Abstract
An optimal cooling rate is a critical factor affecting the survival of biological cells during cryopreservation. In this paper, a system for on-board cooling-rate-controlled cryopreservation under low-temperature (-80 °C) environments is developed with disposable, biocompatible polydimethylsiloxane (PDMS) storage chambers on top of localized heaters on printed circuit boards. The assembly allows the storage chambers to be removed from the temperature-controlled board and transferred from a −80 °C freezer to a liquid nitrogen tank for long-term cryopreservation. The use of PDMS enables the insertion of a syringe needle for loading samples, and during freezing, seeding extracellular ice formation. The PDMS storage chambers were fabricated using a polymethyl methacrylate mold made using a laser cutting machine. For each device, a copper thin film was deposited on a fiberglass epoxy substrate using electroless plating, and patterned using photolithography techniques into a micro-serpentine shape. The copper film functioned simultaneously as a resistive heating element and a temperature sensor with a proportional-integral-derivative feedback control program embedded in a microcontroller to semi-actively control the transient temperature profiles during the freezing process for multiple samples with different cooling rate requirements. The results show that the proposed devices are able to maintain a stable cooling rate down to 1 °C/min, which covers the optimal range for some mammalian cell types with low cell membrane permeability, for which low cooling rates are required. A heat transfer simulation was established to model transient and spatial temperature profiles of the device during freezing. Preliminary biological tests on yeast cells and their survival rates after on-board cryopreservation suggest that the prototype device can be a low-cost, reliable, and convenient tool for laboratory use in cryopreservation.
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