A novel microporous polyurethane blood conduit: biocompatibility assessment of the UTA arterial prosthesis by an organo-typic culture technique.

An organotypic culture assay has been used to assess the biocompatibility and cytotoxicity of an arterial prosthesis developed at the University of Texas-Arlington (the UTA graft) from a structurally modified polyurethane (PU) elastomer (Tecoflex). The cell culture test was applied to the UTA graft after sterilization by ethylene oxide and by gamma radiation in two separate series. First, small specimens of the prosthesis were incubated for 7 days on a semisolid nutrient medium with their luminal surface in direct contact with endothelium explanted from the aorta of chick embryos. Second, the possibility of cytotoxic contaminants being leached from the polyurethane was assessed by immersing the biomaterial in the liquid culture medium for 5 days at 37 degrees C prior to conducting the organo-typic culture assay on a standard control surface. The structure of the UTA polyurethane prosthesis is porous, but the graft wall is impervious because it contains closed (i.e., noncommunicating) pores. In addition, four other vascular prostheses were included in the study for comparison. They were the Hydrophilic Mitrathane PU graft with a similar impervious, closed pore structure, an experimental Hydrophobic Mitrathane PU graft with a fibrous, open pore structure, and the commercial Impra and Reinforced Goretex expanded PTFE grafts. Following 7 days of cell culture, the biocompatibility and cytotoxicity of the various biomaterials were measured in terms of the area of migrating cells, the density of cells surrounding the explants, and the level of cell adhesion. Comparison of the results against control cultures demonstrated that the UTA graft, along with the other four prostheses, does not release cytotoxic extractables. Microscopic observations of its cultured surface indicated that the UTA graft promotes a high density of cell growth over a limited area, similar to the Hydrophilic Mitrathane graft. This level of biocompatibility is considered inferior to that of the two PTFE and the Hydrophobic Mitrathane prostheses, which promote more extensive cell migration, greater cell adhesion, and cell growth in a continuous single layer.

A novel microporous polyurethane vascular graft: in vivo evaluation of the UTA prosthesis implanted as infra-renal aortic substitute in dogs.

A novel microporous polyurethane blood conduit developed at the University of Texas at Arlington was implanted as an infra-renal substitute in dogs. The prosthesis was fabricated by precipitating a solution of the polymer with dry nitrogen onto a rotating mandrel. The grafts were sterilized either by gamma radiation (series I) or ethylene oxide (series II); they were implanted for the following prescheduled periods: 4, 24, 48 hours, and 1 week (short-term) and 2, 4 weeks, 3 and 6 months (medium-term). The thrombohematological characteristics of each animal were evaluated prior to implantation and confirmed that the index of blood coagulability was normal. In the short-term group, five out of eight grafts were patent and three were partially occluded; four grafts in the medium-term group were patent; one was partially occluded; and three were thrombosed at retrieval. One week after implantation, the prostheses were surrounded by an external capsule, which was present mainly at the two anastomoses. The external capsule covered the entire graft at 3 months. No kinking of the grafts was observed and the presence of a mild yellow stain related to bilirubin uptake was detected at 2 weeks, 1, 3, and 6 months. Histological studies have revealed the formation of a thin internal capsule at both anastomoses, 2 weeks postimplantation, which was not anchored to the graft wall. In the medium-term group, the thrombosed grafts failed to develop an internal capsule, whereas the patent graft exhibited a thick internal capsule made of neocollagenous tissue over the entire graft. This new microporous polyurethane prosthesis did not perform satisfactorily as an infra-renal substitute in dogs and its in vivo stability requires further assessment. Thus, the concept of a polyurethane with closed pores does not achieve what was anticipated.

Physical properties and testing methods for PTFE cardiovascular patches.

Patching after endarterectomy, especially carotid artery surgery, is a common procedure to repair and close the surgical site. Both synthetic and natural materials can be used, but saphenous vein is preferred due to its greater long-term patency. In situations where it is not possible to use the saphenous vein, both Dacron and expanded polytetrafluoroethylene (ePTFE) patches have been used successfully. Expanded PTFE patches are readily available, soft and pliable, have excellent biocompatibility and do not require preclotting prior to implantation. Comparison of two types of ePTFE patches versus natural vessel show that they have more than adequate properties for their intended use.

T lymphocyte modification with the UTA microporous polyurethane vascular prosthesis: in vivo studies in rats.

Sequential quantification of blood T cell subsets by immunocytofluorometry was used to investigate the immune response of microporous polyurethane vascular prostheses after intraperitoneal implantation in rats. The experimental prosthesis, as developed by the University of Texas-Arlington group (UTA), and the Mitrathane prosthesis, as developed by Matrix Med., were implanted for 1, 2 and 6 weeks and compared with ePTFE and wounded rats without prostheses (control group). The implants were examined for histopathology by light microscopy. The percentages of CD4-(helper) and CD8-(suppressor) bearing cells of the PTFE group were significantly lower (p less than 0.05) than the control group 1 week post-implantation. The UTA and the Mitrathane grafts exhibited a significant decrease in both T cell subsets at 1 week, and CD4-bearing cells at 2 weeks. At 6 weeks, T cell subsets were similar among all groups. The ratio of CD4/CD8- cells was similar among all groups except for the PTFE group, which was lower than the control group after 1 week. Histological examination of Mitrathane and UTA grafts showed an acute phase of inflammation which lasted at least 2 weeks. Some foreign body giant cells (FBGC) were present 2 weeks post-implantation, and encapsulation was greater than that observed with PTFE grafts. On the other hand, PTFE grafts exhibited a different pattern of inflammation compared to polyurethane grafts. PTFE implants exhibited a moderate chronic inflammatory response for the first week, as shown by the formation of FBGC. At 2 and 6 weeks, the grafts were encapsulated by a thin layer of collagenous tissue and FBGC were still present around the implants, mostly located in contact with the reinforcing mesh.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo study of an elastomer end-coated polytetrafluoroethylene vascular prosthesis.

Polyurethane end-coated polytetrafluoroethylene (PTFE) grafts (elastomer PTFE grafts) were implanted in 12 female adult mongrel dogs to assess patency, intimalization, tissue incorporation, and technical suitability of the material as a vascular graft. Each dog had bilateral aortoiliac grafts placed, one a standard PTFE and the other an elastomer PTFE graft. The length of the grafts was 7-8 cm and the diameter was 6 mm. The grafts were harvested at intervals to 120 days postoperatively. The elastomer PTFE grafts showed superior longitudinal elasticity, retention of shape, and no graft tearing with suture tension; however, no significant difference in bleeding was noted at the anastomoses between the standard and elastomer PTFE grafts. Satisfactory patency was obtained with both standard (8/10) and elastomer PTFE grafts (9/10) at 90-120 days. No significant difference in the thickness of intima and the length of pannus ingrowth was noted between the standard and elastomer PTFE grafts. No outer tissue incorporation was seen at the elastomer-treated graft segments as opposed to the well-incorporated untreated segments. In conclusion, elastomer end-coating of a PTFE vascular prosthesis provided excellent handling characteristics without detracting from patency; however, the lack of outer tissue incorporation may be a potential disadvantage in its clinical use.

Fabrication and characterization of small-diameter vascular prostheses.

We have developed a process to fabricate polyurethane vascular grafts of various dimensions and porosities in our laboratory. A primary feature of the presented fabrication technique is the ability to control surface porosity and roughness, and bulk mechanical properties. The method is based on the spray application of a fine mixture of polymer solution and nitrogen gas bubbles onto a lathe-mounted mandrel. The technique was successfully tested with Tecoflex, a linear segmented aliphatic polyurethane. Other urethane polymers can be used as well. Several polymer coats are applied in a semiautomated process, at the end of which the polymer coating is dried and the tube is slipped off the mandrel. It is the purpose of this paper to describe the fabrication process and present results of the evaluation of grafts. Wall structure was evaluated using scanning electron microscopy and compliance was measured in a specially designed testing apparatus. We developed methods to quantify kink resistance and suture retention capacity of the grafts. These characteristics were correlated with graft fabrication variables: mandrel rpm, horizontal speed of the spray nozzle, gas and polymer solution flow rates. We were able to routinely produce 3-6-mm-ID grafts with 0.5-1.2-mm wall thickness and average bulk pore sizes of 10-250 microns; the wall porosity could be varied between 30 and 70%. Compliance values of the grafts were comparable with corresponding values of carotid and femoral arteries of dogs.

Synthetic vascular graft fabrication by a precipitation-flotation method.

The authors report a new technique for fabricating synthetic vascular grafts. It involves spraying a polymer solution (generated by mixing polymer solution and nitrogen gas in a spray nozzle) onto the surface of a flowing nonsolvent liquid (water): polymer fibers form during precipitation of the spray drops as they travel on the water surface, until picked up by a partially submerged rotating mandrel. Depending on process conditions, these fibers may aggregate to form a continuous layer or remain separated until they are picked up. A number of independent process variables allow control of characteristics of the conduits: gas and polymer solution feed rates, nozzle traverse speed, nonsolvent (water) flow rate, spray-mandrel spacing, and mandrel rpm. The SEM reveals that the graft wall consists of numerous fused polymeric fibers arrayed in both the circumferential and axial directions. The inner surface resembles a mesh of closely spaced fused fibers. The conduits have walls with interconnected pores (water permeabilities between 0.05 to 7.0 ml/min-cm2); nonporous surfaces also can be made. Tensile stress of the grafts at failure (in radial direction) varied between 0.05 to 2.3 MPa, whereas elongation at break ranged between 150 to 600%, depending on the porosity and fabrication conditions. A major advantage of this technique is its ability to produce grafts of a wide variety of fiber sizes and fusion characteristics in an inexpensive, safe, and reliable fashion.