Trypan blue

Trypan blue is an azo dye. It is a direct dye for cotton fabrics. In life sciences, it is used as a vital dye to selectively stain dead tissue or cells blue. Live cells or tissue with intact cell membranes do not stain. Because cells are highly selective for membrane-crossing compounds, a viable cell does not absorb trypan blue; however, it does cross the membrane in a dead cell. Therefore, dead cells appear a characteristic blue color under a microscope. Since living cells are excluded from staining, this staining method is also known as the dye exclusion method.