How to Design a Freshwater Underwater Electrode Array for Localized mV Voltage Control

Hi all, first of all I am really no expert (I am a biologist), so I apologize in case for the naivety of my inquiry. I have some ideas in mind for a project and I would like to get some inputs in order to see what's feasible or not and what's the best way to do it.

I would need to design an underwater electrode array (no salty water, only fresh) that would produce "localized" or patterned voltage changes in the order of mV (1-20 mV). It does not really matter if the changes are positive or negative (they should be heterogeneous across the array though), however it could be nice to be able to control their sign. They definitely would need to be switched ON and OFF by some external trigger but they should also have the chance to stay "ON" for long times (hours or days). They should be shaped in a way that would allow an array arrangement (I was thinking about some sort of "tile"-like flat electrode). The tricky part would be to have the circuit insulated but still able to produce detectable signals from the outside. The water pressure would not be super high... I plan to use them in very shallow water (20cm) on a 80x80cm surface.

I though one way could be to use LEDs as when they light ON they cause a drop in the circuit voltage of some few mV (1-5 I guess). LEDs would have to advantage to be also visible and so if one has an array (as I plan to build) one could identify where the voltage change occurred. the array will be put in a square glass tank and the activity of the electrodes will be monitored through other recording electrodes.

Ultimately the idea is to have an array of electrodes which activity can be patterned and programmed via software (or a simpler arduino-like solution).

Any feedback on this or any idea on which materials/electrodes would be best to use in such a configuration ?
As an alternative, would it make sense to use electromagnetic field through the glass tank ? I hope all this doesn't sound too stupid

Thanks a lot !

Hello Livio! How are you? Thanks for asking here on EEWeb!By the way, I just need some further information to make a meaningful appraisal of your idea.
What do you need the electrode array for? Is it a proof of concept or do you need it to function for a research endeavor related to biology?Thanks and best regards,Justin

Thanks Justin, just proof of concept

I agree with Justin that it would help to have a clearer idea what you are trying to do.That being said, there are several parts to this design which it may be better to address separately.First, you will need a circuit to create the voltage that you want. This could include a way to adjust the voltage over some range. Your idea of using LEDs may not work since they are usually have 2 or 3 volts across them. I would be inclined to use a potentiometer and a simple op-amp circuit for this
Then you need a way to distribute the voltage to the electrodes and select what electrodes to send the voltage to. Depending on how many electrodes you have, you could use a set of DIP switches (one for each electrode). You may also need more op-amps to buffer the signal.You need to select materials for the electrodes and figure out how to attach the wires. This is likely to require some experimentation. Water is not generally friendly to electronics so you would want to seal the connections or maybe the electrodes.You also said you want to be able to trigger the voltage on and off so you need some sort of timer circuit.Finally, you will need a power supply.If you provide a bit more detail, we can help with more specific ideas.

Ok thanks both. Here I attach a schematic representation of the setup. Briefly: the array would be submerged underwater (depth 20-40 cm max). A gold coated PCB design will be used to create a grid of electrodes (hexagonal arrangement of circular electrodes could also be a good idea, instead of a rectangular grid). So far for the material I have no better option in mind. The main requirements would be: resistant to prolonged water immersion (i.e. several days before eventual cleaning). It should be able to produce +/- 10-20 mV at each electrode without getting encrusted or coated too much with electrolites (eventually cleaning could be done once a week or 2). It should be possible to pattern the activation of these electrodes both topographically but also in terms of intensity (this latter is not a strict requirement, all electrodes could also produce the same current intensity - or potential change - if the circuit gets too complicated). There should be the chance to shift (at least on X and Y axes) the pattern based on the movement of objects over the array. The object movement could be tracked using a camera and OpenCV. Eventually the coordinates of the tracked object can be attributed to the center-of-origin of the array so that it shifts according to the object movement. It could be nice to be able to apply rotations as well (in case of non circular objects). For this reason, the activity pattern should be set using some kind of coordinate-system reflecting the real coordinates of the array.
I hope is clear enough, 
Thanks for your feedback !

Livio

I think the biggest problem will be metal migration, even in non salty water this will happen. Would you be able to reverse the direction of current flow every few minutes ?

not really but I could add in the water tank some other electrode to act as a "cleaner" ...alternatively I was thinking to use to some conductive textile to coat the  PCB but in that case I am not sure it will avoid the problem and maybe it could also distort the electric field "resolution"created by the activated pattern of electrodes.

I forgot the attachment sorry. If you have any input on the design of course I appreciate !