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).
Line-operated Iontophoretic Microinjector
Model Iontophoretic Microinjector