Mechanism of failure of biocompatible-treated surfaces

Heparin coating of vascular prostheses reduces thromboemboli

BACKGROUND

Synthetic conduits made from currently available materials are suboptimal for use in small-diameter vascular reconstruction because of their high surface thrombogenicity, which leads to failure.

METHODS

In this study control, heparin-irrigated, or heparin-bonded expanded polytetrafluoroethylene (ePTFE) grafts (4 mm long by 1 mm inner diameter) were implanted to reconstruct the iliac artery in male rats. The cremaster muscle was isolated as an island flap based on branches of the iliac artery downstream from the graft. Emboli were quantitated by using intravital fluorescent microscopy of the cremaster muscle's microcirculation.

RESULTS

The mean number of emboli observed per animal during a 20-minute period was 91 for the control group, 84 for the heparin-irrigated group, and 22 for the tridodecylmethylammonium chloride (TDMAC)-heparin group. The mean area of each embolus was 1057 microns 2 for control, 940 microns 2 for heparin-irrigated, and 808 microns 2 for TDMAC-heparin-coated grafts (p < 0.05 for TDMAC-heparin versus control or heparin-irrigated).

CONCLUSIONS

A TDMAC-heparin coating of ePTFE microvascular prostheses significantly reduces downstream microemboli.

Polyethylene/hydrophilic polymer blends for biomedical applications

Polyethylene blends with poly(2-hydroxyethyl methacrylate) [poly(HEMA)] or poly(2,3-dihydroxypropyl methacrylate) [poly(DHPMA)] were prepared by swelling polyethylene with HEMA or 2,3-epoxypropyl methacrylate (EPMA) and by polymerization of the respective monomers. Poly(EPMA) in blends was hydrolysed to poly(DHPMA) with acetic acid. The blends had similar surface and bulk compositions. Swelling with water and surface wettability were proportional to the content of the hydrophilic component; at the same content the polyethylene/poly(DHPMA) blends appeared more hydrophilic than those of polyethylene/poly(HEMA). Thrombus formation in contact with blood examined ex vivo and in vivo was considerably slower on the blends than on unmodified polyethylene. The tests indicated optima in composition; the best biological response was achieved with the blends containing about 14% poly(HEMA) or 16% poly(DHPMA).

On the mechanism of enhanced thromboresistance of polymeric materials in the presence of heparin

Polymeric materials with covalently immobilized heparin were shown to display enhanced thromboresistance in vitro and in vivo experiments. This property of heparin-containing polymers is due to the specific effect of immobilized heparin for every step of interaction of a polymer with blood. The presence of heparin substantially changes the character of adsorbed proteins on a polymer surface and the number of adhered platelets. Thromboresistance enhancement is largely carried out by the interaction of immobilized heparin with plasma proteins which is accompanied by the decrease in total blood coagulant activity, by the decrease in fibrinogen, prothrombin and thrombin concentrations, and by the supression of fibrinstabilizing factor activity. The free heparin content in blood is not changed. It was found that immobilized heparin forms complexes with fibrinogen, thrombin and plasmin that produce lytic action on unstabilized fibrin.