Alveolar (pneumocyte) type II cells are widely studied because they synthesize and secrete the phospholipidrich lung surfactant, which lines the air-alveolar interface and prevents alveolar collapse by lowering surface tension at low lung volumes. 

Isolation of these cells from both adult and foetal lung is an important prerequisite for their culture and study. Viscardi et al, who successfully developed a Nycodenz® gradient technique, emphasised the importance of the non-toxic, noninvasive properties of this gradient medium and pointed out that although Percoll.

Gradients had been used previously for purifying these cells, the potentially toxic nature of a polyvinyl-pyrollidone-coated silica colloid was of considerable concern in studies of their function.