A sensor is provided for sensing layer removal from a surface. A head has an abrasive portion for contacting the surface and a conducting portion inside the head. This device has a sensor design which provides self-generation of a sensor signal with no power supply to the device head. This simplifies the design of the sensor head, and means it can have a design which is easy to clean with no parts which are easily damaged. In one set of examples, the device is a skin treatment device.