Hollow Microfibers Research Articles

Electromagnetic wave absorption enhancement of double-layer structural absorbers based on carbon nanofibers and hollow Co2Y hexaferrite microfibers

Novel double-layer microwave absorbers possessing excellent absorption performances have been designed based on carbon nanofibers (CNFs) and hollow Ba2Co2Fe12O22 microfibers (Co2Y MFs). The electromagnetic wave absorbing characteristics of double-layer absorbers with different layer thicknesses and compositions are investigated over the 2–18 GHz range. Efficient coupling and synergistic effects between the absorbing layer and matching layer lead to a better impedance matching and in turn improve microwave absorption. The optimized double-layer structures exhibit far superior microwave absorbing properties with stronger absorption intensity and wider effective bandwidth compared with the corresponding single-layer ones. When the absorbing layer (1.15 mm in thickness) is filled with 5 wt% CNFs and the matching layer (0.85 mm) is filled with 67 wt% Co2Y MFs, a minimum reflection loss (RL) value of −85.5 dB is achieved at 16.1 GHz and the RL less than −10 dB is obtained in the 10.4–18 GHz. Conversely, as the absorbing layer (1.75 mm) is filled with 67 wt% Co2Y MFs and the matching layer (0.25 mm) is filled with 5 wt% CNFs, the minimum RL value reaches −68.7 dB at 14.8 GHz and the absorption frequency range with RL values below −10 dB is up to 10 GHz (from 8 to 18 GHz). Moreover, the position, intensity and effective bandwidth of the absorption peaks for the double-layer absorbers can be readily tuned by adjusting the stacking order, the thickness of each layer and total absorber thickness, showing good potential for practical application.

A microfluidic strategy to fabricate ultra-thin polyelectrolyte hollow microfibers as 3D cellular carriers.

Microfluidics-based microfibers have been widely used as bottom-up scaffolds for tissue engineering applications. Different forms of microfibers with certain thickness of shell have been developed during the past decade. Ultra-thin microfiber, as a special and promising carrier of cells, was less explored. In this work, by using the interfacial ionic interaction between sodium alginate (NaA) and chitosan (CS), a novel ultra-thin polyelectrolyte hollow microfiber with the diameter of ~200 μm and the shell thickness of 1.3 ± 0.3 μm was fabricated via a microfluidic device for liver tissue engineering. The fluorescence of FITC labeled CS confirmed the inner CS layer of the fabricated microfiber and the SEM results illustrated its ultra-thin characteristic. Although there are only two layers in the ultra-thin polyelectrolyte hollow microfiber, the following cells encapsulation experiments indicated that it could bear cells loading and the hollow space of the microfibers could encapsulate sufficient number of cells for tissue engineering applications. The presence of inner CS layer in the microfiber promoted cell adhesion and ultra-thin shell characteristic facilitated the exchange of nutrient substance and O2 and thus promoted cell proliferation. HepG2 cells encapsulated in the microfibers maintained favorable viability, proliferation ability and hepatic specific functions during 10 days' culture. These results suggest that the established polyelectrolyte microfibers hold great potential applications in the field of liver tissue engineering. We believe this work will lead to the development of innovative methodologies and materials for both cell culture and biomedical application.

Micro/nano microfiber synthetic leather base with different nanofiber diameters

The microfiber synthetic leather base (MSLB) was prepared by mixing hollow segmented pie microfibers and polyacrylonitrile electrospun nanofibers. Micro/nano hybrid leather base was prepared to simulate the three-dimensional (3D) structure of natural leather. The diameter of the polyacrylonitrile nanofibers was 950 nm, 450 nm and 200 nm, respectively. The effects of nanofibers of different diameters on the performances of MSLB were investigated. The results indicated that polyacrylonitrile nanofibers were randomly dispersed in MSLB. Differential scanning calorimetry further demonstrated the presence of polyacrylonitrile nanofibers in MSLB, and thermogravimetric analysis showed that the addition of polyacrylonitrile nanofibers had a little influence on the thermal stability of MSLB. There was no significant change in the water contact angle of the MSLB surface before and after the addition of nanofibers. As the diameter of the nanofibers decreased from 950 nm to 200 nm, the static water vapor transmission rate was increased by 28.2%, the softness was increased by 39.74%, and air permeability was decreased by 17.16%. When the nanofiber diameter is 950 nm, the moisture absorption performance of MSLB was better, which was 768.99%. Moreover, the tear strength was also significantly enhanced by 105.76%.

Regenerative and durable small-diameter graft as an arterial conduit

Despite significant research efforts, clinical practice for arterial bypass surgery has been stagnant, and engineered grafts continue to face postimplantation challenges. Here, we describe the development and application of a durable small-diameter vascular graft with tailored regenerative capacity. We fabricated small-diameter vascular grafts by electrospinning fibrin tubes and poly(ε-caprolactone) fibrous sheaths, which improved suture retention strength and enabled long-term survival. Using surface topography in a hollow fibrin microfiber tube, we enable immediate, controlled perfusion and formation of a confluent endothelium within 3-4 days in vitro with human endothelial colony-forming cells, but a stable endothelium is noticeable at 4 weeks in vivo. Implantation of acellular or endothelialized fibrin grafts with an external ultrathin poly(ε-caprolactone) sheath as an interposition graft in the abdominal aorta of a severe combined immunodeficient Beige mouse model supports normal blood flow and vessel patency for 24 weeks. Mechanical properties of the implanted grafts closely approximate the native abdominal aorta properties after just 1 week in vivo. Fibrin mediated cellular remodeling, stable tunica intima and media formation, and abundant matrix deposition with organized collagen layers and wavy elastin lamellae. Endothelialized grafts evidenced controlled healthy remodeling with delayed and reduced macrophage infiltration alongside neo vasa vasorum-like structure formation, reduced calcification, and accelerated tunica media formation. Our studies establish a small-diameter graft that is fabricated in less than 1 week, mediates neotissue formation and incorporation into the native tissue, and matches the native vessel size and mechanical properties, overcoming main challenges in arterial bypass surgery.

Microfluidic Fabrication of Biomimetic Helical Hydrogel Microfibers for Blood-Vessel-on-a-Chip Applications.

Nature has created many perfect helical microstructures, including DNA, collagen fibrils, and helical blood vessels, to achieve unique physiological functions. While previous studies have developed a number of microfabrication strategies, the preparation of complex helical structures and cell-laden helical structures for biomimetic applications remains challenging. In this study, a one-step microfluidics-based methodology is presented for preparing complex helical hydrogel microfibers and cell-laden helical hydrogel microfibers. Several types of complex helical structures, including multilayer helical microfibers and superhelical hollow microfibers with helical channels, are prepared by simply tuning the flow rates or modifying the geometry of microfluidic device. With the decent perfusability, the hollow microfibers may simulate the structural characteristics of helical blood vessels and create swirling blood flow in a blood-vessel-on-chip setup. Such hydrogel-based helical microstructures may potentially be used in areas such as blood vessel tissue engineering, organ-on-chips, drug screening, and biological actuators.

The Application of Hollow Segmented Pie Bicomponent Spunbond Hydro-Entangled Microfiber Nonwovens for Microfiber Synthetic Leather Apparel

The polyester/nylon 6 hollow segmented pie bicomponent spunbond hydro-entangled microfiber nonwoven (PET/PA6 HSBSYMNW) was prepared and used as a leather base to produce microfiber synthetic leather for apparel. The structure and properties of PET/PA6 HSBSYMNW were investigated. SEM analysis indicated the fiber diameter size was between 2.2∼5.5 μm. Its performance met the demand of nonwoven textiles for synthetic leather. The obtained microfiber synthetic leather was arranged in a four-layer configuration, and its air permeability, water vapor transmission rate (WVT), thermal insulation, and hand properties were better than commercial garment synthetic leathers with different leather bases. The method achieved a more environmentally-friendly and high-performance preparation of microfiber synthetic leather used for apparel.

Microfluidics-Based Fabrication of Cell-Laden Hydrogel Microfibers for Potential Applications in Tissue Engineering.

Fibrous hydrogel scaffolds have recently attracted increasing attention for tissue engineering applications. While a number of approaches have been proposed for fabricating microfibers, it remains difficult for current methods to produce materials that meet the essential requirements of being simple, flexible and bio-friendly. It is especially challenging to prepare cell-laden microfibers which have different structures to meet the needs of various applications using a simple device. In this study, we developed a facile two-flow microfluidic system, through which cell-laden hydrogel microfibers with various structures could be easily prepared in one step. Aiming to meet different tissue engineering needs, several types of microfibers with different structures, including single-layer, double-layer and hollow microfibers, have been prepared using an alginate-methacrylated gelatin composite hydrogel by merely changing the inner and outer fluids. Cell-laden single-layer microfibers were obtained by subsequently seeding mouse embryonic osteoblast precursor cells (MC3T3-E1) cells on the surface of the as-prepared microfibers. Cell-laden double-layer and hollow microfibers were prepared by directly encapsulating MC3T3-E1 cells or human umbilical vein endothelial cells (HUVECs) in the cores of microfibers upon their fabrication. Prominent proliferation of cells happened in all cell-laden single-layer, double-layer and hollow microfibers, implying potential applications for them in tissue engineering.

Simple fabrication of inner chitosan-coated alginate hollow microfiber with higher stability.

Sodium alginate (NaA) has been widely used as microfiber-based scaffold material. However, Ca-alginate microfiber might disintegrate in the physiological environment due to the loss of calcium ions, which will limit its long-term application in tissue engineering. In this work, to enhance the stability of Ca-alginate microfiber in the physiological environment, an inner chitosan coating was introduced to Ca-alginate hollow microfiber by one step via a microfluidic device. A more stable composite microfiber with double cross-linking layers was generated. The stability of the microfiber was studied in the PBS solution (pH 7.4) to identify the coating effect on the hollow structure. The results revealed that chitosan component bonded an NaA layer to form a stable polyelectrolyte complex membrane in the inner wall of the microfiber, which stabilized the hollow region even though the Ca-alginate shell was disintegrated by PBS solution. In addition, the introduction of chitosan coating improved the inner environment of the low affinity of alginate to cell surfaces and facilitated the cell adhesion and culture in the microfiber. HepG2 cells in the microfibers displayed favorable cell viability and proliferation ability. We believe that this work will lead to the development of innovative methodologies and materials for both cell culture and tissue engineering application. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B:2527-2536, 2019.

Plasmonic TiN boosting nitrogen-doped TiO2 for ultrahigh efficient photoelectrochemical oxygen evolution

Achieving a high photocatalytic activity toward photoelectrochemical (PEC) water splitting has become a formidable challenge for titanium oxide (TiO2) owing to its poor photoresponse to visible light and low electrical conductivity. Herein, we report the first demonstration of nonmetal TiN as a plasmonic booster to significantly enhancing the PEC water splitting performance of TiO2. A unique multiscaled architecture organized by interweaving hollow microfiber monolith and hierarchical TiN/N-TiO2 nanorod arrays is fabricated by a facile seamless nitridation process. The conductive TiN not only affords plasmon resonance on the N-TiO2 to enable high photoactivity in a broadband UV–vis light region, but also assists in the charge generation-separation-transportation-injection efficiency of TiO2 for enhanced water oxidation kinetics. The TiN/N-TiO2 heterostructure manifests an unprecedented high and durable photocurrent density of 3.12 mA cm−2 at 1.23 V (vs. reversible hydrogen electrode (RHE)) under standard AM 1.5 G illumination and substantiates an outstanding visible-light-driven photocurrent density of 1.63 mA cm−2 without the use of any hole scavenger and cocatalysts. This study will enrich the fundamental understanding of nonmetal plasmonic effect in and beyond the field of PEC water splitting.

Microscopic hollow hydrogel springs, necklaces and ladders: a tubular robot as a potential vascular scavenger

A non-coaxial microfluidic method is presented to generate hollow hydrogel microfibers for potential application as a tubular vascular scavenger.

Flexible Asymmetric Microsupercapacitors from Freestanding Hollow Nickel Microfiber Electrodes

AbstractThe increasing demand for flexible and wearable microelectronics is a major driving force for the development of high‐performance small‐volume energy sources. Microsupercapacitors exhibit significant potential in energy storage as they provide high power and good cycle stability. Yet, most microsupercapacitors display low energy densities limiting their practical use in microelectronics. Here, synthesis of high surface area freestanding electrodes comprising hollow nickel microfibers is demonstrated. The microfiber nickel electrodes constitute a platform for flexible asymmetric microsupercapacitors exhibiting excellent mechanical resilience as well as high energy density (0.11 mWh cm−2) and power density (37.5 mW cm−2). The freestanding nature and extensive surface area of the electrodes contribute to pronounced areal and volumetric capacitance, energy storage values, and stability after numerous (hundreds) physical bending cycles. The simple preparation scheme and use of an inexpensive building block (e.g., nickel) underscore potential uses as light and flexible energy storage devices.

Hierarchical SnO2 nanoclusters wrapped functionalized carbonized cotton cloth for symmetrical supercapacitor

As an excellent free-standing carbon source, commercialized cotton cloth can be carbonized to fabricate conductive carbon cloth composed of hollow carbon microfibers. In view of the hydrophobicity of carbon cloth, a functionalized carbonized cotton cloth (FCC) is prepared by acidification treatment, and acted as a flexible substrate to grow hierarchical SnO2 nanoclusters by a solvothermal reaction and calcination process. In this SnO2 wrapped FCC (FCC@SnO2) composite, the oxygen-containing groups in carbon microfibers provide numerous anchoring sites for growing SnO2 nanoparticles, meanwhile, the carbon microfibers provide conductive channels for the fast transfer of electrons and ions. The SnO2 nanoclusters effectively contribute their pseudocapacitance. As free-standing electrodes in a symmetrical two-electrode supercapacitor, the FCC@SnO2 composite exhibits a higher capacitance of 197.7 F g−1 or 1265.3 mF cm−2 at 1 A g−1, much higher than that of FCC (100.3 F g−1 or 411.2 mF cm−2); furthermore, its capacitance remains 95.5% after cycling for 5000 cycles at 15 A g−1. The FCC@SnO2 composite is easier preparation and low cost, which can be utilized as self-supporting flexible electrodes for high performance supercapacitors.

Monodisperse and Tiny Co2N0.67 Nanocrystals Uniformly Embedded over Two Curving Surfaces of Hollow Carbon Microfibers as Efficient Electrocatalyst for Oxygen Evolution Reaction

Rational design of highly efficient electrocatalysts for oxygen evolution reaction (OER) is critical for both water splitting and rechargeable metal–air batteries. Especially, developing excellent earth-abundant catalysts for OER is an ongoing challenge. We report a series of cobalt nitride (Co2N0.67)–carbon composites with different textural properties as high-activity OER materials herein. Experimental results demonstrate that the specific morphologies of different carbon matrices (such as carbon nanotubes (CNTs), graphene (GN), macroporous carbons (MPCs), and hollow carbon microfibers (CMFs)) play crucial roles in determining the overall OER catalytic efficacies of corresponding composites. Both SEM and TEM characterization results revealed that monodisperse and tiny Co2N0.67 nanoparticles (NPs) indeed grew uniformly over both the inner and outer walls of CMFs because of the widespread edges and defect sites. This is beneficial to fully expose and utilize active sites on each Co2N0.67 NP. In addition, ...

Spider-Inspired Multicomponent 3D Printing Technique for Next-Generation Complex Biofabrication.

The shortage of tissue resources is currently a serious challenge that limits the clinical therapy to patients with tissue loss or end-stage organ failure. The booming development of 3D printing offers unprecedented hope for tissue engineering since it can construct cells and biomaterials into a 3D tissue-mimicking object with precise control over size and shape. However, it is still challenging to fabricate artificial living tissues or organs due to the extreme complexity of biological tissues. Herein, we propose a new concept of spider-inspired 3D printing technique (SI-3DP) for continuous multicomponent 3D printing based on in situ gelation at a multibarrel printing nozzle. The printing process allows for rapid construction of 3D architectures composed of different inks in the desired position. To present the potential in biomedical applications, the SI-DIP also prints vessel-like hollow hydrogel microfibers and cell-laden hollow fibers, indicating good biocompatibility of this technique. The newly developed SI-3DP technique is envisioned to promote the development of next-generation complex biofabrication.

A Living 3D In Vitro Neuronal Network Cultured inside Hollow Electrospun Microfibers.

Micro/nanofiber-based scaffolds are widely used as a physical graft for axon growth in vitro for nerve tissue engineering. In this study, the fabrication of hollow coaxial microfibers with a 3D structure for directional neuronal cell culture is reported. The coaxial microfibers with poly-ε-caprolactone (PCL) sheath and polyvinyl alcohol (PVA) mixed with PC 12 cells as core are prepared using a coaxial double cylinder based on the liquid-liquid coflowing electrospinning method. By optimizing the electrospinning conditions, uniform microfibers with an average diameter of 54.6 µm are obtained using a PCL concentration of 20% and a tip-to-collector distance of 6 cm with a voltage of 6 kV. After selectively dissolving the PVA core, pheochromocytoma 12 (PC12) cells are successfully cultured inside these fibers. It is experimentally confirmed that such 3D hollow fibers can support and guide PC12 cells to grow, proliferate, and differentiate inside the hollow fibers and form 3D-connected neuronal networks with axons extended along the hollow fibers. This implies that complex nerve connection could be constructed using the low cost and powerful liquid-liquid coflowing electrospinning method, which open the door to culture well-controlled nerve connections as in vitro models for neural tissue regeneration, brain disease models, and so on.

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.

Hydrothermal Synthesis of Hollow Al2O3 Microfibers for Thermal Insulation Materials

Abstract Hollow Al2O3 microfibers were successfully synthesized via a novel hydrothermal method using cotton fiber as a template followed by annealing. The hollow Al2O3 microfibers annealed at 1200 °C for 5 h contained no impurity phases, and the Al2O3 composing the microfibers was confirmed to exhibit the trigonal unit cell of α-Al2O3 with R$\bar{3}$c space-group symmetry. The synthesized hollow Al2O3 microfibers were 5–15 µm in diameter, with walls 500–800 nm thick; the walls were composed of Al2O3 primary particles 100–200 nm in diameter. The specific heat capacity of the synthesized hollow Al2O3 microfibers was approximately the same as that reported in the literature for α-Al2O3. In addition, the annealing temperature of the hollow Al2O3 microfibers was studied to elucidate their mechanism of formation. The chemical and physical properties of the synthesized hollow Al2O3 microfibers indicate that they can be used as a thermal insulation material.

Hollow core micro-fiber for optical wave guiding and microfluidic manipulation

We report the fabrication and characterization of hollow core micro-fibers (HCMFs) for use as optical waveguides and microfluidic channels meanwhile. By one-step heating-drawing process from commercial capillary with diameter of hundreds of micrometers, HCMFs with diameters down to subwavelength scale can be fabricated with excellent repeatability. The HCMFs reveal good optical waveguide properties as waveguides with low optical losses by evanescently coupled from silica fiber taper within the visible to near-infrared spectral range. Meanwhile, the wavelength-scale-diameter hollow core of the HCMF can be used as a microfluidic channel to host liquid with trace amounts, which has an intense interaction with the guiding light. Flexible waveguide properties and different fractions of evanescent fields of HCMFs can be achieved by filling the core with various liquids with different refractive indexes (RIs), which might be useful in various applications. For example, we use our HCMF to detect the fluorescence intensity for monodisperse micro-particles solutions with an effective detection volume of femtoliter scale. Our work provides a promising candidate in constructing miniaturized optical devices, which can be used for detection of ultra-low-volume samples.

Coaxial extrusion bioprinting of 3D microfibrous constructs with cell-favorable gelatin methacryloyl microenvironments

Bioinks with shear-thinning/rapid solidification properties and strong mechanics are usually needed for the bioprinting of three-dimensional (3D) cell-laden constructs. As such, it remains challenging to generate soft constructs from bioinks at low concentrations that are favorable for cellular activities. Herein, we report a strategy to fabricate cell-laden constructs with tunable 3D microenvironments achieved by bioprinting of gelatin methacryloyl (GelMA)/alginate core/sheath microfibers, where the alginate sheath serves as a template to support and confine the GelMA pre-hydrogel in the core during the extrusion process, allowing for subsequent UV crosslinking. This novel strategy minimizes the bioprinting requirements for the core bioink, and facilitates the fabrication of cell-laden GelMA constructs at low concentrations. We first showed the capability of generating various alginate hollow microfibrous constructs using a coaxial nozzle setup, and verified the diffusibility and perfusability of the bioprinted hollow structures that are important for the tissue engineering applications. More importantly, the hollow alginate microfibers were then used as templates for generating cell-laden GelMA constructs with soft microenvironments, by using GelMA pre-hydrogel as the bioink for the core phase during bioprinting. As such, GelMA constructs at extremely low concentrations (<2.0%) could be extruded to effectively support cellular activities including proliferation and spreading for various cell types. We believe that our strategy is likely to provide broad opportunities in bioprinting of 3D constructs with cell-favorable microenvironments for applications in tissue engineering and pharmaceutical screening.

Axon mimicking hydrophilic hollow polycaprolactone microfibres for diffusion magnetic resonance imaging

Highly hydrophilic hollow polycaprolactone (PCL) microfibres were developed as building elements to create tissue-mimicking test objects (phantoms) for validation of diffusion magnetic resonance imaging (MRI). These microfibres were fabricated by the co-electrospinning of PCL-polysiloxane-based surfactant (PSi) mixture as shell and polyethylene oxide as core. The addition of PSi had a significant effect on the size of resultant electrospun fibres and the formation of hollow microfibres. The presence of PSi in both co-electrospun PCL microfibre surface and cross-section, revealed by X-ray energy dispersive spectroscopy (EDX), enabled water to wet these fibres completely (i.e., zero contact angle) and remained active for up to 12 months after immersing in water. PCL and PCL-PSi fibres with uniaxial orientation were constructed into water-filled phantoms. MR measurement revealed that water molecules diffuse anisotropically in the PCL-PSi phantom. Co-electrospun hollow PCL-PSi microfibres have desirable hydrophilic properties for the construction of a new generation of tissue-mimicking dMRI phantoms.