Biomedical applications
The pH ISFET chip is very suitable to be built-in in a catheter as has been done in the
period 1975-1985 by the pacemaker company Cordis in Roden, the Netherlands. The
reference electrode was made in the tip of the catheter which contained a polyHEMA
plug, a KCl gel and a Ag/AgCl electrode. The Al2O3 ISFET was mounted behind a
side window near the tip of the 6F catheter
Each ISFET was factory tested in vitro before sterilisation, and the essential
parameters, which will be dealt with below, are stored in a PROM (Programmable
Read Only Memory) which is an integral part of the ISFET connector. Experience
obtained from animal as well as clinical experiments has shown that the in vitro characterisation is also valid for the in vivo use of the devices. Therefore, the data
stored in the PROM connector also have an in vivo valuation and are used by the
floating signal conditioner to convert the ISFET output signal to the blood pH.
The signal conditioner applies the ISFET in the constant drain current mode, with
constant drain-source voltage, Vds, resulting in a gate-source voltage, Vgs, which
directly reflects the pH-sensitive interfacial potential at the gate surface.
If no temperature sensitivity and time drift would occur, the equation handled by the
signal conditioner can be simply: