The example provided by Alec_t is the closest to the study (taking all consideration into account)
I do understand that the focus and primary objective is
in fine the replication of a study, when there are no references other than the present work, this complicate the matter further.
If the configuration is not the focus and any circuit producing similar output is valid then clearly the original configuration is not the issue.
If there is a reasoning (portability, constraint due to external factors, temperatures or stability) who are taken into account then there is a reason to at least duplicate as closely as possible with a similar configuration.
Only focusing on µA circuits published there are example (from current limiting diodes such as j500, DC- DC controller with adjustable constant current) The issue in this case is voltage values and deviations.
"the constant current becomes 1μA and the voltage across the sample remains below 1V for all sample resistances up to 1 Megohm." This is exactly the answer.
A few patents discuss the issue of skin impedance (
in vivo in this case but
in vitro tissues depending on state of tissues can have a wide range of values as data indicate)
There is a patent that focus on DC - DC converter to ensure that the controller circuit avoid voltage drop and current regulation.
The controller circuit in this case is described as: series pass regulator comprising MOSFET transistor, resistors, capacitor and operational amplifier.