Polymeric Surface Modifications of Tantalum Stents

Purpose:

To compare two kinds of polymer-coated tantalum stents with bare tantalum stents (control) to determine if the coatings can improve thromboresistance.

Methods:

Twenty-seven Fontaine-Dake stents were balloon expanded in three 8-mm × 80-cm.polytetrafluoroethylene (PTFE) grafts; 9 stents were bare tantalum (T); 9 were coated with polyetherurethane (PL); and 9 were coated with parylene (PA). There were 9 stents placed in each graft as follows: 3 tantalum, 3 polyetherurethane, and 3 parylene. In swine whose platelets had been radiolabeled with indium 111, the ends of each stented graft were connected to 14F femoral and venous sheaths to create an ex vivo fistula. Each graft was exposed to blood for 30, 60, and 120 minutes. At the end of each test period, the stented grafts were disconnected from the sheaths, flushed with saline until clear, and then flushed with formalin. The stents were removed from the grafts, and a radionuclide well counter recorded radionuclide counts from each stent type at each period of blood contact. These values were converted to platelet density per 1000 mUm2. Stents were then photographed and scanned with electron microscopy (EM) for qualitative analysis. Possible significant differences in platelet adhesion with the three types of stents (both between stent groups and within stent groups) were examined using a two-tailed Student's f-test.

Results:

There were significantly fewer platelets adsorbed on PA versus T at all time periods (p < 0.005); on PL versus T at 60 and 120 minutes (p < 0.005); and on PA versus PL at 30 and 120 minutes (p < 0.0005). There was no significant difference in platelet density within each stent group (p = 0.1). Mean platelet density (number of platelets per 1000 mUm2 ± SD) was as follows: at 30 minutes: T = 1891 ± 965; PL = 373 ± 193; and PA = 27 ± 3; at 60 minutes: T = 6226 ± 1621; PL = 1573 ± 793; and PA = 1185 ± 710; at 120 minutes: T = 5307 ± 591; PL = 3164 ± 318; and PA = 180 ± 100. Gross inspection of the 120-minute groups demonstrated focal areas of thrombus on T, less on PL, and none on PA. Scanning EM demonstrated extensive platelet accumulation covering T at all time periods, less on PL, and even less on PA.

Conclusions:

Polymeric surface modification of tantalum stents with parylene and/or polyetherurethane can improve the acute thromboresistance of these devices; parylene appears to be the more thromboresistant of the two coatings.

Polymeric surface modifications of tantalum stents

PURPOSE

To compare two kinds of polymer-coated tantalum stents with bare tantalum stents (control) to determine if the coatings can improve thromboresistance.

METHODS

Twenty-seven Fontaine-Dake stents were balloon expanded in three 8-mm x 80-cm polytetrafluoroethylene (PTFE) grafts; 9 stents were bare tantalum (T); 9 were coated with polyetherurethane (PL); and 9 were coated with parylene (PA). There were 9 stents placed in each graft as follows: 3 tantalum, 3 polyetherurethane, and 3 parylene. In swine whose platelets had been radiolabeled with indium 111, the ends of each stented graft were connected to 14F femoral and venous sheaths to create an ex vivo fistula. Each graft was exposed to blood for 30, 60, and 120 minutes. At the end of each test period, the stented grafts were disconnected from the sheaths, flushed with saline until clear, and then flushed with formalin. The stents were removed from the grafts, and a radionuclide well counter recorded radionuclide counts from each stent type at each period of blood contact. These values were converted to platelet density per 1000 microns 2. Stents were then photographed and scanned with electron microscopy (EM) for qualitative analysis. Possible significant differences in platelet adhesion with the three types of stents (both between stent groups and within stent groups) were examined using a two-tailed Student's t-test.

RESULTS

There were significantly fewer platelets adsorbed on PA versus T at all time periods (p < 0.005); on PL versus T at 60 and 120 minutes (p < 0.005); and on PA versus PL at 30 and 120 minutes (p < 0.0005). There was no significant difference in platelet density within each stent group (p = 0.1). Mean platelet density (number of platelets per 1000 microns 2 +/- SD) was as follows: at 30 minutes: T = 1891 +/- 965; PL = 373 +/- 193; and PA = 27 +/- 3; at 60 minutes: T = 6226 +/- 1621; PL = 1573 +/- 793; and PA = 1185 +/- 710; at 120 minutes: T = 5307 +/- 591; PL = 3164 +/- 318; and PA = 180 +/- 100. Gross inspection of the 120-minute groups demonstrated focal areas of thrombus on T, less on PL, and none on PA. Scanning EM demonstrated extensive platelet accumulation covering T at all time periods, less on PL, and even less on PA.

Use of polyetherurethane to improve the biocompatibility of vascular stents

PURPOSE

To investigate potential differential growth on neointima following overdilating arterial trauma with polyetherurethane-coated versus bare metallic stents in swine.

METHODS

Twelve specially constructed tantalum stents, 6 coated with polyetherurethane block copolymer and 6 uncoated, were overdilated by 25% in 12 normal renal arteries of six swine. The stents were harvested 8 weeks after implantation and prepared for histologic examination. Neointimal thickness was quantified and analyzed for significant differences between coated and uncoated prostheses.

RESULTS

All specimens demonstrated fractures of the internal elastic lamina consistent with vascular injury. There was significantly less neointimal formation (0.0001 < p < 0.05) in coated specimens as compared to uncoated controls in each test animal.

CONCLUSIONS

The vascular response to overdilating stent trauma appears to be moderated with the use of polyetherurethane block copolymer as compared to control.

Decreased platelet adherence of polymer-coated tantalum stents

PURPOSE

To compare the acute thrombogenicity of polyurethane-coated stents with that of bare tantalum stents.

MATERIALS AND METHODS

Thirty stents (15 coated with polyurethane) were balloon expanded in 8-mm x 80-cm sections of polytetrafluoroethylene grafts (six stents per graft). Under sterile conditions, 8- and 10-F vascular sheaths were placed percutaneously in the femoral artery and vein of a 100-lb (45-kg) swine. Platelets were labeled with indium-111 and reinjected for quantitative assay. The stented grafts were connected to the arterial and venous sheaths to create an ex vivo fistula. Each fistula was opened for 5, 10, 30, 60, or 120 minutes, exposing heparinized blood to fully expanded stents. The fistulas were closed, flushed with saline until clear, and fixed with formalin. Stents were explanted, placed in a radionuclide well counter, and scanned with electron microscopy.

RESULTS

Quantitatively, there were considerably fewer platelets on coated versus uncoated stents. At 60 minutes, coated stents averaged 12.93 platelets per 1,000 microns 2 compared with 75.88 platelets per 1,000 microns 2 for bare metallic stents. At 120 minutes, there were 23.22 platelets/1,000 microns 2 versus 102.31 platelets/1,000 microns 2, respectively. Electron microscopy of coated stents demonstrated few scattered platelets at 5, 10, and 30 minutes. There was a uniform layer of platelets at 60 and 120 minutes. Uncoated stents demonstrated random areas of platelet clumping at 5, 10, and 30 minutes. At 60 and 120 minutes, uncoated stents showed extensive layering of platelets and fibrin.

CONCLUSION

Polyurethane coating decreases platelet adhesion, relative to bare tantalum, at all time intervals tested.