Development of a passive device for freezing large amounts of transplantable skin at one time in a −70 °C mechanical refrigerator

Abstract

A simple device has been developed for the simultaneous cooling of up to 9120 cm 2 of allograft skin in a flat package format. The device, named an insulated alternating-offset heat sink device, is composed of a stack of interleaved layers of 2.0-mm-thick packets of skin and 3.18-mm-thick aluminum heat sinks (each 33.0 cm long by 22.9 cm wide). Four skin packets are placed in a single layer on each heat sink plate, and the number of plates can be varied to accommodate different numbers of skin packets. Every heat sink protrudes 6.3 cm of its 33.0-cm length beyond the skin packets to make a fin for heat convection, but adjacent plates alternate the direction of their fin protrusion so that the layers of plates alternate in their 6.3-cm offset. Insulation layers of 2.54-cm-thick expanded polystyrene are placed on the exposed surfaces of the top and bottom heat sinks in the stack, and the stack is held together by rubber bands. The device is cooled in a −70 °C mechanical refrigerator. Maximal cooling rates of −1.8 °C min −1 are obtained for both 6- and 11-plate devices, and −3.0 °C min −1 for a 2-plate device. The exothermic temperature plateaus associated with skin cooled in these devices are 1.5–1.8 min in duration. Skin cooled by this technique maintains levels of glucose oxidation similar to those associated with skin cooled by liquid nitrogen vapor at a controlled rate of −1 °C min −1, provided rapid warming is employed after −70 °C storage. The development of this device provides a method for the simple, low-cost cryopreservation of the large amounts of allograft skin obtained from a cadaveric donor.