 Acid oxidation of carbon nanotubes can also be used in filtration to form buckypaper, but requires a high degree of surface acidic groups in order to obtain efficient dispersal in aqueous solution.An alternative casting method was developed in 2008 to produce buckypaper that did not require the use of surfactants or the acid oxidation of carbon nanotubes in order to obtain high-purity buckypaper for biomedical applications.The frit-compression system was adapted from a Solid phase extraction (SPE) column, where a suspension of carbon nanotubes is squeezed between two polypropylene frits (70 micrometre pore diameter) inside a syringe column.