No abstract

Thermal conductivity of dimethyl-sulfoxide (DMSO) solution is measured in this study using a transient hot wire technique, where DMSO is a key ingredient in many cryoprotective agent (CPA) cocktails. Characterization of thermal properties of cryoprotective agents is essential to the analysis of cryopreservation processes, either when evaluating experimental data or for the design of new protocols. Also presented are reference measurements of thermal conductivity for pure water ice and glycerol. The thermal conductivity measurement setup is integrated into the experimentation stage of a scanning cryomacroscope apparatus, which facilitates the correlation of measured data with visualization of physical events. Thermal conductivity measurements were conducted for a DMSO concentration range of 2M and 10M, in a temperature range of -180°C and 25°C. Vitrified samples showed decreased thermal conductivity with decreasing temperature, while crystalline samples showed increased thermal conductivity with decreasing temperature. These different behaviors result in up to a tenfold difference in thermal conductivity at -180°C. Such dramatic differences can drastically impact heat transfer during cryopreservation and their quantification is therefore critical to cryobiology.

The thermal conductivity of the cryoprotective agent (CPA) cocktail DP6 in combination with synthetic ice modulators (SIMs) is measured in this study, using a transient hot-wire method. DP6 is a mixture of 3M dimethyl sulfoxide (DMSO) and 3M propylene glycol, which received significant attention in the cryobiology community in recent years. Tested SIMs include 6% 1,3Cyclohexanediol, 6% 2,3Butanediol, and 12% PEG400 (percentage by volume). This study integrates the scanning cryomacroscope for visual verification of crystallization and vitrification events. It is demonstrated that the thermal conductivity of the vitrifying CPA cocktail decreases monotonically with the decreasing temperature down to −180°C. By contrast, the thermal conductivity of the crystalline material increases with decreasing temperature in the same temperature range. Results of this study demonstrate that the thermal conductivity may vary by three fold between the amorphous and crystalline phases of DP6 below the glass transition temperature of DP6 (Tg = −119°C). The selected SIMs demonstrate the ability to inhibit crystallization in DP6, even at subcritical cooling rates. An additional ice suppression capability is observed by the Euro-Collins as a vehicle solution, disproportionate to its volume ratio in the cocktail. The implication of the observed thermal conductivity differences between the amorphous and crystalline phases of the same cocktail on cryopreservation simulations is significant in some cases and must be taken into account in thermal analyses of cryopreservation protocols.

This study focuses on thermal analysis of the problem of scaling up from the vitrification of rabbit kidneys to the vitrification of human kidneys, where vitrification is the preservation of biological material in the glassy state. The basis for this study is a successful cryopreservation protocol for a rabbit kidney model, based on using a proprietary vitrification solution known as M22. Using the finite element analysis (FEA) commercial code ANSYS, heat transfer simulations suggest that indeed the rabbit kidney unquestionably cools rapidly enough to be vitrified based on known intrarenal concentrations of M22. Scaling up 21-fold, computer simulations suggest less favorable conditions for human kidney vitrification. In this case, cooling rates below -100 °C are sometimes slower than 1 °C/min, a rate that provides a clear-cut margin of safety at all temperatures based on the stability of rabbit kidneys in past studies. Nevertheless, it is concluded in this study that vitrifying human kidneys is possible without significant ice damage, assuming that human kidneys can be perfused with M22 as effectively as rabbit kidneys. The thermal analysis suggests that cooling rates can be further increased by a careful design of the cryogenic protocol and by tailoring the container to the shape of the kidney, in contrast to the present cylindrical container. This study demonstrates the critical need for the thermal analysis of experimental cryopreservation and highlights the unmet need for measuring the thermophysical properties of cryoprotective solutions under conditions relevant to realistic thermal histories.

This study explores thermal design aspects of nanowarming-assisted recovery of the heart from indefinite cryogenic storage, where nanowarming is the volumetric heating effect of ferromagnetic nanoparticles excited by a radio-frequency electromagnet field. This study uses computation means, while focusing on the human heart and the rat heart models. The underlying nanoparticle loading characteristics are adopted from a recent, proof-of-concept experimental study. While uniformly distributed nanoparticles can lead to uniform rewarming, and thereby minimize adverse effects associated with ice crystallization and thermomechanical stress, the combined effects of heart anatomy and nanoparticle loading limitations present practical challenges which this study comes to address. Results of this study demonstrate that under less-than-ideal conditions, nonuniform nanoparticles warming may lead to a subcritical rewarming rate in some parts of the domain, excessive heating in others, and increased exposure potential to cryoprotective agents (CPAs) toxicity. Nonetheless, results of this study also demonstrate that computerized planning of the cryopreservation protocol and container design can help mitigate the associated adverse effects, with examples relating to adjusting the CPA and/or nanoparticle concentration, and selecting heart container geometry and size. In conclusion, nanowarming provides superior conditions for organ recovery from cryogenic storage, which comes with an elevated complexity of protocol planning and optimization.

The objective of this study is to provide thermal conductivity data for CPA-based nanofluids for the benefit of the analyses of cryopreservation by vitrification. Thermal conductivity measurements were conducted using a hot-wire technique on an experimentation platform of the cryomacroscope, to correlate measurements with observed physical effects such as crystallization and fracturing. Tested materials in this study include the CPA cocktails M22, VS55, DP6, and DP6+sucrose. Nanofluids in this study include the above CPA cocktails as base solutions, when mixed with either iron-oxide nanoparticles (IONP) or silica-coated iron-oxide nanoparticles (sIONP). Results of this study demonstrated the addition of sIONP to any of the CPA cocktails tested did not significantly affect its thermal conductivity, its tendency to vitrify or, conversely, its tendency to form rewarming phase crystallization (RPC). Fractures were observed with cryomacroscopy at the onset of rewarming for DP6+sIONP under carefully controlled rewarming conditions without RF activation, despite the inherent opacity of the sIONP solutions. It is likely that using RF heating in order to accelerate rewarming while unifying the temperature distribution would prevent fracture and RPC. However, sIONP were not activated in this study, as the RF heating mechanism would interfere with thermal conductivity measurements. The addition of IONP to DP6 appears to hinder the tendency of the CPA to vitrify, which is a detrimental effect. But it is unlikely that uncoated nanoparticle solutions will be used in practical applications.

No abstract

The current study is aimed at developing a device and technique to measure the thermal conductivity of materials relevant to cryopreservation — the preservation of biomaterials at very low temperatures. It is well established that ice formation is the cornerstone of low-temperature injury [1]. In an effort to improve the outcome of cryopreservation, ice crystallization can be controlled by the addition of cryoprotective agents (CPAs), such as dimethyl sulfoxide (DMSO). CPA solutions are characterized by exponentially increasing viscosity with the decreasing temperature. If cooled rapidly enough, the crystalline phase can be completely suppressed and the material is trapped in a solid-like state known as vitrification (vitreous in Latin means glassy). While correlating the quality of the cryopreserved product with the thermal history may be straightforward to obtain in small specimens, characterized by close-to-uniform temperature distribution, analysis of larger specimens requires integration of mathematical tools to estimate the spatial temperature distribution at any instant along the cryogenic protocol. The data developed in the current study is aimed at enabling the corresponding thermal analysis, while exploring the variation in thermal conductivity between the crystalline and glassy states.