Yang Zhang, Gierad Laput, and Chris Harrison have developed Electrick: low-cost touch sensing using Electric Field Tomography (EFT). In their paper on the topic, the authors show techniques and examples for adding interactivity to otherwise static objects.

The electrically conductive material used can be spray or brush-coated into irregularly-shaped objects (compared to more typical applications like smartphones and tablets), including laser-cut or 3d-printed items. A custom PCB featuring a Kinetis K20 MCU running Teensy 3.2 firmware hosts the voltage-controlled current source (VCCS), direct digital synthesizer (DDS), an ADC preamp, and a Bluetooth module for wireless data transmission. Read more on Chris or Yang‘s web sites, or check out the video below.