Electrochemical processing, packaging and characterization of high-density implantable neural probes for in-vivo recording

Implantable neural probes represent one the most revolutionary technology of the last years, having experienced a large growth because of the wide range of applications in which they can be adopted, spacing from the treatment of neurological disease to the Internet of Things. They aim to record or stimulate the neurons activity using thin needles to be implanted into the brain cortex, with a depth and location depending on the particular device purposes. The biggest challenge for a more massive use of human machine interfaces for biomedical applications is related to the capability of developing small devices with a high density of recording/stimulation sites, able to last long in a biological medium. That happens because unconventional materials for microfabrication processes have to be used, even if micromachining is not well developed for such materials. Thus, it is the case of stainless steel used to give robustness and flexibility to devices substrates, as well as Parylene C to ensure biocompatibility and good insulation performance: both of them have been adopted to realize the devices used in this thesis work. After the fabrication process, some customization procedures need to be adopted to improve quality of the surfaces. Electropolishing and electroetching are two valide candidates for this purpose: the procedures have been implemented to get hybrid flexible/stiff devices with smooth edges, reducing the possibility to damage the recording sites or changing the electrical insulator properties. In terms of packaging, correctly interfacing the device with the electrical setup for the recording operation could be challenging due the fragility of the device itself. This operation needs to be customized in such a way that the mechanical stress for the device is minimized, without losing in signal quality, in terms of parasitics and yield of the packaging procedure. Adhesive flip chip bonding and Zero Insertion Force (ZIF) connectors have been used to obtain this result, by interfacing the probe back-end with customized flexible Printed Circuit Boards (PCBs). Then, each device needs to be electrically characterized, to ensure good performance before the in-vivo implantation or to modify the fabrication process if major problems are encountered during the result analysis. Electrochemical Impedance Spectroscopy (EIS) is the technique adopted in this thesis work to analyze both the response of one single recording site and the crosstalk effects between two traces. The implemented setup uses a PCB board to switch between all the probe channels, resulting in an automatic routine that makes the process fast and increases reliability. Another important aspect of implantable devices characterization is the estimation of the mean time to failure after the insertion in a biological tissue. The device aging has been artificially carried on and the probe electrical properties have been analyzed over time starting from different aging conditions. Finally, the last step of the characterization is to test how each recording site responds to an electrical stimulus that emulates the neuron activity, before performing the in-vivo characterization directly on animals brain.