A flexible electrode array consisting of a thin metal film on a polymer substrate has been developed for neural implantation in rats. The biocompatible arrays record cortical brain signals from awake and mobile rats in order to gather significant neurological data. Four point bend testing of the metal-Kapton system has been used to characterize the interfacial toughness, and therefore the mechanical durability, of the array. Several different adhesion layers on were evaluated using this method. Use of a titanium-tungsten interlayer increases the mixed-mode fracture toughness from approximately 1 J/m2 to approximately 2 J/m2, while a titanium interlayer provides a toughness of more than 4 J/m2. Gold-Kapton arrays were implanted in rats for periods exceeding 200 days, and neural recordings were taken frequently. The arrays exhibit excellent long-term reliability, with no decrease in signal recording capability over the course of the implantation.