Percoll

Percoll does not notably diffuse over time, resulting in the formation of very stable gradients. 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. However, in 1996, Pharmacia sent out a letter to laboratories stating that Percoll should be used for research purposes only, not clinical. Pharmacia had not marketed it as a sperm preparation product; it had been repackaged and sold as such by third-party manufacturers. The FDA Warning Letter was due to concerns that PVP might cause damage to sperm (an unknown issue), and also that some batches of Percoll contained high levels of endotoxin (a known issue). 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. Since then it has been replaced with other colloids in the ART industry.^[7]