NEAR-FIELD ELECTROSPINNING

Inkjet printing (Singh et al., 2010; De Gans et al., 2004; Zhan et al., 2016) directly transfers functional materials onto a plate or stereoscopic substrate at room temperature, and does not require a mask-plate or photolithography process. Therefore, this noncontact technology has become a growing focus in the fifield of micro/nano fabrication. Thanks to these characteristics, inkjet printing technology can meet the developmental demands of precise integration, multifunction composition, and stereoscopic fabrication for micro/nano systems.

Electrospinning is a versatile printing method that utilizes an electrical fifield to draw a fifine jet from a viscoelastic solution. With great advantages, electrospun nanofifibers have displayed application potential in the fifields of micro energy systems (Li et al., 2017; Zhang et al., 2016b), tissue engineering (Tang et al., 2016; Torres-Giner et al., 2016), wearable sensors (Jian et al., 2017), nanodevices (Min et al., 2015; Miao et al., 2010), microelectromechanical systems (MEMSs) (Hess et al., 2011), etc. But, the chaotic motion of the charged jet and the random deposition of the nanofifiber are the biggest obstacles to the application of electrospun nanofifibers. Several methods have been developed to control the deposition behavior of the as-spun charged jet, such as assistant electrodes (Deitzel et al., 2001), rotational collectors (Wang et al., 2014), parallel electrode collectors (Zhao et al., 2016a), inducing tip collectors (Kim et al., 2010), etc. Owing to the lack of precise deposition control for a single nanofifiber, the aforementioned methods cannot meet the requirements of integration fabrication of micro/nanodevices.

In 2006, Sun et al. (2006) invented a novel technology, named near-fifield electrospinning (NFES), to realize controllable deposition of a charged thin jet at a low voltage. When the distance between the spinneret and the collector is shortened, the nanofifiber can be deposited precisely onto the collector before the jet steps into the chaotic motion stage. A solid probe is used as the spinneret for NFES, which increases the strength of the electrical fifield and provides strong restriction to enhance the stability of the charged jet. The straight jet can be used to direct write a predesigned pattern by controlling the motion trajectory of the collector. The NFES process can be operated under the observation of a microscope to increase the position precision of the direct-written nanofifiber. The visual operation improves the process compatibility of electrospinning, and promotes the application of electrohydrodynamic printing technology into micro/nanosystem integration fabrication.

In this chapter, the recent advances in NFES are reviewed and summarized, including themechanism, operation process, and applications. The NFES technology provides new prospects for the fast, low-cost, and precise integration fabrication of micro/nanosystem.

The ejection process of near-fifield electrospinning (NFES). (A) Schematic of the NFES apparatus with a solid tungsten probe. (B) Scanning electron microscopy micrograph of the tungsten probe tip. (C) An optical photo of a solution droplet attached on the probe tip. (D) The stable jet that ejects from the spinneret. (E) The continuous jet with a smaller droplet. © 2006 American Chemical Society

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