Controlled ice nucleation in cryopreservation – A review

High-speed cryo-microscopy reveals that ice-nucleating proteins of Pseudomonas syringae trigger freezing at hydrophobic interfaces

Ice-nucleating proteins (INpro) trigger the freezing of supercooled water droplets relevant to atmospheric, biological, and technological applications. The high ice nucleation activity of INpro isolated from the bacteria Pseudomonas syringae could be linked to the aggregation of proteins at the bacterial membrane or at the air-water interface (AWI) of droplets. Here, we imaged freezing onsets, providing direct evidence of these proposed mechanisms. High-speed cryo-microscopy identified the onset location of freezing in droplets between two protein-repellent glass slides. INpro from sterilized P. syringae (Snomax) statistically favored nucleation at the AWI of the droplets. Removing cellular fragments by filtration or adding surfactants increased the frequency of nucleation events at the AWI. On the other hand, cultivated intact bacteria cells or lipid-free droplets nucleated ice without an affinity to the AWI. Overall, we provide visual evidence that INpro from P. syringae trigger freezing at hydrophobic interfaces, such as the AWI or the bacterial membrane, with important mechanistic implications for applications of INpro.

Synergistic Effects of the Superhydrophilic and Superhydrophobic Components on the Antifreezing Performances of Latex Particles and Anti-Icing Properties of Latex Films

The development of new materials for antifreezing and anti-icing applications is a big challenge in industry and academic area. Inspired by the antifreeze proteins, latex particles with superhydrophilic zwitterionic shells and superhydrophobic cores are synthesized by reversible addition-fragmentation chain transfer emulsion polymerization, and the applications of the latex particles in antifreezing and anti-icing applications are investigated. In antifreezing study, the critical aggregate temperature (CAT) of the latex particles decreases, and the separation of the melting and freezing temperature of ice increases with the particle concentration. Enzyme molecules can be cryopreserved in the particle solution, and their bioactivities are well maintained. Latex particles are casted into latex films with dynamic surfaces. Anti-icing performances, including antifrosting properties, freezing delay time, and ice adhesion strengths, are studied; and the water-treated latex films present stronger anti-icing properties than other films, due to the synergistic effects of the superhydrophilic and superhydrophobic components. In addition, latex particles with zwitterionic shells and poly(n-butyl methacrylate) cores, and latex particles with small molecular surfactant on the surfaces are synthesized. The antifreezing performances of the latex particles and anti-icing properties of the latex films are compared.

Cryopreservation-induced delayed injury and cell-type-specific responses during the cryopreservation of endothelial cell monolayers

The cryopreservation of endothelial cell monolayers is an important step that bridges the cryopreservation of cells in suspension to that of tissues. Previous studies have identified clear distinctions in freezing mechanisms between cells in suspension and in monolayers, as well as developed novel protocols for monolayer cryopreservation. Recently, our group has shown that human umbilical vein endothelial cell (HUVEC) and porcine corneal endothelial cell (PCEC) monolayers grown on Rinzl plastic substrate can be cryopreserved in 5% dimethyl sulfoxide, 6% hydroxyethyl starch, and 2% chondroitin sulfate, following a slow-cooling protocol (-1 °C/min) with rapid plunge into liquid nitrogen from -40 °C. However, membrane integrity assessments were done immediately post thaw, which may result in an overestimation of cell viability due to possible delayed injury responses. Here, we show that for the optimal protocol condition of plunge at the -40 °C interrupt temperature, HUVEC and PCEC monolayers exhibited no significant immediate post-thaw injuries nor delayed injury responses during the 24-h post-thaw overnight culture period. HUVEC monolayers experienced no significant impact to their natural growth rate during the post-thaw culture, while PCEC monolayers experienced significantly higher growth than the unfrozen controls. The difference in the low-temperature responses between HUVEC and PCEC monolayers was further shown under high temperature plunge conditions. At these suboptimal plunge temperatures, HUVEC monolayers exhibited moderate immediate membrane injury but a pronounced delayed injury response during the 24-h post-thaw culture, while PCEC monolayers showed significant immediate membrane injury but no additional delayed injury response during the same period. Therefore, we provide further validation of our group's previously designed endothelial monolayer cryopreservation protocol for HUVEC and PCEC monolayers, and we identify several cell-type-specific responses to the freezing process.

Cryopreserved Thyroid Tissue Autotransplant in Pediatric Age Patients: A Feasibility Study and Literature Review

BACKGROUND AND AIMS

This paper aims to study an alternative solution to hormonal replacement therapy in specific groups of patients who underwent thyroidectomy during childhood or adulthood. After cryopreservation, thyroid autotransplantation could be an alternative solution which would allow us to use the ability of the thyroid tissue of producing hormones according to the physiological needs of the body.

MATERIALS AND METHODS

A feasibility study about the effects of the most modern cryopreservation techniques on the structural and functional integrity of the follicular cells of the thyroid tissue has been carried out. Patients who could benefit from the treatment have been found for both autotransplant techniques. Additionally, a literature review has been conducted.

RESULTS

The histological analysis has shown that cryopreservation does not alter the original architecture, and the culture examination that cell viability is successfully preserved. Moreover, both thyroid autotransplantation studies on animals and those on humans that were found in the literature have shown good results regarding the viability and functionality of the transplant.

CONCLUSIONS

The viability of cryopreserved thyroid tissue found in this study is encouraging. Further studies to evaluate the levels of FT3, FT4 and thyroglobulin in thyroid tissue after cryopreservation are needed to verify that the secretory properties of the thyrocytes have been maintained intact. Furthermore, autotransplanted cases found in the literature do not have a long-term follow-up.

Relevance of controlled cooling and freezing phases in T-cell cryopreservation

When cells are cryopreserved, they go through a freezing process with several distinct phases (i.e., cooling until nucleation, ice nucleation, ice crystal growth and cooling to a final temperature). Conventional cell freezing approaches often employ a single cooling rate to describe and optimize the entire freezing process, which neglects its complexity and does not provide insight into the effects of the different freezing phases. The aim of this work was to elucidate the impact of each freezing phase by varying different process parameters per phase. Hereto, spin freezing was used to freeze Jurkat T cells in either a Me2SO-based or Me2SO-free formulation. The cooling rates before ice nucleation and after total ice crystallization impacted cell viability, resulting in viability ranging from 26.7% to 52.8% for the Me2SO-free formulation, and 22.5%-42.6% for the Me2SO-based formulation. Interestingly, the degree of supercooling upon nucleation did not exhibit a significant effect on cell viability in this work. However, the rate of ice crystal formation emerged as a crucial factor, with viability ranging from 2.4% to 53.2% for the Me2SO-free formulation, and 0.3%-53.2% for the Me2SO-based formulation, depending on the freezing rate. A morphological study of the cells post-cryopreservation was performed using confocal microscopy, and it was found that cytoskeleton integrity and cell volume were impacted, depending on the formulation-process parameter combination. These findings underscore the importance of scrutinizing all cooling and freezing phases, as each phase impacted post-thaw viability in a distinct way, depending of the specific formulation used.

Monitoring Aqueous Sucrose Solutions Using Droplet Microfluidics: Ice Nucleation, Growth, Glass Transition, and Melting

Freezing and freeze-drying processes are commonly used to extend the shelf life of drug products and to ensure their safety and efficacy upon use. When designing a freezing process, it is beneficial to characterize multiple physicochemical properties of the formulation, such as nucleation rate, crystal growth rate, temperature and concentration of the maximally freeze-concentrated solution, and melting point. Differential scanning calorimetry has predominantly been used in this context but does have practical limitations and is unable to quantify the kinetics of crystal growth and nucleation. In this work, we introduce a microfluidic technique capable of quantifying the properties of interest and use it to investigate aqueous sucrose solutions of varying concentration. Three freeze-thaw cycles were performed on droplets with 75-μm diameters at cooling and warming rates of 1 °C/min. During each cycle, the visual appearance of the droplets was optically monitored as they experienced nucleation, crystal growth, formation of the maximally freeze-concentrated solution, and melting. Nucleation and crystal growth manifested as increases in droplet brightness during the cooling phase. Heating was associated with a further increase as the temperature associated with the maximally freeze-concentrated solution was approached. Heating beyond the melting point corresponded to a decrease in brightness. Comparison with the literature confirmed the accuracy of the new technique while offering new visual data on the maximally freeze-concentrated solution. Thus, the microfluidic technique presented here may serve as a complement to differential scanning calorimetry in the context of freezing and freeze-drying. In the future, it could be applied to a plethora of mixtures that undergo such processing, whether in pharmaceutics, food production, or beyond.

Systematic cryopreservation study of cardiac myoblasts in suspension

H9c2 myoblasts are a cell line derived from embryonic rat heart tissue and demonstrate the ability to differentiate to cardiac myotubes upon reduction of the serum concentration (from 10% to 1%) and addition of all-trans retinoic acid in the growth medium. H9c2 cells are increasingly being used as an easy-to-culture proxy for some functions of cardiomyocytes. The cryobiology of cardiac cells including H9c2 myoblasts has not been studied as extensively as that of some cell types. Consequently, it is important to characterize the cryobiological response and systematically develop well-optimized cryopreservation protocols for H9c2 cells to have optimal and consistent viability and functionality after thaw for high quality studies with this cell type. In this work, an interrupted slow cooling protocol (graded freezing) was applied to characterize H9c2 response throughout the cooling profile. Important factors that affect the cell response were examined, and final protocols that provided the highest post-thaw viability are reported. One protocol uses the common cryoprotectant dimethyl sulfoxide combined with hydroxyethyl starch, which will be suitable for applications in which the presence of dimethyl sulfoxide is not an issue; and the other protocol uses glycerol as a substitute when there is a desire to avoid dimethyl sulfoxide. Both protocols achieved comparable post-thaw viabilities (higher than 80%) based on SYTO 13/GelRed flow cytometry results. H9c2 cells cryopreserved by either protocol showed ability to differentiate to cardiac myotubes comparable to fresh (unfrozen) H9c2 cells, and their differentiation to cardiac myotubes was confirmed with i) change in cell morphology, ii) expression of cardiac marker troponin I, and iii) increase in mitochondrial mass.

Comparison of liquid nitrogen-free slow freezing protocols toward enabling a practical option for centralized cryobanking of ovarian tissue

Geographically distributed ovarian tissue cryobanks remain limited due to the high facility and staff costs, and cold transportation to centers is associated with ischemia-induced tissue damage that increases with transport distance. It is ideal to perform the cryopreservation procedure at a tissue removal site or local hospital before shipment to cost-effective centralized cryobanks. However, conventional liquid nitrogen-based freezers are not portable and require expensive infrastructure. To study the possibility of an ovarian tissue cryopreservation network not dependent on liquid nitrogen, we cryopreserved bovine ovarian tissue using three cooling techniques: a controlled rate freezer using liquid nitrogen, a liquid nitrogen-free controlled rate freezer, and liquid nitrogen-free passive cooling. Upon thawing, we evaluated a panel of viability metrics in frozen and fresh groups to examine the potency of the portable liquid nitrogen-free controlled and uncontrolled rate freezers in preserving the ovarian tissue compared to the non-portable conventional controlled rate freezer. We found similar outcomes for reactive oxygen species (ROS), total antioxidant capacity (TAC), follicular morphology, tissue viability, and fibrosis in the controlled rate freezer groups. However, passive slow cooling was associated with the lowest tissue viability, follicle morphology, and TAC, and the highest tissue fibrosis and ROS levels compared to all other groups. A stronger correlation was found between follicle morphology, ovarian tissue viability, and fibrosis with the TAC/ROS ratio compared to ROS and TAC alone. The current study undergirds the possibility of centralized cryobanks using a controlled rate liquid nitrogen-free freezer to prevent ischemia-induced damage during ovarian tissue shipment.

Successful expansion and cryopreservation of human natural killer cell line NK-92 for clinical manufacturing

Natural killer (NK) cells have recently shown renewed promise as therapeutic cells for use in treating hematologic cancer indications. Despite this promise, NK cell manufacturing workflows remain largely manual, open, and disconnected, and depend on feeders, as well as outdated unit operations or processes, often utilizing research-grade reagents. Successful scale-up of NK cells critically depends on the availability and performance of nutrient-rich expansion media and cryopreservation conditions that are conducive to high cell viability and recovery post-thaw. In this paper we used Cytiva hardware and media to expand the NK92 cell line in a model process that is suitable for GMP and clinical manufacturing of NK cells. We tested a range of cryopreservation factors including cooling rate, a range of DMSO-containing and DMSO-free cryoprotectants, ice nucleation, and cell density. Higher post-thaw recovery was seen in cryobags over cryovials cooled in identical conditions, and cooling rates of 1°C/min or 2°C/min optimal for cryopreservation in DMSO-containing and DMSO-free cryoprotectants respectively. Higher cell densities of 5x107 cells/ml gave higher post-thaw viability than those cryopreserved at either 1x106 or 5x106 cells/ml. This enabled us to automate, close and connect unit operations within the workflow while demonstrating superior expansion and cryopreservation of NK92 cells. Cellular outputs and performance were conducive to clinical dosing regimens, serving as a proof-of-concept for future clinical and commercial manufacturing.

Understanding the impact of ammonium ion substitutions on heterogeneous ice nucleation

Understanding the mechanisms underpinning heterogeneous ice nucleation in the presence of ionic inclusions is important for fields such as cryopreservation and for improved models of climate and weather prediction. Feldspar and ammonium are both present in significant quantities in the atmosphere, and experimental evidence has shown that feldspar can nucleate ice from ammonium-containing solutions at temperatures warmer than water alone. In recent work, Whale hypothesised that this increase in nucleation temperature is due to an increase in configurational entropy when an ammonium ion is included in the ice hydrogen bond network (T. F. Whale, J. Chem. Phys., 2022, 156, 144503). In this work, we investigate the impact of the inclusion of an ammonium ion on the hydrogen bond network by direct enumeration of the number of structures found using Rick's algorithm. We also determine the energy of these systems and thus compare the effects of enthalpy and entropy to test Whale's hypothesis. We find that the inclusion of an ammonium ion increases the total number of configurations under conditions consistent with a realistic surface charge. We also find that the enthalpic contribution is dominant in determining the location of the ammonium ion within the structure, although we note that this neglects other practicalities of ice nucleation.

Cryopreservation of organoids: Strategies, innovation, and future prospects

Organoid technology has demonstrated unique advantages in multidisciplinary fields such as disease research, tumor drug sensitivity, clinical immunity, drug toxicology, and regenerative medicine. It will become the most promising research tool in translational research. However, the long preparation time of organoids and the lack of high-quality cryopreservation methods limit the further application of organoids. Although the high-quality cryopreservation of small-volume biological samples such as cells and embryos has been successfully achieved, the existing cryopreservation methods for organoids still face many bottlenecks. In recent years, with the development of materials science, cryobiology, and interdisciplinary research, many new materials and methods have been applied to cryopreservation. Several new cryopreservation methods have emerged, such as cryoprotectants (CPAs) of natural origin, ice-controlled biomaterials, and rapid rewarming methods. The introduction of these technologies has expanded the research scope of cryopreservation of organoids, provided new approaches and methods for cryopreservation of organoids, and is expected to break through the current technical bottleneck of cryopreservation of organoids. This paper reviews the progress of cryopreservation of organoids in recent years from three aspects: damage factors of cryopreservation of organoids, new protective agents and loading methods, and new technologies of cryopreservation and rewarming.

An open-hardware community ice nucleation cold stage for research and teaching

Aerosol particles with rare specific properties act as nuclei for ice formation. The presence of ice nucleating particles in the atmosphere leads to heterogeneous freezing at warm temperatures and thus these particles play an important role in modulating microphysical properties of clouds. This work presents an ice nucleation cold stage instrument for measuring the concentration of ice nucleating particles in liquids. The cost is ∼ $10 k including an external chiller. Using a lower cost heat sink reduces the cost to ∼ $6 k. The instrument is suitable for studying ambient ice nucleating particle concentrations and laboratory-based process-level studies of ice nucleation. The design plans allow individuals to self-manufacture the cold-stage using 3D printing, off-the-shelf parts, and a handful of standard tools. Software to operate the instrument and analyze the data is also provided. The design is intended to be simple enough that a graduate student can build it as part of a course or thesis project. Costs are kept to a minimum to facilitate use in classroom demonstrations and laboratory classes.

Cryopreservation of assay-ready hepatocyte monolayers by chemically-induced ice nucleation: preservation of hepatic function and hepatotoxicity screening capabilities

Cell culture plays a critical role in biomedical discovery and drug development. Primary hepatocytes and hepatocyte-derived cell lines are especially important cellular models for drug discovery and development. To enable high-throughput screening and ensure consistent cell phenotypes, there is a need for practical and efficient cryopreservation methods for hepatocyte-derived cell lines and primary hepatocytes in an assay-ready format. Cryopreservation of cells as adherent monolayers in 96-well plates presents unique challenges due to low volumes being susceptible to supercooling, leading to low recovery and well-to-well variation. Primary cell cryopreservation is also particularly challenging due to the loss of cell viability and function. In this study, we demonstrate the use of soluble ice nucleator materials (IN) to cryopreserve a hepatic-derived cell line (HepG2) and primary mouse hepatocytes, as adherent monolayers. HepG2 cell recovery was near 100% and ∼75% of primary hepatocytes were recovered 24 hours post-thaw compared to just 10% and 50% with standard 10% DMSO, respectively. Post-thaw assessment showed that cryopreserved HepG2 cells retain membrane integrity, metabolic activity, proliferative capacity and differentiated hepatic functions including urea secretion, cytochrome P450 levels and lipid droplet accumulation. Cryopreserved primary hepatocytes exhibited reduced hepatic functions compared to fresh hepatocytes, but functional levels were similar to commercial suspension-cryopreserved hepatocytes, with the added benefit of being stored in an assay-ready format. In addition, normal cuboidal morphology and minimal membrane damage were observed 24 hours post-thaw. Cryopreserved HepG2 and mouse hepatocytes treated with a panel of pharmaceutically active compounds produced near-identical dose-response curves and EC50 values compared to fresh hepatocytes, confirming the utility of cryopreserved bankable cells in drug metabolism and hepatotoxicity studies. Cryopreserved adherent HepG2 cells and primary hepatocytes in 96 well plates can significantly reduce the time and resource burden associated with routine cell culture and increases the efficiency and productivity of high-throughput drug screening assays.

Slow Freezing of Preserved Boar Sperm: Comparison of Conventional and Automated Techniques on Post-Thaw Functional Quality by a New Combination of Sperm Function Tests

The slow freezing of boar sperm is the only way to preserve genetic material for extended periods; this can be achieved with exposure to liquid nitrogen vapors (conventional) or by using automated freezing equipment. The aim was to compare the effect of both techniques on post-thaw functionality. Boar sperm devoid of seminal plasma and resuspended in lactose-egg yolk-glycerol medium were cryopreserved. Conventional: straws were exposed to LN2 vapors; automated: using a drop curve of -39.82 °C·min-1 for 113 s from -5 to -80 °C during the critical period; and subsequent immersion in NL2. Cell viability, cholesterol flow, mitochondrial membrane potential (MMP), lipid peroxidation, peroxynitrite, superoxide anion levels, phosphatidylserine translocation, and caspase activation were evaluated by flow cytometry. In addition, total motility (TM) and progressive motility (PM) were determined by the SCA system immediately (T0), 60 (T60), and 120 min (T120) post-thawing. Automated freezing significantly reduces cholesterol flow and free radical and lipid peroxidation levels, making it possible to preserve motility for 120 min of incubation. At the same time, viability, acrosome integrity, MMP, and caspase activation did not differ from the conventional technique. In conclusion, controlling the temperature drop curve using automated freezing equipment reduces oxidative/nitrosative stress, preserving membrane fluidity and sperm motility.

Does supercooled water retain its universal nucleation behavior under shear at high pressure?

Understanding the nucleation of homogeneous flow systems at high pressures is vital in protein crystallization and cryopreservation, where high pressure prevents the freezing of biological samples. This study examines the behavior of ice nucleation under shear at various pressures and explores the universal nucleation behavior of the sheared systems applied to supercooled water at higher pressures. In this study, the nucleation rates for the TIP4P/Ice model via a seeding method based on extended classical nucleation theory (CNT) are computed at pressures of 1, 100, 500, 700, and 1000 bar and a constant temperature of 240 K. Using extended CNT with explicitly embodying the shear rate, we analyzed the dependence of pressure on the transport and thermodynamic properties. In line with previous studies, we observed that Δμliq-ice and viscosity decrease while diffusivity increases with an increase in pressure. Furthermore, we showed that the dependence of the nucleation rate on shear at higher pressure is non-monotonic, with the maximum at optimal shear rates between 107 and 108 s-1. Our results demonstrate a non-monotonic pressure dependence of the optimal shear rates, which could originate from a violation of the Stokes-Einstein relation.

Slow Cooling and Controlled Ice Nucleation Enabling the Cryopreservation of Human T Lymphocytes with Low-Concentration Extracellular Trehalose

Cryopreservation of human T lymphocytes has become a key strategy for supporting cell-based immunotherapy. However, the effects of ice seeding on the cryopreservation of cells under relatively slow cooling have not been well researched. The cryopreservation strategy with a nontoxic, single-ingredient, and injectable cryoprotective solution remains to be developed. We conducted ice seeding for the cells in a solution of normal saline with 1% (v/v) dimethyl sulfoxide (Me2SO), 0.1 M trehalose, and 4% (w/v) human serum albumin (HSA) under different slow cooling rates. With the positive results, we further applied seeding in the solution of 0.2 M trehalose and 4% (w/v) HSA under the same cooling rates. The optimal concentration of trehalose in the Me2SO-free solutions was then investigated under the optimized cooling rate with seeding, with control groups without seeding, and in a freezing container. In vitro toxicity of the cryoprotective solutions to the cells was also tested. We found that the relative viability of cells (1% [v/v] Me2SO, 0.1 M trehalose and 4% [w/v] HSA) was improved significantly from 88.6% to 94.1% with ice seeding, compared with that without seeding (p < 0.05). The relative viability of cells (0.2 M trehalose and 4% [w/v] HSA) with seeding was significantly higher than that without seeding, 96.3% and 92.0%, respectively (p < 0.05). With no significant difference in relative viability between the solutions of 0.2 M trehalose or 0.3 M trehalose with 4% (w/v) HSA (92.4% and 94.6%, respectively, p > 0.05), the solution of 0.2 M trehalose and 4% (w/v) HSA was selected as the optimized Me2SO-free solution. This strategy could cryopreserve human T lymphocytes without any toxic cryoprotectant and boost the application of cell products in humans by intravenous injection, with the osmolality of the low-concentration cryoprotective solution close to that of human plasma.

Observing ice structure of micron-sized vapor-deposited ice with an x-ray free-electron laser

The direct observation of the structure of micrometer-sized vapor-deposited ice is performed at Pohang Accelerator Laboratory x-ray free electron laser (PAL-XFEL). The formation of micrometer-sized ice crystals and their structure is important in various fields, including atmospheric science, cryobiology, and astrophysics, but understanding the structure of micrometer-sized ice crystals remains challenging due to the lack of direct observation. Using intense x-ray diffraction from PAL-XFEL, we could observe the structure of micrometer-sized vapor-deposited ice below 150 K with a thickness of 2-50 μm grown in an ultrahigh vacuum chamber. The structure of the ice grown comprises cubic and hexagonal sequences that are randomly arranged to produce a stacking-disordered ice. We observed that ice with a high cubicity of more than 80% was transformed to partially oriented hexagonal ice when the thickness of the ice deposition grew beyond 5 μm. This suggests that precise temperature control and clean deposition conditions allow μm-thick ice films with high cubicity to be grown on hydrophilic Si3N4 membranes. The low influence of impurities could enable in situ diffraction experiments of ice nucleation and growth from interfacial layers to bulk ice.

Characterizing Surface Ice-Philicity Using Molecular Simulations and Enhanced Sampling

The formation of ice, which plays an important role in diverse contexts ranging from cryopreservation to atmospheric science, is often mediated by solid surfaces. Although surfaces that interact favorably with ice (relative to liquid water) can facilitate ice formation by lowering nucleation barriers, the molecular characteristics that confer icephilicity to a surface are complex and incompletely understood. To address this challenge, here we introduce a robust and computationally efficient method for characterizing surface ice-philicity that combines molecular simulations and enhanced sampling techniques to quantify the free energetic cost of increasing surface-ice contact at the expense of surface-water contact. Using this method to characterize the ice-philicity of a family of model surfaces that are lattice matched with ice but vary in their polarity, we find that the nonpolar surfaces are moderately ice-phobic, whereas the polar surfaces are highly ice-philic. In contrast, for surfaces that display no complementarity to the ice lattice, we find that ice-philicity is independent of surface polarity and that both nonpolar and polar surfaces are moderately ice-phobic. Our work thus provides a prescription for quantitatively characterizing surface ice-philicity and sheds light on how ice-philicity is influenced by lattice matching and polarity.

Engineered Compounds to Control Ice Nucleation and Recrystallization

One of the greatest concerns in the subzero storage of cells, tissues, and organs is the ability to control the nucleation or recrystallization of ice. In nature, evidence of these processes, which aid in sustaining internal temperatures below the physiologic freezing point for extended periods of time, is apparent in freeze-avoidant and freeze-tolerant organisms. After decades of studying these proteins, we now have easily accessible compounds and materials capable of recapitulating the mechanisms seen in nature for biopreser-vation applications. The output from this burgeoning area of research can interact synergistically with other novel developments in the field of cryobiology, making it an opportune time for a review on this topic.

Chemically Induced Extracellular Ice Nucleation Reduces Intracellular Ice Formation Enabling 2D and 3D Cellular Cryopreservation

3D cell assemblies such as spheroids reproduce the in vivo state more accurately than traditional 2D cell monolayers and are emerging as tools to reduce or replace animal testing. Current cryopreservation methods are not optimized for complex cell models, hence they are not easily banked and not as widely used as 2D models. Here we use soluble ice nucleating polysaccharides to nucleate extracellular ice and dramatically improve spheroid cryopreservation outcomes. This protects the cells beyond using DMSO alone, and with the major advantage that the nucleators function extracellularly and hence do not need to permeate the 3D cell models. Critical comparison of suspension, 2D and 3D cryopreservation outcomes demonstrated that warm-temperature ice nucleation reduces the formation of (fatal) intracellular ice, and in the case of 2/3D models this reduces propagation of ice between adjacent cells. This demonstrates that extracellular chemical nucleators could revolutionize the banking and deployment of advanced cell models.

RSeeds: Rigid Seeding Method for Studying Heterogeneous Crystal Nucleation

Heterogeneous nucleation is the dominant form of liquid-to-solid transition in nature. Although molecular simulations are most uniquely suited to studying nucleation, the waiting time to observe even a single nucleation event can easily exceed the current computational capabilities. Therefore, there exists an imminent need for methods that enable computationally fast and feasible studies of heterogeneous nucleation. Seeding is a technique that has proven to be successful at dramatically expanding the range of computationally accessible nucleation rates in simulation studies of homogeneous crystal nucleation. In this article, we introduce a new seeding method for heterogeneous nucleation called Rigid Seeding (RSeeds). Crystalline seeds are treated as pseudorigid bodies and simulated on a surface with metastable liquid above its melting temperature. This allows the seeds to adapt to the surface and identify favorable seed-surface configurations, which is necessary for reliable predictions of crystal polymorphs that form and the corresponding heterogeneous nucleation rates. We demonstrate and validate RSeeds for heterogeneous ice nucleation on a flexible self-assembled monolayer surface, a mineral surface based on kaolinite, and two model surfaces. RSeeds predicts the correct ice polymorph, exposed crystal plane, and rotation on the surface. RSeeds is semiquantitative and can be used to estimate the critical nucleus size and nucleation rate when combined with classical nucleation theory. We demonstrate that RSeeds can be used to evaluate nucleation rates spanning many orders of magnitude.

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.

Anomalous Vapor and Ice Nucleation in Water at Negative Pressures: A Classical Density Functional Theory Study

In contrast to the abundance of work on the anomalous behavior of water, the relationship between the water's thermodynamic anomalies and kinetics of phase transition from metastable water is relatively unexplored. In this work, we have employed classical density functional theory to provide a unified and coherent picture of nucleation (both vapor and ice) from metastable water at negative pressure conditions. Our results suggest a peculiar nonmonotonic temperature dependence of vapor-liquid surface tension at temperatures where vapor-liquid coexistence is metastable with respect to the ice phase. The vapor nucleation barrier on isochoric cooling also shows a nonmonotonic temperature dependence. We further report that, for low density isochores, the temperature of the minimum vapor nucleation barrier (TΔΩv/min*) does not coincide with the temperature of maximum density (TMD) where metastability is maximum. The difference between the TΔΩv/min* and the TMD, however, decreases with increasing the density of the isochore. The vapor nucleation barrier along isobars shows an interesting crossover behavior in the vicinity of the Widom line on lowering the temperature. Our results on the ice nucleation suggest an anomalous retracing behavior of the nucleation barrier along isotherms at negative pressures and theoretically validate the recent findings that the reentrant ice(Ih)-liquid coexistence line can induce a drastic change in the kinetics of ice nucleation. Thus, this study establishes a direct connection between the metastable water's thermodynamic anomalies and the (vapor and ice) nucleation kinetics. In addition, this study provides deeper insights into the origin of the isothermal compressibility maximum on isochoric cooling.

Ice Nucleation Promotion Impact on the Ice Recrystallization Inhibition Activity of Polyols

Heterogeneous ice nucleation occurs vis-à-vis nucleating agents already present in solution yet can occur within a rather broad range of temperatures (0 to ca. -38 °C). Controlling this temperature and the subsequent growth of resulting ice crystals is crucial for the survival of biological organisms (certain insects, fish, and plants that endure subzero temperatures), as well as in the context of medical cryopreservation and food science. In these environments, uncontrolled crystal shape and size can rupture the cell membrane causing irreversible and catastrophic damage. Antifreeze (AF) proteins and synthetic AF analogs address this issue to restrict crystal growth and to shape ice crystals. Yet, if the nucleation temperature is not controlled and occurs in a lower temperature range, nascent ice crystals will have grown to a significantly larger size before the AF agents can be active on their surface to halt or slow the Ostwald ripening process during recrystallization. At a higher nucleation temperature, diffusion of AF macromolecules is enhanced, and dynamic crystal shaping can start earlier, producing smaller crystals overall. While antifreeze proteins, the inspiration for these synthetic analogs, are always applied in a salt buffer aqueous environment (most typically phosphate-buffered saline (PBS) buffer), the heterogeneous nucleation events are stochastic and occur within a wide temperature range. Silver iodide (AgI), however, is a highly effective ice nucleation promoter as its crystal lattice structure is a 98% lattice match to the basal plane of hexagonal ice (Ih) crystals acting as a template for water molecule orientation and decreasing the interfacial free energy. Here, we expose the advantage of purposely seeding such nascent ice crystals with AgI at a defined and higher temperature (-7 °C) in ultrapure water (UPW) such that nucleation can only come from AgI (and also in AgI/PBS), resulting in the most potent synthetic IRI observed to date (at concentrations as low as 0.001 mg·mL^-1).

A highly active mineral-based ice nucleating agent supports in situ cell cryopreservation in a high throughput format

Cryopreservation of biological matter in microlitre scale volumes of liquid would be useful for a range of applications. At present, it is challenging because small volumes of water tend to supercool, and deep supercooling is known to lead to poor post-thaw cell viability. Here, we show that a mineral ice nucleator can almost eliminate supercooling in 100 µl liquid volumes during cryopreservation. This strategy of eliminating supercooling greatly enhances cell viability relative to cryopreservation protocols with uncontrolled ice nucleation. Using infrared thermography, we demonstrate a direct relationship between the extent of supercooling and post-thaw cell viability. Using a mineral nucleator delivery system, we open the door to the routine cryopreservation of mammalian cells in multiwell plates for applications such as high throughput toxicology testing of pharmaceutical products and regenerative medicine.

Control strategies of ice nucleation, growth, and recrystallization for cryopreservation

The cryopreservation of biomaterials is fundamental to modern biotechnology and biomedicine, but the biggest challenge is the formation of ice, resulting in fatal cryoinjury to biomaterials. To date, abundant ice control strategies have been utilized to inhibit ice formation and thus improve cryopreservation efficiency. This review focuses on the mechanisms of existing control strategies regulating ice formation and the corresponding applications to biomaterial cryopreservation, which are of guiding significance for the development of ice control strategies. Herein, basics related to biomaterial cryopreservation are introduced first. Then, the theoretical bases of ice nucleation, growth, and recrystallization are presented, from which the key factors affecting each process are analyzed, respectively. Ice nucleation is mainly affected by melting temperature, interfacial tension, shape factor, and kinetic prefactor, and ice growth is mainly affected by solution viscosity and cooling/warming rate, while ice recrystallization is inhibited by adsorption or diffusion mechanisms. Furthermore, the corresponding research methods and specific control strategies for each process are summarized. The review ends with an outlook of the current challenges and future perspectives in cryopreservation. STATEMENT OF SIGNIFICANCE: Ice formation is the major limitation of cryopreservation, which causes fatal cryoinjury to cryopreserved biomaterials. This review focuses on the three processes related to ice formation, called nucleation, growth, and recrystallization. The theoretical models, key influencing factors, research methods and corresponding ice control strategies of each process are summarized and discussed, respectively. The systematic introduction on mechanisms and control strategies of ice formation is instructive for the cryopreservation development.

Experiments and simulations demonstrating the rapid ultrasonic rewarming of frozen tissue cryovials

The development of methods to safely rewarm large cryopreserved biological samples remains a barrier to the widespread adoption of cryopreservation. Here, experiments and simulations were performed to demonstrate that ultrasound can increase rewarming rates relative to thermal conduction alone. An ultrasonic rewarming setup based on a custom 444 kHz tubular piezoelectric transducer was designed, characterized, and tested with 2 ml cryovials filled with frozen ground beef. Rewarming rates were characterized in the -20 °C to 5 °C range. Thermal conduction-based rewarming was compared to thermal conduction plus ultrasonic rewarming, demonstrating a tenfold increase in rewarming rate when ultrasound was applied. The maximum recorded rewarming rate with ultrasound was 57° C/min, approximately 2.5 times faster than with thermal conduction alone. Coupled acoustic and thermal simulations were developed and showed good agreement with the heating rates demonstrated experimentally and were also used to demonstrate spatial heating distributions with small (<3° C) temperature differentials throughout the sample when the sample was below 0° C. The experiments and simulations demonstrate the potential for ultrasonic cryovial rewarming with a possible application to large volume rewarming, as faster rewarming rates may improve the viability of cryopreserved tissues and reduce the time needed for cells to regain normal function.

Ice-templated synthesis of multicomponent porous coatings via vapour sublimation and deposition polymerization

A multicomponent vapour-deposited porous (MVP) coating with combined physical and biochemical properties was fabricated based on a chemical vapour sublimation and deposition process. Multiple components are used based on their natural thermodynamic properties, being volatile and/or nonvolatile, resulting in the sublimation of water vapour (from an iced template), and a simultaneous deposition process of poly-p-xylylene occurs upon radical polymerization into a disordered structure, forming porous coatings of MVP on various substrates. In terms of physical properties, the coating technology exhibits adjustable hydrophobicity by tuning the surface morphology by timed control of the sublimation of the iced template layer from a substrate. However, by using a nonvolatile solution during fabrication, an impregnation process of the deposited poly-p-xylylene on such a solution with tuning contact angles produces an MVP coating with a customizable elastic modulus based on deformation-elasticity theory. Moreover, patterning physical structures with adjustable pore size and/or porosity of the coatings, as well as modulation and compartmentalization to introduce necessary boundaries of microstructures within one MVP coating layer, can be achieved during the proposed fabrication process. Finally, with a combination of defined solutions comprised of both volatile and nonvolatile multicomponents, including functional biomolecules, growth factor proteins, and living cells, the fabrication of the resultant MVP coating serves devised purposes exhibiting a variety of biological functions demonstrated with versatility for cell proliferation, osteogenesis, adipogenesis, odontogenesis, spheroid growth of stem cells, and a complex coculture system towards angiogenesis. Multicomponent porous coating technology is produced based on vapour sublimation and deposition upon radical polymerization that overturns conventional vapour-deposited coatings, resulting in only dense thin films, and in addition, the versatility of adjusting coating physical and chemical properties by exploiting the volatility mechanism of iced solution templates and accommodation of solute substances during the fabrication process. The MVP coating and the proposed fabrication technique represent a simple approach to provide a prospective interface coating layer for materials science and are attractive for unlimited applications.

Bioinspired Threonine-Based Polymers with Potent Ice Recrystallization Inhibition Activity

Ice growth mitigation is a pervasive challenge for multiple industries. In nature, ice-binding proteins (IBPs) demonstrate potent ice growth prevention through ice recrystallization inhibition (IRI). However, IBPs are expensive, difficult to produce in large quantities, and exhibit minimal resilience to nonphysiological environmental stressors, such as pH. For these reasons, researchers have turned to bioinspired polymeric materials that mimic IBP behavior. To date, however, no synthetic polymer has rivaled the ability of native IBPs to display IRI activity at ultralow nanomolar concentrations. In this work, we study the IRI activity of peptides and polypeptides inspired by common ice-binding residues of IBPs to inform the synthesis and characterization of a potent bioinspired polymer that mimics IBP behavior. We show first that the threonine polypeptide (pThr) displays the best IRI activity in phosphate-buffered saline (PBS). Second, we use pThr as a molecular model to synthesize and test a bioinspired polymer, poly(2-hydroxypropyl methacrylamide) (pHPMA). We show that pHPMA exhibits potent IRI activity in neutral PBS at ultralow concentrations (0.01 mg/mL). pHPMA demonstrates potent IRI activity at low molecular weights (2.3 kDa), with improved activity at higher molecular weights (32.8 kDa). These results substantiate that pHPMA is a robust molecule that mitigates ice crystal growth at concentrations similar to native IBPs.

Pollen derived macromolecules serve as a new class of ice-nucleating cryoprotectants

Cryopreservation of biological material is vital for existing and emerging biomedical and biotechnological research and related applications, but there remain significant challenges. Cryopreservation of cells in sub-milliliter volumes is difficult because they tend to deeply supercool, favoring lethal intracellular ice formation. Some tree pollens are known to produce polysaccharides capable of nucleating ice at warm sub-zero temperatures. Here we demonstrated that aqueous extractions from European hornbeam pollen (pollen washing water, PWW) increased ice nucleation temperatures in 96-well plates from ≈ − 13 °C to ≈ − 7 °C. Application of PWW to the cryopreservation of immortalized T-cells in 96-well plates resulted in an increase of post-thaw metabolic activity from 63.9% (95% CI [58.5 to 69.2%]) to 97.4% (95% CI [86.5 to 108.2%]) of unfrozen control. When applied to cryopreservation of immortalized lung carcinoma monolayers, PWW dramatically increased post-thaw metabolic activity, from 1.6% (95% CI [− 6.6 to 9.79%]) to 55.0% (95% CI [41.6 to 68.4%]). In contrast to other ice nucleating agents, PWW is soluble, sterile and has low cytotoxicity meaning it can be readily incorporated into existing cryopreservation procedures. As such, it can be regarded as a unique class of cryoprotectant which acts by inducing ice nucleation at warm temperatures.

Chemical approaches to cryopreservation

Cryopreservation of cells and biologics underpins all biomedical research from routine sample storage to emerging cell-based therapies, as well as ensuring cell banks provide authenticated, stable and consistent cell products. This field began with the discovery and wide adoption of glycerol and dimethyl sulfoxide as cryoprotectants over 60 years ago, but these tools do not work for all cells and are not ideal for all workflows. In this Review, we highlight and critically review the approaches to discover, and apply, new chemical tools for cryopreservation. We summarize the key (and complex) damage pathways during cellular cryopreservation and how each can be addressed. Bio-inspired approaches, such as those based on extremophiles, are also discussed. We describe both small-molecule-based and macromolecular-based strategies, including ice binders, ice nucleators, ice nucleation inhibitors and emerging materials whose exact mechanism has yet to be understood. Finally, looking towards the future of the field, the application of bottom-up molecular modelling, library-based discovery approaches and materials science tools, which are set to transform cryopreservation strategies, are also included.

Phase transitions and mechanisms of cryoprotection of serum-/xeno-free media based on dextran and dimethyl sulfoxide

The development of serum-/xeno-free media may help avoid the drawbacks of using serum and its components, such as probable contamination, instability of composition, or difficulty in sterilization. The objectives of this research were to investigate the use of combinations of a permeating cryoprotective agent (Me₂SO) and non-permeating (polyvinyl alcohol, polyvinylpyrrolidone, polyethylene glycol, hydroxyethyl starch, dextran) polymers for cryopreservation of interstitial cells (ICs) of rat testis, and to propose the mechanism of cryoprotection of such compositions. In the course of this study, the best combination was 100 mg/ml dextran (M.m. 40 kDa) (Dex40) with 0.7 M Me₂SO in Ham's F12. The ICs were additionally cooled and warmed to different end temperatures (-30, -50, -50 and -196 °C) to determine which temperature intervals contributed most to the IC loss. Then, the cryoprotective action of this serum-/xeno-free medium was investigated in comparison with serum or albumin-containing media by differential scanning calorimetry (DSC) and thermomechanical analysis (TMA). The results showed that the medium based on Dex40 did not decrease the amount of ice formed. However, it could undergo other phase separation and phase transformation to form glassy states. Potential cell-damaging physical processes such as eutectic crystallization/melting, recrystallization of NaCl and/or Me₂SO derivatives, found in serum-containing media and taking place in specific temperature intervals, were not observed in the Dex40 based media. This was in good correlation with indicators of cell survival. Additionally, the application of Dex40 allowed using Me₂SO in lower concentrations (0.7 M) than required for serum-containing media (1.4 M), which may decrease the toxicity of serum-/xeno-free media.

The role of structural order in heterogeneous ice nucleation

The freezing of water into ice is a key process that is still not fully understood. It generally requires an impurity of some description to initiate the heterogeneous nucleation of the ice crystals. The molecular structure, as well as the extent of structural order within the impurity in question, both play an essential role in determining its effectiveness. However, disentangling these two contributions is a challenge for both experiments and simulations. In this work, we have systematically investigated the ice-nucleating ability of the very same compound, cholesterol, from the crystalline (and thus ordered) form to disordered self-assembled monolayers. Leveraging a combination of experiments and simulations, we identify a “sweet spot” in terms of the surface coverage of the monolayers, whereby cholesterol maximises its ability to nucleate ice (which remains inferior to that of crystalline cholesterol) by enhancing the structural order of the interfacial water molecules. These findings have practical implications for the rational design of synthetic ice-nucleating agents.

The freezing of water into ice is still not fully understood. Here, we investigate the role of structural disorder within the biologically relevant impurities that facilitate this fundamental phase transition.

Studies on the identification of frostbitten and frozen meat after defrosting by the spectrophotometric method for determining DNA in muscle tissue extracts

Changes in the state of meat during freezing are determined by the phase transition of water into ice and an increase in the concentration of substances dissolved in the liquid phase. The process of crystal formation leads to a change in the physical characteristics of the meat and may be accompanied by changes in its structural properties. The effect of the proportion of frozen water on the permeability of the membranes of muscle fibers of frostbitten and frozen meat has been established. The ratio of optical densities at wavelengths of 270 nm and 290 nm (R) can be used to judge the thermal state of the meat. It is shown that the value of R after defrosting frozen meat is 2 times higher than for frostbitten meat.

Cryopreservation of Induced Pluripotent Stem Cell-Derived Dopaminergic Neurospheres for Clinical Application

BACKGROUND

Pluripotent stem cell (PSC)-derived dopaminergic (DA) neurons are an expected source of cell therapy for Parkinson's disease. The transplantation of cell aggregates or neurospheres, instead of a single cell suspension has several advantages, such as keeping the 3D structure of the donor cells and ease of handling. For this PSC-based therapy to become a widely available treatment, cryopreservation of the final product is critical in the manufacturing process. However, cryopreserving cell aggregates is more complicated than cryopreserving single cell suspensions. Previous studies showed poor survival of the DA neurons after the transplantation of cryopreserved fetal ventral-mesencephalic tissues.

OBJECTIVE

To achieve the cryopreservation of induced pluripotent stem cell (iPSC)-derived DA neurospheres toward clinical application.

METHODS

We cryopreserved iPSC-derived DA neurospheres in various clinically applicable cryopreservation media and freezing protocols and assessed viability and neurite extension. We evaluated the population and neuronal function of cryopreserved cells by the selected method in vitro. We also injected the cells into 6-hydroxydopamine (6-OHDA) lesioned rats, and assessed their survival, maturation and function in vivo.

RESULTS

The iPSC-derived DA neurospheres cryopreserved by Proton Freezer in the cryopreservation medium Bambanker hRM (BBK) showed favorable viability after thawing and had equivalent expression of DA-specific markers, dopamine secretion, and electrophysiological activity as fresh spheres. When transplanted into 6-OHDA-lesioned rats, the cryopreserved cells survived and differentiated into mature DA neurons, resulting in improved abnormal rotational behavior.

CONCLUSION

These results show that the combination of BBK and Proton Freezer is suitable for the cryopreservation of iPSC-derived DA neurospheres.

Bioinspired Ice-Binding Materials for Tissue and Organ Cryopreservation

Cryopreservation of tissues and organs can bring transformative changes to medicine and medical science. In the past decades, limited progress has been achieved, although cryopreservation of tissues and organs has long been intensively pursued. One key reason is that the cryoprotective agents (CPAs) currently used for cell cryopreservation cannot effectively preserve tissues and organs because of their cytotoxicity and tissue destructive effect as well as the low efficiency in controlling ice formation. In stark contrast, nature has its unique ways of controlling ice formation, and many living organisms can effectively prevent freezing damage. Ice-binding proteins (IBPs) are regarded as the essential materials identified in these living organisms for regulating ice nucleation and growth. Note that controversial results have been reported on the utilization of IBPs and their mimics for the cryopreservation of tissues and organs, that is, some groups revealed that IBPs and mimics exhibited unique superiorities in tissues cryopreservation, while other groups showed detrimental effects. In this perspective, we analyze possible reasons for the controversy and predict future research directions in the design and construction of IBP inspired ice-binding materials to be used as new CPAs for tissue cryopreservation after briefly introducing the cryo-injuries and the challenges of conventional CPAs in the cryopreservation of tissues and organs.

Step-bunching instability of growing interfaces between ice and supercooled water

Step-bunching instability (SBI) is one of the interfacial instabilities driven by self-organization of elementary step flow associated with crystal-growth dynamics, which has been observed in diverse crystalline materials. However, despite theoretical suggestions of its presence, no direct observations of SBI for simple melt growth have been achieved so far. Here, with the aid of a type of optical microscope and its combination with a two-beam interferometer, we realized quantitative in situ observations of the spatiotemporal dynamics of the SBI. This enables us to examine the origin of the SBI at the level of the step–step interaction. We also found that the SBI spontaneously induces a highly stable spiral growth mode, governing the late stage of the growth process.

Ice-crystal growth in supercooled water is one of the most familiar examples of phase-transition dynamics, playing essential roles in various natural phenomena on Earth. Despite its fundamental importance, the microscopic view at the elementary step level remains elusive. Here, using an advanced optical microscope, we find self-organization of elementary steps during ice-crystal growth, called step-bunching instability (SBI), driven by the competition between step dynamics, interfacial stiffness, and latent heat diffusions. We also find that the SBI transiently induces screw dislocations and resulting spiral growth in the late stage of the growth process. Furthermore, quantitative observations with a two-beam interferometer allow us to obtain insights into the relative importance of the various mechanisms of the step–step interactions. Our finding offers a significant clue to understanding the general mechanism of melt growth beyond ice-crystal growth, inseparably involving several broad research fields, including cryobiological, geophysical, and material branches.

Ice Dendrite Growth Atop a Frozen Drop under Natural Convection Conditions

Condensation frosting is a type of icing encountered ubiquitously in our daily lives. Understanding the dynamics of condensation frosting is essential in developing effective technologies to suppress frost accretions that compromise heat transfer and system integrity. Here, we present an experimental study on ice dendrite growth atop a single frozen drop, an important step affecting the subsequent frosting process, and the properties of fully-developed frost layers. We evaluate the effect of natural convection by comparing the growth dynamics of ice dendrites on the surface of a frozen drop with three different orientations with respect to gravity. The results show that both the average deposition rate and its spatial variations are profoundly altered by surface orientations. Such behavior is confirmed by a numerical simulation, showing how gravity-assisted (hindered) vapor diffusion yields the deposition outcomes. These findings benefit the optimization of anti-/de- frosting technologies and the rational design of heat exchangers.

Freeform Cell-Laden Cryobioprinting for Shelf-Ready Tissue Fabrication and Storage

One significant drawback of existing bioprinted tissues is their lack of shelf-availability caused by complications in both fabrication and storage. Here, we report a cryobioprinting strategy for simultaneously fabricating and storing cell-laden volumetric tissue constructs through seamlessly combining extrusion bioprinting and cryopreservation. The cryobioprinting performance was investigated by designing, fabricating, and storing cell-laden constructs made of our optimized cryoprotective gelatin-based bioinks using a freezing plate with precisely controllable temperature. The in situ freezing process further promoted the printability of cell-laden hydrogel bioinks to achieve freeform structures otherwise inconvenient with direct extrusion bioprinting. The effects of bioink composition on printability and cell viability were evaluated. The functionality of the method was finally investigated using cell differentiation and chick ex ovo assays. The results confirmed the feasibility and efficacy of cryobioprinting as a single-step method for concurrent tissue biofabrication and storage.

ESEM Methodology for the Study of Ice Samples at Environmentally Relevant Subzero Temperatures: "Subzero ESEM"

Frozen aqueous solutions are an important subject of study in numerous scientific branches including the pharmaceutical and food industry, atmospheric chemistry, biology, and medicine. Here, we present an advanced environmental scanning electron microscope methodology for research of ice samples at environmentally relevant subzero temperatures, thus under conditions in which it is extremely challenging to maintain the thermodynamic equilibrium of the specimen. The methodology opens possibilities to observe intact ice samples at close to natural conditions. Based on the results of ANSYS software simulations of the surface temperature of a frozen sample, and knowledge of the partial pressure of water vapor in the gas mixture near the sample, we monitored static ice samples over several minutes. We also discuss possible artifacts that can arise from unwanted surface ice formation on, or ice sublimation from, the sample, as a consequence of shifting conditions away from thermodynamic equilibrium in the specimen chamber. To demonstrate the applicability of the methodology, we characterized how the true morphology of ice spheres containing salt changed upon aging and the morphology of ice spheres containing bovine serum albumin. After combining static observations with the dynamic process of ice sublimation from the sample, we can attain images with nanometer resolution.

Differentiation of Human iPS Cells Into Sensory Neurons Exhibits Developmental Stage-Specific Cryopreservation Challenges

Differentiation of human induced pluripotent stem cells (hiPSCs) generates cell phenotypes valuable for cell therapy and personalized medicine. Successful translation of these hiPSC-derived therapeutic products will rely upon effective cryopreservation at multiple stages of the manufacturing cycle. From the perspective of cryobiology, we attempted to understand how the challenge of cryopreservation evolves between cell phenotypes along an hiPSC-to-sensory neuron differentiation trajectory. Cells were cultivated at three different stages to represent intermediate, differentiated, and matured cell products. All cell stages remained ≥90% viable in a dimethyl sulfoxide (DMSO)-free formulation but suffered ≥50% loss in DMSO before freezing. Raman spectroscopy revealed higher sensitivity to undercooling in hiPSC-derived neuronal cells with lower membrane fluidity and higher sensitivity to suboptimal cooling rates in stem cell developmental stages with larger cell bodies. Highly viable and functional sensory neurons were obtained following DMSO-free cryopreservation. Our study also demonstrated that dissociating adherent cultures plays an important role in the ability of cells to survive and function after cryopreservation.

Hypothermic Stem Cell Storage Using a Polypeptide Thermogel

We report a polypeptide-based thermogel as a new tool for hypothermic storage of stem cells at ambient temperature (25 °C). Stem cells were suspended in the sol state (10 °C) of an aqueous poly(ethylene glycol)-poly(l-alanine) (PEG-PA) solution (4.0 wt %) in phosphate-buffered saline (PBS), which turned into a stem cell-incorporated gel by a heat-induced sol-to-gel transition. The cell harvesting procedure from the thermogels was simply performed through a gel-to-sol transition by diluting and cooling the system. More than 99% of stem cells died in PBS and Pluronic F127 thermogel (control thermogel) when the cells were stored at 25 °C for 7 days. The cell recovery rate from the PEG-PA thermogel (64%) was significantly greater than that from the commercially available HypoThermosol FRS preservation solution (HTS) (26%). Additionally, the surviving stem cells from the PEG-PA thermogel were healthier than those from HTS in terms of (1) expression of stemness biomarkers (NANOG, OCT4, and SOX2), (2) proliferation rate, and (3) differentiation potentials into osteogenic, chondrogenic, and adipogenic lineages. Membrane stabilization was suggested as a cell protection mechanism in the cytocompatible PEG-PA thermogel. The PEG-PA thermogel provides a convenient cytocompatible way for the storage and recovery of cells and thus is a promising tool for the transportation and short-term banking of cells.

Antifreeze proteins and their biomimetics for cell cryopreservation: Mechanism, function and application-A review

Cell-based therapy is a promising technology for intractable diseases and health care applications, in which cryopreservation has become an essential procedure to realize the production of therapeutic cells. Ice recrystallization is the major factor that affects the post-thaw viability of cells. As a typical series of biomacromolecules with ice recrystallization inhibition (IRI) activity, antifreeze proteins (AFPs) have been employed in cell cryopreservation. Meanwhile, synthesized materials with IRI activity have emerged in the name of biomimetics of AFPs to expand their availability and practicality. However, fabrication of AFPs mimetics is in a chaotic period. There remains little commonality among different AFPs mimetics, then it is difficult to set guidelines on their design. With no doubt, a comprehensive understanding on the antifreezing mechanism of AFPs in molecular level will enable us to rebuild the function of AFPs, and provide convenience to clarify the relationship between structure and function of these early stage biomimetics. In this review, we would discuss those previously reported biomimetics to summarize their structure characteristics concerning the IRI activity and attempt to develop a roadmap for guiding the design of novel AFPs mimetics.

Towards increasing stallion sperm longevity by storage at subzero temperatures in the absence of ice

The aim of cell preservation technologies is to slow down damaging reactions by lowering the storage temperature. Upon dilution in a stabilizing extender, stallion sperm can be stored at refrigerator temperatures for several days. Cryopreservation allows storage for decades, but freezing and thawing cause damage and viability losses. It is assumed that by storing cells at subzero temperatures in a non-frozen supercooled state, the damaging effects of ice formation can be avoided. In this study, we have investigated if stallion sperm can be stored at -10°C in the absence of ice, and compared viability during supercooled storage with that during storage at 5°C. We found that addition of 2% Ficoll-400 to buffered saline and covering with mineral oil depressed the sample freezing point and inhibited surface-catalyzed nucleation. This allowed storage in a supercooled state at -10°C for up to 7 days. Supplementing specimens with sperm, however, increased the incidence of sample freezing. Nonetheless, with 50×10⁶ sperm mL^-1, about 40% of the samples turned out to be non-frozen. Adding 100 mM sucrose was found to preserve sperm membrane intactness during supercooled storage, although this resulted in lower percentages as found with refrigerated storage. Sperm motility appeared to be lost during supercooled storage but could be partly restored by substituting buffered saline with a milk-based extender as base medium. Percentages of membrane intact sperm, however, were found to be lower. Supercooled storage holds promise for semen preservation, but further optimization of the storage solution is required to preserve sperm motility.

Antifreeze and Rheological Properties of Injectable Triblock Copolymer Hydrogels with Supramolecular Junctions

ABC triblock copolymers composed of hydrophobic poly(ε-caprolactone) (PCL), zwitterionic poly(carboxybetaine methacrylate) midblock, and P(PEGMA-UPy_0.15 ) containing supramolecular ureidopyrimidinone moieties, poly(ε-caprolactone-block-carboxybetaine methacrylate-block-[poly(ethylene glycol) methyl ether methacrylate-co-(α-methacryloyl-ω-(6-(3-(6-methyl-4-oxo-1,4-dihydropyrimidin-2-yl)ureido)hexylcarbamoyloxy)poly(ethylene glycol))]), are investigated to achieve multifunctional antifreeze hydrogels. The PCL and P(PEGMA-UPy_0.15 ) blocks induce the formation of physical network with a hierarchical nanostructure comprising hydrophobic PCL cores and supramolecular junctions, respectively. The super-hydrophilic nature of polyzwitterion midblocks and the confinement effect of the supramolecular junctions enhance the antifreeze performance, where the majority of water molecules remains supercooled below sub-zero temperature. The hydrogel relaxation characterized over a wide range of timescale reveals that the facile dynamics of the supramolecular junctions lead to the self-healing and injectability of the hydrogels. In conjunction with the biodegradable PCL cores, the antifreeze and rheological characteristics of the triblock copolymer hydrogels provide significant potential to use for cryo-preservable and bio-injectable drug storage and delivery.

Faster Nucleation of Ice at the Three-Phase Contact Line: Influence of Interfacial Chemistry

Controlling the nucleation of ice is important in many areas including atmospheric sciences, cryopreservation, food science, and infrastructure protection. Presently, we conduct controlled experiments and analysis to uncover the influence of surface chemistry at the three-phase line on ice nucleation. We show that ice nucleation is faster upon replacing the air at the water-air interface with oils like silicone oil and almond oil. We show via statistically meaningful and carefully designed experiments that ice nucleation occurs at a higher temperature at an aluminum-water-silicone oil (or almond oil) interface as compared to an aluminum-water-air interface. We show that the location of ice nucleation can be controlled (in situations with multiple locations for ice nucleation) by controlling the interfacial chemistry at the three-phase line. We develop a model (which utilizes classical nucleation theory) to study the combined influence of two interfaces on a seed crystal of ice originating at the three-phase contact line. This model can evaluate the thermodynamic competition between nucleation at the three -phase line and heterogeneous nucleation at an interface. The model shows that three-phase contact lines usually result in a higher driving force than heterogeneous nucleation, which speeds up nucleation kinetics. Overall, our experiments and modeling uncover several useful insights into the influence of three-phase lines on nucleation during contact freezing.