COAXIAL ELECTROSPINNING

Electrospinning is a very effective technique for fabricating continuous ultrafifine fifibers with diameters down to the micro/nanometer scale from polymers, ceramics, and composites since it was put forward (Yu et al., 2004; Greiner and Wendorff, 2007; Reneker and Yarin, 2008; Wang et al., 2011; Ding et al., 2016). Compared with other micro/nanosized object fabrication approaches, electrospinning has outstanding advantages of method simplicity, material versatility, easy large area fabrication, high throughput of continuous fifibers, low cost, etc. In addition, the electrospinning technique is also notable for its facile structural tenability of products. By tuning the materials system, solvent, solution concentration, electrostatic fifield strength, collector confifiguration, environmental humidity and temperature, as well as some other parameters, routine electrospinning can generate micro/nanofifibers with cylindrical, ribbon, spindle, porous, and other complex structures. However, the simple single-nozzle electrospinning setup forms just a single flfluid jet, which can generate only a solid fifiber, without inner structure, in principle. These simply structured nanofifibers largely restricted the applications in many fifields. Since the late 1990s, a good number of reports have demonstrated that nanomaterials with hollow inner structures or heterogeneous coreeshell structures often exhibit unusual properties compared with their solid counterparts. Researchers were thus inspired by the idea that it was necessary not only to change the chemical composition, but also to further tune the innerstructures of the electrospun nanofifibers. Apparently, routine single-nozzle electrospinning was not capable of making fifibers with complex inner structures and compositions. In 2002, a revolutionary coaxial  electrohydrodynamic jetting technique was proposed by I.G. Loscertales, A. Barrero, and colleagues, by which coreeshell capsules were successfully fabricated from a charged coaxial jet of immiscible liquids (Loscertales et al., 2002). This newfangled technology immediately sparked great interests of electrospinning researchers and was applied to generate continuous core-shell or hollow fifibers in short time, which was named coaxial electrospinning or simpler co-electrospinning. Fig 5.1 illustrated a schematic of coaxial electrospinning device (Zhang et al., 2004).

This newfangled technology immediately sparked a great interest in electrospinning researchers and in a short period of time was applied to generating continuous coreeshell or hollow fifibers, which was named coaxial electrospinning or, more simply, coelectrospinning. In the same period, another side-by-side electrospinning method was also put forward, which could make “Janus” nanofifibers with heterogeneous compositions. As the second generation of electrospinning technique, coelectrospinning and side-by-side electrospinning showed a powerful versatility in creating a good variety of ultrafifine fifibers with interesting hollow, coreeshell, and many other intriguing structures and compositions. Then researchers began to consider whether more complex structures could be created by this facile method. In this regard, Zhao, Jiang, and colleagues further proposed the third-generation multiflfluidic (coaxial) electrospinning technique in 2007. By this multiflfluidic strategy, electrospun fifibers with multichannel structures or multiple coreeshell structures were successfully generated, which further extended the possibilities of the electrospinning technique. Nowadays, hollow, coreeshell, side-by-side, multichannel, wire-in-tube, multicoreeshell, multishell, bamboolike, and ultraporous fifibers and many other interesting variants have been successfully fabricated by coaxial, side-by-side, or multiflfluidic electrospinning. These novel structured fifibers have found broad applications in biomaterials, drug delivery, highly effificient absorption and catalysis, energy materials, sensors, and many other areas. In this chapter, we will introduce the developing route of coelectrospinning and its products with different structures and compositions. After discussing fabrication, we will describe some representative applications of the complexly structured fifibers. At the end, we give a brief perspective on the future development of electrospinning.

The schematic diagram of the coaxial electrospinning apparatus with coaxial jets of two immiscible liquids. The Taylor cone of the core-shell jets from the two menisci and usually produce the core-shell structured micro/nano products (Zhang et al., 2004).

Paper link：https://www.sciencedirect.com/book/9780323512701/electrospinning-nanofabrication-and-applications