Circuit for Current Injection and Voltage Reading
The circuit looks OK as far as it goes, but I have two observations. 

a) Is the material capable of polarisation? If so, then uni-directional current will steadily change the apparent conductivity over reading time. Examples would be damp earth for instance, or skin under gel electrodes. 

A mitigation for this would be either to make your current source switchable polarity, or make the analogue multiplexers switchable between ground and current source. While full AC would work, it would complicate detection by your DAQ, and short periods of DC excitation, reversed between readings to provide zero average current, may work well enough.

b) If your multiplexers are switchable between GND and the current source, and you use a bipolar current source, then you need only 1:8 multiplexers, not 1:16, which may be a worthwhile hardware or tracking saving. Connect the outputs of the two multiplexers alternately to the electrodes, and you still have full control over which pair of electrodes receives which polarity of excitation.