Main Morphological Characteristics of Tubular Polymeric Scaffolds to Promote Peripheral Nerve Regeneration—A Scoping Review

The “nerve guide conduits” (NGC) used in nerve regeneration must mimic the natural environment for proper cell behavior. Objective: To describe the main morphological characteristics of polymeric NGC to promote nerve regeneration. Methods: A scoping review was performed following the Preferred Reporting Items for Systematic reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR) criteria in the PubMed, Web of Science, Science Direct, and Scientific Electronic Library Online (SciELO) databases. Primary studies that considered/evaluated morphological characteristics of NGC to promote nerve regeneration were included. Result: A total of 704 studies were found, of which 52 were selected. The NGC main morphological characteristics found in the literature were: (I) NGC diameter affects the mechanical properties of the scaffold. (II) Wall thickness of NGC determines the exchange of nutrients, molecules, and neurotrophins between the internal and external environment; and influences the mechanical properties and biodegradation, similarly to NGC (III) porosity, (IV) pore size, and (V) pore distribution. The (VI) alignment of the NGC fibers influences the phenotype of cells involved in nerve regeneration. In addition, the (VII) thickness of the polymeric fiber influences neurite extension and orientation. Conclusions: An NGC should have its diameter adjusted to the nerve with wall thickness, porosity, pore size, and distribution of pores, to favor vascularization, permeability, and exchange of nutrients, and retention of neurotrophic factors, also favoring its mechanical properties and biodegradability.


Ethical Issues
This literature review is part of the authors' research projects that will include experimental methodologies in vitro and in vivo (Universidad de La Frontera Ethics Committee Approval Numbers: 091/19 and 074/20). However, for this review, formal ethical approval is not required, as no primary data were collected.

Eligible Criteria
Primary in vitro and animal studies were included where the general objective was to study the influence of one or more morphological characteristics of a tubular polymeric scaffold (NGC) on peripheral nerve regeneration. Full-text articles with no limits on the publication date, written in English or Spanish, were included for the analysis. Articles were excluded that study nerve regeneration in the central nervous system (CNS), that evaluates a biomaterial and not a morphological characteristic, those that describe accessory morphological characteristics not inherent to the morphology of an NGC and non-polymer scaffold construction material.

Article Selection and Data Extraction
Two independent reviewers analyzed articles obtained in the systematic search process by reviewing the titles and abstracts. Articles that fulfilled the eligibility criteria were analyzed in full text to confirm their relevance. In cases of disagreement between the two reviewers, a third reviewer was invited to help resolve the discrepancies of opinion. The following information was collected from the full-text articles comprising the final selection: authors, publication years, study design, morphological characteristics of tubular polymeric scaffolds, and their influence on the nerve regenerative. The tables used in data extraction were designed by the authors of this review to obtain data relevant to the topic studied.

Study Selection
The article search and selection process are summarized in Figure 1. The total number of articles found in the databases used was 697, and 7 additional articles were included after the manual search, totaling 704 studies, of which 198 were duplicates.

Characteristics of the Selected Studies
The present literature gathered and summarized the evidence in the literature that indicates seven main characteristics that should be considered for the elaboration of an NGC as they are determinants for regenerative success. The characteristics of the polymeric NGC are listed in Tables 1-5. NGC diameters influenced tube collapse, nerve regeneration, and decreased muscle reinnervation.  After the initial reading by title, 192 articles were excluded, of which 41 were systematic reviews, 64 in vitro studies of biomaterials with no regenerative outcomes, 52 studies assessed regeneration of the CNS, and 35 studied the inclusion of adjuvant molecules or characteristics in the NGC.
Among the articles available for abstract evaluation (314 in total), 174 were excluded, of which 7 were literature reviews, 74 were CNS studies, 48 evaluated new scaffold elaboration techniques that did not assess their inherent morphology, and 45 did not evaluate the influence of a morphological characteristic on nerve regeneration.
After reading full-text articles, 88 were excluded, of which 52 did not establish a comparison of measures of morphological characteristics, 13 described a method of scaffold elaboration techniques, 17 did not evaluate nerve regeneration, and 6 were secondary studies.
Finally, in this review, 52 articles were included that corresponded to experimental studies in vitro or in vivo that met the previously defined criteria.

Characteristics of the Selected Studies
The present literature gathered and summarized the evidence in the literature that indicates seven main characteristics that should be considered for the elaboration of an NGC as they are determinants for regenerative success. The characteristics of the polymeric NGC are listed in Tables 1-5.  In vitro: stem cells differentiated to Schwann cell-like cells. In vivo: SD rat sciatic nerve injury.

Wall thicknesses
Increasing the thickness of the wall increased stiffness and limited the permeability of the canal, so it did not show any positive effect on the biological response of the regenerating nerve.  [25] In vivo: sciatic nerve defects in SD rats.

Immersion precipitation method
Conduits with asymmetric porosity (The external surface of the NGC has a larger pore size than the lumen surface).
Asymmetric PLGA NGC showed a stable supporting structure, inhibiting exogenous cell invasion during the regeneration, higher regenerated axons at the mid-conduit, and distal nerve site. Asymmetric structure in the NGC wall enhanced the removal of the blockage of the waste drain from the inner inflamed wound in the early stage. Poly(lactic-co-glycolic acid) PLGA and Pluronic F127

Modified immersion precipitation
Conduits with asymmetric porosity-nanopores on the inner surface and micropores on the outer surface.
Asymmetric NGC influenced optimal mechanical properties and hydrophilicity and affected nutrient permeability.
Asymmetric porosity NGC influenced the nutrients and oxygen permeation, and proliferation of SC. Also prevented fibrous scar tissue invasion. Unidirectional permeability NGC showed more myelin fibers than the high bidirectional patency NGC. Oh et al., 2008 [8] In vivo: Sciatic nerve of SD rats Poly(lactic-co-glycolic acid) (PLGA) and Pluronic F127

Modified immersion precipitation method
Conduits with asymmetric porosity.
Asymmetric porosity NGC influenced the infiltration of fibrous tissue, neurotrophic factors, and nutrients. Allowing vascular growth for effective delivery of nutrients and oxygen, resulting in rapid and continuous axonal growth.     The alignment of scaffold fibers influenced retrograde nerve conduction speed, motor, and sensory function. Bilayer NGC (random and aligned nanofibers layers) are more tear-resistant in surgical procedures due to isotropic mechanical properties. It was unclear if random nanofibers could interfere with the aligned nanofibers in the extension pattern of the neurites and interfere in nerve regeneration.
Radhakrishnan et al., 2015 [52] In vitro: SC Poly(lactide-co-glycolide) Electrospinning Aligned scaffold fibers favored the adhesion, proliferation, and morphology of cells. Additionally, the expression of myelination markers and maturation of SC was regulated.
Yan et al., 2015 [53] In vivo: Sciatic nerve of adult SD rats Poly(L-lactic acid-co-e-caprolactone) Electrospinning Alignment of the scaffold fibers influenced cell growth orientation, myelination, and neuropathic pain post-injury.

Gelatin Electrospinning
The alignment of the scaffold fibers influenced adhesion and proliferation cell. The polymeric scaffold diameter is a determining factor in the mechanical properties of the tubular scaffold in nerve regeneration [14,21]. Five studies compared in vitro and/or in vivo wall thicknesses in the rat sciatic nerve [13,15,16,22,23]. They determined its influence mainly on the exchange of nutrients, molecules, and growth factors between the internal and external environment. Furthermore, wall thicknesses influence mechanical properties and biodegradation.
A scaffold must provide structural support to regenerating axons in order to facilitate a higher rate of regeneration [47,48]. Therefore, scaffolds must demonstrate adequate mechanical properties. Natural polymers have decreased mechanical properties and rapid degradation rates, which limits their exclusive use [47,58,60]. However, the presence of synthetic polymers improves these properties [47], which are also favored by the morphological characteristics of the scaffold, such as the alignment of the fibers [48,49,58]. Therefore, the method of mixing synthetic polymers and natural polymers has been widely used with varying degrees of success [47][48][49]54,58]. This mixture improves the bioactivity, biocompatibility, biodegradability of the scaffold, and improves the interaction of the scaffold with the cells [48,58].
As has been shown in the literature, the composition of the scaffold will directly influence nerve regeneration; however, the morphological characteristics of the NGC are also determining factors for regenerative success. Therefore, this literature review sought and gathered the main morphological characteristics of a polymeric NGC that can influence nerve regeneration. In total, seven main characteristics were identified in the selected studies. The following discussion will address these main aspects relating to nerve regeneration.

Scaffold Diameter/Adjustment
Commercially available NGC is an accepted strategy for overcoming short gaps in nerve repair in which poor clinical results are not uncommon. An unrecognized cause of their failures is the dimension of the canal around the diameter of the nerve [14,21]. Thus, the choice of the diameter of the NGC will depend on the diameter of the injured nerve. The adjustment of the scaffold is an important factor to consider [21]. The implantation of an NGC of small internal diameters can influence the quality of nerve regeneration and maturation and is technically difficult [14,21,22]; faced with this intraoperative dilemma, different sizes of conduits would often have to be chosen [21].
The selection of a large canal technically facilitates implantation to the nerve. However, it would lead to a pronounced collapse of the nerve canal, independent of the polymeric material used, resulting in poor regenerative consequences, muscle atrophy, lower muscle weight, and weak contraction force will occur [21].
On the other hand, a slightly larger channel may represent the most likely type of sizing error in a clinical setting. The NGC size would have the theoretical benefit of being able to accommodate nerve end swelling or scaffold wall swelling and could offer a reasonable fit. However, there are no positive effects compared to better-fitting polymeric tubes [14,21].
The NGC mechanical properties and an eventual collapse of the tubular structure also depend on the scaffold diameter [14,21]. The adjustment of the tubular scaffold could directly influence nerve regeneration, the number, and diameter of regenerating axons and their myelination, as well as subsequent muscle reinnervation (Figure 2) [21].

Scaffold Wall Thickness
Biomolecular signals are required to stimulate cells to regenerate damaged tissue [62]. A promising strategy to improve nerve regeneration obtained with NGC includes the integration of neurotrophic factors (NTFs) [15,23,63,64] as they play an important role in the control of survival, migration, proliferation, and differentiation of cells involved in nerve regeneration [15,65,66] as well as various nutrients important to the cell survival [15].
Increased wall thickness will lead to greater retention of growth factors within the lumen, which may improve the survival capacity of neurons [15,23]. However, these conditions lead to a decrease in the amount of oxygen and the exchange between the internal and external environment of necessary nutrients in the lumen, such as glucose and lysozyme [28,32]. Nevertheless, thinner NGC can be useful for surgical maneuvers [13], but mechanical failures can occur, as revealed in vivo experiments that led to the collapse of the NGC [7,13,32].
Therefore, a suitable NGC wall thickness must provide sufficient mechanical resistance with a minimum thickness that allows its manipulation and the sufficient diffusion of nutrients to ensure that nutrients, molecules, and oxygen reach the preselected Schwann cells (SCs) and regenerate neural tissue while retaining neurotrophic factors [15,16,22,28,32,34].

Porosity of the Scaffold
The architecture of the scaffold affects cellular behavior in nerve regeneration. Therefore, the design and selection of a scaffold with defined porosity will be critical to achieving positive results in nerve repair [8,25]. Porosity is defined as the percentage of void space; it is determined as the ratio of the volume of the pore space divided by the total volume of the object [31].
The porosity of the scaffold plays a vital role in the exchange of oxygen, nutrients, neurotrophic factors, between the internal and external environment [2,5,11,34], which stimulate and promote cell orientation, infiltration, and migration, in addition to providing a positive influence for axonal growth after nerve injury [5,8,11,24,25,27,28,32,34]. This shows that regenerating axons traverse much longer spaces if the NGC are permeable to the medium [24].
The lack of porosity in the NGC walls decreases and affects nerve regeneration [23,25,30,34]. By contrast, a very porous duct will allow trophic factors to diffuse from the lumen of the NGC to the external environment this loss will prevent the axons from reaching their optimal growth [23]. Therefore, the porosity is a crucial parameter that determines both the diffusion of hydrophilic proteins and the permeability of small molecules such as glucose [28] through the wall to provide an optimal medium for nerve regeneration [28,33].

Pore Size
The pore size of the NGC determines which molecules pass through the graft from the surrounding tissue to the regenerating nerve [25]. Large pores can promote vascularization within NGC [34]; however, if the pores are too large, there is also the risk of cellular infiltration and blockage of the nerve duct by fibrous tissue, providing a less permissive environment for axonal growth, resulting in a low density and number of nerve fibers [26,32,34]. Nevertheless, small pore sizes in the NGC walls decrease and affect nerve regeneration [23,25,30,34]. Therefore, pore size is important in the architecture of a tubular scaffold because it determines the exchange of molecules, growth factors, and nutrients through the wall [24].
A desirable characteristic of an NGC is biodegradability, and this is directly proportional to porosity and pore size [33,67]. Therefore, it is important to consider the size of the pores in different layers of the NGC to obtain desirable results in nerve regeneration.

Distribution and Orientation of Scaffold Pores
Several studies have shown that the interconnected pores of the permeable ducts can increase the exchange of nutrients between the light and the external environment, prevent cellular infiltration that can impede the extension of the axon, and retain neurotrophic factors in the lumen of the NGC [8,26,27]. However, these interconnected pores must be "asymmetrical", thus the external surface of the NGC should have a larger pore size than the lumen NGC surface.
The method used for the elaboration of NGC with the distribution of polymeric asymmetric pores is the immersion precipitation method. In its basic form, dip precipitation is carried out by dipping a thin film of a concentrated polymer solution into a non-solvent bath. This method achieves asymmetrically porous NGC with selective permeability (to prevent infiltration of fibrous tissue but impregnate nutrients) and hydrophilicity (for effective nutrient permeability) [8,11,27,29,30].
Experimental studies have shown that the NGC requires greater permeability for outflow than for inflow to remove debris, reduce inflammation at the injury site, and accelerate nerve regeneration [8,11,25,26,30]. In addition, these fluidity characteristics allow the exchange of fluids, nutrients, and oxygen, while minimizing fibrous tissue ingrowth, but retain neurotrophic factors, resulting in larger axon diameter and number, a greater number of blood vessels, thicker myelin sheath, faster axonal growth, and SC proliferation [9,45]. Thus, this asymmetry of pore distribution favors these biological events and, consequently, nerve regeneration [8,11,25,[29][30][31].

Diameter of Polymer Fiber
A biomimetic NGC should mimic the structure and function of the ECM, defining the optimal architecture to maintain cell organization, viability, invasion, proliferation, and differentiation [39]. The ECM in the peripheral nerves acts as a three-dimensional scaffold consisting of polysaccharide fibers and proteins (collagen fibers, elastin), ranging from tens to hundreds of nanometers [6].
Electrospinning is a promising technique to mimic the structure of the ECM [7,44,45] due to the ability to generate polymeric fibers on a micro-or nano-scale, providing a three-dimensional space with more adhesion sites for growth cells [7,59].
Understanding the influence of micro and nanoscale topography on cell behavior is crucial for the design of functional scaffolds for nerve tissue engineering [29]. Different studies have reported that the polymeric fiber diameter of NGC could alter cell differentiation, morphology, growth, proliferation, and migration [3,5,7,[35][36][37][38][39]46,50,[52][53][54][55][56][58][59][60]. Furthermore, the average diameter of electrospun fibers in scaffolds is a control variable, useful for manipulating the release profile of growth factors [40]. Therefore, the polymeric fiber diameter is of great importance to promoting nerve regeneration, and this is achieved by varying different electrospinning parameters, such as concentration of the polymer, flow rate, voltage, distance, and collector speed [7,35,36,39]. Among the variables of the polymeric solution, it was shown that the diameter of the fiber is directly proportional to the polymer concentration [35,38].
The diameter of the polymer fiber also influences the mechanical properties of fibrous scaffolds influencing the ultimate tensile strength (UTS), elastic modulus (EM), and elongation at break (EB) [40]. There is evidence that cells respond to a natural-like environment. Similarly, natural characteristics could give cells physical clues that favor the differentiation of stem cells and the growth of neurites [7].

Alignment of the Polymer Fibers
During nerve regeneration, axon growth and neurite outgrowth occur in response to chemical and physical signals derived from the local microenvironment [45]. Therefore, the topography of the fibers within the designed scaffold plays a crucial role in mimicking the ECM [47,48]. The superiority of an electrospun fibrous NGC architecture was demonstrated, which can be conveniently arranged to provide better topographic cues for cells in nerve regeneration [6,7,37,40,42,44]. The native ECM has a specific architecture, which is important for tissue function. Therefore, following a nerve injury, a well-defined architecture is necessary to mimic the ECM accurately to guide nerve regeneration. This is the reason the orientation of polymers fibers of NGC is crucial to promoting nerve regeneration [42,44].

Biodegradable Properties
A successful nerve guide must comply with established parameters of biodegradation, because bio-durable NGC has the disadvantage of remaining in the body, causing an inflammatory response, and consequently a second intervention may be necessary for its extraction, which can cause an injury to the regenerated nerve [14]. In contrast, NGCs that degrade very rapidly do not provide optimal guidance in the required time. Figure 3a,b represent scaffold biodegradation at a controllable rate to avoid the need for a second surgery, and to ensure nerve regeneration at a controllable rate according to axonal growth rates towards the distal stump [14], guaranteeing that the canal maintains its shape and protects the tissue until functional recovery is achieved [15]. elongation at break (EB) [40]. There is evidence that cells respond to a natural-like environment. Similarly, natural characteristics could give cells physical clues that favor the differentiation of stem cells and the growth of neurites [7].

Alignment of the Polymer Fibers
During nerve regeneration, axon growth and neurite outgrowth occur in response to chemical and physical signals derived from the local microenvironment [45]. Therefore, the topography of the fibers within the designed scaffold plays a crucial role in mimicking the ECM [47,48]. The superiority of an electrospun fibrous NGC architecture was demonstrated, which can be conveniently arranged to provide better topographic cues for cells in nerve regeneration [6,7,37,40,42,44]. The native ECM has a specific architecture, which is important for tissue function. Therefore, following a nerve injury, a well-defined architecture is necessary to mimic the ECM accurately to guide nerve regeneration. This is the reason the orientation of polymers fibers of NGC is crucial to promoting nerve regeneration [42,44].

Biodegradable Properties
A successful nerve guide must comply with established parameters of biodegradation, because bio-durable NGC has the disadvantage of remaining in the body, causing an inflammatory response, and consequently a second intervention may be necessary for its extraction, which can cause an injury to the regenerated nerve [14]. In contrast, NGCs that degrade very rapidly do not provide optimal guidance in the required time. Figure 3a,b represent scaffold biodegradation at a controllable rate to avoid the need for a second surgery, and to ensure nerve regeneration at a controllable rate according to axonal growth rates towards the distal stump [14], guaranteeing that the canal maintains its shape and protects the tissue until functional recovery is achieved [15].  Biodegradability is mainly determined by the polymeric material. Non-degradable polymers have been widely used for the study of nerve regeneration due to their inert and mechanical properties. However, non-degradable polymers may induce chronic inflammatory response and pain by nerve compression because they remain in situ without degradation, which could require a reoperative surgery for the conduit removal. Recent researches for NGC fabrications have concentrated on biodegradable synthetic polymers, including polylactic acid (PLA), poly (glycolic acid) (PGA), poly (lactic-co-glycolic acid) (PLGA), and poly (phosphoester) [27,48]. One of the most effective methods to develop new scaffolds is the alloy of synthetic and natural polymers that can promote nerve regeneration as natural polymers improve the hydrophilicity of composite scaffolds and increase the rate of degradation [31,48,58].
Various studies have shown that the biodegradable property of NGC will also be influenced by the morphological characteristics of the scaffold [31,33,40,67]. Table 6 represents the behavior of the biodegradability of an NGC with the different morphological characteristics. The overall results indicated that biodegradability is inversely proportional to fiber diameter [40] and is directly proportional to porosity and pore size, where the higher the pore size and porosity, the high the degradation rate [31,33,67], due to auto-catalyzed degradation as larger pore sizes allow more PBS solution to be contained per unit volume of scaffolds [33]. On the other hand, the orientation of the fiber plays a major role in the tailoring of the scaffold properties such as degradability. The degradation was greater in random fibers compared to aligned fibers [44]. Therefore, it is important to consider the morphological characteristics of the NGC to obtain desirable results in nerve regeneration. As gradual biodegradation is desired at a controllable rate according to axonal growth rates to ensure that the canal maintains its shape and protects the tissue until functional recovery is achieved [15,33,48] as the mechanical properties deteriorated with the degradation of NGC [33,43].

Mechanical Properties
The type of polymeric material plays a significant role in the mechanical properties of the scaffold [27,34]. However, the morphological characteristics of NGC are determinants in the mechanical properties of the tubular scaffold, to create an environment conducive to nerve regeneration.
Controlling the fit of the tubular scaffold will protect regenerating axons from compression owing to the collapse of the tubular structure oversizing [21]. Similarly, the wall thickness directly influences this mechanical resistance, important to maintain a stable support structure for nerve regeneration [27]. A very thin wall thickness collapses without additional force (Figure 4a). It is well known that with the increase in conduit wall thickness, the strength of the conduit to resist compression increases accordingly [32]. However, the exchange of nutrients and oxygen between light and the external environment decreases with the increasing thickness of the conduit wall and the increased rigidity is a factor for the failure of the conduit [16]. The behavior of the wall thickness and the tubular scaffold adjustment in the collapse of the NGC is graphically represented in Figure 4. Figure 4a shows a typical collapse of the NGC oversized with imperfect adjustment. Oppositely, Figure 4b shows the behavior of an NGC adjusted to the peripheral nerve with an adequate wall thickness, managing to maintain its tubular construction and exhibiting a complete adjustment of the lumen. Therefore, a minimum wall thickness is required that allows the exchange of molecules necessary for nerve regeneration but that provides balanced mechanical resistance for the neural scaffold to resist pressure from handling, suturing, and surrounding tissue [2,16]. allows the exchange of molecules necessary for nerve regeneration but that provides balanced mechanical resistance for the neural scaffold to resist pressure from handling, suturing, and surrounding tissue [2,16]. The mechanical properties of NGC should provide sufficient resistance to tolerate surgical procedures, subsequent tissue movements associated with patient movement, especially when tissue begins to infiltrate through scaffolds and axonal extension occur [29,31,58]. These mechanical properties, such as Young's modulus, yield stress, yield strain, ultimate stress, and ultimate strain, are determined by the morphological characteristics of NGC [33]. Mechanical properties were inversely proportional to pore size and porosity [33]. On the other hand, the bending stiffness was significantly affected by the porosity [34] and wall thickness [16]. Figure 5 shows the behavior of the bending stiffness of NGC according to morphological characteristics. The higher the porosity and the lower the wall thickness, the bending stiffness is significantly lower [16,34] (Figure 5b). Consequently, the greater the wall thickness the flexural rigidity is significantly higher [16] (Figure 5a). As is known, a thin wall thickness and higher porosity and pore size facilitate the better mass transfer of nutrients, growth factors, and oxygen. However, mechanical properties are compromised as porosity increases and wall thickness decreases, therefore there must be a balance between facilitating nutrient exchange and sufficient mechanical properties to maintain the tubular structure until nerve regeneration [33].  The mechanical properties of NGC should provide sufficient resistance to tolerate surgical procedures, subsequent tissue movements associated with patient movement, especially when tissue begins to infiltrate through scaffolds and axonal extension occur [29,31,58]. These mechanical properties, such as Young's modulus, yield stress, yield strain, ultimate stress, and ultimate strain, are determined by the morphological characteristics of NGC [33]. Mechanical properties were inversely proportional to pore size and porosity [33]. On the other hand, the bending stiffness was significantly affected by the porosity [34] and wall thickness [16]. Figure 5 shows the behavior of the bending stiffness of NGC according to morphological characteristics. The higher the porosity and the lower the wall thickness, the bending stiffness is significantly lower [16,34] (Figure 5b). Consequently, the greater the wall thickness the flexural rigidity is significantly higher [16] (Figure 5a). As is known, a thin wall thickness and higher porosity and pore size facilitate the better mass transfer of nutrients, growth factors, and oxygen. However, mechanical properties are compromised as porosity increases and wall thickness decreases, therefore there must be a balance between facilitating nutrient exchange and sufficient mechanical properties to maintain the tubular structure until nerve regeneration [33].
allows the exchange of molecules necessary for nerve regeneration but that provides balanced mechanical resistance for the neural scaffold to resist pressure from handling, suturing, and surrounding tissue [2,16]. The mechanical properties of NGC should provide sufficient resistance to tolerate surgical procedures, subsequent tissue movements associated with patient movement, especially when tissue begins to infiltrate through scaffolds and axonal extension occur [29,31,58]. These mechanical properties, such as Young's modulus, yield stress, yield strain, ultimate stress, and ultimate strain, are determined by the morphological characteristics of NGC [33]. Mechanical properties were inversely proportional to pore size and porosity [33]. On the other hand, the bending stiffness was significantly affected by the porosity [34] and wall thickness [16]. Figure 5 shows the behavior of the bending stiffness of NGC according to morphological characteristics. The higher the porosity and the lower the wall thickness, the bending stiffness is significantly lower [16,34] (Figure 5b). Consequently, the greater the wall thickness the flexural rigidity is significantly higher [16] (Figure 5a). As is known, a thin wall thickness and higher porosity and pore size facilitate the better mass transfer of nutrients, growth factors, and oxygen. However, mechanical properties are compromised as porosity increases and wall thickness decreases, therefore there must be a balance between facilitating nutrient exchange and sufficient mechanical properties to maintain the tubular structure until nerve regeneration [33].  Moreover, the distribution and thickness of the fiber also directly influence the mechanical properties of NGC. Coarse fibers have significantly more elastic modulus [49]. Figure 6 represents the elastic modulus of an NGC. The diameter of the fiber is directly proportional to the elastic modulus of the NGC. Thus, the greater the thickness of the fiber, the greater the elastic modulus, therefore the more rigid will be the NGC.
Moreover, the distribution and thickness of the fiber also directly influence the mechanical properties of NGC. Coarse fibers have significantly more elastic modulus [49]. Figure 6 represents the elastic modulus of an NGC. The diameter of the fiber is directly proportional to the elastic modulus of the NGC. Thus, the greater the thickness of the fiber, the greater the elastic modulus, therefore the more rigid will be the NGC. Furthermore, thick fibers have significantly more ultimate tensile strength (UTS) and elongation at break (EB) [40]. Figure 7 shows a schematic diagram for the stress-strain curve of NGC that comparing thin and thick fibers. The strength of an NGC considers the relationship between the external loads applied to materials (Figure 7a) and the resulting deformation or change in material dimensions. In Figure 7b the uphill slope represents the increase in traction in the NGC. Ultimate tensile strength is the maximum in the curve, which means the maximum tension that the NGC can sustain in tension. Thin fibers have a lower maximum tensile strength than thick fibers. The downward slope of the curve after UTS is caused by the instability ty caused by the reduction of the cross-section, which will lead to the break of NGC represented with EB. NGC with thin fibers has significantly less elongation at break (EB) than thick fibers. Therefore, increasing the fiber diameter could significantly improve the mechanical properties of fibrous scaffolds [40].
The mechanical properties provided by the randomly oriented and aligned electrospun fibers differed considerably in all studies [43,44,48,58]. The flexibility and elongation for the aligned electrospun fibers were all greater than those for the random fibers ( Figure  5a,b) [44], however, Young's modulus and tensile strength are controversial in the literature [44,49,58]. Aligned fibers are significantly favorable for nerve regeneration, however, despite the good flexibility and elongation properties of aligned NGCs that allow movement in the injured area, it is difficult to use them for surgical applications due to their insufficient mechanical resistance, independent of the polymeric material, due to the fewer contact points for the aligned nanofibers [2,43]. Several studies have studied the use of a bilayer NGC of aligned nanofibers on the luminal surface, capable of improving the proliferation, migration, and differentiation of neuronal cells and random nanofibers on the external surface to improve mechanical properties [2,51]. Furthermore, thick fibers have significantly more ultimate tensile strength (UTS) and elongation at break (EB) [40]. Figure 7 shows a schematic diagram for the stress-strain curve of NGC that comparing thin and thick fibers. The strength of an NGC considers the relationship between the external loads applied to materials (Figure 7a) and the resulting deformation or change in material dimensions. In Figure 7b the uphill slope represents the increase in traction in the NGC. Ultimate tensile strength is the maximum in the curve, which means the maximum tension that the NGC can sustain in tension. Thin fibers have a lower maximum tensile strength than thick fibers. The downward slope of the curve after UTS is caused by the instability ty caused by the reduction of the cross-section, which will lead to the break of NGC represented with EB. NGC with thin fibers has significantly less elongation at break (EB) than thick fibers. Therefore, increasing the fiber diameter could significantly improve the mechanical properties of fibrous scaffolds [40].
The mechanical properties provided by the randomly oriented and aligned electrospun fibers differed considerably in all studies [43,44,48,58]. The flexibility and elongation for the aligned electrospun fibers were all greater than those for the random fibers (Figure 5a,b) [44], however, Young's modulus and tensile strength are controversial in the literature [44,49,58]. Aligned fibers are significantly favorable for nerve regeneration, however, despite the good flexibility and elongation properties of aligned NGCs that allow movement in the injured area, it is difficult to use them for surgical applications due to their insufficient mechanical resistance, independent of the polymeric material, due to the fewer contact points for the aligned nanofibers [2,43]. Several studies have studied the use of a bilayer NGC of aligned nanofibers on the luminal surface, capable of improving the proliferation, migration, and differentiation of neuronal cells and random nanofibers on the external surface to improve mechanical properties [2,51]. Ultimate tensile strength is the maximum in the curve, which means the maximum tension that the NGC can sustain in tension. The downward slope of the curve after UTS is caused by the instability caused by the reduction of the cross-section, which will lead to the break of NGC represented with EB. Coarse fibers have significantly higher ultimate tensile strength (UTS) and elongation at break (EB).

Limitations of the Study
The present literature review aimed to obtain, organize and understand the existing evidence in the literature on the morphological characteristics of NGC that would favor nerve regeneration. Although we were systematic in our review, we may have missed publications due to the large number of studies evaluating NGC for nerve regeneration. However, we believe that this was minimized due to the sensitive search strategy used, the additional manual search of the literature, and the double-independent review process used. In addition, the evidence described in this study gives a clear statement of the importance of the seven main morphological characteristics of tubular scaffold to promote peripheral nerve regeneration; therefore, the loss of any publication will not significantly alter what is described in this study. Furthermore, the authors understand that this review fulfills the function of being a first step in the organization and understanding of the evidence in the literature; this scoping review could serve as a basis for the development of future systematic reviews on the same topic.

Conclusions
Currently, advances in tissue engineering allow the development of a wide range of intelligent polymeric materials as candidates for the development of nerve guide channels. One challenge is making the NGC design better meet the needs of the affected tissue. This review may be crucial to understanding the main morphological characteristics of the NGC to successfully promote nerve regeneration. A morphologically ideal NGC should have its diameter adjusted to the nerve in which it will be implanted and present a wall thickness, porosity, pore size, and asymmetric distribution of pores, to favor vascularization, permeability, and exchange of nutrients and retention of neurotrophic factors. Thus, avoiding undesirable cell infiltration and always considering the need to maintain biomechanical properties and biodegradability. In polymeric NGC prepared by electrospinning, the thickness of the polymeric fibers on a nanoscale and micrometric scale as well as the alignment of these fibers suggest favoring peripheral nerve regeneration. This scaffold design is challenging given the need to create a supporting structure for cells to proliferate and grow, without affecting the overall function of the resulting structure. In the future, more in vitro and in vivo controlled studies are necessary that incorporate the Ultimate tensile strength is the maximum in the curve, which means the maximum tension that the NGC can sustain in tension. The downward slope of the curve after UTS is caused by the instability caused by the reduction of the cross-section, which will lead to the break of NGC represented with EB. Coarse fibers have significantly higher ultimate tensile strength (UTS) and elongation at break (EB).

Limitations of the Study
The present literature review aimed to obtain, organize and understand the existing evidence in the literature on the morphological characteristics of NGC that would favor nerve regeneration. Although we were systematic in our review, we may have missed publications due to the large number of studies evaluating NGC for nerve regeneration. However, we believe that this was minimized due to the sensitive search strategy used, the additional manual search of the literature, and the double-independent review process used. In addition, the evidence described in this study gives a clear statement of the importance of the seven main morphological characteristics of tubular scaffold to promote peripheral nerve regeneration; therefore, the loss of any publication will not significantly alter what is described in this study. Furthermore, the authors understand that this review fulfills the function of being a first step in the organization and understanding of the evidence in the literature; this scoping review could serve as a basis for the development of future systematic reviews on the same topic.

Conclusions
Currently, advances in tissue engineering allow the development of a wide range of intelligent polymeric materials as candidates for the development of nerve guide channels. One challenge is making the NGC design better meet the needs of the affected tissue. This review may be crucial to understanding the main morphological characteristics of the NGC to successfully promote nerve regeneration. A morphologically ideal NGC should have its diameter adjusted to the nerve in which it will be implanted and present a wall thickness, porosity, pore size, and asymmetric distribution of pores, to favor vascularization, permeability, and exchange of nutrients and retention of neurotrophic factors. Thus, avoiding undesirable cell infiltration and always considering the need to maintain biomechanical properties and biodegradability. In polymeric NGC prepared by electrospinning, the thickness of the polymeric fibers on a nanoscale and micrometric scale as well as the alignment of these fibers suggest favoring peripheral nerve regeneration. This scaffold design is challenging given the need to create a supporting structure for cells to proliferate and grow, without affecting the overall function of the resulting structure. In the future, more in vitro and in vivo controlled studies are necessary that incorporate the morphological characteristics discussed in this review to confirm how to favor nerve regeneration. This characterized NGC model will help to reduce the number of in vitro and in vivo experimental studies, as it will provide the initial morphological parameters in its elaboration ensuring regenerative success.

Conflicts of Interest:
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.