Apple file's a Haptic patent related to their Taptic Engine that introduces the use of an "Inductance Sensor"
Last Thursday a new Apple patent relating to their haptics device (the Taptic Engine), introduced the use of a new sensor called an inductance sensor. It could be used to give users the feel of a keyboard when using a virtual keyboard. In the long-term it could be used for a new type of metal button.
Source: patentlyapple.com