The inkjet printing technology has a print head to eject and direct liquid droplets at a precise location on a paper or flexible substrate. Inkjet printing methods are not just limited to printing of color inks on paper, but they also apply to printing of functional devices. Alternatively, inkjet printing methods or additive manufacturing can help rapid prototyping of MEMS devices, albeit larger at a sub-millimeter size. Successful making of MEMS devices required many runs and optimization of established micro-fabrication processes. Silicon micro-machining of MEMS devices is done in batch and often has a low yield at the initial trials. Yet, silicon micro-machining is expensive and tedious in steps, involving photolithography, material addition and material subtraction. MEMS are mostly fabricated on silicon substrate, which is the choice for fabricating integrated microelectronics. MEMS consist of moving micro-structural elements, integrated electrodes and even active material under the control of integrated microelectronics. The actuation principles include : (1) electrostatic attraction to close the air gap between a movable electrode and a stationery one (2) converse piezoelectric effect to induce a strain in the piezoelectric ceramic subjected to an electric field (3) solid thermal expansion as induced by resistive heating.
Sensing principles of MEMS include one of the following : (1) capacitive sensing to measure an electrode displacement with the output of capacitance change (2) piezo-resistive sensing to measure a stress or strain in a deformable element with the output of a resistance change (3) piezoelectric sensing to measure the stress in a piezoelectric ceramic with the output of electric charge. For example, a micro-mirror array is used to switch pixels of the light projection display (i.e., Texas Digital Light Processor). MEMS actuators can convert an electrical input into a controllable output of mechanical response. For example, MEMS accelerometers and gyroscopes are used in a smartphone to sense the orientation/rotation of a display screen.
MEMS sensors can convert an input of a mechanical measurand into an electrical output signal. Micro-electro-mechanical systems (MEMS) are sensors or actuators of micro-meter to sub-millimeter size.
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