This study investigates thermomechanical stress in cryopreservation by vitrification of the heart, while exploring the effects of nanowarming-assisted recovery from cryogenic storage. This study expands upon a recently published study, combining experimental investigation and thermal analysis of cryopreservation on a rat heart model. Specifically, this study focuses on scenarios with variable concentrations of silica-coated iron-oxide nanoparticles (sIONPs), while accounting for loading limitations associated with the heart physiology, as well as the properties of cryoprotective agent (CPA) solution and the geometry of the container. Results of this study suggest that variable sIONP concentration based on the heart physiology will elevate mechanical stresses when compared with the mathematically simplified, uniform distribution case. The most dangerous part of rewarming is below glass transition and at the onset of nanowarming past the glass transition temperature on the way for organ recovery from cryogenic storage. Throughout rewarming, regions that rewarm faster, such as the chambers of the heart (higher sIONP concentration), undergo compressive stresses, while the slower rewarming regions, such as the heart myocardium (low sIONP concentration), undergo tension. Being a brittle material, the vitrified organ is expected to fail under tension in lower stresses than in compression. Unfortunately, the location and magnitude of the maximum stress in the investigated cases varied, while general rules were not identified. This investigation demonstrates the need to tailor the thermal protocol of heart cryopreservation on a case-by-case basis, since the location, orientation, magnitude, and instant at which the maximum mechanical stress is found cannot be predicted a priori. While thermomechanical stress poses a significant risk to organ integrity, careful design of the thermal protocol can be instrumental in reducing the likelihood of structural damage, while taking full advantage of the benefits of nanowarming.

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.

The thermal conductivities of five metal powders for powder bed additive manufacturing (Inconel 718, 17-4 stainless steel, Inconel 625, Ti-6Al-4V, and 316L stainless steel) were measured using the transient hot wire method. These measurements were conducted with three infiltrating gases (argon, nitrogen, and helium) within a temperature range of 295-470 K and a gas pressure range of 1.4-101 kPa. The measurements of thermal conductivity indicate that the pressure and the composition of the gas have a significant influence on the effective thermal conductivity of the powder, but that the metal powder properties and temperature do not. Our measurements improve the accuracy upon which laser parameters can be optimized in order to improve thermal control of powder beds in selective laser melting processes, especially in overhanging and cellular geometries where heat dissipation by the powder is critical.

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.

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 3 M dimethyl sulfoxide (DMSO) and 3 M 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 degrees 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 (T-g = -119 degrees 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. (C) 2016 Published by Elsevier Inc.

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 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.

A pilot study was conducted to investigate the dependency of thermal conductivity on temperature, phase of state, and solution concentration in the range of 2 M (classical cryopreservation) and up to 10 M Me2SO (simulative of highly concentrated cocktails used for vitrification). Cryoprotective agents (CPAs), such as dimethyl sulfoxide (Me2SO) are used to control ice formation—the cornerstone of cryoinjury. When cooled, the CPA may crystallize in low concentrations and relatively low cooling rates, or vitrify (form glass—be trapped in an amorphous state) in high concentrations and relatively high cooling rates. Analysis of cryopreservation protocols and explanation of related physical events may be assisted by computer simulations of the thermal process. Unfortunately, the physical properties of CPAs necessary for thermal analysis represent a largely uncharted area. Furthermore, the difference in thermal conductivity between an amorphous and crystalline material of the same composition can vary by orders of magnitude. A transient hot-wire technique was used to measure thermal conductivity of Me2SO solutions in a controlled-rate cooler. Physical events in the sample along the cryopreservation protocol were recorded with the application of the scanning cryomacroscope to confirm whether the sample was vitrified (transparent) or crystallized (opaque). Selected cooling and rewarming rates were chosen to promote either vitrification or crystallization, based on literature data. Thermal conductivity measurements were made continuously during the rewarming phase of the protocol, in the temperature range of −100 to +20 °C. Thermal conductivity of crystalline Me2SO was found to be five-fold higher than that of amorphous Me2SO, with relatively little dependency on the concentration. In the amorphous state, thermal conductivity decreases with the increasing concentration: at 15°C ranging from 0.24 to 0.31 W/m K for 10 M and 7.05 M Me2SO, respectively, and at −100 °C ranging from 0.22 to 0.26 W/m K for 10 M and 7.05 M Me2SO, respectively. The decrease in thermal conductivity with decreasing temperature is consistent with literature data for other amorphous materials such as SiO2, Se, and PMMA. The measured thermal conductivity of 2 M Me2SO above −4 °C is consistent with the dependency of thermal conductivity on concentration for the other solutions in the liquid phase. The onset of crystallization based on a water–Me2SO phase diagram, −4 °C, correlates well with the measured increase in thermal conductivity of the mixture around this temperature. The increased thermal conductivity upon phase transition is consistent with literature data on pure water, although thermal conductivity of pure water increases by a factor of four (from 0.566 to 2.25 W/m K), whereas the thermal conductivity of 2 M Me2SO increases by a factor of three, from 0.47 W/m K to higher than 1.4 W/m K. The integration of the transient hot-wire method into scanning cryomacroscopy developed in this study represents the only available method for measuring thermal conductivity in cryogenic temperature while visually verifying the phase of state. Source of funding: This study is supported, in part by, Award Number R21RR026210, National Center for Research Resources (NCRR) and R21GM103407, National Institute of General Medical Sciences (NIGMS). Conflict of interest: None declared. [email protected]