BIO20: In Vitro Thrombogenicity Testing of Various Silicone Dispersions for VAD Inflow Cannula

Abstract

Study: Our group is developing next-generation ventricular cannula for use with the PediaFlow™ miniature totally magnetically levitated ventricular assist device. Various formulations of silicone elastomers are being investigated based on criteria of durometer and tear strength. In addition to mechanical properties, the dispersions employ various curing systems (e.g., platinum cure, acetoxy…) and solvent carriers (e.g., heptane, xylene…) which may affect their thrombogenicity. This study aimed to assess thrombus deposition on candidate materials with recalcified blood in vitro. Methods: Preliminary studies assessed a flat specimen of one-part acetoxy cure room-temperature-vulcanizing silicone compared to latex (positive control) and PTFE (negative control). Material samples 4.0x4.5 cm in size were cut lengthwise and both halves were placed in 10 mL no-additive BD Vacutainer blood collection tube as shown in Figure 1a. Tubes were filled with sheep whole blood in CPD (Lampire Biological Laboratories). The blood was heparinized and recalcified to achieve a target ACT value between 250-300 s. The tubes were rotated on a modified hot dog roller, shown on Figure 1b, at 2.5 RPM for 30 minutes. Afterwards samples were rinsed and assessed for gross thrombus deposition. Based on these preliminary studies, the protocol was optimized for further comparative testing of the remaining silicone candidates – which will be presented at the conference. Results: Figure 1d shows specimens after 30 minutes. While there was an appreciable difference between the positive and negative control materials, an artifact was suspected due to blood trapped between the sample and the wall of the tube. To circumvent this issue, an improved protocol was developed using short segments of PVC (Tygon®) tubing coated with the materials of interest. Latex tubing and uncoated PVC tubing will be used as the positive and negative controls, respectively.Figure 1. (a) Material sample placement in a 10 ml Vacutainer blood collection tube. (b) A hot dog roller with the heating element disabled was used to slowly spin the tubes. (c) The latex (positive control), PTFE (negative control), and Silicone samples before the test. (d) The three material samples after 30 minutes in whole blood.