title: Desiccation kinetics and biothermodynamics of glass forming trehalose solutions in thin films.
abstract: In this study, the desiccation kinetics of aqueous trehalose solutions were investigated numerically by solving the coupled heat and mass transfer problem with a moving interface using the finite element method. The free volume models for vapor pressure and mutual diffusion coefficient were incorporated into the model to account for the effect of glass transition on the heat and mass transport process that ultimately determines the desiccation kinetics. It was found that the temperature in the film could drop significantly upon the initiation of drying due to the absorption of latent heat associated with water evaporation although the spatial distribution of temperature in the solution is very homogeneous. On the contrary, the spatial distribution of water content in the solution is non-homogeneous, particularly at the solution-vapor interface where an extremely thin layer of skin with extremely low molecular mobility usually forms during drying. The solution film can be dried to approximately 6-10 wt.% residual water within minutes for thin films; but drying times depends strongly on the initial film thickness, initial solution concentration, temperature, and convective coefficient. Desiccation to below 6 wt.% residual water is very slow due to the retarded water mobility in the extremely thin skin where the solution is in the glassy state. Since the water mobility in a trehalose solution or glass with 6-10% residual water is still high enough to allow degradative reactions to occur in a relatively short time at room temperature, it is important that the samples should be kept at a temperature around 0 degrees C or lower for storage after drying. Furthermore, approaches that might enable further quick reduction of the residual water to less than 6-10 wt.% are also proposed so that a sample could be preserved at super-zero or even room temperature. The established models and the reported results will be useful for the development of effective protocols for lyopreservation of biomaterials including living cells using trehalose as the excipient.
authors.affiliation: Center for Engineering in Medicine and Surgical Services, Massachusetts General Hospital, Harvard Medical School, 114 16th St, Charlestown, MA 02129, USA. xmhe@sc.edu
authors.forename: Xiaoming
authors.initials: X
authors.lastname: He
authors.forename: Alex
authors.initials: A
authors.lastname: Fowler
authors.forename: Michael
authors.initials: M
authors.lastname: Menze
authors.forename: Steve
authors.initials: S
authors.lastname: Hand
authors.forename: Mehmet
authors.initials: M
authors.lastname: Toner
mesh_terms: D001672:Biocompatible Materials / Q000737:chemistry*; D003198:Computer Simulation; D003890:Desiccation; D005898:Glass / Q000737:chemistry; D007700:Kinetics; D008422:Materials Testing; D008567:Membranes, Artificial; D008956:Models, Chemical; D044367:Phase Transition; D013816:Thermodynamics; D014199:Trehalose / Q000737:chemistry*
chemical_list: D001672:Biocompatible Materials; D008567:Membranes, Artificial; D014199:Trehalose