A patch for sampling one or more analytes through the skin of a patient comprises an electrode layer (7) for positioning adjacent to the skin of a patient； and means (1) for actuating the electrode layer (7) to induce the withdrawal of analyte through the skin by reverse iontophoresis. A first reservoir (4) in the patch contains an electrically conducting medium such as a liquid electrolyte, which can be controllably delivered onto a surface of the electrode layer (7) adjacent to the skin to increase the conductivity between the electrode layer (7) and the skin. Means (11) are provided for transporting the analytes to a location (13) where they are to be analysed. The patch may comprise a second reservoir containing a drug for transdermal delivery to the patient. An actuator (2) may stretch and/or compress the reservoirs (4) to expel their contents. The actuator (2) may comprise a generally planar mesh formed from a shape memory alloy.
