Pcb wave motors and piezo acoustic actuators have been around for quite a while. Usally making up in size and simplicity what they lack in torque and speed.
Recent development call for a new look at the plausibility to utilize these motion technologies in 3d printing. Lacking absolute position control these motors depend on a feedback loop which enable very high precision.
Here's an illustration of how a linear traveling wave motor could look when applied as an direct drive in a filament extruder.