A role for microwave processing in the dry preservation of mammalian cells.

Dry preservation involves removing water from samples so that degradative biochemical processes are slowed and extended storage is possible. Recently this approach has been explored as a method for preserving living mammalian cells. The current work explores the use of microwave processing to enhance evaporation rates and to improve drying uniformity, thereby overcoming some of the challenges in this field. Mouse macrophage cells (J774) were pre-incubated in full complement media containing 50 mM trehalose, for 18-h, to allow for endocytosis of trehalose. Droplets of experimental and control (no intracellular trehalose) cell suspensions were placed on coverslips in a microwave cavity. Water was evaporated using intermittent microwave heating (600 W, 30 s intervals). Samples were dried to various moisture levels, rehydrated, and then survival was assessed after a 45-min recovery period using Calcein-AM/PI fluorescence and Trypan Blue exclusion assays. The metabolic activity of dried cells (4.3 gH(2)O/gdw) was assessed after rehydration using a resazurin reduction assay. Apoptosis levels were also measured. Post- rehydration survival correlated with the final moisture content achieved, consistent with other drying methods. Intracellular trehalose provided protection against injury associated with moisture loss. Metabolic assays revealed normal growth in surviving cells, and these survival levels were consistent with results from apoptosis assays (P > 0.05). Brightfield and fluorescence images of microwave-dried samples revealed a uniform distribution of cells within the dried matrix and profilometry analysis demonstrated that solids were uniformly distributed throughout the sample. Microwave-processing successfully facilitated rapid and uniform dehydration of cell-based samples.