What is the principle of real-time viability assay?

Real-time viability assay is used to determine the ability of tissues, cells, or organs to maintain or recover a state of survival. It is based on the principle that only live, viable cells possess intact cell membranes that exclude certain dyes such as eosin, trypan blue, and propidium. Dead cells take up the dye as they do not possess intact cell membranes and are unable to exclude the dye. 

In a real-time viability assay, the cell suspension is mixed with specific dyes and examined visually in real time to determine whether cells exclude or take up the dye.