Neural probes made of flexible and soft materials typically have a limited shank length, restricting access to deeper brain areas. In this study, we developed single-shank polyimide-based neural probes with an ultra-long implantable shank, enabling penetration depths of several centimeters. The probes are composed of multiple components assembled using gold-gold thermocompression bonding after wafer-level fabrication, with the assembled devices exceeding a length of 300 mm. The tapered shank is approximately 200 µm wide, 15 µm thick and contains 32 linearly placed iridium-oxide microelectrodes (diameter: 30 µm; electrode pitch: 100 µm or 150 µm). Different probe designs have been realized based on the in-house microfabrication process (single or dual metal layers sandwiched between polyimide layers) and different microelectrode layouts (edge and center design). Here, we demonstrate the functionality of the devices based on preliminary results of in vitro and in vivo experiments. Functional microelectrodes had an average impedance magnitude of 200.45 ± 95.52 kΩ at 1 kHz (n=68 sites). The acute electrophysiological performance of the probes was validated in neocortical and hippocampal areas of anesthetized rats. We were able to record high quality local field potentials (including cortical slow waves and hippocampal gamma activity), as well as single- and multi-unit activity. We could isolate the activity of multiple individual neurons in these recordings with average spike waveform amplitudes of 110 µV. Concurrently with probe validation, we are developing methods for accurate and reliable probe implantation, as well as a brain tissue equivalent phantom to aid in implantation tests.