Note that for this type of potentiometric sensors it is esssential that the sense material
(the membrane) is conducting, because the circuit of measurement should be
“closed”. Insulating materials, which are sensitive to ions, can not be used.
Miniaturized versions of the glass-membrane electrode appear to be less stable and
moreover for in vivo monitoring these relative fragile electrodes can not be applied.
Therefore especially for biomedical applications new concepts had to be developed,
of which the ISFET is the first example (research started around 1970 by the author).