Wireless Temperature-Sensing System for Liquid-Nitrogen Biobanks

Liquid-nitrogen biobanks need temporary, spatially distributed temperature records, but conventional batteries and radios become unreliable far below their rated operating temperatures. We developed a finite-duration wireless logging system that separates sensing and communication in both space and time. A remote PT1000 probe follows the cryogenic environment, whereas a polytetrafluoroethylene (PTFE)/aerogel enclosure delays cooling of the battery and electronics; data are stored locally during exposure and retrieved by Bluetooth Low Energy only after warm-up. This differentiated thermal-path and staged-communication architecture is the principal novelty of this work. A transient node model reproduced the internal cooling trend, with a mean absolute error (MAE) of 3.74 °C, a root mean square error (RMSE) of 3.92 °C, and r = 0.9986, with a 123.5 s difference in the time to reach −50 °C. Nine nodes logged for 38.4–47.2 min (mean 44.2 min), and all reconnected after 35 min of warm-up at approximately 25 °C. In a one-node indoor engineering test, historical records were recovered without packet loss over 2–10 m, and 18 locations in an operating biobank yielded retrievable temperature histories. The system therefore supports short, non-real-time mapping and workflow assessment; it is not a substitute for fixed real-time alarm or metrological monitoring systems.