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I would suggest cryopreserving the iPCS in the vapor phase above the liquid nitrogen in the long run. The temperature of the vapor phase can reach between -140°C and -180°C. Although -80°C is another common temperature to store mammalian cells, the cells tend to reduce viability within months of storage. Research has reported that adding 10% Ficoll 70 to the 10% DMSO containing cryoprotectant makes cells frozen at -80°C for one year without loss of viability, compared with liquid nitrogen storage. You may find more details on the paper below.