AbstractThe implantable ventricular assist systems currently undergoing clinical readiness testing shuttle the displaced gas between the non‐blood side of the pumping diaphragm and an elastic chamber generally called a “compliance chamber” or variable volume device. The movement of the stored gas allows the pump to fill and empty without compression or expansion of the gas behind the pump diaphragm. The material used for the construction of compliance chambers should be fatigue resistant to withstand the 63 million flexes per year of the blood pump. The material should also be biocompatible and highly impervious to gases. Significant diffusion of gases from the compliance system necessitates external make‐up gases to somehow be added to the internal system. Material selection is complicated by the fact that most fatigue‐resistant elastomers also have high gas permeability. In order to solve this problem, bilayer compliance chambers have been developed using biocompatible and fatigue‐resistant polyolefin rubber comolded with relatively impervious b