Percoll

Percoll was developed from previously reported uses of colloidal silica nanoparticles coated with polysaccharides or polymers for rate zonal, isopycnic, or equilibrium centrifugal separations.

[4] Percoll itself specifically consists of polydisperse silica nanoparticles 15–30 nm diameter (23% w/w in water) which have been coated with polyvinylpyrrolidone (PVP).

Therefore, both discontinuous and continuous Percoll density gradients can be prepared weeks in advance, allowing great reproducibility & ease of use.

[6] Percoll was previously used in assisted reproductive technology (ART) to select sperm from semen by density gradient centrifugation, for use in techniques such as in vitro fertilization or intrauterine insemination.

The latter concern also applies to the use of Percoll with any other cells that might be injected back into a patient, since endotoxin can cause severe inflammation and fever.