Peripheral nerve repair using a poly(organo)phosphazene tubular prosthesis

Determination of surface charge properties of PC-12 cells by electrophoresis

The electrophoretic behavior of pheochromocytoma (PC-12) cells was investigated both experimentally and theoretically. Cell mobility in aqueous media at different pHs and ionic concentrations was measured, and a model, which assumed that the cell surface contains both acidic and basic functional groups, was proposed. As a result, it was revealed that the experimental data gathered can be described satisfactorily by assuming that the cell surface contains two types of monovalent acidic functional groups and one basic functional group. The values of the dissociation constants of the acidic and basic groups are found to be close to those of acidic amino acids, which indicates that the acidic amino acids may play an important role in the surface electrical properties of PC-12 cells.

Low temperature formation of hydroxyapatite-poly(alkyl oxybenzoate)phosphazene composites for biomedical applications

The formation of biodegradable composites which may be suitable as bone analogs is described. Polyphosphazene-hydroxyapatite (HAp) composites were produced via an acid-base reaction of tetracalcium phosphate and anhydrous dicalcium phosphate in the presence of polyphosphazenes bearing alkyl ester containing side-groups. The polyphosphazenes used were poly(ethyl oxybenzoate)phosphazene (PN-EOB) and poly(propyl oxybenzoate) phosphazene (PN-POB). The effects of temperature and the proportions of polymers, PN-EOB and PN-POB on the kinetics, reaction chemistry and phase evolution during the formation of stoichiometric HAp were studied. Kinetics, phase evolution and microstructural development were evaluated using isothermal calorimetry, X-ray diffraction and scanning electron microscopy, respectively. Analysis of solution chemistry revealed that the increases in the pH during the formation of SHAp, resulted in partial hydrolysis of the polymer surfaces, which led in turn to the formation of a calcium cross-linked polymer surface. The calcium cross-linked polymer surface appeared to facilitate the nucleation and growth of apatite deposits on the polymer. The current study illustrates the in situ formation of HAp in the presence of polyphosphazenes, where HAp is chemically bonded to the polymer.

Use of Skeletal Muscle Tissue in Peripheral Nerve Repair: Review of the Literature

The management of peripheral nerve injury continues to be a major clinical challenge. The most widely used technique for bridging defects in peripheral nerves is the use of autologous nerve grafts. This technique, however, necessitates a donor nerve and corresponding deficit. Many alternative techniques have thus been developed. The use of skeletal muscle tissue as graft material for nerve repair is one example. The rationale regarding the use of the skeletal muscle tissue technique is the availability of a longitudinally oriented basal lamina and extracellular matrix components that direct and enhance regenerating nerve fibers. These factors provide superiority over other bridging methods as vein grafts or (non)degradable nerve conduits. The main disadvantages of this technique are the risk that nerve fibers can grow out of the muscle tissue during nerve regeneration, and that a donor site is necessary to harvest the muscle tissue. Despite publications on nerve conduits as an alternative for peripheral nerve repair, autologous nerve grafting is still the standard care for treatment of a nerve gap in the clinical situation; however, the use of the skeletal muscle tissue technique can be added to the surgeon's arsenal of peripheral nerve repair tools, especially for bridging short nerve defects or when traditional nerve autografts cannot be employed. This technique has been investigated both experimentally and clinically and, in this article, an overview of the literature on skeletal muscle grafts for bridging peripheral nerve defects is presented.

Nano-biotechnology: carbon nanofibres as improved neural and orthopaedic implants

For the continuous monitoring, diagnosis, and treatment of neural tissue, implantable probes are required. However, sometimes such neural probes (usually composed of silicon) become encapsulated with non-conductive, undesirable glial scar tissue. Similarly for orthopaedic implants, biomaterials (usually titanium and/or titanium alloys) often become encapsulated with undesirable soft fibrous, not hard bony, tissue. Although possessing intriguing electrical and mechanical properties for neural and orthopaedic applications, carbon nanofibres/nanotubes have not been widely considered for these applications to date. The present work developed a carbon nanofibre reinforced polycarbonate urethane (PU) composite in an attempt to determine the possibility of using carbon nanofibres (CNs) as either neural or orthopaedic prosthetic devices. Electrical and mechanical characterization studies determined that such composites have properties suitable for neural and orthopaedic applications. More importantly, cell adhesion experiments revealed for the first time the promise these materials have to increase neural (nerve cell) and osteoblast (bone-forming cell) functions. In contrast, functions of cells that contribute to glial scar-tissue formation for neural prostheses (astrocytes) and fibrous-tissue encapsulation events for bone implants (fibroblasts) decreased on PU composites containing increasing amounts of CNs. In this manner, this study provided the first evidence of the future that CN formulations may have towards interacting with neural and bone cells which is important for the design of successful neural probes and orthopaedic implants, respectively.

Adhesive and mechanical properties of hydrogels influence neurite extension

Photopolymerizable polyethylene glycol (PEG) hydrogels conjugated with bioactive ligands were examined for their use as scaffolds in peripheral nerve regeneration applications. The bioactivity and mechanical properties of PEG hydrogels can be tailored through the integration of bioactive factors (adhesion ligands, proteolytic sites, growth factors) and the alteration of PEG concentrations, respectively. For peripheral nerve regeneration, it will be important to determine the type and concentration of the bioactive molecules required to improve neurite extension. In this study, cell adhesion ligands (RGDS, IKVAV, and YIGSR) were covalently attached to PEG hydrogels. Both the type and concentration of cell adhesion ligand used affected neurite extension. Extension from PC12 cells was greater on hydrogels with RGDS incorporated than IKVAV, and the optimal concentration for each ligand was different. Cells adhered to but did not extend neurites on hydrogels with YIGSR. Cells did not adhere to hydrogels containing RGES. Furthermore, different combinations of these ligands affected neurite extension to different degrees. The mechanical properties of the hydrogels also significantly affected neurite extension. PC12 cells grown on more flexible hydrogels exhibited the greatest degree of neurite extension. PEG hydrogels have thus been developed with varying biochemical and mechanical properties that may enhance nerve regeneration.

Change in electrophoretic mobility of PC12 cells after culturing on PVA membranes modified with different diamines

The cell-biomaterial interaction is of extreme importance in regulating the numerous functions necessary for cell adhesion, growth, and differentiation. In the current study, electrophoresis was used to investigate the interactions between cells and biomaterials by measuring the change in electrophoretic mobility of pheochromocytoma (PC12) cells after they were cultured on the poly (vinyl alcohol) membranes modified with different diamines. Variations in cellular activity and electrophoretic mobility of cultured cells were compared. It was found that the intracellular metabolism and the cell surface charge properties were altered after cells contacting biomaterials and the variation of the latter occurred earlier than that of the former. Although the precise mechanism by which the variation of electrophoretic mobility of cultured PC12 cells was unknown, the biomaterials could influence the cell mobility within a short incubation time. It was hypothesized that changes in extracellular matrix components of cell surface may be in part responsible.

Facial nerve regeneration through progesterone-loaded chitosan prosthesis. A preliminary report

Biodegradable nerve guides have represented new treatment alternatives for nerve repairing. They are gradually biodegradable, exert biological effects directly to the injured nerve, and act as drug- or cell-delivery devices. Furthermore, progesterone (PROG) has been demonstrated to promote injured peripheral nerve regeneration. In this study, it was hypothesized that PROG delivered from chitosan prostheses provides better facial nerve regenerative response than chitosan prostheses with no PROG. As there are no reports on the use of the former as nerve-guide material in the regeneration of injured nerves, this is the main objective of the present work. Chitosan prostheses containing PROG were used to bridge 10-mm gaps in rabbit facial nerves. The regenerated nerves were evaluated 45 days after implantation in animals with the use of light microscopy and morphometric analysis. Gas chromatography was used in order to quantify PROG content in prosthesis prior to and after implantation in subcutaneous tissue at different periods of up to 60 days. In addition, the prosthesis walls were evaluated with histological techniques in order to assess their integrity and the surrounding tissue reaction. Chitosan prostheses allowed PROG release during the time needed for nerve regeneration. At 45 days myelinated nerve fibers were observed in both the proximal and distal stumps. This parameter and the N ratio were higher in the progesterone-treated group when compared to that of the vehicle control. Findings indicate that chitosan prostheses were useful in nerve regeneration, acting as a long-lasting PROG delivery device a faster nerve regeneration.

Biocompatibility of fluorinated poly(organophosphazene)

We investigated neutrophil and platelet adhesion on a fluorinated poly(organophosphazene) in vitro. The results suggested that neutrophil and platelet adhesion on the poly(organophosphazene) only occurred on a few occasions, as observed by SEM. We demonstrated that the fluorinated poly(organophosphazene) showed excellent biocompatibility compared with the poly(organophosphazene) without the fluorinated side groups or PDMS. Additionally, we estimated the competitive plasma protein adsorption to the fluorinated poly(organophosphazene) using a gold-colloid-labeled immunoassay. Interestingly, the fluorinated poly(organophosphazene) film selectively adsorbed albumin when compared with gamma-globulin and fibrinogen, suggesting that a selective albumin adsorption on the film is responsible for the suppression of platelet adhesion.

Tyrosine-Bearing Polyphosphazenes

Tyrosine-functionalized polyphosphazenes were synthesized, and their hydrolytic stability, pH-sensitive behavior, and hydrogel-forming capabilities were investigated. The physical and chemical properties of the polymers varied with the type of linkage between the tyrosine unit and phosphazene backbone. Poly[(ethyl glycinat-N-yl)(ethyl tyrosinat-N-yl)phophazenes] (linkage via the amino group of tyrosine) were found to be hydrolytically erodible. The rate of hydrolysis was dependent on the ratio of the two side groups, the slowest rate being associated with the highest concentration of tyrosine. The hydrolysis products were identified as phosphates, tyrosine, glycine, ammonia, and ethanol derived from the ester group. The hydrolytically stable phenolic-linked tyrosine derivatives were prepared from N-t-BOC-L-tyrosine methyl ester and alkoxy-based cosubstituents. Polyphosphazenes with both propoxy and phenolic-linked tyrosine side groups showed a pH-sensitive solubility behavior, which was dependent on the ratio and nature of the two side groups. The polymer was soluble in aqueous media below pH 3 and above pH 4. From pH 3-4, the polymer was insoluble. Replacement of propoxy by trifluoroethoxy units yielded a polymer that was insoluble in aqueous media at all pH values. Replacement of propoxy by methoxyethoxyethoxy groups gave a polymer that was soluble at all pH values. Exposure of both the propoxy and methoxyethoxyethoxy polymers to calcium ions in aqueous media caused gel formation due to ionic cross-linking through the carboxylate groups.

Biocompatibility of implantable synthetic polymeric drug carriers: focus on brain biocompatibility

Numerous polymeric biomaterials are implanted each year in human bodies. Among them, drug delivery devices are potent novel powerful therapeutics for diseases which lack efficient treatments. Controlled release systems are in direct and sustained contact with the tissues, and some of them degrade in situ. Thus, both the material itself and its degradation products must be devoid of toxicity. The knowledge and understanding of the criteria and mechanisms determining the biocompatibility of biomaterials are therefore of great importance. The classical tissue response to a foreign material leads to the encapsulation of the implant, which may impair the drug diffusion in the surrounding tissue and/or cause implant failure. This tissue response depends on different factors, especially on the implantation site. Indeed, several organs possess a particular immunological status, which may reduce the inflammatory and immune reactions. Among them, the central nervous system is of particular interest, since many pathologies still need curative treatments. This review describes the classical foreign body reaction and exposes the particularities of the central nervous system response. The recent in vivo biocompatibility studies of implanted synthetic polymeric drug carriers are summarized in order to illustrate the behavior of different classes of polymers and the methodologies used to evaluate their tolerance.

Adhesion and growth of human Schwann cells on trimethylene carbonate (co)polymers

Seeding of artificial nerve grafts with Schwann cells is a promising strategy for bridging large nerve defects. The aim of the present study was to evaluate the adhesion and growth of human Schwann cells (HSCs) on 1,3-trimethylene carbonate (TMC) and epsilon-caprolactone copolymers, with the final goal of using these materials in the development of an artificial nerve graft. The adhesion, proliferation, and morphology of HSCs on copolymers containing 10 and 82 mol % of TMC and on the parent homopolymers were investigated. HSCs adhered faster and in greater numbers on the copolymer with 82 mol % of TMC and on the TMC homopolymer compared with the other (co)polymers. On all polymer films, cell adhesion was lower than on gelatin (positive control). Despite differences in cell adhesion, cells displayed exponential growth on all tested surfaces, with similar growth rates. Cell numbers doubled approximately every 3 days on all substrates. When the polymer films were coated with fibronectin, no significant differences in cell adhesion and proliferation were observed between coated polymer surfaces and gelatin. The results indicate that all tested materials support the adhesion and proliferation of HSCs and can in principle be used for the preparation of flexible and slowly degrading nerve guides.

Bioactive poly(L-lactic acid) conduits seeded with Schwann cells for peripheral nerve regeneration

This study attempted to enhance the efficacy of peripheral nerve regeneration using our previously tested poly(L-lactic acid) (PLLA) conduits by incorporating them with allogeneic Schwann cells (SCs). The SCs were harvested, cultured to obtain confluent monolayers and two concentrations (1 x 10(4) and 1 x 10(6) SC/ml) were combined with a collagen matrix (Vitrogen) and injected into the PLLA conduits. The conduits were then implanted into a 12 mm right sciatic nerve defect in rats. Three control groups were used: isografts, PLLA conduits filled with collagen alone and empty silicone tubes. The sciatic functional index (SFI) was calculated monthly through four months. At the end of second and fourth months, the gastrocnemius muscle was harvested and weighed for comparison and the graft conduit and distal nerve were harvested for histomorphologic analysis. The mean SFI demonstrated no group differences from isograft control. By four months, there was no significant difference in gastrocnemius muscle weight between the experimental groups compared to isograft controls. At four months, the distal nerve demonstrated a statistically lower number of axons mm2 for the high and low SC density groups and collagen control. The nerve fiber density was significantly lower in all of the groups compared to isograft controls by four months. The development of a "bioactive" nerve conduit using tissue engineering to replace autogenous nerve grafts offers a potential approach to improved patient care. Although equivalent nerve regeneration to autografts was not achieved, this study provides promising results for further investigation.

Peripheral nerve injury: a review and approach to tissue engineered constructs

Eleven thousand Americans each year are affected by paralysis, a devastating injury that possesses associated annual costs of $7 billion (American Paralysis Association, 1997). Currently, there is no effective treatment for damage to the central nervous system (CNS), and acute spinal cord injury has been extraordinarily resistant to treatment. Compared to spinal cord injury, damage to peripheral nerves is considerably more common. In 1995, there were in excess of 50,000 peripheral nerve repair procedures performed. (National Center for Health Statistics based on Classification of Diseases, 9th Revision, Clinical Modification for the following categories: ICD-9 CM Code: 04.3, 04.5, 04.6, 04.7). These data, however, probably underestimate the number of nerve injuries appreciated, as not all surgical or traumatic lesions can be repaired. Further, intraabodominal procedures may add to the number of neurologic injuries by damage to the autonomic system through tumor resection. For example, studies assessing the outcome of impotency following radical prostatectomy demonstrated 212 of 503 previously potent men (42%) suffered impotency when partial or complete resection of one or both cavernosal nerve(s). This impotency rate decreased to 24% when the nerves were left intact (Quinlan et al., J. Urol. 1991;145:380-383; J. Urol. 1991;145:998-1002).

Preparation of EVAL membranes with smooth and particulate morphologies for neuronal culture

In this work, the in vitro interaction of cerebellar granule neurons prepared from 7-day-old Wistar rats and poly ethylene-co-vinyl alcohol (EVAL) membranes was investigated. Cells were cultured in smooth and particulate EVAL membranes for up to 7 days. Particulate membranes were prepared by using 1-octanol to precipitate EVAL solutions in DMSO. Such a membrane was microporous characterized by a packed bed of particles. Voids left between the aggregated particles formed a continuous and interconnected porous network. Crystallization of the EVAL polymer induced by 1-octanol is responsible for the formation of particulate morphology. The membrane structure and its relationship with cells were examined by scanning electron microscopy and the MTT assay. It was observed that the particulate membrane was more favorable for the neuron culture than the smooth membrane. Neurons seeded on the particulate membrane were able to regenerate with formation of an extensive neuritic network. Therefore, the particulate structure may spatially mediate cellular response that can promote neuronal cell attachment, differentiation and neuritic growth, indicating that the particulate structure should be useful as a new polymer scaffold for nerve repair.

Clinical long-term in vivo evaluation of poly(L-lactic acid) porous conduits for peripheral nerve regeneration

It was the purpose of this study to evaluate the clinical long-term effects of PLLA degradation in vivo on nerve regeneration in the rat sciatic nerve model. Thirty-one Sprague Dawley rats were utilized. Two groups of animals were selected. The control group of 10 animals received a 12 mm reversed isograft into the right sciatic nerve from 5 donor animals. The experimental group (n = 21) received a 12 mm empty PLLA conduits placed into a 12 mm defect in the right sciatic nerve. The left leg served as an internal control. Walking track analysis was performed monthly through 8 months. At the end of 4 and 8 months, animals in the control isograft and experimental group had the medial and lateral gastrocnemius muscles harvested and weighed for comparison. The midconduit/isograft and the distal nerve in these same animals were harvested and histomorphologically analyzed. Multiple samples were collected and expressed as means +/- standard error. A two-sample t-test and Wilcoxon rank sum test was used to compare the variables. Significance level was set at alpha = 0.05. After Bonferroni correction for multiple testing, a p value of < or = 0.01 was considered statistically significant. Throughout all time periods, the PLLA conduit remained structurally intact and demonstrated tissue incorporation and vascularization. There was no evidence of conduit collapse or breakage with limb ambulation. Moreover, there was no evidence of conduit elongation at 8 months as previously observed with the 75:25 poly(DL-lactic-co-glycolic acid) (PLGA) conduits. The mean absolute value of the sciatic functional index (SFI) demonstrated no group differences from isograft controls measured over the 8 months except at 3 months where the isograft values were higher (p = 0.0379) and at 7 months were the isograft group was significantly lower (p = 0.0115). At 4 and 8 months, the weight of the gastrocnemius muscles of the experimental group was not significantly different from isografts. At 4 months the number of axons/mm2 and nerve fiber density was not significantly different between the isograft control and experimental groups in either the midconduit/isograft or distal nerve. At 8 months the number of axons/mm2 was significantly lower in the isograft compared to the midconduit experimental group (p = 0.006). The number of axons/mm2 in the distal nerve and the nerve fiber density in the midconduit and distal nerve were not significantly different between the two groups. The study confirmed our initial hypothesis that PLLA conduits are a viable scaffold for clinical long-term nerve gap replacement. We are critically aware however that longer evaluation of polymer degradation is warrented. Further studies on these individual nerve components are continuing, with the ultimate goal being the fabrication of a bioactive conduit that meets or exceeds the functional results of isografts.

The role of cell density in the survival of cultured cerebellar granule neurons

The dependence for survival of cerebellar granule neurons on the cell density was examined both experimentally and theoretically. The results of batch experiments revealed that the cell survival index (CSI) was inappreciable, if cell density was below a critical level. If cell density exceeded this critical value, CSI increased with the increase in cell density. In addition, CSI was significantly increased by using a conditioned medium from the dense cultures. This suggests that not only cell density promotes survival of neurons, but also an increased concentration of growth factors produced by neurons has a direct effect on the survival of the neurons. A quantitative model describing the distribution of the growth factor at different cell densities was proposed to investigate the role of cell density in the survival of the neurons. We showed the existence of a critical level for cell density, and good agreement in the improvement of CSI was found between the theoretical prediction and the experimental result. Finally, the average concentration of growth factor necessary for cell survival based on our model was in a reasonable range compared to the practice of the addition of neurotrophic factors to the medium of cultured cerebellar granule neurons.

Peripheral nerve regeneration using bioresorbable macroporous polylactide scaffolds

The ability of DRG-derived neurons to survive and attach onto macroporous polylactide (PLA) foams was assessed in vitro. The foams were fabricated using a thermally induced polymer-solvent phase separation. Two types of pore structures, namely oriented or interconnected pores, can be produced, depending on the mechanism of phase separation, which in turn can be predicted by the thermodynamics of the polymer-solvent pair. Coating of the porous foams with polyvinylalcohol (PVA) considerably improved the wettability of the foams and allowed for cell culture. The in vitro biocompatibility of the PVA-coated supports was demonstrated by measuring cell viability and neuritogenesis. Microscopic observations of the cells seeded onto the polymer foams showed that the interconnected pore networks were more favorable to cell attachment than the anisotropic ones. The capacity of highly oriented foams to support in vivo peripheral nerve regeneration was studied in rats. A sciatic nerve gap of 5-mm length was bridged with a polymer implant showing macrotubes of 100 microm diameter. At 4 weeks postoperatively, the polymer implant was still present. It was well integrated and had restored an anatomic continuity. An abundant cell migration was observed at the outer surface of the polymer implant, but not within the macrotubes. This dense cellular microenvironment was found to be favorable for axogenesis.

Growth factor delivery for tissue engineering

A tissue-engineered implant is a biologic-biomaterial combination in which some component of tissue has been combined with a biomaterial to create a device for the restoration or modification of tissue or organ function. Specific growth factors, released from a delivery device or from co-transplanted cells, would aid in the induction of host parenchymal cell infiltration and improve engraftment of co-delivered cells for more efficient tissue regeneration or ameliorate disease states. The characteristic properties of growth factors are described to provide a biological basis for their use in tissue engineered devices. The principles of polymeric device development for therapeutic growth factor delivery in the context of tissue engineering are outlined. A review of experimental evidence illustrates examples of growth factor delivery from devices such as microparticles, scaffolds, and encapsulated cells, for their use in the application areas of musculoskeletal tissue, neural tissue, and hepatic tissue.

Challenges to nerve regeneration

Peripheral nerve injuries can result from mechanical, thermal, chemical, congenital, or pathological etiologies. Failure to restore these damaged nerves can lead to the loss of muscle function, impaired sensation, and painful neuropathies. Current surgical strategies for the repair of critical nerves involve the transfer of normal donor nerve from an uninjured body location. However, these "gold standard" methods for tissue restoration frequently are limited by tissue availability, risk of disease spread, secondary deformities, and potential differences in tissue structure and size. One possible alternative to autogenous tissue replacement is the development of engineered constructs to replace those elements necessary for axonal proliferation, including a scaffold, support cells, induction factors, and extracellular matrices. Despite advances and contributions in the field of tissue engineering, results to date with nerve conduits have failed to equal the nerve regeneration achieved with autogenous grafts for large distances. We review the current challenges to tissue-engineered constructs. Each of the four components is reviewed and approaches are outlined. Semin. Surg. Oncol. 19:312-318, 2000.

Engineering strategies for peripheral nerve repair

Tissue engineering in the peripheral nervous system unites efforts by physicians, engineers, and biologists to create either natural or synthetic tubular nerve guidance channels as alternatives to nerve autografts for the repair of peripheral nerve defects. Guidance channels help direct axons sprouting from the regenerating nerve end, provide a conduit for diffusion of neurotropic and neurotrophic factors secreted by the damaged nerve stumps, and minimize infiltration of fibrous tissue. In addition to efforts to control these physical characteristics of nerve guidance channels, researchers are optimizing the incorporation of biologic factors and engineering interactive biomaterial that can specifically stimulate the regeneration process. Current and future research will ultimately result in biologically active and interactive nerve guidance channels that can support and enhance peripheral nerve regeneration over longer, more clinically relevant defect lengths.

Poly(DL-lactide-epsilon-caprolactone) nerve guides perform better than autologous nerve grafts

The aim of this study was to compare the speed and quality of nerve regeneration after reconstruction using a biodegradable nerve guide or an autologous nerve graft. We evaluated nerve regeneration using light microscopy, transmission electron microscopy and morphometric analysis. Nerve regeneration across a short nerve gap, after reconstruction using a biodegradable nerve guide, is faster and qualitatively better, when compared with nerve reconstruction using an autologous nerve graft. Therefore, we conclude that in the case of a short nerve gap (1 cm), reconstruction should be carried out using a biodegradable nerve guide constructed of a copolymer of DL-lactide and epsilon-caprolactone.

Growth factor delivery for tissue engineering

A tissue-engineered implant is a biologic-biomaterial combination in which some component of tissue has been combined with a biomaterial to create a device for the restoration or modification of tissue or organ function. Specific growth factors, released from a delivery device or from co-transplanted cells, would aid in the induction of host parenchymal cell infiltration and improve engraftment of co-delivered cells for more efficient tissue regeneration or ameliorate disease states. The characteristic properties of growth factors are described to provide a biological basis for their use in tissue engineered devices. The principles of polymeric device development for therapeutic growth factor delivery in the context of tissue engineering are outlined. A review of experimental evidence illustrates examples of growth factor delivery from devices such as microparticles, scaffolds, and encapsulated cells, for their use in the application areas of musculoskeletal tissue, neural tissue, and hepatic tissue.

Modulation of peripheral nerve regeneration: a tissue-engineering approach. The role of amnion tube nerve conduit across a 1-centimeter nerve gap

A new type of a biodegradable nerve graft conduit material, the amnion tube, has been developed in our laboratory. To test the tube in the peripheral nerve regeneration process, it was initially applied across a 1-cm sciatic nerve gap in rats and was compared with other nerve conduit materials. We used male Sprague-Dawley rats as our animal model. The experiment included 66 rats that were randomly assigned into five groups: autograft (n = 17), amnion tube (n = 19), silicone tube (n = 20), no repair (n = 7), and sham group (n = 3). The process of peripheral nerve regeneration was evaluated at 2, 4, 10, and 17 weeks following injury and repair by using morphologic and functional assessments of the outcome of nerve regeneration in each animal. Nerve regeneration across the amnion tube nerve conduit was comparable with that seen in autograft and superior to that of the silicone group. A uniform nerve tissue was seen filling and crossing the amnion conduit, and the regenerated nerve from the proximal stump reached the distal end and was undifferentiated from the normal nerve tissues. At 4 months, the amnion tube biodegraded and no longer could be identified and differentiated from the nerve tissues. The amnion tube animal group showed a number of axons very close to that in the nerve autograft group (37,157 versus 33,054). Functional recovery at a 2- to 4-week interval was significantly statistically higher only in the amnion tube animal group (p = 0.01). However, the improvement disappeared between 10 and 17 weeks. In conclusion, the amnion tube is a potential ideal nerve conduit material secondary to its unique characteristics: it contains important neurotropic factors, is biodegradable, provokes a very weak immune response, is semiflexible, is readily available, and is easily manufactured into different sizes and diameters.

Host response to tissue engineered devices

The two main components of a tissue engineered device are the transplanted cells and the biomaterial, creating a device for the restoration or modification of tissue or organ function. The implantation of polymer/cell constructs combines concepts of biomaterials and cell transplantation. The interconnections between the host responses to the biomaterial and transplanted cells determines the biocompatibility of the device. This review describes the inflammatory response to the biomaterial component and immune response towards transplanted cells. Emphasis is on how the presence of the transplanted cell construct affects the host response. The inflammatory response towards a biomaterial can impact the immune response towards transplanted cells and vice versa. Immune rejection is the most important host response towards the cellular component of tissue engineered devices containing allogeneic, xenogeneic or immunogenic ex vivo manipulated autologous cells. The immune mechanisms towards allografts and xenografts are outlined to provide a basis for the mechanistic hypotheses of the immune response towards encapsulated cells, with antigen shedding and the indirect pathway of antigen presentation predominating. A review of experimental evidence illustrates examples of the inflammatory response towards biodegradable polymer scaffold materials, examples of devices appropriately integrated as assessed morphologically with the host for various applications including bone, nerve, and skin regeneration, and of the immune response towards encapsulated allogeneic and xenogeneic cells.

The development of bioartificial nerve grafts for peripheral-nerve regeneration

This article describes recent, significant scientific advances leading to the development of the bioartificial nerve graft. Schwann cells, which play an active role in the repair and function of peripheral nerves, are used to seed a synthetic, often resorbable conduit, which is then used to bridge and repair nerve gaps caused by injury or disease. By enhancing the rate and extent of regeneration, the bioartificial nerve graft holds great promise for improving recovery in the peripheral (and central) nervous system.

Polyorganophosphazene microspheres for drug release: polymer synthesis, microsphere preparation, in vitro and in vivo naproxen release

Microsphere preparation for naproxen slow release was investigated using two newly prepared biodegradable polyorganophosphazenes, derivatized at the phosphorus atoms with phenylalanine ethyl ester and imidazole at molar ratios of 71/29 and 80/20. The polymers were prepared by substitution of the chloride atoms of polydichlorophosphazene with a phenylalanine ethyl ester-imidazole mixture followed, after 7 or 48 h reaction, by the addition of excess imidazole. Three methods of microsphere preparation have been considered: spray-drying, emulsion/solvent evaporation and emulsion/solvent evaporation-extraction. Microparticles obtained by spray-drying were found to possess a narrow distribution size with a mean diameter of 2-5 microm. Their internal structure consisted of a porous or empty core depending upon the solvent used for the preparation. Furthermore the microspheres prepared with this technique rapidly released the entrapped naproxen independently of the used polymer, the drug loading or the preparation process. On the other hand microspheres prepared by solvent evaporation or solvent evaporation-extraction showed a distribution size ranging between 10 and 100 microm. By the appropriate choice of pH and solvent composition of the external phase, naproxen could be entrapped, in these microspheres, with a yield higher of 80%. The polymer composition dictates the in vitro release rate of naproxen from the particles, which was faster when the microspheres were prepared with the polymer at higher imidazole content. In vivo experiments, carried out by subcutaneous implantation in rats of microspheres prepared by solvent evaporation, demonstrated that a constant level of naproxen in plasma could be maintained up to 400 h at a suitable concentration for antinflammatory activity.

Bioartificial nerve grafts based on absorbable guiding filament structures--early observations

Gaps 10 mm wide in the sciatic nerves of 64 rats were bridged by bioartificial nerve grafts consisting of a silicone tube containing seven longitudinally placed filaments made of non-absorbable, (polyamide [Ethilon]) or absorbable, material (polydioxanone [PDS], polyglactin [Vicryl], and catgut). The purpose was to study the organisation of axonal growth inside the tube along such filaments. After two and four weeks histological techniques were used to study the contents of the tube and at four weeks immunohistological techniques were used to confirm the presence of axons distal to the tube. In all experimental groups axons had traversed the tube and reached the distal segment after four weeks. Inside the tube axons were organised in multiple minifascicles in all groups, but there were no axons growing in direct contact with the filaments. We conclude that resorbable filaments placed inside a silicone tube do not disturb axonal growth across the tube.

Anti-inflammatory activity of polyphosphazene-based naproxen slow-release systems

A biocompatible and biodegradable polyphosphazene bearing phenylalanine ethyl ester, imidazole and chlorine (10.7:1:2.5 molar ratio) as substituents of the phosphorus atoms of the polymer backbone was studied for the preparation of polymeric naproxen slow-release systems. Discs 2.5 cm in diameter and 0.5 mm (thin) or 0.65 mm (thick), loaded, respectively, with 20 and 13.5% naproxen, showed different drug release kinetics, the thin matrices releasing naproxen at a faster rate and for a shorter time. In-vivo studies in rats demonstrated the pharmacological efficacy of these two different delivery systems in the inhibition of acute or chronic inflammatory diseases. Subcutaneous implantation of the thin matrices in rats was found to reduce carrageenan oedema induced both 1 h and 7 days after implantation. Rats implanted with thick matrices showed a reduction in chronic inflammation caused by adjuvant arthritis. Approximately 78% inhibition of arthritic oedema was found 28 days after subcutaneous administration of the matrices whereas 28.7% inhibition was found after daily oral administration of naproxen. Blood levels of naproxen in arthritic rats after matrix implantation showed the presence of drug up to day 28. These positive results have encouraged us to study a controlled-release system suitable for use in man.