Iontophoretic injector designs.

These devices are for microinjecting soluble ionized material into cells. They deliver a precision regulated current of a few nanoamperes in the face of varying electrode resistance up to the compliance voltage limit of the device. In practice, when the driving voltage (compliance voltage of the current source) exceeds a few volts there is likely to be a problem that higher voltages will not cure. With very fine electrodes, once a few volts is reached the compliance voltage often quickly rises to the limiting compliance (supply) level and the current will no longer be what is set. This behaviour may be due to blockage of the electrode and power dissipation in the very small space causing cavitation and very low conductivity as a result.

These designs do work:  a number of devices have been built and are still in use since 1990. The designs are offered "as is" for whatever use can be made of them. I cannot consult on the building and troubleshooting of devices. The circuitry is not really that complicated. Pay attention to the floating "command voltage" supply in the line-operated unit. The battery-operated unit uses a different method in which the electrode is held at a "virtual ground" by the dual-FET buffer/op-amp sinking a current equivalent to that supplied by the command voltage and the series resistor. Note that the output is a monitor of the compliance-voltage drive level, and in all cases up to saturation, the delivered current should be precisely what is SET. With no injection current active the unit makes a fair electrode amplifier for measuring membrane potentials of static or slowly changing cells, but it is not really designed for fast potential measurements.

This circuit (without the command voltage and series resistor) is similar to the front-end configuration of some old pH meter designs (Fisher Accumet 600 for instance).

Schematic, Line-operated Iontophoretic Microinjector

Schematic, Battery Model Iontophoretic Microinjector

Dale Callaham