Piezoelectric motors have been developed in the last years by many enterprises. This new technology is very interesting since it offers high forces and precision with miniature dimensions.
To have an idea, let’s consider SQUIGGLE micromotors 
developed by Newscale Technologies. Both linear and rotary micromotors are able to produce up to 5 N forces while being smaller than a coin. Despite their tiny dimensions (1.8 x 1.8 x 6 mm), these devices can be controlled with nanometer precision and are very robust.
As reported on the aforementioned website, “piezoelectric actuators change shape when electrically excited. A SQUIGGLE motor consists of several piezoelectric ceramic actuators attached to a threaded nut, with a mating threaded screw inside. Applying power to the actuators creates ultrasonic vibrations, causing the nut to vibrate in an orbit“.
The field of application of such micromotors is huge and it becomes especially interesting in the case of biomedical devices 😉
andreacollo 