An electrothermal heater mat for anti-icing or de-icing of a helicopter rotor blade or other aerodynamic surface comprises a substrate, such as a flexible polyimide sheet, bearing tracks of a material of selected electrical resistivity, these tracks being formed by printing onto the substrate with a thermosetting ink loaded with electrically conductive (e.g. carbon) particles. Electrical bus bars/terminals for the supply of electrical energy to the resistive tracks may also be printed, using an ink loaded with particles of higher conductivity material such as copper or silver.