Currently many methods of drying are under investigation for the anhydrous preservation of mammalian cells, including convective drying (Bhowmick et al., 2003), vacuum drying (Elliott et al., 2002), room temperature desiccation cabinet drying (Acker et al., 2002), and freeze-drying (Wolkers et al., 2001). Independent of the drying method, cells are typically in an aqueous suspension at the onset. Generally the change in ‘system’ weight (cells and extracellular solution) is measured gravimetrically and this moisture loss in the system is assumed to equate to the loss of moisture within the suspended cells. This approach assumes that the intra- and extra cellular water contents are equivalent and that the dried sample is homogenous. These assumptions are not always valid. Oftentimes cells are seen to pool towards the middle of a droplet and macroscopic heterogeneities of moisture content within the droplet are sometimes visible by eye. Although important advances in mammalian cell preservation have been made using this approach, a convenient method of determining localized water content within such samples would advance the state-of-the-art in drying technologies. Because sugars are a common excipient used in drying, the current study investigates the spatial distribution of moisture content in dried sugar matrices using pyranine as a fluorescent probe of the local water environment. The pyranine molecule has two fluorescent states, emitting at 511 nm (green) and 440 nm (blue). Using calibration curves based on ratiometric measurements of blue to green pyranine fluorescence emissions the effect of drying substrate on moisture content uniformity is explored in pyranine-doped sucrose solutions.

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