The working principle of Micropipette depends on what kind of Micropipette you are using. Air Displacement Micropipette works on the air displacement principle. It consists of a piston which aspirates and dispenses liquid as the air pocket moves up and down respectively. The internal mechanism of the Micropipette does not come in direct contact with the sample.
In Positive Displacement Micropipettes, the piston comes in the direct contact of the sample. The disposable tip in a positive displacement micropipette is a microsyringe composed of a capillary and a piston (movable inner part) which directly displaces the liquid.