(23e) Microfluidic Phase Diagram of Sucrose Droplets: Freezing, Freeze-Concentrated Glass Transition, and Melting Temperatures

In the manufacturing of pharmaceuticals, drug product attributes must be controlled to ensure their stability, safety, and efficacy. The majority of biopharmaceuticals are stored either in the frozen or freeze-dried state to extend shelf life beyond those in the liquid state. As a result, there is a need to accurately characterize the physicochemical properties related to the liquid–solid phase transition. To this end, differential scanning calorimetry has previously been used extensively to identify important thermal features of a solution, such as the freeze-concentrated glass transition temperature. For some solutions, however, the signal is weak and multiple experiments at different experimental conditions may be needed if the signal can be detected at all. Techniques using microscopy can probe the visual appearance of the sample, but they have not been able to extract quantitative information about thermal events.

In this work, we demonstrate a new technique that can simultaneously track the sample optically while obtaining quantitative information about its thermal features. We monitored the visual appearance of sucrose droplets for a range of concentrations over several cycles of temperature that span the nucleation, freeze-concentrated glass transition, and melting temperatures. Droplets were generated according to our previous work [1] with diameters of 75 μm, and temperature was controlled at a rate of 1 K min^−1 for cooling and warming. At these picoliter volumes, a high degree of supercooling was observed compared to bulk solutions (micro- to milliliter), and nucleation occurred near or below the freeze-concentrated glass transition temperature of the sucrose–water system, depending on the concentration of sucrose. Based on changes in the brightness of droplets in the recorded images, temperatures of interest were extracted. Nucleation and glass transition temperatures were identified by the onset of significant increases in intensity due to crystal nucleation or growth, respectively, while the melting temperature was associated with a decrease in pixel intensity due to crystal dissolution. In addition, with the same technique, we quantified the linear
growth rate of ice crystals in highly-concentrated sucrose droplets.

Overall, we introduced a method capable of visualizing and quantifying important thermal features of the sucrose–water phase diagram. Such a technique could be extended to real pharmaceutical formulations and aid in the design of processes such as freeze-drying that are carried out for the preservation and storage of drug products.

[1] F. N. Isenrich, N. Shardt, M. Rösch, J. Nette, S. Stavrakis, C. Marcolli, Z. A. Kanji, A. J. deMello, U. Lohmann, The Microfluidic Ice Nuclei Counter Zürich (MINCZ): a platform for homogeneous and heterogeneous ice nucleation, Atmospheric Measurement Techniques 15 (18) (2022) 5367–5381