Differential sensing with imprinted and non-imprinted organic electrochemical transistors reduces calibration drift in wearable sweat cortisol sensors
The output of a wearable cortisol sensor drifts over days for reasons unrelated to cortisol, so a calibration made on the first day soon stops being valid. In our differential organic electrochemical transistor, a PEDOT:PSS transistor coated with a molecularly imprinted polymer (MIP) for cortisol is paired with one coated with a non-imprinted polymer (NIP) of the same composition, and the NIP response is subtracted. Over four calibrations in artificial sweat across 14 days, the response of the MIP alone shifted by 15% to 22% of its range on day 1, whereas the differential output shifted by 1% to 10%, and the variation between days at 1 to 10nM fell by 65% to 70% on average for individual pairs. In sweat from one participant, read against the calibration from day 1, both outputs tracked the direction of visible changes in salivary cortisol, while the MIP alone diverged from the differential output, consistent with the laboratory drift.