With the MTG, cooling rate depends on the ice interface velocity (V) and the thermal gradient (G).
The ice interface velocities were: 0.02, 0.2 and 2 mm/s, resulting in calculated cooling rates of 0.51, 5.1 and 51°C/min, respectively (cooling rate = G×V).
We evaluated the survival of HUCB MNCs after freeze-thawing and freeze-drying at different cooling rates, resulting from different ice interface velocities (V).
Figure 1 depicts the effect of ice interface velocity on cell survival, according to fluorescent Syto/PI staining after freeze-thawing and freeze-drying.
At a slow ice interface velocity (0.02 mm/s), large ice crystals are formed, minimizing the size of the unfrozen fraction, and thus causing mechanical damage to the cells and cell-shearing.
On the other hand, at the high ice interface velocity (2 mm/s), the size of the unfrozen fraction is large but the likelihood of intracellular ice formation increases, since water has less time to diffuse out of the cells during the freezing process [48].
When we evaluated the effect of ice interface velocity (Figure 1) on the viability of freeze-thawed and freeze-dried HUCB-derived MNCs, we obtained an inverted U-shaped viability curve in which the highest survival rate was achieved at the intermediate velocity of 0.2 mm/s.
Further away from the ice interface the liquid phase has a normal solute concentration with an undepressed freezing point.
Their ice growth never plateaued and their simulations are too short to allow proper sampling or protein diffusion at the ice interface.
Depending on the solute composition and the rate of growth of the ice front, solute rejection (including rejection of structures such as cells) can occur ahead of the advancing ice interface [5,14].
This osmotic gradient concentrates the solute in the liquid phase just in advance of the ice front that depresses the freezing point immediately in front of the ice interface [ 7].
