A medical substance delivery device including a movable control member 10, at least one transducer 16 responsive to movement of said control member 10 to convert mechanical energy into electrical energy to output an electrical signal, a drive circuit 20 for receiving said electrical signal and for outputting an output drive signal, and a display device 14 for receiving said output drive signal and displaying a variable image that varies consequent on said relative movement. The transducer 16 may be a piezoelectric element and the display 14 may be a bi-stable display e.g. an electronic paper display so that the display function is self-powered without a requirement for a battery or the like. The parameter displayed for a medical substance delivery device may be for example a dose volume, a count of does delivered, progress and/or completion of a dose, etc.
