ABSTRACT
The most common method for separating cells based on physical properties is centrifugation. In differential centrifugation, cells are suspended in a homogeneous solution and centrifuged, causing cells to sediment at different rates based on their size and density. If the cells are assumed to be a perfect sphere in a Newtonian §uid, the sedimentation velocity (V) can be calculated from a balance of forces acting on the cell, including the buoyancy force, gravitational force, and drag force:
= ρ − ρV r g2
9 ( )c f
(6.1)
where r is the cell radius μ is the §uid viscosity ρc and ρf are the densities of the cell and §uid, respectively g is the centrifugal acceleration
In density gradient centrifugation, a cell suspension is added to the top of a tube ¦lled with a solution varying from low density at the top to high density at the bottom
(Figure 6.1a). As the sample is centrifuged, bands of cells with similar sedimentation rates form. Differential and density gradient centrifugation can be used to separate cells in bulk, however, with relatively low purity.
