The pumping diaphragm of the Texas Heart Institute (THI) E-Type ALVAD must perform the dual functions of providing a flexible blood interface and isolating the electrical actuator from adjacent fluids. Thus, protection is required against fluid leakage and moisture diffusion to prevent corrosion and damage to electrical actuator components. Average diffusion rates up to 1 ml per day through currently used elastomeric diaphragm materials have been measured during static in-vitro and in-vivo tests. To circumvent this problem, an improved pumping diaphragm has been recently developed for use with the electrically-actuated THI E-Type ALVAD. This trilaminar diaphragm consists of a composite Biomer and butyl rubber design. A.010 inch layer of butyl rubber (characterized by an extremely low diffusion rate for water, approximately 0 ml per day) is positioned between two Biomer layers (.020 and.010 inches in thickness). Initial invitro and in-vivo studies, in calves, indicate that this composite diaphragm provides an excellent barrier to water permeation, without sacrificing biocompatibility or structural integrity under conditions of chronic flexure.