Improved Temperature Stability in Gas-Phase Nitrogen Refrigerators: Use of a Copper Heat Shunt

Abstract

Long-term storage of cells and tissues is usually carried out in liquid nitrogen refrigerators. This practice has developed to ensure stable temperatures below −185°C. Recent events have demonstrated a significant risk of transmission of infective agents between samples if storage is carried out by immersion in liquid nitrogen. Unfortunately, gas-phase storage does not provide the same uniformity of temperature, nor as low a temperature as immersion, and this may affect stability during long-term preservation. Temperature stability can be improved and tank temperature lowered by providing a low-resistance heat path from the gas space to the liquid. Inventory systems made from efficient heat conductors such as aluminum can help achieve this if the lower compartments are immersed in the liquid nitrogen, but avoidance of this is desired. In this report is described a copper heat shunt that is compatible with existing gas-phase inventory systems and maintains temperatures below −185°C at all levels within the gas space. Additionally, the heat shunt reduces fluctuations in temperature when the refrigerator is opened and it maintains a temperature below −160°C, even when the refrigerator is open for extended periods.