Tracking cryogenic tank failure from inception to critical warming: a site-specific analysis of sample-storage risk

Cryogenic storage is essential for the long-term preservation of gametes, embryos, and reproductive tissues in assisted reproductive technology (ART). Liquid-nitrogen [N_₂(l)] storage dewars rely on high-vacuum insulation to maintain cryogenic conditions and predictable evaporation rates. Progressive deterioration of vacuum integrity may initially present as accelerated N_₂(l) loss while temperatures at the sample-storage level remain within the cryogenic range. Consequently, conventional temperature monitoring alone may provide a narrower intervention window than multimodal systems capable of identifying earlier physical indicators of tank failure.

To characterize the progression of an induced vacuum failure from its earliest detectable signs to localized warming at clinically representative sample-storage locations, and to evaluate whether combined vacuum-integrity, N_₂(l) weight, and sample-proximal temperature monitoring provides earlier warning than conventional temperature monitoring alone.

A new, unused XC 47/11 liquid nitrogen storage dewar (MVE Biological Solutions, Ball Ground, GA, USA), designated Device 7108, was evaluated under controlled environmental conditions. Baseline evaporation testing was conducted before vacuum-failure induction. During a closed-lid control trial, the dewar required 110.84 days to decline from 44.0 L of N₂(l) to depletion, corresponding to an average evaporation rate of 0.397 L/day. Additional 48-h characterization trials demonstrated N₂(l) losses of 0.7 L with the lid closed and 2.4 L with the lid open. For the vacuum-failure simulation, the dewar was filled with 45.8 L of medical-grade N₂(l) and stabilized for 32 h before a controlled vacuum breach was induced. Empty S-Cryolock® vitrification devices (Biotech, Inc., Alpharetta, GA, USA) were arranged in clinically representative storage assemblies. Vacuum integrity was not measured directly with pressure transducers. The vacuum failure was induced by creating a full-thickness perforation through the vacuum-port plug of the otherwise new, previously unmodified dewar using a 1/16-in (1.59 mm) drill bit. Changes associated with loss of vacuum integrity were assessed non-invasively from external thermal divergence. N_₂(l) mass and temperature were continuously monitored using a NIST-traceably calibrated load cell and thermocouples positioned at predefined internal, external, and sample-proximal locations, including directly on S-Cryolock® devices.

Vacuum degradation and accelerated N_₂(l) loss were detectable before conventional thermal-alarm thresholds were reached. Early external temperature drifts were observed within minutes of breach induction, including changes of 1.1 °C and 1.6 °C, at monitored tank locations, with subsequent progression to a 7.3 °C deviation. As the failure evolved, sample-proximal thermocouples recorded sequential warming at the vitrification-device level. First-stage alerts near - 180 °C were observed at multiple sample-storage locations, including - 176.2 °C, - 179.0 °C, - 177.7 °C, and - 173.0 °C. A second-stage alert near - 160 °C was subsequently recorded at - 158.1 °C. At one monitored location, temperature increased progressively from - 179.9 to - 168.4 °C across sequential alerts, demonstrating localized heat propagation within the storage assembly. During the failure simulation, the remaining N_₂(l) volume declined below the predefined 5-L safety threshold to 4.7 L. Combined vacuum-integrity and weight-loss monitoring identified deteriorating tank performance before the predefined conventional internal-temperature alarm threshold was reached.

Progressive vacuum failure produced detectable changes in vacuum integrity and N_₂(l) loss before clinically concerning warming developed at representative sample-storage locations. Multimodal monitoring that integrates vacuum-awareness sensing, continuous weight-based measurement, and sample-proximal temperature surveillance may extend the available intervention window beyond that provided by conventional temperature monitoring alone. Earlier detection may enable verification, escalation, nitrogen replenishment, or specimen transfer before localized warming approaches temperatures associated with increasing risk to vitrified reproductive material.