Polymer Self-Assembly Induced Enhancement of Ice Recrystallization Inhibition

Higher molecular weight pectin inhibits ice crystal growth and its effect on the microstructural and physical properties of pectin cryogels

Understanding the formation of ice crystals is essential for tailoring the microstructure and physical properties of cryogels. This study investigated the effects and mechanisms of pectin molecular weight (Mw) on impacting ice crystal formation. Pectin fractions various Mw (10.13-212.20 kDa) were prepared by hydrothermal method. The solution of high Mw pectin fractions exhibited higher contact angle, lower water freedom, and stronger adsorption of water molecules. The splat experiment and molecular dynamic (MD) results confirmed that higher Mw pectin have stronger ice crystal growth inhibition activity than lower Mw pectin. Furthermore, the pore size distribution of the cryogel increased from 98-203 μm to 105-267 μm as the molecular weight decreased from 212.2 kDa to 121.0 kDa. Additionally, in the higher Mw pectin cryogel, stronger mechanical strength was observed. These findings suggested that changing the molecular weight of pectin has the potential to regulate the ice crystal growth, microstructure and physical properties of frozen products.

The Valence-Dependent Activity of Colloidal Molecules as Ice Recrystallization Inhibitors

Inspired by advances in cryopreservation techniques, which are essential for modern biomedical applications, there is a special interest in the ice recrystallization inhibition (IRI) of the antifreeze protein (AFPs) mimics. There are in-depth studies on synthetic materials mimicking AFPs, from simple molecular structure levels to complex self-assemblies. Herein, we report the valence-dependent IRI activity of colloidal organic molecules (CMs). The CMs were prepared through polymerization-induced particle-assembly (PIPA) of the ABC-type triblock terpolymer of poly(acryloxyethyl trimethylammonium chloride)-b-poly(benzyl acrylate)-b-poly(diacetone acrylamide) (PATAC-b-PBzA-b-PDAAM) at high monomer conversions. Stabilized by the cationic block of PATAC, the strong intermolecular H-bonding and incompatibility of the PDAAM block with PBzA contributed to the in situ formation of Janus particles (AX1) beyond the initial spherical seed particles (AX0), as well as the high valency clusters of linear AX2 and trigonal AX3. Their distribution was controlled mainly by the polymerization degrees (DPs) of PATAC and PDAAM blocks. IRI activity results of the CMs suggest that the higher fraction of AX1 results in the better IRI activity. Increasing the fraction of AX1 from 27% to 65% led to a decrease of the mean grain size from 39.8% to 10.9% and a depressed growth rate of ice crystals by 58%. Moreover, by replacing the PDAAM block with the temperature-responsive one of poly(N-isopropylacrylamide) (PNIPAM), temperature-adjustable IRI activity was observed, which is well related to the reversible transition of AX0 to AX1, providing a new idea for the molecular design of amphiphilic polymer nanoparticle-based IRI activity materials.

Poly(l-proline)-Stabilized Polypeptide Nanostructures via Ring-Opening Polymerization-Induced Self-Assembly (ROPISA)

Poly(proline) II helical motifs located at the protein-water interface stabilize the three-dimensional structures of natural proteins. Reported here is the first example of synthetic biomimetic poly(proline)-stabilized polypeptide nanostructures obtained by a straightforward ring-opening polymerization-induced self-assembly (ROPISA) process through consecutive N-carboxyanhydride (NCA) polymerization. It was found that the use of multifunctional 8-arm initiators is critical for the formation of nanoparticles. Worm-like micelles as well as spherical morphologies were obtained as confirmed by dynamic light scattering (DLS), transmission electron microscopy (TEM), and small angle X-ray scattering (SAXS). The loading of the nanostructures with dyes is demonstrated. This fast and open-vessel procedure gives access to amino acids-based nanomaterials with potential for applications in nanomedicine.

Effect of Added Salt on the RAFT Polymerization of 2-Hydroxyethyl Methacrylate in Aqueous Media

We report the effect of added salt on the reversible addition-fragmentation chain transfer (RAFT) polymerization of 2-hydroxyethyl methacrylate (HEMA) in aqueous media. More specifically, poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC26) was employed as a salt-tolerant water-soluble block for chain extension with HEMA targeting PHEMA DPs from 100 to 800 in the presence of NaCl. Increasing the salt concentration significantly reduces the aqueous solubility of both the HEMA monomer and the growing PHEMA chains. HEMA conversions of more than 99% could be achieved within 6 h at 70 °C regardless of the NaCl concentration when targeting PMPC26-PHEMA800 vesicles at 20% w/w solids. Significantly faster rates of polymerization were observed at higher salt concentration owing to the earlier onset of micellar nucleation. Transmission electron microscopy (TEM) was used to construct a pseudo-phase diagram for this polymerization-induced self-assembly (PISA) formulation. High-quality images required cross-linking of the PHEMA chains with glutaraldehyde prior to salt removal via dialysis. Block copolymer spheres, worms, or vesicles can be accessed at any salt concentration up to 2.5 M NaCl. However, only kinetically trapped spheres could be obtained in the presence of 3 M NaCl because the relatively low HEMA monomer solubility under such conditions leads to an aqueous emulsion polymerization rather than an aqueous dispersion polymerization. In this case, dynamic light scattering studies indicated a gradual increase in z-average diameter from 26 to 86 nm when adjusting the target PHEMA degree of polymerization from 200 to 800. When targeting PMPC26-PHEMA800 vesicles, increasing the salt content up to 2.5 M NaCl leads to a systematic reduction in the z-average diameter from 953 to 92 nm. Similarly, TEM analysis and dispersion viscosity measurements indicated a gradual reduction in worm contour length with increasing salt concentration for PMPC26-PHEMA600 worms. This new PISA formulation clearly illustrates the importance of added salt on aqueous monomer solubility and how this affects (i) the kinetics of polymerization, (ii) the morphology of the corresponding diblock copolymer nano-objects, and (iii) the mode of polymerization in aqueous media.

Development of Macromolecular Cryoprotectants for Cryopreservation of Cells

Cryopreservation is a common way for long-term storage of therapeutical proteins, erythrocytes, and mammalian cells. For cryoprotection of these biosamples to keep their structural integrity and biological activities, it is essential to incorporate highly efficient cryoprotectants. Currently, permeable small molecular cryoprotectants such as glycerol and dimethyl sulfoxide dominate in cryostorage applications, but they are harmful to cells and human health. As acting in the extracellular space, membrane-impermeable macromolecular cryoprotectants, which exert remarkable membrane stabilization against cryo-injury and are easily removed post-thaw, are promising candidates with biocompatibility and feasibility. Water-soluble hydroxyl-containing polymers such as poly(vinyl alcohol) and polyol-based polymers are potent ice recrystallization inhibitors, while polyampholytes, polyzwitterions, and bio-inspired (glyco)polypeptides can significantly increase post-thaw recovery with reduced membrane damages. In this review, the synthetic macromolecular cryoprotectants are systematically summarized based on their synthesis routes, practical utilities, and cryoprotective mechanisms. It provides a valuable insight in development of highly efficient macromolecular cryoprotectants with valid ice recrystallization inhibition activity for highly efficient and safe cryopreservation of cells.

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.

Synthesis of Poly(propylene oxide)-Poly(N,N'-dimethylacrylamide) Diblock Copolymer Nanoparticles via Reverse Sequence Polymerization-Induced Self-Assembly in Aqueous Solution

Sterically-stabilized diblock copolymer nanoparticles comprising poly(propylene oxide) (PPO) cores are prepared via reverse sequence polymerization-induced self-assembly (PISA) in aqueous solution. N,N'-Dimethylacrylamide (DMAC) acts as a cosolvent for the weakly hydrophobic trithiocarbonate-capped PPO precursor. Reversible addition-fragmentation chain transfer (RAFT) polymerization of DMAC is initially conducted at 80% w/w solids with deoxygenated water. At 30-60% DMAC conversion, the reaction mixture is diluted to 5-25% w/w solids. The PPO chains become less solvated as the DMAC monomer is consumed, which drives in situ self-assembly to form aqueous dispersions of PPO-core nanoparticles of 120-190 nm diameter at 20 °C. Such RAFT polymerizations are well-controlled (Mw/Mn ≤ 1.31), and more than 99% DMAC conversion is achieved. The resulting nanoparticles exhibit thermoresponsive character: dynamic light scattering and transmission electron microscopy studies indicate the formation of more compact spherical nanoparticles of approximately 33 nm diameter on heating to 70 °C. Furthermore, 15-25% w/w aqueous dispersions of such nanoparticles formed micellar gels that undergo thermoreversible (de)gelation on cooling to 5 °C.

Poly(l-alanine-co-l-threonine succinate) as a Biomimetic Cryoprotectant

We synthesized a series of [(l-Ala)x-co-(l-Thr succinate)y] (PATs), which are analogous to natural antifreezing glycoprotein with the structure of [l-Ala-l-Ala-l-Thr disaccharide]n, by varying the composition and degree of succinylation while fixing their molecular weight (Mn) and Ala/Thr ratio at approximately 10-12 kDa and 2:1, respectively. We investigated their ice recrystallization inhibition (IRI), ice nucleation inhibition (INI), dynamic ice shaping (DIS), thermal hysteresis (TH), and protein cryopreservation activities. Both IRI and INI activities were greater for PATs with higher l-Ala content (PATs-3 and PATs-4) than those with lower l-Ala content (PATs-1 and PATs-2). DIS activity with faceted crystal growth was clearly observed in PATs-2 and PATs-4 with a high degree of succinylation. TH was small with <0.1 °C for all PATs and slightly greater for PATs with a high l-Ala content. Except for PATs-1, the protein (lactate dehydrogenase, LDH) stabilization activity was excellent for all PATs studied, maintaining LDH activity as high as that of fresh LDH even after 15 freeze-thaw cycles. To conclude, the cryo-active biomimetic PATs were synthesized by controlling the l-Ala content and degree of succinylation. Our results showed that PATs with an l-Ala content of 65-70% and degree of succinylation of 12-19% exhibited the cryo-activities of IRI, INI, and DIS, and particularly promising properties for the cryoprotection of LDH protein.

Self-assembly of tamarind seed polysaccharide via enzymatic depolymerization and degalactosylation enhanced ice recrystallization inhibition activity

Polysaccharides are becoming potential candidates for developing food-grade cryoprotectants due to their extensive accessibility and health-promoting effects. However, unremarkable ice recrystallization inhibition (IRI) activity and high viscosity limit their practical applications in some systems. Our previous study found a galactoxyloglucan polysaccharide from tamarind seed (TSP) showing moderate IRI activity. Herein, the enhancement of the IRI performance of TSP via enzymatic depolymerization and degalactosylation-induced self-assembly was reported. TSP was depolymerized and subsequently removed ∼40 % Gal, which induced the formation of supramolecular rod-like fiber self-assembles and exhibited a severalfold enhancement of IRI. Ice shaping assay did not show obvious faceting of ice crystals, indicating that both depolymerized and self-assembled TSP showed very weak binding to ice. Molecular dynamics simulation confirmed the absence of molecular complementarity with ice. Further, it highlighted that degalactosylation did not cause significant changes in local hydration properties of TSP from the view of a single oligomer. The inconsistency between molecular simulation and macroscopic IRI effect proposed that the formation of unique supramolecular self-assemblies may be a key requirement for enhancing IRI activity. The findings of this study provided a new opportunity to enhance the applied potential of natural polysaccharides in food cryoprotection.

Azobenzene-Containing Liquid Crystalline Twisted Ribbons via Polymerization-Induced Hierarchical Self-Assembly

Polymerization-induced self-assembly incorporating liquid crystallization, as a polymerization-induced hierarchical self-assembly (PIHSA) method to produce polymeric particles with anisotropic morphologies facilely and efficiently, has drawn wide attention recently. However, the means of regulating the morphologies of liquid crystalline (LC) polymer assemblies still need to be explored. Herein, a route is presented to fabricate the twisted ribbons via PIHSA containing azobenzene based on poor reversible addition-fragmentation chain transfer (RAFT) control, called poorly controlled PIHSA. Cyano-4-(dodecylsulfanylthiocarbonyl)sulfanyl pentanoic acid-2-(2-pyridyldithio) ethyl ester is used as the RAFT agent with poor controllability, and the morphological evolution from ribbons to twisted ribbons can be observed in the corresponding PIHSA system. The formation mechanism of the twisted ribbons is studied systematically and the broad molecular weight distribution is considered to be the decisive factor. Moreover, the supramolecular chirality induced by symmetry breaking is also related to the twist of the ribbons. This study enriches the methods of controlling the morphologies of LC polymer particles and is helpful for further clarifying the mechanism of PIHSA.

Self-Assembled Nanostructures of Homo-Oligopeptide as a Potent Ice Growth Inhibitor

This study reports the formation of self-assembled nanostructures with homo-oligopeptides consisting of amino acids (i.e., alanine, threonine, valine, and tyrosine), the resulting morphologies (i.e., spherical shape, layered structure, and wire structure) in aqueous solution, and their potential as ice growth inhibitors. Among the homo-oligopeptides investigated, an alanine homo-oligopeptide (n = 5) with a spherical nanostructure showed the highest ice recrystallization inhibition (IRI) activity without showing a burst ice growth property and with low ice nucleation activity. The presence of nanoscale self-assembled structures in the solution showed superior IRI activity compared to an amino acid monomer because of the higher binding affinity of structures on the growing ice crystal plane. Simulation results revealed that the presence of nanostructures induced a significant inhibition of ice growth and increased lifetime of hydrogen bonding compared with unassembled homo-oligopeptide. These results envision extraordinary performance for self-assembled nanostructures as a desirable and potent ice growth inhibitor.

Does freezing induce self-assembly of polymers? A molecular dynamics study

This work investigates the freezing-induced self-assembly (FISA) of polyvinyl alcohol (PVA) and PVA-like polymers using molecular dynamics simulations. In particular, the effect of the degree of supercooling, degree of polymerization, polymer type, and initial local concentration on the FISA was studied. It was found that the preeminent factor responsible for FISA is not the diffusion of the polymers away from the nucleating ice front, but the increase in the polymer's local concentration upon freezing of the solvent (water). At a higher degree of supercooling, the polymers are engulfed by the growing ice front, impeding their diffusion into the supercooled solution and finally inhibiting their self-assembly. Conversely, at a relatively lower degree of supercooling, the rate of diffusion of the polymers into the supercooled solution is higher, which increases their local concentration and results in FISA. FISA was also observed to depend on the polymer-solvent interactions. Strongly favorable solute-solvent interactions hinder the self-assembly, whereas unfavorable solute-solvent interactions promote the self-assembly. The polymer and aggregate morphology were investigated using the radius of gyration, end-to-end distance, and asphericity analysis. This study brings molecular insights into the quintessential factors governing self-assembly via freezing of the solvent, which is a novel self-assembly technique especially suitable for biomedical applications.

Enhanced Cryopreservation Efficacies of Ice Recrystallization Inhibiting Nanogels

Development of new cryopreservation technologies holds significant potential to revolutionize the fields of cell culture, tissue engineering, assisted reproduction, and transfusion medicine. The current gold standard small-cell permeating cryopreservation agents (CPAs) demonstrate promising cryopreservation efficacies but are cytotoxic and immunogenic at the concentrations required for cryopreservation applications. In comparison, new cell impermeable CPAs of nanodimensions demonstrate outstanding potential to overcome the drawbacks of existing CPAs. In this study, we report the synthesis of vitamin B5 analogous methacrylamide (B5AMA)-incorporated nanogels as a potential solution to address the commonly observed limitations of existing CPAs. The stimuli-responsive poly(B5AMA) nanogels prepared by radical polymerization demonstrated significant ice recrystallization inhibition efficacies and showed either superior or comparable cryopreservation efficacies compared to the traditional cryoprotectant DMSO/glycerol in both eukaryotic and prokaryotic cells.

Synthetic exploration of electrophilic xanthylation via powerful N-xanthylphthalimides

Organic xanthates are broadly applied as synthetic intermediates and bioactive molecules in synthetic chemistry. Electrophilic xanthylation represents a promising approach but has rarely been explored mainly due to the lack of powerful electrophilic reagents. Herein, synthetic exploration of electrophilic xanthylation via powerful N-xanthylphthalimides was investigated. This strategy might provide a new avenue to less-concerned but meaningful electrophilic xanthylation in organic synthesis. With the help of these powerful reagents, electrophilic xanthylation of a wide range of substrates including aryl/alkenyl boronic acids, β-keto esters, 2-oxindole, and alkyl amines, as well as previously inaccessible phenols (first report) was achieved under mild reaction conditions. Notably, this simple electrophilic xanthylation of alkyl amine substrates will occur in the desulfuration reaction, consistent with the previously reported methods. Similarly, xanthamide and thioxanthate groups could also be transformed into desired nucleophiles via this electrophilic reagent strategy. The broad substrate scope, excellent functional group compatibility and late-stage functionalization of bioactive or functional molecules made them very attractive as general reagents which will allow rapid incorporation of SC(S)R (R = OEt, Oalkyl, NEt2 and SEt) into the target molecules.

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.

Ice nucleation in aqueous solutions of short- and long-chain poly(vinyl alcohol) studied with a droplet microfluidics setup

Poly(vinyl alcohol) (PVA) has ice binding and ice nucleating properties. Here, we explore the dependence of the molecular size of PVA on its ice nucleation activity. For this purpose, we studied ice nucleation in aqueous solutions of PVA samples with molar masses ranging from 370 to 145 000 g mol-1, with a particular focus on oligomer samples with low molar mass. The experiments employed a novel microfluidic setup that is a follow-up on the previous WeIzmann Supercooled Droplets Observation on a Microarray (WISDOM) design by Reicher et al. The modified setup introduced and characterized here, termed nanoliter Bielefeld Ice Nucleation ARraY (nanoBINARY), uses droplet microfluidics with droplets (96 ± 4) µm in diameter and a fluorinated continuous oil phase and surfactant. A comparison of homogeneous and heterogeneous ice nucleation data obtained with nanoBINARY to those obtained with WISDOM shows very good agreement, underpinning its ability to study low-temperature ice nucleators as well as homogeneous ice nucleation due to the low background of impurities. The experiments on aqueous PVA solutions revealed that the ice nucleation activity of shorter PVA chains strongly decreases with a decrease in molar mass. While the cumulative number of ice nucleating sites per mass nm of polymers with different molar masses is the same, it becomes smaller for oligomers and completely vanishes for dimer and monomer representatives such as 1,3-butanediol, propan-2-ol, and ethanol, most likely because these molecules become too small to effectively stabilize the critical ice embryo. Overall, our results are consistent with PVA polymers and oligomers acting as heterogeneous ice nucleators.

Ice recrystallization inhibition mechanism of zwitterionic poly(carboxybetaine methacrylate)

Understanding the ice recrystallization inhibition (IRI) mechanism is of fundamental importance for the rational design of novel antifreeze protein mimetics and reducing IR-related damage. In this communication, using quantitive experimental methods and molecular dynamics simulations we demonstrate that zwitterionic poly(carboxybetaine methacrylate) (PCBMA) can serve as a novel IRI-active substance. This work unravels the atomic-level details of the IRI mechanism of zwitterionic antifreeze protein mimetics and provides insight into the development of next-generation antifreeze protein mimetics.

Microcurvature Controllable Metal-Organic Framework Nanoagents Capable of Ice-Lattice Matching for Cellular Cryopreservation

Ice-binding proteins (IBPs) produced by psychrophilic organisms to adapt for the survival of psychrophiles in subzero conditions have received illustrious interest as a cryopreservation agent required for cells and tissues to completely recover after freezing/thawing. Depressing water-freezing point and avoiding ice-crystal growth affect their activities which are closely related to the presence of ice crystal well-matched binding moiety. The interaction of IBPs with ice and water is critical in enhancing their freeze avoidance against cell or tissue damage. Metal-organic frameworks (MOFs) with a controllable lattice at the molecular level and a size at the nanometer scale can offer periodically ordered ice-binding sites by modifying organic linkers and controlling microcurvature at the ice surface. Herein, zirconium (Zr)-based MOF-801 nanoparticles (NPs) with good biocompatibility were used as a cryoprotectant that is well dispersed and colloidal-stable in an aqueous solution. The MOF NP size was precisely controlled, and 10, 35, 100, and 250 nm NPs were prepared. The specific IBPs-mimicking pendants (valine and threonine) were simply introduced into the MOF NP-surface through the acrylate-based functionalization to endow with hydrophilic and hydrophobic dualities. When small-sized MOF-801 NPs were attached to ice, they confined ice growth in high curvature between the adsorption sites because of the decreased radius of the convex area of the growth region, leading to highly enhanced ice recrystallization inhibition (IRI). Surface-functionalized MOF NPs could increase the number of anchored clathrate water molecules with hydrophilic/hydrophobic balance of the ice-binding moiety, effectively inhibiting ice growth. The MOF-801 NPs were biocompatible with various cell lines regardless of concentration or NP surface-functionalization, whereas the smaller-sized surface-functionalized NPs showed a good cell recovery rate after freezing/thawing by induction of IRI. This study provides a strategy for the fabrication of low-cost, high-volume antifreeze nanoagents that can extend useful applications to organ transplantation, cord blood storage, and vaccines/drugs.

Nanoarchitectonics of RGO-Wrapped CNF/GO Aerogels with Controlled Pore Structures by PVA-Assisted Freeze-Casting Approach for Efficient Sound and Microwave Absorption

Acoustic absorption materials play an important role in eliminating the negative effects of noise. Herein, a polyvinyl alcohol (PVA)-assisted freeze-casting was developed for controllably fabricating reduced graphene oxide wrapped carbon nanofiber (RGO@CNF)/graphene oxide composite aerogel. During the freeze-casting, PVA was used as an icing inhibitor to control the size of ice crystals. While the concentration of PVA increased from 0 to 15 mg ⋅ ml^-1 , the average pore size of the aerogel was reduced from 154 to 45 μm. Due to the modulation of the pore size and composition, the propagation path and friction loss for sound were optimized, especially at low frequency. For instance, the normalized sound absorption coefficient of RGO@CNF/GO-10 achieves 0.79 (250-6300 Hz). The sample also exhibits a desirable microwave absorbing property whose maximum reflection loss is -47.3 dB (9.44 GHz, d=3.0 mm). Prospectively, this synthetic strategy can be extended to develop other forms of elastic aerogel with a controlled pore size.

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.

Infrared spectroscopic analysis of hydrogen-bonding interactions in cryopreservation solutions

BACKGROUND

In this study we investigated hydrogen bonding interactions in hydrated and frozen solutions of different cryoprotective agents (CPAs) including dimethyl sulfoxide, glycerol, ethylene glycol, propylene glycol, and trehalose. We also investigated the effect of CPAs on ice crystal growth during storage and correlated this with storage stability of liposomes.

METHODS

FTIR spectroscopy was used to study hydrogen bonding interactions in CPA solutions in H₂O and D₂O, and their thermal response was analyzed using van 't Hoff analysis. The effect of CPAs on ice crystal growth during storage was investigated by microscopy and correlated with storage stability of liposomes encapsulated with a fluorescent dye.

RESULTS

Principal component analyses demonstrated that different CPAs can be recognized based on the shape of the OD band region only. Chemically similar molecules such as glycerol and ethylene glycol closely group together in a principal component score plot, whereas trehalose and DMSO appear as condensed separated clusters. The OH/OD band of CPA solutions exhibits an overall shift to higher wavenumbers with increasing temperature and changed fractions of weak and strong hydrogen interactions. CPAs diminish ice crystal formation in frozen samples during storage and minimize liposome leakage during freezing but cannot prevent leakage during frozen storage.

CONCLUSIONS

CPAs can be distinguished from one another based on the hydrogen bonding network that is formed in solution. DMSO-water mixtures behave anomalous compared to other CPAs that have OH groups. CPAs modulate ice crystal formation during frozen storage but cannot prevent liposome leakage during frozen storage.

Poly(vinyl alcohol) Molecular Bottlebrushes Nucleate Ice

Ice binding proteins (IBP) have evolved to limit the growth of ice but also to promote ice formation by ice-nucleating proteins (INPs). IBPs, which modulate these seemingly distinct processes, often have high sequence similarities, and molecular size/assembly is hypothesized to be a crucial determinant. There are only a few synthetic materials that reproduce INP function, and rational design of ice nucleators has not been achieved due to outstanding questions about the mechanisms of ice binding. Poly(vinyl alcohol) (PVA) is a water-soluble synthetic polymer well known to effectively block ice recrystallization, by binding to ice. Here, we report the synthesis of a polymeric ice nucleator, which mimics the dense assembly of IBPs, using confined ice-binding polymers in a high-molar-mass molecular bottlebrush. Poly(vinyl alcohol)-based molecular bottlebrushes with different side-chain densities were synthesized via a combination of ring-opening metathesis polymerization (ROMP) and reversible addition-fragmentation chain-transfer (RAFT) polymerization, using "grafting-to" and "grafting-through" approaches. The facile preparation of the PVA bottlebrushes was performed via selective hydrolysis of the acetate of the poly(vinyl acetate) (PVAc) side chains of the PVAc bottlebrush precursors. Ice-binding polymer side-chain density was shown to be crucial for nucleation activity, with less dense brushes resulting in colder nucleation than denser brushes. This bio-inspired approach provides a synthetic framework for probing heterogeneous ice nucleation and a route toward defined synthetic nucleators for biotechnological applications.

Polysaccharide-Derived Ice Recrystallization Inhibitors with a Modular Design: The Case of Dextran-Based Graft Polymers

Ice recrystallization inhibitors inspired from antifreeze proteins (AFPs) are receiving increasing interest for cryobiology and other extreme environment applications. Here, we present a modular strategy to develop polysaccharide-derived biomimetics, and detailed studies were performed in the case of dextran. Poly(vinyl alcohol) (PVA) which has been termed as one of the most potent biomimetics of AFPs was grafted onto dextran via thiol-ene click chemistry (Dex-g-PVA). This demonstrated that Dex-g-PVA is effective in IRI and its activity increases with the degree of polymerization (DP) (sizes of ice crystals were 18.846 ± 1.759 and 9.700 ± 1.920 μm with DPs of 30 and 80, respectively) and fraction of PVA. By means of the dynamic ice shaping (DIS) assay, Dex-g-PVA is found to engage on the ice crystal surfaces, thus the ice affinity accounts for their IRI activity. In addition, Dex- g-PVA displayed enhanced IRI activity compared to that of equivalent PVA alone. We speculate that the hydrophilic nature of dextran would derive PVA in a stretch conformation that favors ice binding. The modular design can not only offer polysaccharides IRI activity but also favor the ice-binding behavior of PVA.

Polymer-tethered glycosylated gold nanoparticles recruit sialylated glycoproteins into their protein corona, leading to off-target lectin binding

Upon exposure to biological fluids, the fouling of nanomaterial surfaces results in non-specific capture of proteins, which is particularly important when in contact with blood for in vivo and ex vivo applications. It is crucial to evaluate not just the protein components but also the glycans attached to those proteins. Polymer-tethered glycosylated gold nanoparticles have shown promise for use in biosensing/diagnostics, but the impact of the glycoprotein corona has not been established. Here we investigate how polymer-tethered glycosylated gold nanoparticles interact with serum proteins and demonstrate that the protein corona introduces new glycans and hence off-specific targeting capability. Using a panel of RAFT-derived polymers grafted to the gold surface, we show that the extent of corona formation is not dependent on the type of polymer. In lectin-binding assays, a glycan (galactose) installed on the chain-end of the polymer was available for binding even after protein corona formation. However, using sialic-acid binding lectins, it was found that there was significant off-target binding due to the large density of sialic acids introduced in the corona, confirmed by western blotting. To demonstrate the importance, we show that the nanoparticles can bind Siglec-2, an immune-relevant lectin post-corona formation. Pre-coating with (non-glycosylated) bovine serum albumin led to a significant reduction in the total glycoprotein corona. However, sufficient sialic acids were still present in the residual corona to lead to off-target binding. These results demonstrate the importance of the glycans when considering the protein corona and how 'retention of the desired function' does not rule out 'installation of undesired function' when considering the performance of glyco-nanomaterials.

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.

Reverse Sequence Polymerization‐Induced Self‐Assembly in Aqueous Media

We report a new aqueous polymerization‐induced self‐assembly (PISA) formulation that enables the hydrophobic block to be prepared first when targeting diblock copolymer nano‐objects. This counter‐intuitive reverse sequence approach uses an ionic reversible addition–fragmentation chain transfer (RAFT) agent for the RAFT aqueous dispersion polymerization of 2‐hydroxypropyl methacrylate (HPMA) to produce charge‐stabilized latex particles. Chain extension using a water‐soluble methacrylic, acrylic or acrylamide comonomer then produces sterically stabilized diblock copolymer nanoparticles in an aqueous one‐pot formulation. In each case, the monomer diffuses into the PHPMA particles, which act as the locus for the polymerization. A remarkable change in morphology occurs as the ≈600 nm latex is converted into much smaller sterically stabilized diblock copolymer nanoparticles, which exhibit thermoresponsive behavior. Such reverse sequence PISA formulations enable the efficient synthesis of new functional diblock copolymer nanoparticles.

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.

Antifreezing Hydroxyl Monolayer of Small Molecules on a Nanogold Surface

The rational design of ice recrystallization inhibition (IRI) materials is challenging due to the poor understanding of the IRI mechanism at the molecular level. Here we report several new findings about IRI. (1) A dense hydroxyl monolayer of small molecules, e.g. 6-aza-2-thiothymine (ATT), adsorbed on a nanogold surface was demonstrated, for the first time, to have IRI activity. Five structural analogues adsorbed on groups nanogold with outward hydroxyl or methyl were created to evidence the origin of IRI activity. (2) Their IRI mechanism is closely related to the density of hydroxyls on a nanogold surface. However, the hydrophobic interaction in our model is not essential for macroscopic IRI activity. (3) A molecular dynamics simulation elucidates the hydroxyl density dependent IRI trajectories underlying the experimental observations, and the radial distribution function reveals that the methyl even slightly hinders the formation of hydrogen bonding due to a hydrophobic interaction. This work sheds more light on the IRI mechanism that should help in the customization of novel IRI materials.

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.

(Bio)degradable and Biocompatible Nano-Objects from Polymerization-Induced and Crystallization-Driven Self-Assembly

Polymerization-induced self-assembly (PISA) and crystallization-driven self-assembly (CDSA) techniques have emerged as powerful approaches to produce a broad range of advanced synthetic nano-objects with high potential in biomedical applications. PISA produces nano-objects of different morphologies (e.g., spheres, vesicles and worms), with high solids content (∼10-50 wt %) and without additional surfactant. CDSA can finely control the self-assembly of block copolymers and readily forms nonspherical crystalline nano-objects and more complex, hierarchical assemblies, with spatial and dimensional control over particle length or surface area, which is typically difficult to achieve by PISA. Considering the importance of these two assembly techniques in the current scientific landscape of block copolymer self-assembly and the craze for their use in the biomedical field, this review will focus on the advances in PISA and CDSA to produce nano-objects suitable for biomedical applications in terms of (bio)degradability and biocompatibility. This review will therefore discuss these two aspects in order to guide the future design of block copolymer nanoparticles for future translation toward clinical applications.

Local detection of pH-induced disaggregation of biocompatible micelles by fluorescence switch ON

Fluorogenic nanoparticles (NPs) able to sense different physiological environments and respond with disaggregation and fluorescence switching OFF/ON are powerful tools in nanomedicine as they can combine diagnostics with therapeutic action. pH-responsive NPs are particularly interesting as they can differentiate cancer tissues from healthy ones, they can drive selective intracellular drug release and they can act as pH biosensors. Controlled polymerization techniques are the basis of such materials as they provide solid routes towards the synthesis of pH-responsive block copolymers that are able to assemble/disassemble following protonation/deprotonation. Ring opening metathesis polymerization (ROMP), in particular, has been recently exploited for the development of experimental nanomedicines owing to the efficient direct polymerization of both natural and synthetic functionalities. Here, we capitalize on these features and provide synthetic routes for the design of pH-responsive fluorogenic micelles via the assembly of ROMP block-copolymers. While detailed photophysical characterization validates the pH response, a proof of concept experiment in a model cancer cell line confirmed the activity of the biocompatible micelles in relevant biological environments, therefore pointing out the potential of this approach in the development of novel nano-theranostic agents.

Poly(l-alanine-co-l-lysine)-g-Trehalose as a Biomimetic Cryoprotectant for Stem Cells

Poly(l-alanine-co-l-lysine)-graft-trehalose (PAK_T) was synthesized as a natural antifreezing glycopolypeptide (AFGP)-mimicking cryoprotectant for cryopreservation of mesenchymal stem cells (MSCs). FTIR and circular dichroism spectra indicated that the content of the α-helical structure of PAK decreased after conjugation with trehalose. Two protocols were investigated in cryopreservation of MSCs to prove the significance of the intracellularly delivered PAK_T. In protocol I, MSCs were cryopreserved at -196 °C for 7 days by a slow-cooling procedure in the presence of both PAK_T and free trehalose. In protocol II, MSCs were preincubated at 37 °C in a PAK_T solution, followed by cryopreservation at -196 °C in the presence of free trehalose for 7 days by the slow-cooling procedure. Polymer and trehalose concentrations were varied by 0.0-1.0 and 0.0-15.0 wt %, respectively. Cell recovery was significantly improved by protocol II with preincubation of the cells in the PAK_T solution. The recovered cells from protocol II exhibited excellent proliferation and maintained multilineage potentials into osteogenic, chondrogenic, and adipogenic differentiation, similar to MSCs recovered from cryopreservation in the traditional 10% dimethyl sulfoxide system. Ice recrystallization inhibition (IRI) activity of the polymers/trehalose contributed to cell recovery; however, intracellularly delivered PEG-PAK_T was the major contributor to the enhanced cell recovery in protocol II. Inhibitor studies suggested that macropinocytosis and caveolin-dependent endocytosis are the main mechanisms for the intracellular delivery of PEG-PAK_T. ¹H NMR and FTIR spectra suggested that the intracellular PEG-PAK_Ts interact with water and stabilize the cells during cryopreservation.

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.

Simultaneous Synthesis and Self-Assembly of Bottlebrush Block Copolymers at Room Temperature via Photoinitiated RAFT Dispersion Polymerization

Bottlebrush polymers exhibiting unique properties have attracted considerable attention for applications in many research areas. Herein, we report the first simultaneous synthesis and self-assembly of bottlebrush block copolymers at room temperature via photoinitiated polymerization-induced self-assembly (photo-PISA) using multifunctional macromolecular chain transfer agents (macro-CTAs). Compare with linear block copolymers, the bottlebrush block copolymers could promote the formation of higher-order morphologies (e.g. vesicles) when targeting similar degrees of polymerization (DPs). Moreover, a higher polymerization rate was observed in the case of bottlebrush block copolymers. Gel permeation chromatography (GPC) analysis showed that good polymerization control was maintained when synthesizing bottlebrush block copolymers by photo-PISA. Finally, the obtained bottlebrush block copolymer vesicles were used as seeds for further chain extension and multicomponent nanoparticles with a sponge internal structure were formed. We expect this study will not only expand polymer architectures employed in PISA, but also provides a new strategy to synthesize polymer nanoparticles with unique structures. This article is protected by copyright. All rights reserved.

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.

Polymerization-induced self-assembly and disassembly during the synthesis of thermoresponsive ABC triblock copolymer nano-objects in aqueous solution

Polymerization-induced self-assembly (PISA) has been widely utilized as a powerful methodology for the preparation of various self-assembled AB diblock copolymer nano-objects in aqueous media. Moreover, it is well-documented that chain extension of AB diblock copolymer vesicles using a range of hydrophobic monomers via seeded RAFT aqueous emulsion polymerization produces framboidal ABC triblock copolymer vesicles with adjustable surface roughness owing to microphase separation between the two enthalpically incompatible hydrophobic blocks located within their membranes. However, the utilization of hydrophilic monomers for the chain extension of linear diblock copolymer vesicles has yet to be thoroughly explored; this omission is addressed for aqueous PISA formulations in the present study. Herein poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate) (G-H) vesicles were used as seeds for the RAFT aqueous dispersion polymerization of oligo(ethylene glycol) methyl ether methacrylate (OEGMA). Interestingly, this led to polymerization-induced disassembly (PIDA), with the initial precursor vesicles being converted into lower-order worms or spheres depending on the target mean degree of polymerization (DP) for the corona-forming POEGMA block. Moreover, construction of a pseudo-phase diagram revealed an unexpected copolymer concentration dependence for this PIDA formulation. Previously, we reported that PHPMA-based diblock copolymer nano-objects only exhibit thermoresponsive behavior over a relatively narrow range of compositions and DPs (see Warren et al., Macromolecules, 2018, 51, 8357–8371). However, introduction of the POEGMA coronal block produced thermoresponsive ABC triblock nano-objects even when the precursor G-H diblock copolymer vesicles proved to be thermally unresponsive. Thus, this new approach is expected to enable the rational design of new nano-objects with tunable composition, copolymer architectures and stimulus-responsive behavior.

Chain extension of linear AB diblock copolymer vesicles by seeded RAFT aqueous dispersion polymerization using a hydrophilic monomer C leads to polymerization-induced disassembly to form lower-order thermoresponsive ABC triblock copolymer nano-objects.

Potent Time-Dependent Ice Recrystallization Inhibition Activity of Cellulose Nanocrystals in Sucrose Solutions

Exploring novel materials with ice recrystallization inhibition (IRI) activity in several fields often starts with a quantitative analysis of ice crystal size change by a splat assay or sandwich assay on a short time scale from 0.5 to 1 h. This study found that this time scale was insufficient to evaluate the IRI activity of cellulose nanocrystals (CNCs) in a model ice cream system-25.0% sucrose solution. No IRI activity was observed in CNCs incubated with ice crystals on a short time scale of 0.5-2.0 h. However, over longer time scales, the growth of ice crystals was entirely inhibited by 1.0% CNCs (between 2 and 24 h) and 0.5% CNCs (between 24 and 72 h) with corresponding final crystal sizes of 25 and 40 μm, respectively. Additionally, ice shaping was observed on a long exposure time, but not on a short exposure time. The findings presented here can be explained by a time-dependent surface coverage of CNCs on ice crystals. The data here indicate the importance of choosing a suitable exposure time for evaluating the IRI activity of new materials and prompt a better understanding of IRI mechanisms involving CNCs.

Computational Reverse Engineering Analysis for Scattering Experiments (CREASE) on Vesicles Assembled from Amphiphilic Macromolecular Solutions

In this paper we present the development and validation of the "Computational Reverse-Engineering Analysis for Scattering Experiments" (CREASE) method for analyzing scattering results from vesicle structures that are commonly found upon assembly of synthetic, biomimetic, or bioderived amphiphilic copolymers in solution. The two-step CREASE method takes the amphiphilic polymer chemistry and small-angle scattering intensity profile, I exp(q), as input and determines the vesicles' structural features on multiple length scales ranging from assembled vesicle wall's individual layer thicknesses to the monomer-level packing and distribution of polymer conformations. In the first step of CREASE, a genetic algorithm (GA) is used to determine the relevant vesicle dimensions from the input macromolecular solution information and I exp(q) by identifying the structure whose computed scattering profile best matches the input I exp(q). Then in the second step, the GA-determined dimensions are used for molecular reconstruction of the vesicle structure. To validate CREASE for vesicles, we test CREASE on input scattering intensity profiles generated mathematically (termed as in silico I exp(q) vs q) from a variety of vesicle sizes with known dimensions. We also test CREASE on in silico I exp(q) vs q generated from vesicles with dispersity in all relevant dimensions, resembling real experiments. After successful validation of CREASE, we compare the CREASE-determined dimensions against those obtained from the traditional approach of fitting the scattering intensity profile to relevant analytical model in SASVIEW package. We show that CREASE performs better than or as well as the core-multishell analytical model's fitting in SASVIEW in determining vesicle dimensions with dispersity. We also show that CREASE provides structural information beyond those possible from traditional scattering analysis using the core-multishell model, such as the distribution of solvophilic monomers between the vesicle wall's inner and outer layers in the vesicle wall and the chain-level packing within each vesicle layer.

Poly(l-Ala-co-l-Lys) Exhibits Excellent Ice Recrystallization Inhibition Activity

The control of ice recrystallization is very important in cryo-technological fields such as the food industry, biopharmaceuticals, and cell storage. Ice recrystallization inhibition (IRI) compounds are therefore designed to limit the growth of ice crystals, decrease the crystal size, and control the crystal shape. To improve the IRI activity of cryo-systems, various synthetic polymers such as biomimetic polypeptides from polar fish, facially amphiphilic polymers, polyampholytes, poly(vinyl alcohol) derivatives, and block copolymers with hydrophilic-hydrophobic balance have been developed. Except for graphene oxide, poly(vinyl alcohol) has thus far exhibited the best performance among these polymers. Herein, poly(l-alanine-co-l-lysine) (PAK) was shown to exhibit a similar IRI activity to that of poly(vinyl alcohol). Moreover, in contrast to the needle-shaped ice crystals generated by the aqueous PVA solution, the PAK solution was shown to generate cubic-to-spherical shaped ice crystals. Furthermore, neither poly(l-alanine-co-l-aspartic acid) (PAD) nor poly(ethylene glycol) (PEG) with a similar molecular weight provided any significant IRI activity. Examination by FTIR and circular dichroism spectroscopies indicated that the PAK forms α-helices, whereas the PAD forms random coils in water. Further, a dynamic ice shaping study suggested that PAK strongly interacts with ice crystals, whereas PAD and PEG only weakly interact. These results suggest that PAK is an important compound with superior IRI activity and that this activity is dependent upon the functional groups and secondary structure of the polypeptides.

Shape-shifting thermoreversible diblock copolymer nano-objects via RAFT aqueous dispersion polymerization of 4-hydroxybutyl acrylate

2-Hydroxypropyl methacrylate (HPMA) is a useful model monomer for understanding aqueous dispersion polymerization. 4-Hydroxybutyl acrylate (HBA) is an isomer of HPMA: it has appreciably higher aqueous solubility so its homopolymer is more weakly hydrophobic. Moreover, PHBA possesses a significantly lower glass transition temperature than PHPMA, which ensures greater chain mobility. The reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization of HBA using a poly(ethylene glycol) (PEG113) precursor at 30 °C produces PEG113-PHBA200-700 diblock copolymer nano-objects. Using glutaraldehyde to crosslink the PHBA chains allows TEM studies, which reveal the formation of spheres, worms or vesicles under appropriate conditions. Interestingly, the partially hydrated highly mobile PHBA block enabled linear PEG113-PHBA x spheres, worms or vesicles to be reconstituted from freeze-dried powders on addition of water at 20 °C. Moreover, variable temperature 1H NMR studies indicated that the apparent degree of hydration of the PHBA block increases from 5% to 80% on heating from 0 °C to 60 °C indicating uniform plasticization. In contrast, the PHPMA x chains within PEG113-PHPMA x nano-objects become dehydrated on raising the temperature: this qualitative difference is highly counter-intuitive given that PHBA and PHPMA are isomers. The greater (partial) hydration of the PHBA block at higher temperature drives the morphological evolution of PEG113-PHBA260 spheres to form worms or vesicles, as judged by oscillatory rheology, dynamic light scattering, small-angle X-ray scattering and TEM studies. Finally, a variable temperature phase diagram is constructed for 15% w/w aqueous dispersions of eight PEG113-PHBA200-700 diblock copolymers. Notably, PEG113-PHBA350 can switch reversibly from spheres to worms to vesicles to lamellae during a thermal cycle.

Protein-, (Poly)peptide-, and Amino Acid-Based Nanostructures Prepared via Polymerization-Induced Self-Assembly

Proteins and peptides, built from precisely defined amino acid sequences, are an important class of biomolecules that play a vital role in most biological functions. Preparation of nanostructures through functionalization of natural, hydrophilic proteins/peptides with synthetic polymers or upon self-assembly of all-synthetic amphiphilic copolypept(o)ides and amino acid-containing polymers enables access to novel protein-mimicking biomaterials with superior physicochemical properties and immense biorelevant scope. In recent years, polymerization-induced self-assembly (PISA) has been established as an efficient and versatile alternative method to existing self-assembly procedures for the reproducible development of block copolymer nano-objects in situ at high concentrations and, thus, provides an ideal platform for engineering protein-inspired nanomaterials. In this review article, the different strategies employed for direct construction of protein-, (poly)peptide-, and amino acid-based nanostructures via PISA are described with particular focus on the characteristics of the developed block copolymer assemblies, as well as their utilization in various pharmaceutical and biomedical applications.

Size and Shape Control of Ice Crystals by Amphiphilic Block Copolymers and Their Implication in the Cryoprotection of Mesenchymal Stem Cells

Precise control over the size and shape of ice crystals is a key factor to consider in designing antifreezing and cryoprotecting molecules for cryopreservation of cells. Here, we report that a poly(ethylene glycol)-poly(l-alanine) (PEG-PA) block copolymer exhibits excellent cryoprotecting properties for stem cells and antifreezing properties for water. As the molecular weight of PA increased from 500, 760, and 1750 Da (P1, P2, and P3) at the same PEG molecular weight of 5000 Da, the β-sheet content decreased and α-helix content increased. Comparing P2 (PEG-PA; 5000-760) and P4 (PEG-PA: 1000-750), β-sheets increased as the PEG block length decreased. The critical micelle concentration of the PEG-PA block copolymers was in a range of 0.5-3.0 mg/mL and was proportional to the hydrophobicity of the PEG-PA block copolymers. The P1, P2, and P3 self-assembled into spherical micelles, whereas P4 formed micelles with cylindrical morphology. The difference in the block copolymer structure affected ice recrystallization inhibition (IRI) activity and cryopreservation of cells. IRI activity was assayed via mean largest grain size (MLGS), and interactions between polymers and ice crystal surfaces were studied by dynamic ice-shaping studies. The MLGS decreased to 58 → 53 → 45 → 35 → 23% of that of PBS, as the polymer (PEG-PA 5000-500) concentration increased from 0.0 (PBS; control) → 1.0 → 5.0 → 10 → 30 → 50 mg/mL. The MLGS of PEG 5k solutions (negative control) decreased to 74 → 71 → 64 → 44 → 37% of that of PBS in the same concentration range. P3 and P4 with a longer hydrophobic PA block developed elongated ice crystals at above 30 mg/mL. The dynamic ice-shaping study exhibited that ice crystals became needle-shaped, as the hydrophobicity of the polymer increased as in P2-P4. The cell recovery in the P1 system after cryopreservation at -196 °C for 7 days was 87% of that of the dimethyl sulfoxide (DMSO) 10% system (positive control). The cell recovery was 48% for the P2 system and drastically decreased to less than 30% of that of the DMSO 10% system in the P3, P4, PEG 5k, PEG 1k, PVA 80H, and PVA 100H systems. Current studies suggest that IRI activity, round ice crystal shaping, and membrane stabilization activity of P1 cooperatively provide excellent cell recovery among the candidate systems. Recovered stem cells exhibited excellent proliferation and multilineage differentiation into osteocytes, chondrocytes, and adipocytes. To conclude, the PEG-PA (5000-500) block copolymer is suggested to be a promising antifreezing cryoprotectant for stem cells.