Needleless Electrospinning: Developments and Performances

Abstract

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 & 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 “Taylor cone” (Taylor, 1969)). When the electric force overcomes the surface tension of the polymer solution, the polymer solution ejects off the tip of the “Taylor cone” 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). Electrospun nanofibers exhibit many unique characteristics, such as high surface-to-mass ratio, high porosity with excellent pore interconnectivity, flexibility with reasonable strength, extensive selection of polymer materials, ability to incorporate other materials (e.g. chemicals, polymers, biomaterials and nanoparticles) into nanofibers through electrospinning, and ability to control secondary structures of nanofibers in order to prepare nanofibers with core/sheath structure, side-by-side structure, hollow nanofibers and nanofibers with porous structure (Chronakis, 2005). These characteristics enable electrospun nanofibers to find applications in filtrations, affinity membranes, recovery of metal ions, tissue engineering scaffolds, release control, catalyst and enzyme carriers, sensors and energy storage (Fang et al., 2008). In spite of the wide applications, electrospun nanofibers are produced at a low production rate when conventional needle electrospinning setup is used, which hinders their commercialization.