# Electrospun Nanofibers for Integrated Sensing, Storage, and Computing Applications
## 2 Different Types of Electrospun Nanofiber Process and Structures
### Aligned Nanofibers by Special Collectors  
Compared with the original unordered nanofibers, highly aligned nanofibers elicit considerable fascination among researchers on account of the improved performance resulting from their directionality. For example, one recent study indicated that the photoelectric devices made by aligned nanofibers tended to exhibit higher sensitivity than devices fabricated by the common messy nanofibers [45]. Among diverse approaches to realizing the aligned structure of fibers, near-field electrospinninging stands out due to the ability to precisely control the positions of deposited nanofibers. Conventional electrospinning is usually conducted in the far-field mode with the distance between the needle and the collector larger than 5 cm, while near-field electrospinninging works for a corresponding distance of 500 \(\upmu\)m-5 cm, causing the thorough change of the stage at which the fibers will be deposited on the collector. A typical scheme of near-field electrospinninging with a movable collector can be seen in Figure 7a [46]. With the decrease in spinning distance, the straight jet was put onto the collector in the absence of whipping instability. On this condition, the electric field between the spinneret and the collector is more concentrated, and the applied voltage and the accordingly flow rate could be reduced. In addition, near-field electrospinninging allows for the desired fiber distribution by adjusting the average motion speed of the needle (S\({}_{\text{n}}\)) and the relative movement speed of the collector (S\({}_{\text{c}}\)), and this was demonstrated by using an electrospun solution blend of PEO, water, and ethanol [47]. However, when the S\({}_{\text{c}}\) was smaller than S\({}_{\text{n}}\), the oscillation motion of the jet was inevitable, and wild coil fibers were deposited on the collector (Figure 7b). With the increase in the moving speed of the collector to 0.36 m/s, one aligned straight fiber of 0.49 \(\upmu\)m diameter was achieved because of the vanishment of the unstable oscillation (Figure 7c). By carefully matching the motion speed of the jet and collector, a specified pattern of the nanofibers could be obtained (Figure 7d,e) [48; 49]. Figure 7: **(a)** Schematic setup image for near-field electrospinninging to directly write fibers on a collecting board. Reprinted with permission from Reference [46]. Copyright 2021 Wiley-VCH GmbH. **(b–d)** SEM images of the PEO nanofibers were deposited on a silicon substrate when the collector moved at speeds of **(b)** 0.03 m/s, and **(c)** 0.36 m/s, respectively. Reprinted with permission from Reference [47]. Copyright 2010 IOP Publishing. **(d)** SEM image of PEO fibers deposited in a pattern by controlling the motion of the collector along with both X and Y directions during near-field electrospinninging. Reprinted with permission from Reference [48]. Copyright 2011, American Chemical Society. **(e)** The latticed pattern of polystyrene nanofibers by near-field electrospinningning. Reprinted with permission from Reference [49]. Copyright 2012 Elsevier Ltd.  
Apart from near-field electrospinning, change of the collectors is also an efficient approach to producing aligned nanofibers. Researchers came up with an intriguing method utilizing a Cu wire-framed drum acting as a collector, different from the conventional electrospinning setup [50]. As shown in Figure 8a,b, the drum had two circular nonconducting disks with a hole cut in the center. Each disk had many saw-cut notches and two disks were mounted on the end of a pipe separately. Using this kind of peculiarly designed collector, aligned poly (methyl methacrylate) (PMMA) fibers were fabricated, and the fiber diameter ranged from 2.5 to 3.5 \(\upmu\)m [51] (Figure 8c,d). Moreover, adopting collectors consisting of a pair of electrodes with an insulating gap (Figure 8e) was the most common method to obtain aligned nanofibers by exploiting the electrostatic forces associated with the electric field [52].