Modularized passive cold devices with phase change materials for efficient vaccine delivery in remote areas

Ensuring effective vaccine delivery in resource-limited and remote areas remains a critical challenge, particularly where reliable refrigeration is unavailable. Vaccine carriers have been the most widely used equipment in the last-mile delivery. Previous design of vaccine carriers has shown outstanding performance and energy efficiency, but the cost and the utility of the developed vaccine carriers remain a critical issue to widespread applications. Thus, the aim of this study is to explore a passive cold device for last-mile delivery of vaccines, employing phase change materials (PCMs), under varying environmental conditions. Through experimental and numerical investigations, the effects of different PCMs including ice, tetradecane, and a mixture of tetradecane and docosane on cold storage duration, thermal stability, and temperature uniformity are systematically examined. Results reveal that the longest storage durations can reach up to 24.1 h under ambient temperature of 24 °C using PCM of ice. The optimum PCM layout of a single unit extends the storage time by 3.3 % under ambient temperature of 24 °C. This study also examines the effects of packing configurations on vaccine storage performance. The three-pack configuration is found to enhance thermal buffering, particularly at the middle vaccine level, ensuring safe vaccine storage under challenging conditions. The optimal three-pack configuration improves thermal resistance and extends storage time by 9.9 % compared to a single unit with ambient temperature of 40 °C. And the PU foam and XPS foam can substantially improve the storage time and the storage time per unit cost compared to EVA foam, with the longest storage time of 42.2 h and highest storage time per unit cost of 19.5 h/AUD using XPS foam in layout 2 under ambient temperature of 24 °C. Under challenging environmental conditions, the study shows that vaccine storage time decreases by more than 50 % as ambient temperatures increase from 24 °C to 50 °C. The findings offer valuable guidance for designing energy-efficient and cost-effective passive cold devices for last-mile delivery of vaccine.