Systematic evaluation of surgical insertion of flexible neural probe arrays into deeper brain targets using length modulation methods

<i>Objective</i>. Penetrating polymer-based microelectrode arrays (pMEAs) offer the potential for long term high-quality electrophysiological recordings of dynamic neural activity. Compared to rigid metal wire and silicon MEAs, improved device-tissue interface stability has been reported. However, accurate surgical placement of long, thin shanks in deeper brain regions is challenging as flexibility is achieved at the expense of axial stiffness. This study systematically evaluates then compares two pMEA placement strategies-dissolvable dip coating and molded brace, both with bare, exposed pMEA tips-to address the need for consistent, reliable, and accurate surgical targeting. These methods were selected based on the criteria of ease of fabrication, surgical feasibility, and mechanical performance.<i>Approach</i>. Sham (mechanical model with no electrodes) and fully functional pMEAs with shanks up to 5.5 mm long were fabricated and then modified using biodegradable polyethylene glycol (PEG) to support implantation. PEG was applied to shanks by motorized dip coating or a mechanical mold. Dissolution time and insertion in agarose gel brain models and rat cortex were evaluated followed by targeting of dip coated pMEAs to the rat hippocampus.<i>Main results</i>. Dip coating at high withdrawal speeds achieved uniform coating on shanks. Both strategies yielded similar critical buckling forces and insertion forces for single shank and arrayed pMEAs. Dip coated pMEAs were successfully placed in hippocampal regions without severe tissue damage as confirmed by histology and recordings obtained.<i>Significance</i>. Dip coating is a simpler method to prepare pMEAs for surgical targeting of deep brain regions compared to the bracing technique, as it does not require both a specialized mold and application process. This work provides a guide for researchers using single or multi-shank pMEAs to an accessible insertion strategy for implanting into deep brain regions in rodents and other small animal models.