multiplexing solution for high number of inputs in a grid pattern?
If your biosensors have low low Zout, you can simply scan the contacts, one contact at a time with confidence the multiplexor input capacitance will be fully charged by the biosensor. Your scan time would be 1/(250Hz * 16) or 4,000Hz or 250uSeconds.

If your multiplexors (perhaps X and Y) and amplifier Cin total 30pF, you can have 10Meg Ohm biosensor Rout, producing 300uS Tau.

So, are your biosensor well described by 10Meg Ohm Rout, able to charge 30pF thru the cell wall to that 10mV or 100mV activation potential? 

Note a single TAU of settling has 37% error. You need 90% (2 tau) or 3 or 4 tau, if you need 1% data measurement.

At 4 Tau, at 30pF, for 60 microsecond Tau, the biosensor R can only be 2 Mega Ohms.