Due to their unique glass-forming tendency, carbohydrate-based solutions introduce certain challenges during desiccation. Depending on various factors such as their chemical properties, the glass transition temperature and the surface interactions nonhomogeneities within the drying solution arise. These effects range from the formation of very sharp concentration gradients within the processed product (the biopreservation solution containing the macromolecules, cells, etc. to be preserved) to cracking, which physically destroys the product. We are interested in engineering carbohydrate solutions to decrease the molecular mobility around proteins, membranes and in biological systems for preservation purposes.

	It is known that secondary flows are formed within a drying sessile droplet on a hydrophilic surface. Due to these secondary flows, solutes in the medium are mainly collected on the periphery of the drying droplet causing heterogeneities. Mainly, the concentrations of the solutes in the periphery of the droplet are significantly higher than those in the center. This generates mechanical stresses on the surface of the droplet causing cracks (Figure 4). Cracks disrupt the continuum within the dried sample by creating surfaces with excess free energy, which could result in locally increased chemical reaction rates yielding to further compartmentalization and crystallization. The factors affecting the secondary flows within the sample are the low relative humidity in the environment and circumferential pinning of the droplet, which is a function of drop-surface contact angle. It was shown that also by changing the surface characteristics, crack formation may be eliminated (for details, see Aksan & Toner 2004). Establishing thermodynamic equilibrium during storage (for a material, which is inherently unstable like a glass) presents another challenge. The main problems encountered during storage are crystallization, denaturation and aggregation of the stored ingredients.

Just to give a simple example to the instabilities encountered during drying of sessile droplets, let’s look at two droplets of identical initial volume (200 nl) and identical initial sugar concentration (Ci=15% w/w). The droplets are deposited on the surface of a quartz crystal and are being dried diffusively (please click on the images for the movies). As seen in the movies, the apex height of the trehalose droplet decreases gradually during desiccation, while for the dextran droplet the response is quite complicated (for detailed examination of the phenomena, see Aksan, et al. Langmuir, 2005).

Molecular Mobility
For a supersaturated solution, crystallization is the energetically most favorable path. However, if the concentration increases very rapidly (or the temperature drops very fast) a meta-stable “glassy” state can be reached. For a glass-forming system, the transition from a dilute to a concentrated solution diffusion mechanism is determined by the concentration corresponding to the cross-over temperature, Tc, predicted by the Mode Coupling Theory. At the cross-over temperature there is a transition from liquid-like to solid-like dynamics. Note that Tc~(1.14-1.6)Tg for most glass-forming solutions, where Tg is the glass-transition temperature. Diffusion in very high concentration solutions (close to glass transition temperature) is governed by the frequency of jumping between the cages surrounding the tagged molecule (either the solvent or a small solute) and is comparable to the time the molecule spends entrapped in the cage rattling (ß-relaxation). This is similar to the mechanism of diffusion in crystalline systems, where the diffusing molecule jumps between the crystal defects (vacancies). Frequency of jumping is inversely related to the structural relaxation (a-relaxation) time, of the matrix. Temperature dependence of distinguishes between the “fragile” and “strong” glasses, where the variation in with temperature is steeper in the former case.

We are interested in engineering carbohydrate solutions to decrease the molecular mobility around proteins, membranes and in biological systems for preservation solutions.