Controlling Equilibrium Vitrification Using Electrical Impedance Spectroscopy

Currently, there is an important need for efficient, affordable, and real-time cryoprotectant monitoring methods in biobanking and organ preservation applications. The precise sensing and control of temperature and cryoprotectant concentration is of utmost importance for the successful implementation of cryopreservation protocols and the design of automated devices for this purpose, especially in liquidus tracking (LT). Electrical impedance spectroscopy (EIS) is proposed and studied here as a noninvasive and affordable method for the real-time monitoring of cryoprotectant concentration. In this work, EIS has been used to provide a complete characterization of the most frequently used cryoprotectants (dimethyl sulfoxide and ethylene glycol), at different concentrations in phosphate buffer saline (from 0% to 90% v/v) and different temperatures (from -40 °C, -17 °C, and -10 °C to 20 °C). Our results show that EIS can successfully control cryoprotectant perfusion by measuring impedance magnitude. An increase in impedance magnitude and a leftward phase shift in the frequency response are observed when cryoprotectant concentration is increased. A decrease on the conductivity of the bulk solution is obtained when temperatures are lowered. These results provide a direct relationship between the cryoprotectant concentration and electrical impedance, thus closing the control loop in perfusion control. Measuring impedance magnitude at near-zero phase angle, corrected against temperature, can verify the proper addition of cryoprotectants in the state-of-the-art cryoprotection techniques.