Stability of the amorphous state in the system water--glycerol--dimethylsulfoxide

Analysis of cryopreservation media thermophysical characteristics after ultra-rapid cooling through differential scanning calorimetry

Cryoprotective agents play a critical role in minimizing cell damage caused by ice formation during cryopreservation. However, high concentrations of CPAs are toxic to cells and tissues. Required concentrations of CPAs can be reduced by utilizing higher cooling and warming rates, but insight into the thermophysical properties of biological solutions in the vitrification method is necessary for the development of cryopreservation protocols. Most studies on thermophysical properties under ultra-rapid cooling conditions have been qualitatively based on visualization. Differential scanning calorimetry methods are ideal for studying the behavior of biomaterials in various freezing conditions quantitatively and accurately, though previous studies have been predominantly restricted to slower cooling rates. Here, we developed an ultra-rapid cooling method for DSC that can achieve minimal cooling rates exceeding 2000 °C/min. We investigated the thermophysical vitrification behavior of ternary solutions of phosphate buffer saline (1X), dimethyl sulfoxide or glycerol and ice blocking polymers (X-1000 or Z-1000). We quantified the impact of solute concentration on ice crystal formation during rapid cooling. Our findings support the expectation that increasing the solute concentration reduces the amount of ice formation, including devitrification. Devitrification increases from 0 % to 40 % (v/v) Me2SO and then reduces significantly. The relative amounts of devitrification to the total ice formation are 0 %, 60 %, 0 % in 20 %, 40 %, 60 % (v/v) Me2SO, and 2 %, 48 %, 49 % in 20 %, 40 %, 60 % (v/v) glycerol, respectively. The results suggest that at low concentrations, such as below 20 % (v/v) for Me2SO or glycerol, increasing the warming rate after ultra-rapid freezing is not essential to eliminate devitrification. Furthermore, ice blocking polymers do not reduce ice formation substantially and cannot eliminate devitrification under ultra-rapid cooling conditions. In conclusion, our results provide insights into the impact of solute concentration on ice formation and devitrification during rapid cooling, which can be practical for optimizing cryopreservation protocols.

Extended Temperature Range of the Ice-Binding Protein Activity

Ice-binding proteins (IBPs) are expressed in various organisms for several functions, such as protecting them from freezing and freeze injuries. Via adsorption on ice surfaces, IBPs depress ice growth and recrystallization and affect nucleation and ice shaping. IBPs have shown promise in mitigating ice growth under moderate supercooling conditions, but their functionality under cryogenic conditions has been less explored. In this study, we investigate the impact of two types of antifreeze proteins (AFPs): type III AFP from fish and a hyperactive AFP from an insect, the Tenebrio molitor AFP, in vitrified dimethylsulfoxide (DMSO) solutions. We report that these AFPs depress devitrification at -80 °C. Furthermore, in cases where devitrification does occur, AFPs depress ice recrystallization during the warming stage. The data directly demonstrate that AFPs are active at temperatures below the regime of homogeneous nucleation. This research paves the way for exploring AFPs as potential enhancers of cryopreservation techniques, minimizing ice-growth-related damage, and promoting advancements in this vital field.

Problems of human spermatozoa cryopreservation: research methods, solutions

Cryopreservation of male gametes is one of the most important methods of assisted reproductive technologies, which allows preserving gametes for research or further use. However, the fertilizing ability of spermatozoa after cryopreservation decreases by 30-70%, which makes it urgent to search for new substances with cryoprotective properties. The review considers the main causes of cell damage during cryopreservation. The relevance of methods for assessing the formation of crystals and the physicochemical properties of cryoprotective media depending on various compositions is discussed. The problem of stabilization of the spermatozoa membrane during cryopreservation is considered. A possible solution to the problem of membrane integrity may consist in modification of the basic cryoprotective media with yolk emulsion or development of methods for saturation of the membrane phospholipid layer with cholesterol.

Technologies for Vitrification Based Cryopreservation

Cryopreservation is a unique and practical method to facilitate extended access to biological materials. Because of this, cryopreservation of cells, tissues, and organs is essential to modern medical science, including cancer cell therapy, tissue engineering, transplantation, reproductive technologies, and bio-banking. Among diverse cryopreservation methods, significant focus has been placed on vitrification due to low cost and reduced protocol time. However, several factors, including the intracellular ice formation that is suppressed in the conventional cryopreservation method, restrict the achievement of this method. To enhance the viability and functionality of biological samples after storage, a large number of cryoprotocols and cryodevices have been developed and studied. Recently, new technologies have been investigated by considering the physical and thermodynamic aspects of cryopreservation in heat and mass transfer. In this review, we first present an overview of the physiochemical aspects of freezing in cryopreservation. Secondly, we present and catalog classical and novel approaches that seek to capitalize on these physicochemical effects. We conclude with the perspective that interdisciplinary studies provide pieces of the cryopreservation puzzle to achieve sustainability in the biospecimen supply chain.

Supplemented phase diagrams for vitrification CPA cocktails: DP6, VS55 and M22

DP6, VS55 and M22 are the most commonly used cryoprotective agent (CPA) cocktails for vitrification experiments in tissues and organs. However, complete phase diagrams for the three CPAs are often unavailable or incomplete (only available for full strength CPAs) thereby hampering optimization of vitrification and rewarming procedures. In this paper, we used differential scanning calorimetry (DSC) to measure the transition temperatures including heterogeneous nucleation temperatures (T_het), glass transition temperatures (T_g), rewarming phase crystallization (devitrification and/or recrystallization) temperatures (T_d) and melting temperatures (T_m) while cooling or warming the CPA sample at 5 °C/min and plotted the obtained transition temperatures for different concentrations of CPAs into the phase diagrams. We also used cryomicroscopy cooling or warming the sample at the same rate to record the ice crystallization during the whole process, and we presented the cryomicroscopic images at the transition temperatures, which agreed with the DSC presented phenomena.

Vitrification Ability of Combined and Single Cryoprotective Agents

Cryoprotective agents (CPA) are an important part of many current vitrification methods. The vitrification ability of CPAs influences the probability of the glass transition and water crystallization occurrence. Thermal characteristics and the vitrification ability of two combined CPAs (PVS2 and PVS3), common plant vitrification solutions, and four single CPAs (ethylene glycol, DMSO, glycerol, and sucrose), the components of the mentioned PVSs, were evaluated utilizing a differential scanning calorimetry (DSC) during standard cooling/warming rates of 10 °C min-1. The effect of solute concentration on their vitrification ability was shown in the CPAs tested. Four typical concentration regions at which the glassy state and/or crystallization occurred were defined. We suggest the solute concentration of 0.7 g g-1 as the universal vitrification concentration, characterized by an actual Tg of CPA solution and limited water crystallization. Knowledge of the thermal properties of CPAs allows the design of new combined CPAs with the required vitrification ability respecting the cryopreservation method used and the characteristics of the cryopreserved sample.

Principles of Ice-Free Cryopreservation by Vitrification

Vitrification is an alternative to cryopreservation by freezing that enables hydrated living cells to be cooled to cryogenic temperatures in the absence of ice. Vitrification simplifies and frequently improves cryopreservation because it eliminates mechanical injury from ice, eliminates the need to find optimal cooling and warming rates, eliminates the importance of differing optimal cooling and warming rates for cells in mixed cell type populations, eliminates the need to find a frequently imperfect compromise between solution effects injury and intracellular ice formation, and can enable chilling injury to be "outrun" by using rapid cooling without a risk of intracellular ice formation. On the other hand, vitrification requires much higher concentrations of cryoprotectants than cryopreservation by freezing, which introduces greater risks of both osmotic damage and cryoprotectant toxicity. Fortunately, a large number of remedies for the latter problem have been discovered over the past 35 years, and osmotic damage can in most cases be eliminated or adequately controlled by paying careful attention to cryoprotectant introduction and washout techniques. Vitrification therefore has the potential to enable the superior and convenient cryopreservation of a wide range of biological systems (including molecules, cells, tissues, organs, and even some whole organisms), and it is also increasingly recognized as a successful strategy for surviving harsh environmental conditions in nature. But the potential of vitrification is sometimes limited by an insufficient understanding of the complex physical and biological principles involved, and therefore a better understanding may not only help to improve present outcomes but may also point the way to new strategies that may be yet more successful in the future. This chapter accordingly describes the basic principles of vitrification and indicates the broad potential biological relevance of this alternative method of cryopreservation.

On the crystal structures and phase transitions of hydrates in the binary dimethyl sulfoxide–water system

Neutron powder diffraction data have been collected from a series of flash-frozen aqueous solutions of dimethyl sulfoxide (DMSO) with concentrations between 25 and 66.7 mol% DMSO. These reveal the existence of three stoichiometric hydrates, which crystallize on warming between 175 and 195 K. DMSO trihydrate crystallizes in the monoclinic space group P21/c, with unit-cell parameters at 195 K of a = 10.26619 (3), b = 7.01113 (2), c = 10.06897 (3) Å, β = 101.5030 (2)° and V = 710.183 (3) Å3 (Z = 4). Two of the symmetry-inequivalent water molecules form a sheet of tiled four- and eight-sided rings; the DMSO molecules are sandwiched between these sheets and linked along the b axis by the third water molecule to generate water–DMSO–water tapes. Two different polymorphs of DMSO dihydrate have been identified. The α phase is monoclinic (space group P21/c), with unit-cell parameters at 175 K of a = 6.30304 (4), b = 9.05700 (5), c = 11.22013 (7) Å, β = 105.9691 (4)° and V = 615.802 (4) Å3 (Z = 4). Its structure contains water–DMSO–water chains, but these are polymerized in such a manner as to form sheets of reniform eight-sided rings, with the methyl groups extending on either side of the sheet. On warming above 198 K, α-DMSO·2H2O undergoes a solid-state transformation to a mixture of DMSO·3H2O + anhydrous DMSO, and there is then a stable eutectic between these two phases at ∼203 K. The β-phase of DMSO dihydrate has been observed in a rapidly frozen eutectic melt and in very DMSO-rich mixtures. It is observed to be unstable with respect to the α-phase; above ∼180 K, β-DMSO·2H2O converts irreversibly to α-DMSO·2H2O. At 175 K, the lattice parameters of β-DMSO·2H2O are a = 6.17448 (10), b = 11.61635 (16), c = 8.66530 (12) Å, β = 101.663 (1)° and V = 608.684 (10) Å3 (Z = 4), hence this polymorph is just 1.16% denser than the α-phase under identical conditions. Like the other two hydrates, the space group appears likely, on the basis of systematic absences, to be P21/c, but the structure has not yet been determined. Our results reconcile 60 years of contradictory interpretations of the phase relations in the binary DMSO–water system, particularly between mole fractions of 0.25–0.50, and confirm empirical and theoretical studies of the liquid structure around the eutectic composition (33.33 mol% DMSO).

Investigation of Electromagnetic Resonance Rewarming Enhanced by Magnetic Nanoparticles for Cryopreservation

The lack of an effective rewarming technique restricted the successful cryopreservation of organ or large tissues by vitrification. The conversion of electromagnetic (EM) energy into heat provides a possible solution for the rewarming process for the cryopreservation. In this work, an EM resonance rewarming system was set up with dynamic feedback control and power feeding optimization. In addition, we take advantage of magnetic nanoparticles (MNPs) to absorb magnetic field energy to further enhance the energy conversion efficiency. We achieved a >200 °C min^-1 rewarming rate for tens of milliliters of cryopreserved samples. Besides, we also investigated the effect of nanoparticle size and concentration based on thermal properties by analyzing the contribution of nanoparticles and the utilization of field energy. The closed system reduced the possible concomitant side effects when increasing the number of nanoparticles or increasing the EM source power. With the remarkably low dosage of nanoparticles (0.1 mg mL^-1 Fe) compared to that for other MNP-based rewarming applications, this study opens the door to new approaches for exploring novel techniques for tissue and organ preservation.

Characterization of Laser Gold Nanowarming: A Platform for Millimeter-Scale Cryopreservation

Preventing ice formation during cryopreservation by vitrification has led to the successful storage and banking of numerous cellular- and tissue-based biomaterials. In their breakthrough work, Peter Mazur's group achieved over 90% survival by using a laser warming technique for 100 μm mice oocytes that were cooled in 0.1 μL droplets with 2.3 M CPA and extracellularly loaded India ink (laser absorber). Laser warming can provide rapid and uniform warming rates to "outrun" damaging ice crystal growth. Here we generalize Mazur's technique for microliter-sized droplets using laser nanowarming to rewarm millimeter-scale biomaterials when loaded extracellularly and/or intracellularly with biocompatible 1064 nm resonant gold nanoparticles. First, we show that droplets containing low-concentration cryoprotectants (such as 2 M propylene glycol ± 1 M trehalose) can be rapidly cooled at rates up to 90 000 °C/min by plunging into liquid nitrogen to achieve either a visually transparent state (i.e., vitrified) or a cloudy with ice (i.e., nonvitrified) state. Both modeling and experiments were then used to characterize the laser nanowarming process for different laser energy (2-6 J), pulse length (1-20 ms), droplet volume (0.2-1.8 μL), cryoprotectant (2-3 M), and gold concentration (0.77 × 10¹⁷-4.8 × 10¹⁷ nps/m³) values to assess physical and biological success. Physical success was achieved by finding conditions that minimize cloudiness and white spots within the droplets during cooling and warming as signs of damaging ice formation and ice crystallization, respectively. Biological success was achieved using human dermal fibroblasts to find conditions that achieve ≥90% cell viability normalized to controls postwarming. Thus, physical and biological success can be achieved using this platform cryopreservation approach of rapid cooling and laser gold nanowarming in millimeter-scale systems.

Vitrification Assessment: Thermal Analysis of Cryoprotective Aqueous Solutions 1,2 Propanediol and Ethylene Glycol

Cryopreservation of viable cells and cell materials is being developed for biological and biopharmaceutical applications. The inhibition of ice formation during the cooling and warming phase of vitrified living biological samples is important for their survival. The tendency to form glasses (glass transition temperature, T_g) upon cooling in the vitrification solution and the stability of the amorphous state upon warming to determine the critical cooling rate (V_ccr) and critical warming rates (V_cwr) are evaluated. The study of thermal properties of ethylene glycol (EG) and 1,2-propanediol (PD) solutions were performed to improve vitrification through better understanding of their molecular mobility and viscosity. Two sets of aqueous solutions were tested. In group A, 35% EG (w/w) was added to different PD concentrations (5%, 10%, and 15%). In group B, 20% PD (w/w) was combined with varying concentrations of EG (20%, 24%, 27%, and 30%). Using the semiempirical model of Boutron, the values of V_ccr and V_cwr for group A were 10, 8, <2.5°C/min, and 1.65 × 10⁵, 678, 32°C/min, respectively. For group B, the values were 24, 10, <2.5, <2.5°C/min, and 9.5 × 10³, 144, 48, 7°C/min, respectively. While the values of V_ccr and V_cwr for 40% EG were 123 and 8.84 × 10⁵°C/min, respectively. The methyl group in PD enhanced the vitreous state, lowering the melting point. Adding a small concentration of PD (3%) to VM3 vitrification solution improved and increased the T_g and enhanced their thermal stability. Analyzing the thermal properties of cryoprotectant is useful when designing the cryopreservation protocols.

Hydrogel Encapsulation Facilitates Rapid-Cooling Cryopreservation of Stem Cell-Laden Core-Shell Microcapsules as Cell-Biomaterial Constructs

Core-shell structured stem cell microencapsulation in hydrogel has wide applications in tissue engineering, regenerative medicine, and cell-based therapies because it offers an ideal immunoisolative microenvironment for cell delivery and 3D culture. Long-term storage of such microcapsules as cell-biomaterial constructs by cryopreservation is an enabling technology for their wide distribution and ready availability for clinical transplantation. However, most of the existing studies focus on cryopreservation of single cells or cells in microcapsules without a core-shell structure (i.e., hydrogel beads). The goal of this study is to achieve cryopreservation of stem cells encapsulated in core-shell microcapsules as cell-biomaterial constructs or biocomposites. To this end, a capillary microfluidics-based core-shell alginate hydrogel encapsulation technology is developed to produce porcine adipose-derived stem cell-laden microcapsules for vitreous cryopreservation with very low concentration (2 mol L-1 ) of cell membrane penetrating cryoprotective agents (CPAs) by suppressing ice formation. This may provide a low-CPA and cost-effective approach for vitreous cryopreservation of "ready-to-use" stem cell-biomaterial constructs, facilitating their off-the-shelf availability and widespread applications.

Progress and challenges of fish sperm vitrification: A mini review

To survive low temperature is required for a long-term storage (cryopreservation), cells should be vitrified to a state in which intracellular water is solidified without ice crystal formation. Two different approaches are described for fish sperm cryopreservation: 1) sperm conventional cryopreservation, in which extracellular water is partially crystallized and 2) sperm vitrification, in which both intra- and extra-cellular liquids are vitrified. Sperm vitrification has been applied to some fish species with limited success. Traditional vitrification requires rapid cooling/warming rates, small sample carriers, and using high permeable cryoprotectant concentrations. The latter cause cytotoxic effects which must be well managed and will require continuous effort to match an appropriate cryoprotectant with suitable apparatus and warming methods. Novel cryoprotectant-free sperm vitrification approach has been applied to several fishes. This review summarizes development of basic procedures and discusses advantages and disadvantages of vitrification when applied it to fish sperm.

Comparison of non-ideal solution theories for multi-solute solutions in cryobiology and tabulation of required coefficients

Thermodynamic solution theories allow the prediction of chemical potentials in solutions of known composition. In cryobiology, such models are a critical component of many mathematical models that are used to simulate the biophysical processes occurring in cells and tissues during cryopreservation. A number of solution theories, both thermodynamically ideal and non-ideal, have been proposed for use with cryobiological solutions. In this work, we have evaluated two non-ideal solution theories for predicting water chemical potential (i.e. osmolality) in multi-solute solutions relevant to cryobiology: the Elliott et al. form of the multi-solute osmotic virial equation, and the Kleinhans and Mazur freezing point summation model. These two solution theories require fitting to only single-solute data, although they can make predictions in multi-solute solutions. The predictions of these non-ideal solution theories were compared to predictions made using ideal dilute assumptions and to available literature multi-solute experimental osmometric data. A single, consistent set of literature single-solute solution data was used to fit for the required solute-specific coefficients for each of the non-ideal models. Our results indicate that the two non-ideal solution theories have similar overall performance, and both give more accurate predictions than ideal models. These results can be used to select between the non-ideal models for a specific multi-solute solution, and the updated coefficients provided in this work can be used to make the desired predictions.

The Scanning Cryomacroscope - A Device Prototype for the Study of Cryopreservation

A new cryomacroscope prototype-a visualization device for the in situ analysis of cryopreserved biological samples-is presented in the current study. In order to visualize samples larger than the field of view of the optical setup, a scanning mechanism is integrated into the system, which represents a key improvement over previous cryomacroscope prototypes. Another key feature of the new design is in its compatibility with available top-loading controlled-rate cooling chambers, which eliminates the need for a dedicated cooling mechanism. The objective for the current development is to create means to generate a single digital movie of an experimental investigation, with all relevant data overlaid. The visualization capabilities of the scanning cryomacroscope are demonstrated in the current study on the cryoprotective agent dimethyl sulfoxide and the cryoprotective cocktail DP6. Demonstrated effects include glass formation, various regimes of crystallization, thermal contraction, and fracture formation.

Effect of common cryoprotectants on critical warming rates and ice formation in aqueous solutions

Ice formation on warming is of comparable or greater importance to ice formation on cooling in determining survival of cryopreserved samples. Critical warming rates required for ice-free warming of vitrified aqueous solutions of glycerol, dimethyl sulfoxide, ethylene glycol, polyethylene glycol 200 and sucrose have been measured for warming rates of order 10-10⁴ K/s. Critical warming rates are typically one to three orders of magnitude larger than critical cooling rates. Warming rates vary strongly with cooling rates, perhaps due to the presence of small ice fractions in nominally vitrified samples. Critical warming and cooling rate data spanning orders of magnitude in rates provide rigorous tests of ice nucleation and growth models and their assumed input parameters. Current models with current best estimates for input parameters provide a reasonable account of critical warming rates for glycerol solutions at high concentrations/low rates, but overestimate both critical warming and cooling rates by orders of magnitude at lower concentrations and larger rates. In vitrification protocols, minimizing concentrations of potentially damaging cryoprotectants while minimizing ice formation will require ultrafast warming rates, as well as fast cooling rates to minimize the required warming rates.

Application of the multisolute osmotic virial equation to solutions containing electrolytes

The prediction of multisolute solution behavior of solutions containing electrolytes is important in many areas of research, including cryopreservation. In this study, the use of a novel form of the osmotic virial equation for multisolute solutions containing an electrolyte is investigated and compared to a rigorous electrolyte solution theory, the Pitzer-Debye-Huckel equation. For aqueous solutions containing a small molecule (either dimethyl sulfoxide or glycerol) and sodium chloride, the multisolute osmotic virial equation, which utilizes only two parameters to capture the electrolyte solution behavior, is shown to be as accurate as the Pitzer-Debye-Huckel equation, which utilizes six empirical parameters and multiple functions to capture the electrolyte solution behavior. In addition, an approach based on the multisolute osmotic virial equation to investigate the effect of electrolyte concentration on macromolecule solution behavior is presented and applied to aqueous solutions of hydroxyethyl starch and sodium chloride. The multisolute osmotic virial equation is shown to be an accurate, straightforward predictive solution theory for important multisolute solutions containing electrolytes.

Statistical prediction of the vitrifiability and glass stability of multi-component cryoprotective agent solutions

Long-term biologic storage of articular cartilage has proven elusive due to cellular degradation over time or acute damage during attempts at cryopreservation. Vitrification is one option that may result in successful cryopreservation but difficulty with cryoprotective agent (CPA) toxicity at high concentrations of a single cryoprotectant has hindered development of successful protocols. This study was designed to determine the vitrifiability and glass stability of solutions containing combinations of commonly used CPAs and to document CPA interactions that occur. One hundred and sixty-four multi-CPA combination solutions of 6-9 M were evaluated for vitrifiability and glass stability using direct visualization after immersion in liquid nitrogen for 30 min and upon warming. Binary and ordinal logistic regression analysis was used to statistically analyze each CPA for its ability to vitrify and its effect on glass stability in multi-component CPA solutions. Propylene glycol had the greatest incremental contribution to vitrification while formamide had the least contribution. A threshold was established whereby the ability of a solution to vitrify could be determined by calculation. Glass stability was not as clearly defined due to variability in the results; however, contributions of interactions between CPAs to the glass stability of solutions were determined. This study provided values that predict if a solution will vitrify. Furthermore, the glass stability of solutions containing multiple CPAs do not behave as linear additions of binary solutions and interactions between CPAs have a significant effect on the glass stability of these solutions. These variables should be considered when designing vitrification solutions.

Review of biomaterial thermal property measurements in the cryogenic regime and their use for prediction of equilibrium and non-equilibrium freezing applications in cryobiology

It is well accepted in cryobiology that the temperature history and cooling rates experienced in biomaterials during freezing procedures correlate strongly with biological outcome. Therefore, heat transfer measurement and prediction in the cryogenic regime is central to the field. Although direct measurement of temperature history (i.e. heat transfer) can be performed, accuracy is usually achieved only for local measurements within a given system and cannot be readily generalized to another system without the aid of predictive models. The accuracy of these models rely upon thermal properties which are known to be highly dependent on temperature, and in the case of significant cryoprotectant loading, also on crystallized fraction. In this work, we review the available thermal properties of biomaterials in the cryogenic regime. The review shows a lack of properties for many biomaterials in the subzero temperature domain, and especially for systems with cryoprotective agents. Unfortunately, use of values from the limited data available (usually only down to -40 degrees C) lead to an underestimation of thermal property change (i.e. conductivity rise and specific heat drop due to ice crystallization) with lower temperatures. Conversely, use of surrogate values based solely on ice thermal properties lead to an overestimation of thermal property change for most biomaterials. Additionally, recent work extending the range of available thermal properties to -150 degrees C has shown that the thermal conductivity will drop in both PBS and tissue (liver) due to amorphous/glassy phases (versus crystalline) of biomaterials with the addition of cryoprotective additives such as glycerol. Thus, we investigated the implications of using approximated or constant property values versus measured temperature-dependent values for predicting temperature history during freezing in PBS (phosphate-buffered saline) and porcine liver with and without cryoprotectants (glycerol). Using measured property values (thermal conductivity, specific heat, and latent heat of phase change) of porcine liver, a standard was created which showed that values based on surrogate ice properties under-predicted cooling times, while constant properties (i.e. based on limited data reported near the freezing point) over-predicted cooling times. Additionally, a new iterative numerical method that accommodates non-equilibrium cooling effects as a function of time and position (i.e. crystallization versus amorphous phase) was used to predict temperature history during freezing in glycerol loaded systems. Results indicate that in addition to the increase in cooling times due to the lowering of thermal diffusivity with more glycerol, non-equilibrium effects such as the prevention of maximal crystallization (i.e. amorphous phases) will further increase required cooling times. It was also found that the amplified effect of non-equilibrium cooling and crystallization with system size prevents the thermal history to be described with non-dimensional lengths, such as was possible under equilibrium cooling. These results affirm the need to use accurate thermal properties that incorporate temperature dependence and crystallized fraction. Further studies are needed to extract thermal properties of other important biomaterials in the subzero temperature domain and to develop accurate numerical methods which take into account non-equilibrium cooling events encountered in cryobiology when partial or total vitrification occurs.

Application of the osmotic virial equation in cryobiology

The multisolute osmotic virial equation is the only multisolute thermodynamic solution theory that has been derived from first principles and can make predictions of multisolute solution behaviour in the absence of multisolute solution data. Other solution theories either (i) include simplifying assumptions that do not take into account the interactions between different types of solute molecules or (ii) require fitting to multisolute data to obtain empirical parameters. The osmotic virial coefficients, which are obtained from single-solute data, can be used to make predictions of multisolute solution osmolality. The osmotic virial coefficients for a range of solutes of interest in cryobiology are provided in this paper, for use with concentration units of both molality and mole fraction, along with an explanation of the background and theory necessary to implement the multisolute osmotic virial equation.

Thermodynamic aspects of vitrification

Vitrification is a process in which a liquid begins to behave as a solid during cooling without any substantial change in molecular arrangement or thermodynamic state variables. The physical phenomenon of vitrification is relevant to both cryopreservation by freezing, in which cells survive in glass between ice crystals, and cryopreservation by vitrification in which a whole sample is vitrified. The change from liquid to solid behavior is called the glass transition. It is coincident with liquid viscosity reaching 10(13) Poise during cooling, which corresponds to a shear stress relaxation time of several minutes. The glass transition can be understood on a molecular level as a loss of rotational and translational degrees of freedom over a particular measurement timescale, leaving only bond vibration within a fixed molecular structure. Reduced freedom of molecular movement results in decreased heat capacity and thermal expansivity in glass relative to the liquid state. In cryoprotectant solutions, the change from liquid to solid properties happens over a approximately 10 degrees C temperature interval centered on a glass transition temperature, typically near -120 degrees C (+/-10 degrees C) for solutions used for vitrification. Loss of freedom to quickly rearrange molecular position causes liquids to depart from thermodynamic equilibrium as they turn into a glass during vitrification. Residual molecular mobility below the glass transition temperature allows glass to very slowly contract, release heat, and decrease entropy during relaxation toward equilibrium. Although diffusion is practically non-existent below the glass transition temperature, small local movements of molecules related to relaxation have consequences for cryobiology. In particular, ice nucleation in supercooled vitrification solutions occurs at remarkable speed until at least 15 degrees C below the glass transition temperature.

A Multisolute Osmotic Virial Equation for Solutions of Interest in Biology

The osmotic virial equation was used to predict osmolalities of solutions of interest in biology. The second osmotic virial coefficients, Bi, account for the interactions between identical solute molecules. For multisolute solutions, the second osmotic virial cross coefficient, Bij, describes the interaction between two different solutes. We propose to use as a mixing rule for the cross coefficient the arithmetic average of the second osmotic virial coefficients of the pure species, so that only binary solution measurements are required for multisolute solution predictions. Single-solute data were fit to obtain the osmotic virial coefficients of the pure species. Using those coefficients with the proposed mixing rule, predictions were made of ternary solution osmolality, without any fitting parameters. This method is shown to make reasonably accurate predictions for three very different ternary aqueous solutions: (i) glycerol + dimethyl sulfoxide + water, (ii) hemoglobin + an ideal, dilute solute + water, and (iii) bovine serum albumin + ovalbumin + water.

Thermal properties of ethylene glycol aqueous solutions

Preventing ice crystallization by transforming liquids into an amorphous state, vitrification can be considered as the most suitable technique allowing complex tissues, and organs cryopreservation. This process requires the use of rapid cooling rates in the presence of cryoprotective solutions highly concentrated in antifreeze compounds, such as polyalcohols. Many of them have already been intensively studied. Their glass forming tendency and the stability of their amorphous state would make vitrification a reality if their biological toxicity did not reduce their usable concentrations often below the concentrations necessary to vitrify organs under achievable thermal conditions. Fortunately, it has been shown that mixtures of cryoprotectants tend to reduce the global toxicity of cryoprotective solutions and various efficient combinations have been proposed containing ethanediol. This work reports on the thermal properties of aqueous solutions with 40, 43, 45, 48, and 50% (w/w) of this compound measured by differential scanning calorimetry. The glass forming tendency and the stability of the amorphous state are evaluated as a function of concentration. They are given by the critical cooling rates v(ccr)above which ice crystallization is avoided, and the critical warming rates v(cwr) necessary to prevent ice crystallization in the supercooled liquid state during rewarming. Those critical rates are calculated using the same semi-empirical model as previously. This work shows a strong decrease of averaged critical cooling and warming rates when ethanediol concentration increases, V(ccr) and V(cwr) = 1.08 x 10 (10) K/min for 40% (w/w) whereas V(ccr) = 11 and V(cwr) = 853 K/min for 50% (w/w). Those results are compared with the corresponding properties of other dialcohols obtained by the same method. Ethylene glycol efficiency is between those of 1,2-propanediol and 1,3-propanediol.

Thermal study of simple amino-alcohol solutions

Widely regarded as the most promising approach to long-term cryopreservation of organs for transplantation, vitrification is a process where liquid is transformed into a disordered solid state free from crystals, known as the amorphous state. The vitreous state is obtained by rapid cooling to cryogenic temperatures in the presence of antifreeze substances called cryoprotectants, such as polyalcohols, which are known to be very good vitrification agents. This work reports on the thermal properties of a new class of compounds, the amino-alcohols, studied for its similarity to the structure of the equivalent polyalcohols. We studied by differential scanning calorimetry the glass-forming tendency and stability of the amorphous state for de-ionized water solutions containing 2-amino-1-ethanol and 3-amino-1-propanol at the concentrations of 35%, 40%, 43%, and 45% (w/w). A comparison is made with previous results obtained by Mehl [Cryobiology 27 (1990) 687-688] on the same compounds under different experimental conditions. The results are also compared with those obtained by Boutron [Cryobiology 30 (1993) 87-97] for the corresponding dialcohols. A further comparison is made with a few results obtained for the 1-amino-2-propanol and the 2-amino-1-propanol tested under the same conditions.

Water crystallization within rat precision-cut liver slices in relation to their viability

This study examined whether tissue vitrification, promoted by partitioning within the tissue, could be the mechanism explaining the high viability of rat liver slices, rapidly frozen after preincubation with 18% Me2SO or VS4 (a 7.5 M mixture of Me2SO, 1,2-propanediol, and formamide with weight ratio 21.5:15:2.4). To achieve this, we first determined the extent to which crystallization or vitrification occurred in cryoprotectant solutions (Me2SO and VS4) and within liver slices impregnated with these solutions. Second, we determined how these events were related to survival of slices after thawing. Water crystallization was evaluated by differential scanning calorimetry and viability was determined by histomorphological examination of the slices after culturing at 37 degrees C for 4 h. VS4-preincubated liver slices indeed behaved differently from bulk VS4 solution, because, when vitrified, they had a lower tendency to devitrify. Vitrified VS4-preincubated slices that were warmed sufficiently rapid to prevent devitrification had a high viability. When VS4 was diluted (to 75%) or if warming was not fast enough to prevent ice formation, slices had a low viability. With 45% Me2SO, low viability of cryopreserved slices was caused by cryoprotectant toxicity. Surprisingly, liver slices preincubated with 18% Me2SO or 50% VS4 had a high viability despite the formation of ice within the slice. In conclusion, tissue vitrification provides a mechanism that explains the high viability of VS4-preincubated slices after ultrarapid freezing and thawing (>800 degrees C/min). Slices that are preincubated with moderately concentrated cryoprotectant solutions (18% Me2SO, 50% VS4) and cooled rapidly (100 degrees C/min) survive cryopreservation despite the formation of ice crystals within the slice.

Glass-forming tendency in the system water-dimethyl sulfoxide

The glass-forming tendency on cooling and the stability of the wholly amorphous state on warming have been previously reported for many cryoprotective solutions. However, unlike the other solutions, those of dimethyl sulfoxide (Me(2)SO) have not been studied on cooling. In this paper, the glass-forming tendency of Me(2)SO aqueous solutions has been measured for solutions containing 40, 43, 45, and 47.5% (w/w) Me(2)SO. At a concentration of 45% (w/w), the glass-forming tendency decreases in the following order: levo-2, 3-butanediol, 1,3-butanediol, 1,2-propanediol, 1,2,3-butanetriol, dimethyl sulfoxide, dimethylformamide, diethylformamide, 1, 4-butanediol, ethylene glycol, glycerol, 1,3-propanediol. New measurements have also been made on warming the Me(2)SO solutions.

Glass-forming tendency and stability of aqueous solutions of diethylformamide and dimethylformamide

The glass-forming tendency on cooling and the stability of the wholly amorphous state on warming of aqueous solutions of diethylformamide and of dimethylformamide have been studied by calorimetry. With diethylformamide, only ice formation is observed except on warming at the lowest rate of 2.5 degreesC/min, where occasionally a hydrate forms also. The hydrate was observed up to 10 degreesC/min with 50% diethylformamide. With dimethylformamide hydrates form even at high warming rates. The last hydrate melts at -47.7 degreesC. The warming thermograms are much more complicated than for diethylformamide. For the glass-forming tendency on cooling, as well as for the stability of the wholly amorphous state on warming, these two compounds, at concentrations of 40, 45, or 50% (w/w) in water, are more efficient than glycerol and ethylene glycol, but less than 1,2-propanediol and levo-2,3-butanediol. On warming, they are comparable to DMSO. Pure diethylformamide could not be crystallized, whereas, conversely, pure dimethylformamide could not be vitrified. Curiously, the glass transition of aqueous solutions of diethylformamide increases and then decreases with the diethylformamide concentration in water, contrary to other cryoprotectants, for which it always increases or decreases. Diethyl- and dimethylformamide could be interesting cryoprotectants if they are not too toxic when added before cryopreservation, and in the case of dimethylformamide, if one can avoid damage due to its hydrates. Copyright 1998 Academic Press.

Crystallization of ice in aqueous solutions of glycerol and dimethyl sulfoxide 2: ice crystal growth kinetics

The crystallization of ice in aqueous solutions of glycerol and dimethyl sulfoxide (Me2SO) has been studied using a combined DSC-video microscope technique. The solutions investigated were 50w/w% glycerol and 45w/w% Me2SO; both of these solutions have a solute concentration of approximately 16 mol%. The rates of growth of the external surfaces of ice crystals from both of these solutions were determined over broad temperature ranges. The growth rates were found to be generally independent of time, particularly at lower temperatures. The ice crystal growth rate in the glycerol solution became negligible at a significantly higher temperature than in the Me2SO solution. Addition of anti-freeze protein from the winter flounder at concentrations of 1.7 and 9.9 mg g-1 was found to have no significant effect on the ice crystal growth rates in 50w/w% glycerol solutions.

Some insight into the physical basis of the cryoprotective action of dimethyl sulfoxide and ethylene glycol

In the determination of the solid-liquid phase equilibria in the aqueous mixtures of dimethyl sulfoxide (Me2SO) and ethylene glycol (EG) one often encounters the problem of equilibrium crystallization. In the present report the above aqueous solutions are equilibrated for crystallization in a dielectric cell during which the dielectric method is used for monitoring the extent of crystallization. The melting temperatures are then measured by using the dielectric technique in combination with the differential scanning calorimeter. The equilibrium phase diagram of Me2SO is found to be eutectic with two compounds formed of water and Me2SO in the ratio of 3:1 and 2:1. In the case of EG solutions it is eutectic with a 1:1 compound formation. It is suggested that the greater depression of the freezing point of water due to the complex formation and hence the attendant increase in the viscosity near the freezing point is the reason for the sluggish crystallization in these solutions. The variation of the glass transition temperature with composition is also examined in the above solutions along with the aqueous solutions of a number of other cryoprotectants. The glass-forming tendency of these solutions is discussed in terms of complex formation. An attempt is made to distinguish between good and bad glass-forming additives in terms of complex formation and ice clathrate formation.

Vitrification properties of solutions of ethylene glycol in saline containing PVP, Ficoll, or dextran

Vitrification solutions which are used for cells or embryos generally contain cryoprotectant, physiological saline, and one or more macromolecular solutes. The macromolecules modify the vitrification tendencies of these solutions, but there is little detailed information on the vitrification properties of ethylene glycol solutions containing the additives PVP, Ficoll, and dextran. This study therefore added ethylene glycol to 0.9% NaCl in water (saline) and used differential scanning calorimetry to determine the lowest concentration at which the solution would remain vitreous when a warming rate of 10 degrees C/min was used. In the absence of other additives 59 wt% ethylene glycol (EG) in saline formed a stable glass. When ethylene glycol was replaced by the polymers Ficoll and/or dextran on a weight for weight basis, the resulting solution vitrified less readily than an EG-saline solution even though the total solute concentration was kept constant. The total solute concentration required to form a stable vitreous solution increased as the Ficoll 70,000 and 400,000 MW or dextran 78,000 MW content increased (5, 10, and 20 wt%). Ficoll and dextran had little or no effect on the glass transition and melting points of the solutions. In the presence of PVP vitrification occurred at a total solute concentration of 59 wt% (PVP 360,000 MW) or 60 wt% (PVP 40,000 MW) for all three tested PVP concentrations (5, 10, and 20 wt%). Although this indicates that PVP and EG have comparable vitrification properties, the melting and the glass transition temperature of the solutions rose as the PVP content increased. When 1 m sucrose was added to saline and 0, 5, 10, or 20 wt% PVP 40,000 MW vitrification was achieved with 31, 26, 23, and 15% EG, respectively, indicating that the total solute concentration required for vitrification could be estimated with reasonable accuracy from the sum of the individual components. We conclude that the tested polymers differ in how they interact with ethylene glycol-based vitrification solutions.

Cryoprotection of red blood cells by a 2,3-butanediol containing mainly the levo and dextro isomers

A 2,3-butanediol containing 96.7% (w/w) racemic mixture of the levo and dextro isomers and only 3.1% (w/w) of the meso isomer (called 2,3-butanediol 97% dl) has been used for the cryoprotection of red blood cells. The erythrocytes were cooled to -196 degrees C at rates between 2 and 3500 degrees C/min, followed by slow or rapid warming. Up to 20% (w/w) of this polyalcohol, only the classical peak of survival is observed, as with up to 20% (w/w) 1,2-propanediol or 1,3-butanediol. Twenty percent 2,3-butanediol 97% dl can protect red blood cells very efficiently. The maximum survival, of 90%, as with 20% glycerol, is a little lower than with 20% 1,2-propanediol and higher than with 20% 1,3-butanediol. Fifteen percent 2,3-butanediol protects fewer red blood cells than 15% glycerol or 1,2-propanediol, with a maximum survival of about 80%. The best cryoprotection by 30% 2,3-butanediol 97% dl is obtained at the slowest cooling and warming rates, where survival approaches 90%. After a minimum, an increase of survival is observed at the fastest cooling rates, which would correspond to complete vitrification. These rates are lower than with 30%, 1,2-propanediol or 1,3-butanediol, in agreement with the higher glass-forming tendency of 2,3-butanediol 97% dl solutions. In agreement with the remarkable physical properties of its aqueous solutions, the present experiments also suggest that 2,3-butanediol containing mainly the levo and dextro isomers could be a very useful cryoprotectant for organ cryopreservation. However, it would perhaps be better to use it in combination with other cryoprotectants, since it is a little more toxic than glycerol or 1,2-propanediol at high concentrations.

Physical aging of glassy state: DSC study of vitrified glycerol systems

After annealing during the glass transition temperature (Tg) range or at sub-Tg temperatures, a "Tg overshoot" has been observed by differential scanning calorimetry in vitrified 55 and 80% (W/W) glycerol solutions. The temperature dependence of the overshoot is most pronounced immediately below the Tg and always increases in amplitude with lengthened annealing periods, whether annealed during Tg range or below. Of particular note is the anomalous shift in devitrification temperature (Td) to lower temperatures following increased time of annealing at sub-Tg temperatures. The Tg obtained on warming also occurs at higher temperatures with increased annealing times. The results allow us to suggest the existence of the time and temperature dependency of the glassy state. Accordingly, it may be desirable to maintain vitrified biological systems at temperatures sufficiently below Tg so that the extent of relaxation in the glass system is avoided or minimized during cryopreservation. The relaxation effects must be taken into account prior to warming a biological system after a long-term cryopreservation. These effects should also be considered in studies of vitrification solutions.

Theoretical prediction of devitrification tendency: determination of critical warming rates without using finite expansions

Previously, critical warming rates vcr above which ice did not have enough time to crystallize had been roughly evaluated for many wholly amorphous aqueous solutions. These evaluations were obtained by extrapolation of the linear variation of the devitrification temperature Td with log v, where v is the warming rate, observed experimentally between 2.5 and 80 degrees C/min. Theory also gives such a linear variation, but only using the first term of a finite expansion. The other terms can be neglected only for small variations of Td. These evaluations were sufficient for classification of the solutions, but large errors were made in vcr. A new and more accurate method of determination of the variation of Td with v is presented here. The general equation giving in our models the derivative of the quantity of ice formed versus temperature T is differentiated, instead of integrated using a finite expansion. This gives an explicit expression of v versus Td assuming that the ratio xd of the quantity of ice formed at Td to the total quantity of ice formed on warming is constant. Experimentally, xd is constant within a good approximation. Theoretical curves representing the variation of Td with v have been drawn for solutions of 35 or 45% (w/w) 1,2-propanediol in water. Td never reaches the temperature of the end of melting Tm, but as v tends toward infinity, Td tends toward an asymptotic value of 0.96Tm for 35% solute. For that solution, above about 10(3) degrees C/min, Td deviates appreciably from linearity with log v, but 1/Td remains almost linear with log v up to Td congruent to 0.95Tm. Therefore, systematic comparison of the theoretical variation of Td with v with a linear variation of 1/Td with log v has been done, varying the parameters of the equations within the entire experimental range. Similar conclusions can be given for all the solutions. Experimentally for Td = 0.95Tm, the quantity of ice crystallized is generally less than 0.1% of the solution, reaching 1% only once. Therefore, a new definition of the critical warming rate vcr has been used, corresponding to extrapolation of the linear variation of 1/T with log v up to Td = 0.95Tm. New values of vcr have been calculated for all the binary systems previously studied. The order of the solutions is almost the same, but the new values of vcr are significantly smaller than the former.

Experimental dissection of devitrification in aqueous solutions of 1,3-butanediol

Devitrification is a major problem which must be overcome for successful organ cryopreservation. Devitrification can be initiated on fracture planes and on bubbles, but the focus of attention here is on devitrification by ordinary heterogeneous and homogeneous mechanisms, which are the most relevant for organ preservation by vitrification. The purpose of the present studies was to define the devitrification process: to determine nucleation rates, ice-crystal growth rates, and the distribution of ice-crystal size and to evaluate the applicability of existing quantitative models of these processes which have successfully approximated the behavior of other aqueous systems. The present work was done using differential scanning calorimetry and cryomicroscopy. The amount of ice formed has been estimated for highly concentrated solutions. Kinetic parameters are presented here for isothermal conditions and continuous heating rate experiments. The classical theory based on the Johnson-Avrami equation has been evaluated and the results are compared with the theory of Boutron. The agreement is good for the continuous heating rate conditions, but results differ for the isothermal conditions.

Phenomena at the advancing ice-liquid interface: solutes, particles and biological cells

Ice formation in aqueous solutions and suspensions involves a number of significant changes and processes in the residual liquid. The resulting effects were described concerning the redistribution of dissolved salts, the behaviour of gaseous solutes and bubble formation, the rejection and entrapment of second-phase particles. This set of conditions is also experienced by biological cells subjected to freezing. The influences of ice formation in that respect and their relevance for cryopreservation were considered as well. A model of transient heat conduction and solute diffusion with a planar ice front, propagating through a system of finite length was found to be in good agreement with measured salt concentration profiles. The spacing of the subsequently developing columnar solidification pattern was of the same order of magnitude as the pertubation wavelengths predicted from the stability criterion. Non-planar solidification of binary salt solutions was described by a pure heat transfer model under the assumption of local thermodynamic equilibrium. The rejection of gaseous solutes and the resulting gas concentration profile ahead of a planar ice front has been estimated by means of a test bubble method, yielding a distribution coefficient of 0.05 for oxygen. The nucleation of gas bubbles has been observed to occur at slightly less than 20-fold supersaturation. The subsequent radial growth of the bubbles obeys a square-root time dependence as expected from a diffusion controlled model until the still expanding bubbles become engulfed by the advancing ice-liquid interface. The maximum bubble radii decrease for increasing ice front velocities. The transition between repulsion and entrapment of spherical latex particles by an advancing planar ice-front has been characterized by a critical value of the velocity of the solidification interface. The critical velocity is inversely proportional to the particle radius as suggested by models assuming an undisturbed ice front. The increase of the critical velocity for increasing thermal gradients shows good agreement with a theoretically predicted square-root type of dependence. Critical velocities have also been measured for yeast and red blood cells. The effect of freezing on biological cells has been analyzed for human lymphocytes and erythrocytes. The reduction of cell volume observed during non-planar freezing agrees reasonably well with shrinkage curves calculated from a water transport model. The probability of intracellular ice formation has been characterized by threshold cooling rates above which the amount of water remaining within the cell is sufficient for crystallization.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of 1,2-propanediol as a cryoprotectant on the freezing of mouse oocytes

Cryopreservation of mammalian eggs has been successfully accomplished using 1,2-propanediol (PG). Effects of holding times of 0 and 30 min at -40 degrees C and storage times of 1 d and 1 mo at -196 degrees C were investigated in combination with various concentrations of PG (1.0, 1.5, and 2.0M) to determine the survival and fertilizability of mouse oocytes rapidly frozen and thawed in straws. A rapid one-step dilution using 0.5 M sucrose solution inside the straws was used following the thawing of oocytes. A significant effect of PG concentration was found between 1.0 M and 1.5 or 2.0 M (P<0.01), but no significance was discovered between 1.5 M and 2.0 M (P>0.05) on subsequent survival and fertilizability of frozen and thawed mouse oocytes. With 2.0 M PG, the best survival rate (58.3%) and fertilizability rate (19.0%) were obtained by holding at -40 degrees C for 30 min and by storage at -196 degrees C for 1 d. Thirty minutes of holding at -40 degrees C reduced oocyte damage during the procedure but not significantly (P>0.05). In addition, there was no significant difference in the various storage periods (P>0.05). This study demonstrated that mammalian oocytes can be cryopreserved in the presence of 1,2-propanediol by utilizing a rapid freezing and thawing procedure.