Electrode Impedance Measurements and Electroplating

Knowing the impedance of electrodes used in electrical stimulation and recording experiments can be very important for a number of reasons including assessing the voltage delivered in current controlled experiments and vice versa. Measuring the impedance will also quickly let you now if there is an unintented open or short circuit. Additionally, impedance measurements made over time will give you an idea about the extent of gliosis which may be particularly useful in in vivo studies.

Measuring Electrode Impedances with NeuroRighter

1. When you open NeuroRighter, on the top left you would see the tab 'System'. When you select that, you will see a drop down menu with an option - 'Hardware settings'. Click on that. Now you will see a pop up which asks 'This will overwrite the curent spike filter. Do you want to proceed?'. Say Yes. When you do that you will see a Settings screen with a Tab called Misc. Once you select this tab, you will see a screen similar to what you see here:

2. Check the box which says Enable Impedance measurements. Then select the NI-DAQ Device that you intent you use for impedance measurements. Make sure that the device you select has pins ai2 and ai3 connected to the V and I terminals of the stimulator interface board (either BNC or screw terminals). V should be connected to ai2 and I should be connected to ai3. Now, you will see a new tab 'Impedance' created alongside the already existing tabs in the software's GUI. 

3. Next, you will have to select a device for sending stimulation output to your electrode (A frequency varying sinusoidal output will be sent to the electrode for measuring impedance). You can do this by selecting System>Hardware Settings> Output.Here, select the check box which says 'Use stimulator' and choose the device that you will use for sending output to your electrode setup. 

4. If you have a MUX circuit connected to your electrodes, Neurorighter will be able to measure the impedance of all channels (electrodes) one after another. You will also be able to measure the impedance of one particular channel (selected through the MUX). If this is the case, you will have to set that up in hardware by selecting the System>Hardware Settings> Output tab and selecting the type of MUX (8 channel or 16 channel). Impedance measurements can also be done in the absence of the MUX circuit. For this you will simply need to connect the stimulation output from the stimulator interface board directly to the electrode that you want to measure the impedance of and a return electrode should be connected to ground. The circuit design of a simple PCB that will let you measure the impedance of individual electrodes (selected through switches) of a MCS 64 channel MEA is uploaded in this page (see file Impedance_Electroplating_switch_board.zip below). The spring pins for this PCB can be purchased from digikey http://www.digikey.com/. Part number

MILL-MAX:  0934-0-15-20-74-14-26-0 ; CONN PIN SPRING-LOAD .198 20GOLD. You need 4 of this.

5. In this screen, you will also be able to select the Device that does I/V control. I/V control is a signal (5 V for current controlled or 0 V for voltage controlled) that is sent to simulator interface board to let it know if you wish to perform current controlled or voltage controlled stimulation. The default is set to voltage controlled. If you wish to change this to current controlled, then connect a device's pin: to the I/V wire that goes to the stimulator interface board (see page on hardware setup for a better description of this wire). Now select this device in the drop-down menu in the software.Hit Accept to save the changes.

6. Now select the newly created Impedance tab. You will see a screen as below: Select the channel that you want to measure the impedance of. If you want to measure the impedance of all channels, select the All channels check box. Remember this can be done only if you have a MUX circuit wired up and have set the software settings correctly to reflect this.If you don't have a MUX circuit and want to measure the impedance of an electrode wired up as explained in the above steps, select channel 1. Select if you wish curent or voltage controlled stimulation. Also select the values of 

Impedance Testing/Electroplating Switchboard

Aaron Morris designed a PCB that can be used to interface NeuroRighter's hardware with an MCS 60 channel planar MEA. This is useful for procedures in which you would not want to use a an expensive multichannel amplifier for MEA interfacing (e.g. sonicoplating http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2871717/). It is also useful for those who do not have an extra amplifier laying around and still want to perform impedance measurements or electroplating.