Table 1.

Studies evaluating the NGC wall thicknesses.

Study	Type of Study	NGC Material	Quantitative Parameters	Main Outcome
Rutkowski et al., 2008 [35]	In vitro (In silico): Schwann cell cultures. Dorsal root ganglia (DRG) of Sprague.	Bioartificial nerve graft (BNG) composed of a tubular conduit of poly-D, L-lactide	Computer model predicting the wall thickness, porosity, and Schwann cell seeding density needed to maximize the axon extension rate and ensuring sufficient nutrients to the neurons.	Low porosity, greater wall and Schwann cell layer thickness led to a decrease in the amount of oxygen available to the axons and greater NGF concentrations.
Rutkowski et al., 2008 [36]	In vitro: SC Cultures. DRG of Sprague–Dawley rats.	Bioartificial nerve graft (BNG) composed of a tubular conduit of poly-D, L-lactide with Schwann cells	Porosities: 0.55–0.95 Wall thickness: 0.56–1.47 mm	Reduced axon growth in conduits with wall thicknesses greater than 0.81 mm, a greater wall thickness, and lower porosities have a detrimental effect on the growth of the axons.
Mobasseri et al., 2015 [25]	In vitro: stem cells differentiated to Schwann cell-like cells. In vivo: Sprague–Dawley rat sciatic nerve injury (n = 24).	Poly ε-caprolactone (PCL) and polylactic acid (PLA)	Wall thickness: 70, 100, 120, 210 µm	Increasing the wall thickness also increases stiffness and limits the permeability of the canal, so it did not show any positive effect on the biological response of the regenerating nerve.
Pateman et al., 2015 [23]	In vitro: SC and DRG. In vivo: common fibular nerve of mouse (n = 18).	poly(ethylene glycol) (PEG)	Wall thickness: 250 μm	NGC with 1 mm (internal diameter), 5 mm (long) and 250 μm wall thickness supported reinnervation through a 3 mm lesion space after 21 days, similar results to an autograft control.
Den Dunnen et al., 1995 [37]	In vivo: Sciatic nerve of rat (n = 24).	50% lactic acid (LA) and 50% e-caprolactone (CL)	Type 1: N° of dip-coated: 2, Int. diameter: 1.23 mm Wall thickness: 0.34 mm. Type 2: N° of dip-coated: 3, Int. diameter:1.18 mm Wall thickness: 0.43 mm. Type 3: N° of dip-coated: 4, Int. diameter: 1.15 mm Wall thickness: 0.64 mm. Type 4: N° of dip-coated: 5 Int. diameter: 1.12 mm Wall thickness: 0.68 mm.	Better nerve regeneration with Type 1 (large amount of targeted neural tissue, a minimal amount of fibrous or scar tissue, and a normal amount and distribution of blood). Type 2: more fibrous tissue and had less favorable nerve regeneration due to nerve compression. Types 3 and 4: bone exposed by severe self-mutilation. Due to the swelling, the NGC lumen completely disappeared.
Den Dunnen et al., 1998 [33]	In vivo: Sciatic nerve of rat (n = 30).	Copolymer of DL-lactide and e-caprolactone	Type 1: N° of dip-coated: 2, Int. diameter: 1.23 mm Wall thickness: 0.34 mm. Type 2: N° of dip-coated:3, Int. diameter:1.18 mm Wall thickness: 0.43 mm. Type 3: N° of dip-coated:4, Int. diameter: 1.15 mm Wall thickness: 0.64 mm. Type 4: N° of dip-coated: 5 Int. diameter: 1.12 mm Wall thickness: 0.68 mm.	Thicker NGC wall: swelling of the degrading biomaterial will be so severe that the NGC becomes occluded. Thinner NGC wall: the nerve guides collapsed. Peripheral nerve regeneration across a 10 mm nerve gap inside a P(DLLA-e-CL) nerve guide was faster and qualitatively better in comparison with a 7 mm long autologous nerve graft.