Hollow Microfibers Research Articles

Production and cross-sectional characterization of aligned co-electrospun hollow microfibrous bulk assemblies

The development of co-electrospun (co-ES) hollow microfibrous assemblies of an appreciable thickness is critical for many practical applications, including filtration membranes and tissue-mimicking scaffolds. In this study, thick uniaxially aligned hollow microfibrous assemblies forming fiber bundles and strips were prepared by co-ES of polycaprolactone (PCL) and polyethylene oxide (PEO) as shell and core materials, respectively. Hollow microfiber bundles were deposited on a fixed rotating disc, which resulted in non-controllable cross-sectional shapes on a macroscopic scale. In comparison, fiber strips were produced with tuneable thickness and width by additionally employing an x–y translation stage in co-ES. Scanning electron microscopy (SEM) images of cross-sections of fiber assemblies were analyzed to investigate the effects of production time (from 0.5h to 12h), core flow rate (from 0.8mL/h to 2.0mL/h) and/or translation speed (from 0.2mm/s to 5mm/s) on the pores and porosity. We observed significant changes in pore size and shape with core flow rate but the influence of production time varied; five strips produced under the same conditions had reasonably good size and porosity reproducibility; pore sizes didn't vary significantly from strip bottom to surface, although the porosity gradually decreased and then returned to the initial level.

Shape‐memory properties of degradable electrospun scaffolds based on hollow microfibers

Multifunctional thermo‐responsive and degradable porous materials exhibiting a shape‐memory effect are explored in biomedicine as actively moving scaffolds or switchable substrates. One example are electrospun shape‐memory polymer‐based scaffolds comprising solid microfibers or nanofibers. In this work, we explored whether fibrous scaffolds composed of hollow microfibers can be prepared from a degradable shape‐memory copolyetheresterurethane named PDC, which is composed of crystallizable oligo(p‐dioxanone) (OPDO) hard and oligo(ε‐caprolactone) (OCL) switching segments. Scaffolds based on PDC microfibers with identical outer diameter around 1.4 ± 0.3 µm and different hollowness of 0%, 13%, and 33% related to the outer diameter (determined by scanning electron microscopy) were prepared by coaxial electrospinning using poly(ethylene glycol) (PEG) as sacrificial core. Thermal characterization of the scaffolds by differential scanning calorimetry (DSC) and thermogravimetric analysis confirmed a successful removal of PEG. DSC results revealed that the degree of crystallinity increased with increasing microfiber hollowness. The Young's modulus and the failure stress of the prepared scaffolds determined by tensile tests at ambient temperature and 50 °C were found to increase with rising hollowness, while the elongation at break decreased. Cyclic, thermomechanical uniaxial tensile tests showed a pronounced dual‐shape effect for all tested materials. Scaffolds comprising microfibers with a hollowness of 33% exhibited the highest shape recovery ratio. Here, we could demonstrate that the degree of hollowness of microfibers, which alters the degree of macromolecular chain orientation, is a suitable design parameter to tailor the mechanical properties as well as the shape‐memory performance of electrospun shape‐memory polymer fibrous scaffolds. Copyright © 2015 John Wiley & Sons, Ltd.

Competitive influence of surface area and mesopore size on gas-sensing properties of SnO2 hollow fibers

In this work, the effects of surface area and mesopore size on gas-sensing properties of SnO2 hollow microfibers assembled by nanocrystals were investigated. When the sintering time was increased from 2 to 24 h, the specific surface area (SSA) of SnO2 microfibers decreased from 103.6 to 59.8 m2 g−1, whereas the mesopore diameter gradually increased from 2.8 to 10.9 nm. Interestingly, it was found that their gas-sensing properties to ppb-level formaldehyde were determined by both SSA and mesopore size. The gas response increased firstly and then decreased with decreasing SSA and increasing mesopore size and reached the maximum value when the sintering time was 11 h. When the sintering time was <11 h, mesopore size (<8.5 nm) dominated sensing behavior by controlling gas diffusion rate. Once the sintering time was more than 11 h, the decreased SSA (<70.8 m2 g−1) dominated sensing performance by influencing the surface reaction activity. Therefore, the competitive influence of surface area and mesopore size on gas-sensing properties of mesoporous SnO2 microfibers was revealed. This work could provide a new understanding for microstructural design of the mesoporous gas-sensing metal oxide materials.

Biomimetic phantom for cardiac diffusion MRI.

PurposeDiffusion magnetic resonance imaging (MRI) is increasingly used to characterize cardiac tissue microstructure, necessitating the use of physiologically relevant phantoms for methods development. Existing phantoms are generally simplistic and mostly simulate diffusion in the brain. Thus, there is a need for phantoms mimicking diffusion in cardiac tissue.Materials and MethodsA biomimetic phantom composed of hollow microfibers generated using co‐electrospinning was developed to mimic myocardial diffusion properties and fiber and sheet orientations. Diffusion tensor imaging was carried out at monthly intervals over 4 months at 9.4T. 3D fiber tracking was performed using the phantom and compared with fiber tracking in an ex vivo rat heart.ResultsThe mean apparent diffusion coefficient and fractional anisotropy of the phantom remained stable over the 4‐month period, with mean values of 7.53 ± 0.16 × 10‐4 mm2/s and 0.388 ± 0.007, respectively. Fiber tracking of the 1st and 3rd eigenvectors generated analogous results to the fiber and sheet‐normal direction respectively, found in the left ventricular myocardium.ConclusionA biomimetic phantom simulating diffusion in the heart was designed and built. This could aid development and validation of novel diffusion MRI methods for investigating cardiac microstructure, decrease the number of animals and patients needed for methods development, and improve quality control in longitudinal and multicenter cardiac diffusion MRI studies. J. MAGN. RESON. IMAGING 2016;43:594–600.

Vascular-like network prepared using hollow hydrogel microfibers

One major challenge in the field of tissue engineering was the creation of volumetric tissues and organs invitro. To achieve this goal, the development of a three-dimensional vascular-like network that extended throughout the tissue-engineered construct was essential to supply sufficient oxygen and nutrients to all of the cells in the constructs. For sufficient oxygenation and nutrition of the tissue-engineered constructs, the distance between each microvessel-like channel in the network should ideally be within 100-200μm. In addition, the medium or blood should be perfused through the microchannels as soon as possible after the seeding of cells into the templates (scaffolds) of the constructs. In the present study, we proposed a novel technique for fabricating an engineered vascular-like network that satisfied these two requirements. The network comprised assembled hollow alginate hydrogel microfibers with mammalian cells enclosed in the gel portions. We controlled the distance between each flow microchannel (hollow core portions and interspace of the microfibers) to be within 150μm by using microfibers with a gel thickness of approximately 50μm. Furthermore, we confirmed that medium could be perfused into the flow channels quickly (within 10min) after immobilization of the cells in the assembly. A human hepatoblastoma cell line (HepG2) proliferated in the gel portions of the microfibers and maintained their specific function during perfusion culture for 7days. These results showed that the novel vascular-like networks fabricated here had the potential to allow the creation of volumetric tissues invitro.

A microfluidic device approach to generate hollow alginate microfibers with controlled wall thickness and inner diameter

Alginate is a natural polymer with inherent biocompatibility. A simple polydimethylsiloxane (PDMS) microfluidic device based self-assembled fabrication of alginate hollow microfibers is presented. The inner diameter as well as wall thickness of the microfibers were controlled effortlessly, by altering core and sheath flow rates in the microfluidic channels. The gelation/cross-linking occured while the solutions were ejected. The microfibers were generated spontaneously, extruding out of the outlet microchannel. It was observed that the outer diameter was independent of the flow rates, while the internal diameter and wall thickness of the hollow fibers were found to be functions of the core and sheath flow rates. At a constant sheath flow, with increasing core flow rates, the internal diameters increased and the wall thicknesses decreased. At a fixed core flow, when sheath flow rate increased, the internal diameters decreased and the wall thickness increased. The immobilization of enzymes in such hollow microfibers can be a potential application as microbioreactors.

Fickian-Based Empirical Approach for Diffusivity Determination in Hollow Alginate-Based Microfibers Using 2D Fluorescence Microscopy and Comparison with Theoretical Predictions.

Hollow alginate microfibers (od = 1.3 mm, id = 0.9 mm, th = 400 µm, L = 3.5 cm) comprised of 2% (w/v) medium molecular weight alginate cross-linked with 0.9 M CaCl2 were fabricated to model outward diffusion capture by 2D fluorescent microscopy. A two-fold comparison of diffusivity determination based on real-time diffusion of Fluorescein isothiocyanate molecular weight (FITC MW) markers was conducted using a proposed Fickian-based approach in conjunction with a previously established numerical model developed based on spectrophotometric data. Computed empirical/numerical (Dempiricial/Dnumerical) diffusivities characterized by small standard deviations for the 4-, 70- and 500-kDa markers expressed in m2/s are (1.06 × 10−9 ± 1.96 × 10−10)/(2.03 × 10−11), (5.89 × 10−11 ± 2.83 × 10−12)/(4.6 × 10−12) and (4.89 × 10−12 ± 3.94 × 10−13)/(1.27 × 10−12), respectively, with the discrimination between the computation techniques narrowing down as a function of MW. The use of the numerical approach is recommended for fluorescence-based measurements as the standard computational method for effective diffusivity determination until capture rates (minimum 12 fps for the 4-kDa marker) and the use of linear instead of polynomial interpolating functions to model temporal intensity gradients have been proven to minimize the extent of systematic errors associated with the proposed empirical method.

Capillarity-induced mechanical behaviors of a polymer microtube surrounded by a droplet

The capillary force of a liquid drop has a great impact on the mechanical behaviors of a polymer microtube. To further explore this capillary effect, we examine the buckling condition and finite deformation of a hollow microfiber surrounded by a droplet. The Eulerian rod model and thin-walled shell model are both adopted to predict the critical value of the capillary force acting on the microfiber. According to the Mooney-Rivlin model, we calculate the true axial stress of the microtube under the combined action of surface tension and Laplace pressure. The numerical results show that the value of the true axial stress is closely related to the Young’s contact angle, droplet volume and characteristic sizes of the microtube. Our findings address that proper control over surface wettability may improve the performance optimization of micro-devices, and these analyses may produce ideas in the areas of nanofabrication, electrospinning and tissue engineering.

Dexamethasone encapsulated coaxial electrospun PCL/PEO hollow microfibers for inflammation regulation

One of the major challenges in the field of biomaterials is to develop strategies to regulate the innate host inflammatory response. Coaxial electrospun PCL/PEO hollow fibers containing dexamethasone were evaluated for a local delivery of dexamethasone to reduce adverse inflammation responses often resulting from biomaterial implantation. Core–shell PCL/PEO hollow fibers with a highly porous surface were successfully developed using coaxial electrospinning. The release of dexamethasone exhibited a burst release during the first 0.5–3 h followed by a sustained release over more than 12 days. In vitro study showed that the dexamethasone encapsulated coaxial PCL/PEO hollow fibers significantly reduced cell proliferation of LPS-stimulated macrophages and meanwhile significantly decreased the mRNA expression levels of the pro-inflammatory cytokines TNF-α and IL-1β. The dexamethasone encapsulated coaxial PCL/PEO hollow microfibers are promising biomaterials for implantation to combat the acute inflammation responses which take place during first 24–48 h after biomaterial implantation and meanwhile to regulate possible hostile chronic inflammations, thus increasing the efficacy and longevity of the implants in vivo.

Self-assembly of hollow PNIPAM microgels to form discontinuously hollow fibers

A new kind of hollow hydrogel microfiber with discontinuous hollow structure was prepared by an ice-segregation-induced self-assembly process. Monodisperse thermo-responsive hollow poly(N-isopropylacrylamide) (PNIPAM) microgels were first synthesized by seed precipitation polymerization using colloidal SiO2 nanoparticles as seeds, followed by removing the silica cores of the formed SiO2/PNIPAM core/shell composite microgels with hydrofluoric acid. Then, the discontinuously hollow hydrogel microfibers were produced by unidirectional freezing of 1 wt% hollow PNIPAM microgel aqueous dispersion in liquid nitrogen bath, followed by freeze-drying to remove the formed ice crystals. Many orderly arrayed dents were observed on the surfaces of the hydrogel microfibers by field-emission scanning electron microscopy, indicating that they are constructed by closely packed monodisperse hollow PNIPAM microgels. The effect of freezing method and the hollow microgel concentration in the aqueous dispersion on the morphological structure of the hollow hydrogel microfibers was investigated.

Magnetic nanocomposite Ba-ferrite/α-iron hollow microfiber: A multifunctional 1D space platform for dyes removal and microwave absorption

A facile gel-thermal selective reduction process was designed for the preparation of magnetic nanocomposite Ba-ferrite/α-iron hollow microfibers. The nanocomposite Ba-ferrite/α-iron microfibers were obtained after the reduction of the as-prepared Ba-ferrite/α-Fe2O3 microfibers at 375°C for 1h. SEM and TEM revealed that the nanocomposite Ba-ferrite/α-iron microfibers were constructed by plate-like and spherical particles with hollow structure. Dye removal experiments demonstrate that the combination of Ba-ferrite and α-iron can lead to larger numbers of methyl blue dye molecules removal from aqueous solution. This enhanced removal performance can be attributed to the unique nanocomposite structure, peculiar electron transfer, adsorption and reduction. These binary microfibers can also be used as absorbents for microwave absorption. For the Ba-ferrite/α-iron microfibers with designed mass ration of 1/4 reduced at 375°C and a specimen thickness of 3.0mm, the optimal reflection loss (RL) reaches −20.6dB at 13.1GHz, and the bandwidth exceeding −10dB covers the whole X-band and 54% Ku-band. Hence, these magnetic nanocomposite Ba-ferrite/α-iron hollow microfibers can provide a multifunctional 1D space platform for dyes removal and microwave absorption.

Simulation and verification of macroscopic isotropy of hollow alginate-based microfibers

A simulation of tensile strength of various alginate-based hollow microfibers using FEA analysis has been conducted with the hypothesis of macroscopic isotropy and linear elastic-plastic behavior. Results of student t-tests indicated that there was no significant difference between the experimental and simulated tensile strengths (p = 0.37, α = 0.05), while there was a significant reduction in elasticity as a result of chitosan coating (p = 0.024, α = 0.05). The hypothesis of macroscopic isotropy was verified by highly correlated (R2 ≥ 0.92) theoretical and experimental elongation at break measurements, findings that could be extended to the failure analysis of alginate microfibers used in regenerative medicine.

Microwave absorption of sandwich structure based on nanocrystalline SrFe12O19, Ni0.5ZnO.5Fe2O4 and alpha-Fe hollow microfibers.

The microwave absorption properties of sandwich structural absorbers based on the nanocrystalline strontium ferrite (SrFe12O19), NiZn ferrite (Ni0.5Zn0.5Fe2O4) and alpha-iron (alpha-Fe) hollow microfibers with diameters of 1-3 microm have been investigated in the frequency range of 2-18 GHz. The sandwich absorbers composed of nanocrystalline ferrite hollow microfibers as the outer or inner layer, and the nanocrystalline alpha-Fe hollow microfibers as the interlayer, have strong microwave absorption with a broad band and thin thickness. Their microwave absorption properties in 2-18 GHz are mainly influenced by the arrangement, each layer thickness and total thickness. It finds that the sandwich absorber with 1.6 mm thick SrFe12O19 microfibers as the outer layer, 0.2 mm thick alpha-Fe microfibers as the interlayer and 0.2 mm thick Ni0.5Zn0.5Fe2O4 microfibers as the inner layer, exhibits an optimal reflection loss (RL) value of -120.1 dB at 13.2 GHz and the bandwidth with RL exceeding -10 dB covers 83% of X-band (8.2-12.4 GHz) and the whole K(u)-band (12.4-18 GHz). This enhancement microwave absorption can be attributed to the unique coupling of the nanocrystalline ferrite and alpha-Fe hollow microfibers arising from the shape anisotropy, interface and small size effects.

Rapid Decolorization of Methyl Blue in Aqueous Solution by Recyclable Microchannel-like La0.8K0.2FeO3 Hollow Microfibers

A novel catalyst of microchannel-like La0.8K0.2FeO3 hollow microfibers for methyl blue (MB) decolorization in aqueous solution via the catalytic wet air oxidation (CWAO) process was prepared by the citrate–gel process. The microchannel-like La0.8K0.2FeO3 hollow microfibers were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), nitrogen adsorption–desorption, and X-ray photoelectron spectroscopy (XPS) methods. The hollow microfibers have a diameter of about 2 μm, length of tens of micrometers, and hollow structure, and the ratio of hollow diameter to fiber diameter is around 0.5. There are at least two kinds of oxygen species on the hollow microfibers according to XPS analysis. The results show these microfibers have a high activity for MB decolorization and most MB can be decolorized in about 30 min under ambient conditions. The rapid decolorization of MB by La0.8K0.2FeO3 hollow microfibers is largely due to the unique microchannel-like structure of the microfibers and active oxygen on the microfiber surface. The MB decolorization data follow well with the second-order reaction model, and the calculated activation energy for MB decolorization process by the Arrhenius equation is 14.27 kJ/mol. The catalytic activity of the microfibers did not exhibited an obvious degradation after cycling ten times.

Bandwidth Enhancement in Microwave Absorption of Binary Nanocomposite Ferrites Hollow Microfibers

The binary Ba0.5Sr0.5Fe12O19 (BSFO)/Ni0.5Zn0.5Fe2O4 (NZFO) nanocomposite ferrites hollow microfibers with high aspect ratios have been prepared by the gel precursor transformation process. These microfibers possess a high specific surface area about 45.2 m2 g(-1), and a ratio of the hollow diameter to the fiber diameter estimated about 5/7. The binary nanocomposite ferrites are formed after the precursor calcined at 750 degrees C for 3 h. Their minimum reflection loss (RL) is -38.1 dB at 10.4 GHz. The microwave absorption bandwidth with RL value exceeding -20 dB covers the whole X-band (8.2-12.4 GHz) and Ku-band (12.4-18 GHz). This enhancement in microwave absorption can be attributed to the exchange-coupling interaction, interfacial polarization and small size effect in nanocomposite hollow microfibers.

Silica-based branched hollow microfibers as a biomimetic extracellular matrix for promoting tumor cell growth in vitro and in vivo.

A novel scaffold composed of loosely branched hollow silica microfibers that has been proven to be highly biocompatible is proposed for the 3D culture of cancer cells. The MCF-7 cancer cells can grow and proliferate freely inside the scaffold in the form of multicellular spheroids. MCF-7 cancer cells cultured on the current 3D silica scaffold retained significantly more oncological characters than those cultured on the conventional 2D substrate and can serve as in vitro tumor model for studying cancer treatment.

Modeling Diffusivity Through Alginate-Based Microfibers: A Comparison of Numerical and Analytical Models Based on Empirical Spectrophotometric Data

The study of mass transport across hollow and solid 3D microfibers to study metabolic profiles is a key aspect of tissue engineering approach. A new modified numerical mathematical model based on Fickian equations in cylindrical coordinates has been proposed for determining the membrane diffusivity of 2% (w/v) alginate-based stents cross-linked with 10% CaCl2. Based on the economical and direct spectrophotometric measurements, using this model, inward diffusivities ranging from 5.2x10-14 m2/s 2.93x10-12m2/s were computed for solutes with Stokes radii ranging between 0.36 to 3.5 nm, diffusing through bare alginate and alginate-chitosan-alginate microfibers. In parallel an analytical solution to the cylindrical Fickian equation was derived to validate the numerical solution using experimental diffusion data from a solid stent. Excellent agreement was found between the numerical and analytical models with a maximum calculated residual value of 4%. Using these models, a flexible computational platform is proposed to conduct custom diffusion and MW cut-off characterization across micro-porous microfibers not limited to alginate in composition.

5AM2-D-2 微生物屋外培養のための中空マイクロファイバーの製作(5AM2-D 微細加工技術)

Microorganisms' mass cultivation in open-environment is demanded to accelerate the use of microorganisms itself and its metabolite to reduce the dependence on fossil resources. In tissue engineering, cell-laden microfibers which grow biological cell within the fiber has a high profile. One approach is to structure fiber with calcium alginate. In order to apply this technique in microorganisms' cultivation, we proposed a process to fabricate hollow microfiber in this study. We also experimentally verified that porous surfaced hollow microfiber worked as protection for inside microorganisms from viruses. Our results show that hollow microfiber has a potential to culture microorganisms massively in open-environment.

Coaxially Electrospun Axon-Mimicking Fibers for Diffusion Magnetic Resonance Imaging

The study of brain structure and connectivity using diffusion magnetic resonance imaging (dMRI) has recently gained substantial interest. However, the use of dMRI still faces major challenges because of the lack of standard materials for validation. The present work reports on brain tissue-mimetic materials composed of hollow microfibers for application as a standard material in dMRI. These hollow fibers were fabricated via a simple and one-step coaxial electrospining (co-ES) process. Poly(ε-caprolactone) (PCL) and polyethylene oxide (PEO) were employed as shell and core materials, respectively, to achieve the most stable co-ES process. These co-ES hollow PCL fibers have different inner diameters, which mainly depend on the flow rate of the core solution and have the potential to cover the size range of the brain tissue we aimed to mimic. Co-ES aligned hollow PCL fibers were characterized using optical and electron microscopy and tested as brain white matter mimics on a high-field magnetic resonance imaging (MRI) scanner. To the best of our knowledge, this is the first time that co-ES hollow fibers have been successfully used as a tissue mimic or phantom in diffusion MRI. The results of the present study provide evidence that this phantom can mimic the dMRI behavior of cellular barriers imposed by axonal cell membranes and myelin; the measured diffusivity is compatible with that of in vivo biological tissues. Together these results suggest the potential use of co-ES hollow microfibers as tissue-mimicking phantoms in the field of medical imaging.

Structure and Catalytic Soot Combustion of Perovskite La-Mn-O Hollow Microfibers via Gel-Precursor Transformation Process

The nanocrystalline perovskite La-Mn-O hollow microfibers were prepared by the gel-precursor transformation process from reagents of metal salts and citric acid. The gel precursor and resultant products were characterized by Fourier transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy. The specific surface area was measured by the Brunauere-Emmette-Teller method. The catalytic performance of soot combustion was evaluated by thermo-gravimetric analysis under model conditions. The nanocrystalline La-Mn-O hollow microfibers calcined at 650 °C for 6 h are characterized with diameters of 2-8 µm, aspect ratios (length/diameter) about 5-15, a micro-tunnel with an estimated ratio 1/3 of the hollow diameter to fiber diameter, and a high specific surface area of 36.7 m2/g that is 1.9 times higher than the counterpart nanosized powder. This nanocrystalline La-Mn-O hollow microfibers catalyst exhibit a high catalytic activity for the soot combustion, with a low T50 of 397°C, which is largely owing to the high surface area and the micro-tunnel structure.