A multiplexed, transdermal extraction and detection system for non-invasive monitoring of one or more substances in a subject, the system comprising: (i) an array of individually addressable sensor pixels, each sensor pixel comprising: (a) a substrate comprising a set of electrodes (1, 2) for applying a current to the subject's skin for performing iontophoresis to transdermally extract one or more substances from the interstitial fluid by electro-migration and/or by electro-osmosis; (b) a reservoir containing a volume of gel for receiving the one or more transdermally extracted substances; and (c) a set of detection electrodes (2, 3, 4) for electrochemical detection of the concentration of the one or more transdermally extracted substances present in the reservoir; wherein the array of sensor pixels is dimensioned so that at least one of the sensor pixels is capable of extracting the one or more substances via a low-resistance preferential pathway associated with a skin appendage at the subject's skin such as a skin pore, hair follicle, or sweat gland; and (ii) a data processing system configured to: analyse concentrations of the one or more substances present in the individual gel reservoirs of each sensor pixel in the array; based on the concentrations, determine which of the sensor pixels extracted the one or more substances via a preferential pathway; and use the concentrations of the one or more substances extracted via a preferential pathway to estimate the concentration of the one or more substances in the subject. (i) an array of individually addressable sensor pixels, each sensor pixel comprising: (a) a substrate comprising a set of electrodes (1, 2) for applying a current to the subject's skin for performing iontophoresis to transdermally extract one or more substances from the interstitial fluid by electro-migration and/or by electro-osmosis; (b) a reservoir containing a volume of gel for receiving the one or more transdermally extracted substances; and (c) a set of detection electrodes (2, 3, 4) for electrochemical detection of the concentration of the one or more transdermally extracted substances present in the reservoir; wherein the array of sensor pixels is dimensioned so that at least one of the sensor pixels is capable of extracting the one or more substances via a low-resistance preferential pathway associated with a skin appendage at the subject's skin such as a skin pore, hair follicle, or sweat gland; and (a) a substrate comprising a set of electrodes (1, 2) for applying a current to the subject's skin for performing iontophoresis to transdermally extract one or more substances from the interstitial fluid by electro-migration and/or by electro-osmosis; (b) a reservoir containing a volume of gel for receiving the one or more transdermally extracted substances; and (c) a set of detection electrodes (2, 3, 4) for electrochemical detection of the concentration of the one or more transdermally extracted substances present in the reservoir; (ii) a data processing system configured to: analyse concentrations of the one or more substances present in the individual gel reservoirs of each sensor pixel in the array; based on the concentrations, determine which of the sensor pixels extracted the one or more substances via a preferential pathway; and use the concentrations of the one or more substances extracted via a preferential pathway to estimate the concentration of the one or more substances in the subject. analyse concentrations of the one or more substances present in the individual gel reservoirs of each sensor pixel in the array; based on the concentrations, determine which of the sensor pixels extracted the one or more substances via a preferential pathway; and use the concentrations of the one or more substances extracted via a preferential pathway to estimate the concentration of the one or more substances in the subject.