Choosing opamp with sufficient output current for Voltage Reference with high impedance load
The short-circuit current of the LTC2055 is actually several mA. Check the datasheet. It's micropower, but not that much. 

The current is limited to Avdd/78K + 1.5V/100K by the 78K resistance between the op-amp and the electrode. From that I conclude that your Avdd must be about 10V. 

You can reduce the resistances R15-R17 and increase the current. I have no idea whether that is safe from a biological point of view, but halving each of them would double the current.