As an alternative to the autograft, allografts from cadaveric donors could avoid donor site morbidity but would be rejected by the host immune system due to human leukocyte antigen (HLA) mismatch. To avoid graft rejection, the tissue needs to be decellularized, removing the HLA epitopes presented on the cell surface but processed nerve allografts maintain the extracellular matrix and its structure, which is advantageous as discussed in Section 4.3 (Whitlock et al., 2009). Cell removal can be achieved by chemical processing with a detergent or physical methods such as irradiation, lyophilization or thermal treatment. Moore and colleagues showed that the processing method has an influence on graft performance, with chemical decellularization leading to better graft function than physical processing (Moore et al., 2011).

Decellularized allograft resulted in regeneration superior to bioartificial collagen-based conduits, but still inferior to autograft in rat models (Whitlock et al., 2009; Giusti et al., 2012). More recently however, a direct comparison in digital nerve repair in human patients with a commercially available collagen NGC (NeuraGen by Integra) versus an allograft (Avance by AxoGen) showed similar results for both products (Rbia et al., 2019). However, both products failed to achieve an outcome that was evaluated as excellent in most patients. This and further limitations related to availability and storage of allografts as well as the high cost, create a strong incentive to engineer synthetic NGCs that can be mass produced and are available off the shelf at different sizes, sufficient quantity and accessible cost.

The most abundant extracellular matrix (ECM) protein Collagen type I is widely implemented as a biomaterial for tissue repair. Collagen consists of a triple helix of three polypeptide chains. Fibrils are formed by cross-links between telopeptides that contribute to the relatively high moduli of telo-collagen, as discussed in depth elsewhere (Gelse et al., 2003).

Collagen provides attachment sites for neurons, that have been shown to contribute to neurite outgrowth (Ivins et al., 2000). Additionally, collagen can act as a scaffold for growth factors and cytokines and thus play an important role in mimicking a biologically relevant healing environment.

A collagen nerve guidance conduit was developed for the regeneration of a 4 mm nerve gap and demonstrated comparable nerve growth to the autograft method in rats and monkeys (Archibald et al., 1991). This product was FDA-approved and commercialized under the name NeuraGen and allowed for regeneration of nerve gaps of 6–18 mm in human patients (Lohmeyer et al., 2007).

As gelatin is denatured collagen that lacks the triple helix structure, it seems that it could be used for NGC construction as well. However, its weak mechanical properties and its tendency to move quickly away from the implant site make additional modification and cross-linking necessary to increase mechanical stability of the NGC (Ko et al., 2017). A clear advantage of both collagen and gelatin is their biodegradability by enzymes present in the healing milieu (van Amerongen et al., 2006; Kuwahara et al., 2011).

Laminin, a glycoprotein of the basal lamina secreted by Schwann cells, has been described as a particularly favorable substrate for adhesion, migration, and regeneration of axons (Mammadov et al., 2016). Laminin is naturally present within the lesioned nerve and exerts its function through interaction with integrin receptors that are upregulated on the neuron cell membrane upon PNI such as integrins α6β1 and α7β1 (Wallquist et al., 2004; Gardiner et al., 2005). The 18 laminin isoforms have varying affinities for different integrins, which have been reviewed in detail elsewhere (Nieuwenhuis et al., 2018). Rather than as a base material, laminin is used in the tissue engineering field to functionalize other synthetic or natural polymer networks (Barros et al., 2019; Drzeniek et al., 2021). Drzeniek et al. (2021) have demonstrated covalent linking of laminin into a methacrylated collagen hydrogel, which improved the material’s bioactive properties. Chang et al. (2020) have recently used laminin to functionalize a synthetic PCL NGC, demonstrating its use as a pro-regenerative additive in PNI.

HA is a highly hydrophilic glycosaminoglycan of the ECM, that contributes to the cushioning function of cartilage due to its high-water content. Already applied clinically in several indications reviewed by Abatangelo et al. (2020) HA is suitable for implantation, injection and tissue engineering purposes due to its viscoelastic properties, biocompatibility, biodegradability and bioactivity. In the field of peripheral nerve regeneration, hyaluronic acid was used as a conduit filler to facilitate axon migration and myelination in the regeneration of a 10 mm rat sciatic nerve gap (Wang et al., 1998). HA can also be implemented to overcome extraneural scarring, as it has been shown to reduce adhesion of the nerve to the neural bed (Ikeda et al., 2003; Zor et al., 2014).

Chitosan is a linear polysaccharide derived from chitin, the major component of the exoskeleton of crustaceans and insects, through chemical or enzymatic processes. Its reactive groups make chitosan accessible to a myriad of chemical modifications and biofabrication techniques, resulting in fibers, beads, films, gels, scaffolds or nanoparticles (reviewed by El Knidri et al. (El Knidri et al., 2018)). Additionally, chitosan possesses an antimicrobial activity probably due to its positive charge and interactions with the negatively charged cell membrane and thus could be a useful component of implantable biomedical devices (D'Almeida et al., 2017). In peripheral nerve regeneration the positive charge of chitosan interacts with the negatively charged axons and significantly improved functional outcome in human patients in a randomized controlled trial of primary surgical nerve repair (Neubrech et al., 2018). In preclinical models chitosan has also been shown to improve regeneration of critical sized nerve injuries (30 mm dog sciatic nerve) and to reduce neuroma formation and fibrosis (Wang et al., 2005; Marcol et al., 2011). Limiting to the use of chitosan are its weak mechanical properties and the low mechanical strength of chitosan is even reduced in physiological environments (Madihally and Matthew, 1999; El Knidri et al., 2018). This challenge can be overcome by additional chemical modification, cross-linking or hybrid use of chitosan with other materials. Thus, most recent NGC designs implement combinations of chitosan and collagen or gelatin (Singh et al., 2019; Itai et al., 2020).

Silk is a protein fiber produced by insects, among others silkworms, spiders and bees, to construct webs and cocoons. Its major component is silk fibroin, a semicrystalline protein, which has been studied for peripheral nerve applications mostly in the form of a silk fibroin protein solution from the silkworm Bombyx mori or the spider Nephila clavipes (Radtke et al., 2011; Radtke, 2016). Silk has been FDA-approved for decades and is used as a surgical suture material. In contrast to the other natural biomaterials mentioned above, silk convinces due to its extremely high tensile strength paired with natural proteolytic degradability (Wongpinyochit et al., 2018). Importantly, silk is biocompatible with nerve and has been shown to support attachment and survival of neurons and Schwann cells not only in vitro (Yang et al., 2007) but also in a large animal model (Radtke et al., 2011).

The protein structure of silk can be modified through genetic engineering and its many functional groups offer a myriad of opportunities for functionalization with additional bioactive domains or growth factors (Kong et al., 2020). In a recent study, a silk fibroin-HA-composite matrix was proposed and implanted subcutaneously into immunocompetent mice to assess immunogenicity. The addition of silk fibroin to HA resulted in a faster regrowth of blood vessels and synthesis of new ECM compared to HA alone, without triggering any excessive inflammation. This demonstrates that silk could be used in multimodal NGC designs to improve the bioactivity and mechanical strength of other materials (Gisbert Roca et al., 2020). The modifiable properties of silk can be implemented for functionalization with neurotrophic factors. A controlled-release-NGC implant successfully restored motor function and reduced distal muscle atrophy in a 10 mm rat sciatic nerve gap study, as the controlled release of GDNF from the proximal conduit led to retrograde neuroprotection not noticed in the plain silk conduit group (Carvalho et al., 2021).

Beyond preclinical rodent models, Kornfeld et al. (2021b) have recently demonstrated that spider silk nerve implants can support axonal regeneration in a 6 cm nerve defect in adult sheep with comparable efficacy to autologous nerve grafts. This result represents one of the most translationally mature implementations of a biomaterial graft in a large nerve gap and paves the way for translation of silk-based conduits into clinical practice.

Organic polymers such as polyaniline, PEDOT and polypyrrole owe their electrical properties to their conjugated bonds as discussed by Le and colleagues (Le et al., 2017). When included in an NGC without an external electricity source, these materials do not actively actuate the healing nerve, instead they can respond to and conduct electrical activity of interfacing neurons. Recently, Vijayavenkataraman et al. (2019)a have demonstrated that neural crest stem cells on conductive a polypyrrole-PCL substrate differentiated into peripheral neurons without any external electrical stimulation. The advantage of these organic polymers is their biocompatibility, the disadvantage being their lack of biodegradability and biochemical motifs that can be recognized by cells. To overcome the latter issue, Khor and colleagues attempted almost three decades ago to impregnate entire animal tissues with polypyrrole, but found that the coated tissues did not conduct electricity, likely due to the discontinuity of the polypyrrole in the surface layer of the tissue (Khor et al., 1995). To integrate the conductive polymers homogenously into a biologically relevant ECM, more recent studies blend them with collagen or other biopolymers prior to gelation (Vijayavenkataraman et al., 2019b; Zarei et al., 2021). With increasing proportions of the conductive polymer, conductivity improves, while biological properties are reduced. To add biological cues without disrupting conductivity, conductive polymers can also be functionalized with cell-adhesive peptides such as the laminin-derived YIGSR motif (Green et al., 2009). Alternatively, biodegradability can be introduced, by co-polymerizing the conductive polymer with a biodegradable polymer, instead of blending (Durgam et al., 2010; Vijayavenkataraman et al., 2019b). Recently conductive polymers have been gaining attention in the nerve regeneration community and the implementation of different combinations of conductive materials in NGCs has been shown to enhance axonal growth in vivo (Sun et al., 2019). Furthermore, Zhao and colleagues have shown that not only neurons, but also Schwann cells (SCs) respond to the electrical stimulation (Zhao et al., 2020).

Peripheral nerve interfaces’ primary function is to interrogate or actuate the peripheral nervous system with electrode arrays for applications such as neuropathic pain management, nerve recording for limb prosthetics or replacement of peripheral nerve function for bladder control, as reviewed by Paggi and colleagues (Paggi et al., 2021). However, depending on their design and geometry, the interfaces may be used in synergy with a regenerative implant to actively stimulate regenerating axons or monitor healing success both as a research question and as a medical theranostic device. Such strategies involve either a sieve electrode which allows for the passage of growing axons (Lago et al., 2005; MacEwan et al., 2016) or a multichannel electrode conduit (Musick et al., 2015). Although implantable nerve interfaces often face similar problems as NGCs, including mechanical compatibility, biocompatibility and immunogenicity, the material choice for neural interfaces is not typically motivated by questions of bioactivity. Instead, a metal electrode such as gold or platinum is supported by a bioinert substrate such as silicone or polyimide (MacEwan et al., 2016; Paggi et al., 2021). Approaches that combine a neural interface with neuroregenerative functionality would have to draw from both research fields, pairing a spatiotemporally precise interface with a bioactive and biodegradable polymer.

As the name suggests, neurotrophins such as NGF, BDNF or NT-3, exert primarily trophic effects on neurons, although some axon guiding effects have also been discussed (Guthrie, 2007; Lykissas et al., 2007).

The most studied member of the neurotrophin family, Nerve Growth Factor (NGF), acts mostly on sensory and sympathetic neurons by promoting neurite sprouting and elongation. Xia and Lv, (2018) loaded an electrospun nanofibrous scaffold with VEGF and NGF, allowing NGF to be released continuously for more than a month. The NGF condition induced stronger proliferation of neural crest stem cells in vitro and better functional recovery after sciatic nerve gap injury in vivo. NGF was also tested as an axon guidance molecule, showing a chemoattractant function on growth cones in vitro (Dontchev and Letourneau, 2002). However, this effect could not be reliably confirmed in a milieu required for ganglionic cell growth, resulting in no directionality and disorganized growth (Fornaro et al., 2020). Brain Derived Neurotrophic Factor (BDNF) is known for its involvement in hippocampal neurogenesis and its protective role in neuronal survival after a PNI, as higher expression of BDNF in Schwann cells and dorsal root ganglia (DRG) were detected after PNI (Kobayashi et al., 2008). Lopes et al. (2017) used tetanus toxin-conjugated nerve targeting nanoparticles to overexpress BDNF DNA in a nerve crush injury model. Their approach led not only to a significantly higher count of myelinated axons but also protected the denervated muscle. This shows that neurotrophins can be valuable therapeutic targets for PNI both on a protein and a gene therapy level.

Neurotrophin-3 (NT-3) can enhance Schwann cell migration and ensure their survival. Donsante et al. (2020) showed that a continuous release of NT-3 from collagen-based electrospun fibers over a 2 week-period led to increased axon counts in the distal nerve. In a co-culture of DRG-derived neurons and then in dorsal root ganglia (DRG) explants NT-3 ensured ordinate and oriented axonal elongation (Fornaro et al., 2020). These recent studies indicate NT-3 as a potential axon guiding component for next generation anisotropic NGCs, although further studies are needed to confirm its axon guiding function in vivo and determine the optimal time and mode of delivery.

Glial cell line-Derived Neurotrophic Factor (GDNF), a neurotrophic factor produced by Schwann cells (SCs) has been shown to promote the survival of sensory neurons and axon outgrowth from DRG explants in vitro (Leclere et al., 1997) but when released at high concentrations from an NGC it impeded nerve regeneration in vivo (Kong et al., 2021). Kong and colleagues discuss this unexpected observation in the context of the “candy store effect,” a term used to describe the entrapment of growing axons in a microenvironment oversaturated with growth factor (Eggers et al., 2013; Kong et al., 2021). It has been suggested that to harness the pro-regenerative function of GDNF a concentration gradient or continuous delivery at low levels via gene therapy is needed (Shi et al., 2010; Eggers et al., 2013).

This last example reminds us, that observations from in vitro studies on neurons or explanted dorsal root ganglions (DRG) treated with recombinant growth factors cannot always be directly translated into conclusions about the factor’s beneficial role in a therapeutic application. The optimal dose, timing, release kinetic and localization may be entirely different for each therapeutic protein and the current challenge lies in understanding and modulating the application-specific pharmacokinetics and pharmacodynamics. While there is a strong consensus on the pro-regenerative potential of soluble factors such as neurotrophic growth factors or the matrix remodeling enzyme chondroitinase in PNI, these potent factors are not a one one-size-fits-all solution and their biological function is dependent on the timing, microanatomical localization and often gradient.

An anisotropic aspect could be added to NGC implants by exposing growth cones to chemical gradients of chemoattractants that determine growth directionality rather than growth rate only. Netrin-1 (Ntn1) and its receptor “Deleted in colorectal carcinoma” have been shown to attract the tip of a sprouting axon in vitro and promote peripheral nerve regeneration, as well as Schwann cell proliferation and migration (Lv et al., 2015; Wang et al., 2019). Other classes of molecules, such as slits act as axon repellent cues, preventing disorganized growth (Long et al., 2004). Lykissas et al. (2007) discussed that while axon attracting effects have also been reported as an additional function for some members of the neurotrophin family, other classes of soluble molecules that predominantly guide axon growth cones constitute a novel and largely unexplored opportunity in the NGC field. Ntn1 was first used for NGC design in a recent study by Huang and colleagues. A graphene mesh-supported double-network hydrogel scaffold was engineered in which Ntn1 promoted Schwann cell migration successfully and guided their alignment, outperforming even the autologous graft (Huang et al., 2021). Thus, chemoattractant agents should be investigated in further studies, as many in vitro findings on axon guidance are yet to be validated in an in vivo PNI model.

Because every growing tissue is dependent on nutrient supply, sufficient vascularization has an important supportive role in the process of nerve regeneration. As Yapici et al. (2017) have shown, vascularized conduits significantly outperformed non-vascularized conduits in a sciatic nerve gap model. Therefore, Vascular Endothelial Growth Factor (VEGF) was used in combination with mesenchymal stem cells (MSCs) to functionalize autogenous vein grafts. The VEGF group resulted in a higher degree of regeneration of a 10 mm nerve gap, compared to MCSs alone (Eren et al., 2016). Beside its pro-angiogenic activity, VEGF can stimulate axonal outgrowth and promote Schwann cell proliferation and migration, suggesting an entire palette of additional mechanisms through which VEGF could enhance nerve regeneration (Sondell et al., 1999; Muratori et al., 2018).

If the PNI is older and repair occurs with a significant delay, a glial scar consisting of chondroitin sulfate proteoglycans (CSPG) and glycosaminoglycans can interfere with axon regeneration even after successful readaptation of transected nerve ends. Chondroitinase ABC, an enzyme derived from the bacterium Proteus vulgaris, catalyzes the degradation of the polysaccharides. Enzymatic removal of the CSPG scar by chondroitinase treatment is an established strategy in the field of spinal cord repair, but also in PNI (Kostereva et al., 2016; Donsante et al., 2020). Chondroitinase may be an especially valuable factor in a setting of delayed repair because its mode of action could complement the pro-regenerative or trophic approaches discussed above. A recent study has identified an additional mechanism through which the enzyme can disinhibit a BDNF receptor, leading to increased neuroplasticity (Lesnikova et al., 2021).

SCs are the principal glial cells of the peripheral nervous system (PNS) and ensure the formation of myelin sheaths around the axons and the accelerated conduction of nerve impulses as reviewed elsewhere (Berrocal et al., 2013; Fallon and Tadi, 2021). A recent study shows that SCs exist in functionally diverse differentiation states, as they can dedifferentiate into a precursor-like, proliferating state, which can downregulate myelin genes and remove pre-existing myelin debris that would inhibit axonal regrowth (Arthur-Farraj et al., 2012; Stratton et al., 2018). Beside their ability to myelinate regenerated axons, SCs enhance axonal sprouting and form bands of Büngner, important guiding structures for the growing axon. To mimic the bands, SCs could be compressed into bundles and surrounded by a hydrogel microcolumn. Such tissue engineered Büngner bands accelerated axonal growth in vitro up to 8 fold and resulted in significantly longer neurite length than an NGC with unaligned SCs (Panzer et al., 2020). An early study of SCs for PNI treatment reported that in vitro cultured SCs would align along the axis of the NGC in structures reminiscent of Büngner’s bands. The same study also found that only syngeneic SCs promoted the growth of myelinated axons, while heterologous SCs induced a host immune response that impaired regeneration (Guénard et al., 1992). The importance of autologous SC sources poses a considerable challenge in terms of NGC availability, standardized product quality control and mass fabrication.

iPSCs are a personalized self-renewing source of stem cells which enables autologous treatment. iPSCs can be reprogrammed from patient-derived somatic cells and then differentiated into a broad spectrum of cell types, enabling researchers to generate large quantities of cells that are difficult to harvest, such as SCs (Takahashi et al., 2007). On the downside, iPSCs’ pluripotent state has demonstrated the risk of teratoma formation if some undifferentiated iPSCs remain in the transplanted iPSC-derived cell population. Different strategies, including cell sorting or the introduction of suicide genes into iPSCs prior to differentiation are being investigated to provide the safety, necessary for human application of iPSC-derived cells (Bedel et al., 2017). For preclinical evaluation in PNI, iPSCs were differentiated into neural crest stem cells (NCSCs) or SCs without teratoma formation. Huang et al. (2017) used these two cell types to create a tissue-engineered NGC that successfully regenerated a 10 mm gap in a rat model. NCSCs, showed stronger paracrine signaling than the further differentiated SCs, pointing out the importance of selecting the iPSC differentiation state.

Because generating patient-derived iPSCs and differentiating them into a desired cell type takes time, their usefulness for PNI management is limited. Strategies for off-the-shelf clinical use and for complete depletion of undifferentiated iPSCs needs to be developed to fully unleash their potential.

MSCs are adult multipotent stromal cells isolated from bone marrow, adipose or perinatal tissues. MSCs have attracted attention due to their well-documented clinical safety, rich pro-regenerative secretome, and immunomodulatory properties (Drzeniek et al., 2021). Additionally they can differentiate into SC-like cells when exposed to a combination of soluble factors and produce even more growth factors in this pre-differentiated state (Drzeniek et al., 2021; Liu et al., 2022). Despite many functional similarity and a similar mode of action, MSCs derived from different sources differ in characteristics such as expansion speed and hemocompatibility, as reviewed by Moll and colleagues (Moll et al., 2019).

Intravenously infused MSCs can migrate to the site of injury (Marconi et al., 2012; Matthes et al., 2013), but are often transplanted into the vicinity of the lesioned nerve during the reconstructive surgery, resulting in a more concentrated effect and reduced local fibrosis (Wang et al., 2015; Cooney et al., 2016). MSCs are a natural source of pro-angiogenic and regenerative growth factors and act as a “living drug factory” for the healing nerve (Drzeniek et al., 2021). Their paracrine effects result in a better electrophysiological and functional outcome and micromorphological intactness of the healed nerve (Marconi et al., 2012; Matthes et al., 2013; Cooney et al., 2016; Bucan et al., 2019). Because the local microenvironment can affect MSC survival and function in vivo, protective carrier materials can shield MSCs from the harsh microenvironment, hold them in place and stimulate them to augment their growth factor secretion (Mao et al., 2017; Drzeniek et al., 2021). In recent years, it has been proposed to transplant MSC-derived vesicles, exosomes, instead of the entire living cell. These exosomes have also been shown to promote peripheral nerve regeneration, similarly to the parent cell (Bucan et al., 2019). Another recent study shows that exosomes from MSC-derived SC-like cells play a supporting role by protecting endogenous SCs from oxidative stress and promoting angiogenesis (Liu et al., 2022).

An easy way to functionalize the NGC lumen is with bulk hydrogel, which can provide a cell friendly environment, allows for easy incorporation of biochemical cues and soluble factors, but does not provide any topographical cues for directional determination and may even reduce the permissiveness of the NGC lumen for growing axons (Yoo et al., 2020; Drzeniek et al., 2021). Therefore, the opinion on bulk fillers is controversial and evidence which favors spatial patterning of hydrogels is growing (Yoo et al., 2020).

The use of molded grooves on the inner NGC wall has been proposed with the rationale that grooves could not only provide anisotropic physical guidance to sprouting axons but also increase the surface area for cell attachment, cell-material interactions and potentially for controlled release of incorporated growth factors. In practice however, patterning 10 µm grooves on the inner NGC wall did not yield a relevant effect compared to an unpatterned chitosan NGC and failed to outperform the autograft control (Li et al., 2018). Similarly, in an older study a micropatterned inner lumen designed to regenerate a 10 mm rat sciatic nerve gap could only impact functional outcome when implemented in synergy with pre-seeded SCs (Rutkowski et al., 2004).

As an alternative to molded groves, longitudinally oriented collagen strips were 3D-printed on a porous PLCL membrane. It could be shown that the 3D-printed collagen lines led to a better axonal regeneration and remyelination than bulk collagen hydrogel filling, indicating that spatially controlled patterning of substrates that promote cell attachment can be a promising strategy that combines structural and substrate-mediated cues (Yoo et al., 2020). Similarly, in a very recent study a grooved PLCL conduit in combination with a patterned gradient of a laminin-derived peptide showed synergistic effects on aligned migration of SCs in vitro and significantly accelerated nerve recovery in vivo (Zhang et al., 2021).

It seems that the benefit of grooves manifests in combination with other factors such as biochemical cues or cells, or maybe acts on axons indirectly by influencing SC biology. Early in vitro studies had found that groove width versus depth differently influence the alignment of SCs versus neurites (Miller et al., 2001a; Miller et al., 2001b). In vivo studies often lack such comparisons and possibly successful use of grooves in NGC design would require more attention to detail and systematic optimization in vivo.

Microchannels mimic the nerve fascicular (perineural) anatomy and prevent axon dispersion and can be introduced into the scaffold either by precise 3D bioprinting or through more conventional fabrication such as directional freeze drying or molding (Figure 2) (Hu et al., 2009; Wray et al., 2012; Zhang et al., 2022). Chang et al. (2017) used an NGC with micromolded intraluminal channels that resulted in increased axon diameter and myelin layer thickness compared to a control NGC without internal channels. While additional aligned nanofibers within the microchannels contributed only slightly to regeneration, adding an additional neurotrophic factor gradient, yielded an effect comparable to autograft. Similarly, the combination of a multichannel design with controlled release of 4-aminopyridine, a potassium channel blocker thought to improve nerve conduction in neurological disorders such as multiple sclerosis, was comparable to autograft in a 15 mm sciatic nerve gap rat model (Manoukian et al., 2021). Here the multichannels were fabricated by unidirectional freeze drying. For a simple yet clever spiral-shaped design, a grooved surface was rolled to fabricate multichannels which were combined with aligned nanofibers, but the structural cues alone failed to compete with the autograft control (Shah et al., 2019). The disadvantage of the aforementioned fabrication techniques is that the material architecture needs to be fabricated before therapeutic cells can be added to the conduit. In contrast to freeze drying, 3D bioprinting allows to integrate cells directly into the biomaterial. Recently, Zhang and colleagues used bioprinting to fabricate an advanced NGC (Zhang et al., 2022). SCs were incorporated into a methacrylated gelatin bioink and printed directly into the multi-channeled conduit.

These studies show that different fabrication techniques can be used to achieve an internal microchannel structure that mimics the natural anatomy of guiding sheaths. Emerging study design combining different modalities of bioactive cues stress the importance of synergistic biological and structural cues for NGC constructs that could promote nerve regeneration across larger gaps.