Design and fabrication of tubular scaffolds via direct writing in a melt electrospinning mode

Flexible tubular structures fabricated from solution electrospun fibers are finding increasing use in tissue engineering applications. However it is difficult to control the deposition of fibers due to the chaotic nature of the solution electrospinning jet. By using non-conductive polymer melts instead of polymer solutions the path and collection of the fiber becomes predictable. In this work we demonstrate the melt electrospinning of polycaprolactone in a direct writing mode onto a rotating cylinder. This allows the design and fabrication of tubes using 20 μm diameter fibers with controllable micropatterns and mechanical properties. A key design parameter is the fiber winding angle, where it allows control over scaffold pore morphology (e.g. size, shape, number and porosity). Furthermore, the establishment of a finite element model as a predictive design tool is validated against mechanical testing results of melt electrospun tubes to show that a lesser winding angle provides improved mechanical response to uniaxial tension and compression. In addition, we show that melt electrospun tubes support the growth of three different cell types in vitro and are therefore promising scaffolds for tissue engineering applications.     

Electrospinning of polycaprolatone nanofibers with DMF additive: The effect of solution proprieties on jet perturbation and fiber morphologies

Electrospinning is a straightforward method to produce sub-micrometer or nanoscale fiber. Polycaprolactone (PCL), an important polymer in biomedical applications, has been electrospun in several solvent systems. N,Ndimethylformamide (DMF) is often used as an additive in the solvent system to prepare PCL nanofibers. The adding of the DMF changes the physical properties of the solution. To trace and understand the influence of these changes on the jet formation as well as the resultant fibers morphologies, a model of jet perturbation based on the Plateau-Rayleigh Instability theory was established to explicate the formation of the particles/fibers and some experiments for testing the solution properties and fibers morphologies were carried out. With the adding of DMF in dichloromethane (DCM)/DMF mixed solvents, the solution surface tensions increase while solution viscosities decrease, which triggers the change of electrospinning to electrospraying in general. However, according to the obtained results, the addition of the DMF makes it easier to induce the transformation of particles electrospraying to fibers electrospinning with smaller diameter. This is attributed to the higher dielectric constant, lower vapor pressure, and higher electric conductivity of DMF. The theoretical model and experimental results strengthen the relations of solution properties, jet moving behaviors, and the resultant fiber morphologies.     

An investigation on the comparison of wet spinning and electrospinning: Experimentation and simulation

Polysulfonamide (PSA) has been widely used in many fields because of its excellent thermodynamic properties. In this study, PSA fibers were prepared separately via two different spinning ways, including conventional wet spinning and electrospinning. Fluid motion of wet spinning and electrostatic field of electrospinning were modeled using finite element analysis to investigate the spinning process. The properties of fabricated PSA fibers were characterized systematically by scanning electron microscope (SEM), fourier transform infrared spectrometer (FTIR), X-ray diffraction (XRD), thermal gravity analysis (TGA) and electronic strength tester. Based on the simulation and theoretical analysis of spinning process, it was found that the extruding force of the wet spinning is larger than that of the electrospinning. The larger extruding force makes the alignment of macromolecules inside fiber relatively uniform, and a higher proportion of crystallization happens. Accordingly, the mechanical properties and thermal stability of PSA fibers could be improved due to a higher proportion of crystallization. The experimental results of mechanical strength and TG test are coincided with the simulation results. PSA fiber prepared by wet spinning has better thermal stability and mechanical properties than that fabricated by electrospinning.     

Preparation of self-clustering highly oriented nanofibers by needleless electrospinning methods

Polyacrylonitrile (PAN) oriented nanofibers were produced by homemade needleless electrospinning device. Spiral coils were adopted to replace the traditional spinning needles in this equipment. The tracks of multi-jets were controlled by adjusting the microcurrent during the eletrospinning process. The microcurrent value and the motion track of the spinning jet during the spinning process were observed, the fiber morphology and the mechanical properties of fiber membranes were measured. The results revealed that the average diameters of the electrospun fibers were increased from 490 nm to 740 nm. with the addition of organic salt. Meanwhile, the self-clustering phenomenon was obviously observed, and the mechanical properties of obtained fibers were also altered, the tensile strength was improved from 3.63 MPa to 23.90 MPa, while the strain decreased from 74.6 % to 27.1 %.     

Large scaled fabrication of microfibers by air-suction assisted needleless melt electrospinning

A cone-shape spinneret with air-suction assisted was used for the production of ultrafine fibers by melt electrospinning. The influence of the applied voltages on the number of jets and the effects of the different types of air flow (air blowing and air suction) on the fiber bundle were studied. It was demonstrated that the breadth of the diameter distribution of the fibers decreases markedly and the production rate was also improved when air suction and higher applied voltages were applied. Therefore this new melt electrostatic spinning equipment can meet the need of some special applications and industrial mass production of nanofibers.     

Electrical properties of ultrafine nylon-6 nanofibers prepared via electrospinning

We report on the preparation and electrical characterization of nylon-6 nanofibers via electrospinning technique. During electrospinning, the polymer solution became highly ionized and emerging out of the micro-tip syringe by forming mesh-like ultrafine nanofibers structure in between the main fibers. The resultant nylon-6 nanofibers were well-oriented with uniform structure. The diameter of the ultrafine nanofibers (7 to 40 nm) is one order less than those of main fibers (100 to 200 nm). The current-voltage (I-V) measurements revealed a linear curve with an occurrence of negative differential resistance (NDR) behavior. The existence of NDR region in the nylon-6 nanofibers can be attributed to the tunneling current through the ultrafine structures. The fabrication of nanofibers, in the form of ultrafine mesh-like form, is relatively fast and inexpensive, and it paves the way to build up of new dimension for nano device applications.     

Electric current in polymer solution jet and spinnability in the needleless electrospinning process

In this study, the electric current of polymer solution jet was measured during a needleless rod electrospinning process using memory oscilloscope recording. According to the results, electric current of solution jet increases as the polyurethane (PU) and tetraethylammonium bromide (TEAB) salt concentration increase. Especially at 17.5 and 20 wt% PU concentrations, electric current increases dramatically with TEAB concentration. Also, there is a strong relationship between the electric current in the solution jet, spinnability and the spinning performance of the roller electrospinning. Thus, the spinnability of polymer solutions can be easily estimated using this simple method.     

Encapsulation of Phytoncide in Nanofibers by Emulsion Electrospinning and their Antimicrobial Assessment

Phytoncides are volatile organic compounds released from trees and plants and are well known for their natural antibacterial activity. In this study, emulsion electrospinning was used to encapsulate phytoncide in the core of nanofibers, with the aim of developing environmentally friendly, functional nanofibers with a sustained release of the encapsulated component. Core/sheath structured phytoncide/poly(vinyl alcohol) nanofibers were successfully prepared by emulsion electrospinning using an ordinary single-nozzle electrospinning setup. An oil-in-water emulsion of an aqueous solution of poly(vinyl alcohol) (as the aqueous phase) and phytoncide (as the oil phase) was used to prepare the core/sheath structured nanofibers. Nanocomposite fibers were electrospun under various spinning conditions and emulsion formulations to find the suitable processing conditions for fabricating nanofibers with core/sheath structures. The resulting nanofibers exhibited a well-aligned core/sheath structure with fiber diameters of 250-350 nm. The release profile of phytoncide from the core of nanofibers over a 21 day period showed that phytoncide was released in a sustained manner over 14 days. The core/sheath structured phytoncide/poly(vinyl alcohol) nanofibers exhibited 99.9 % bacterial reduction against both Staphylococcus aureus and Escherichia coli, indicating that the encapsulated phytoncide in the fiber provided strong antimicrobial effects.     

Effects of wet-spinning conditions on structures,mechanical and electrical properties of multi-walled carbon nanotube composite fibers

A series of composite fibers composed of multi-walled carbon nanotube (MWCNT) and poly(vinyl alcohol) (PVA) are prepared by varying co-flowing wet-spinning conditions such as spinning geometry and PVA concentration, which affect aligning shear stress for MWCNTs during the wet-spinning. Then, structural features, mechanical and electrical performances of MWCNT/PVA composite fibers are investigated as a function of the aligning shear stress of the wet-spinning process. SEM images of the composite fibers exhibit that MWCNTs are wetted effectively with PVA chains. Polarized Raman spectra confirm that the alignment of MWCNTs is enhanced along the composite fiber axis with increasing the aligning shear stress of the spinning process. Accordingly, initial moduli and tensile strengths of the composite fibers are significantly increased with the increment of the aligning shear stress. In addition, it is found that electrical conductivities of MWCNT/PVA composite fibers increase slightly with the aligning shear stress, which is associated with the formation of efficient electrical conduction paths caused by well-aligned MWCNTs along the composite fiber axis.     

An Investigation into Scalability Production of Ultra-Fine Nanofiber Using Electrospinning Systems

An integrated experimental and modeling approach was utilized to study scalable production of nanofibers via electrospinning. Two concepts have been investigated to study the fabrication of PAN nanofibers, which are needle-based and orifice-guided electrospinning to utilize the optimum setup. Moreover, it was observed that the natural flow rate of electrospinning does not scale linearly with number of needles (unlike polymer processing methods such as dry spinning), which was explained based on the partial pressure of the solvent vapor, peculiar to multi-needle setup, and the stress relaxation in the solution. In addition, it was demonstrated that the minimum voltage required to continuously electrospun fibers increases as the distance between needles is reduced, which was explained by the shielding effect of neighboring needles and elucidated by the Finite Element Analysis (FEA) models. Nano-fibers with diameters less than 100 nm were produced in this investigation.     

Morphological aspects of polymer fiber mats obtained by air flow rotary-jet spinning

Fiber mats were obtained by using a modified rotary-jet spinning system, which allows a forced air flow produced by an air compressor to interfere with the polymer jets. The main focus of current studies rely on the range of morphological and dimensional characteristics of fibers that may be expected when using this new technical setup of a rotary-jet based spinning system. In fact, this work represents a proof of concept study regarding the potential of an air flow modified rotary-jet spinning for obtaining continuous fibers and nonwoven mats. The morphological examinations by scanning electron microscopy were proved the efficiency of this technique on obtaining relative homogeneous fiber mats from different raw compositions of pure and admixed, natural and synthetic polymers with different molecular masses and polydispersity degrees, like gelatin, polyurethane, and poly (vinyl chloride). The feasibility of air flow rotary-jet spinning was also tested for simultaneous independent deposition of mixed fiber mats from solutions of two polymers made in different solvents, and it was found that by carefully selecting the ratio of polymers through spinnerets number, this technique could be successfully used even in difficult solvent conditions otherwise incompatible with traditional spinning techniques. The distribution of fiber diameters was varying between nanometer scales (100–700 nm) in the case of pure polyurethane and micrometer ranges (2–12 µm) for gelatin-polyurethane mixed mats, which are convenient for various applications, from dressings and scaffolding to different filter systems. Besides the already known advantages of rotary-jet versus electrospinning, the air flow allows the control of solvent evaporation, extending the applicative range of this technique.     

Fiber formation model for PVP (polyvinyl pyrrolidone) electrospinning. I. Critical voltage

The major concern in electrospinning process is the occurrence of jet formation. In order to study the physical properties of the solution and process parameters, critical voltage models in electrospinning are presented. Critical voltage models are developed to predict the onset of electrospinning. Rayleigh’s instability phenomena at the moment of droplet shape change, Zeleny’s research for relationship between surface tension and electric field, and Taylor’s critical voltage model are the theoretical backgrounds of this proposition. SIC 1, SIC 2, GSP models are suggested to calculate the critical voltage according to the shapes of polymer drop. This newly developed critical voltage model for polymer jet during electrospinning could predict other processing parameters, such as, TCD, radius and length of nozzle, and is verified by comparing experimental values using polyvinyl pyrrolidone (PVP) solution through electrospinning.     

Role of elasticity in control of diameter of electrospun PAN nanofibers

A series of poly(acrylonitrile-co-methylacrylate) copolymers (PAN) with varying molecular weight were synthesized by radical copolymerization using α-α′-azobisisobutyronitrile (AIBN) as initiator. These copolymers were dissolved at different concentrations in DMF and electrospun at Minimum electrospinning voltage (MEV) to correlate electrical energy required to perform the mechanical work during the spinning of the fibers. The diameters of the resultant fibers were correlated with the Berry number and average number of entanglements per chain of the spinning solution. It was observed that number of entanglements per chain, which represents the capacity of the polymer system to store elastic energy, could correlate well with the ultimate diameter of the fiber. Interestingly, the diameters of the nanofibers were found to increase linearly with increase in number of entanglements per chain with two distinct regions having transition of the slope at number of entanglement value of 3.5.     

Carbon nanotube/poly(vinyl alcohol) fibers with a sheath-core structure prepared by wet spinning

A method for manufacturing sheath-core structured fibers was developed using wet spinning techniques. The core portion of a fiber was prepared using a carbon nanotube (CNT) solution while the sheath used a fiber-forming polymer such as polyvinyl alcohol (PVA). Preparation methods of CNT solutions were investigated and it was found that dispersivity and concentration played an important role in the formation and spinning of fiber??s core. CNT solution prepared using a surfactant with high molecular weight such as sodium lignosulfonate (SLS) was most effective and the CNT concentration was as high as 30 g/l. Fiber processing conditions were optimized and it was determined that stretching fibers in the coagulation bath was a significant step in the formation of a solid and well structured core. Drawn fibers were so strong and flexible that they could be woven into a fabric for potential use as a pressure sensor. These results are relevant for practical applications, such as the development of large-area fabric sensors. Furthermore, the described procedure to produce sheath-core CNT fibers is scalable as wet spinning methods have been widely used in the fiber industry.     

Fiber spiral motion in a swirl die melt-blowing process

Melt blowing is a major process for producing nanofibrous nonwovens. Compared to another technology for producing nanofibrous nonwovens, electrospinning, melt blowing applies high-speed air flow field to attenuate the extruded polymer jet. It is known that the essential electrospinning mechanism is a rapidly whipping jet in an electric field. While there are few studies on the fiber whipping in the melt-blowing process. In this study, a high-speed camera was used to capture the fiber path below a single-orifice melt-blowing swirl die. The spiral path of the fiber was revealed. The characteristics of the whipping amplitude, whipping frequency, and the fiber velocity were obtained. Fiber diameter reduction ratio contributed by the spiral path was calculated by establishing a mathematical model. The study indicates that spiral path of the whipping plays an important role in fiber attenuation near the die.     

Effect of polystyrene mixing on mechanical properties and structural changes in melt spinning

To increase the spinning speed of poly(trimethylene terephthalate) (PTT) fibers, polystyrene (PS) was selected as an additive polymer in the PTT matrix. Mixing of the immiscible PS with PTT led to an increase in spinning speed up to 5,500 m/min. PS was employed to improve the extensibility of the matrix PTT in the spinning process as it can prevent PTT molecular orientation. Experimental results show that the mixing of PS achieved this. The elongation at break of spun fibers increased with the amount of PS. PS addition prevented fiber orientation, especially amorphous orientation, and improved drawability, and as such, increased spinning speed up to 5,500 m/min.     

Fabrication and surface modification of melt-electrospun poly(D,L-lactic-co-glycolic acid) microfibers

In this study, biodegradable poly(D,L-lactic-co-glycolic acid) (PLGA) fibers were prepared by a melt-electrospinning and treated with plasma in the presence of either oxygen or ammonia gas to modify the surface of the fibers. The effects of processing parameters on the melt-electrospinning of PLGA were examined in terms of fiber morphology and diameter. Among the processing parameters, the spinning temperature and mass flow rate had a significant effect on the average fiber diameter and its distribution. The water contact angle of melt-electrospun PLGA fibers decreased significantly from 123 ° to 55 ° (oxygen plasma treatment) or to 0 ° (ammonia plasma treatment) by plasma treatment for 180 sec, while their water content increased significantly from 2.4 % to 123 % (oxygen plasma treatment) or to 189 % (ammonia plasma treatment). Ammonia gas-plasma enhanced the surface hydrophilicity of PLGA fibers more effectively compared to oxygen gas-plasma. X-ray photoelectron spectroscopy analysis supported that the number of polar groups, such as hydroxyl and amino groups, on the surface of PLGA fibers increased after plasma treatment. Overall, the microfibrous PLGA scaffolds with appropriate surface hydrophilicity and fiber diameter could be fabricated by melt electrospinning and subsequent plasma treatment, without a significant deterioration of fiber structure and dimensional stability. This approach of controlling the surface properties and structures of fibers could be useful in the design and tailoring of novel scaffolds for tissue engineering.     

Towards Well-aligned electrospun PAN/MWCNTs composite nanofibers: Design,fabrication, and development

A proper collector is designed and examined in electrospinning process to produce electrospun nanofibers with favored mechanical propertied. The quality of product was controlled by changing and optimizing the process variables, namely electrospinning time, gap distance, and collector rotating speed in a manner that well-aligned yarns were fabricated from polyacrylonitrile (PAN) dilute solutions. It was found that the tensile characteristics of fabricated yarns are greatly dependent on the process variables. Incorporation of multi-walled carbon nanotubes (MWCNTs) into the polymer solution revealed improvement to the yarn strength because of enhancement in alignment of the filaments. The state of fiber alignment and dispersion of MWCNTs were detected by means of scanning electron microscopy. It was illustrated that combination of nanofibers and microfibers gives PAN/MWCNTs composite nanofibers with high surface area and high porosity to satisfy sophisticated users.     

Matrix-fibril morphology development of polypropylene/poly(butylenes terephthalate) blend fibers at different zones of melt spinning process and its relation to mechanical properties

Different shapes of dispersed phase such as sphere, laminar and fibrillar can form in the matrix phase of polymer blends. Production of blend fibers in melt spinning process can result more effective in fibrillar phase morphology formation than in other processes. In this research, the matrix-fibril morphology development during the melt spinning of polypropylene/poly(butylenes terephthalate) was studied. The shapes of blend dispersed phase collected from different zones of the melt spinning line were evaluated by scanning electron micrographs (SEM) and rheological mechanical spectra (RMS). The results showed that fibrillar shape could not be created in the PP/PBT blend fiber samples exited from the spinneret orifice (gravity spun fibers) at low contents (5 percent) of the PBT dispersed phase. However, a complete fibrillar structure was formed in all the as-spun PP/PBT blend fiber samples (melt drawn). The rheological evaluations confirmed a network structure resulting from fibril formation for the samples with high contents (20–40 %) of the PBT dispersed phase and the formation of spherical shape with low contents (5–10 %) of the PBT dispersed phase in matrix of the blend fibers. It was observed that the flow fields of processing zones and blend ratio, in producing the blend fibers, have intensive effects on morphological variations; besides there was a strong relation between the mechanical and morphological properties.     

The effects of operating parameters on the fabrication of polyacrylonitrile nanofibers in electro-centrifuge spinning

Introduced recently, electro centrifuge is a new method for nanofiber production. In the electro-centrifuge method, fibers are produced by the simultaneous use of electrical and centrifugal forces. In this research, the effective parameters in the production of PAN nanofibers diameter and the influence of each of them have been discussed. These parameters are voltage, rotation speed, flow rate of exiting solution from nozzle and viscosity of solution. Also the capability of fiber production by this method is compared with the conventional electrospinning system. Results show that a significant enhancement can be achieved by proper adjustments of the polymer solution viscosity, applied voltage, and rotational velocity in fiber production rate. To exemplify, in a PAN polymer solution, the increased production rate of electro centrifuge varied from 193 to 1200 percent, as compared with a similar electrospinning method in which the polymer concentration and applied voltage varied in a range of 13 to 16 wt% and 15 to 10 kV, respectively.