Capacitive touch sensors are entirely in the domain of DIY, requiring little more than a carefully-chosen conductive surface ...

The sensor forms an image and OpenCV detects the actual touch configuration. It appears you can use the raw data from the Arduino, too, but it might be a little harder.

The EasyTouch capacitive touch sensor can be used with either the Arduino platform or the Raspberry Pi mini PC using the EasyPlug breakout board if required.