VAD Biomer blood sacs: mechanical tests and ultrastructural observations

Ventricular Assist Devices (VADs) are a support to the heart function and are used as bridge to transplantation for a brief period in patients awaiting a donor heart. They consist of two sacs with two artificial valves that allow blood flow, the whole connected to a specific electronic equipment. This study examines the performance of a segmented-polyurethane elastomer, namely Biomer, that has been extensively used to manufacture pneumatically actuated VAD blood sacs. Specimens obtained from three VAD blood sacs explanted from three human patients and specimens from one unimplanted blood sac were investigated for mechanical performance of the material and for the changes it undergoes as a result of mechanical stress and interaction with the surrounding biological environment. The application of tensile strength at break yielded no relevant differences between unimplanted and explanted specimens. Biomer exhibited a high elongation at break for both groups, with somewhat lower values for explanted specimens. SEM observations carried out on the inner surface--in contact with blood--of unstressed explanted specimens showed no thrombotic deposits, calcifications, microbial contamination or structural changes. The complementary broken surfaces of stressed specimens showed a distinctive fracture pattern.