A Supramolecular Ice Growth Inhibitor

Advances in single ice crystal shaping materials: From nature to synthesis and applications in cryopreservation

Antifreeze (glyco) proteins [AF(G)Ps], which are widely present in various extreme microorganisms, can control the formation and growth of ice crystals. Given the significance of cryogenic technology in biomedicine, climate science, electronic energy, and other fields of research, scientists are quite interested in the development and synthesis high-efficiency bionic antifreeze protein materials, particularly to reproduce their dynamic ice shaping (DIS) characteristics. Single ice crystal shaping materials, a promising class of ice-controlling materials, can alter the morphology and growth rate of ice crystals at low temperatures. This review aims to highlight the development of single ice crystal shaping materials and provide a brief comparison between a series of natural and bionic synthetic materials with DIS ability, which include AF(G)Ps, polymers, salts, and nanomaterials. Additionally, we summarize their applications in cryopreservation. Finally, this paper presents the current challenges and prospects encountered in developing high-efficiency and practical single ice crystal shaping materials. STATEMENT OF SIGNIFICANCE: The formation and growth of ice crystals hold a significant importance to an incredibly broad range of fields. Therefore, the design and fabrication of the single ice crystal shaping materials have gained the increasing popularity due to its key role in dynamic ice shaping (DIS) characteristics. Especially, single ice crystal shaping materials are considered one of the most promising candidates as ice inhibitors, presenting tremendous prospects for enhancing cryopreservation. In this work, we focus on the molecular characteristics, structure-function relationships, and DIS mechanisms of typical natural and biomimetic synthetic materials. This review may provide inspiration for the design and preparation of single ice crystal shaping materials and give guidance for the development of effective cryopreservation agent.

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.

Micro-thermography for imaging ice crystal growth and nucleation inside non-transparent materials

Ice crystal growth and nucleation rate measurements are usually done using light microscopy in liquid and transparent samples. Yet, the understanding of important practical problems depends on monitoring ice growth inside solid materials, for example how rapid ice growth leads to structural damage of food, or how the final structure of cementitious materials is affected by ice during curing. Imaging crystal growth inside solid materials cannot be done with visible light and is intrinsically more challenging than visible light imaging. Thermography is a technique that uses thermal (infrared) cameras to monitor temperature changes in a material, and it has been used to provide a qualitative description of ice propagation with a low spatial resolution. Here, we describe a method that uses a novel micro-thermography system to image ice nucleation and growth inside non-transparent samples. This method relies on two major components: a cold stage with accurate temperature control (±0.001 °C) and a thermal camera with high spatial and temperature resolution. Our experiments include imaging of ice formation and growth in pure water first and then inside plant leaves used as a model for a non-transparent material. An ice growth rate of 2.2 mm/s was measured inside a plant leaf at -12 °C, and ice nucleation in single plant cells was observed as a hotspot having a diameter of 160 µm. The results presented here provide an experimental proof that high-quality imaging of ice growth is achievable, thus paving the way for quantitative measurements of ice growth kinetics and ice nucleation in solid materials.

Inhibiting ice recrystallization by amyloid protein fibrils

Ice recrystallization is harmful to the quality of frozen foods and the cryopreservation of cells and biological tissues, requiring biocompatible materials with ice recrystallization inhibition (IRI) activity. Emerging studies have associated IRI activity with amphiphilic structures. We propose amphiphilic amyloid protein fibrils (APFs) may be IRI-active. APFs were prepared from whey protein isolate (WPI) in water (W-APFs) and in trifluoroethanol (TFE-APFs). W-APFs and TFE-APFs were more IRI-active than WPI over a concentration range of 2.5-10.0 mg/mL. Both APFs showed stronger IRI activity at pH 3.0 than at pH 5.0, 7.0, and 10.0, which was ascribed to the effect of water dispersibility and fibril length. The reduced IRI activity of the two APFs with increasing NaCl content was caused by fibril aggregation. Ice binding by APFs was absent or very weak. Ordered water was observed for the two APFs, which might be essential for IRI activity. Our findings may lead to the use of APFs as novel ice recrystallization inhibitors.

Ice recrystallization inhibition activity in bile salts

Ice recrystallization inhibitors are novel cryoprotective agents that can reduce the freezing damage of cells, tissues, and organs in cryopreservation. To date, potent ice recrystallization inhibition (IRI) activity has been found on antifreeze (glyco)proteins, polymers, nanomaterials, and a limited number of chemically synthesized small molecules. This paper reports a relatively potent IRI activity on a group of small biological molecules - bile salts. The IRI activity increased as the number of hydroxyl groups decreased in bile salts. Among sodium cholate (NaC), sodium deoxycholate (NaDC), sodium chenodeoxycholate (NaCC), and sodium lithocholate (NaLC), the least hydrophilic NaLC at a concentration of 25.0 mM entirely blocked the ice growth in phosphate-buffered saline (PBS) under test conditions. The IRI activity of bile salts was not related to viscosity or gelation. No IRI activity was found below the critical micelle concentration. The IRI activity was independent of liquid crystal formation. No ice shaping and thermal hysteresis were observed on any bile salts, but NaC and NaLC could increase the ice nucleation temperature. The findings add bile salts to the existing material list of ice recrystallization inhibitors.

Janus regulation of ice growth by hyperbranched polyglycerols generating dynamic hydrogen bonding

In this study, a new phenomenon describing the Janus effect on ice growth by hyperbranched polyglycerols, which can align the surrounding water molecules, has been identified. Even with an identical polyglycerol, we not only induced to inhibit ice growth and recrystallization, but also to promote the growth rate of ice that is more than twice that of pure water. By investigating the polymer architecture and population, we found that the stark difference in the generation of quasi-structured H₂O molecules at the ice/water interface played a crucial role in the outcome of these opposite effects. Inhibition activity was induced when polymers at nearly fixed loci formed steady hydrogen bonding with the ice surface. However, the formation-and-dissociation dynamics of the interfacial hydrogen bonds, originating from and maintained by migrating polymers, resulted in an enhanced quasi-liquid layer that facilitated ice growth. Such ice growth activity is a unique property unseen in natural antifreeze proteins or their mimetic materials.

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.

Minimalistic ice recrystallisation inhibitors based on phenylalanine

Ice recrystallisation inhibition (IRI) is typically associated with ice binding proteins, but polymers and other mimetics are emerging. Here we identify phenylalanine as a minimalistic, yet potent, small-molecule IRI capable of inhibiting ice growth at just 1 mg mL^-1. Facial amphiphilicity is shown to be a crucial structural feature, with para-substituents enhancing (hydrophobic) or decreasing (hydrophilic) IRI activity. Both amino and acid groups were found to be essential. Solution-phase self-assembly of Phenylalanine was not observed, but the role of self-assembly at the ice/water interface could not be ruled out as a contributing factor.

Small-molecule fulvic acid with strong hydration ability for non-vitreous cellular cryopreservation

The exploitation of biocompatible ice-control materials especially the small molecules for non-vitreous cryopreservation remains challenging. Here, we report a small molecule of fulvic acid (FA) with strong hydration ability, which enables non-vitreous cellular cryopreservation by reducing ice growth during freezing and reducing ice recrystallization/promoting ice melting during thawing. Without adding any other cryoprotectants, FA can enhance the recovery of sheep red blood cells (RBCs) by three times as compared with a commercial cryoprotectant (hydroxyethyl starch) under a stringent test condition. Investigation of water mobility reveals that the ice-control properties of FA can be ascribed to its strong bondage to water molecules. Furthermore, we found that FA can be absorbed by RBCs and mainly locates on membranes, suggesting the possible contribution of FA to cell protection through stabilizing membranes. This work bespeaks a bright future for small-molecule cryoprotectants in non-vitreous cryopreservation application.

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FA shows strong hydration ability

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FA reduces ice growth/recrystallization and promotes ice melting

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FA can be absorbed by RBCs and mainly locates on membranes

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FA enables non-vitreous cellular cryopreservation

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.

Ice Recrystallization Inhibition by Amino Acids: The Curious Case of Alpha- and Beta-Alanine

Extremophiles produce macromolecules which inhibit ice recrystallization, but there is increasing interest in discovering and developing small molecules that can modulate ice growth. Realizing their potential requires an understanding of how these molecules function at the atomistic level. Here, we report the discovery that the amino acid l-α-alanine demonstrates ice recrystallization inhibition (IRI) activity, functioning at 100 mM (∼10 mg/mL). We combined experimental assays with molecular simulations to investigate this IRI agent, drawing comparison to β-alanine, an isomer of l-α-alanine which displays no IRI activity. We found that the difference in the IRI activity of these molecules does not originate from their ice binding affinity, but from their capacity to (not) become overgrown, dictated by the degree of structural (in)compatibility within the growing ice lattice. These findings shed new light on the microscopic mechanisms of small molecule cryoprotectants, particularly in terms of their molecular structure and overgrowth by ice.

Gold Nanoprobes Exploring the Ice Structure in the Aqueous Dispersion of Poly(Ethylene Glycol)-Gold Hybrid Nanoparticles

Ice structures and their formation process are fundamentally important to cryobiology, geoscience, and physical chemistry. In this work, we synthesized gold nanoprobes by grafting water-soluble polyethylene glycol (PEG) onto spherical gold nanoparticles and analyzed the structure of ice formation in the vicinity of the resulting hybrid PEG-Au nanoparticles (AuPEGNPs). Temperature-dependent in situ small-angle X-ray scattering (SAXS) indicated that AuPEGNPs, like PEG, caused the formation of bulk spherulite ice. Unlike for PEG, we observed the formation of lamellar ice with a periodicty of 4.6 nm, which is thermodynamically less stable than the bulk form. The lamellar ice formed after AuPEGNP agglomeration during cooling at -19 °C, and it remained during subsequent heating from -20 to -11 °C and melted at around -10 °C, far below the melting temperature of bulk ice. We explain different effects of AuPEGNP and free PEG on ice formation by the topological differences. The highly concentrated PEG chains on the agglomerated Au cores lead to the formation of PEG-hydrates that assemble into lamellar ice with a periodicity of 4.6 nm.

Synthesis of poly(vinyl alcohol) by blue light bismuth oxide photocatalysed RAFT. Evaluation of the impact of freeze/thaw cycling on ice recrystallisation inhibition

Poly(vinyl alcohol), PVA, is the most potent polymeric ice recrystallisation inhibitor (IRI), mimicking a complex function of ice binding proteins. The IRI activity of PVA scales with its molecular weight and hence broad molecular weight distributions in free radical-derived PVAs lead to activity measurements dominated by small amounts of heavier fractions. Well-defined PVA can be prepared by thermally initiated RAFT/MADIX polymerization using xanthates by the polymerization of the less activated monomer vinyl acetate. The low conversions and molecular weights obtained during this approach, often requires feeding of additional initiator and bulk polymerization. Here we employ bismuth oxide photo-RAFT in solution, using blue light (450 nm), rather than previously reported white light, to obtain a library of PVA's. The use of blue light enabled quantitative conversion and acceptable dispersities. Purple light (380 nm) was also used, but asymmetric molecular weight distributions were obtained in some cases. High concentrations of high molecular weight PVA is known to form cryogels during freeze/thaw which has led to speculation this might limit the use of PVA in environments where the temperature cycles e.g. the construction industry. After 4 freeze/thaw cycles there was only small changes in observable IRI for all synthesised PVAs and two commercial standards. In an extended test, activity was retained after 100 freeze/thaw cycles, mitigating concerns that PVA could not be used in situations where freeze/thaw cycles occur. This work presents a convenient method to obtain well-defined PVAs for cryoscience studies compared to conventional thermal-RAFT and indicates that cryogelation concerns may not prevent their use.

Poly(vinyl alcohol) is synthesised using blue light bismuth oxide photo-RAFT, to high conversion. The ice recrystallisation inhibition activity of these polymers was shown to be retained over 100 freeze/thaw cycles.

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.

Adsorption-Inhibition of Clathrate Hydrates by Self-Assembled Nanostructures

The mechanism by which safranine O (SFO), an ice growth inhibitor, halts the growth of single crystal tetrahydrofuran (THF) clathrate hydrates was explored using microfluidics coupled with cold stages and fluorescence microscopy. THF hydrates grown in SFO solutions exhibited morphology changes and were shaped as truncated octahedrons or hexagons. Fluorescence microscopy and microfluidics demonstrated that SFO binds to the surface of THF hydrates on specific crystal planes. Cryo-TEM experiments of aqueous solutions containing millimolar concentrations of SFO exhibited the formation of bilayered lamellae with an average thickness of 4.2±0.2 nm covering several μm² . Altogether, these results indicate that SFO forms supramolecular lamellae in solution, which might bind to the surface of the hydrate and inhibit further growth. As an ice and hydrate inhibitor, SFO may bind to the surface of these crystals via ordered water molecules near its amine and methyl groups, similar to some antifreeze proteins.

Dynamic Anti-Icing Surfaces (DAIS)

Remarkable progress has been made in surface icephobicity in the recent years. The mainstream standpoint of the reported antiicing surfaces yet only considers the ice-substrate interface and its adjacent regions being of static nature. In reality, the local structures and the overall properties of ice-substrate interfaces evolve with time, temperature and various external stimuli. Understanding the dynamic properties of the icing interface is crucial for shedding new light on the design of new anti-icing surfaces to meet challenges of harsh conditions including extremely low temperature and/or long working time. This article surveys the state-of-the-art anti-icing surfaces and dissects their dynamic changes of the chemical/physical states at icing interface. According to the focused critical ice-substrate contacting locations, namely the most important ice-substrate interface and the adjacent regions in the substrate and in the ice, the available anti-icing surfaces are for the first time re-assessed by taking the dynamic evolution into account. Subsequently, the recent works in the preparation of dynamic anti-icing surfaces (DAIS) that consider time-evolving properties, with their potentials in practical applications, and the challenges confronted are summarized and discussed, aiming for providing a thorough review of the promising concept of DAIS for guiding the future icephobic materials designs.

Hyperactive Antifreeze Proteins Promote Ice Growth before Binding to It

The antifreeze mechanism of antifreeze proteins (AFPs) evolved by organisms has been widely studied. However, detailed knowledge of the synergy between AFPs and ice crystals still remains fragmentary. In the present contribution, the cooperative effect of the hyperactive insect antifreeze protein TmAFP and ice crystals on the interfacial water during the entire process of inhibiting ice growth is systematically investigated at the atomic level and compared with its low activity mutant and a nonantifreeze protein. The results indicate a significant synergy between TmAFP and ice crystals, which enables the TmAFP to promote the ice growth before adsorbing on the surfaces of the ice crystals, while the mutant and the nonantifreeze protein cannot promote the ice growth due to the lack of this synergy. When TmAFP approaches the ice surface, the interfacial water is induced by both the AFP and the ice crystals to form the anchored clathrate motif, which binds TmAFP to the ice surface, resulting in a local increase in the curvature of the ice surface, thereby inhibiting the growth of ice. In this study, three stages, namely, promotion, adsorption, and inhibition, are observed in the complete process of TmAFP inhibiting ice growth, and the synergistic mechanism between protein and ice crystals is revealed. The results are helpful for the design of antifreeze proteins and bioinspired antifreeze materials with superior performance.

Polymer Self-Assembly Induced Enhancement of Ice Recrystallization Inhibition

Ice binding proteins modulate ice nucleation/growth and have huge (bio)technological potential. There are few synthetic materials that reproduce their function, and rational design is challenging due to the outstanding questions about the mechanisms of ice binding, including whether ice binding is essential to reproduce all their macroscopic properties. Here we report that nanoparticles obtained by polymerization-induced self-assembly (PISA) inhibit ice recrystallization (IRI) despite their constituent polymers having no apparent activity. Poly(ethylene glycol), poly(dimethylacrylamide), and poly(vinylpyrrolidone) coronas were all IRI-active when assembled into nanoparticles. Different core-forming blocks were also screened, revealing the core chemistry had no effect. These observations show ice binding domains are not essential for macroscopic IRI activity and suggest that the size, and crowding, of polymers may increase the IRI activity of "non-active" polymers. It was also discovered that poly(vinylpyrrolidone) particles had ice crystal shaping activity, indicating this polymer can engage ice crystal surfaces, even though on its own it does not show any appreciable ice recrystallization inhibition. Larger (vesicle) nanoparticles are shown to have higher ice recrystallization inhibition activity compared to smaller (sphere) particles, whereas ice nucleation activity was not found for any material. This shows that assembly into larger structures can increase IRI activity and that increasing the "size" of an IRI does not always lead to ice nucleation. This nanoparticle approach offers a platform toward ice-controlling soft materials and insight into how IRI activity scales with molecular size of additives.

High Sub-Zero Organ Preservation: A Paradigm of Nature-Inspired Strategies

The field of organ preservation is filled with advancements that have yet to see widespread clinical translation, with some of the more notable strategies deriving their inspiration from nature. While static cold storage (SCS) at 2 °C to 4 °C is the current state-of-the-art, it contributes to the current shortage of transplantable organs due to the limited preservation times it affords combined with the limited ability of marginal grafts (i.e. those at risk for post-transplant dysfunction or primary non-function) to tolerate SCS. The era of storage solution optimization to minimize SCS-induced hypothermic injury has plateaued in its improvements, resulting in a shift towards the use of machine perfusion systems to oxygenate organs at normothermic, sub-normothermic, or hypothermic temperatures, as well as the use of sub-zero storage temperatures to leverage the protection brought forth by a reduction in metabolic demand. Many of the rigors that organs are subjected to at low sub-zero temperatures (-80 °C to -196 °C) commonly used for mammalian cell preservation have yet to be surmounted. Therefore, this article focuses on an intermediate temperature range (0 °C to -20 °C), where much success has been seen in the past two decades. The mechanisms leveraged by organisms capable of withstanding prolonged periods at these temperatures through either avoiding or tolerating the formation of ice has provided a foundation for some of the more promising efforts. This article therefore aims to contextualize the translation of these strategies into the realm of mammalian organ preservation.

Advanced Nanomaterials-Assisted Cell Cryopreservation: A Mini Review

Cell cryopreservation is of vital significance both for transporting and storing cells before experimental/clinical use. Cryoprotectants (CPAs) are necessary additives in the preserving medium in cryopreservation, preventing cells from freeze-thaw injuries. Traditional organic solvents have been widely used in cell cryopreservation for decades. Given the obvious damage to cells due to their undesirable cytotoxicity and the burdensome post-thaw washing cycles before use, traditional CPAs are more and more likely to be replaced by modern ones with lower toxicity, less processing, and higher efficiency. As materials science thrives, nanomaterials are emerging to serve as potent vehicles for delivering nontoxic CPAs or inherent CPAs comparable to or even superior to conventional ones. This review will introduce some advanced nanomaterials (e.g., organic/inorganic nanoCPAs, nanodelivery systems) utilized for cell cryopreservation, providing broader insights into this developing field.

Ice Growth Acceleration by Antifreeze Proteins Leads to Higher Thermal Hysteresis Activity

Since some antifreeze proteins and glycoproteins (AF(G)Ps) cannot directly bind to all ice crystal planes, they change ice crystal morphology by minimizing the area of the crystal planes to which they cannot bind until crystal growth is halted. Previous studies found that growth along the c-axis (perpendicular to the basal plane, the crystal plane to which these AF(G)Ps cannot bind) is accelerated by some AF(G)Ps, while growth of other planes is inhibited. The effects of this growth acceleration on crystal morphology and on the thermal hysteresis activity are unknown to date. Understanding these effects will elucidate the mechanism of ice growth inhibition by AF(G)Ps. Using cold stages and an infrared laser, ice growth velocities and crystal morphologies in AF(G)P solutions were measured. Three types of effects on growth velocity were found: concentration-dependent acceleration, concentration-independent acceleration, and concentration-dependent deceleration. Quantitative crystal morphology measurements in AF(G)P solutions demonstrated that the adsorption rate of the proteins to ice plays a major role in determining the morphology of the bipyramidal crystal. These results demonstrate that faster adsorption rates generate bipyramidal crystals with diminished basal surfaces at higher temperatures compared to slower adsorption rates. The acceleration of growth along the c-axis generates crystals with smaller basal surfaces at higher temperatures leading to increased growth inhibition of the entire crystal.

Ice recrystallisation inhibiting polymer nano-objects via saline-tolerant polymerisation-induced self-assembly

Chemical tools to modulate ice formation/growth have great (bio)-technological value, with ice binding/antifreeze proteins being exciting targets for biomimetic materials. Here we introduce polymer nanomaterials that are potent inhibitors of ice recrystallisation using polymerisation-induced self-assembly (PISA), employing a poly(vinyl alcohol) graft macromolecular chain transfer agent (macro-CTA). Crucially, engineering the core-forming block with diacetone acrylamide enabled PISA to be conducted in saline, whereas poly(2-hydroxypropyl methacrylate) cores led to coagulation. The most active particles inhibited ice growth as low as 0.5 mg mL-1, and were more active than the PVA stabiliser block alone, showing that the dense packing of this nanoparticle format enhanced activity. This provides a unique route towards colloids capable of modulating ice growth.

X-ray diffraction to probe the kinetics of ice recrystallization inhibition

Understanding the nucleation and growth of ice is crucial in fields ranging from infrastructure maintenance, to the environment, and to preserving biologics in the cold chain. Ice binding and antifreeze proteins are potent ice recrystallization inhibitors (IRI), and synthetic materials that mimic this function have emerged, which may find use in biotechnology. To evaluate IRI activity, optical microscopy tools are typically used to monitor ice grain size either by end-point measurements or as a function of time. However, these methods provide 2-dimensional information and image analysis is required to extract the data. Here we explore using wide angle X-ray scattering (WAXS/X-ray powder diffraction (XRD)) to interrogate 100's of ice crystals in 3-dimensions as a function of time. Due to the random organization of the ice crystals in the frozen sample, the number of orientations measured by XRD is proportional to the number of ice crystals, which can be measured as a function of time. This method was used to evaluate the activity for a panel of known IRI active compounds, and shows strong agreement with results obtained from cryo-microscopy, as well as being advantageous in that time-dependent ice growth is easily extracted. Diffraction analysis also confirmed, by comparing the obtained diffraction patterns of both ice binding and non-binding additives, that the observed hexagonal ice diffraction patterns obtained cannot be used to determine which crystal faces are being bound. This method may help in the discovery of new IRI active materials as well as enabling kinetic analysis of ice growth.

Effect of surface charge density on the ice recrystallization inhibition activity of nanocelluloses

Recently nanocelluloses have been found to possess ice recrystallization inhibition (IRI) activity, which have several potential applications. The present study focuses on the relationship between the surface charge density (SCD) of nanocelluloses and IRI activity. Cellulose nanocrystals (CNCs) and 2, 2, 6, 6-tetramethylpiperidine-1-oxyl oxidized cellulose nanofibrils (TEMPO-CNFs) with similar degrees of polymerization (DP) or fibril lengths but with different SCDs were prepared and characterized for IRI activity. When the SCD of CNCs was progressively reduced, an initial increase of IRI activity was observed, followed by a decrease due to fibril aggregation. CNCs with a low SCD became IRI active at increased unfrozen water fractions and higher annealing temperatures. TEMPO-CNFs with a low SCD also had higher IRI activity. Additionally, lowering pH to protonate the carboxylate groups of TEMPO-CNFs enhanced the IRI activity. These research findings are important in producing nanocelluloses with enhanced IRI activity and understanding their structure-activity relationship.

The properties, biotechnologies, and applications of antifreeze proteins

By natural selection, organisms evolve different solutions to cope with extremely cold weather. The emergence of an antifreeze protein gene is one of the most momentous solutions. Antifreeze proteins possess an importantly functional ability for organisms to survive in cold environments and are widely found in various cold-tolerant species. In this review, we summarize the origin of antifreeze proteins, describe the diversity of their species-specific properties and functions, and highlight the related biotechnology on the basis of both laboratory tests and bioinformatics analysis. The most recent advances in the applications of antifreeze proteins are also discussed. We expect that this systematic review will contribute to the comprehensive knowledge of antifreeze proteins to readers.

Unraveling Molecular Mechanism on Dilute Surfactant Solution Controlled Ice Recrystallization

Ice recrystallization (IR) is ubiquitous, playing an important role in many areas of science, such as cryobiology, food science, and atmospheric physics. However, controllable ice recrystallization remains a challenging task largely due to an incomplete understanding of the physical mechanism associated with ice recrystallization. Herein, we explore the molecular mechanism underlying the controlling of ice recrystallization by using different small amphiphilic molecules (surfactants) through joint experimental measurements and molecular dynamics simulation. Our experiment shows that in nonionic/zwitterionic surfactant solutions, the mean size of the recrystallized ice grains increases monotonically with the concentration of surfactants, whereas in the ionic surfactant solutions, the mean size of the recrystallized ice grains tends to increase first and then decrease with increasing the concentration, yielding a peak typically at ∼5 μM. Further sequential ice affinity purification experiments and molecular dynamics simulations show that the surfactants actually do not bind to ice directly. Rather, the different spatial distributions of counter ions and molecular surfactants in the interfacial regions (ice-water interface and water-air interface) and bulk region can markedly affect the mean size of the recrystallized ice grain.

Combinatorial Biomaterials Discovery Strategy to Identify New Macromolecular Cryoprotectants

Cryoprotective agents (CPAs) are typically solvents or small molecules, but there is a need for innovative CPAs to reduce toxicity and increase cell yield, for the banking and transport of cells. Here we use a photochemical high-throughput discovery platform to identify macromolecular cryoprotectants, as rational design approaches are currently limited by the lack of structure-property relationships. Using liquid handling systems, 120 unique polyampholytes were synthesized using photopolymerization with RAFT agents. Cryopreservation screening identified "hit" polymers and nonlinear trends between composition and function, highlighting the requirement for screening, with polymer aggregation being a key factor. The most active polymers reduced the volume of dimethyl sulfoxide (DMSO) required to cryopreserve a nucleated cell line, demonstrating the potential of this approach to identify materials for cell storage and transport.

Peptidic Antifreeze Materials: Prospects and Challenges

Necessitated by the subzero temperatures and seasonal exposure to ice, various organisms have developed a remarkably effective means to survive the harsh climate of their natural habitats. Their ice-binding (glyco)proteins keep the nucleation and growth of ice crystals in check by recognizing and binding to specific ice crystal faces, which arrests further ice growth and inhibits ice recrystallization (IRI). Inspired by the success of this adaptive strategy, various approaches have been proposed over the past decades to engineer materials that harness these cryoprotective features. In this review we discuss the prospects and challenges associated with these advances focusing in particular on peptidic antifreeze materials both identical and akin to natural ice-binding proteins (IBPs). We address the latest advances in their design, synthesis, characterization and application in preservation of biologics and foods. Particular attention is devoted to insights in structure-activity relations culminating in the synthesis of de novo peptide analogues. These are sequences that resemble but are not identical to naturally occurring IBPs. We also draw attention to impactful developments in solid-phase peptide synthesis and 'greener' synthesis routes, which may aid to overcome one of the major bottlenecks in the translation of this technology: unavailability of large quantities of low-cost antifreeze materials with excellent IRI activity at (sub)micromolar concentrations.

Bioinspired Ice Growth Inhibitors Based on Self-Assembling Peptides

Antifreeze proteins (AFPs) are widely found in organisms living in subzero environments. Their strong ability to inhibit ice growth and recrystallization have inspired considerable bioinspired efforts to engineer artificial ice growth inhibitors for cryopreservation. However, it remains challenging to engineer biocompatible and cost-effective synthetic ice growth inhibitors to meet the increasing needs of cryoprotectants in biomedical research and industry. Here we report the design of artificial ice growth inhibitors based on self-assembling peptides. We demonstrate the importance of threonine residues as well as their spatial arrangement for effective ice binding. The engineered self-assembling ice growth inhibiting peptides show moderate ice inhibiting activity including suppression of ice growth rates and retardation of recrystallization of ice crystals. The applications of these peptides in cryopreservation of enzymes and cells were also demonstrated.

Enhancement of Macromolecular Ice Recrystallization Inhibition Activity by Exploiting Depletion Forces

Antifreeze (glyco) proteins (AF(G)Ps) are potent inhibitors of ice recrystallization and may have biotechnological applications. The most potent AF(G)Ps function at concentrations a thousand times lower than synthetic mimics such as poly(vinyl alcohol), PVA. Here, we demonstrate that PVA’s ice recrystallization activity can be rescued at concentrations where it does not normally function, by the addition of noninteracting polymeric depletants, due to PVA forming colloids in the concentrated saline environment present between ice crystals. These depletants shift the equilibrium toward ice binding and, hence, enable PVA to inhibit ice growth at lower concentrations. Using theory and experiments, we show this effect requires polymeric depletants, not small molecules, to enhance activity. These results increase our understanding of how to design new ice growth inhibitors, but also offer opportunities to enhance activity by exploiting depletion forces, without re-engineering ice-binding materials. It also shows that when screening for IRI activity that polymer contaminants in buffers may give rise to false positive results.

Mimicking the Ice Recrystallization Activity of Biological Antifreezes. When is a New Polymer "Active"?

Antifreeze proteins and ice-binding proteins have been discovered in a diverse range of extremophiles and have the ability to modulate the growth and formation of ice crystals. Considering the importance of cryoscience across transport, biomedicine, and climate science, there is significant interest in developing synthetic macromolecular mimics of antifreeze proteins, in particular to reproduce their property of ice recrystallization inhibition (IRI). This activity is a continuum rather than an "on/off" property and there may be multiple molecular mechanisms which give rise to differences in this observable property; the limiting concentrations for ice growth vary by more than a thousand between an antifreeze glycoprotein and poly(vinyl alcohol), for example. The aim of this article is to provide a concise comparison of a range of natural and synthetic materials that are known to have IRI, thus providing a guide to see if a new synthetic mimic is active or not, including emerging materials which are comparatively weak compared to antifreeze proteins, but may have technological importance. The link between activity and the mechanisms involving either ice binding or amphiphilicity is discussed and known materials assigned into classes based on this.

Site-specific conjugation of antifreeze proteins onto polymer-stabilized nanoparticles

Antifreeze proteins (AFPs) have many potential applications, ranging from cryobiology to aerospace, if they can be incorporated into materials. Here, a range of engineered AFP mutants were prepared and site-specifically conjugated onto RAFT polymer-stabilized gold nanoparticles to generate new hybrid multivalent ice growth inhibitors. Only the SNAP-tagged AFPs lead to potent 'antifreeze' active nanomaterials with His-Tag capture resulting in no activity, showing the mode of conjugation is essential. This versatile strategy will enable the development of multivalent AFPs for translational and fundamental studies.

Multivalent Presentation of Ice Recrystallization Inhibiting Polymers on Nanoparticles Retains Activity

Poly(vinyl alcohol) (PVA) has emerged as the most potent mimic of antifreeze (glyco)proteins ice recrystallization inhibition (IRI) activity, despite its lack of structural similarities and flexible, rather than rigid, backbone. The precise spacing of hydroxyl groups is hypothesized to enable PVA to recognize the prism planes of ice but not the basal plane, due to hydroxyl pattern matching of the ice surface giving rise to the macroscopic activity. Here, well-defined PVA derived from reversible addition-fragmentation chain-transfer (RAFT) polymerization is immobilized onto gold nanoparticles to enable the impact of nanoscale assembly and confinement on the observed IRI activity. Unlike previous reports using star-branched or bottle-brush PVAs, the nanoparticle-PVA retains all IRI activity compared to polymers in solution. Evidence is presented to show that this is due to the low grafting densities on the particle surface meaning the chains are free to explore the ice faces, rather than being constrained as in star-branched polymers. These results demonstrate a route to develop more functional IRI's and inclusion of metallic particle cores for imaging and associated applications in cryobiology.

A Freezing-Induced Turn-On Imaging Modality for Real-Time Monitoring of Cancer Cells in Cryosurgery

Cryosurgery has attracted much attention for the treatment of tumors owing to its clear advantages. However, determining the volume of frozen tissues in real-time remains a challenge, which greatly lowers the therapeutic efficacy of cryosurgery and hinders its broad application for the treatment of cancers. Herein, we report a freezing-induced turn-on strategy for the selective real-time imaging of frozen cancer cells. As a type of aggregation-induced emission (AIE) fluorogen, TABD-Py molecules interact specifically with ice crystals and form aggregates at the ice/water interface. Consequently, bright fluorescent emission appears upon freezing. TABD-Py molecules are enriched mostly in the cancer cells and exhibit high biocompatibility as well as low cytotoxicity; therefore, a freezing-induced turn-on imaging modality for cryosurgery is developed, which will certainly maximize the therapeutic efficacy of cryosurgery in treating tumors.

Betaine Combined with Membrane Stabilizers Enables Solvent-Free Whole Blood Cryopreservation and One-Step Cryoprotectant Removal

Cryopreservation of red blood cells (RBCs) is fundamentally important to modern transfusion medicine. Currently, organic solvent glycerol is utilized as the state-of-the-art cryoprotectant (CPA) for RBC cryopreservation. However, glycerol must be removed before RBC transfusion to avoid intravascular hemolysis via a time-consuming deglycerolization process with specialized equipment (e.g., ACP 215), thus limiting the clinical use of frozen RBCs. Herein, we report novel biocompatible CPA formulations combining betaine with membrane stabilizers (disaccharides or amino acids), which can achieve outstanding efficiency for RBC cryopreservation directly using whole blood without any separation process. Most importantly, because of the osmotic regulation capacity of betaine, a simple and fast one-step method can be used for CPA removal, which is significantly superior to the current multistep deglycerolization process. This work offers a promising solution for highly efficient and solvent-free RBC cryopreservation and holds great potential for improving the long-term storage and long-distance distribution of RBCs.