2.1.1 Peptides

Peptides are the intermediates of protein hydrolysis. They consist of amino acids and can be enzymatically degraded in vivo, with the resultant products, also being amino acids, readily assimilated by cells. Consequently, they exhibit high biocompatibility within biological systems.⁸¹ Under specific conditions, certain peptides can self-assemble to form ordered high-dimensional structures, providing scaffolding for cell adhesion and growth.⁸² This property offers potential in designing and fabricating hydrogels with designated functionalities. Additionally, peptides can mimic the functional domains of natural proteins, providing peptide-based hydrogels with specific bioactive properties.⁸³ Hence, they have a high potential for tissue engineering, biological factors delivery, and biosensing applications.

Peptide hydrogels have been extensively investigated. However, the synthesis of self-assembled functional peptide hydrogels with corresponding protein sequences based on the molecular structure of known high-performance materials remains unclear. Wei et al.⁸⁴ developed a silk fibroin peptide (SF16) hydrogel containing a filamentous protein sequence, which could support PC12 cells in secreting matrixes, promoting cell proliferation in damaged neural sites, regulating neural-associated cell differentiation, and enhancing neurite growth. In a self-assembled 3D peptide hydrogel with dual functionality achieved by the adhesion protein modification motif IKVAV, the brain-derived NGF peptide site RGI was able to significantly enhance the adhesion of SCs to the material, promote the expression of NGFs, BDNFs, ciliary neurotrophic factor, peripheral myelin protein 22, neuropilin 2, and other related genes, and successfully repair a 10 mm sciatic nerve defect in vivo.⁵² Owing to their derivation from proteins and the multiple binding sites of peptide hydrogels, a synergistic effect between IKVAV and RGI was achieved, promoting axonal and myelin regeneration and accelerating motor function recovery.⁵² Peptide co-assembly with mimic peptide epitopes has also been proposed to functionalize hydrogels. Lu et al.⁵¹ used this approach to obtain hydrogels modified with BDNF and vascular endothelial growth factors (VEGFs), which provided a 3D neurovascular microenvironment for the growth of endothelial and neuronal cells and similarly promoted PNI repair. Compared with self-assembled and co-assembled peptides, self-sorting peptides can selectively interact with specific ligands to produce complex assemblies with precise components.⁸⁵ While it has been confirmed that these hydrogels can promote tissue self-healing, their application in PNI repair requires further verification.⁸⁶

Peptides can assemble into high-dimensional structures that support cell adhesion and proliferation.⁸⁷ Upon degradation, they yield amino acids that nourish cells. By mimicking active sites, peptides modulate cellular behavior, thus demonstrating strong interactions with cells and emerging as promising materials for tissue engineering.⁸⁸ However, while peptides are more stable than proteins, they still exhibit sensitivity to temperature, pH, and mechanical stimuli.⁸⁹ As biomaterials for repair, their limited stability can result in the leakage or premature release of encapsulated substances during the repair process. Furthermore, the metabolic instability of their degradation products may induce inflammation.^90-92 Therefore, it is necessary to further observe the release behavior of peptide hydrogels in real-time with a monitoring device and examine their efficacy on the encapsulation of substances.

2.1.2 Alginate

Alginates are biologically inert materials composed of homopolymers, and the G residues within them can be cross-linked to produce hydrogels.⁹³ They can then be chemically decomposed and reconstituted into a nanoporous structure like a basement membrane, which physically restricts the encapsulated cells and affects their migration, proliferation, and differentiation.^{93, 94} However, after alginates absorb a large volume of water and form gels, they are prone to deformations by movements or external pressures due to their high hydrophilicity. These deformations can damage the neural stem cells they carry or the ones at the implantation site.⁶¹ Jin et al.⁹⁵ prepared a new biosafe hydrogel with graphene and sodium alginates that exhibited favorable conductivity and mechanical properties, and anti-inflammatory functions. It promoted the release of neurotrophic substances and up-regulated the expression of growth factors while simulating the neural growth microenvironment. Another study used alginates and collagen to prepare hydrogels containing derived exosomes critical in regulating neural differentiation. The alginates were shown to have good biocompatibility that facilitated the growth of SCs within its microstructure and provided conditions for the secretion of neurotrophic factors acting on tropomyosin receptor kinase A receptors.⁹⁶ Raimondo et al.⁹⁷ revealed that injectable alginate hydrogels could load VEGFs and insulin-like growth factor-1 and transport them to damaged sites in vivo to promote the regeneration of muscle fibers and peripheral nerves. Alginate also exhibits pronounced hydrophilicity, enabling it to form hydrogels that structurally resemble the ECM.⁹³ This characteristic, and its excellent biocompatibility, supports cell adhesion, proliferation, differentiation, and controlled drug release.⁹⁸ In addition, the native polysaccharides of alginate possess inherent anti-inflammatory properties. However, its limited mechanical strength restricts the standalone application of pure alginate hydrogels.⁹³ Still, through different profile designs and material modifications, alginate hydrogels could obtain favorable physicochemical properties, bioactivity, and biological factor loading performance, which would benefit their use in PNI repair.

2.1.3 Collagen

Collagen is the most abundant component in ECMs. It can be converted to a hydrogel via self-fibrosis under physiological conditions achieved through changing external conditions, such as pH and temperature.⁹⁹ Thus, collagen hydrogels are bionanomimetic materials that can mimic the physiological cellular microenvironment and are widely used in regenerative medication and PNI repair. Masand et al.¹⁰⁰ proposed using polysalivary acid and the human natural killer cell epitope HNK-1 to prepare functionalized collagen hydrogels. They discovered that these hydrogels promoted PNI repair by improving axon number, motor neuron targeting, and axon myelination. Zhang et al.¹⁰¹ improved the physical structure of collagen hydrogels via the 3D method to fabricate loaded gingiva-derived mesenchymal stem cells (GMSCs). Their results showed that stem cells could migrate and integrate into the ECM of natural neural stem cells through the hydrogel. This significantly promoted the recovery of neural function and axon regeneration under the regulation of the Notch signaling pathway. As a natural component of the ECM, collagen hydrogels exhibit superior biocompatibility compared to alginate ones.¹⁰² Nevertheless, like alginates, collagen hydrogels are highly sensitive to mechanical stimuli, mainly exhibiting limited tensile resistance. Furthermore, being a natural protein, collagen is readily recognized and degraded by matrix metalloproteinases (MMPs) with collagenase activity, leading to rapid in vivo degradation.¹⁰³ This rapid breakdown poses challenges for prolonged processes such as neural repair. Therefore, various chemical cross-linking strategies might enhance their properties, rendering them more suitable for PNI repair.

2.1.4 Gelatin

Gelatin is a decomposed product of collagen that does not have a triple helix structure with the same molecular components as collagen and is considered an ideal collagen substitute.¹⁰⁴ Some researchers have developed a 3D photo-cross-linked, gelatin-based hydrogel that can encapsulate SCs. This hydrogel exhibits high viscosity and strength, and SC viability can be retained. Moreover, it promotes SC proliferation, neurite extension, and glial cell recruitment after being filled into damaged areas.¹⁰⁵ Compared with conventional nerve conduits, the double network of gelatin, when combined with alginate-loaded netrin-1 hydrogels (as prepared by Huang et al.¹⁰⁶), not only targets axon pathfinding and neuronal migration but effectively induces cannula formation, which makes it a potentially superior material for neural repair than autologous grafts. In addition, gelatin can be biodegraded by cell-derived enzymes (e.g., MMPs), which contribute to cell-mediated matrix remodeling.^{104, 107} As gelatin is derived from collagen, it offers biocompatibility and mechanical properties comparable to collagen, with the added benefits of reduced production costs and adjustable degradation rates.^{103, 108} However, gelatin hydrogels have the same disadvantages as collagen in terms of mechanical strength. Compared to collagen hydrogels, gelatin ones form pores larger than cells. While these scaffolds provide a 3D structure at the macroscopic level, the large pore size means individual cells effectively experience a 2D culture microstructure.⁹⁴ Still, gelatin has more prospective applications than collagen for preparing biological factor- and cell-laden materials.

2.1.5 Hyaluronic acid

HA is extremely biosafe and has strong anti-inflammatory abilities and influence on cell migration and differentiation due to its binding sites with cells overexpressing CD44.⁶¹ This feature is the basis for preparing relevant hydrogel materials established on neural regeneration promotion theories, such as recruiting SCs. Compared with other natural macromolecules, HA has poor autonomic cross-linking ability,⁶¹ which needs stricter and more requirements for surface modification, performance improvement, and development applications. Huang et al.¹⁰⁹ demonstrated that a simple HA-laminin hydrogel, acting as a hollow nerve conduit, can increase the thickness of regenerating fiber sheath membranes by inhibiting functional axonal loss and increasing the density of regenerating nerve fibers. Liu et al.¹¹⁰ found that photo-cross-linked HA methacrylate hydrogels carrying stem cell-derived exosomes could promote PNI repair and downregulate the expression of inflammatory factors, such as interleukin-1 β and tumor necrosis factor. HA hydrogels with an elastic modulus of 0.78 kPa can promote exosome release through their weak rigidity, which could provide a new direction for developing HA-based hydrogels. In addition, HA hydrogels with a high molecular weight (more than 10⁷ Da) have stronger anti-inflammatory and anti-angiogenic effects than low ones.⁶¹ Overall, HA hydrogels exhibit superior bioactivity compared to collagen, gelatin, and sodium alginate. While they suffer from the mechanical limitations inherent to natural hydrogels, their mechanical properties can be enhanced by altering cross-linking methods, incorporating nanomaterials, or blending with high-performance polymers.^{94, 111} It might also be feasible to improve the function of these materials by modulating their intrinsic properties, such as molecular weight. Moreover, proper cross-linked groups could be selected to modify these materials when HA is used to promote PNI repair.

2.1.6 Agarose

Agarose is a natural, biocompatible polysaccharide extracted from red algae. It can self-coagulate at 37°C without cross-linking agents, so its toxicity can be avoided. This polymer also has a high water absorption capacity, strong biological activity, and characteristics similar to natural ECMs, which can promote cell growth, differentiation, and proliferation.¹¹² Agarose-made scaffolds are widely used in nerve defects but are not enriched in ECM molecules, cells, or factors to enhance growth.¹¹³ Therefore, growth factors, stem cells, and agarose can form a composite scaffold to enhance the repair effect. Gao et al. implanted a BMSC-BDNF-multichannel agarose-guided scaffold for PNI regeneration into a 15-mm-long rat sciatic nerve space, a composite scaffold that mimicked the 3D structure of the peripheral nerve. The BMSCs secreted matrix, fibronectin, collagen, and BDNF, which promoted axonal growth and successfully guided long-distance linear axonal regeneration.¹¹⁴

2.1.7 Cellulose

Cellulose is the most abundant biopolymer in plant cell walls and is produced by animals, fungi, and bacteria. It has good biocompatibility and biological activity, and its nonbiodegradable characteristics make it suitable for nerve injury recovery.¹¹⁵ Zheng et al. used oxidized hydroxyethyl cellulose and chitosan (CS) to cross-link polymerization. Within a specific range, the mechanical properties and porosity of polymeric materials are directly proportional to the proportion of oxidized hydroxyethyl cellulose, and the swelling behavior and degradation rate of polymeric materials decreases with an increase in oxidized hydroxyethyl cellulose.¹¹⁶ Therefore, cross-linking cellulose with other hydrogels is helpful to improve the characterization of the materials.

Studies have also shown that electrical stimulation can promote nerve regeneration.¹¹⁷ Asiaticoside liposomes can interfere with the DNA synthesis of fibroblasts and inhibit their differentiation and proliferation, reducing collagen synthesis and inhibiting the development of scar tissue.^{118, 119} Overall, the best cell proliferation, axon growth, and cell differentiation occur in this hydrogel group.

2.1.8 Decellularized extracellular matrix (dECM)

dECM has been widely used as a reliable alternative material for PNI repair in clinical surgery. Since the decellularization technique can preserve numerous bioactive components in the ECM, such as polysaccharides, proteins, and growth factors, dECM-based biomaterials can preserve the intrinsic microenvironment of tissues, promote the proliferation and differentiation of SCs and induce neural regeneration.¹²⁰ However, its application has some limitations, including poor preservation effects of active components in the initial preparation, sterilization, and preservation processes, poor reproducibility of homogeneous dECM, and a weak ability to continuously release NGFs. Given these restraints, the material was first used as a lumen filler for nerve conduits or a transplantation vehicle for SCs.^{121, 122} More recently, however, dECM has gradually been used in hydrogels. Commonly used dECM hydrogels are of porcine neurogenic origin and exhibit favorable in vitro properties in promoting the axonal growth of dorsal root ganglion (DRG) cells and the myelination of axons by SCs¹²³ (Figure 1). Meder et al.¹²⁵ developed a decellularized, porcine nerve-derived hydrogel filler. They evaluated the recovery of peripheral innervation functioning 24 weeks post-injury and found improved distribution of peripheral nerve matrixes within the catheter, enhanced electrophysiological responses, and more nascent axons than in the control group. Xue et al.¹²⁶ proposed a theory related to the facilitative effect of the synergistic action of dECM hydrogels and their components on the behavioral regulation of DRGs and SCs. However, despite their high ductility, dECM hydrogels lose the natural structure of intact ECMs and are susceptible to bacterial contamination.

As a natural material that can mimic the tissue microenvironment, decellularized matrices offer structures conducive to cell adhesion, mechanical properties supportive of tissue integration, and bioactivity-promoting neural regeneration. The primary limitations to their development and application lie in their fabrication and associated costs. Striking a balance between safety, immune modulation, and retention of active components during fabrication is crucial for developing hydrogels with translational significance.

2.2.1 Polyvinyl alcohol (PVA)

PVA is widely used as a tissue engineering material to prepare eye drops, artificial joints, and tissue barriers. However, its application is limited by its low mechanical stress threshold due to traditional physical cross-linkage, low purity, and inhomogeneous cross-linking caused by chemical cross-linkages.^127-129 Oxidative modification, double cross-linkages, and multi-walled carbon nanotube implantations are common methods to improve its performance.^{61, 130} Stocco et al.¹³⁰ used 1% oxidant-treated PVA to develop PNI repair cannulas with improved elasticity and suture resistance. These cannulas were shown to maintain their shapes during implantation and promoted regeneration of axons at the injury site, allowing the formation of myelinated nerve fibers similar to the original nerve size. Porzionato et al.¹³¹ suggested that nerve cannulas prepared from oxidized PVA were more conducive to releasing ciliary neurotrophic factors than unoxidized PVA. Additionally, Aregueta-Robles et al.¹³² found that cross-linked PVA with biomacromolecule silk glue and gelatin improved its recruitment of and adhesion to SCs. Recently, hydrogels with multi-walled carbon nanotubes doped with PVA were confirmed to have better ductility, electrical conductivity, and enhanced self-healing ability after injury.^133-135 However, further investigations into the application of these hydrogels in PNI repair are necessary, especially regarding its regulation of the inflammatory response of damaged nerves and the induction and regulation of cells such as SCs.

PVA hydrogels demonstrate limitations in mechanical properties, degradability, and support for cell adhesion.¹³⁶ However, oxidative modifications provide controlled tuning of PVA's mechanical and degradable characteristics while blending with other materials, which can modify its mechanical properties and cell adhesion potential.^{137, 138} These approaches present potential directions for the development of hydrogels for neural repair. Consequently, enhancing the biocompatibility of composite hydrogels containing PVA is imperative.

2.2.2 Polyethylene glycol (PEG)

Compared to PVA, PEG and its derivatives are the most widely used synthetic biocompatible hydrogel materials. PEG promotes cell rearrangements with its highly spreadable and tractable structures during degradation,¹³⁹ which provides the theoretical basis for PEG hydrogels's ability to fill along nerve alignments during injury repair. PEG hydrogels are widely used in PNI repair owing to their convenient preparation processes, relatively low costs, simple chemical modifications, and easily tunable mechanical properties.⁶¹ Studies have confirmed that the fusion of allogeneic peripheral nerve grafts with PEG can reduce the immune response, significantly inhibit Wallerian degeneration, rapidly re-establish axonal conduction, re-innervate the injured nerve to the target organ, and even restore body functioning to its original level.^{33, 140} Unfortunately, original PEG hydrogels and their derivatives cannot support cell adhesion due to their biological inertness. Hence, modification through cross-linkages is needed to improve their properties and enhance their bioactivity to allow cell adhesion.⁶¹ PEG hydrogels conjugated with integrin-binding motifs such as RGD or MMPs are more conducive to multicellular aggregation in larger volumes.¹⁴¹ Based on these findings, PEG hydrogels and their derivatives will be better applied in PNI repair after eliminating these defects.

2.2.3 Polycaprolactone (PCL)

PCL has been widely used in PNI repair for controlled drug release, cell carriage, and mechanical support amongst other applications and targeted enhancement of its mechanical properties may be the focus of future research.⁶¹ Niu et al.¹⁴² prepared a PNI-repair scaffold cross-linked with PCL and PEG, which increased its surface area and enhanced its cell adhesion capacity. Various PCL materials loaded with urolithin-A,¹⁴³ encapsulated with magnesium ions,¹⁴⁴ and modified with type I collagen and mineralized collagen,¹⁴⁵ have been prepared and examined. These conduits play an important role in recruiting SCs and guiding nerve fiber regeneration while effectively maintaining drug concentrations and efficacy.

PCL is extensively used in synthetic hydrogels for PNI repair due to its clinically relevant mechanical strength, inherent softness, ease of fabrication, prolonged biodegradation cycles, and favorable biocompatibility.¹⁴⁶ However, it exhibits inferior hydrophilicity compared to PVA, which could influence its interactions with cells and biomolecules. Surface modification has emerged as an effective strategy to address this limitation. Therefore, strategies such as PCL surface modification to create structures conducive to cell adhesion are likely to be the primary directions for developing PCL hydrogels for neural repair in the future.¹⁴⁷

2.3.1 DNA

The most important feature of DNA hydrogels is that they can carry a large amount of inherited information.^{92, 148} This may have positive implications for regulating cell distribution, genetics, and metabolism at the implantation site. However, these hydrogels have disadvantages such as difficult and expensive preparation costs, susceptibility to external stimuli such as environmental changes that may modify genetic information and physiological functions, and undefined safety when producing regulatory responses in organisms.⁶¹ Liu et al.¹²⁴ developed a novel delivery system composed of DNA hydrogels, VEGFs, and NGFs. They exploited the difference in the degradation rates of X- and T-type DNA to achieve the bidirectional release of VEGFs and NGFs, which promoted the repair of PNIs. Presently, most DNA hydrogels are used for drug delivery. Although research on the preparation and practical application of DNA hydrogels remains immature, it has received considerable attention for its unique molecular recognition, ability to protect biologically active, functional proteins, and modifiability based on multiple recognition sites.¹⁴⁹ In future studies, it will be necessary to investigate the development and preparation of DNA hydrogels from the perspective of molecular recognition and protection. Moreover, further research should focus on the regulatory mechanisms, preparation costs, and degradation rate of DNA in vivo. The application of DNA hydrogels as direct filler materials or cell carriers should also be explored. In summary, the mechanical properties of DNA hydrogels are inferior, even when compared to other hydrogels with suboptimal mechanical attributes, such as those based on gelatin or collagen. Moreover, their compromised stability, susceptibility to enzymatic degradation in vivo, and high fabrication and purification costs have constrained their broader applications. Incorporating other materials to enhance their stability and mechanical strength could be a direction for future improvements. Given DNA's nanoscale programmability, which facilitates superior cellular and molecular interactions, hydrogels with a DNA component have promising potential in tissue regeneration fields, including neural repair.^{150, 151}

In general, the biological activity of natural hydrogels is better than artificial ones. However, artificial hydrogels have unique advantages due to their controllable characterization, such as the mechanical strength and viscosity of the material (details are listed in Table 3 ^{62, 64, 65, 69, 72-80}). The specific conditions of the materials may also be affected by other conditions, such as temperature, pH, treatment method, etc.