Ureido-Derivatized Polymers Based on Both Poly(allylurea) and Poly(l-citrulline) Exhibit UCST-Type Phase Transition Behavior under Physiologically Relevant Conditions

Controllable transformation of UCST and LCST behaviors in polyampholyte hydrogels enabled by an association-disassociation theory-based switch mechanism

The potential temperature-sensitive characteristics of polyampholyte hydrogels have not been explored yet, despite their excellent mechanical properties and universality as supramolecular materials. Here, polyampholyte hydrogels were prepared with anionic and cationic monomers at high concentrations and their thermosensitive behaviors were investigated systematically. The results of this study break through the traditional understanding that hydrogels prepared from zwitterionic copolymers could only exhibit UCST characteristics. Moreover, the "association-disassociation" theory was presented to explain the abnormal phenomenon, which could endow a controllable switch for transforming UCST and LCST in polyampholyte hydrogels; the thermosensitive properties of the polyampholyte hydrogels arise from the competition of "association force" and "disassociation force", based on which the polyampholyte hydrogels could be endowed opposite thermosensitive properties by regulating the monomer concentration and monomer ratio. Accordingly, essential conditions required to form physically crosslinked UCST hydrogels could be concluded: satisfactory solubility of monomers; high-enough monomer concentration; appropriate hydrophilicity of ion pairs and suitable monomer ratio.

Zwitterionic nanoparticles for thermally activated drug delivery in hyperthermia cancer treatment

Hyperthermia is considered a promising strategy to boost the curative outcome of traditional chemotherapeutic treatments. However, this thermally mediated drug delivery is still affected by important limitations. First, the poor accumulation of the conventional anticancer formulations in the target site limits the bioavailability of the active ingredient and induces off-site effects. In addition, some tumoral scenarios, such as ovarian carcinoma, are characterized by cell thermotolerance, which induces tumoral cells to activate self-protecting mechanisms against high temperatures. To overcome these constraints, we developed thermoresponsive nanoparticles (NPs) with an upper critical solution temperature (UCST) to intracellularly deliver a therapeutic payload and release it on demand through hyperthermia stimulation. These NPs were synthesized via reversible addition-fragmentation chain transfer (RAFT) emulsion polymerization and combine polyzwitterionic stabilizing segments and an oligoester-based biodegradable core. By leveraging the pseudo-living nature of RAFT polymerization, important physicochemical properties of the NPs were controlled and optimized, including their cloud point (Tcp) and size. We have tuned the Tcp of NPs to match the therapeutic needs of hyperthermia treatments at 43 °C and tested the nanocarriers in the controlled delivery of paclitaxel, a common anticancer drug. The NPs released almost entirely the encapsulated drug only following 1 h incubation at 43 °C, whereas they retained more than 95% of the payload in the physiological environment (37 °C), thus demonstrating their efficacy as on-demand drug delivery systems. The administration of drug-loaded NPs to ovarian cancer cells led to therapeutic effects outperforming the conventional administration of non-encapsulated paclitaxel, which highlights the potential of the zwitterionic UCST-type NPs as an innovative hyperthermia-responsive drug delivery system.

UCST-Type Thermoresponsive Sol-Gel Transition Triblock Copolymer Containing Zwitterionic Polymer Blocks

Thermoresponsive sol-gel transition polymers are of significant interest because of their fascinating biomedical applications, including as drug reservoirs for drug delivery systems and scaffolds for tissue engineering. Although extensive research has been conducted on lower critical solution temperature (LCST)-type sol-gel transition polymers, there have been few reports on upper critical solution temperature (UCST)-type sol-gel transition polymers. In this study, we designed an ABA-type triblock copolymer composed of a poly(ethylene glycol) (PEG) block and zwitterionic polymer blocks that exhibit UCST-type thermoresponsive phase transitions. A sulfobetaine (SB) monomer with both ammonium and sulfonate (-SO3) groups in its side chain or a sulfabetaine (SaB) monomer with both ammonium and sulfate (-OSO3) groups in its side chain was polymerized from both ends of the PEG block via reversible addition-fragmentation chain-transfer (RAFT) polymerization to obtain PSB-PEG-PSB and PSaB-PEG-PSaB triblock copolymers, respectively. Although an aqueous solution containing the PSB-PEG-PSB triblock copolymer showed an increase in viscosity upon cooling, it did not undergo a sol-to-gel transition. In contrast, a sol-to-gel transition was observed when a phosphate-buffered saline containing PSaB-PEG-PSaB was cooled from 80 °C to 25 °C. The PSaB blocks with -OSO3 groups exhibited a stronger dipole-dipole interaction than conventional SB with -SO3 groups, leading to intermolecular association and the formation of a gel network composed of PSaB assemblies bridged with PEG. The fascinating UCST-type thermoresponsive sol-gel transition properties of the PSaB-PEG-PSaB triblock copolymer suggest that it can provide a useful platform for designing smart biomaterials, such as drug delivery reservoirs and cell culture scaffolds.

Spontaneous Liquid Droplet-to-Gel Transition of Citrulline Polypeptide Complexed with Nucleic Acids

Inside cells, proteins complex with nucleic acids to form liquid droplets resulting from liquid-liquid phase separation. The presence of mutated proteins can change the state of these liquid droplets to solids or gels, triggering neurodegenerative diseases. The mechanism of the liquid to solid or gel transition is still unclear. Solutions of poly(l-ornithine-co-l-citrulline) (PLOC) copolymers, which exhibit upper critical solution temperature-type behavior, change state upon cooling. In this study, we evaluated the effect of nucleic acids complexed with PLOC on phase changes. In the presence of nucleic acids, such as polyC and polyU, PLOC formed liquid droplets at low temperatures. The droplets dissolved at temperatures above the phase separation temperature. The phase separation temperature depended on the chemical structure of the nucleobase, implying that electrostatic and hydrogen bonding interactions between the nucleic acid and PLOC influenced phase separation. Furthermore, the liquid droplets spontaneously changed to gel-like precipitates due to spontaneous release of nucleic acids from the complex. The rate of the liquid droplet-to-gel transition depended on the magnitude of electrostatic and hydrogen bonding interactions between PLOC and nucleic acid. PLOC complexed with mRNA also underwent a liquid droplet-to-gel transition upon the release of mRNA. This work provides insights into the mechanism of pathogenic transitions of the cellular droplets.

A Unified Description of Salt Effects on the Liquid-Liquid Phase Separation of Proteins

Protein aggregation via liquid-liquid phase separation (LLPS) is ubiquitous in nature and is intimately connected to many human diseases. Although it is widely known that the addition of salt has crucial impacts on the LLPS of proteins, full understanding of the salt effects remains an outstanding challenge. Here, we develop a molecular theory that systematically incorporates the self-consistent field theory for charged macromolecules into the solution thermodynamics. The electrostatic interaction, hydrophobicity, ion solvation, and translational entropy are included in a unified framework. Our theory fully captures the long-standing puzzles of the nonmonotonic salt concentration dependence and the specific ion effect. We find that proteins show salting-out at low salt concentrations due to ionic screening. The solubility follows the inverse Hofmeister series. In the high salt concentration regime, protein continues salting-out for small ions but turns to salting-in for larger ions, accompanied by the reversal of the Hofmeister series. We reveal that the solubility at high salt concentrations is determined by the competition between the solvation energy and translational entropy of the ion. Furthermore, we derive an analytical criterion for determining the boundary between the salting-in and salting-out regimes, which is in good agreement with experimental results for various proteins and salt ions.

A Review on Thermal Properties of Hydrogels for Electronic Devices Applications

Hydrogels, as a series of three-dimensional, crosslinked, hydrophilic network polymers, exhibit extraordinary properties in softness, mechanical robustness and biocompatibility, which have been extensively utilized in various fields, especially for electronic devices. However, since hydrogels contain plenty of water, the mechanical and electrochemical properties are susceptible to temperature. The thermal characteristics of hydrogels can significantly affect the performance of flexible electronic devices. In this review, recent research on the thermal characteristics of hydrogels and their applications in electronic devices is summarized. The focus of future work is also proposed. The thermal stability, thermoresponsiveness and thermal conductivity of hydrogels are discussed in detail. Anti-freezing and anti-drying properties are the critical points for the thermal stability of hydrogels. Methods such as introducing soluble ions and organic solvents into hydrogels, forming ionogels, modifying polymer chains and incorporating nanomaterials can improve the thermal stability of hydrogels under extreme environments. In addition, the critical solution temperature is crucial for thermoresponsive hydrogels. The thermoresponsive capacity of hydrogels is usually affected by the composition, concentration, crosslinking degree and hydrophilic/hydrophobic characteristics of copolymers. In addition, the thermal conductivity of hydrogels plays a vital role in the electronics applications. Adding nanocomposites into hydrogels is an effective way to enhance the thermal conductivity of hydrogels.

Nanocarriers for drug-delivery systems using a ureido-derivatized polymer gatekeeper for temperature-controlled spatiotemporal on-off drug release

Accidental chemotherapy extravasation exacerbates the side effects of anticancer drugs. Therefore, drug-delivery nanocarriers should be designed to avoid persistent drug release at off-target sites and promote burst drug release at on-target. Considering these requirements, poly(allylamine)-co-poly(allylurea) (PAU), a ureido-derivatized temperature responsive polymer with upper critical solution temperature (UCSTs), is an ideal material. This report describes the fabrication, characterization, and in vitro cellular toxicity of PAU polymer-grafted magnetic mesoporous silica nanoparticles as drug-delivery nanocarriers. A UCST of 43 °C and an ultranarrow transition temperature range of 39-43 °C was realized, ensuring that the nanocarriers suppressed undesirable leakage to below 10 % of the drug loading for 8 h in the absence of a thermal stimulus. A drug release burst of up to 75 % of the drug loading was achieved within 30 min after the stimulus, reducing the viability of the in vitro cancer cells to 12 %. Therefore, the ureido-derivatized polymer is one of the most suitable gatekeepers for drug-delivery nanocarriers.

One-step immobilization of β-glucosidase in crude enzyme solution by recyclable UCST-responsive polymer with enhanced uniformly biocatalytic performance

In this study, the upper critical solution temperature (UCST)-responsive polymers poly (ethylene oxide) monomethyl ether-block-poly(acrylamide-co-acrylonitrile) (PEG-b-p(AAM-co-AN) were synthesized and successfully utilized to immobilize β-glucosidase in crude enzyme solution. These UCST-responsive β-glucosidase biocatalysts (PEG-b-p(AAM-co-AN@LytA-Glu) have specific UCST with tunable transition temperature, which could be tuned the separation temperature to the desired temperature range. The P2 @ LytA-Glu with an UCST of about 42.9 ℃ was exploited by one-step covalent immobilization of β-glucosidase in crude enzyme solution. The prepared P2 @ LytA-Glu exhibited significantly improved temperature, pH, storage, and operation stabilities compared with that of free enzyme. The catalytic rate of P2 @ Glu-LytA was 14.5% higher than that of P2-Glu (immobilized pure β-glucosidase), which indicated that one-step immobilization of crude enzyme directly from crude enzyme solution was feasible, and it can greatly save the purification step and reduce the experimental cost. The engineered UCST-responsive immobilized enzymes are potentially useful for the practical green biocatalysis.

The UCST phase transition of a dextran based copolymer in aqueous media with tunable thermoresponsive behavior

A hydrogen bonded UCST polymer has been developed by grafting of methacrylamide and acrylic acid on dextran via free radical polymerization.

Poly(N-cyanoethylacrylamide), a new thermoresponsive homopolymer presenting both LCST and UCST behavior in water

We have recently demonstrated that poly(N-cyanomethylacrylamide) (PCMAm) synthesized by reversible addition-fragmentation chain transfer (RAFT) radical polymerization exhibits a typical upper critical solution temperature (UCST)-type transition in water with a very...

Nonionic star polymers with upper critical solution temperature in aqueous solutions

Novel UCST star poly(2-ureido methacrylates) synthesized via the ARGET ATRP technique showed enhanced trapping abilities of model drug molecules.

Functional polydiynes prepared by metathesis cyclopolymerization of 1,7-dihalogen-1,6-heptadiyne derivatives

The MCP route used for the polymerization of 1,6-heptadiynes was successfully applied to the polymerization of 1,7-dihalogen-1,6-heptadiynes, and the target polymers were obtained in high yield with high molecular weight and unique UCST behavior.

Improving the flame retardancy and accelerating the degradation of poly (lactic acid) in soil by introducing fully bio-based additives

In this study, a novel bio-based flame retardant LC-PA is prepared by the Mannich reaction between phytic acid (PA) and L-citrulline (LC). LC-PA is combined with tannic acid (TA) and introduced into PLA to improve fire performance and accelerate biodegradability. Compared with control PLA, the PLA composite containing 10% LC-PA/TA increases the LOI value to 26.9%, reaches a V-0 rating in the UL-94 test, and reduces the peak heat release rate and total heat release by 24.5% and 21.1%, respectively. More importantly, the introduction of LC-PA/TA accelerates the degradation rate of PLA in soil, which is of significance for biodegradable materials. The addition of LC-PA/TA can attract water and provide a suitable energy source for microbial proliferation, accelerating the hydrolysis and microbial degradation of PLA. This work provides a practical approach for high flame retardancy and rapid biodegradability in the soil to the bio-based polymer.

Nonionic Polymer with Flat Upper Critical Solution Temperature Behavior in Water

We rationally designed a monomer that when polymerized formed a well-defined nonionic polymer [poly(2-(methacryloyloxy) ethylureido glycinamide), PMEGA] by reversible addition fragmentation chain transfer with a flat and tunable upper critical solution temperature (UCST) in water. The monomer was made in one pot from commercially available compounds and with ease of purification. Strong hydrogen-bonding side groups on the polymer produced sharp coil-to-globule transitions upon cooling below its UCST. Ideal random copolymers produced with butyl methacrylate also showed flat UCST profiles, in which the UCST increased with a greater butyl methacrylate copolymer composition from 7 to 65 °C. In the presence of NaCl, the UCST decreased linearly with NaCl concentration due to the "salting-in" effect, and it was found that the slopes from the linear decrease of UCST were nearly identical for all copolymer compositions. This new polymer and its copolymers support the hypothesis that strong hydrogen bonding between the side groups allowed the flat UCST to be readily tuned with a high level of predictability. We postulate that this polymer system may provide wide biological applicability similar to that found for the well-used flat lower critical solution temperature (LCST) of poly(N-isopropylacrylamide).

Preparation of a thermo-responsive drug carrier consisting of a biocompatible triblock copolymer and fullerene

A triblock copolymer (PEG-b-PUEM-b-PMPC; EUM) comprising poly(ethylene glycol) (PEG), thermo-responsive poly(2-ureidoethyl methacrylate) (PUEM), and poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC) blocks was synthesized via controlled radical polymerization. PEG and PMPC blocks exhibit hydrophilicity and biocompatibility. The PUEM block exhibits an upper critical solution temperature (UCST). PMPC can dissolve hydrophobic fullerenes in water to form a complex by grinding PMPC and fullerene powders. Fullerene-C₇₀ (C₇₀) and EUM were ground in a mortar and phosphate-buffered saline (PBS) was added to synthesize a water-soluble complex (C₇₀/EUM). C₇₀/EUM has a core-shell-corona structure, whose core is a complex of C₇₀ and PMPC, the shell is PUEM, and corona is PEG. The maximum C₇₀ concentration dissolved in PBS was 0.313 g L^-1 at an EUM concentration of 2 g L^-1. The C₇₀/EUM hydrodynamic radius (R_h) was 34 nm in PBS at 10 °C, which increased due to the PUEM block's UCST phase transition with increasing temperature, and R_h attained a constant value of 38 nm above 36 °C. An anticancer drug, doxorubicin, was encapsulated in the PUEM shell by hydrophobic interactions in C₇₀/EUM at room temperature, which can be released by heating. The generation of singlet oxygen (¹O₂) from C₇₀/EUM upon visible-light irradiation was confirmed using the singlet oxygen sensor green indicator. Water-soluble C₇₀/EUM may be used as a carrier that releases encapsulated drugs when heated and as a photosensitizer for photodynamic therapy.

Chitosan-Based Biocompatible Copolymers for Thermoresponsive Drug Delivery Systems: On the Development of a Standardization System

Chitosan is a natural polysaccharide that is considered to be biocompatible, biodegradable and non-toxic. The polymer has been used in drug delivery applications for its positive charge, which allows for adhesion with and recognition of biological tissues via non-covalent interactions. In recent times, chitosan has been used for the preparation of graft copolymers with thermoresponsive polymers such as poly-N-vinylcaprolactam (PNVCL) and poly-N-isopropylamide (PNIPAM), allowing the combination of the biodegradability of the natural polymer with the ability to respond to changes in temperature. Due to the growing interest in the utilization of thermoresponsive polymers in the biological context, it is necessary to increase the knowledge of the key principles of thermoresponsivity in order to obtain comparable results between different studies or applications. In the present review, we provide an overview of the basic principles of thermoresponsivity, as well as a description of the main polysaccharides and thermoresponsive materials, with a special focus on chitosan and poly-N-Vinyl caprolactam (PNVCL) and their biomedical applications.

Thermoresponsive Polymers Based on Tertiary Amine Moieties

Thermoresponsive polymers exhibiting unique reversible phase transition properties in aqueous solution in response to temperature stimuli have been extensively investigated. In the past two decades, thermoresponsive polymers based on tertiary amine moieties have achieved considerable progress and become an important family of thermoresponsive polymers, including tertiary amine functionalized poly((meth)acrylamide)s, poly((meth)acrylate)s, poly(styrene)s, poly(vinyl alcohol)s, and poly(ethylene oxide)s, which exhibit lower critical solution temperature and/or upper critical solution temperature in water or aliphatic alcohols. Their phase transition behavior can be modulated by the solution pH and CO₂ due to the protonation of tertiary amine moieties in acidic condition and deprotonation in alkaline condition and the charged ammonium bicarbonate formed by the tertiary amine moieties and CO₂ . The aim of this review is to summarize the recent progress in the thermoresponsive polymers based on tertiary amine moieties.

Upper Critical Solution Temperature-Type Responsive Cyclodextrins with Characteristic Inclusion Abilities

Water-soluble and thermoresponsive macrocycles with stable inclusion toward guests are highly valuable to construct stimuli-responsive supramolecular materials for versatile applications. Here, we develop such macrocycles - ureido-substituted cyclodextrins (CDs) which exhibit unprecedented upper critical solution temperature (UCST) behavior in aqueous media. These novel CD derivatives showed good solubility in water at elevated temperature, but collapsed from water to form large coacervates upon cooling to low temperature. Their cloud points are greatly dependent on concentration and can be mediated through oxidation and chelation with silver ions. Significantly, the amphiphilicity of these CD derivatives is supportive to host-guest binding, which affords them inclusion abilities to guest dyes. The inclusion complexation remained nearly intact during thermally induced phase transitions, which is in contrast to the switchable inclusion behavior of lower critical solution temperature (LCST)-type CDs. Moreover, ureido-substituted CDs were exploited to co-encapsulate a pair of guest dyes whose fluorescence resonance energy transfer process can be switched by the UCST phase transition. We therefore believe these novel thermoresponsive CDs may form a new strategy for developing smart macrocycles and allow for exploring smart supramolecular materials.

Simulations of the Upper Critical Solution Temperature Behavior of Poly(ornithine-co-citrulline)s Using MARTINI-Based Coarse-Grained Force Fields

Poly(ornithine-co-citrulline)s are ureido-based polymers, which were shown to exhibit tunable upper critical solution temperature (UCST) behavior, a property that can be exploited to develop thermoresponsive nanoparticles for controlled drug delivery systems. To gain insight into the driving forces that govern the formation and dissolution processes of poly(ornithine-co-citrulline) nanoparticles, a molecular dynamics (MD) simulation study has been carried out using MARTINI-based protein coarse-grained models. Multi-microsecond simulations at temperatures ranging from 280 to 370 K show that the fully reparametrized version 3.0 of MARTINI force field is able to capture the dependence on temperature of poly(ornithine-co-citrulline) aggregation and dissolution, while version 2.2 could not account for it. Furthermore, the phase separation observed in these simulations allowed us to extrapolate a phase diagram based on the Flory-Huggins theory of polymer solution, which could help in future rational design of drug delivery nanoparticles based on poly(amino acid)s.

Stimulus-Responsive Polymers Based on Polypeptoid Skeletons

Polypeptoids have attracted a lot of atteSDntion because of their unique structural characteristics and special properties. Polypeptoids have the same main chain structures to polypeptides, making them have low cytotoxicity and excellent biocompatibility. Polypeptoids can also respond to external environmental changes by modifying the configurations of the side chains. The external stimuli can be heat, pH, ions, ultraviolet/visible light and active oxygen or their combinations. This review paper discussed the recent research progress in the field of stimulus-responsive polypeptoids, including the design of new stimulus-responsive polypeptoid structures, controlled actuation factors in response to external stimuli and the application of responsive polypeptoid biomaterials in various biomedical and biological nanotechnology, such as drug delivery, tissue engineering and biosensing.

Synthesis of Temperature-Responsive Polymers Containing Piperidine Carboxamide and N,N-diethylcarbamoly Piperidine Moiety via RAFT Polymerization

In this study, poly(N-acryloyl-nipecotamide) (PNANAm), poly(N-acryloyl-isonipecotamide) (PNAiNAm), and poly(N-acryloyl-N,N-diethylnipecotamide) (PNADNAm) are synthesized as novel temperature-responsive polymers using reversible addition-fragmentation chain-transfer polymerization. Aqueous solutions of these three polymers are examined via temperature-dependent optical transmittance measurements. The PNANAm sample with a hydrophilic terminal group shows an upper critical solution temperature (UCST) in phosphate-buffered saline (PBS) when its molecular weight (M_n ) is 7600 or higher, whereas PNANAm (M_n < 7600) is soluble. The UCST is influenced by molecular weight and the polymer concentration. In contrast, PNANAm sample with nonionic terminal group shows UCST, when M_n is below 7600, suggesting that the terminal nonionic group possibly increases UCST of PNANAm. The urea addition experiment suggests that the driving force for expression of UCST of PNANAm is the formation of inter-and intramolecular hydrogen bonds among the polymer chains. PNAiNAm is soluble in PBS but exhibits an UCST in an appropriate concentration of ammonium sulfate. In contrast, PNADNAm exhibits a lower critical solution temperature. Comparing the chemical structure of these polymers and their phase transition behaviors suggests that the carboxamide group position in the piperidine ring could determine the UCST expression. These results could help design temperature-responsive polymers with a desired the cloud point temperature.

An Inverse Thermogelling Bioink Based on an ABA-Type Poly(2-oxazoline) Amphiphile

Hydrogels are key components in several biomedical research areas such as drug delivery, tissue engineering, and biofabrication. Here, a novel ABA-type triblock copolymer comprising poly(2-methyl-2-oxazoline) as the hydrophilic A blocks and poly(2-phenethyl-2-oxazoline) as the aromatic and hydrophobic B block is introduced. Above the critical micelle concentration, the polymer self-assembles into small spherical polymer micelles with a hydrodynamic radius of approx 8-8.5 nm. Interestingly, this specific combination of hydrophilic and hydrophobic aromatic moieties leads to rapid thermoresponsive inverse gelation at polymer concentrations above a critical gelation concentration (20 wt %) into a macroporous hydrogel of densely packed micelles. This hydrogel exhibited pronounced viscoelastic solid-like properties, as well as extensive shear-thinning, rapid structure recovery, and good strain resistance properties. Excellent 3D-printability of the hydrogel at lower temperature opens a wide range of different applications, for example, in the field of biofabrication. In preliminary bioprinting experiments using NIH 3T3 cells, excellent cell viabilities of more than 95% were achieved. The particularly interesting feature of this novel material is that it can be used as a printing support in hybrid bioink systems and sacrificial bioink due to rapid dissolution at physiological conditions.

Ureido Functionalization through Amine-Urea Transamidation under Mild Reaction Conditions

Ureido-functionalized compounds play an indispensable role in important biochemical processes, as well as chemical synthesis and production. Isocyanates, and KOCN in particular, are the preferred reagents for the ureido functionalization of amine-bearing compounds. In this study, we evaluate the potential of urea as a reagent to graft ureido groups onto amines at relatively low temperatures (<100 °C) in aqueous media. Urea is an inexpensive, non-toxic and biocompatible potential alternative to KOCN for ureido functionalization. From as early as 1864, urea was the go-to reagent for polyurea polycondensation, before falling into disuse after the advent of isocyanate chemistry. We systematically re-investigate the advantages and disadvantages of urea for amine transamidation. High ureido-functionalization conversion was obtained for a wide range of substrates, including primary and secondary amines and amino acids. Reaction times are nearly independent of substrate and pH, but excess urea is required for practically feasible reaction rates. Near full conversion of amines into ureido can be achieved within 10 h at 90 °C and within 24 h at 80 °C, and much slower reaction rates were determined at lower temperatures. The importance of the urea/amine ratio and the temperature dependence of the reaction rates indicate that urea decomposition into an isocyanic acid or a carbamate intermediate is the rate-limiting step. The presence of water leads to a modest increase in reaction rates, but the full conversion of amino groups into ureido groups is also possible in the absence of water in neat alcohol, consistent with a reaction mechanism mediated by an isocyanic acid intermediate (where the water assists in the proton transfer). Hence, the reaction with urea avoids the use of toxic isocyanate reagents by in situ generation of the reactive isocyanate intermediate, but the requirement to separate the excess urea from the reaction product remains a major disadvantage.

Design of an LCST-UCST-Like Thermoresponsive Zwitterionic Copolymer

Thermoresponsive polymers that possess both UCST- and LCST-like behaviors have generally been designed using diblock copolymers that are mostly composed of an LCST-like polymer and a UCST polymer. Herein, we prepared an LCST-UCST-type polymer composed of UCST-like thermoresponsive zwitterionic sulfabetaine methacrylate and nonthermoresponsive PEG methacrylate, ZB-PEG, by reversible addition-fragmentation chain transfer (RAFT) copolymerization. By adjusting the PEG composition, ZB-PEG formed a mesoglobule, a microglobule, and the dissociated states in phosphate-buffered saline (PBS). These states were found to be reversible via temperature control. Moreover, this behavior showed high reversibility and succeeded in stabilizing horseradish peroxidase (HRP) in the dilute condition. Such thermoresponsive ZB-PEG can be applied over a wide range of applications in biotechnology and other fields.

Light-Regulated Liquid-Liquid Phase Separation for Spatiotemporal Protein Recruitment and Cell Aggregation

We have previously shown that the upper critical solution temperature-type thermoresponsive ureido polymers such as polyallylurea and poly(2-ureidoethylmethacrylate) derivatives show liquid-liquid phase separation (LLPS), also known as simple coacervation, under physiological conditions below their phase-separation temperatures (T_p). The addition of the polymer-rich coacervate droplets that result from LLPS to a monolayer cell culture induced aggregation of cells into multicellular spheroids. In this study, we prepared a ureido copolymer, poly(vinylamine-co-vinylurea), with azobenzene substituents (Azo-PVU) and demonstrated light-guided assembly and disassembly of LLPS coacervates. Azo-PVUs with T_p values ranging from 10 to 52 °C were prepared by changing the azobenzene content. Ultraviolet light caused a decrease in the T_p of Azo-PVU because of trans-to-cis photoisomerization of the azobenzene and irradiation with visible light increased the T_p. Thus, LLPS of Azo-PVU was reversibly controlled. The coacervate droplets deposited on a dish surface were immediately dissolved by targeted UV irradiation (owing to a decrease in the T_p). Spatially controlled recruitment of proteins on the dish surface was achieved when protein solution was added to the light-patterned surface. Furthermore, the light-guided deposition of coacervates resulted in the spatiotemporal transformation of monolayer cells to aggregates. This light-controlled LLPS will allow the preparation of novel liquid-based materials for biomolecular and cellular engineering.

Evaluation of Cooling-Induced Liquid-Liquid Phase Separation of Ureido Polymers as a Cold-Shock Stress Granules Model

Many intracellular reactions occur in membrane-less organelles that form due to liquid-liquid phase separation (LLPS). Cold-shock stress granules, which are membrane-less organelles, are formed in response to a significant decrease in temperature and recruit biomolecules for regulation of their activities. The authors have reported that synthetic ureido copolymers exhibit cooling-induced LLPS under physiologically relevant conditions. In this study, influences of the cooling-induced LLPS of ureido polymers on enzymatic activity is investigated to evaluate whether the ureido polymers can mimic cold-shock stress granules. The enzyme β-galactosidase (β-Gal) is efficiently entrapped into phase-separated coacervates of ureido polymers upon cooling. The activity of β-Gal is significantly suppressed by the entrapment. The enzymatic activity is recovered after heating, which dissolves the coacervate. Thus, the LLPS formed by ureido polymers are a suitable model for cold-shock stress granules.

Anion-induced thermoresponsiveness in cationic polycysteine and DNA binding

This study describes the synthesis of an l-cysteine-based water-soluble cationic polypeptide, an investigation of its thermoresponsive behaviour in the presence of added anions and its polyplexation with DNA.

Aggregation of a double hydrophilic block glycopolymer: the effect of block polymer ratio

Double hydrophilic block glycopolymers (DHBGs) composed of glycopolymers and polyethylene glycol (PEG) aggregate in aqueous solution. However, there are no guidelines to direct and design DHBG aggregation. Herein, we investigated the effect of the ratio of glycopolymer length to PEG length on the structure, and report that structure size could be influenced by the block polymer ratio. Nine kinds of DHBG with different glycopolymers and PEG lengths were synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization. The aggregation capability of DHBG was investigated by transmission electron microscopy (TEM) and dynamic light scattering (DLS). In all cases, the DHBGs formed the spherical structures, even when the PEG and glycopolymer lengths were quite different. The size of the structure was controlled by the ratio of the PEG length to the glycopolymer length. The aggregation of the DHBGs was induced by hydrogen bonding between the sugar moieties. The aggregation of the DHBG was affected by temperature and concentration.

Tunable LCST/UCST-Type Polypeptoids and Their Structure-Property Relationship

Bioinspired thermoresponsive polymeric materials with tunable phase-transition behaviors are highly desirable for biomedical applications. Here, we reported a facile approach for the synthesis of both lower critical solution temperature (LCST) and upper critical solution temperature (UCST) types of thermoresponsive polypeptoids with tunable phase-transition temperature in the range of 29--55 °C. The introduction of alkyl groups and ethylene glycol (EG) units results in a controlled phase-transition behavior under fairly mild conditions. A very sharp transition (ΔT ≤ 1.5 °C) is observed by simply adjusting pH and the alkyl chain length. In particular, the carboxyl-containing polypeptoids display designable UCST behavior, which can be finely tuned in both water and methanol. All these features make the obtained polymers beneficial for practical applications. More interestingly, we demonstrate that the hydrophilic EG group behaves as an excellent regulator to tune the UCST behavior, while the hydrophobic alkyl residues show remarkable capability to regulate the LCST behavior of the system. We hope that such systematic structure-property studies will enable the design of smart polymer materials to meet the specific needs of future applications.

Control of Mitochondrial Localization Using Thermoresponsive Sulfobetaine Polymer

Fast intracellular migration and controlled localization of molecules represent significant challenges for future applications of drug discovery and related fields. In this study, thermoresponsive sulfobetaine polymers with pyridinium cations are evaluated as biocompatible and mitochondria-localizing agents. Among the polymers, poly(3-(4-(2-methacrylamido)ethyl pyridinio-1-yl)propane-1-sulfonate), P(E-PySMAAm)14k (M_n  = 14 000 g mol^-1 ) exhibit thermoresponsiveness with an upper critical solution temperature like behavior in cell culture medium containing serum with minimal cytotoxicity. Upon the addition of P(E-PySMAAm)14k to HeLa cells at temperatures above the clearing point at 37 °C, effective localization is observed in mitochondria. However, increased intensity but nonspecific localization is observed below the clearing point at 4 °C. Doxorubicin is conjugated to the P(E-PySMAAm) and achieves effective mitochondrial delivery while maintaining drug efficacy. Such sulfobetaine polymers represent promising tools for intracellular delivery of molecules.

On Going to a New Era of Microgel Exhibiting Volume Phase Transition

The discovery of phenomena of volume phase transition has had a great impact not only on bulk gels but also on the world of microgels. In particular, research on poly(N-isopropylacrylamide) (PNIPAM) microgels, whose transition temperature is close to body temperature, has made remarkable progress in almost 35 years. This review presents some breakthrough findings in microgels that exhibit volume phase transitions and outlines recent works on the synthesis, structural analysis, and research direction of microgels.

Bundling Process of Citrulline Polypeptides upon UCST-Type Phase Separation

Ureido-modified poly(l-citrulline) (l-ornithine-co-l-citrulline denoted by PlOC) shows UCST-type phase separation behavior even under physiologically relevant conditions, which forms an α-helix structure above its phase separation temperature (T_p) but transforms into a solid-like aggregation composed of regular hexagonal packed cylinders below the T_p. This morphological transformation is characteristic of the phase separation behavior, but the mechanism behind it has remained incompletely understood. Here, we studied the phase separation behavior using small-angle X-ray scattering (SAXS) measurements. To analyze the SAXS data, we employed the modified unified model proposed previously, which decomposes the scattering profile into each structural element, such as the α-helices and their aggregation formed via hydrogen-bonding interactions between the ureido groups. The aggregation level is dependent on the temperature (T) and grouped into three classes: (1) mass-fractal aggregation composed of the α-helix (T > T_p), (2) spherical aggregation composed of the hexagonal packed cylinder (T < T_p), and (3) micro-order agglomeration formed by mutual fusion of the spherical aggregation, which appears as a solid-like aggregation. The SAXS analysis suggested that the transformation from the dispersed state as the α-helix to the agglomeration containing hierarchical structures occurs in a stepwise manner when the temperature falls below the T_p, which might also be transition behavior similar to the process of protein folding through folding intermediates.

Poly(ε-caprolactone) with pH and UCST responsiveness as a 5-fluorouracil carrier

Stimuli-responsive polymers with excellent biocompatibility and biodegradability are highly demanded as carriers for controlled drug delivery.

Thermoresponsive properties of poly(acrylamide-co-acrylonitrile)-based diblock copolymers synthesized (by PISA) in water

UCST-type poly(acrylamide-co-acrylonitrile) diblock copolymers synthesized in water (by PISA) can not only undergo reversible temperature-induced chain dissociation, but also temperature-induced morphological transition.