While sweat has been recognized as an interesting and easily accessible biofluid for non-invasive health monitoring, current lab-on-chip solutions require excessive sweat quantities to perform. That is, they either require the patient to sweat through exercise, or they must collect sweat and data over large skin areas.
That's two issues the researchers at the École Polytechnique Fédérale de Lausanne (EPFL), incidentally also staff at Swiss startup Xsensio SA, focused on solving. In a paper titled "Three-Dimensional Integrated Ultra-Low-Volume Passive Microfluidics with Ion-Sensitive Field-Effect Transistors for Multiparameter Wearable Sweat Analyzers" published in the ACS Nano journal, the researchers detail the heterogeneous integration of ISFET sensors with a biocompatible microfluidic interface to enhance the collection of sweat, all on a single silicon chip less than one square centimetre.
It has been shown that biomarkers in sweat can be directly correlated to their concentrations in blood, hence the choice of sweat as an analyte. The prototype described in the paper was aimed at two biomarkers, sodium and potassium, useful to individuate hormonal changes which prelude ovulation but also to diagnose cystic fibrosis.
Ion-sensitive FETs (ISFETs) have no metallic gate, instead the gate oxide is directly exposed to a liquid environment and the gate electrode is replaced with a reference electrode (RE). Hence the potential at the silicon surface is a function of the RE bias and the influence of the ions or charged molecules inside the analyte.