1. A system for positioning a cell or organism, which comprises: a microfluidic device comprising: a) a microstructure layer comprising a bottom layer and a plurality of microstructures on said bottom layer, wherein each of the microstructures comprises a plurality of columns on said bottom layer; and b) a cover layer comprising a positioning well and at least one inlet pool, wherein said at least one inlet pool and said positioning well are in fluid connection by a microchannel between said microstructure layer and said cover layer, wherein said cover layer is connected to said microstructure layer to form said microchannel, said microchannel being provided on the upper surface of said microstructure layer and/or on the lower surface of said cover layer, and said positioning well aligns with and is in fluid connection with said microstructures; and a cell or organism in at least one of the plurality of microstructures.