A custom-built controlled-rate freezer for small sample cryopreservation studies

A custom-built controlled-rate freezing machine is described. It operates by automatically raising and lowering the sample carrier in a static column of liquid nitrogen vapour. It is controlled by a computer-assisted thermocouple feedback system that operates a stepper motor driving the sample carrier. The cooling protocol is divided into three phases: cooling from +5 to −5°C, initiation of ice nucleation, and cooling from −5 to −80°C. Experiments are described to validate the device over a range of different cooling rates. A freezing protocol is established to cool samples in plastic straws over a range of rates up to 80°C min−1, with rapid and consistent absorption of the latent heat.