Trypan blue is a dye used to determinecell viability by staining dead cells and tissues. In a live cell, trypan blue is not able to penetrate the cell membrane. On the other hand, trypan blue can penetrate through the cell membrane of a dead cell and enter the cytoplasm. Therefore, the blue color can only be seen with dead cells. Trypan blue has a wide range of uses including cell counting, dying cotton textiles, and can also be used in cataract surgery.