Patterned Metal Grids for Flexible and Transparent Neural Microelectrode Arrays

Flexible and transparent microelectrodes can provide large-scale neural recordings with temporal and spatial resolution when used alongside functional calcium imaging. Patterned metal grids defined by direct laser writing (DWL) are a promising approach for these electrodes, as they resort to standard microfabrication processes and materials, allowing the possibility of mass production. For these reasons, a study exploring transparent grid-based electrodes using DWL for measuring electrocorticography signal was performed. Patterned metal grids with 1 μm of linewidth and 22 μm of spacing between lines showed a sheet resistance of 6 Ω/sq and a transmittance of 81% at 550 nm. The grids were transferred to a 5 μm Parylene-C membrane using an optimized procedure that involves an oxygen plasma pre-treatment. This procedure ensures mechanical robustness and stability of the grids. Finally, a flexible and transparent prototype was fabricated with a microelectrode array composed by 16 electrodes with 500 μm of diameter. These microelectrodes shown an impedance of 10 kΩ at 1 kHz in saline solution and they are highly conformal facilitating in vivo implantation and the recording of neural activity in the mouse cerebellum surface. To conclude, patterned metal grids based-electrodes exhibit a promising performance compared to transparent conductive oxides or graphene. Moreover, the introduction of DLW enables easy and fast manipulation of grid shape and dimensions without the need of physical masks, while keeping large scale compatibility, which is important for tools used in neuroscience community. ### Competing Interest Statement The authors have declared no competing interest. * AFM : Atomic Force Microscopy DLW : Direct Laser Writing ECoG : Electrocorticography EIS : Electrochemical Impedance Spectroscopy FDTD : Finite Difference Time Domain ITO : Indium Tin Oxide MEA : Microelectrode Array PMG : Patterned Metal Grid PR : Photoresist PVA : Polyvinyl Alcohol RIE : Reactive Ion Etching RMS : Root Mean Square SEM : Scanning Electron Microscopy ZIF : Zero Insertion Force