What medium should I use when isolating exosomes using ultracentrifugation and a density gradient?

For best results, most researchers use sucrose or another metabolically inert medium such as iodixanol, which is non-ionic and non-toxic to cells. Unlike sucrose, iodixanol is less viscous, and is capable of forming iso-osmotic solutions at all densities, thereby better preserving the size of the vesicles in the gradient.¹ This medium is currently available under the brand name OptiPrep™, a sterile, endotoxin-tested solution of 60% iodixanol in water with a density of 1.32 g/mL. The high density of OptiPrep™ facilitates the fractionation of cells by flotation from a dense load zone through either a continuous or discontinuous gradient, or through a simple density barrier.

[1] Dettenhofer M, Yu XF. Highly purified human immunodeficiency virus type 1 reveals a virtual absence of Vif in virions. J Virol. 73; 1460-7: (1999).