Nerve regeneration through biodegradable polyester tubes.

One approach to repair of transected nerves is to attempt extrinsic guidance of axons across the gaps. We inserted the proximal and distal stumps of severed mouse sciatic nerves into opposite ends of biodegradable polyester tubes. The nerves and ensheathing tubes were examined after postoperative survival times of as long as 2 years. Myelinated fiber number in each successfully regenerated nerve was measured and correlated with the tube's residual lumen size. In selected regenerated nerves axonal sizes and myelin sheath widths were sampled and compared with control values. Swelling and deformation of tube walls occurred in nearly all tubes. Successful regeneration was obtained through more than half of the implants, and was more probable in tubes with larger initial lumens. Myelinated fiber number in regenerated nerves ranged from 231 to 3561 (normally 3900 to 4200); larger values again were found in tubes with larger initial lumens. Mean axonal areas in regenerated nerves were roughly half of normal, though myelin sheaths became appropriately thick. We concluded that the more biodegradable a tube, the more likely it was to incur distortion and luminal narrowing. Tube composition per se seemed of importance mainly as it related to maintenance of adequate luminal size over the length of the degrading tubes; luminal adequacy, not tube composition, seemed paramount in determining the extent of nerve regeneration.

Nontoxic nerve guide tubes support neovascular growth in transected rat optic nerve.

Nontoxic, bioresorbable "nerve guide" tubes were used to bridge the transected optic nerves of adult rats. Nerve guides were fabricated as polymers of synthetic poly D,L-lactates with 2% triethyl citrate added as a plasticizer. The local environment was manipulated further by the addition of the proteins collagen, fibrinogen, and anti-Thy-1 antibody to the nerve guide lumens at the time of operation. Neovascular growth through the nerve guide lumens was quantified with the aid of a computer-controlled microscope. Neovascular growth was greater in the nerve guides to which proteins had been added, compared with initially empty nerve guides. These experiments demonstrated the effectiveness of these nerve guide tubes in supporting and directing neovascular growth in the mammalian central nervous system, and suggested that specific alterations of the local environment within the nerve guide lumen can affect the extent of neovascular growth.

Nerve regeneration through synthetic biodegradable nerve guides: regulation by the target organ.

The successful regeneration of a multifascicular, complete peripheral nerve through a tubular synthetic biodegradable nerve guide across a gap of 10 mm in the rat sciatic nerve is reported. The importance of the distal nerve as a source of target-derived neuronotrophic factors necessary for the successful regeneration of the proximal regenerating nerve is emphasized. A simplified research model for further investigation into and manipulation of the biological processes of nerve regeneration is described. The potential clinical utilization of this model in the management of peripheral nerve injuries and, ultimately, central nervous system lesions is mentioned.

Peripheral nerve repair with bioresorbable prosthesis.

Using the transected sciatic nerve model in adult mice, regeneration of a large bundle of axons organized into the form of a nerve with myelinated and unmyelinated axons, Schwann cells, fibroblasts, collagen, blood vessels, and connective tissue sheaths has been achieved with bioresorbable microtubular guidance channels over gaps of 5 mm in nonimmobilized animals. After 4-6 wks postoperatively, the regenerated nerve cable contains on the order of 40% as many myelinated axons as were measured in the proximal nerve stumps. With the channels used so far in this model, regenerating axons pass into the distal stump in about 3-6 wks postoperatively. The guidance channels used consist of synthetic polyesters and/or polyester composites including glycolic and lactic acid polymers, and polyesters derived from Krebs Cycle dicarboxylic acids. Inflammatory response to these materials has been minimal. Biodegradation/resorption rates can be controlled so as to be compatible with axon growth rates.

Development of blood-compatible elastomers. V. Surface structure and blood compatibility of avcothane elastomers.

The Avcothane 51 elastomer, a member of a series of proprietary materials best characterized as polyurethane/poly(dialkylsiloxane) block copolymers, displays considerable hemocompatibility without any incorporated anticoagulants. In the form of intra-aortic balloons, the elastomer was implanted in several thousands of cardiac patients without intolerable hematologic effects. Hemocompatibility has been assumed to result from a predominantly dispersion-type surface force field whose intensity fluctuates within small domains, maintaning adsorbed blood proteins in an unstable state. The relative hemocompatibility of films, which were obtained from a prepolymer solution cast on substrates impenetrable to the solvent, is a function of the effective surface molecular structure. This can vary as a function of preparative conditions (temperature and rate of evaporation), and has been correlated with an anisotropic distribution of the silicone component in cured films. The concentration of this component in surface layers was quantified independently by IRATR spectroscopy and electron-microprobe analysis, giving consistent results. An IRATR index, which is computed from the ratio of absorptivities measured at 13.00 and 12.62 mu and is inversely proportional to the relative silicone content of surface layers, was found to correlate with the apparent hemocompatibility determined by different in vitro methods. Optimized reproducible hemocompatibility is attained by strict process controls.

Effects of polymer surface molecular structure and force-field characteristics on blood interfacial phenomena.

To quantify the effects of major surface structural factors influencing interfacial reactions induced by polymers in native blood, model surfaces of solvent-cast films of two analogous poly(ether urethanes) and three homologous polyamides (nylon 4, 6/6, and 12) were exposed ex vivo to canine blood under the well-defined hemodynamic conditions of the Stagnation Point Flow Experiment. The selected surfaces allow for incremental changes in properties and were characterized by their "Composite Surface Free ENergy Function," gamma'S, which describes the surface force field as the sum of the mean dispersion (gammaSd) and polar (gammaSp) contributions and is computed from wettability spectra obtained with ultrapure diagnostic liquids. Blood interfacial effects were measured by the shear-limited diameter of the white cell circle formed around the stagnation point, the flow parameter at which symmetric aggregation occurred, and the surface-number density of platelets, [P s], remaining adherent under fixed conditions. At identical flows, within each group of polymers, both the WBC-circle diameter and [P s] scale with gamma Sp/gamma'S, implying that 1) only the magnitude but not the interaction mechanism varies as a function of incremental structural and surface changes, 2) the primary determinant of surface-induced effects is the polar force contribution, and 3) the magnitude of gamma'S is secondary if gammaSd/gamma'S is sufficiently great.

Thermodynamics of native protein/foreign surface interactions. IV. Calorimetric and microelectrophoretic study of human fibrinogen sorption onto glass and LTI-carbon.

1. According to a working hypothesis put forward in the previous papers of this series2-4, the initial phases of native blood/foreign surface interactions have been considered within the framework of a physicochemical model of contact activation at the molecular level. The salient features of this hypothesis are: a) the arrival and adsorption of native plasma proteins on a contact surface overwhelmingly precedes that of the cellular blood components; b) the interaction energy that arises between a particular foreign surface and native plasma proteins settling on it, is a characteristic quantity depending upon the effective surface molecular structure as well as the nature of the proteins; c) the "intensity" of native protein/foreign surface interactions can be treated in terms of thermodynamic quantities since these energy terms are independent of the type of forces acting between protein and surface; and d) depending upon the degree to which the adsorption of a native protein is thermodynamically favored by enthalpy and/or entropy factors, the interaction energy can be utilized to induce conformational changes of varying degree in the sorbed protein. 2. Using glass and low temperature isotropic (LTI) carbon adsorbents, i.e., a known procoagulant and a relevant biomaterial, respectively, the adsorption properties and the potential surface-induced conformational changes of high-purity native human fibrinogen (clottability greater than or equal to 92%) were studied, at 25 degrees C, by 3 independent methods. In all of the experiments performed, a) both adsorbents were employed in the form of particles less than or equal to 1.0 mu representing specific surface areas of 9.85 M2/Gm and 27.7 (nominal) M2/Gm for the glass and LTI-carbon powders, respectively, and b) the fibrinogen was absorbed from a standaridized buffer (pH = 7.2, ionic strength 0.05) using the same fixed surface area/protein solution volume ratio with a given adsorbent. 3. The 25 degrees C adsorption isotherms of fibrinogen obtained for both the glass and LTI-carbon powder adsorbents are not amenable to Langmuir type analysis but indicate multilayer sorption with a possible change in binding mechanism after the completion of the first sorbed monolayer. The adsorptivities attained at first monolayer coverage were slightly greater on glass (approximately 0.76 mug/cm2) than on LTI-carbon (approximately 0.52 mug/cm2). 4. Using a custom-built, thermistorized, isothermal-jacketed microcalorimeter routinely capable of resolving temperature changes of 0.00001 degrees C in 100 ml of aqueous sample volume, the "intensity" of interaction between fibrinogen and each of the microparticulate adsorbents was studied by the direct measurement of the net overall enthalpy changes, hI(SLP)25 arising as a result of protein adsorption. From these values, the net heat of protein sorption has been determined...

The intravascular magnetic suspension of a test device for in vivo hemocompatibility evaluation of biomaterials.

To advance hemocompatibility evaluation techniques, a new in vivo method has been developed for the dynamic testing of candidate biomaterials in suitably-sized experimental animals. One of the salient features this method is that the material to be evaluated constitutes the blood contact surface of a slender body of revolution which is coaxially suspended in a large canine vessel by electromagnetic forces only. The insertion site of the specimen is distal and downstream to the test region, reducing the influence of thrombotic tissue substances. These experimental conditions also insure that the only chronically exposed foreign surface is that of the test material, whose interactions with blood components are not affected by contact with the vessel intima. As demonstrated in simulated Circulation Model experiments, macroscopic thromboembolic phenomena induced by the test material can be continuously monitored. Preliminary in vivo trials have verified the validity of the underlying principles; the feasibility of the required surgical techniques, and the adequate performance of the suspension system.