These systems have both electrical and mechanical components. Today there are many more fabrication techniques and materials available.
Sensors are non-invasive while actuators modify the environment.
Micro sensors are useful because their physical size allows them to be less invasive. Micro actuators are useful because the amount of work they perform on the environment is small and therefore can be very precise. Polysilicon Resonator Transducers are an example of a MEMS sensor using a stress controlled thin film polysilicon process which is a modification of integrated circuit fabrication techniques. The sensor uses a mechanically free beam of polysilicon whose resonant frequency can be measured electronically. It is interesting to note that the first MEMS device was a gold resonating MOS gate structure.