Steven P. Callahan

Maintaining the integrity of cryogenically preserved biological materials is critical, as even brief, undetected temperature excursions in storage can compromise sample viability. Existing monitoring systems may miss transient thaw–refreeze events, posing serious quality risks. To address this, we developed and validated frozen indicator tubes that visually signal deviations from the frozen state, serving as a cost-effective backup to electronic monitors. Our first method uses an aqueous dye solution that immobilizes the dye when frozen; any thawing causes the dye to disperse, providing a clear, external visual cue of a partial or complete thaw. For ultra-low-temperature storage (−80 °C), we introduced a second method using an ethanol-based solution calibrated to indicate thaw events. This system detects temperature rises of 10 °C or more sustained for at least fifteen minutes—conditions that may jeopardize sample stability. When paired with standard monitoring systems, these indicator tubes offer an added layer of protection by providing simple, reliable, and immediate visual confirmation of critical temperature breaches. This innovation enhances confidence in cryogenic storage protocols and supports the long-term preservation of sensitive biological materials.