Tobias Nyberg: Low Impedance 3D Polymer Microelectrodes for Neural Interfacing

Tobias Nyberg
School of Technology and Health, KTH Royal Institute of Technology, Stockholm, Sweden

Low Impedance 3D Polymer Microelectrodes for Neural Interfacing

Low impedance is a desirable characteristic for electrodes used in neural interfacing. A strategy to reduce the impedance of electrodes is to create a rough surface of the electrode which increases the area accessible to the electrolyte but retains the geometrical surface area of the electrode. We have fabricated conducting hydrogel electrodes where the bulk is accessible to the electrolyte thus giving an opportunity to use the height of the electrode as a parameter to decrease impedance. A low impedance of the interface is desirable for recording purposes. Correspondingly the low impedance is desirable for stimulation as it reduces the potential needed to drive a stimulus current over the interface; lowering the possible amount of noxious electrochemical products formed by elevated stimulus potentials. The stimulation and recording properties of conjugated polymer hydrogel microelectrodes based on polypyrrole or poly(3,4-ethylenedioxythiophene) (PEDOT) blends with poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) were investigated. The stimulation efficiency of PEDOT based electrodes at low stimulation potentials was evaluated for in vitro neural networks of cerebral cortex neurons and compared to the stimulation efficiency of indium tin oxide (ITO) electrodes. Polymer hydrogel microelectrodes with an impedance of less than 10 kohm at 1 kHz for a geometrical surface area of 1000 square micrometers were fabricated. Charge storage capacity of the polymer hydrogel electrodes was found to be proportional to the deposition charge, showing the high permeability of the bulk material. Polymer electrode stimulation evoked a greater response from neural networks than stimulation from ITO electrodes for the same potentials. Successful interfacing using polymer electrodes could be maintained for several months.