ON-CHIP CRYOPRESERVATION OF CELLS

MEMS (Microelectromechanical Systems)-assisted, on-chip cryopreservation of biological cells under low temperature environments has been successfully demonstrated. In the proof-of-concept experiment, microheaters are used as both the resistive heating elements and temperature monitors to control the temperature profile of microenvironments under liquid nitrogen cooling. A two-step cryopreservation protocol is applied to the controlled microenvironment and 7~12 times better survival rate has been achieved for yeast cells after the air thawing process as compared with a reference sample without the MEMS-assisted control. As such, this scheme could have potential applications in onchip cryopreservation processes, including those for sperm, embryo, or lab-on-a-chip. INTRODUCION Cryopreservation of biological cells including freezing and thawing processes has achieved great progresses over the past few decades [1]. The freezing process consists of two main steps after exposing cells to the cryoprotective solution: (1) to control the cooling of the specimen to an intermediate subzero temperature around -20~-40C depending on cell species; (2) to rapidly freeze the specimen to the storage temperature typically below -130C. At the meantime, one complementary holding step can be added into the main steps to improve survival rate by holding the sample at the intermediate subzero temperature for a certain time to extend the dehydration of cells. On the other hand, the recovery process or the thawing process depends highly on the cooling procedure but rapid thawing is preferred and a 37C water bath is the typical setup for the thawing process. During the freezing and thawing steps, there are two main mechanisms that could damage cells and should be avoided as illustrated in Fig. 1 that is based on the mouse ova model [1]. As temperature cools down in an aqueous suspension of living cells, ice forms first in the environment surrounding cells resulting in increasing concentration of solutes outside cells [2]. If the cooling rate is too slow, osmosis removes water from cells excessively, leading to a harmful high electrolyte concentration inside cells. If the cooling rate is too fast, intracellular ice could form and becomes the recrystallization sites of large ice crystals later during the thawing process and could damage the cell membranes. The ideal cooling rate, where the survival of cells is optimal, is slow enough that the intracellular ice may not form and rapid enough that cells are not damaged by high concentrations solutes or excessive shrinkage. The optimal condition is typically determined empirically and depends on the type of cells. Because the low temperature effects on biological cells have not been fully understood, the survival rate after the cryopreservation of cells could be different between laboratories or batches with same experimental protocols. The advantage of using MEMS-assisted cryopreservation is the feasibility to provide better and uniform temperature control locally and precisely via microheater so as to achieve better survival rate. Furthermore, the preservation process can be shortened and programmed directly with onchip microelectronics. These attributes could fit well as the future platform for cryogenic lab-on-a-chip and BioMEMS applications.