Each well is covered with a very thin layer of gold. The research prototype was a chip 17 millimetres square and 310 microns thick and contained 34 wells, though a chip this size could accommodate 1,000 wells. This gold coating is the clever part.
Gold is a chemically inert substance.
It doesn't rust, and this chemical laziness explains why it is non-toxic. The chip is designed as a microcircuit in which the golden lids of the wells form anodes. Changing the voltage of the circuit allows the controlled dissolution of the gold. There are no moving parts. The researchers show that the circuit can be designed in such a way that the contents of only selected wells can be released, rather than all the wells at once.