The practicability of an implantable artificial heart is strongly dependent on the availability of a high performance, compact, and lightweight thermal insulation system to contain the radioisotope energy source. The insulation system must limit heat losses to less than 0.024 W/cm² to maintain an acceptable thermal efficiency with the Stirling engine heat source at 870°C (1600°F). On the basis of the overall system requirements, vacuum metal foil insulation is the only acceptable choice. Concentric nickel foil cups were utilized in place of the standard spiral-wrapped foil design with mitered corners, thereby eliminating a serious heat leak and the structural problems generally associated with the corners. Initial total system thermal performance tests have been very encouraging and predict that a system heat loss of 2 W can be met with about 35 to 40 foil cup layers. Since the implantable system is to be designed for a sealed ten-year lifetime under static vacuum, measurements are also being made to determine the long-time performance and outgassing characteristics of the nickel packages.