Peripheral nerve regeneration by microbraided poly(L-lactide-co-glycolide) biodegradable polymer fibers

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/poly(ε-caprolactone) blends for tissue engineering applications in the form of hollow fibers

In this work, hollow fibers to be used as guides for tissue engineering applications were produced by dry-jet-wet spinning of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/poly(epsilon-caprolactone) (PHBHV/PCL) solutions in chloroform with various weight ratios between the components (PHBHV/PCL 100/0; 80/20; 60/40; 50/50; 40/60; 20/80; 0/100 w/w). Fibers obtained from PHBHV/PCL blends had a low degree of surface and bulk porosity, depending on composition. Physicochemical characterization involving scanning electron microscopy and differential scanning calorimetry (DSC) showed that PHBHV/PCL blends are compatible. Interactions between blend components were studied by Fourier transform infrared total reflectance spectroscopy, DSC analysis, and polarized optical microscopy analysis. Homogeneity of blend composition was assessed by IR-chemical imaging analysis. PHBHV/PCL samples were found to be weakly hydrophilic and their biocompatibility was proved by in vitro tests using mouse fibroblasts. Mechanical properties of PHBHV/PCL blends were investigated by stress-strain tests, showing an increasing ductility of blend samples with increasing PCL amount. Hollow fibers supported fibroblasts attachment and proliferation depending on composition and porosity degree.

Fabrication of patterned multi-walled poly-l-lactic acid conduits for nerve regeneration

Topographical cues in the micron and nanoscale regime represent a powerful and effective method for controlling neuron and glial cell behavior. Previous studies have shown that contact guidance can facilitate axon pathfinding, accelerate neurite growth and induce glial cell alignment. In this paper, we exploit the concept of haptotaxis via implementation into three-dimensional neural based scaffolds. Polymeric poly-l-lactic acid (PLLA) conduits possessing multiple intralumenal walls and precise topography along the longitudinal axis were fabricated using solvent casting, physical imprinting and a rolling-fusing method. Measurements made on scanning electron micrographs show the conduits demonstrate a transparency factor (void to polymer ratio) of up to 87.9% and an increase in surface area of four to eight times over comparably sized hollow conduits. Intralumenal wall thickness was approximately 20 microm and physical parameters such as the number of lumens, conduit length and diameter were controllable. These results imply that the structures are conducive for cellular infiltration and proliferation. Although PLLA was used, the manufacturing techniques are highly flexible and are compatible with multiple polymer-solvent systems. Thus, the proposed conduits can be custom tailored to resorb in parallel with the healing process. Applications for these scaffolds include autograft substitutes for peripheral nerve transection or potential use in spinal cord related injuries.

Guidance of glial cell migration and axonal growth on electrospun nanofibers of poly-ε-caprolactone and a collagen/poly-ε-caprolactone blend

Our long-term goal is to develop an artificial implant as a conduit for axonal regeneration after peripheral nerve injury. In this study, biodegradable, aligned poly-epsilon-caprolactone (PCL) and collagen/PCL (C/PCL) nanofibers designed as guidance structures were produced by electrospinning and tested in cell culture assays. We compared fibers of 100% PCL with fibers consisting of a 25:75% C/PCL blend. To test their biocompatibility, assays of cell adhesion, survival, migration, effects on cell morphology, axonal growth and axonal guidance were performed. Both types of eletrospun fibers supported oriented neurite outgrowth and glial migration from dorsal root ganglia (DRG) explants. Schwann cell migration, neurite orientation, and process formation of Schwann cells, fibroblasts and olfactory ensheathing cells were improved on C/PCL fibers, when compared to pure PCL fibers. While the velocity of neurite elongation from DRG explants was higher on PCL fibers, analysis of isolated sensory neurons showed significantly better axonal guidance by the C/PCL material. The data demonstrate that electrospun fibers composed of a collagen and PCL blend represent a suitable substrate for supporting cell proliferation, process outgrowth and migration and as such would be a good material for artificial nerve implants.

Thyroid Hormone in Biodegradable Nerve Guides Stimulates Sciatic Nerve Regeneration: A Potential Therapeutic Approach for Human Peripheral Nerve Injuries

It has been already demonstrated that thyroid hormone (T3) is one of the most important stimulating factors in peripheral nerve regeneration. We have recently shown that local administration of T3 in silicon tubes at the level of the transected rat sciatic nerve enhanced axonal regeneration and improved functional recovery. Silicon, however, cannot be used in humans because it causes a chronic inflammatory reaction. Therefore, in order to provide future clinical applications of thyroid hormone in human peripheral nerve lesions, we carried out comparative studies on the regeneration of transected rat sciatic nerve bridged either by biodegradable P(DLLA-(-CL) or by silicon nerve guides, both guides filled with either T3 or phosphate buffer. Our macroscopic observation revealed that 85% of the biodegradable guides allowed the expected regeneration of the transected sciatic nerve. The morphological, morphometric and electrophysiological analysis showed that T3 in biodegradable guides induces a significant increase in the number of myelinated regenerated axons (6862 +/- 1831 in control vs. 11799 +/- 1163 in T3-treated). Also, T3 skewed the diameter of myelinated axons toward larger values than in controls. Moreover, T3 increases the compound muscle action potential amplitude of the flexor and extensor muscles of the treated rats. This T3 stimulation in biodegradable guides was equally well to that obtained by using silicone guides. In conclusion, the administration of T3 in biodegradable guides significantly improves sciatic nerve regeneration, confirming the feasibility of our technique to provide a serious step towards future clinical application of T3 in human peripheral nerve injuries.

Fabrication and characterization of hydrophilized porous PLGA nerve guide conduits by a modified immersion precipitation method

Nerve guide conduits (NGCs) with selective permeability and hydrophilicity were fabricated using poly(lactic-co-glycolic acid) (PLGA) and Pluronic F127 by a modified immersion precipitation method developed by our laboratory. The hydrophilized porous PLGA tubes as NGCs were fabricated by immersing a water-saturated rod-shape alginate hydrogel into PLGA/Pluronic F127 mixture solution (in tetraglycol). The PLGA/Pluronic F127 mixture was precipitated outside the alginate hydrogel rod by the diffusion of water from the hydrogel rod into PLGA/Pluronic F127 mixture solution. The inner diameter and wall thickness of tubes could be easily controlled by adjusting the diameter of alginate hydrogel rod and immersion time, respectively. It was observed that the tube wall has an asymmetric column-shape porous structure. The inner surface of the tube had nano-size pores ( approximately 50 nm), which can effectively prevent from fibrous tissue infiltration but permeate nutrients and retain neurotrophic factors, while the outer surface had micro-size pores ( approximately 50 microm), which can allow vascular ingrowth for effective supply of nutrients and oxygen into the tube. From the investigations of mechanical property, water absorbabiliy, and model nutrient permeability of the tubes, the hydrophilized PLGA/F127 (3 wt %) tube seems to be a good candidate as a NGC for the effective permeation of nutrients as well as the good mechanical strength to maintain a stable support structure for the nerve regeneration.

Novel tubular composite matrix for bone repair

Tissue engineering develops organ replacements to overcome the limitations associated with autografts and allografts. The work presented here details the development of biodegradable, porous, three-dimensional polymer-ceramic-sintered microsphere matrices to support bone regeneration. Poly(lactide-co-glycolide)/hydroxyapatite microspheres were formed using solvent evaporation technique. Individual microspheres were placed in a cylindrical mold and sintered at various temperatures. Scaffolds were characterized using scanning electron microscopy, mercury porosimetry, and mechanical testing in compression. After varying the temperature of sintering, a single temperature was selected and the time of sintering was varied. Mechanical testing indicated that as the sintering temperature or time was increased, the elastic modulus, compressive strength, maximum compressive load, and energy at failure significantly increased. Furthermore, increasing the sintering temperature or time resulted in a decreased porosity and the spherical morphology of the microspheres was lost as the microspheres blended together. To more closely mimic the bone marrow cavity observed in native bone tissue, tubular composite-sintered microsphere matrices were formed. These scaffolds demonstrated no statistically significant difference in compressive mechanical properties when compared with cylindrical composite-sintered microsphere matrices of the same dimension. One potential application for these scaffolds is bone regeneration.

Biomaterials and Strategies for Nerve Regeneration

Nerve regeneration is a complex biological phenomenon. Once the nervous system is impaired, its recovery is difficult and malfunctions in other parts of the body may occur because mature neurons do not undergo cell division. To increase the prospects of axonal regeneration and functional recovery, researches have focused on designing "nerve guidance channels" or "nerve conduits." When developing ideal tissue-engineered nerve conduits, several components come to mind. They include a biodegradable and porous channel wall, the ability to deliver bioactive growth factors, incorporation of support cells, an internal oriented matrix to support cell migration, intraluminal channels to mimic the structure of nerve fascicles, and electrical activities. This article reviews the factors that are critical for nerve repair, and the advanced technologies that are explored to fabricate nerve conduits. To more accurately mimic natural repair in the body, recent studies have focused on the use of various advanced approaches to create ideal nerve conduits that combine multiple stimuli in an effort to better mimic the complex signals normally found in the body.

Materials for Peripheral Nerve Regeneration

Recent efforts in scientific research in the field of peripheral nerve regeneration have been directed towards the development of artificial nerve guides. We have studied various materials with the aim of obtaining a biocompatible and biodegradable two layer guide for nerve repair. The candidate materials for use as an external layer for the nerve guides were poly(caprolactone) (PCL), a biosynthetic blend between PCL and chitosan (CS) and a synthesised poly(ester-urethane) (PU). Blending PCL, which is a biocompatible synthetic polymer, with a natural polymer enhanced the system biocompatibility and biomimetics, fastened the degradation rates and reduced the production costs. Various novel block poly(ester-urethane)s are being synthesised by our group with tailored properties for specific tissue engineering applications. One of these poly(ester-urethane)s, based on a low molecular weight poly(caprolactone) as the macrodiol, cycloesandimethanol as the chain extender and hexamethylene diisocyanate as the chain linker, was investigated for the production of melt extruded nerve guides. We studied natural polymers such as gelatin (G), poly(L-lysine) (PL) and blends between chitosan and gelatin (CS/G) as internal coatings for nerve guides. In vitro and in vivo tests were performed on PCL guides internally coated either with G or PL to determine the differences in the quality of nerve regeneration associated with the type of adhesion protein. CS/G natural blends combined the good cell adhesion properties of the protein phase with the ability to promote nerve regeneration of the polysaccharide phase. Natural blends were crosslinked both by physical and chemical crosslinking methods. In vitro neuroblast adhesion tests were performed on CS/G film samples, PCL/CS and PU guides internally coated with G to evaluate the ability of such materials towards nerve repair.

Nerve regeneration with the use of a poly(l-lactide-co-glycolic acid)-coated collagen tube filled with collagen gel

AIM

The aim of this study was to develop a novel artificial nerve conduit and to evaluate its efficiency based on the promotion of peripheral nerve regeneration in rabbits.

MATERIAL AND METHODS

The nerve conduit was made of a poly (l-lactide-co-glycolic acid)-coated collagen tube filled with collagen gel. The conduits were implanted into a 15 mm gap in the peroneal nerves of five rabbits. On the contralateral side, the defects were bridged with collagen-filled vein grafts.

RESULTS

Twelve weeks postoperatively nerve regeneration was superior to the vein graft in the PLGA-coated collagen tube, both morphologically and electrophysiologically.

CONCLUSION

The results indicate the superiority of the PLGA-coated collagen tube over vein grafts. Furthermore, they show that entubulation repair with this type of tube can support nerve regeneration over a nerve gap distance of at least 15 mm.

An in vivo evaluation of a biodegradable genipin-cross-linked gelatin peripheral nerve guide conduit material

We evaluated peripheral nerve regeneration using a biodegradable nerve conduit, which was made of genipin-cross-linked gelatin. The genipin-cross-linked gelatin conduit (GGC) was dark blue in appearance, which was concentric and round with a rough outer surface whereas its inner lumen was smooth. After subcutaneous implantation on the dorsal side of the rat, the GGC only evoked a mild tissue response, forming a thin tissue capsule surrounding the conduit. Biodegradability of the GGC and its effectiveness as a guidance channel were examined as it was used to repair a 10 mm gap in the rat sciatic nerve. As a result, tube fragmentation was not obvious until 6 weeks post-implantation and successful regeneration through the gap occurred in all the conduits at the three experimental periods of 4, 6, and 8 weeks. Histological observation showed that numerous regenerated nerve fibers, mostly unmyelinated and surrounded by Schwann cells, crossed through and beyond the gap region 6 weeks after operation. Peak amplitude and area under the muscle action potential curve both showed an increase as a function of the recovery period, indicating that the nerve had undergone adequate regeneration. Thus, the GGC can not only be an effective aids for regenerating nerves but can also lead to favorable nerve functional recovery.

Development of fibrous biodegradable polymer conduits for guided nerve regeneration

The technique of microbraiding with modification was employed as a novel method for the fabrication of fibrous tubular scaffolds for nerve tissue engineering purposes. The biodegradable polymers used in this study were poly(L-lactide-co-glycolide) (10:90) and chitosan. The polymeric fibers were microbraided around a Teflon mandrel to make it as a tubular construct. The conduits were then studied for their surface morphology, swelling behaviour and biocompatibility. The surface morphology was analysed by scanning electron microscope, swelling behaviour by weight increase due to water uptake and biocompatibility by in vitro cytotoxicity assessment in terms of cell morphology and cell viability by the MTT assay of polymer extract treated cells. These conduits may also be used for regeneration of tissues, which require tubular scaffolds such as blood vessel, spinal cord, intestine etc.