Although platelet-rich plasma (PRP) is relatively easy to produce by centrifugation of whole blood, yields of platelets may be variable, because many of them are trapped within the erythrocyte layer. 

Although they can be recovered by washing these cells with isotonic saline, it is a general rule that to avoid activation of the platelets, the number of centrifugations and resuspensions should be kept to a minimum. Another problem is that aspiration of the PRP must be performed carefully to avoid contamination from leucocytes in the buffy coat which lies atop the erythrocytes. 

To provide a highly purified platelet fraction from human blood Ford et al [1] layered whole blood over a density barrier of Nycodenz® (ρ = 1.063 g/ml) that allowed the erythrocytes and leucocytes to pellet during centrifugation at 350g. The platelets, because of their small size, sediment much more slowly; they form a broad band extending into the density barrier from just above the interface. 

The platelets recovered from this density barrier method have been used directly in aggregation studies; the Nycodenz® did not interfere with this process [1]. An iodixanol barrier of the same density can be substituted for the Nycodenz®; this has no effect on the separation or yield of platelets.