Polymer-based laminar probes with an ultra-long flexible spiral-shaped cable for in vivo neural recordings

Compared to conventional rigid silicon probes, flexible penetrating neural implants exhibit improved mechanical compliance with brain tissue, enabling high-quality neural recordings over extended periods of time. However, the length of the implantable shank and extension cable of most flexible devices falls short of clinical electrodes used in diagnostics and treatment of neurological disorders. In this study, we demonstrate the design, fabrication, and in vivo validation of polyimide-based neural probes with a thin spiral-shaped cable that reaches a length of 27 centimeters, comparable to that of clinical electrodes. The spiral probe comprises a 3.9-mm-long, 75–280-µm-wide and 10-µm-thick implantable shank with 24 linearly placed gold microelectrodes (diameter: 20 µm; electrode pitch: 150 µm) placed either close to the shank edge or centered on the shank. The electrical impedance of the microelectrodes was ∼266 kΩ at 1 kHz measured in vitro. In acute rodent experiments, we recorded high-quality local field potentials (e.g., cortical slow waves and hippocampal gamma activity), single and multiunit activities. In addition, the probes could simultaneously detect the activity of about 10 well-isolated single units, with an average spike amplitude of 65 µV. Furthermore, devices chronically implanted in rats successfully recorded spiking activity over several consecutive days. As demonstrated here, the developed spiral probes can be applied to various tasks, such as the laminar analysis of brain oscillations or the study of the sleep-wake cycle in naturally sleeping animals. In conclusion, our flexible probe design may stimulate the development of novel implantable devices for application in large animal models or clinical settings.