P37: Development of a Large Diameter In Vitro Flow Loop Thrombogenicity Test System

Purpose: To accommodate a wide range of medical device sizes for in vitro thrombogenicity testing, a larger flow loop system using 9.5 mm inner diameter (ID) tubing was developed and evaluated based on the 6.4 mm ID system that we previously used. Methods: Four cardiopulmonary bypass roller pumps were used concurrently to drive four different flow loops constructed with 9.5 mm ID polyvinyl chloride (PVC) tubing. To ensure that the four pumps produced a consistent blood flow rate (around 530 mL/min), a simplified dynamic roller-occlusion method was developed and applied. ACDA-anticoagulated bovine blood from live donors was obtained commercially, shipped overnight, and used within 24-36 hours of the blood draw. Immediately before starting each flow loop test, the blood was recalcified and heparinized to a donor-specific concentration (0.5 – 0.8 U/mL). The donor-specific heparin level was selected based on an established static pre-test and verified by an initial flow loop test with the following acceptance criteria: thrombus surface coverages of ≤10% on the negative control (PTFE) and ≥50% on the positive control (latex) materials, and platelet (PLT) reduction of ≤30% for the negative control and ≥90% for the positive control. For the dynamic testing, the heparinized blood was recirculated at 20 rpm for 1 hour at room temperature through each PVC tubing loop containing a test material. Six materials (3.2 mm OD, 12 cm long) with different thrombogenic potentials were assessed: negative control polytetrafluoroethylene (PTFE), positive control latex, BUNA rubber, silicone, abraded PTFE, and 3D printed nylon. Thrombogenicity potentials of the test materials were evaluated after the recirculation by measuring the thrombus surface coverage on test materials and platelet count reduction in the blood samples. Results: The in vitro flow loop test system effectively differentiated the known thrombogenic materials (latex, BUNA) from the historically thromboresistant PTFE (p<0.05 for PLT count reduction and thrombus surface coverage, Figure 1). Additionally, the 3D printed nylon and silicone test samples were shown to have an intermediate thrombogenicity level with significantly less thrombus surface coverage and PLT count reduction than latex and BUNA, but significantly more thrombus surface coverage than the PTFE (p<0.05). While the average PLT count reductions for the 3D printed nylon and silicone were also greater than that of PTFE, the differences were not statistically significant. Conclusion: The preliminary data suggest that the 9.5 mm ID test system is capable of differentiating materials of varying thrombogenicity potentials, and the different pumps can produce consistent results when appropriate occlusion settings are used. This system will be further investigated in an ASTM interlaboratory study to develop standardized best practices for performing in vitro dynamic thrombogenicity testing of medical devices and materials.Figure 1. A) Platelet count reduction, and B) Thrombus surface coverage on test material after 1 hour blood recirculation at room temperature. Data are shown as a mean ± standard error. n=4 for all materials, except BUNA and silicone (n=3). *p<0.05.