Cells can be very sensitive - not only to culture conditions, but also to physical forces acting on them. These so-called shear forces can damage cells and thus influence their viability and proliferation - in the worst case leading to cell lysis or cell death. Shear forces cannot be avoided when pipetting cells in cell culture. The extent to which cells can sustain shear forces differs between different cell types. In general, cells should be pipetted slowly and carefully to reduce the shear forces acting on the cells at the tip orifice. Generally, the smaller the tip diameter, the greater the chance of shear forces acting on the cells. Additionally, make sure to pipette slowly and in a controlled manner. The aspiration of the cell suspension is more critical - so if you need to work faster, leave it at a slow aspiration of the cell suspension and consider a faster delivery. Electronic liquid handling devices offer controlled and smooth piston movements that are difficult to achieve and reproduce with mechanical devices.
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