A morphological study on the effects of collagen gel matrix on regeneration of severed rat sciatic nerve in silicone tubes

Sciatic nerve regeneration by using collagen type I hydrogel containing naringin

In the present study, collagen hydrogel containing naringin was fabricated, characterized and used as the scaffold for peripheral nerve damage treatment. The collagen was dissolved in acetic acid, naringin added to the collagen solution, and cross-linked with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide powder (EDC; 0.10 mM) to form the hydrogel. The microstructure, swelling behavior, biodegradation, and cyto/hemocompatibility of the fabricated hydrogels were assessed. Finally, the healing efficacy of the prepared collagen hydrogel loaded with naringin on the sciatic nerve crush injury was assessed in the animal model. The characterization results showed that the fabricated hydrogels have a porous structure containing interconnected pores with the average pore size of 90 µm. The degradation results demonstrated that about 70% of the primary weight of the naringin loaded hydrogel had been lost after 4 weeks of storage in PBS. The in vitro study showed that the proliferation of Schwann cells on the collagen/naringin hydrogel was higher than the control group (tissue culture plate) at both 48 and 72 h after cell seeding and even significantly higher than pure collagen 72 h after cell seeding (*p < 0.005, **p < 0.001). The animal study implied that the sciatic functional index reached to -22.13 ± 3.00 at the end of 60th days post-implantation which was statistically significant (p < 0.05) compared with the negative control (injury without the treatment) (-82.60 ± 1.06), and the pure collagen hydrogel (-59.80 ± 3.20) groups. The hot plate latency test, the compound muscle action potential, and wet weight-loss of the gastrocnemius muscle evaluation confirmed the positive effect of the prepared hydrogels on the healing process of the induced nerve injury. In the final, the histopathologic examinations depicted that the collagen/naringin hydrogel group reduced all the histological changes induced from the nerve injury and showed more resemblance to the normal sciatic nerve, with well-arranged fibers and intact myelin sheath. The overall results implied that the prepared collagen/naringin hydrogel can be utilized as a sophisticated alternative to healing peripheral nerve damages.

Regeneration of sciatic nerve crush injury by a hydroxyapatite nanoparticle-containing collagen type I hydrogel

The current study aimed to enhance the efficacy of peripheral nerve regeneration using a hydroxyapatite nanoparticle-containing collagen type I hydrogel. A solution of type I collagen, extracted from the rat tails, was incorporated with hydroxyapatite nanoparticles (with the average diameter of ~212 nm) and crosslinked with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) to prepare the hydrogel. The Schwann cell cultivation on the prepared hydrogel demonstrated a significantly higher cell proliferation than the tissue culture plate, as positive control, after 48 h (n = 3, P < 0.005) and 72 h (n = 3, P < 0.01). For in vivo evaluation, the prepared hydrogel was administrated on the sciatic nerve crush injury in Wistar rats. Four groups were studied: negative control (with injury but without interventions), positive control (without injury), collagen hydrogel and hydroxyapatite nanoparticle-containing collagen hydrogel. After 12 weeks, the administration of hydroxyapatite nanoparticle-containing collagen significantly (n = 4, P < 0.005) enhanced the functional behavior of the rats compared with the collagen hydrogel and negative control groups as evidenced by the sciatic functional index, hot plate latency and compound muscle action potential amplitude measurements. The overall results demonstrated the applicability of the produced hydrogel for the regeneration of peripheral nerve injuries.

Tissue-Engineered Peripheral Nerve Interfaces

Research on neural interfaces has historically concentrated on development of systems for the brain; however, there is increasing interest in peripheral nerve interfaces (PNIs) that could provide benefit when peripheral nerve function is compromised, such as for amputees. Efforts focus on designing scalable and high-performance sensory and motor peripheral nervous system interfaces. Current PNIs face several design challenges such as undersampling of signals from the thousands of axons, nerve-fiber selectivity, and device-tissue integration. To improve PNIs, several researchers have turned to tissue engineering. Peripheral nerve tissue engineering has focused on designing regeneration scaffolds that mimic normal nerve extracellular matrix composition, provide advanced microarchitecture to stimulate cell migration, and have mechanical properties like the native nerve. By combining PNIs with tissue engineering, the goal is to promote natural axon regeneration into the devices to facilitate close contact with electrodes; in contrast, traditional PNIs rely on insertion or placement of electrodes into or around existing nerves, or do not utilize materials to actively facilitate axon regeneration. This review presents the state-of-the-art of PNIs and nerve tissue engineering, highlights recent approaches to combine neural-interface technology and tissue engineering, and addresses the remaining challenges with foreign-body response.

Porous and Nonporous Nerve Conduits: The Effects of a Hydrogel Luminal Filler With and Without a Neurite-Promoting Moiety

Nerve conduits prefilled with hydrogels are frequently explored in an attempt to promote nerve regeneration. This study examines the interplay in vivo between the porosity of the conduit wall and the level of bioactivity of the hydrogel used to fill the conduit. Nerve regeneration in porous (P) or nonporous (NP) conduits that were filled with either collagen only or collagen enhanced with a covalently attached neurite-promoting peptide mimic of the glycan human natural killer cell antigen-1 (m-HNK) were compared in a 5 mm critical size defect in the mouse femoral nerve repair model. Although collagen is a cell-friendly matrix that does not differentiate between neural and nonneural cells, the m-HNK-enhanced collagen specifically promotes axon growth and appropriate motor neuron targeting. In this study, animals treated with NP conduits filled with collagen grafted with m-HNK (CollagenHNK) had the best overall functional recovery, based on a range of histomorphometric observations and parameters of functional recovery. Our data indicate that under some conditions, the use of generally cell friendly fillers such as collagen may limit nerve regeneration. This finding is significant, considering the frequent use of collagen-based hydrogels as fillers of nerve conduits.

Biocompatibility and characterization of a peptide amphiphile hydrogel for applications in peripheral nerve regeneration

Peripheral nerve injury is a debilitating condition for which new bioengineering solutions are needed. Autografting, the gold standard in treatment, involves sacrifice of a healthy nerve and results in loss of sensation or function at the donor site. One alternative solution to autografting is to use a nerve guide conduit designed to physically guide the nerve as it regenerates across the injury gap. Such conduits are effective for short gap injuries, but fail to surpass autografting in long gap injuries. One strategy to enhance regeneration inside conduits in long gap injuries is to fill the guide conduits with a hydrogel to mimic the native extracellular matrix found in peripheral nerves. In this work, a peptide amphiphile (PA)-based hydrogel was optimized for peripheral nerve repair. Hydrogels consisting of the PA C₁₆GSH were compared with a commercially available collagen gel. Schwann cells, a cell type important in the peripheral nerve regenerative cascade, were able to spread, proliferate, and migrate better on C₁₆GSH gels in vitro when compared with cells seeded on collagen gels. Moreover, C₁₆GSH gels were implanted subcutaneously in a murine model and were found to be biocompatible, degrade over time, and support angiogenesis without causing inflammation or a foreign body immune response. Taken together, these results help optimize and instruct the development of a new synthetic hydrogel as a luminal filler for conduit-mediated peripheral nerve repair.

The effect of glycomimetic functionalized collagen on peripheral nerve repair

Increasing evidence suggests that the improper synaptic reconnection of regenerating axons is a significant cause of incomplete functional recovery following peripheral nerve injury. In this study, we evaluate the use of collagen hydrogels functionalized with two peptide glycomimetics of naturally occurring carbohydrates-polysialic acid (PSA) and human natural killer cell epitope epitope (HNK-1)-that have been independently shown to encourage nerve regeneration and axonal targeting. Our novel biomaterial was used to bridge a critical gap size (5 mm) in a mouse femoral nerve injury model. Functional recovery was assessed using gait and hind limb extension, and was significantly better in all glycomimetic peptide-coupled collagen conditions versus non-functional scrambled peptide-coupled collagen, native collagen, and saline controls. Analysis of cross-sections of the regenerated nerve demonstrated that hydrogels coupled with the PSA glycomimetic, but not HNK, had significant increases in the number of myelinated axons over controls. Conversely, hydrogels coupled with HNK, but not PSA, showed improvement in myelination. Additionally, significantly more correctly projecting motoneurons were observed in groups containing coupled HNK-1 mimicking peptide, but not PSA mimicking peptide. Given the distinct morphological outcomes between the two glycomimetics, our study indicates that the enhancement of recovery following peripheral nerve injury induced by PSA- and HNK-functionalized collagen hydrogels likely occurs through distinct mechanisms.

Differences between the effect of anisotropic and isotropic laminin and nerve growth factor presenting scaffolds on nerve regeneration across long peripheral nerve gaps

Anisotropic scaffolds of agarose hydrogels containing gradients of laminin-1 (LN-1) and nerve growth factor (NGF) molecules were used to promote sciatic nerve regeneration across a challenging 20mm nerve gap in rats. Step and continuous gradient anisotropic scaffolds were fabricated and characterized, and regeneration was compared to that in isotropic scaffolds with uniform concentrations of LN-1 and NGF and sciatic nerve grafts harvested from syngenic rats. Polysulfone tubular guidance channels were used to present the agarose-based scaffolds to the nerve stumps. Four months after implantation, regenerating axons were observed in animals implanted with anisotropic scaffolds with gradients of both LN-1 and NGF molecules and nerve grafts, but not in animals with isotropic scaffold implants. Also, the scaffolds with gradients of either LN-1 or NGF, with the other component being uniformly distributed in the scaffold, did not elicit axonal regeneration. The total number of myelinated axons was similar for the anisotropic scaffold and the nerve graft conditions, with the anisotropic scaffolds having a higher density of axons than the nerve grafts. Axonal diameter distribution was similar for the anisotropic scaffolds and the nerve grafts. The nerve grafts and anisotropic scaffolds resulted in better functional outcome compared to isotropic scaffolds as measured by the relative gastrocnemius muscle weight (RGMW). Additionally the state of neuromuscular junctions as assessed by pre- and post-synaptic staining revealed that both the anisotropic scaffolds performed as well as nerve grafts.

Tissue engineered nerve constructs: where do we stand?

Driven by enormous clinical need, interest in peripheral nerve regeneration has become a prime focus of research and area of growth within the field of tissue engineering. While using autologous donor nerves for bridging peripheral defects remains today's gold standard, it remains associated with high donor site morbidity and lack of full recovery. This dictates research towards the development of biomimetic constructs as alternatives. Based on current concepts, this review summarizes various approaches including different extracellular matrices, scaffolds, and growth factors that have been shown to promote migration and proliferation of Schwann cells. Since neither of these concepts in isolation is enough, although each is gaining increased interest to promote nerve regeneration, various combinations will need to be identified to strike a harmonious balance. Additional factors that must be incorporated into tissue engineered nerve constructs are also unknown and warrant further research efforts. It seems that future directions may allow us to determine the "missing link".