A device for delivering an agent to the skin of a subject comprising: (a) an anhydrous agent, wherein the agent is a pharmaceutical agent or a cosmetic agent； (b) an anhydrous heating component； and (c) a reservoir containing a first solvent and a second solvent, wherein in use solvent released from the reservoir hydrates the anhydrous agent to produce a solution and hydrates the anhydrous heating component which reacts exothermically thereby heating the solution, is provided. Preferably in use the first solvent is removed from the solution over time thereby concentrating the agent in the solution. Preferably the device comprises (i) a backing layer, wherein the backing layer is permeable to vapour of the first solvent； (ii) a solid substrate, wherein in use a portion of the first solvent is removed from the solution by adsorption onto the solid substrate； or (iii) a copolymer, wherein in use a portion of the first solvent is separated from the solution by selective absorption by the copolymer. Preferably the device comprises (i) a backing layer, wherein the backing layer is permeable to vapour of the first solvent； and (ii) a solid substrate, in particular activated carbon, wherein in use a portion of the first solvent is removed from the solution by adsorption onto the solid substrate. Preferably the first solvent is a solvent with a lower boiling point than water, in particular ethanol, and the second solvent is water. Preferably the anhydrous heating component is calcium chloride and/or magnesium sulphate.
