Designing a reliable and effective ventricular assist device (VAD) presents many technical challenges. Problems include anatomical fit, sepsis, reliability, durability, low heat dissipation, high efficiency, minimal maintenance and monitoring. The HeartSaver VAD™ currently being developed by the Cardiovascular Devices Division of the University of Ottawa Heart Institute and World Heart Corporation is an innovative, new design which overcomes many of the technical challenges inherent to previous VAD designs. The current design concept uses silicone oil as a hydraulic fluid to displace a flexible polyurethane diaphragm which pumps blood out of the device. This design concept overcomes some difficult problems with VAD design. However, it also raises questions relating to the fact that polyurethanes are typically highly permeable materials and the loss of hydraulic fluid may adversely effect the performance of the VAD. This thesis documents an investigation into the permeability of silicone oil (decamethyltetrasiloxane) through a polymer diaphragm (Biospan - a segmented polyether-polyurethane) into whole bovine blood. The development of a test apparatus to determine the permeability rate of silicone oil through the diaphragm and a simple method for determining the diffusion coefficient of candidate biomaterials are presented. A method to extract silicone oil from whole blood for gas chromatography/mass spectrum (GC/MS) analysis is also discussed. The results from the experiments show that the silicone oil/polyurethane/whole blood system has a permeability coefficient of 4.1x10"n cm2/s, a diffusion coefficient of 3.1xl0"08 cm2/s and a solubility coefficient of .0013. Using these values (which are specific only for the test model) the loss of silicone oil through a 60.5 cm2 diaphragm with a thickness of .060 cm over one year would be 1.31 ml. The total volume of hydraulic fluid in the device is approximately 295 ml and the loss of 1.31 ml during a year is insignificant. The thesis concluded that analysis of blood for silicone oil contamination can be performed using GC/MS. Silicone oil does permeate through segmented polyetherrpolyurethanes at a slow rate.