Natural-synthetic composite hydrogels are interpolymer complexes in which block copolymerization or physical interaction occurs between synthetics and natural polymers. These composite hydrogels combine the biocompatibility of natural hydrogels with the mechanically tunable properties of synthetic hydrogels. For example, the addition of polylysine to poly (ethylene glycol) (PEG; Wei et al., 2021), or chitosan to methacrylamide can increase the viscosity of the material (Saleem et al., 2022).

Gelatin is widely used in tissue engineering and drug delivery systems because of its high hemocompatibility, biodegradability and no other by-products after in vivo degradation, non-immunogenicity, and the same components and biological properties as collagen. Wang et al. designed a hydrogel composed of gelatin-hydroxyphenylpropionic acid (Gtn-HPA) conjugates formed by partial oxidative coupling of 3,4-hydroxyphenylpropionic acid catalyzed by H₂O₂ and horseradish peroxidase. It was found that the proliferation rate of mesenchymal stem cells (MSCs) increased as the stiffness of the hydrogel decreased, and the neurogenesis of MSCs was also strongly affected by the stiffness. The hardness of this material can be modulated by changing the concentration of H₂O₂ without changing the concentration of the polymer precursor, thus controlling the proliferation rate and differentiation of MSCs in gelatin (Wang et al., 2010).

Methacrylated gelatin (GelMA), obtained by preparing methacrylic anhydride with gelatin, is a photosensitive biohydrogel material (Kurian et al., 2022). Cai et al. prepared a conductive, longitudinally grooved GelMA-MXene nerve conduit and explored its role in SCI repair. It was found that the grooved GelMA-MXene effectively promoted the adhesion, directed proliferation and differentiation of neural stem cells (NSCs) in vitro. Its ability to effectively repair complete transection of SCI was demonstrated when GelMA-MXene loaded with NSCs was implanted in the injured spinal cord site. This group exhibited significant neural recovery and significantly higher BBB scores compared to other groups. Therefore, GelMA-MXene with microgroove structure and loaded NSC is a promising strategy for the treatment of SCI (Cai et al., 2022).

Hydrogels also are combined with nanoparticles (organic polymer nanoparticles, inorganic nanoparticles, and metal/metal oxide nanoparticles) to obtain more excellent properties (Wang Y. et al., 2022). Organic polymer nanoparticles include dendritic macromolecules and hyperbranched polyesters. Inorganic nanoparticles include hydroxyapatite, silica, silicate and calcium phosphate. And metal/metal oxide nanoparticles include gold, silver and iron oxide. Among them, metal/metal oxide nanoparticles are particularly favored because of their corrosion resistance, oxidation resistance, high specific surface area, surface chemistry and functionalization (Tan et al., 2019).

Studies have shown that the incorporation of magnetic Fe₃O₄ nanoparticles into chitosan or polyethylene glycol hydrogels can promote the survival and differentiation of bone marrow mesenchymal stem cells (Tan et al., 2019; Barrett-Catton et al., 2021). Li et al. prepared MnO₂ nanoparticles-dotted hydrogels (MnO₂ NPs) by dispersing MnO₂ nanoparticles in peptide-modified hyaluronic acid hydrogels. MnO₂ NPs alleviated the oxidative environment, which effectively improved the viability of MSCs. MSCs transplanted in multifunctional gels produced significant motor function recovery in a large span rat spinal cord transection model and induced in vivo integration of implanted MSCs as well as neural differentiation, resulting in efficient regeneration of central neurospinal tissues (Li L. et al., 2019).

Zinc oxide nanoparticles (ZnO NPs) have been widely reported to achieve satisfying anti-inflammatory functions. Lin et al. constructed a novel ZnO NPs-Gel composite hydrogel and detected the antioxidant markers of SOD, GSH, Nrf2 and HO-1 by ELISA and protein blotting, and demonstrated that the hydrogel effectively down-regulated ROS intensity and improved the oxidative microenvironment at the injury site. In addition, apoptosis of the damaged spinal cord was inhibited. Thus, metal oxide nanoparticle composite hydrogels represent a promising strategy for the treatment of CNS disorders by comprehensively modulating the complications of the pathological microenvironment (Lin et al., 2021).

Nanotubes are widely used in biomedical fields for their low toxicity, high biocompatibility and environmental friendliness. Due to their stable tubular morphology, unique charge distribution and crystal structure, they can be dispersed in hydrogels. Studies have shown that the combination of nanotubes and chitosan has little effect on the pore structure but increases the number of pores, which facilitates cell survival (Waris et al., 2022). Different studies combined elosite nanotubes with different hydrogels such as chitosan, alginate and collagen to form nanotube composite hydrogels. These studies confirmed the biocompatibility and safety of the composite hydrogels and showed that the addition of elosite nanotubes could improve the mechanical strength of the composite hydrogels (Kazemi-Aghdam et al., 2021; Machowska et al., 2022).

Some studies have incorporated carbon nanotubes into gelatin-methacryloyl prepolymer formulations to form biocompatible hydrogels after UV irradiation. The photocrosslinking process of the prepolymer provided delivery of the formulation at the site of injury. This composite hydrogel scaffold improved the reconnection process, provided a site for nerve cell development, and delivered methononin to improve inflammation. It was found that the incorporation of carbon nanotubes effectively promoted neurite growth (de Vasconcelos et al., 2020).

Effective and accurate drug delivery to SCI tissues is crucial to restore neurological deficits. Burst release phenomenon may lead to unexpected side effects. Hydrogels perform well in supporting cell adhesion and guiding cell migration to the affected area, but there is still room for improvement in sustained drug release. Sustained-release microspheres (MS), on the other hand, facilitate sustained and stable drug release (Wang et al., 2011) and also improve the precision of drug delivery (Wu et al., 2023). Therefore, a lot of studies have combined sustained-release MS with hydrogels to form composite hydrogel scaffolds capable of sustained drug release. Wen et al. prepared HA hydrogels with a longitudinal multichannel structure and carried poly (lactic-co-glycolic acid) (PLGA) sustained-release MS encapsulated with vascular endothelial growth factor (VEGF) to form HA composite hydrogels. The HA composite hydrogel was implanted into the SCI area of rats as the experimental group, and the rat model of SCI without the implantation material and the HA hydrogel implantation alone were used as the control group. It was found that the composite hydrogel integrated well with the spinal cord tissue, induced vascular and nerve fiber regeneration, and could promote the recovery of motor function after SCI in rats (Wen et al., 2016).

Zhang et al. developed and optimized injectable PLGA MS loaded with melatonin (Mel), which was further mixed with Laponite hydrogel to form a composite hydrogel scaffold to reduce the sudden release of MS. The MS was able to achieve stable and prolonged Mel release as well as synergistic Lap hydrogel to repair neural function in SCI by in situ injection (Zhang et al., 2021).

Composite hydrogels with the addition of different materials possess corresponding advantages, including mechanical properties, electrical conductivity, oxidation resistance, and extended release ability (Table 1). Most of the current composite hydrogels have only one of these advantages, and there is no one that combines all of them. However, it is not a simple addition of the strengths of each, but also needs to consider the interaction of different additives and the overall impact on the composite hydrogels. Research on the preparation of composite hydrogels with better overall performance is still ongoing.

Temperature-responsive hydrogels undergo sol–gel or gel–sol transitions with subtle changes in ambient temperature, and their temperature-dependent phase transitions are mainly driven by hydrophilic and hydrophobic interactions. Near the critical temperature, hydrophilic polymers lose a large amount of water and form gels dominated by hydrophobic interactions (Rahmani et al., 2023). ZHENG et al. prepared poly (lactic acid)-poly (ethylene glycol)-poly (lactic acid) (PLA–PEG–PLA) as injectable temperature-sensitive hydrogel materials, which successfully inhibited the expression of inflammatory cytokines in the early stage of injury, reduced the apoptosis of neurons in spinal cord injured rats and promoted the functional recovery (Zheng et al., 2021).

Photoresponsive hydrogels utilize hydrogel factors encoded by photosensitive groups (e.g., photocracking groups and photoisomerization groups) to trigger phase transitions (Hoque et al., 2019). Wang et al. prepared F127 hydrogel loaded with extracellular vesicles for SCI treatment. F127 is a copolymer of poly(citrate-polyethyleneimine), which is activated in the presence of rapidly cross-linked and cured into gels by UV and visible light in the presence of a photoinitiator. This delivery system has been shown to inhibit fibrotic scar formation, reduce inflammation, and promote myelin and axonal regeneration (Wang et al., 2021).

For hydrogels used in SCI therapy, in addition to good loading capacity and long duration retention in vivo, the ability for time-specific controlled release is required. pH-responsive hydrogels have many acidic or basic groups that can rapidly receive or release protons in the environment, thus enabling an intelligent response to solution pH. It is stable under neutral conditions but can be degraded in the inflammatory microenvironment of SCI with low pH to achieve pH-responsive drug release from the inflammatory site of SCI (Chang et al., 2022). A pH and photothermal dual-responsive drug-loaded nanoparticle, injectable collagen hydrogel system was fabricated to treat SCI. The system uses ZIF-8 as the carrier material, a ligand compound formed by the self-assembly of the ligand 2-methylimidazole and a zinc-based compound, which has good drug release in acidic inflammatory sites and maintains a stable state until it reaches the affected area. Hydrogel degradation was able to raise the pH of the microenvironment，promote proliferation of neural stem cells and protect neurons. The loaded drug paclitaxel played an effective role in promoting axonal lengthening and neuronal differentiation. The results showed that the system resulted in higher BBB scores in rats with SCI (Chen and Dai, 2023). Rogina et al. prepared a pH-responsive chitosan-HAP hydrogel with NaHCO₃ as the gelling agent, which not only achieved rapid gelation of chitosan-HAP-based hydrogels within 4 min, but also served a pH-passive targeting function to react with the micro-acidic environment and achieve drug release (Rogina et al., 2017).

Electrically charged drug molecules under an electric field move in the opposite direction of charge, which is the key mechanism of drug release from electrically responsive hydrogels. These gels constructed from conductive polymers such as polypyrrole and polyaniline also contract under electrical stimulation to induce drug release (Mo et al., 2022). Conductive hydrogels have promising applications in the repair of electrically active biological tissues such as cardiac muscle, skeletal muscle and nerves. This is because conductive hydrogel matrices can improve electrical signaling between adjacent cells, thus facilitating the reconstruction of tissue motor and sensory signals (Kiyotake et al., 2022; Feng et al., 2023). Proper electrical conductivity facilitates neurite growth, and better axon regeneration length, axon growth rate, and axon remyelination formation were observed at conductivity of 1–10 s/m (Zhou et al., 2018). Cai et al. developed a unique and simple agarose/gelatin/polypyrrole (Aga/Gel/PPy, AGP3) hydrogel with electrical conductivity and modulus similar to that of spinal cord similarly. Gelation occurs through non-covalent interactions and physical cross-linking characteristics make AGP3 hydrogel injectable. In vitro cultures showed that AGP3 hydrogel exhibited excellent biocompatibility and promoted the differentiation of NSCs toward neurons while inhibiting the over proliferation of astrocytes. In vivo implantation of AGP3 hydrogel completely covered the tissue defects and reduced the injured luminal area (Yang B. et al., 2022; Yang S. et al., 2022).

Neurotrophic factors mainly include nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophic factor-3 (NT-3), and neurotrophic factor-4 (NT-4). In recent years, hydrogels combined with neurotrophic factors have been increasingly used in SCI repair, and studies have shown that neurotrophic factors can improve neuronal survival in the area of SCI, induce differentiation of stem cells to neuronal cells, promote axonal regeneration, have anti-apoptotic, and mediate cell differentiation (Park et al., 2022). For example, temperature-sensitive hydrogels loaded with NGF can enhance neuroprotection and promote neuroregeneration in rat spinal cord contusion models by inhibiting apoptosis in response to endoplasmic reticulum stress (Zhao et al., 2016). This composite tissue engineering scaffold can not only bridge the lesion site, but also promote the endogenous regeneration process through various signaling pathways (Madhusudanan et al., 2020). He et al. designed a novel anti-inflammatory peptide and BDNF-modified hyaluronic acid-methyl-fiber hydrogel to promote neural regeneration in rats with SCI, which had a small swelling rate and sustained release of BDNF. Compared with hydrogel treatment alone, the hydrogel combined with BDNF significantly improved neurological function and histomorphology, reduced inflammatory cytokine expression and cavitation, and increased neuronal survival in rats with SCI (He et al., 2019; Huang et al., 2021). NT-3 promoted the recovery of motor neurons after SCI and the sprouting of downstream native spinal cord neurons, protected the downstream spinal cord tract, and improved hindlimb motor function. Li et al. applied natural biomaterial-chitosan complex neurotrophic factors (NTs) to a series of novel active biomaterials, which could release neurotrophic factors in a controlled manner for a long period of 14 weeks, improve the local microenvironment of the injury area, activate endogenous neural stem cells and recruit them to migrate to the injury site, guiding them to differentiate into mature neurons. The newborn neurons could form functional neural loops with host cells eventually allowing functional recovery (Hao et al., 2017; Wang et al., 2023). In addition, encapsulation of NT-3 in PLGA nanoparticles combined with methylcellulose and hyaluronic acid injectable hydrogel for SCI can achieve slow release of NT-3 into the environment where dorsal root ganglion cells survive and achieve long-term sustained stimulation of nerve cells (Stanwick et al., 2012). It is worth noting that the release of NT-3 is closely related to the degradation time of hydrogels, and the development of hydrogels with appropriate degradation rate combined with neurotrophic factors can be more beneficial for SCI repair.

Cell transplantation alone has inevitable problems such as immune rejection and the inability of cell flow to remain in the damaged area for a long period of time, making it impossible to treat the injured area in a sustainable manner. By combining hydrogels and cells, it is possible to stabilize the cells at the site of injury while enhancing the effect of hydrogels and better simulating the ECM environment (Suzuki et al., 2023). Compared with adult cells, stem cells have more potential applications, and cells commonly used in central nervous system injury repair cell therapy include NSCs/neural group cells (NPCs), induced pluripotent stem cells (iPSCs), and MSCs (He et al., 2022; Silva et al., 2023).

NSCs/NPCs can differentiate into astrocytes, oligodendrocytes, and neurons to build new neural networks, increase neuronal connectivity and achieve functional recovery. Transplantation of stromal glia loaded with NSCs such as the SCI area in rats was able to detect new neuronal regeneration while promoting improved motor function in rats (Wang et al., 2020). Compared with the simple transplantation of NSCs, the injection of 3D printed biomimetic hydrogel combined with NSCs into the SCI rat model can make the NSCs have a higher survival rate, and can well mimic the natural structure in the spinal cord and promote axonal regeneration (Ashammakhi et al., 2019; Koffler et al., 2019). In further studies, it was found that developing hydrogel-loaded NSCs that mimic the natural microenvironment has better therapeutic effects, and ECM proteins may help regulate integrin expression and regulation of AKT/ERK-related signaling pathways in NSCs, promoting proliferation and differentiation of NSCs and formation of new neural networks in SCI-injured regions (Xu et al., 2021).

IPSCs are pluripotent stem cells that can differentiate into therapeutic target cells such as neurons and glial cells when stimulated by different cellular microenvironments. IPSCs are often used as an alternative to NSCs in SCI tissue engineering repair, mainly because iPSCs can differentiate into NSCs by induction and can be cultured in large numbers in vitro without ethical implications (Ramotowski et al., 2019). A study by Kong et al. generated iPSC from dermal fibroblasts and transplanted their derived NSCs into SCI rat lesions, which effectively suppressed inflammation and reduced fibrotic tissue (Kong et al., 2021). Overall, iPSCs are already capable of being derived into NSCs and are similar to NSCs in terms of therapeutic effects, but their potential for multidirectional differentiation, especially to tumorigenicity, limits their application in tissue engineering hydrogels (Nagoshi et al., 2020).

MSCs are a kind of dominant seed cells with multi-directional differentiation potential, immune regulation, and biological anti-inflammatory effects, which can differentiate into many cell types such as bone, cartilage, neurons, and myocardium. They also have the advantages of easy access and preservation. Currently, they are widely used in SCI tissue engineering repair. By combined with hydrogels, MSCs are able to release vesicles with anti-inflammatory and antioxidant properties that can enhance the survival and proliferation of endogenous NSCs, thus promoting recovery from SCI (Boido et al., 2019; Zhang et al., 2020). MSCs can differentiate into neurons, but unlike NSCs, they have immature characteristics, and hydrogel combined with MSCs is more valued for the anti-inflammatory and pro-angiogenic properties of MSCs.

In the organism, cells encounter a large number of soluble small molecules from the aqueous microenvironment. Small molecules are distributed in the microenvironment on which the cells depend and are produced by cellular autocrine or paracrine secretion, with a fraction of small molecules produced by endocrine secretion in the circulatory system. These small molecules either diffuse freely in aqueous media or are fixed in the ECM and are tightly controlled in space and time. The local concentration, spatial distribution and biological activity of small molecules play an important role in the regulation of different cellular behaviors. VEGF has been shown to promote the proliferation of neuronal precursors, and injection of hydrogels loaded with VEGF into SCI sites significantly promoted angiogenesis and axonal growth, contributing to the restorative effects of the hydrogels (des Rieux et al., 2014). Furthermore, the addition of Transforming Growth Factor-β1 to hydrogels stimulates fibroblast differentiation to myofibroblasts, which contributes to wound healing and fibrosis development (Park et al., 2022). Some small molecule drugs are also frequently loaded on hydrogels for SCI therapeutic studies, such as serine protease (Kwiecien et al., 2020), cabazitaxel (Li X. L. et al., 2019), minocycline and paclitaxel (Nazemi et al., 2020).

Compared to single growth factors, exosomes seem to be more popular. Exosomes are extracellular vesicles secreted by cells, which are rich in various proteins and miRNAs and are widely involved in regulating various cellular pathways (Liu et al., 2021). In addition to anti-inflammation and pro-angiogenesis, exosomes can also activate neural stem cells and induce differentiation into neurons. Li et al. (2021) found that hippocampal niche exosomes can promote the differentiation of neural stem cells into neurons, and further studies showed that miR-3,559-3P and miR-6,324 in hippocampal niche exosomes have a pro-NSC differentiation effect (Cheng et al., 2021). Not only as small molecules used in tissue engineering for SCI treatment, exosomes can also be used as carriers to load siRNA, miRNA and antagonists and deliver them to the site of injury through the “homing” function of exosomes, so that the hydrogel can play a better therapeutic role (Huang et al., 2020).

Compared with MSCs, MSCs exosomes (MSCs-Exo) not only have anti-inflammatory and homing properties, but also can pass through the blood-spinal cord barrier to exert a therapeutic effect. Hydrogels combined with MSCs-Exo also reduced inflammation, enhanced neural stem cell recruitment, and promoted axon regeneration (Li et al., 2020), while overcoming some disadvantages of hydrogels, such as the aggravating effect of conductive hydrogels on inflammation at the injured site (Fan et al., 2022). It has more advantages than MSCs in the treatment of SCI (Table 2).