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Vaccines are very sensitive to temperature, needing to be held between 2 and 80C to maintain potency. In developing countries where electricity and fuel supplies are unreliable, many vaccines are ruined due to thermal exposure. These are also the locations where vaccines are needed the most, yet often many of the vaccines given are ineffectve. Long holdover vaccine coolers are designed to maintain a proper internal temperature during long periods of power loss. The most prevalent technology is the ice-lined cooler, but in the field these often have problem with freezing the vaccines. A vaccine cooler was designed that modifies the ice-jacket idea by separating the ice compartment and the vaccine chamber, connecting them through a heat transfer regulating device. The objective of this research is to design and prototype the heat transfer regulating device. After several design iterations a cooling loop filled with R-134a made of 1/8 piping, a 0.055 in ID capillary, and a Clippard normally-closed valve was combined with a modified car thermostat, using peanut oil as its working fluid, to create a thermosyphon type heat transfer device with a safety shutoff to prevent freezing. The prototype was manufactured an