In the fields of neurology and regenerative medicine, peripheral nerve injury (PNI) poses a significant obstacle [1]. Neurological deficits often occur as a consequence of various factors, such as trauma, surgery, or underlying medical conditions [2]. The integrity of the peripheral nervous system (PNS) is crucial for transmitting sensory information and motor commands throughout the body [3]. An individual's quality of life can be affected by any disturbance to the integrity of the PNS.

Assessing and repairing peripheral nerves, which consist of axons, connective tissue, and supporting cells, present distinctive difficulties due to their intricate composition. The severity of peripheral nerve injuries varies, with complete neurotmesis being particularly severe and often resulting in poor neurological recovery [4]. The successful recovery of neurotmesis depends largely on the effective regeneration of the damaged nerve, with suturing techniques serving as a crucial foundation for nerve regrowth [5]. To recover nerve continuity, epineurium or perineurium neurorrhaphy is the choice for the treatment of neurotmesis [6]. This method has certain shortcomings, such as misconnection of nerve functional bundles, escape of regenerated nerve fibers, and connective tissue hyperplasia, which are not conducive to nerve regeneration and nerve function recovery. To address this issue, we devised the technique of conduit small gap tubulization [7]. Instead of anastomosing nerve stumps, this technique involves suturing nerve stumps to the conduit while maintaining a slight distance between them, thereby enhancing the effectiveness of nerve repair.

The development of conduits appropriate for nerve repair holds immense importance in the field of neural tissue engineering [4,8]. There are many options for materials used to prepare nerve conduits, with chitosan and chitin gaining attention. Chitosan is extensively utilized in the biomedical domain due to its remarkable characteristics, including biocompatibility, biodegradability, and antibacterial properties [9]. Moreover, chitosan is obtained from chitin, which can be easily acquired from crustaceans [10]. Therefore, chitosan is an excellent material for preparing nerve conduits. Studies have shown that chitosan can interact with the nerve regeneration microenvironment and improve axon regeneration [11,12]. The chitosan-based conduits developed by our research team demonstrate outstanding biocompatibility and mechanical characteristics [13]. Nevertheless, the nerves frequently lack an adequate provision of bioactive compounds following surgical suturing, thereby restricting the effectiveness of the healing process. Hence, to enhance the efficacy of nerve regeneration, it is essential to furnish damaged nerves with a sufficient amount of biologically active compounds.

Many factors influence the complex process of repairing peripheral nerves, necessitating a favorable microenvironment for regeneration [14]. Overall, the significance of growth factors stands out. Regulating physiological signals, affecting signaling pathways, and influencing tissue regeneration potential govern biological processes such as cell chemotaxis, proliferation, and differentiation [15]. Impairment of tissue healing and regeneration occurs as a consequence of peripheral nerve injury, causing disruption of blood flow to the affected area [16]. VEGF, alternatively referred to as vascular endothelial growth factor, is a powerful inducer of angiogenesis. In the injured region, by promoting the growth of endothelial cells, VEGF aids in new blood vessel formation. This increases the oxygen and nutrient supplies [17]. Facilitating the repair of injuries to peripheral nerves is made possible by brain-derived neurotrophic factor (BDNF). BDNF is a member of the neurotrophin family. BDNF plays a vital role in this process by supporting the survival, development, and restoration of injured nerve fibers [18]. More importantly, VEGF and BDNF can promote nerve regeneration through synergistic effects [19,20]. The synergistic impact of VEGF-induced blood vessel formation and BDNF-supplied NGF can foster a milieu that boosts the viability of neuronal cells, facilitates the extension of axons, and ultimately restores effective neural connections [21,22]. Hence, the concurrent utilization of VEGF and BDGF might be a promising approach for enhancing the severity of nerve injuries.

During peripheral nerve injury treatment, to ensure the long-lasting effect of growth factors on the injury site, it is imperative to discover a means of uninterrupted delivery of growth factors. The microgels are colloidal particles with a diameter ranging from 1 μm to 1000 μm. They are composed of a three-dimensional polymer network that can absorb and retain water [23]. Microgels are widely used in biomedical fields such as drug delivery, tissue engineering, and diagnostic imaging [24]. Growth factors can be loaded into microgels and released continuously, maintaining drug therapeutic levels, improving bioavailability, and reducing dosing frequency and potential side effects [25]. Hence, the distinct characteristics of microgels render them a compelling choice for the delivery of growth factors. There are many options for materials from which microgels are made, such as natural or synthetic polymers. Methacrylic alginate (AlgMA), a versatile biomaterial, is extensively utilized in various fields, including drug delivery, tissue engineering, bioprinting, and wound healing [[26], [27], [28], [29]]. AlgMA is an altered form of alginate that possesses increased functionality and control through the incorporation of methacrylate groups into its molecular composition [30]. This enables it to undergo cross-linking via photopolymerization upon exposure to ultraviolet radiation. AlgMA microgels preserve the outstanding biological characteristics of AlgMA and have been utilized in the advancement of drug transport [31]. Nevertheless, the impact of employing AlgMA microgels for the delivery of growth factors to achieve complete neurotmesis remains uncertain.

In this study, AlgMA microgels were formulated to enable the prolonged release of VEGF and BDNF, and subsequently, their biological impacts were examined via in vitro investigations. Afterwards, we employed chitosan to create conduits. These conduits were subsequently combined with AlgMA microgels that contained two growth factors. Afterwards, we conducted an experiment on rats in which we successfully restored the sciatic nerve after complete neurotmesis. The effect of nerve repair was evaluated by examining neuromuscular histology, target muscle wet weight, the nerve function index, nerve electrophysiology, and other indicators at the 12-week postoperative period.