I suppose that you need an isolated supply with minimum leakage current and almost no load current?

Then, a simple one cell hearing aid battery with a voltage divider will work. You will lose power in the voltage divider, yes. But with miniscule power consumption - does it matter?

Polarity reversal. You can use a mid-point to get a plus and a minus potential. Then switch between plus and minus with a couple of low power FET which you control from an astable FF (old school).

If you need remote control- there's IoT to help.

If you want continuous operation over many months/years, you will need energy harvesting or a plain mains connection. Mains is simple, but then there's the leakage current problem. Two transformers with the low voltage secondary of the first supplying a second transformer and the mid-point of the first secondary connected to the specimen's bulk eliminates the leakage. You do not need a true midpoint tap, a potentiometer with slide grounded works well, sometimes better since you can null out leakage by adjusting the potentiometer.

Then, there must be lots of such products available commercially. No luck there?

Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.

Simple : A voltage divider and a buffer that can supply the load current

any question: email daveross100 at gmail dat com

To have some accuracy, a voltage reference IC should be used, followed by a voltage divider and op amp buffer as suggested by Daveross. Power it from a couple AA battery cells as suggested by Skogsgurra and you have a device that just by switching the electrodes out of it you switch polarity.

Sounds like you're doing some experiments in a biological wet lab.