Thermodynamic analysis of a partial freezing organ preservation protocol

Partial freezing has been proposed in various forms since the 1990s as a high sub-0°C technique for prolonging the viable preservation window of organs relative to conventional hypothermic storage. However, despite the fundamental dependence of this technique on equilibrium thermodynamic behaviors, solution and protocol design have thus far remained largely empirical. The multisolute osmotic virial equation (MSOVE) has previously been used for quantitative protocol design in cryobiology, including the optimization of cryoprotectant addition and removal procedures and the prediction of osmotic responses in multi-solute systems. Here, we apply the MSOVE to model the freezing process of partial freezing solutions utilized in recent rat liver preservation protocols, computing melting points, liquidus curves, equilibrium ice fractions, and solute concentration trajectories down to -20 °C. The calculations provide simple mechanistic insights into observed differences in preservation quality between different pairings of composition and storage temperature, and suggest practical routes to optimize electrolyte content, polymer concentration, and cryoprotectant choice when rationally designing future partial freezing protocols.