Development and preclinical evaluation of bioactive nerve conduits for peripheral nerve regeneration: A comparative study

In severe peripheral nerve injuries, nerve conduits (NCs) are good alternatives to autografts/allografts; however, the results the available devices guarantee for are still not fully satisfactory. Herein, differently bioactivated NCs based on the new polymer oxidized polyvinyl alcohol (OxPVA) are compared in a rat model of sciatic nerve neurotmesis (gap: 5 mm; end point: 6 weeks). Thirty Sprague Dawley rats are randomized to 6 groups: Reverse Autograft (RA); Reaxon®; OxPVA; OxPVA + EAK (self-assembling peptide, mechanical incorporation); OxPVA + EAK-YIGSR (mechanical incorporation); OxPVA + Nerve Growth Factor (NGF) (adsorption). Preliminarily, all OxPVA-based devices are comparable with Reaxon® in Sciatic Functional Index score and gait analysis; moreover, all conduits sustain nerve regeneration (S100, β-tubulin) without showing substantial inflammation (CD3, F4/80) evidences. Following morphometric analyses, OxPVA confirms its potential in PNI repair (comparable with Reaxon®) whereas OxPVA + EAK-YIGSR stands out for its myelinated axons total number and density, revealing promising in injury recovery and for future application in clinical practice.

Stem Cells and Tissue Engineering-Based Therapeutic Interventions: Promising Strategies to Improve Peripheral Nerve Regeneration

Unlike the central nervous system, the peripheral one has the ability to regenerate itself after injury; however, this natural regeneration process is not always successful. In fact, even with some treatments, the prognosis is poor, and patients consequently suffer with the functional loss caused by injured nerves, generating several impacts on their quality of life. In the present review we aimed to address two strategies that may considerably potentiate peripheral nerve regeneration: stem cells and tissue engineering. In vitro studies have shown that pluripotent cells associated with neural scaffolds elaborated by tissue engineering can increase functional recovery, revascularization, remyelination, neurotrophin expression and reduce muscle atrophy. Although these results are very promising, it is important to note that there are some barriers to be circumvented: the host's immune response, the oncogenic properties attributed to stem cells and the duration of the pro-regenerative effects. After all, more studies are still needed to overcome the limitations of these treatments; those that address techniques for manipulating the lesion microenvironment combining different therapies seem to be the most promising and proactive ones.

Peripheral Nerve Regeneration: Opportunities and Challenges

Peripheral nerve injury has detrimental effects on the quality of life for patients and is a worldwide issue with high rates of morbidity. Research on the molecular mechanisms of nerve injury, microsurgical techniques, and advances in stem cell research have led to substantial progress in the field of translational neurophysiology. Current research on peripheral nerve regeneration aims to accelerate peripheral nerve development through pluripotent stem cells and potential use of smart exosomes, pharmacological agents, and bioengineering of nerve conduits. In this article critically reviewed and summarized various methods used for peripheral nerve regeneration and highlight the opportunities and challenges that come along with these strategies.

A Comparative Study of Porcine Small Intestine Submucosa and Cross-Linked Bovine Type I Collagen as a Nerve Conduit

Purpose

We compared 2 commercially available nerve conduits—the Axoguard Nerve Connector, made of porcine small intestine submucosa (SIS), and the NeuraGen Nerve Guide, made of cross-linked bovine type I collagen (Col)—using a rodent model at 4 weeks, specifically focusing on subchronic host responses to the implants.

Methods

A unilateral 5-mm sciatic nerve defect was created in 18 male Lewis rats and was repaired with SIS or Col conduits. After 4 weeks, histological evaluations of morphology, collagen content, macrophage polarization, vascularization, axonal regeneration, and myelination were conducted. To achieve a blinded examination, an independent qualified pathologist evaluated the images that were stained with hematoxylin-eosin, α-smooth muscle actin, and Masson trichrome stains.

Results

The results showed a dominant macrophage type 2 (M2) response in the SIS group and a dominant macrophage type 1 (M1) response in the Col group. The SIS group showed deeper implant vascularization and fibroblast ingrowth than the Col group. Collagen deposition was higher within the lumen of the Col group than the SIS group. All Col conduits were surrounded by a colocalized staining of Masson trichrome and α-smooth muscle actin, forming a capsule-like structure.

Conclusion

Distinctive histological features were identified for each conduit at the cellular level. The SIS conduits had a significantly higher number of host macrophages expressing M2 surface marker CD163, and the Col conduits showed a predominance of host macrophages expressing the M1 surface marker CD80. Data suggest that promoting the M2 response for tissue engineering and regenerative medicine is associated with a remodeling response. In addition, an independent analysis revealed an encapsulation-like appearance around all Col conduits, which is similar to what is seen in breast implant capsules.

Clinical relevance

The biomaterial choice for conduit material can play an important role in the host tissue response, with the potential to impact adverse events and patient outcomes.

Comparison of nerve conduits and nerve graft in digital nerve regeneration: A systematic review and meta-analysis

The goal of this systematic review and meta-analysis was to compare nerve conduits and nerve graft for peripheral nerve regeneration. This type of lesion frequently causes disability due to pain, paresthesia and motor deficit. On the PICO process, "P" corresponded to patients with peripheral digital nerve lesions of any age, gender or ethnicity, "I" to interventions with nerve conduits or nerve graft, "C" to the control group with no treatment, placebo or receiving other treatment, and "O" to outcome assessment of nerve regeneration. Initial search found in 3859 studies, including 2001 duplicates. The remaining 1858 studies were selected by title and/or abstract; 1798 articles were excluded, leaving 60 articles for full-text review. Thirty-nine of these 60 reports were excluded as not meeting our inclusion criteria, and 21 articles were ultimately included in the systematic review. For patients older than 40 years, there was a greater mean improvement on S2PD and M2PD tests with grafting, which seemed to be the better surgical technique, positively impacting prognosis. On the M2PD test, there was significantly greater improvement in 11-17.99 mm defects with grafting (P < 0.001); this finding should guide surgical strategy in peripheral nerve regeneration, to ensure better outcomes.

High-resolution combinatorial 3D printing of gelatin-based biomimetic triple-layered conduits for nerve tissue engineering

Peripheral nerve injury is a common clinical problem often requiring surgical nerve reconstruction. To this end, tissue-engineered conduit has been proved to be crucial for nerve reconstruction. Despite its progress in recent years, the design and fabrication of translational biomimetic nerve conduits is highly challenging. Therefore, this study aims to design and fabricate mechanically-tunable nerve conduits with biomimetic structural features of the human nerve suitable for nerve tissue engineering. Herein, we employed combinatorial approach comprising of electrohydrodynamic (EHD) jet printing, dip-coating, and electrospinning techniques for fabricating triple-layered conduits. The intricate structural details were achieved via high-resolution EHD jet printed PCL filaments with tunable directionality, as the innermost layer; followed by dip coating of gelatin hydrogels to form the middle layer, and lastly, wrapped with electrospun PCL nanofibers as an outer layer of the conduits. The mechanical properties, porosity, and biocompatibility of the fabricated conduits were studied and compared with control. The results of this study confirmed that the combinatorial approach has greater potential to fabricate mechanically-tunable triple-layered conduits with favorable neuronal precursor and vascular cell compatibility.

Application of peripheral nerve conduits in clinical practice: A literature review

Understanding the pathomechanisms behind peripheral nerve damage and learning the course of regeneration seem to be crucial for selecting the appropriate methods of treatment. Autografts are currently the gold standard procedure in nerve reconstruction. However, due to the frequency of complications resulting from autografting and a desire to create a better environment for the regeneration of the damaged nerve, artificial conduits have become an approved alternative treatment method. The aim of this mini-review is to present the nerve scaffolds that have been applied in clinical practice to date, and the potential directions of developments in nerve conduit bioengineering. Articles regarding construction and characterization of nerve conduits were used as the theoretical background. All papers, available in PubMed database since 2000, presenting results of application of artificial nerve conduits in clinical trials were included into this mini-review. Fourteen studies including ≤10 patients and 10 trials conducted on >10 patients were analyzed as well as 24 papers focused on artificial nerve conduits per se. Taking into consideration the experiences of the authors investigating nerve conduits in clinical trials, it is essential to point out the emergence of bioresorbable scaffolds, which in the future may significantly change the treatment of peripheral nerve injuries. Also worth mentioning among the advanced conduits are hybrid conduits, which combine several modifications of a synthetic material to provide the optimal regeneration of a damaged nerve.

Three-dimensional imaging and analysis of entire peripheral nerves after repair and reconstruction

BACKGROUND

We wanted to achieve a three-dimensional (3D), non-destructive imaging and automatic post-analysis and evaluation of reconstructed peripheral nerves without involving cutting and staining processes.

NEW METHOD

We used a laboratory-based micro computed tomography system for imaging, as well as a custom analysis protocol. The sample preparation was also adapted in order to achieve 3D images with true micrometer resolution and suitable contrast.

RESULTS

Analysis of the acquired tomograms enabled the quantitative assessment of 3D tissue structures, i.e., surface morphology, nerve fascicles, nerve tissue volume, geometry, and vascular regrowth. The resulting data showed significant differences between operated animals and non-operated controls.

COMPARISON WITH EXISTING METHODS

Our approach avoids the sampling error associated with conventional 2D visualization approaches and holds promise for automation of the analysis of large series of datasets.

CONCLUSIONS

We have presented a potential way for 3D imaging and analysis of entire regenerated nerves non-destructively, paving the way for high-throughput analysis of therapeutic conditions of treating adult nerve injuries.

Bioabsorbable nerve conduits coated with induced pluripotent stem cell-derived neurospheres enhance axonal regeneration in sciatic nerve defects in aged mice

Aging influences peripheral nerve regeneration. Nevertheless, most basic research of bioabsorbable nerve conduits including commercial products have been performed in very young animals. Results from these studies may not provide information about axonal regeneration in aged tissue, because young nerve tissue holds sufficient endogenous potential for axonal regeneration. The clinical target age for nerve conduit application is most likely going to increase with a rapidly growing elderly population. In the present study, we examined axonal regeneration after sciatic nerve defects in aged and young mice. 5-mm sciatic nerve defects in young (6 weeks old) and aged (92 weeks old) mice were reconstructed using nerve conduits (composed of a poly lactide and caprolactone) or autografts. In addition, in aged mice, sciatic nerve defects were reconstructed using nerve conduits coated with mouse induced pluripotent stem cell (iPSc)-derived neurospheres. Using electrophysiological and histological techniques, we demonstrated axonal regeneration was significantly less effective in aged than in young mice both for nerve conduits and for nerve autografts. However, despite the low regenerative capacity of the peripheral nerve in aged mice, axonal regeneration significantly increased when nerve conduits coated with iPSc-derived neurospheres, rather than nerve conduits alone, were used. The present study shows that aging negatively affects peripheral nerve regeneration based on nerve conduits in mice. However, axonal regeneration using nerve conduits was improved when supportive iPSc-derived neurospheres were added in the aged mice. We propose that tissue-engineered bioabsorbable nerve conduits in combination with iPSc-derived neurospheres hold therapeutic potential both in young and elderly patients. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 1752-1758, 2018.

Evaluation of biodegradable polymer conduits--poly(L-lactic acid)--for guiding sciatic nerve regeneration in mice

Polymeric biomaterials are often used for stimulating nerve regeneration. Among different conduits, poly(lactide acid) - PLA polymer is considered to be a good substrate due to its biocompatibility and resorbable characteristics. This polymer is an aliphatic polyester which has been mostly used in biomedical application. It is an organic compound with low allergenic potential, low toxicity, high biocompatibility and predictable kinetics of degradation. In this study we fabricated and evaluated a PLA microporous hollow fiber as a conduit for its ability to bridge a nerve gap in a mouse sciatic nerve injury model. The PLA conduit was prepared from a polymer solution, throughout extrusion technique. The left sciatic nerve of C57BL/6 mouse was transected and the nerve stumps were placed into a resorbable PLA (PLA group) or a PCL conduit (PCL group), n=5 each group. We have also used another group in which the nerves were repaired by autograft (autograft group, n=5). Motor function was analyzed according to sciatic functional index (SFI). After 56days, the regenerated nerves were processed for light and electron microscopy and morphometric analyses were performed. A quantitative analysis of regenerated nerves showed significant increase in the number of myelinated fibers and blood vessels in animals that received PLA conduit. The PLA group exhibited better overall tissue organization compared to other groups. Presenting well-organized bundles, many regenerating clusters composed of preserved nerve fibers surrounded by layers of compacted perineurium-like cells. Also the SFI revealed a significant improvement in functional recovery. This work suggests that PLA conduits are suitable substrate for cell survival and it provides an effective strategy to be used to support axonal growth becoming a potential alternative to autograft.

Overcoming short gaps in peripheral nerve repair: conduits and human acellular nerve allograft

Nerve conduits and acellular nerve allograft offer efficient and convenient tools for overcoming unexpected gaps during nerve repair. Both techniques offer guidance for migrating Schwann cells and axonal regeneration though utilizing very different scaffolds. The substantially greater amount of animal and clinical data published on nerve conduits is marked by wide discrepancies in results that may be partly explained by a still poorly defined critical repair gap and diameter size. The available information on acellular allografts appears more consistently positive though this tool is also hampered by a longer but also limited critical length. This article reviews the current relative literature and examines pertinent parameters for application of both acellular allograft and nerve conduits in overcoming short nerve gaps.

Tissue engineered constructs for peripheral nerve surgery

BACKGROUND

Tissue engineering has been defined as "an interdisciplinary field that applies the principles of engineering and life sciences toward the development of biological substitutes that restore, maintain, or improve tissue function or a whole organ". Traumatic peripheral nerve injury resulting in significant tissue loss at the zone of injury necessitates the need for a bridge or scaffold for regenerating axons from the proximal stump to reach the distal stump.

METHODS

A review of the literature was used to provide information on the components necessary for the development of a tissue engineered peripheral nerve substitute. Then, a comprehensive review of the literature is presented composed of the studies devoted to this goal.

RESULTS

Extensive research has been directed toward the development of a tissue engineered peripheral nerve substitute to act as a bridge for regenerating axons from the proximal nerve stump seeking the distal nerve. Ideally this nerve substitute would consist of a scaffold component that mimics the extracellular matrix of the peripheral nerve and a cellular component that serves to stimulate and support regenerating peripheral nerve axons.

CONCLUSIONS

The field of tissue engineering should consider its challenge to not only meet the autograft "gold standard" but also to understand what drives and inhibits nerve regeneration in order to surpass the results of an autograft.