Recent progress in the science of complex coacervation

Assembly of biomimetic microreactors using caged-coacervate droplets

Complex coacervates are liquid-like droplets that can be used to create adaptive cell-like compartments. These compartments offer a versatile platform for the construction of bioreactors inspired by living cells. However, the lack of a membrane significantly reduces the colloidal stability of coacervates in terms of fusion and surface wetting, which limits their suitability as compartments. Here, we describe the formation of caged-coacervates surrounded by a semipermeable shell of silica nanocapsules. We demonstrate that the silica nanocapsules create a protective shell that also regulates the molecular transport of water-soluble compounds as a function of nanocapasule size. The adjustable semipermeability and intrinsic affinity of enzymes for the interior of the caged-coacervates allowed us to assemble biomimetic microreactors with enhanced colloidal stability.

Mathematical modeling of microscale biology: Ion pairing, spatially varying permittivity, and Born energy in glycosaminoglycan brushes

Biological macromolecules including nucleic acids, proteins, and glycosaminoglycans are typically anionic and can span domains of up to hundreds of nanometers and even micron length scales. The structures exist in crowded environments that are dominated by multivalent electrostatic interactions that can be modeled using mean-field continuum approaches that represent underlying molecular nanoscale biophysics. We develop such models for glycosaminoglycan brushes using steady state modified Poisson-Boltzmann models that incorporate important ion-specific (Hofmeister) effects. The results quantify how electroneutrality is attained through ion electrophoresis, spatially-varying permittivity hydration forces, and ion-specific pairing. Brush-salt interfacial profiles of the electrostatic potential as well as bound and unbound ions are characterized for imposed jump conditions across the interface. The models should be applicable to many intrinsically-disordered biophysical environments and are anticipated to provide insight into the design and development of therapeutics and drug-delivery vehicles to improve human health.

Polymer-Rich Dense Phase Can Concentrate Metastable Silica Precursors and Regulate Their Mineralization

Multistep mineralization processes are pivotal in the fabrication of functional materials and are often characterized by far from equilibrium conditions and high supersaturation. Interestingly, such 'nonclassical' mineralization pathways are widespread in biological systems, even though concentrating molecules well beyond their saturation level is incompatible with cellular homeostasis. Here, we show how polymer phase separation can facilitate bioinspired silica formation by passively concentrating the inorganic building blocks within the polymer dense phase. The high affinity of the dense phase to mobile silica precursors generates a diffusive flux against the concentration gradient, similar to dynamic equilibrium, and the resulting high supersaturation leads to precipitation of insoluble silica. Manipulating the chemistry of the dense phase allows to control the delicate interplay between polymer chemistry and silica precipitation. These results connect two phase transition phenomena, mineralization and coacervation, and offer a framework to achieve better control of mineral formation.

Nano-Sized Fucoidan Interpolyelectrolyte Complexes: Recent Advances in Design and Prospects for Biomedical Applications

The marine polysaccharide fucoidan (FUC) is a promising polymer for pharmaceutical research and development of novel drug delivery systems with modified release and targeted delivery. The presence of a sulfate group in the polysaccharide makes FUC an excellent candidate for the formation of interpolyelectrolyte complexes (PECs) with various polycations. However, due to the structural diversity of FUC, the design of FUC-based nanoformulations is challenging. This review describes the main strategies for the use of FUC-based PECs to develop drug delivery systems with improved biopharmaceutical properties, including nanocarriers in the form of FUC-chitosan PECs for pH-sensitive oral delivery, targeted delivery systems, and polymeric nanoparticles for improved hydrophobic drug delivery (e.g., FUC-zein PECs, core-shell structures obtained by the layer-by-layer self-assembly method, and self-assembled hydrophobically modified FUC particles). The importance of a complex study of the FUC structure, and the formation process of PECs based on it for obtaining reproducible polymeric nanoformulations with the desired properties, is also discussed.

Polyelectrolyte complexation of two oppositely charged symmetric polymers: A minimal theory

Interplay of Coulomb interaction energy, free ion entropy, and conformational elasticity is a fascinating aspect in polyelectrolytes (PEs). We develop a theory for complexation of two oppositely charged PEs, a process known to be the precursor to the formation of complex coacervates in PE solutions, to explore the underlying thermodynamics of complex formation, at low salts. The theory considers general degrees of solvent polarity and dielectricity within an implicit solvent model, incorporating a varying Coulomb strength. Explicit calculation of the free energy of complexation and its components indicates that the entropy of free counterions and salt ions and the Coulomb enthalpy of bound ion-pairs dictate the equilibrium of PE complexation. This helps decouple the self-consistent dependency of charge and size of the uncomplexed parts of the polyions, derive an analytical expression for charge, and evaluate the free energy components as functions of chain overlap. Complexation is observed to be driven by enthalpy gain at low Coulomb strengths, driven by entropy gain of released counterions but opposed by enthalpy loss due to reduction of ion-pairs at moderate Coulomb strengths, and progressively less favorable due to enthalpy loss at even higher Coulomb strengths. The total free energy of the system is found to decrease linearly with an overlap of chains. Thermodynamic predictions from our model are in good quantitative agreement with simulations in literature.

Numerical Techniques for Applications of Analytical Theories to Sequence-Dependent Phase Separations of Intrinsically Disordered Proteins

Biomolecular condensates, physically underpinned to a significant extent by liquid-liquid phase separation (LLPS), are now widely recognized by numerous experimental studies to be of fundamental biological, biomedical, and biophysical importance. In the face of experimental discoveries, analytical formulations emerged as a powerful yet tractable tool in recent theoretical investigations of the role of LLPS in the assembly and dissociation of these condensates. The pertinent LLPS often involves, though not exclusively, intrinsically disordered proteins engaging in multivalent interactions that are governed by their amino acid sequences. For researchers interested in applying these theoretical methods, here we provide a practical guide to a set of computational techniques devised for extracting sequence-dependent LLPS properties from analytical formulations. The numerical procedures covered include those for the determination of spinodal and binodal phase boundaries from a general free energy function with examples based on the random phase approximation in polymer theory, construction of tie lines for multiple-component LLPS, and field-theoretic simulation of multiple-chain heteropolymeric systems using complex Langevin dynamics. Since a more accurate physical picture often requires comparing analytical theory against explicit-chain model predictions, a commonly utilized methodology for coarse-grained molecular dynamics simulations of sequence-specific LLPS is also briefly outlined.

High-Yield Separation of Extracellular Vesicles Using Programmable Zwitterionic Coacervates

Programmable coacervates based on zwitterionic polymers are designed as dynamic materials for ion exchange bioseparation. These coacervates are proposed as promising materials for the purification of soft nanoparticles such as liposomes and extracellular vesicles (EVs). It is shown that the stimulus-responsiveness of the coacervates and the recruitment of desired molecules can be independently programmed by polymer design. Moreover, the polymeric coacervates can recruit and release intact liposomes, human EVs, and nanoalgosomes in high yields and separate vesicles from different types of impurities, including proteins and nucleic acids. This approach combines the speed and simplicity of precipitation methods and the programmability of chromatography with the gentleness of aqueous two-phase separation, thereby guaranteeing product stability. This material represents a promising alternative for providing a low-shear, gentle, and selective purification method for EVs.

Liquid-liquid and liquid-solid separation in self-assembled chitosan-alginate and chitosan-pectin complexes

The complexation between two oppositely charged polyelectrolytes (PE) can lead liquid-liquid (complex coacervates, CC) or liquid-solid (solid precipitates, SP) phase separations. Herein, the effect of pH (2-11) and ionic strength (I, 0.05-1.0 M KCl) on the associative interactions between chitosan (QL)-alginate (SA) and QL-Pectin (Pec), polysaccharides widely used in biotechnology field, is described. pH and I, exhibited significant effect on the structure and phase transitions by modifying the ionization degree (α), pk_a, and associative interactions between PE. Onset of binding was established at pH_c 9, while continued acidification (pH_τ 5.8) led to simultaneous CC and SP exhibiting a maximum turbidity in both systems. At pH_δ 4.0, QL-Pec showed preferably CC structures whereas QL-SA maintained the CC and SP structures. At pH_ω 2, the associative interactions were suppressed due to the low ionization of Pec and SA. I (1.0 M) significantly diminished the interactions in QL-Pec due to charge screening. Molecular weight, second virial coefficient, hydrodynamic size, ionizable groups, and persistence length of polyion, influenced on the phase behavior of QL-Pec and QL-SA systems. Therefore, CC and SP are found simultaneously in both systems, their transitions can be modulated by intrinsic and environmental conditions, expanding the functional properties of complexed polysaccharides.

A mini-review on bio-inspired polymer self-assembly: single-component and interactive polymer systems

Biology demonstrates meticulous ways to control biomaterials self-assemble into ordered and disordered structures to carry out necessary bioprocesses. Empowering the synthetic polymers to self-assemble like biomaterials is a hallmark of polymer physics studies. Unlike protein engineering, polymer science demystifies self-assembly by purposely embedding particular functional groups into the backbone of the polymer while isolating others. The polymer field has now entered an era of advancing materials design by mimicking nature to a very large extend. For example, we can make sequence-specific polymers to study highly ordered mesostructures similar to studying proteins, and use charged polymers to study liquid-liquid phase separation as in membraneless organelles. This mini-review summarizes recent advances in studying self-assembly using bio-inspired strategies on single-component and multi-component systems. Sequence-defined techniques are used to make on-demand hybrid materials to isolate the effects of chirality and chemistry in synthetic block copolymer self-assembly. In the meantime, sequence patterning leads to more hierarchical assemblies comprised of only hydrophobic and hydrophilic comonomers. The second half of the review discusses complex coacervates formed as a result of the associative charge interactions of oppositely charged polyelectrolytes. The tunable phase behavior and viscoelasticity are unique in studying liquid macrophase separation because the slow polymer relaxation comes primarily from charge interactions. Studies of bio-inspired polymer self-assembly significantly impact how we optimize user-defined materials on a molecular level.

Condensates formed by prion-like low-complexity domains have small-world network structures and interfaces defined by expanded conformations

Biomolecular condensates form via coupled associative and segregative phase transitions of multivalent associative macromolecules. Phase separation coupled to percolation is one example of such transitions. Here, we characterize molecular and mesoscale structural descriptions of condensates formed by intrinsically disordered prion-like low complexity domains (PLCDs). These systems conform to sticker-and-spacers architectures. Stickers are cohesive motifs that drive associative interactions through reversible crosslinking and spacers affect the cooperativity of crosslinking and overall macromolecular solubility. Our computations reproduce experimentally measured sequence-specific phase behaviors of PLCDs. Within simulated condensates, networks of reversible inter-sticker crosslinks organize PLCDs into small-world topologies. The overall dimensions of PLCDs vary with spatial location, being most expanded at and preferring to be oriented perpendicular to the interface. Our results demonstrate that even simple condensates with one type of macromolecule feature inhomogeneous spatial organizations of molecules and interfacial features that likely prime them for biochemical activity.

Molecular Determinants for the Layering and Coarsening of Biological Condensates

Many membraneless organelles, or biological condensates, form through phase separation, and play key roles in signal sensing and transcriptional regulation. While the functional importance of these condensates has inspired many studies to characterize their stability and spatial organization, the underlying principles that dictate these emergent properties are still being uncovered. In this review, we examine recent work on biological condensates, especially multicomponent systems. We focus on connecting molecular factors such as binding energy, valency, and stoichiometry with the interfacial tension, explaining the nontrivial interior organization in many condensates. We further discuss mechanisms that arrest condensate coalescence by lowering the surface tension or introducing kinetic barriers to stabilize the multidroplet state.

Balance of Macrophage Activation by a Complex Coacervate-Based Adhesive Drug Carrier Facilitates Diabetic Wound Healing

Uncontrolled and sustained inflammation disrupts the wound-healing process and produces excessive reactive oxygen species, resulting in chronic or impaired wound closure. Natural antioxidants such as plant-based extracts and natural polysaccharides have a long history in wound care. However, they are hard to apply to wound beds due to high levels of exudate or anatomical sites to which securing a dressing is difficult. Therefore, we developed a complex coacervate-based drug carrier with underwater adhesive properties that circumvents these challenges by enabling wet adhesion and controlling inflammatory responses. This resulted in significantly accelerated wound healing through balancing the pro- and anti-inflammatory responses in macrophages. In brief, we designed a complex coacervate-based drug carrier (ADC) comprising oligochitosan and inositol hexaphosphate to entrap and release antioxidant proanthocyanins (PA) in a sustained way. The results from in vitro experiments demonstrated that ADC is able to reduce LPS-stimulated pro-inflammatory responses in macrophages. The ability of ADC to reduce LPS-stimulated pro-inflammatory responses in macrophages is even more promising when ADC is encapsulated with PA (ADC-PA). Our results indicate that ADC-PA is able to polarize macrophages into an M2 tissue-healing phenotype via up-regulation of anti-inflammatory and resolution of inflammatory responses. Treatment with ADC-PA around the wound beds fine-tunes the balance between the numbers of inducible nitric oxide synthase-positive (iNOS+) and mannose receptor-negative (CD206-) M1 and iNOS-CD206+ M2 macrophages in the wound microenvironment compared to controls. Achieving such a balance between the numbers of iNOS+CD206- M1 and iNOS-CD206+ M2 macrophages in the wound microenvironment has led to significantly improved wound closure in mouse models of diabetes, which exhibit severe impairments in wound healing. Together, our results demonstrate for the first time the use of a complex coacervate-based drug delivery system to promote timely resolution of the inflammatory responses for diabetic wound healing by fine-tuning the functions of macrophages.

Analytical Formulation and Field-Theoretic Simulation of Sequence-Specific Phase Separation of Protein-Like Heteropolymers with Short- and Long-Spatial-Range Interactions

A theory for sequence-dependent liquid-liquid phase separation (LLPS) of intrinsically disordered proteins (IDPs) in the study of biomolecular condensates is formulated by extending the random phase approximation (RPA) and field-theoretic simulation (FTS) of heteropolymers with spatially long-range Coulomb interactions to include the fundamental effects of short-range, hydrophobic-like interactions between amino acid residues. To this end, short-range effects are modeled by Yukawa interactions between multiple nonelectrostatic charges derived from an eigenvalue decomposition of pairwise residue-residue contact energies. Chain excluded volume is afforded by incompressibility constraints. A mean-field approximation leads to an effective Flory-Huggins χ parameter, which, in conjunction with RPA, accounts for the contact-interaction effects of amino acid composition and the sequence-pattern effects of long-range electrostatics in IDP LLPS, whereas FTS based on the formulation provides full sequence dependence for both short- and long-range interactions. This general approach is illustrated here by applications to variants of a natural IDP in the context of several different amino-acid interaction schemes as well as a set of different model hydrophobic-polar sequences sharing the same composition. Effectiveness of the methodology is verified by coarse-grained explicit-chain molecular dynamics simulations.

Phase Transitions in Chemically Fueled, Multiphase Complex Coacervate Droplets

Membraneless organelles are droplets in the cytosol that are regulated by chemical reactions. Increasing studies suggest that they are internally organized. However, how these subcompartments are regulated remains elusive. Herein, we describe a complex coacervate-based model composed of two polyanions and a short peptide. With a chemical reaction cycle, we control the affinity of the peptide for the polyelectrolytes leading to distinct regimes inside the phase diagram. We study the transitions from one regime to another and identify new transitions that can only occur under kinetic control. Finally, we show that the chemical reaction cycle controls the liquidity of the droplets offering insights into how active processes inside cells play an important role in tuning the liquid state of membraneless organelles. Our work demonstrates that not only thermodynamic properties but also kinetics should be considered in the organization of multiple phases in droplets.

Excitonically Coupled Simple Coacervates via Liquid/Liquid Phase Separation

Viscoelastic liquid coacervate phases that are highly enriched in nonconjugated polyelectrolytes are currently the subject of highly active research from biological and soft-materials perspectives. However, formation of a liquid, electronically active coacervate has proved highly elusive, since extended π-electron interactions strongly favor the solid state. Herein we show that a conjugated polyelectrolyte can be rationally designed to undergo aqueous liquid/liquid phase separation to form a liquid coacervate phase. This result is significant both because it adds to the fundamental understanding of liquid/liquid phase separation but also because it opens intriguing applications in light harvesting and beyond. We find that the semiconducting coacervate is intrinsically excitonically coupled, allowing for long-range exciton diffusion in a strongly correlated, fluctuating environment. The emergent excitonic states are comprised of both excimers and H-aggregates.

Droplet Microfluidics for the Label-Free Extraction of Complete Phase Diagrams and Kinetics of Liquid-Liquid Phase Separation in Finite Volumes

Liquid-liquid phase separation of polymer and protein solutions is central in many areas of biology and material sciences. Here, an experimental and theoretical framework is provided to investigate the thermodynamics and kinetics of liquid-liquid phase separation in volumes comparable to cells. The strategy leverages droplet microfluidics to accurately measure the volume of the dense phase generated by liquid-liquid phase separation of solutions confined in micro-sized compartments. It is shown that the measurement of the volume fraction of the dense phase at different temperatures allows the evaluation of the binodal lines that determine the coexistence region of the two phases in the temperature-concentration phase diagram. By applying a thermodynamic model of phase separation in finite volumes, it is further shown that the platform can predict and validate kinetic barriers associated with the formation of a dense droplet in a parent dilute phase, therefore connecting thermodynamics and kinetics of liquid-liquid phase separation.

Phase Behavior of Ion-Containing Polymers in Polar Solvents: Predictions from a Liquid-State Theory with Local Short-Range Interactions

The thermodynamic phase behavior of charged polymers is a crucial property underlying their role in biology and various industrial applications. A complete understanding of the phase behaviors of such polymer solutions remains challenging due to the multi-component nature of the system and the delicate interplay among various factors, including the translational entropy of each component, excluded volume interactions, chain connectivity, electrostatic interactions, and other specific interactions. In this work, the phase behavior of partially charged ion-containing polymers in polar solvents is studied by further developing a liquid-state (LS) theory with local shortrange interactions. This work is based on the LS theory developed for fully-charged polyelectrolyte solutions. Specific interactions between charged groups of the polymer and counterions, between neutral segments of the polymer, and between charged segments of the polymer are incorporated into the LS theory by an extra Helmholtz free energy from the perturbed-chain statistical associating fluid theory (PC-SAFT). The influence of the sequence structure of the partially charged polymer is modeled by the number of connections between bonded segments. The effects of chain length, charge fraction, counterion valency, and specific short-range interactions are explored. A computational App for salt-free polymer solutions is developed and presented, which allows easy computation of the binodal curve and critical point by specifying values for the relevant model parameters.

Rheology and tribology of chitosan/Acacia gum complex coacervates

Acacia gum (Gum Arabic; GA) and chitosan (CTS) form complex coacervates in acidic environments, providing a polymer-rich aqueous material with interesting bio-lubricant properties. We investigate the interplay of the tribology and rheology of these coacervates, demonstrating that they dramatically reduce the friction coefficient between lubricated soft model surfaces as compared to solutions of the individual polymers. We characterize the phase separation behavior using microscopy, electrophoretic mobility and thermogravimetric analysis. The macroscopic rheological behaviour is predominantly viscous and ranges from weakly to strongly shear thinning: viscosity levels and strength of shear thinning increase with decreasing ionic strength, but no apparent yield stress or predominant elasticity were observed even in the absence of salt. Conversely, friction coefficients measured between soft and rough surfaces increase with a rise in ionic strength and can be scaled onto a Stribeck-type master curve across varying ionic strength and pH in the mixed and hydrodynamic lubrication regimes.

Recent advances in essential oil complex coacervation by efficient physical field technology: A review of enhancing efficient and quality attributes

Although complex coacervation could improve the water solubility, thermal stability, bioavailability, antioxidant activity and antibacterial activity of essential oils (EOs). However, some wall materials (such as proteins and polysaccharides) with water solubility and hydrophobic nature limited their application in complex coacervation. In order to improve the properties of EO complex coacervates, some efficient physical field technology was proposed. This paper summarizes the application and functional properties of EOs in complex coacervates, formation and controlled-release mechanism, as well as functions of EO complex coacervates. In particular, efficient physical field technology as innovative technology, such as high pressure, ultrasound, cold plasma, pulsed electric fields, electrohydrodynamic atomization and microwave technology improved efficient and quality attributes of EO complex coacervates are reviewed. The physical fields could modify the gelling, structural, textural, emulsifying, rheological properties, solubility of wall material (proteins and polysaccharides), which improve the properties of EO complex coacervates. Overall, EOs complex coacervates possess great potential to be used in the food industry, including high bioavailability, excellent antioxidant capacity and gut microbiota in vivo, masking the sensation of off-taste or flavor, favorable antimicrobial capacity.

Small-Molecule-based Supramolecular Plastics Mediated by Liquid-Liquid Phase Separation

Plastics are one of the most widely used polymeric materials. However, they are often undegradable and non-recyclable due to the very stable covalent bonds of macromolecules, causing environmental pollution and health problems. Here, we report that liquid-liquid phase separation (LLPS) could drive the formation of robust, stable, and sustainable plastics using small molecules. The LLPS process could sequester and concentrate solutes, strengthen the non-covalent association between molecules and produce a bulk material whose property was highly related to the encapsulated water amounts. It was a robust plastic with a remarkable Young's modulus of 139.5 MPa when the water content was low while became adhesive and could instantly self-heal with more absorbed water. Finally, responsiveness enabled the material to be highly recyclable. This work allowed us to understand the LLPS at the molecular level and demonstrated that LLPS is a promising approach to exploring eco-friendly supramolecular plastics that are potential substitutes for conventional polymers.

Driving force and pathway in polyelectrolyte complex coacervation

There is notable discrepancy between experiments and coarse-grained model studies regarding the thermodynamic driving force in polyelectrolyte complex coacervation: experiments find the free energy change to be dominated by entropy, while simulations using coarse-grained models with implicit solvent usually report a large, even dominant energetic contribution in systems with weak to intermediate electrostatic strength. Here, using coarse-grained, implicit-solvent molecular dynamics simulation combined with thermodynamic analysis, we study the potential of mean force (PMF) in the two key stages on the coacervation pathway for symmetric polyelectrolyte mixtures: polycation-polyanion complexation and polyion pair-pair condensation. We show that the temperature dependence in the dielectric constant of water gives rise to a substantial entropic contribution in the electrostatic interaction. By accounting for this electrostatic entropy, which is due to solvent reorganization, we find that under common conditions (monovalent ions, room temperature) for aqueous systems, both stages are strongly entropy-driven with negligible or even unfavorable energetic contributions, consistent with experimental results. Furthermore, for weak to intermediate electrostatic strengths, this electrostatic entropy, rather than the counterion-release entropy, is the primary entropy contribution. From the calculated PMF, we find that the supernatant phase consists predominantly of polyion pairs with vanishingly small concentration of bare polyelectrolytes, and we provide an estimate of the spinodal of the supernatant phase. Finally, we show that prior to contact, two neutral polyion pairs weakly attract each other by mutually induced polarization, providing the initial driving force for the fusion of the pairs.

An Overview of Coacervates: The Special Disperse State of Amphiphilic and Polymeric Materials in Solution

Individual amphiphiles, polymers, and colloidal dispersions influenced by temperature, pH, and environmental conditions or interactions between their oppositely charged pairs in solvent medium often produce solvent-rich and solvent-poor phases in the system. The solvent-poor denser phase found either on the top or the bottom of the system is called coacervate. Coacervates have immense applications in various technological fields. This review comprises a concise introduction, focusing on the types of coacervates, and the influence of different factors in their formation, structures, and stability. In addition, their physicochemical properties, thermodynamics of formation, and uses and multifarious applications are also concisely presented and discussed.

How can we interpret the relationship between liquid-liquid phase separation and amyotrophic lateral sclerosis?

One of the critical definitions of neurodegenerative diseases is the formation of insoluble intracellular inclusion body. These inclusions are found in various neurodegenerative diseases such as Alzheimer's disease, amyotrophic lateral sclerosis (ALS), Huntington's disease, Parkinson's disease, and frontotemporal dementia (FTD). Each inclusion body contains disease-specific proteins and is also resistant to common detergent treatments. These aggregates are generally ubiquitinated and thus recognized as misfolded by the organism. They are observed in residual neurons at the affected sites in each disease, suggesting a contribution to disease pathogenesis. The molecular mechanisms for the formation of these inclusion bodies remain unclear. Some proteins, such as superoxide dismutase 1 (SOD1) mutant that causes familial ALS, are highly aggregative due to altered folding caused by point mutations. Still, the aggregates observed in neurodegenerative diseases contain wild-type proteins. In recent years, it has been reported that the proteins responsible for neurodegenerative diseases undergo liquid-liquid phase separation (LLPS). In particular, the ALS/FTD causative proteins such as TAR DNA-binding protein 43 kDa (TDP-43) and fused-in-sarcoma (FUS) undergo LLPS. LLPS increases the local concentration of these proteins, and these proteins eventually change their phase from liquid to solid (liquid-solid phase transition) due to abnormal folding during repetitive separation cycles into two phases and recovery to one phase. In addition to the inclusion body formation, sequestration of essential proteins into the LLPS droplets or changes in the LLPS status can directly impair neural functions and cause diseases. In this review, we will discuss the relationship between the LLPS observed in ALS causative proteins and the pathogenesis of the disease and outline potential therapeutic approaches.

Wetting behavior of polyelectrolyte complex coacervates on solid surfaces

The wetting behavior of complex coacervates underpins their use in many emerging applications of surface science, particularly wet adhesives and coatings. Many factors dictate if a coacervate phase will condense on a solid surface, including solution conditions, the nature of the polymer-substrate interaction, and the underlying supernatant-coacervate bulk phase behavior. In this work, we use a simple inhomogeneous mean-field theory to study the wetting behavior of complex coacervates on solid surfaces both off-coexistence (wetting transitions) and on-coexistence (contact angles). We focus on the effects of salt concentration, the polycation/polyanion surface affinity, and the applied electrostatic potential on the wettability. We find that the coacervate generally wets the surface via a first order wetting transition with second order transitions possible above a surface critical point. Applying an electrostatic potential to a solid surface always improves the surface wettability when the polycation/polyanion-substrate interaction is symmetric. For asymmetric surface affinity, the wettability has a nonmonotonic dependence with the applied potential. We use simple scaling and thermodynamic arguments to explain our results.

Molecular logic operations from complex coacervation with aggregation-induced emission characteristics

Leveraging complex coacervation of a polycation and a bivalent anion with aggregation-induced emission characteristics, we accomplish eight basic logic operations with environmental stimuli as inputs, producing Boolean-like fluorescence intensity or turbidity 'outputs' with contrast higher than one order of magnitude. Storage of information of a fluorescent pattern and thermo-sensor applications are also demonstrated.

Understanding the stimuli responsive behavior of polyion grafted nanoparticles in the presence of salt and polyelectrolytes

The design of nanoparticles (NPs) that respond to external stimuli like pH, temperature, and electric or magnetic fields has found immense interest in various fields of nanotechnology like nanomedicine, drug delivery, and cancer therapy. Nanoparticles grafted with polymeric ligands have been extensively used as building blocks in the directed self assembly of nanoparticles. These moieties not only assemble into various morphologies but also respond to a wide range of external stimuli. In this work, we have used coarse grained molecular dynamics simulations to understand the stimuli-responsive behavior of assemblies of NPs grafted with oppositely charged polyions (PGNs) in the presence of salt and polyelectrolytes. At low grafting density, a transformation from ring morphology to form dimers/strings/dispersed NPs was observed upon addition of divalent/trivalent salts. NPs grafted with longer grafts showed higher stability to remain as rings compared to shorter grafts. The change in NP morphology was a direct consequence of preferential interaction of the polyaion grafts with the oppositely charged salt ions compared to the oppositely charged grafts on the NPs. At fixed salt valency, the size of the salt ion, concentration and molecular connectivity played a crucial role in the stimuli responsive behavior of polyion grafted NPs in solutions. Further, in the presence of polyelectrolytes, these transitions occurred at lower monomer valency due to the stronger electrostatic interactions between the grafted chains and oppositely charged free polyelectrolytes in solutions. Disordered and ordered aggregates assemblies formed at higher grafting density were broken into smaller NP assemblies in the presence of salt. Drug encapsulation studies in the presence of salt and polyelectrolytes were performed on model drug moieties in order to demonstrate the potential use of the modelled stimuli responsive nanoparticle assemblies in drug delivery applications.

Recent progress in liquid embolic agents

Vascular embolization is a non-surgical procedure used to treat diseases or morbid conditions related to blood vessels, such as bleeding, arteriovenous malformation, aneurysm, and hypervascular tumors, through the intentional occlusion of blood vessels. Among various types of embolic agents that have been applied, liquid embolic agents are gaining an increasing amount of attention owing to their advantages in distal infiltration into regions where solid embolic agents cannot reach, enabling more extensive embolization. Meanwhile, recent advances in biomaterials and technologies have also contributed to the development of novel liquid embolic agents that can resolve the challenges faced while using the existing embolic materials. In this review, we briefly summarize the clinically used embolic agents and their applications, and then present selected research results that overcome the limitations of the embolic agents in use. Through this review, we suggest the required properties of liquid embolic agents that ensure efficacy, which can replace the existing agents, providing directions for the future development in this field.

Coacervates: recent developments as nanostructure delivery platforms for therapeutic biomolecules

Coacervation is a liquid-liquid phase separation that can occur in solutions of macromolecules through self-assembly or electrostatic interactions. Recently, coacervates composed of biocompatible macromolecules have been actively investigated as nanostructure platforms to encapsulate and deliver biomolecules such as proteins, RNAs, and DNAs. One particular advantage of coacervates is that they are derived from aqueous solutions, unlike other nanoparticle delivery systems that often require organic solvents. In addition, coacervates achieve high loading while maintaining the viability of the cargo material. Here, we review recent developments in the applications of coacervates and their limitations in the delivery of therapeutic biomolecules. Important factors for coacervation include molecular structures of the polyelectrolytes, mixing ratio, the concentration of polyelectrolytes, and reaction conditions such as ionic strength, pH, and temperature. Various compositions of coacervates have been shown to deliver biomolecules in vitro and in vivo with encouraging activities. However, major hurdles remain for the systemic route of administration other than topical or local delivery. The scale-up of manufacturing methods suitable for preclinical and clinical evaluations remains to be addressed. We conclude with a few research directions to overcome current challenges, which may lead to successful translation into the clinic.

How to Predict the Order of Phase Separation of Polyelectrolyte Complexes and Their Miscibility

The mixture of two oppositely charged polyelectrolyte solutions results in complexation that may lead to an associative phase separation, forming a highly concentrated phase in both polyelectrolytes in equilibrium with a dilute phase. In this work, we aim to investigate what controls the order of complexation when more polyelectrolytes of the same charge are present. For this, the effect of the addition of a third oppositely charged polyelectrolyte in a mixture of two polyelectrolytes with the same charge was studied. Our results show that, under certain conditions, the electrostatic complexation takes place selectively, where one polyanion (or polycation) phase separates first, followed by the other phase separation, with both complexes at their 1:1 charge stoichiometry. Infrared analyses of the phase-separated complexes confirmed that, in a mixture of polyanions, poly(styrenesulfonate) is complexed first, followed by poly(acrylate). For polycations, these analyses showed that poly(diallyldimethylammonium) is preferentially complexed over poly(allylamine). These results suggest that electrostatic complexation occurs following the sequence predicted as in an acid/base titration, where the acidic/basic strength of the involved polyions dictates which one is complexed first. In this respect, the order of complexation can be associated with the equivalence pH for each pair, which we propose can be used as a parameter to predict phase separation in polyelectrolyte mixtures. In addition, we have investigated the miscibility of these complex mixtures, confirming that multiphasic complexes are formed whenever the polyions display ionizable groups with different acid/basic strengths and that this can also be related to their equivalence pH.

Coacervation of poly-electrolytes in the presence of lipid bilayers: mutual alteration of structure and morphology

Many intrinsically disordered peptides have been shown to undergo liquid-liquid phase separation and form complex coacervates, which play various regulatory roles in the cell. Recent experimental studies found that such phase separation processes may also occur at the lipid membrane surface and help organize biomolecules during signaling events; in some cases, phase separation of proteins at the membrane surface was also observed to lead to significant remodeling of the membrane morphology. The molecular mechanisms that govern the interactions between complex coacervates and lipid membranes and the impacts of such interactions on their structure and morphology, however, remain unclear. Here we study the coacervation of poly-glutamate (E30) and poly-lysine (K30) in the presence of lipid bilayers of different compositions. We carry out explicit-solvent coarse-grained molecular dynamics simulations by using the MARTINI (v3.0) force-field. We find that more than 20% anionic lipids are required for the coacervate to form stable contact with the bilayer. Upon wetting, the coacervate induces negative curvature to the bilayer and facilitates local lipid demixing, without any peptide insertion. The magnitude of negative curvature, extent of lipid demixing, and asphericity of the coacervate increase with the concentration of anionic lipids. Overall, we observe a decrease in the number of contacts among the polyelectrolytes as the droplet spreads over the bilayer. Therefore, unlike previous suggestions, interactions among polyelectrolytes do not constitute a driving force for the membrane bending upon wetting by the coacervate. Rather, analysis of interaction energy components suggests that bending of the membrane is favored by enhanced interactions between polyelectrolytes with lipids as well as with counterions. Kinetic studies reveal that, at the studied polyelectrolyte concentrations, the coacervate formation precedes bilayer wetting.

Comb Polyelectrolytes Stabilize Complex Coacervate Microdroplet Dispersions

Complex coacervate microdroplets are membraneless compartments that selectively sequester biological molecules from their surroundings and enhance bioreactions. Yet, their use as protocell models and bioreactors has been limited owing to a lack of feasible strategies to prevent their uncontrolled coalescence. Herein, we introduce an approach to mitigate coalescence of complex coacervate microdroplets using comb polyelectrolytes as stabilizers, creating complex coacervate dispersions with months-long stabilities. Tunability of microdroplet size and stability is achieved by the regulation of comb polyelectrolyte concentration and molecular weight. Importantly, the comb polyelectrolyte-stabilized coacervate microdroplets spontaneously sequester and retain proteins over extended periods. Moreover, enhanced catalytic activity of proteins and significant (up to 10-fold) acceleration of bioreactions are achieved in stabilized complex coacervate dispersions, even when stored for up to 48 h. Our findings are expected to expand the utility of complex coacervate microdroplets as artificial protocells, encapsulants, and bioreactors and also facilitate their use in pharmaceutical, agricultural, food, and cosmetics formulations.

On the nature of screening in Voorn–Overbeek type theories

By using a recently formulated Legendre transform approach to the thermodynamics of charged systems, we explore the general form of the screening length in the Voorn–Overbeek-type theories, which remains valid also in the cases where the entropy of the charged component(s) is not given by the ideal gas form as in the Debye–Hückel theory. The screening length consistent with the non-electrostatic terms in the free energy ansatz for the Flory–Huggins and Voorn–Overbeek type theories, derived from the local curvature properties of the Legendre transform, has distinctly different behavior than the often invoked standard Debye screening length, though it reduces to it in some special cases.

Self-Assembly of Miktoarm Star Polyelectrolytes in Solutions with Various Ionic Strengths

We studied the self-assembly of miktoarm star polyelectrolytes with different numbers of arms in solutions with various ionic strengths using coarse-grained molecular dynamic simulations. Spherical micelles are obtained for star polyelectrolytes with fewer arms, whereas wormlike clusters are obtained for star polyelectrolytes with more arms at a low ionic strength environment, with hydrophilic arms showing a stretched conformation. The number of clusters shows an overall decreasing tendency with increasing the number of arms in star polyelectrolytes due to strong electrostatic coupling between polycations and polyanions. The formation of wormlike clusters follows an overall stepwise pathway with an intermittent association-dissociation process for star polyelectrolytes with weak electrostatic coupling. These computational results can provide relevant physical insights to understand the self-assembly mechanism of star polyelectrolytes in solvents with various ionic strengths and to design star polyelectrolytes with functional groups that can fine-tune self-assembled structures for specific applications.

Protected Poly(3-sulfopropyl methacrylate) Copolymers: Synthesis, Stability, and Orthogonal Deprotection

Because of their permanent charge, strong polyelectrolytes remain challenging to characterize, in particular, when they are combined with hydrophobic features. For this reason, they are typically prepared through a postmodification of a fully hydrophobic precursor. Unfortunately, these routes often result in an incomplete functionalization or otherwise require harsh reaction conditions, thus limiting their applicability. To overcome these problems, in this work a strategy is presented that facilitates the preparation of well-defined strong polyanions by starting from protected 3-sulfopropyl methacrylate monomers. Depending on the chemistry of the protecting group, the hydrophobic precursor could be quantitatively converted into a strong polyanion under nucleophilic, acidic, or basic conditions. As a proof of concept, orthogonally protected diblock copolymers were synthesized, selectively deprotected, and allowed to self-assemble in aqueous solution. Further conversion into a fully water-soluble polyanion was achieved by deprotecting the second block as well.