J Electr Bioimp, vol. 1, pp. 84–92, 2010 Supplementary information Supplementary information All simulations were performed taking into account the electrode interface components at thee experimental PRF of 375kHz (frequency at which the solution resistance is extracted during actual measurements). measurements) The electrode interface behaviour was modelled based on a thin layer approximation similar to the one implemented in Cantrell DR, Inayat S, Taflove A, Ruoff RS, Troy JB. Incorporation of the electrodeelectrolyte electrode electrolyte interface into finite finiteelement element models of metal microelectrodes. [Internet]. [Internet]. Journal Journal of neural engineering. 2008; 2008 5(1):54 5(1):5467.Available 67.Available from: http://www.ncbi.nlm.nih.gov/pubmed/18310811 I. Refer to the section Materials and methods (subheading Electrode design and fabrication) fabrication (1.5 ⋅m) for an excitation of 25mV at Fig. 1 Quasistatic simulation imulation (Comsol 4.0a) of the electric field norm for the bipolar electrodes in Ringer’s solution (1.5 375kHz (frequency at which the solution resistance is extracted during actual measurements) measurements).. The penetration depth of electric field is achieved at 8.3m (on the zaxis) z where the electric field is half the value obtained on the axis (y=0) of the microprobe surface (1100V/m). The white band in the plot represents the zone for penetration depth across the geometry. The two thick black lines represent the electrodes. II. Refer to the section Materials and methods (subheading Measurement method and modelling) modelling) The cell constant from simulation of the bipolar electrodes can be calculated as follows. Using Comsol, an integration of current density norm over the return electrode gives a current of ~0.72A A for an applied excitation voltage on the electrodeV electrode a of 25mV 25mV. The he computed solution resistance, resistance Rsc is calculated as: Rsc = where, Ic – computed overall current in the system Leading to an Rsc = 34830. S1 Va Ic Supplementary information: Kasi et al., J Electr Bioimp, 1, 84-92, 2010 We know that cell constant, kbipolar in Ringer’s solution of known resistivity, ρRinger = 1.5⋅m can be calculated knowing the resistivity of the medium and the solution resistance as follows: kbipolar = Rsc ρ Ringer Hence, we obtain a cell constant of 23220m1 from simulation. III. Refer to the section Materials and methods (subheading Measurement apparatus and protocol)   ⋅ ⋅             ⋅                   Fig. 2 Resistivity (directly calculated from impedance at peak resistance frequency) stabilisation with time for measurements at three arbitrary depths (40m, 80m and 130m) in a rat retinal slice is presented above. After the microprobe was displaced to a depth in the retina, a series of impedance spectra were recorded within intervals of 15 seconds each. Based on the above resistivity changes with time, it was established that the time to wait before valid measurements be recorded was 30 seconds. S2 Supplementary information: Kasi et al., J Electr Bioimp, 1, 84-92, 2010 IV. Refer to the section Discussion (First paragraph on reduction of fringing effects) Using Comsol 4.0a, a DC simulation between bipolar electrodes with sharp and rounded corners was simulated. The current density norm – surface and contour plots were generated at an exc excitation itation voltage of 25mV. Fig. 3 Comparison through a DC simulation of bipolar electrodes with sharp corners (left) and rounded corners (right) used in this sstudy. tudy. Sharp electrode edges produce high electric field zones that may lead to fringing effects and local tissue heating and damag damagee during an actual trial. Fig. 4 A zoom in the region of electrode corners reveals an improvement against fringing effects of rounded corner electrodes in comparison with their sharper counterparts. S3