Ultra Micro Neural Stimulation and Recording Electrodes
Authors: Ghazavi Atefeh , Leon Carlos Ricardo , Cogan Stuart F. 
Published: February 2020

With recent interest in increasing selectivity and reducing foreign body reaction to neural implants by using small-scale indwelling electrode arrays, the necessity of using ultramicroelectrodes (UME) in the implant design is inevitable. UMEs have been widely used as sensors for detecting biological elements such as neurotransmitters and oxygen in living tissue. Thus, their behavior in vitro under similar experimental conditions has been well investigated by performing electrochemical experiments such as cyclic voltammetry and chronoamperometry. However, the behavior of UMEs under neural stimulation and recording conditions has not been well characterized.

In this study we have investigated the behavior of UMEs in cyclic voltammetry, electrochemical impedance spectroscopy, chronopotentiometry, and chronoamperometry experiments in phosphate buffered saline (PBS) electrolyte. Comparisons are made between macro, micro, and ultra micro platinum electrode behavior in PBS with ionic reactive species, hydrogen and hydroxyl ions, in galvanostatic and potentiostatic experiments.

As migration is also a charge transfer mechanisms besides diffusion in electrochemical cells with ionic reactive species, the effect of hemispherical diffusion on the total current flux in such conditions has been evaluated by means of finite element analysis. A transient two-dimensional axisymmetric finite element model is developed to solve the Poisson-Nernst-Planck equation (Equations (1-2)) using COMSOL Multiphysics software. It has been assumed that the ionic flux is due to migration and diffusion and there is no convection flux.

where V is the electric potential, ε the permittivity of the electrolyte, e the electron charge, ci the ionic concentration of ion i, Ni the flux of ion i, Di the diffusion coefficient of ion I, R the Boltzmann constant, T the temperature, zi the balance of ion i, umi the mobility of ion I, F the Faraday constant. Butler-Volmer equation (Equation 3) was assigned to the electrode-electrolyte interface to represent the interfacial reactions.

where iloc is the local charge transfer current density, i0 the exchange current density, cr is the concentration of reduced species, co the concentration of oxidized species, crb the bulk concentration of reduced species, cob bulk concentration of oxidized species, η the overpotential, ac the cathodic transfer coefficient. Gouy-Chapman-Stern model was used to simulate the electric double layer. Based on the Gouy-Chapman-Stern model the electric double layer is the capacitance of the Stern and Gouy-Chapman layers in series (Equations (4-5)).

Where CH is the Stern layer capacitance, CGc the Gouy-Chapman layer capacitance, εr the relative permittivity of the electrolyte, ε0 the electric permittivity of free space,λD the Debye length, e the elementary charge, UT the thermal voltage, c0 the concentration of ions in the bulk.

Cyclic voltammetry experiments were performed in PBS electrolyte purged with Argon gas with a scan rate of 50 mV/s. Charge storage capacity of a 10 mm diameter UME, 100 mm diameter microelectrode, and 1.6 mm diameter macroelectrode platinum electrodes (Bioanalytical Systems) in PBS electrolyte were compared. Upon 99% decrease in geometric surface area, the increase in cathodic and anodic charge storage capacities are 82% and 140% from microelectrode to UME, while these increases are only 16% and 11% from macro to microelectrode, respectively.

Figure 1