Biotransport phenomena in freezing mammalian oocytes

Use of membrane transport models to design cryopreservation procedures for oocytes

Oocyte cryopreservation is increasingly being used in reproductive technologies for conservation and breeding purposes. Further development of oocyte cryopreservation techniques requires interdisciplinary insights in the underlying principles of cryopreservation. This review aims to serve this purpose by: (1) highlighting that preservation strategies can be rationally designed, (2) presenting mechanistic insights in volume and osmotic stress responses associated with CPA loading strategies and cooling, and (3) giving a comprehensive listing of oocyte specific biophysical membrane characteristics and commonly used permeation model equations. It is shown how transport models can be used to simulate the behavior of oocytes during cryopreservation processing steps, i.e., during loading of cryoprotective agents (CPAs), cooling with freezing as well as vitrification, warming and CPA unloading. More specifically, using defined cellular and membrane characteristics, the responses of oocytes during CPA (un)loading were simulated in terms of temperature- and CPA type-and-concentration-dependent changes in cell volume and intracellular solute concentration. In addition, in order to determine the optimal cooling rate for slow programmable cooling cryopreservation, the freezing-induced cell volume response was simulated at various cooling rates to estimate rates with tolerable limits. For vitrification, special emphasis was on prediction of the timing of reaching osmotic tolerance limits during CPA exposure, and the need to use step-wise CPA addition/removal protocols. In conclusion, we present simulations and schematic illustrations that explain the timing of events during slow cooling cryopreservation as well as vitrification, important for rationally designing protocols taking into account how different CPA types, concentrations and temperatures affect the oocyte.

Monitoring of freezing patterns within 3D collagen-hydroxyapatite scaffolds using infrared thermography

The importance of cryopreservation in tissue engineering is unceasingly increasing. Preparation, cryopreservation, and storage of tissue-engineered constructs (TECs) at an on-site location offer a convenient way for their clinical application and commercialization. Partial freezing initiated at high sub-zero temperatures using ice-nucleating agents (INAs) has recently been applied in organ cryopreservation. It is anticipated that this freezing technique may be efficient for the preservation of both scaffold mechanical properties and cell viability of TECs. Infrared thermography is an instrumental method to monitor INAs-mediated freezing of various biological entities. In this paper, porous collagen-hydroxyapatite (HAP) scaffolds were fabricated and characterized as model TECs, whereas infrared thermography was proposed as a method for monitoring the crystallization-related events on their partial freezing down to -25 °C. Intra- and interscaffold latent heat transmission were descriptively evaluated. Nucleation, freezing points as well as the degree of supercooling and duration of crystallization were calculated based on inspection of respective thermographic curves. Special consideration was given to the cryoprotective agent (CPA) composition (Snomax®, crude leaf extract from Hippophae rhamnoides, dimethyl sulfoxide (Me₂SO) and recombinant type-III antifreeze protein (AFP)) and freezing conditions ('in air' or 'in bulk CPA'). For CPAs without ice nucleation activity, thermographic measurements demonstrated that the supercooling was significantly milder in the case of scaffolds present in a CPA solution compared to that without them. This parameter (ΔT, °C) altered with the following tendency: 10 Me₂SO (2.90 ± 0.54 ('in air') vs. 7.71 ± 0.43 ('in bulk CPA', P < 0.0001)) and recombinant type-III AFP, 0.5 mg/ml (2.65 ± 0.59 ('in air') vs. 7.68 ± 0.34 ('in bulk CPA', P < 0.0001)). At the same time, in CPA solutions with ice nucleation activity the least degree of supercooling and the longest crystallization duration (Δt, min) for scaffolds frozen 'in air' were documented for crude leaf homogenate (CLH) from Hippophae rhamnoides (1.57 ± 0.37 °C and 21.86 ± 2.93 min compared to Snomax, 5 μg/ml (2.14 ± 0.33 °C and 23.09 ± 0.05), respectively). The paper offers evidence that infrared thermography provides insightful information for monitoring partial freezing events in TECs when using different freezing containers, CPAs and conditions. This may further TEC-specific cryopreservation and optimization of CPA compositions with slow-nucleating properties.

Water-transport and intracellular ice formation of human adipose-derived stem cells during freezing

The key to optimizing the cryopreservation strategy of human adipose-derived stem cells (hADSCs) is to identify the biophysical characteristics during freezing. Systematic freezing experiments were conducted under a cryo-microscope system to investigate the cryoinjury mechanism for hADSCs at different cooling rates. By simultaneously fitting morphological change data to the water-transport equation at 5, 10 and 20 °C/min, the plasma membrane hydraulic conductivity, L_pg, and activation energy, E_Lp, were determined. Moreover, the optimal cooling rate was also predicted by using mathematical model methods. Additionally, the surface-catalyzed nucleation (SCN) parameters were calculated by fitting in numerical models, Ω₀^SCN and k₀^SCN were determined at cooling rates of 30, 45 and 60 °C/min. These results may provide potential application value for cryopreservation of hADSCs.

Soft liquid metal nanoparticles achieve reduced crystal nucleation and ultrarapid rewarming for human bone marrow stromal cell and blood vessel cryopreservation

High warming rates during cryopreservation are crucial and essential for successful vitrification. However, realizing a faster warming rate in low-concentration cryoprotective agents appears to be challenging for conventional warming process through convective heat transfer. Herein, we developed a liquid metal (LM) nanosystem that can act as a spatial source to significantly enhance the warming rates with near-infrared laser irradiation during the warming process. The synthetic Pluronic F127-liquid metal nanoparticles (PLM NPs) displayed multiple performances with uniform particle size, superior photothermal conversion efficiency (52%), repeatable photothermal stability, and low cytotoxicity. Particularly, it is more difficult for the liquid PLM NPs with less surface free energy to form crystal nucleation than other solid NPs such as gold and Fe₃O₄, which is beneficial for the cooling process during cryopreservation. The viability of human bone marrow-derived mesenchymal stem cells postcryopreservation reached 78±3%, which is threefold higher than that obtained by the conventional warming method (25±6%). Additionally, the cells postcryopreservation maintained their normal attachment, proliferation, surface marker expression, and intact multilineage differentiation properties. Moreover, the results of mouse tails including blood vessel cryopreservation showed a relatively improved intact structure when using PLM NP rewarming compared with the results of conventional warming. The new LM nanosystem provides a universal platform for cryopreservation that is expected to have potential for widespread applications including bioengineering, cell-based medicine, and clinical translation. STATEMENT OF SIGNIFICANCE: In this study, we fabricated soft liquid metal nanoparticles with high photothermal conversion efficiency, repeatable photothermal stability, and low cytotoxicity. Particularly, soft liquid metal nanoparticles with less surface free energy and suppression effects of ice formation were first introduced to mediate cryopreservation. Superior ice-crystallization inhibition is achieved as a result of less crystal nucleation and ultrarapid rewarming during the freezing and warming processes of cryopreservation, respectively. Collectively, cryopreservation of human bone marrow stromal cells (HBMSCs) and mouse tails including blood vessels can be successfully performed using this new nanoplatform, showing great potential in the application of soft nanoparticles in cryopreservation.

The application of convolution neural network based cell segmentation during cryopreservation

For most of the cells, water permeability and plasma membrane properties play a vital role in the optimal protocol for successful cryopreservation. Measuring the water permeability of cells during subzero temperature is essential. So far, there is no perfect segmentation technique to be used for the image processing task on subzero temperature accurately. The ice formation and variable background during freezing posed a significant challenge for most of the conventional segmentation algorithms. Thus, a robust and accurate segmentation approach that can accurately extract cells from extracellular ice that surrounding the cell boundary is needed. Therefore, we propose a convolutional neural network (CNN) architecture similar to U-Net but differs from those conventionally used in computer vision to extract all the cell boundaries as they shrank in the engulfing ice. The images used was obtained from the cryo-stage microscope, and the data was validated using the Hausdorff distance, means ± standard deviation for different methods of segmentation result using the CNN model. The experimental results prove that the typical CNN model extracts cell borders contour from the background in its subzero state more coherent and effective as compared to other traditional segmentation approaches.

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.

Modeling and experimental studies of enhanced cooling by medical gauze for cell cryopreservation by vitrification

Vitrification is considered as an important alternative approach to traditional slow freezing method for cryopreservation of cells. A typical cell vitrification procedure involves a non-equilibrium cooling process commonly accomplished in liquid nitrogen, while in which film boiling is believed to greatly hinder heat transfer surrounding the sample, resulting in incomplete vitrification or a much higher critical concentration. In this study, we developed a simple while effective approach, wrapping traditional French-type straw with medical gauze, to greatly enhance convective heat transfer during cooling by suppress film boiling. We further established a coupled heat transfer model for cooling and warming of cell suspensions to investigate the inherent thermodynamic mechanism in this approach. The model describes both the macroscale thermal distributions in extracellular solution and the microscale ice crystallization inside the cells. The simulation indicated that straws wrapped with medical gauze would increase cell survival subject to vitrification cryopreservation by significantly increasing the cooling rate to inhibit intracellular ice formation (IIF). Our experiments on human umbilical vein endothelial cells (HUVECs) further confirmed the predictions in that the cell survival rate was significantly increased by wrapping straws with medical gauze.

Magnetic induction heating of superparamagnetic nanoparticles during rewarming augments the recovery of hUCM-MSCs cryopreserved by vitrification

Cryopreservation by vitrification has been recognized as a promising strategy for long-term banking of living cells. However, the difficulty to generate a fast enough heating rate to minimize devitrification and recrystallization-induced intracellular ice formation during rewarming is one of the major obstacles to successful vitrification. We propose to overcome this hurdle by utilizing magnetic induction heating (MIH) of magnetic nanoparticles to enhance rewarming. In this study, superparamagnetic (SPM) Fe3O4 nanoparticles were synthesized by a chemical coprecipitation method. We successfully applied the MIH of Fe3O4 nanoparticles for rewarming human umbilical cord matrix mesenchymal stem cells (hUCM-MSCs) cryopreserved by vitrification. Our results show that extracellular Fe3O4 nanoparticles with MIH may efficiently suppress devitrification and/or recrystallization during rewarming and significantly improve the survival of vitrified cells. We further optimized the concentration of Fe3O4 nanoparticles and the current of an alternating current (AC) magnetic field for generating the MIH to maximize cell viability. Our results indicate that MIH in an AC magnetic field with 0.05% (w/v) Fe3O4 nanoparticles significantly facilitates rewarming and improves the cryopreservation outcome of hUCM-MSCs by vitrification. The application of MIH of SPM nanoparticles to achieve rapid and spatially homogeneous heating is a promising strategy for enhanced cryopreservation of stem cells by vitrification.

STATEMENT OF SIGNIFICANCE

Here we report the successful synthesis and application of Fe3O4 nanoparticles for magnetic induction heating (MIH) to enhance rewarming of vitrification-cryopreserved human umbilical cord matrix mesenchymal stem cells (hUCM-MSCs). We found that MIH-enhanced rewarming greatly improves the survival of vitrification-cryopreserved hUCM-MSCs. Moreover, the hUCM-MSCs retain their intact stemness and multilineage potential of differentiation post cryopreservation by vitrification with the MIH-enhanced rewarming. Therefore, the novel MIH-enhanced cell vitrification is valuable to facilitate the long-term storage of hUCM-MSCs and possibly many other important cells to meet their ever-increasing demand by the burgeoning cell-based medicine.

Comparison of the Fitting Validity Between the 2P Model and the Nondilute Solution Model Using Statistical Methods in Modeling Cell Membrane Permeabilities

In cryopreservation, the two-parameter (2P) model and the nondilute solution model have been developed to study the membrane transport properties of cells. However, to our knowledge, comparison of the fitting validity has never been made between the two models. In this study, to make this comparison, the permeability parameters of porcine adipose-derived stem cells (pADSCs) were first determined with the two models, and then the errors between the predictions and the experimental data were tested using the Lilliefors test. The results indicate that the 2P model is slightly better than the nondilute solution model in predicting the mass transport across cell membrane. The causes for this phenomenon are discussed and suggestions on using these two models are given.

Alginate Hydrogel Microencapsulation Inhibits Devitrification and Enables Large-Volume Low-CPA Cell Vitrification

Cryopreservation of stem cells is important to meet their ever-increasing demand by the burgeoning cell-based medicine. The conventional slow freezing for stem cell cryopreservation suffers from inevitable cell injury associated with ice formation and the vitrification (i.e., no visible ice formation) approach is emerging as a new strategy for cell cryopreservation. A major challenge to cell vitrification is intracellular ice formation (IIF, a lethal event to cells) induced by devitrification (i.e., formation of visible ice in previously vitrified solution) during warming the vitrified cells at cryogenic temperature back to super-zero temperatures. Consequently, high and toxic concentrations of penetrating cryoprotectants (i.e., high CPAs, up to ~8 M) and/or limited sample volumes (up to ~2.5 μl) have been used to minimize IIF during vitrification. We reveal that alginate hydrogel microencapsulation can effectively inhibit devitrification during warming. Our data show that if ice formation were minimized during cooling, IIF is negligible in alginate hydrogel-microencapsulated cells during the entire cooling and warming procedure of vitrification. This enables vitrification of pluripotent and multipotent stem cells with up to ~4 times lower concentration of penetrating CPAs (up to 2 M, low CPA) in up to ~100 times larger sample volume (up to ~250 μl, large volume).

Effect of Hydroxyapatite Nanoparticles on Biotransport Phenomena in Freezing HeLa Cells

The effect of nanoparticles on subzero biotransport phenomena of living cells is very rare in the literature, although the information is of great importance for the application of nanotechnology in the field of cryobiology. In this study, subzero water transport phenomena in freezing HeLa cells in 1 × phosphate buffered saline (PBS) containing 0%, 0.05%, and 0.1% (w/w) hydroxyapatite (HA) nanoparticles with and without pre-incubation at 37 °C was quantitatively investigated. The results reveal that the presence of HA nanoparticles slightly facilitates the subzero water transport of HeLa cells.

Influence of hydroxyapatite nanoparticles on the viscosity of dimethyl sulfoxide-H2O-NaCl and glycerol-H2O-NaCl ternary systems at subzero temperatures

The viscosity, at subzero temperatures, of ternary solutions commonly used in cryopreservation is tremendously important for understanding ice formation and molecular diffusion in biopreservation. However, this information is scarce in the literature. In addition, to the best of our knowledge, the effect of nanoparticles on the viscosity of these solutions has not previously been reported. The objectives of this study were thus: (i) to systematically measure the subzero viscosity of two such systems, dimethyl sulfoxide (Me2SO)-H2O-NaCl and glycerol-H2O-NaCl; (ii) to explore the effect of hydroxyapatite (HA) nanoparticles on the viscosity; and (iii) to provide models that precisely predict viscosity at multiple concentrations of cryoprotective agent (CPA) in saline solutions at subzero temperatures. Our experiments were performed in two parts. We first measured the viscosity at multiple CPA concentrations [0.3-0.75 (w/w)] in saline solution with and without nanoparticles at subzero temperatures (0 to -30°C). The data exhibited a good fit to the Williams-Landel-Ferry (WLF) equation. We then measured the viscosity of residual unfrozen ternary solutions with and without nanoparticles during equilibrium freezing. HA nanoparticles made the solution more viscous, suggesting applications for these nanoparticles in preventing cell dehydration, ice nucleation, and ice growth during freezing and thawing in cryopreservation.

An improved model for nucleation-limited ice formation in living cells during freezing

Ice formation in living cells is a lethal event during freezing and its characterization is important to the development of optimal protocols for not only cryopreservation but also cryotherapy applications. Although the model for probability of ice formation (PIF) in cells developed by Toner et al. has been widely used to predict nucleation-limited intracellular ice formation (IIF), our data of freezing Hela cells suggest that this model could give misleading prediction of PIF when the maximum PIF in cells during freezing is less than 1 (PIF ranges from 0 to 1). We introduce a new model to overcome this problem by incorporating a critical cell volume to modify the Toner's original model. We further reveal that this critical cell volume is dependent on the mechanisms of ice nucleation in cells during freezing, i.e., surface-catalyzed nucleation (SCN) and volume-catalyzed nucleation (VCN). Taken together, the improved PIF model may be valuable for better understanding of the mechanisms of ice nucleation in cells during freezing and more accurate prediction of PIF for cryopreservation and cryotherapy applications.

Role of cells in freezing-induced cell-fluid-matrix interactions within engineered tissues

During cryopreservation, ice forms in the extracellular space resulting in freezing-induced deformation of the tissue, which can be detrimental to the extracellular matrix (ECM) microstructure. Meanwhile, cells dehydrate through an osmotically driven process as the intracellular water is transported to the extracellular space, increasing the volume of fluid for freezing. Therefore, this study examines the effects of cellular presence on tissue deformation and investigates the significance of intracellular water transport and cell-ECM interactions in freezing-induced cell-fluid-matrix interactions. Freezing-induced deformation characteristics were examined through cell image deformetry (CID) measurements of collagenous engineered tissues embedded with different concentrations of MCF7 breast cancer cells versus microspheres as their osmotically inactive counterparts. Additionally, the development of a biophysical model relates the freezing-induced expansion of the tissue due to the cellular water transport and the extracellular freezing thermodynamics for further verification. The magnitude of the freezing-induced dilatation was found to be not affected by the cellular water transport for the cell concentrations considered; however, the deformation patterns for different cell concentrations were different suggesting that cell-matrix interactions may have an effect. It was, therefore, determined that intracellular water transport during freezing was insignificant at the current experimental cell concentrations; however, it may be significant at concentrations similar to native tissue. Finally, the cell-matrix interactions provided mechanical support on the ECM to minimize the expansion regions in the tissues during freezing.

Numerical simulation of the effect of superparamagnetic nanoparticles on microwave rewarming of cryopreserved tissues

In this study, the microwave rewarming process of cryopreserved samples with embedded superparamagnetic (SPM) nanoparticles was numerically simulated. The Finite Element Method (FEM) was used to calculate the coupling of the electromagnetic field and the temperature field in a microwave rewarming system composed of a cylindrical resonant cavity, an antenna source, and a frozen sample phantom with temperature-dependent properties. The heat generated by the sample and the nanoparticles inside the electromagnetic field of the microwave cavity was calculated. The dielectric properties of the biological tissues were approximated using the Debye model, which is applicable at different temperatures. The numerical results showed that, during the rewarming process of the sample phantom without nanoparticles, the rewarming rate was 29.45°C/min and the maximum temperature gradient in the sample was 3.58°C/mm. If nanoparticles were embedded in the sample, and the cavity power was unchanged, the rewarming rate was 47.76°C/min and the maximum temperature gradient in the sample was 1.64°C/mm. In the presence of SPM nanoparticles, the rewarming rate and the maximum temperature gradient were able to reach 20.73°C/min and 0.68°C/mm at the end of the rewarming under the optimized cavity power setting, respectively. The ability to change these temperature behaviors may prevent devitrification and would greatly diminish thermal stress during the rewarming process. The results indicate that the rewarming rate and the uniformity of temperature distribution are increased by nanoparticles. This could be because nanoparticles generated heat in the sample homogeneously and the time-dependent parameters of the sample improved after nanoparticles were homogeneously embedded within it. We were thus able to estimate the positive effect of SPM nanoparticles on microwave rewarming of cryopreserved samples.

Freezing mammalian cells for production of biopharmaceuticals

Cryopreservation techniques utilize very low temperatures to preserve the structure and function of living cells. Various strategies have been developed for freezing mammalian cells of biological and medical significance. This paper highlights the importance and application of cryopreservation for recombinant mammalian cells used in the biopharmaceutical industry to produce high-value protein therapeutics. It is a primer that aims to give insight into the basic principles of cell freezing for the benefit of biopharmaceutical researchers with limited or no prior experience in cryobiology. For the more familiar researchers, key cell banking parameters such as the cell density and hold conditions have been reviewed to possibly help optimize their specific cell freezing protocols. It is important to understand the mechanisms underlying the freezing of complex and sensitive cellular entities as we implement best practices around the techniques and strategies used for cryopreservation.

Thermostability of biological systems: fundamentals, challenges, and quantification

This review examines the fundamentals and challenges in engineering/understanding the thermostability of biological systems over a wide temperature range (from the cryogenic to hyperthermic regimen). Applications of the bio-thermostability engineering to either destroy unwanted or stabilize useful biologicals for the treatment of diseases in modern medicine are first introduced. Studies on the biological responses to cryogenic and hyperthermic temperatures for the various applications are reviewed to understand the mechanism of thermal (both cryo and hyperthermic) injury and its quantification at the molecular, cellular and tissue/organ levels. Methods for quantifying the thermophysical processes of the various applications are then summarized accounting for the effect of blood perfusion, metabolism, water transport across cell plasma membrane, and phase transition (both equilibrium and non-equilibrium such as ice formation and glass transition) of water. The review concludes with a summary of the status quo and future perspectives in engineering the thermostability of biological systems.