Fiber generators in needleless electrospinning

The conventional electrospinning often uses a needle-like nozzle to produce nanofibers with a very low production rate. Despite the enormous application potential, needle electrospun nanofibers meet difficulties in broad applications in practice, due to the lack of an economic and efficient way to scale up the electrospinning process. Recently, needleless electrospinning has emerged as a new electrospinning mode and shown ability to produce nanofibers on large-scales. It has been established that the fiber generator, also referred to as &ldquo;spinneret&rdquo; in this paper, in needleless electrospinning plays a key role in scaling up the nanofiber production. This paper summarizes the recent advances in the development of needleless spinnerets and their influences on electrospinning process, nanofiber quality, and productivity.<br /

Needleless electrospinning : influences of fibre generator geometry

The fibre generator shape and dimension are key factors affecting the needleless electrospinning process and fibre fineness. In this work, cylinder with rounded rim, disc and ball were used as spinnerets to electrospin polyvinyl alcohol and polyacrylonitrile solutions. A finite element method was used to analyse how the spinneret geometry affected the electric field generated during electrospinning and the associated changes in fibre diameter and productivity. For cylinder spinnerets, increasing the rim radius reduced the discrepancy of electric field intensity between the cylinder end and middle area, which affected the fibre productivity. The electrospinning failed to operate when the rim radius was over 20&thinsp;mm. With decreasing cylinder diameter, the electric field intensity in the middle area increased, improving the fibre productivity. Thinner disc spinnerets increased the electric field intensity, resulting in finer nanofibres and higher productivities. Ball spinnerets produced evenly distributed electric field, but failed to electrospin fibres when the diameters were below 60&thinsp;mm. It has been found that strong and narrowly distributed electric field in the fibre-generating area can significantly facilitate the mass production of quality nanofibres.<br /

Needleless electrospinning : developments and performances

Electrospinning technique has attracted a lot of interests recently, although it was invented in as early as 1934 by Anton (Anton, 1934). A basic electrospinning setup normally comprises a high voltage power supply, a syringe needle connected to power supply, and a counter-electrode collector as shown in Fig. 1. During electrospinning, a high electric voltage is applied to the polymer solution, which highly electrifies the solution droplet at the needle tip (Li &amp; Xia, 2004). As a result, the solution droplet at the needle tip receives electric forces, drawing itself toward the opposite electrode, thus deforming into a conical shape (also known as &ldquo;Taylor cone&rdquo; (Taylor, 1969)). When the electric force overcomes the surface tension of the polymer solution, the polymer solution ejects off the tip of the &ldquo;Taylor cone&rdquo; to form a polymer jet. The charged jet is stretched by the strong electric force into a fine filament. Randomly deposited dry fibers can be obtained on the collector due to the evaporation of solvent in the filament. There are many factors affecting the electrospinning process and fiber properties, including polymer materials (e.g. polymer structure, molecular weight, solubility), solvent (e.g. boiling point, dielectric properties), solution properties (e.g. viscosity, concentration, conductivity, surface tension), operating conditions (e.g. applied voltage, collecting distance, flow rate), and ambient environment (e.g. temperature, gas environment, humidity)

Needleless-electrospinning of PVA nanofibres

In this paper, we demonstrated that a thin metal disk can be used as nozzle to electrospin PVA nanofibres on a large-scale. With the rotation of a disk covered with a thin layer of electrically charged PVA solution, a large number of fibres were electrospun simultaneously from two sides of the disk and deposited on the electrode collector. The fibre production rate can be as high as 6.0 glhr, which is about 270 times higher than that of a corresponding normal needle based electrospinning system (0.022 g/hr). The effects of applied voltage, the distance between the disk nozzle and collector, and PVA concentration on the fibre morphology were examined. The dependency of fibre diameter on the PV A concentration showed a similar trend to that for a conventional electrospinning system using a syringe needle nozzle, but the diameter distribution was slightly wider for the disk electrospun fibres. The profiles of electric field strength in disk electrospinning showed considerable dependence on the disk thickness, with a thin disk exhibiting similar electric field strength profile to that of a needle electrospinning system.<br /

Upward needleless electrospinning of nanofibers

Polyacrylonitrile (PAN) nanofibers were prepared by a needleless electrospinning method using three rotating fiber generators, cylinder, disc and coil. The effects of the spinneret shape on the electrospinning process and resultant fiber morphology were examined. The disc spinneret needed the lowest voltage to initiate fiber formation, followed by the coil and cylinder. Compared to cylinder, the disc and coil produced finer fibers with narrower diameter distribution. The productivity of a coil was 23 g/hr, which was much larger than that of the cylinder spinneret having the same length and diameter. Finite elementary method was used to analyze the electric field. Stronger electric field was found to be formed on disc and coil surface, which concentrated on the disc circumferential edge and coil wire surface, respectively. For cylinder, the high intensity electric field was mainly concentrated on the end area. Concentrated electric field on the fiber generating surface could be used to explain the better electrospinning performance of coil, which may form a new concept for designing needleless electrospinning spinnerets.<br /