Cryopreservation of cells for tissue engineering

Engineered tissue for clinical use must be evaluated, inventoried, and delivered to the recipient. This is time intensive and requires a means of sustaining cell viability prior to implantation. Tissue stored at ambient temperature or warmer requires expensive human involvement to satisfy metabolic demands and risks infection and biological alteration. At low temperatures, i.e., below - 125 degrees C, tissues have no metabolic demands. These cold environments can be conveniently attained with cryogens like liquid nitrogen, and 10+ years of storage without change in viability is common. Cryogenic storage is clinically accepted for blood cells, bone marrow, reproductive cells, skin, cornea, and vascular tissue such as heart valves. The scientific basis for cryopreservation derives from principles of biophysics, engineering, and chemistry. The goal of this review is to identify these principles for tissue engineers and provide sufficient mathematical definition to guide the design of specific cryopreservation protocols for component cells of newly engineered tissues. The discussion assumes cells are "typical" from the standpoint of cryopreservation, which means neither the nucleus nor any vacuole is greater than 50% of the total volume, cellular contents are roughly 15% protein, 15% lipid, and 70% water by mass, and the longest dimension of a cell or cellular aggregate is 1-1000 microm.