Wettability

Microfluidics and wettability are interrelated and mutually reinforcing fields, experiencing synergistic growth. Surface wettability is paramount in regulating microfluidic flows for processing and manipulating fluids at the microscale. Microfluidics, in turn, has emerged as a versatile platform for tailoring the wettability of materials. We present a critical review on the microfluidics-enabled soft manufacture (MESM) of materials with well-controlled wettability and their multidisciplinary applications. Microfluidics provides a variety of liquid templates for engineering materials with exquisite composition and morphology, laying the foundation for precisely controlling the wettability. Depending on the degree of ordering, liquid templates are divided into individual droplets, one-dimensional (1D) arrays, and two-dimensional (2D) or three-dimensional (3D) assemblies for the modular fabrication of microparticles, microfibers, and monolithic porous materials, respectively. Future exploration of MESM will enrich the diversity of chemical composition and physical structure for wettability control and thus markedly broaden the application horizons across engineering, physics, chemistry, biology, and medicine. This review aims to systematize this emerging yet robust technology, with the hope of aiding the realization of its full potential.

Three Polymers from the Sea: Unique Structures, Directional Modifications, and Medical Applications

With the increase of wounds and body damage, the clinical demand for antibacterial, hemostatic, and repairable biomaterials is increasing. Various types of biomedical materials have become research hotspots. Of these, and among materials derived from marine organisms, the research and application of alginate, chitosan, and collagen are the most common. Chitosan is mainly used as a hemostatic material in clinical applications, but due to problems such as the poor mechanical strength of a single component, the general antibacterial ability, and fast degradation speed research into the extraction process and modification mainly focuses on the improvement of the above-mentioned ability. Similarly, the research and modification of sodium alginate, used as a material for hemostasis and the repair of wounds, is mainly focused on the improvement of cell adhesion, hydrophilicity, degradation speed, mechanical properties, etc.; therefore, there are fewer marine biological collagen products. The research mainly focuses on immunogenicity removal and mechanical performance improvement. This article summarizes the source, molecular structure, and characteristics of alginate, chitosan, and collagen from marine organisms; and introduces the biological safety, clinical efficacy, and mechanism of action of these materials, as well as their extraction processes and material properties. Their modification and other issues are also discussed, and their potential clinical applications are examined.

Wettability Difference Induced Out-of-Plane Unidirectional Droplet Transport for Efficient Fog Harvesting

Securing freshwater resources is a global issue for ensuring sustainable development. Fog harvesting is attracting great attention as a method to collect water without any energy input. Previous reports that were inspired by insects and plants have given insights such as the effectiveness of in-plane wettability and structural differences for droplet transport, which might enhance artificial water harvesting efficiency. However, further efforts to transfer droplets while maintaining performance are needed because droplet motion owing to these effects is limited to the in-plane direction. In this study, we report droplet transport between three-dimensional copper wire structures with nanostructured hydrophobic and superhydrophilic features. This mechanism enhanced the fog harvesting capability by more than 20% compared with the cumulative value of individual wires. In addition, the relationship between the droplet height and spacing of wires affected the performance. Our results show the importance of out-of-plane directional droplet transport from the wire surface assisted by differences in wire wettability, which minimizes limiting factors of fog harvesting including clogging and droplet shedding. Furthermore, the proposed arrangement reduces the overall system width compared with that of a two-dimensional arrangement while maintaining the amount of harvested water. These results provide a promising approach to designing large-scale and highly efficient fog harvesters.

Bioinspired Fibers with Controlled Wettability: From Spinning to Application

Our knowledge on spider silks shows the importance of joining heterogeneous structures and surface chemical compositions in preparing fibers, fibrous surfaces, and 3D materials with a controllable wettability. We start our review with spider silk and proceed to the historical development of nature-inspired spinning processes, their products, and their advantages and disadvantages. Relevant wetting states are then summarized in fiber-based systems. Recent applications are reviewed, including one-dimensional spindle-knotted fibers for highly efficient fog harvesting, long-distance transport, and stimulus-responsive wettability and two-dimensional spindle-knotted fibrous systems for water collection, functional surfaces, and filtration. Finally, we offer some perspective on future research trends regarding biomimetic fibers for wetting-controlled engineering.

Review of microfluidic approaches for fabricating intelligent fiber devices: importance of shape characteristics

Shape characteristics, which include the physical dimensions (scale), apparent morphology, surface features, and structure, are essential factors of fibrous materials and determine many of their properties. Microfluidic technologies have recently been proposed as an approach for producing one-dimensional (1D) fibers with controllable shape characteristics and particle alignment, which impart specific functionality to the fiber. Moreover, superfine 1D fibers with a high surface area and ordered structure have many potential applications as they can be directly braided or woven into textiles, clothes, and tissues with two- or three-dimensional (2D or 3D) structures. Previous reviews of microfluidic spinning have not focus on the importance of the shape characteristic on fiber performance and their use in intelligent fiber design. Here, the latest achievements in microfluidic approaches for fiber-device fabrication are reviewed considering the underlying preparation principles, shape characteristics, and functionalization of the fibers. Additionally, intelligent fiber devices with shapes tailored by microfluidic approaches are discussed, including 1D sensors and actuators, luminous fibers, and devices for encoding, energy harvesting, water collection, and tissue engineering applications. Finally, recent progress, challenges, and future perspectives of the microfluidic approaches for fiber device fabrication are discussed.

Controllable microfluidic fabrication of microstructured functional materials

Microstructured functional materials such as microfibers and microparticles are widely used for a myriad of applications. Precise manipulation of the functional components and structure is important for the microstructured functional materials to achieve desired functions for advanced application. This review highlights the recent progress on the controllable microfluidic fabrication of microstructured functional materials from liquid templates. First, microfluidic strategies for controllable generation of liquid templates including laminar jets and emulsion droplets are introduced. Then, strategies for fabricating microfibers and microparticles with diverse structures and advanced functions from the liquid templates are highlighted. These strategies mainly focus on precisely engineering the functional components and microstructures of the microfibers and microparticles by tailoring those of their liquid templates to achieve desired advanced functions. Finally, future development of microfluidic techniques for industrial-scale production of the microstructured functional materials is discussed.

Hourglass-Shaped Microfibers

Heterotypic microfibers have been recognized as promising building blocks for the multifunctionality demanded in various fields, such as environmental and biomedical engineering. We present a novel microfluidics-based technique to generate bio-inspired microfibers with hourglass-shaped knots (named hourglass-shaped microfibers) via the integration of a non-solvent-induced phase separation (NIPS) process. The microfibers with spindle knots (named spindle-microfibers) are generated as templates at a large scale. The morphologies of spindle-microfibers can be precisely regulated by controlling the flow rates of the constituent fluids. After post-treatment of the partially gelled spindle-microfibers in ethanol, the encapsulated oil cores leak from knots, and the fibers morph into an hourglass shape. By controlling the oil core spillage and the template's configurations, a variety of hourglass-shaped microfibers can be obtained with adjustable morphologies and densities ranging from those of cavity-microfibers to those of spindle-microfibers. The hourglass-shaped microfibers preponderate spindle-microfibers in terms of changeable weight, adjustable morphologies, high specific surface areas, and enhanced surface roughness. Their unique macroscale topographies and properties lead to enhanced dehumidification and water collection abilities. This NIPS-integrated microfluidic technique offers a promising and novel way to manufacture microfibers by design, tailoring their structures and properties to suit a desired application.

Bioinspired Nanofibril-Humped Fibers with Strong Capillary Channels for Fog Capture

Bioinspired nanofibril-humped fibers (BNFs) are fabricated by using thermoplastic polyester elastomer and chitosan, via combining the electrospinning technique and fluid coating method to achieve periodic humps composed of interlaced random nanofibrils and a joint composed of aligned nanofibrils, which are highly similar to the micro/nanostructures of wetted spider silk. Especially, nanofibrils can increase the specific area of the hump to capture fog droplets effectively and transport water in channels between the nanofibrils under humid conditions, and thus the fog droplets can coalesce and be highly efficiently transported toward humps for water collection directionally. Such an ability of highly efficient fog capture is attributed to cooperation of an efficient transportation inside the outer shell of BNFs and outside transportation. Inside transportation is induced by anisotropic capillary channels between nanofibrils. When BNFs are wetted, the inside transportation mode is dominated for water collection, induced by anisotropic capillary channels between nanofibrils. BNF web is also used to investigate the droplet transportation in different cross-fiber contact modes in the process of fog capture on a large scale. This study offers an insight into the design of novel materials, which is expected to be developed for some realms of applications, such as fog harvesting engineering, filtration, and others.

Engineering of Hydrogel Materials with Perfusable Microchannels for Building Vascularized Tissues

Vascular systems are responsible for various physiological and pathological processes related to all organs in vivo, and the survival of engineered tissues for enough nutrient supply in vitro. Thus, biomimetic vascularization is highly needed for constructing both a biomimetic organ model and a reliable engineered tissue. However, many challenges remain in constructing vascularized tissues, requiring the combination of suitable biomaterials and engineering techniques. In this review, the advantages of hydrogels on building engineered vascularized tissues are discussed and recent engineering techniques for building perfusable microchannels in hydrogels are summarized, including micromolding, 3D printing, and microfluidic spinning. Furthermore, the applications of these perfusable hydrogels in manufacturing organ-on-a-chip devices and transplantable engineered tissues are highlighted. Finally, current challenges in recapitulating the complexity of native vascular systems are discussed and future development of vascularized tissues is prospected.

Nematic Liquid-Crystal Necklace Structure Made by Microfluidics System

We report a necklace structure made of liquid crystal dispersed in poly(vinyl alcohol) (PVA) aqueous solution, which is fabricated by a microfluidic device. In the necklace structure, liquid crystal droplets that are tens of micrometers in diameter are connected by microtethers, which are birefringent, are not penetrating the droplets, and can be elastically stretched by applying external force. The necklace structure was analyzed by fluorescent confocal microscopy, and the tethers were made of liquid crystal and PVA composite. The elastic constant of the tether was determined by using laser tweezers to stretch the tether. The Whispering Gallery Modes circulating inside individual droplets in the necklace structure were also observed.

Water Harvesting of Bioinspired Microfibers with Rough Spindle-Knots from Microfluidics

Heterostructure rough spindle-knot microfibers (HRSFs) are fabricated via a flexible parallel-nozzle microfluidic method. In this method, the bioinspired HRSF with a roughness gradient between spindle-knots and joints, can be manufactured in large-scale, and with which the size of the spindle-knots and joints can be precisely adjusted by regulating flow rates. The HRSFs, fabricated with chitosan and calcium alginate, have strong mechanical properties and corrosion resistance in acid environment (pH = 5) and alkaline environment (pH = 9), respectively. More attractively, under controlled treatment conditions, the morphology of the spindle-knots on the HRSFs can be effectively managed by changing the composite content of calcium chloride in the fluid. During the water collection process, tiny droplets of moisture can be captured on the surface of the HRSFs, subsequently, the droplets can coalesce and be transported from joint to spindle-knot sections. It is demonstrated that the surface morphology of spindle-knots directly influences the water collection efficiency, where a higher roughness gradient generates higher water collection efficiency. This parallel-nozzle microfluidic technology provides a low-cost and flexible method to manufacture high biocompatibility bioinspired rough spindle-knot microfibers, which has many potential applications in large-scale water collection, sustained drug release, and directional water collection.

Fog Collection on a Bio-inspired Topological Alloy Net with Micro-/Nanostructures

Because of the scarcity of freshwater resources, fog collection as one of the effective methods to solve this issue has attracted widespread concern. Inspired by several natural creatures with the capability to collect water from fog, the bio-inspired water-harvesting materials have aroused considerable attention and been widely developed. Inspired by the directional water droplets transportation to the apex on both shorebirds beaks and wheat awns, the bio-inspired topological alloy net with a V-shaped asymmetric geometry in its mesh was designed for fog collecting. Then, micro-/nano-hierarchical structures were modified on the surface of the netting wire via the cathodic electrodeposition method. Thus, the bio-inspired topological alloy net with micro/nanostructures was fabricated successfully. Through the integration of topological geometry and surface microstructure, not only the water-collection rate is improved by efficient drainage along the designated pathways, but also the issue of mesh clogging is resolved. In addition, a theoretical model was constructed to reveal the mechanism, especially the resultant force arising from the V-shaped structure. This work provides insight into the development of novel fog-collecting materials, which has potential applications in other fields, such as liquid transportation, microfluidics, and interface science.

Microfluidic Approach to Generate a Tadpole-Egg-Shaped Alginate Fiber and Its Application in Tissue Engineering

Herein, we introduce a facile microfluidic technique to produce a hybrid alginate fiber with a tadpole-egg shape. A triple-flow polydimethylsiloxane microfluidic device was constructed to allow the formation of oil droplets inside the alginate stream and was instantaneously gelated with the coaxially adjacent CaCl₂. The fiber entrapping the uniform oil droplets was dehydrated, leading to the formation of a distinct tadpole-egg-shaped structure. A series of diverse fiber architectures was fabricated in a controlled manner based on the flow rates of the relevant flows. The tadpole-egg-shaped alginate fibers were employed as building blocks to create a three-dimensional microwell template for cell cultures. First, the tadpole-egg-shaped alginate fibers containing the oil droplets were half-dipped into a melted agarose solution. After the solidification of the agarose gel, the alginate fibers were degraded by an ethylenediaminetetraacetic acid (EDTA) solution to generate the hemispherical microwells. Mesenchymal stem cells (MSCs) were cultured in the microwells to generate spheroids, which were induced into chondrocytes using transforming growth factor-β3. The formed MSC spheroids exhibited a relatively high ratio of cell viability with more than 95% live cells after 14 days of culture. The success of the chondrogenic differentiation was proven based on staining (Safranin O) and the glycosaminoglycan levels. The latter was significantly higher in spheroids that were induced to form chondrocytes compared to those that were not induced after 21 days of differentiation. Second, we investigated the potential of the tadpole-egg-shaped alginate fibers as microcarriers for applications in drug delivery and implantable technologies. It was revealed that the degradation of the Ca-alginate wall of the hybrid fibers to release the oil droplets required an EDTA solution with a concentration of 500 mM for a 15 min period. This result can be used to further develop the tadpole-egg-shaped alginate fibers as uniform microcarriers with multiple compartments.

Multifunctional Micro/Nanoscale Fibers Based on Microfluidic Spinning Technology

Superfine multifunctional micro/nanoscale fibrous materials with high surface area and ordered structure have attracted intensive attention for widespread applications in recent years. Microfluidic spinning technology (MST) has emerged as a powerful and versatile platform because of its various advantages such as high surface-area-to-volume ratio, effective heat transfer, and enhanced reaction rate. The resultant well-defined micro/nanoscale fibers exhibit controllable compositions, advanced structures, and new physical/chemical properties. The latest developments and achievements in microfluidic spun fiber materials are summarized in terms of the underlying preparation principles, geometric configurations, and functionalization. Variously architected structures and shapes by MST, including cylindrical, grooved, flat, anisotropic, hollow, core-shell, Janus, heterogeneous, helical, and knotted fibers, are emphasized. In particular, fiber-spinning chemistry in MST for achieving functionalization of fiber materials by in situ chemical reactions inside fibers is introduced. Additionally, the applications of the fabricated functional fibers are highlighted in sensors, microactuators, photoelectric devices, flexible electronics, tissue engineering, drug delivery, and water collection. Finally, recent progress, challenges, and future perspectives are discussed.

Hierarchical fibers for water collection inspired by spider silk

Water scarcity plagues two-thirds of the global population. Interestingly, researchers have found that spider silk exhibits excellent water-collection ability owing to its unique structure and chemical components. Based on this characteristic, numerous bioinspired fibers have been fabricated for water collection. Herein, we review the water-collection process for spider silk and recent vital advances in bioinspired fibriform materials, focusing on the water-collection mechanisms of spindle-knot fibers, which exhibit directional droplet transport, hanging mechanism and hanging ability. Also, we evaluated their water-collection abilities on a micro- and macro-scale, which gave a better view for the design of bioinspired water-collection materials. These advances enable the significant use of bioinspired fibers in water collection, which may be applied in several other fields, such as directional transport, tissue engineering, oil-water separation and biosensors.

Engineered tissue micro-rings fabricated from aggregated fibroblasts and microfibres for a bottom-up tissue engineering approach

Tissue rings with incorporated microscaffolds have been engineered as promising building blocks for constructing biological tubes from the bottom up. However, the microscaffolds available for incorporation are very limited at present. In this paper we provide an efficient strategy to first incorporate microfluidic spun Ca-alginate microfibres encapsulating magnetic nanoparticles into self-assembled fibroblast micro-rings. Based on the surface modification, microfibres with a size of ∼40 μm allowed fibroblasts to spread and proliferate along the long axis. The optimal cell seeding density was obtained by evaluating the degree of coverage of fibroblasts on microfibres after 3 days of culture. Then we designed a magnetically guided culture apparatus with multiple annular micro-wells to facilitate cell-driven assembly of microfibres. A manipulation strategy dependent on surface tension was used to pattern microfibres along the micro-wells prior to cell seeding, and magnetic attraction further kept the patterned microfibres from being deposited in the micro-wells during cultivation. Within 3 days of culture, microfibre-incorporated tissue micro-rings were formed in the micro-wells. Quantitative analysis of the formation process revealed liquid-like aggregating behaviours, and incorporated microfibres showed the potential to promote the directed organization of cells in tissue micro-rings. Furthermore, magnetically driven manipulation was used robotically to assemble the micro-rings on a micropillar inserted into the centre of the culture apparatus. After 5 days of culture to allow cell fusion, a biological tubular microstructure was achieved. Microfluidic spinning can generate fibres with a variety of shapes, geometries, and compositions; therefore, our proposed method greatly enriches the variety of microscaffolds available for incorporation into tissue rings to engineer complex artificial organs for tissue engineering and regenerative medicine.

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.

Transformable bubble-filled alginate microfibers via vertical microfluidics

A novel buoyancy-assisted vertical microfluidic setup has been developed to fabricate a new class of transformable bubble-filled hydrogel microfibers. A co-axial flow of an aqueous sodium-alginate solution enveloping an air phase was injected into a quiescent aqueous CaCl2 solution, through a vertically-oriented co-axial glass-capillary setup. This induced instantaneous gelation and produced bubble-filled calcium-alginate fibers. The surface-morphology of the resulting fibers was controlled from smooth to wavy by slowing down the gelation kinetics. The advantage of the buoyancy force acting on the fibers by the trapped air bubbles was taken not only to shape the fibers, but to transform them into several other novel hydrogel structures, such as water-filled segmented fibers, beaded microfibers, and threaded capsules. The ultimate transformability was demonstrated by the fibers being allowed to elongate and then undergo controlled destruction to produce uniform anisotropic micro-particles with a wide range of sizes and shapes from frustums to barrel and cylindrical types.

Fog Harvesting of a Bioinspired Nanocone-Decorated 3D Fiber Network

The bioinspired nanocone-decorated three-dimensional fiber network (N3D) can be fabricated, where an original 3D web is designed, inspired by some newest research findings of spider web, and it is decorated with hydrophilic zinc oxide (ZnO) nanocones inspired by cactus spine. Multilevel high specific surface area exposure on fiber together with the hydrophilic decoration enables it to be more attractive to water molecules. These nanocones can capture fog droplet, generate coalesced droplet, and accordingly make droplet transport efficient because of Laplace pressure difference. Especially, a novel mechanism revealed that after the nanocone-decorated fiber was wetted, that is, a water film formed and immediately broke up into droplets, owing to the force relating to Rayleigh instability. Consequent lower retention surface realizes the formation of fast continuous water flow, rather than the traditional intermittent course. Thus, outstanding fog-harvesting efficiency was achieved on N3D, for example, probably reaching 865.1 kg/m²/day, where the mass of collected water within 2 h can raise up to over 240 times higher than the weight of an original 3D web without nanocones. Such a bioinspired ZnO nanocone-decorated 3D fiber network (i.e., N3D) has potential application to harvest fog water for production or living, for example, water recondensation in cooling water towers and in agricultural irrigation systems, even in water-deficient countries.

Electric Field-Induced Cutting of Hydrogel Microfibers with Precise Length Control for Micromotors and Building Blocks

Microfiber modules with controllable lengths emerged as novel biomimetic platforms and are significant for many tissue engineering applications. However, accurately controlling the length of microfibers on the scale of millimeter or even micrometer still remains challenging. Here, a novel and scalable strategy to generate microfiber modules with precisely tunable lengths ranging from 100 to 3500 μm via an alternating current (AC) electric field is presented. To control the microfiber length, double-emulsion droplets containing a chelating agent (sodium citrate) as a spacing node are first uniformly embedded in the microfibers in a controllable spatial arrangement. This process is precisely tuned by adjusting the flow rates, thus, tailoring the resulting multicompartmental microfiber structure. Next, an AC voltage signal is used to trigger the electric field-induced cutting process, where the time-averaged electrical force acting on the induced bipolar charge from the Maxwell-Wagner structural polarization mechanism breaks the stress balance at the interfaces, rupturing the double-emulsion droplets, and resulting in the burst release of the encapsulated chelating agents into the hydrogel cavity. The outer hydrogel shell is quickly dissolved by a chemical reaction, cutting the long fiber into a series of microfiber units of given length. Furthermore, adding magnetic nanoparticles endows magnetic functionality with these microfiber modules, which are allowed to serve as micromotors and building blocks. This electro-induced cutting method provides a facile strategy for the fabrication of microfibers with desired lengths, showing considerable promise for various chemical and biological applications.

Microfluidic Fabrication of Bioinspired Cavity-Microfibers for 3D Scaffolds

We present a gas-in-water microfluidic method to precisely fabricate well-controlled versatile microfibers with cavity knots (named cavity-microfiber), like tiny-cavity-microfiber, hybrid-cavity-microfiber, cavity-microfiber, and chained microfiber. The cavity-microfibers are endowed with tunable morphologies, unique surface properties, high specific surface area, assembling ability, flexibility, cytocompatibility, and hydroscopicity. We assemble cavity-microfibers as 3D scaffolds for culturing the human umbilical vein endothelial cells (HUVECs) and dehumidifying. The HUVECs on the scaffolds demonstrate good cell viability and 3D HUVECs frameworks, confirming the unique cytocompatibility of cavity-microfiber. And the cavity-microfibers and their scaffolds also demonstrate excellent dehumidifying ability and large-scale dehumidifying, respectively. Our cavity-microfiber can offer a broad range of applications in sensor, wearable electronics, dehumidifying, water collection engineering, drug delivery, biomaterials, and tissue engineering.

Spontaneous directional transportations of water droplets on surfaces driven by gradient structures

Spontaneous directional transportation of droplets on solid surfaces driven by structure gradients has attracted much attention due to its large-scale applications, such as heat transfer, microfluidic devices, water collection, and separation. It also provides new insight for theoretical research into the interactions between droplets and solid surfaces. This review article summarizes recent progress in the spontaneous directional transportation of droplets on surfaces with structure gradients. Currently, surfaces with structure gradients can be divided into three types: wedge corners with a gradient opening angle, wedge-shaped surfaces, and conical substrates. This review focuses on their basic theory, detailed transport processes, fabrication methods, influence factors and application development. Finally, a perspective of this mode of transportation for future development is proposed.

Microfluidic Spun Alginate Hydrogel Microfibers and Their Application in Tissue Engineering

Tissue engineering is focusing on processing tissue micro-structures for a variety of applications in cell biology and the “bottom-up” construction of artificial tissue. Over the last decade, microfluidic devices have provided novel tools for producing alginate hydrogel microfibers with various morphologies, structures, and compositions for cell cultivation. Moreover, microfluidic spun alginate microfibers are long, thin, and flexible, and these features facilitate higher-order assemblies for fabricating macroscopic cellular structures. In this paper, we present an overview of the microfluidic spinning principle of alginate hydrogel microfibers and their application as micro-scaffolds or scaffolding elements for 3D assembly in tissue engineering.

Scalable and Automated Fabrication of Conductive Tough-Hydrogel Microfibers with Ultrastretchability, 3D Printability, and Stress Sensitivity

Creating complex three-dimensional structures from soft yet durable materials enables advances in fields such as flexible electronics, regenerating tissue engineering, and soft robotics. Tough hydrogels that mimic the human skin can bear enormous mechanical loads. By employing a spider-inspired biomimetic microfluidic nozzle, we successfully achieve continuous printing of tough hydrogels into fibers, two-dimensional networks, and even three-dimensional structures without compromising their extreme mechanical properties. The resultant thin fibers demonstrate a stretch up to 21 times of their original length at a water content of 52%, and are intrinsically transparent, biocompatible, and conductive at high stretches. Moreover, the printed robust tough-hydrogel networks can sense strain that are orders of magnitude lower than stretchable conductors by percolations of conductive particles. To demonstrate their potential application, we use printed tough-hydrogel fiber networks as wearable sensors for detecting human motions. The capability to shape tough hydrogels into complex structures by scalable continuous printing opens opportunities for new areas of applications such as tissue scaffolds, large-area soft electronics, and smart textiles.

Necklace-Like Microfibers with Variable Knots and Perfusable Channels Fabricated by an Oil-Free Microfluidic Spinning Process

Fiber materials with different structural features, which in many cases endow the fibers extraordinary functions, are drawing considerable attention from biomedical and material researchers. Here, perfusable necklace-like knotted microfibers are presented for the first time. Additionally, a novel microfluidic spinning method facilitates the production of variable knots and channels. Not only spindle-, but also hemisphere- and petal-knotted microfibers can be controllably fabricated. Generation and perfusion of both Janus channels and helical channel in the knotted microfibers are also shown. With no need of oil and surfactant, the spinning process is highly cytocompatible. The potential bioengineering and biomedical application of the knotted hollow microfiber is demonstrated by its cell-encapsulation feasibility and the unique liver acinus-like diffusion gradient in the knot. The merits of perfusability, cytocompatibility, and structural diversity of the microfibers may open more avenues for further material and biomedical investigation.

Magnetic alginate microfibers as scaffolding elements for the fabrication of microvascular-like structures

Traditional cell-encapsulating scaffolds may elicit adverse host responses and inhomogeneity in cellular distribution. Thus, fabrication techniques for cellular self-assembly with micro-scaffold incorporation have been used recently to generate toroidal cellular modules for the bottom-up construction of vascular-like structures. The micro-scaffolds show advantage in promoting tissue formation. However, owing to the lack of annular cell micro-scaffolds, it remains a challenge to engineer micro-scale toroidal cellular modules (micro-TCMs) to fabricate microvascular-like structures. Here, magnetic alginate microfibers (MAMs) are used as scaffolding elements, where a winding strategy enables them to be formed into micro-rings as annular cell micro-scaffolds. These micro-rings were investigated for NIH/3T3 fibroblast growth as a function of surface chemistry and MAM size. Afterwards, micro-TCMs were successfully fabricated with the formation of NIH/3T3 fibroblasts and extracellular matrix layers on the three-dimensional micro-ring surfaces. Simple non-contact magnetic assembly was used to stack the micro-TCMs along a micro-pillar, after which cell fusion rapidly connected the assembled micro-TCMs into a microvascular-like structure. Endothelial cells or drugs encapsulated in the MAMs could be included in the microvascular-like structures as in vitro cellular models for vascular tissue engineering, or as miniaturization platforms for pharmaceutical drug testing in the future.

STATEMENT OF SIGNIFICANCE

Magnetic alginate microfibers functioned as scaffolding elements for guiding cell growth in micro-scale toroidal cellular modules (micro-TCMs) and provided a magnetic functionality to the micro-TCMs for non-contact 3D assembly in external magnetic fields. By using the liquid/air interface, the non-contact spatial manipulation of the micro-TCMs in the liquid environment was performed with a cost-effective motorized electromagnetic needle. A new biofabrication paradigm of construct of microvascular-like structure. The micro-tubal-shaped structures allowed direct cell-to-cell contact that solved problems of cell-encapsulating scaffolds.

A novel layer-structured scaffold with large pore sizes suitable for 3D cell culture prepared by near-field electrospinning

Electrospinning is a powerful method for preparing porous materials that can be applied as biomedical materials for implantation or tissue engineering or as scaffolds for 3D cell culture experiments. However, this technique is limited in practical applications because the pore size of 3D scaffolds directly prepared by conventional electrospinning is usually less than several tens of micrometres, which may not be suitable for 3D cell culture and tissue growth. To allow for satisfactory 3D cell culture and tissue engineering, the pore size of the scaffold should be controllable according to the requirement of the specific cells to be cultured. Here, we show that layer-structured scaffolds with pore sizes larger than 100μm can be obtained by stacking meshes prepared by direct-writing using the near-field electrospinning (NFES) technique. In the study, we prepared composite scaffolds made of polycaprolactone (PCL) and hydroxyapatite (HAp) via the above-mentioned method and tested the effectiveness of the novel scaffold in cell culture using mouse pre-osteoblast cells (MC3T3-E1). The pore size and the degradability of the PCL/HAp scaffolds were characterized. The results showed that the average pore size of the scaffolds was 167μm, which was controllable based on the required application; the degradation rate was controllable depending on the ratio of PCL to HAp. The biocompatibility of the scaffolds in vitro was studied, and it was found that the scaffolds showed no toxicity and that the cells could effectively attach, proliferate, and differentiate in the 3D skeleton of the scaffolds. Our studies showed that a simple modification of the preparation procedure can lead to a new way to fabricate novel layer-structured 3D scaffolds with controllable structures and pore sizes suitable for practical applications in implantation, tissue engineering and 3D cell culture.

Bioinspired Dynamic Wetting on Multiple Fibers

Natural fibers have versatile strategies for interacting with water media and better adapting to the local environment, and these strategies offer inspiration for the development of artificial functional fibers with diverse applications. Wetting on fibers is a dynamic liquid-moving process on/in fibrous systems with various patterns, and the process is normally driven by the structural gradient, chemical gradient, elasticity of a single fiber, or the synergistic effect of these factors in multiple fibers in an integrated system in which the spatial geometry of the fibers is involved. Compared with the directional liquid movement on a single fiber, wetting on multiple fibers in both the micro- and macroscales is particularly fascinating, with various performances, including directional liquid transport, controllable liquid transfer, efficient liquid encapsulation, and capillary-induced fibrous coalescence. Based on these properties, fibrous materials offer an alternative open system for liquid manipulation that is applicable to various functional liquid materials. Here, recent achievements in bioinspired dynamic wetting on multiple fibers are highlighted, and perspectives on future directions are presented.

Large-scale water collection of bioinspired cavity-microfibers

Large-scale and high-efficient water collection of microfibers with long-term durability still remains challenging. Here we present well-controlled, bioinspired spindle-knot microfibers with cavity knots (named cavity-microfiber), precisely fabricated via a simple gas-in-water microfluidic method, to address this challenge. The cavity-microfiber is endowed with unique surface roughness, mechanical strength, and long-term durability due to the design of cavity as well as polymer composition, thus enabling an outstanding performance of water collection. The maximum water volume collected on a single knot is almost 495 times than that of the knot on the cavity-microfiber. Moreover, the spider-web-like networks assembled controllably by cavity-microfibers demonstrate excellent large-scale and high-efficient water collection. To maximize the water-collecting capacity, nodes/intersections should be designed on the topology of the network as many as possible. Our light-weighted yet tough, low-cost microfibers with high efficiency in directional water transportation offers promising opportunities for large-scale water collection in water-deficient areas.

Spider-silk-mimicking microfibers often suffer from low efficiency and durability in water collection. Here, the authors fabricate robust microfibers with spindle cavity-knots and different topological fiber-networks with improved water-collecting performance

Fog Collection on Polyethylene Terephthalate (PET) Fibers: Influence of Cross Section and Surface Structure

Fog-collecting meshes show a great potential in ensuring the availability of a supply of sustainable freshwater in certain arid regions. In most cases, the meshes are made of hydrophilic smooth fibers. Based on the study of plant surfaces, we analyzed the fog collection using various polyethylene terephthalate (PET) fibers with different cross sections and surface structures with the aim of developing optimized biomimetic fog collectors. Water droplet movement and the onset of dripping from fiber samples were compared. Fibers with round, oval, and rectangular cross sections with round edges showed higher fog-collection performance than those with other cross sections. However, other parameters, for example, width, surface structure, wettability, and so forth, also influenced the performance. The directional delivery of the collected fog droplets by wavy/v-shaped microgrooves on the surface of the fibers enhances the formation of a water film and their fog collection. A numerical simulation of the water droplet spreading behavior strongly supports these findings. Therefore, our study suggests the use of fibers with a round cross section, a microgrooved surface, and an optimized width for an efficient fog collection.

Bioinspired Special Wettability Surfaces: From Fundamental Research to Water Harvesting Applications

Nowadays, the pollution of water has become worse in many parts of the world, which causes a severe shortage of clean water and attracts widespread attention worldwide. Bioinspired from nature, i.e. spider silk, cactus, Namib desert beetle, Nepenthes alata, special wettability surfaces have attracted great interest from fundamental research to water-harvesting applications. Here, recently published literature about creatures possessing water-harvesting ability are reviewed, with a focus on the corresponding water-harvesting mechanisms of creatures in dry or arid regions, consisting of the theory of wetting and transporting. Then a detailed account of the innovative fabrication technologies and bionic water-harvesting materials with special wetting are summarized, i.e. bio-inspired artificial spider silk, bio-inspired artificial cactus-like structures, and bio-inspired artificial Namib desert beetle-like surfaces. Special attentions are paid to the discussion of the advantages and disadvantages of the technologies, as well as factors that affect the amount of water-harvesting. Finally, conclusions, future outlooks and the current challenges for future development of the water-harvesting technology are presented and discussed.

Bioinspired Multifunctional Spindle-Knotted Microfibers from Microfluidics

Heterostructured microfibers with spindle-knots and joints are developed using a novel microfluidic technology, which enables integrative microfiber joint spinning, fluid coating, and knot emulsification. The knots emulsification process can be precisely tunable by adjusting the flow rates. In this way, the size and spacing of the spindle-knots of the microfibers can be achieved with high controllability. More attractively, the construction process benefits from the broad availability of the coating fluids, which determines the compositions of the knots. Thus, the resultant microfibers are imparted with distinctive functions, such as humidity-responsive water capture, thermally triggered water convergence, induced colloidal crystal assembly, and cell microcarrier arrays. These features make such microfibers highly versatile for use in diverse applications.

Flexible Asymmetric Encapsulation for Dehydration-Responsive Hybrid Microfibers

A new class of smart alginate microfibers with asymmetric oil encapsulates is introduced. These fibers are produced by injecting an aqueous alginate solution into an outer aqueous calcium chloride solution to form alginate fibers, which are asymmetrically loaded with oil entities through eccentrically aligned inner capillaries. The fiber morphology and its degree of asymmetry can be tuned via altering the size, location, and frequency of the oil encapsulates. These asymmetric fibers reveal significant potential for applications where conventional symmetric fibers fail to perform. It is shown how asymmetric oil-encapsulated fibers can become dehydration-sensitive, and trigger the release of encapsulates if their hydration level drops below a critical value. It is also shown how the triggered response could be switched off on demand by stabilizing the oil encapsulates. The capability of asymmetric fibers to carry and release multiple cargos in parallel is demonstrated. The fibers loaded with equal-sized spheres are more asymmetric than those containing unequal drops, have a higher tensile strength, and show better potential for a triggered response.

Control of Polymer Phase Separation by Roughness Transfer Printing for 2D Microlens Arrays

Great efforts have been devoted to the control of phase separation between blended polymers in terms of the advantages for engineering functional topologies. A simple and straightforward pathway through roughness transfer printing (RTP) is proposed to realize the control of polymer phase separation. The additional roughness difference, which is introduced by trace agarose transferred from a hydrogel stamp, offers a great effect on the rate of nucleation and coalescence orientation of polymethylmethacrylate (PMMA) protrusions grown from a polydimethylsiloxane (PDMS) network. Using a particular topography of agarose stamp and a proper growth time in toluene atmosphere, a 2D microlens array with high uniformity is obtained that shows great potential for optical applications. Moreover, the control of polymer phase separation was successfully extended to the collection and identification of fingerprints with a high degree of replication.