Propagation of pseudo-intimal linings of vascular prostheses

Internal Coating of Ureteral Stents with Chemical Vapor Deposition of Parylene

Ureteral balloon catheters and ureteral stents are implanted in large quantities on a daily basis. They are the suspected cause for about a quarter of all the nosocomial infections, which lead to approx. 20,000 deaths in Germany alone. To fight these infections, catheters should be made antibacterial. A technique for an antibacterial coating of catheters exhibiting an aspect ratio of up to 200 consists of a thin silver layer, which is deposited out of an aqueous solution, which is followed by a second step: chemical vapor deposition (CVD) of an organic polymeric film, which moderates the release rate of silver ions. The main concern of the second step is the longitudinal evenness of the film. For tubes with one opening as balloon catheters, this issue can be solved by applying a descendent temperature gradient from the opening to the end of the catheter. An alternative procedure can be applied to commercially available ureteral stents, which exhibit small drainage openings in their middle. The same CVD as before leads to a longitudinal homogeneity of about ±10%—at very low costs. This deposition can be modeled using viscous flow.

A rapid in vitro test of the in vivo healing potential of vascular prostheses

An in vitro method for comparing the penetration of bovine fibroblasts seeded on the external surface of porous vascular prostheses was devised. The effects of water porosity reduction and differing manufacturing constructions (warp-knit Dacron, woven Dacron and polytetrafluoroethylene) on the ability of the bovine fibroblasts to penetrate transmurally was investigated. Of the warp-knit external-veloured Dacron prostheses, the highest porosity 140-denier prototype had the highest luminal surface cell count and the lowest porosty 280-denier prototype the lowest luminal surface cell count. The intermediate prototypes had values between these two extremes. The woven Dacron prostheses, which were of even lower porosity but with a much thinner wall, had cell counts midway between the 140-denier and the 280-denier prototypes. The microporous polytetrafluoroethylene prostheses did not allow fibroblast penetration despite adherence of cells to the outer surface. These findings agree with in vivo healing studies of the same materials in the descending thoracic aorta of the dog, demonstrating that this rapid in vitro assay method can help predict the healing potential of a vascular prosthesis.

Interactions of synthetic and natural surfaces with blood in the physiological environment

Efforts to explain blood compatibility with synthetic and natural surfaces based on a single parameter or a single biological test procedure have either been unsuccessful or led to misleading generalizations. The problem reflects the complex interdependence between material's properties, the composition and properties of blood, and in vivo biorheological conditions. Among the initial events that occur when materials contact blood is the very rapid adsorption of plasma proteins; this process effectively influences the subsequent interactions with the formed blood elements, especially the platelets with the proteinated surfaces. In the case of natural surfaces, when the endothelium is damaged, collagen may become exposed that may cause the activation, adhesion, and aggregation of platelets leading to thrombosis. Current evidence indicates that the platelet-aggregating ability of collagen depends on its "multimeric" or fibrillar structure, rather than on the activation of the platelet-bound enzyme system. Under normal conditions, the flowing blood is probably not in direct contact with endothelial cells that line the blood vessel walls, but with an adsorbed layer of plasma proteins. Should a formation of a multilayer of plasma proteins occur following the initial adsorption of a monolayer, this process could be influenced by changes in the solubility of the proteins, especially fibrinogen, the solubility of which is quite low in plasma. The hypothesis is proposed that such changes may be intimately related to the electrical properties of proteins present in the vascular wall and in blood. It is possible that these properties play a much greater role in thrombogenesis and in the problem of blood compatibility than is currently appreciated. Considering synthetic polymers, a number of these have been prepared that exhibit little adverse effects on blood components and, at the same time, retain their physical properties for various periods of time in the physiological environment. These combined biological and physical properties make them useful for various prosthetic and other biomedical applications in surgery and therapy.

Growth of cultured calf aortic smooth muscle cells on cardiovascular prosthetic materials

The growth of cultured calf aortic smooth muscle cells on cardiovascular biomaterials was investigated, using native and oxidized polyacrylonitrile (orlon) fabrics, dacron velour, and Parylene-C coated polypropylene microfabric as substrates. By light microscopic evaluation, surface cell coverage was most complete on microfabric, followed by native orlon, dacron velour, and oxidized orlon. Native orlon supported the greatest total cell growth, as determined by chemically extractable protein, followed by oxidized orlon, dacron velour, and the microfabric. The observed differences appear to be related to the pore size and fiber thickness of the different substrates.