Quantitative assessment of intracellular/extracellular dimethyl sulfoxide concentrations during freezing with low-temperature confocal Raman micro-spectroscopy

Although cryopreservation plays an indispensable role in the clinical application of cell therapy, the research on the osmotic behavior of cells during freezing is still at the level of theoretical models, and quantitative experimental data are still lacking. Therefore, the Raman spectra of dimethyl sulfoxide (DMSO) solutions with different standard concentrations (5%–80% v/v) were recorded experimentally to establish a quantitative evaluation method with the intensity ratio of different labeled peaks to the hydrogen bonding peak (as the internal standard) of water molecules in relation to different DMSO concentrations. By using this method, the characteristics of quantitative changes in intra- and extracellular concentrations under three different freezing methods were explored, including direct freezing, ice seeding freezing and vitrification. It was found that the intracellular concentration (@ −50 °C) after the ice seeding (@ −7 °C) freezing (1 °C min−1) method could reach 41.6%–49.2%, significantly higher than that of the direct freezing method (1 °C min−1 to −50 °C) of 32.4%–39.1%. Moreover, it is worth noting that the quantitative values of concentrations (@ −50 °C) of the ice seeding freezing are more consistent with the primary saturation curve of the DMSO solution. Thus, for the first time, it was revealed from the experimental data that the fundamental reason for the improvement of cell survival after ice seeding operation was pre-dehydration, higher concentration and smaller osmotic pressure difference between the inside and outside of the cell. These results also confirmed the validity of the famous two-factor hypothesis and more work will be carried out in depth.