Flexible 64 channel implantable electrode arrays have been produced which are able to record the surface field potentials from the cortex of rats to map electrical activity evoked from stimulating whiskers. High electrode density is required for adequate spatial resolution, and here a density of 2.2 contacts per square mm is achieved. The electrode arrays are implanted between the dura and the skull over the whisker barrels somatosensory cortex, slightly lateral of the midline and caudal of the bregma. These arrays consist of a polyimide backing, a patterned gold array, and a protective layer of SU-8 photoresist. The reliability of these materials for multi-day implantations requires good adhesion between the layers, and for successful implantation the overall shape must be planar. This paper will describe the processing steps required to generate near zero net residual stress in the device through control of layer thicknesses and heat treatments. Stresses and curvatures have been measured as a function of temperature to gauge suitability of materials thicknesses for both processing and the effect of multi-day implantation. The evoked response from multi-day implantations of the flexible arrays will be reported, with additional discussion of biocompatibility and electrode durability for short or long-term studies.  The adhesion of the gold to polyimide has been assessed via four point bending to improve both processing and reliability of the materials during use, and the impact of adhesion promoting layers and pre-treatments prior to deposition of the 300 nm gold layer will be discussed.