# Electrospun Nanofibers for Integrated Sensing, Storage, and Computing Applications
## 2 Different Types of Electrospun Nanofiber Process and Structures
### Hollow Nanofibers by Coaxial Spinnet  
The special core-shell structure of electrospun nanofibers can bring high conductivity of several orders magnitude higher than those of common solid nanofibers [35]. Hence, a novel technique of coaxial electrospinning has attracted more attention for fabricating continuous core-shell composite nanofibers or further hollow nanofibers. As shown in Figure 4, the key component of the coaxial electrospinning system is the coaxial needle, with one smaller capillary inside and another larger one in coaxial geometry [36]. These two capillaries separately drive different fluids with tunable flow rates. By pumping both inner and outer precursor solution in the two needles, a compound Taylor cone consisting of the core fluid wrapped by the shell fluid would emerge at the end of the tip to generate continuous core-sheath nanofibers on the receiving plate. In this process, adjustable factors, including the physical properties of both inner and outer materials, as well as electrospinning parameters, altogether play important roles in successfully preparing nanofibers of coaxial morphology. Specifically, the immiscibility of the two electrospun solutions ensures the detached structure of the core and shell fiber instead of mixing or inversion. Sufficient viscosity is another solution factor influencing the  
Figure 3: SEM images of The SEM of PLA/ZIF-8 porous fibers with varied ZIF-8 weight percentage: (**a**) 0, (**b**) 1 wt%, (**c**) 2 wt%, (**d**) 3 wt%, and (**e**) 5 wt%. Reprinted with permission from Reference [33]. Copyright 2018 Elsevier B.V. (**f**) SEM image of PS porous electrospun nanofibers (**g**) PAN fibers electrospun into liquid nitrogen from DMF. (**h**) PVDF fibers electrospun into liquid nitrogen followed by drying in vacuo. The inset of (**f–h**) gives an SEM image of the broken end of the fiber at a higher magnification, indicating the fiber is porous throughout. Reprinted with permission from Reference [34]. Copyright 2006 American Chemical Society. production of long and continuous fibers. Moreover, two optimal flow rates of the coaxial fluids should be obtained to keep the inner fluid fully wrapped by the outer one. Different kinds of materials, such as metal salts, eupolymers, and oligomers, have been made in pairs to realize the core-shell structure nanofibers [37; 38; 39; 40]. Figure 5a shows a regular polymer nanofiber blend of poly (vinylidene fluoride) (PVDF) and polyaniline (PANI), and the production of the core-shell nanofibers is also observed by transmission electron microscopy in Figure 5b. Experiment results have proven that a core-shell electrospun fibrous mat had much better conductivity of \(2.75\times 10^{-9}\) S cm\({}^{-1}\) than \(4.08\times 10^{-14}\) S cm\({}^{-1}\) conductivity for a simple electrospun fibrous mat [41]. Notably, coaxial electrospinningn is of great help to produce nanofibers by unspinnable liquids in that these liquids serve as the inner fluid while the outer fluid guides it to spin. For instance, ultrafine Polydimethylsiloxane (PDMS) fibers were prepared by the coaxial electrospinning method using a PDMS and polyvinylpyrrolidone (PVP) as core and sheath materials with the final removal of PVP [42] (Figure 5c-f). Based on core-shell nanofibers, hollow nanofibers can be achieved by the removal of the core materials. In a classic instance, PVP hollow nanofibers derived from PVP/mineral oil core-shell fibers were obtained by being immersed in hexane for 24 h to remove the mineral oil and the hollow structure of these electrospun fibers could be seen in Figure 5g,h [43]. Besides, more complex structures could be constructed in this way with the aid of more spin needles. In one study, multichannel tubes were created by three spinnerets in the layout of an equilateral triangle [44] (Figure 6). A mixture of PVP and Ti(DiPr)\({}_{4}\) was added to the outer spinneret while the paraffin oil was fed into the inner three needles.