A patch for sampling one or more analytes through the skin of a patient comprises an electrode layer for positioning adjacent to the skin of a patient； and means for actuating the electrode layer to induce the withdrawal of analytes through the skin by reverse iontophoresis. A first reservoir 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 adjacent to the skin to increase the conductivity between the electrode layer and the skin. Means are provided for transporting the analytes to a location where they are to be analysed. The patch may comprise a second reservoir containing a drug for transdermal delivery to the patient. An actuator may stretch and/or compress the reservoirs to expel their contents. The actuator may comprise a generally planar mesh formed from a shape memory alloy.
