Table 2.

Studies evaluating the NGC diameter of polymeric fibers.

Study	Type of Study	NGC Material	Technique	Diameter of Fibers	Main Outcome
Wen X. et al., 2005 [28]	In vitro: DRG explants	Poly(acrylonitrile-co-vinyl chloride) (PAN-PVC)	Wet-phase inversion process	Diameters: 5 ± 1.58, 30 ± 1.26, 100 ± 1.41, 200 ± 2.03, 500 ± 3.66 μm	5 and 30 μm-diameter filaments increase the neurite outgrowth and SC migration. Robust and uniformly distributed neuronal growth was achieved with highly directional filaments.
Yang F. et al., 2005 [38]	In vitro: Neural stem cells (NSCs)	Poly(L-lactic acid) (PLLA)	Electrospinning	Aligned fibers: 300 nm nanometer scale 1.5 μm submicron scale	No significant changes to cell orientation were associated with fiber diameters. NSC differentiation was higher for PLLA nanofibers than microfibers, independent of their alignment. The main growing direction of NSC neurites was parallel to nano and microfibers.
Yao L.et al., 2009 [39]	In vitro: PC12 cells	Poly(caprolactone) (PCL)	Electrospinning	Randomly oriented fibers: 4.4 ± 0.5 μm. Aligned oriented fibers: 0.8 ± 0.7, 3.7 ± 0.5, 5.9 ± 0.9, 8.8 ± 0.9 μm.	PC12 cells’ neurites showed similar parallel growth on the aligned fibers irrespective of fiber diameter. Neurite length on aligned fibers (fiber Φ: 3.7 ± 0.5 and 5.9 ± 0.9 μm), was longer than neurite length on randomly oriented fibers.
Wang HB. et al., 2010 [29]	In vitro: Dorsal root ganglia (DRG)	Poly-L-lactic acid (PLLA)	Electrospinning	Highly aligned, electro-spun fiber scaffolds, fiber diameters: Large:1325 + 383 nm interm.:759 + 179 nm small: 293 + 65 nm	Small diameter: did not promote extensive neurite extension or SC migration. Intermediate diameter: promoted long, directed neurite extension independent of SC migration. Large diameter: promoted long, directed neurite extension and SC migration.
Junxia Wang, et al., 2012 [40]	In vitro: human embryonic stem cells (hESCs)	Tussah silk fibroin (TSF)	Electrospinning	Different diameter: 400 and 800 nm	Neurite outgrowth along the fibers was longer on aligned 400 nm TSF-scaffold than on-aligned 800 nm TSF-scaffold. 400 nm aligned TSF scaffold supports survival and promotes neuronal differentiation of hESC-derived NPs.
Daud M.F.B., et al., 2012 [41]	In vitro: I. neuronal or primary SC cultures; II. Neuronal and primary SC in co-culture; III. Isolated DRG cultures, containing both neuronal and SC.	Poly(caprolactone)	Electrospinning	Diameters: 1, 5 and 8 μm	For neuronal cells alone, 8 μm fibers promoted better neurite outgrowth. For neuronal cells plus primary SC or DRG explants, 1 μm fibers supported superior neurite outgrowth, SC migration, and elongation in comparison with 5 and 8 μm fibers.
Gnavi S. et al., 2014 [42]	In vitro: Explant cultures of SC and DRG Ex vivo: SC	Gelatin	Electrospinning	Nanofibrous matrices, diameters: 300 or 600 nm, 1000 or 1300 nm	Nanofibers (300 nm) promoted cell spreading and actin cytoskeleton organization, increasing cellular adhesion and SC proliferation rate. Migration rate and motility, axonal density was greater in cells cultured on microfibers (1300 nm). Microfibers promoted SC migration and axonal outgrowth. Nanofibers promoted SC proliferation and adhesion.
Hu J. et al., 2016 [10]	In vitro: PC12 cells	Poly(ε-caprolactone) (PCL)-Nerve Growth Factor (NGF) and Bovine Serum Albumin (BSA)	Emulsion electrospinning technique	Diameter Random: 343 ± 113 nm; 320 ± 97 nm; 371 ± 95 nm; 343 ± 113 nm Aligned: 354 ± 91 nm; 302 ± 70 nm; 333 ± 90 nm; 320 ± 87 nm.	Aligned nanofibers presented similar diameters to randomly aligned nanofibers, but the aligned nanofibers were more uniform. PC12 neurite length on PCL-NGF and BSA scaffold (diameter 320 ± 87) was higher on aligned nanofibers (70.17 μm) compared to randomly aligned nanofibers (41.67 μm).
Lizarraga LR. et al., 2019 [8]	In vitro: NG108-15 neuronal cells and Schwann cells	Poly(3-hydroxybutyrate) P(3HB) poly(3-hydroxyoctanoate) P(3HO) 25:75 % P(3HO)/P(3HB) blend (PHA blend)	Electrospinning	Highly aligned and uniform fibers diameters: Small: 2.4 ± 0.3 μm Medium: 3.7 ± 0.3μm Large: 13.5 ± 2.3 μm	A direct correlation between fiber diameter and neuronal growth and differentiation was noted. Highly aligned large fibers (13.50 ± 2.33 μm) resulted in better neurite outgrowth and higher neuronal cell differentiation in co-culture. With RN22 SC, the number of NG108-15 cells increased as the fiber diameter increased.
Jiang et al., 2012 [5]	In vivo: sciatic nerve injury model in female Sprague–Dawley rats (n = 26).	Poly(ε-caprolactone) (PCL)	Electrospinning	Microfibers (981  ±  83 nm) and nanofibers (251  ±  32 nm)	Nanofiber NGC resulted in a higher number of myelinated axons, thicker myelin sheaths, an increase in regenerated DRG sensory neurons, and functional recovery compared to microfiber and film NGC. Nanofiber conduits possessed a smaller pore size compared to microfiber conduits.