Limits of pressure-based ice detection during isochoric vitrification

Vitrification under isochoric (constant-volume or volumetrically confined) conditions has emerged as an intriguing new cryopreservation modality, but the physical complexities of the process confound straight-forward interpretation of experimental results. In particular, the signature pressure-based ice detection used in many isochoric techniques becomes paradoxical during vitrification, wherein the emergence of a sharp increase in pressure reliably indicates the presence of ice, but avoidance of this increase does not necessarily indicate its absence. This phenomenon arises from the rich interplay between thermochemical and thermovolumetric effects in isochoric systems, and muddies efforts to confirm the degree to which a sample has vitrified. In this work, we seek to aid interpretation of isochoric vitrification experiments by calculating thermodynamic limits on the maximum amount of ice that may form without being detected by pressure, and by clarifying the myriad physical processes at play. Neglecting kinetic effects, we develop a simplified thermodynamic model accounting for thermal contraction, cavity formation, ice growth, solute ripening, and glass formation, we evaluate it for a range of chamber materials and solution compositions, and we validate against the acutely limited data available. Our results provide both counter-intuitive insights— lower-concentration solutions may contract less while producing more pressure-undetectable ice growth for example— and a general phenomenological framework by which to evaluate the process of vitrification in isochoric systems. We anticipate that the model herein will enable design of future isochoric protocols with minimized risk of pressure-undetectable ice formation, and provide a thermodynamic foundation from which to build an increasingly rigorous multi-physics understanding of isochoric vitrification.