Molecular and structural basis of low interfacial energy of complex coacervates in water

Combining and concentrating nanocelluloses for cryogels with remarkable strength and wet resilience

Cellulose cryogels are promising eco-friendly materials that exhibit low density, high porosity, and renewability. However, the applications of these materials are limited by their lower mechanical and water resistance compared to petrochemical-based lightweight materials. In this work, nanocelluloses were functionalized with cationic and anionic groups, and these nanomaterials were combined to obtain strong and water-resilient cryogels. To prepare the cryogels, anionic and cationic micro- and nanofibrils (CNFs) were produced at three different sizes and combined in various weight ratios, forming electrostatic complexes. The complex phase was concentrated by centrifugation and freeze-dried. Porous and open cellular structures were assembled in all compositions tested (porosity >90 %). Compressive testing revealed that the most resistant cryogels (1.7 MPa) were obtained with equivalent amounts of negatively and positively charged CNFs with lengths between 100 and 1200 nm. The strength at this condition was achieved as CNF electrostatic complexes assembled in thick cells, as observed by synchrotron X-ray tomography. In addition to mechanical strength, electrostatic complexation provided remarkable structural stability in water for the CNF cryogels, without compromising their biodegradability. This route by electrostatic complexation is a practical strategy to combine and concentrate nanocelluloses to tailor biodegradable lightweight materials with high strength and wet stability.

Effect of Polymer Gel Elasticity on Complex Coacervate Phase Behavior

Gels are key materials in biological systems such as tissues and may control biocondensate formation and structure. To further understand the effects of elastic environments on biomacromolecular assembly, we have investigated the phase behavior and radii of complex coacervate droplets in polyacrylamide (PAM) networks as a function of gel modulus. Poly-l-lysine (PLL) and sodium hyaluronate (HA) complex coacervate phases were prepared in PAM gels with moduli varying from 0.035 to 15.0 kPa. The size of the complex coacervate droplets is reported from bright-field microscopy and confocal fluorescence microscopy. Overall, the complex coacervate droplet volume decreases inversely with the modulus. Fluorescence microscopy is also used to determine the phase behavior and concentration of fluorescently tagged HA in the complex coacervate phases as a function of ionic strength (100-270 mM). We find that the critical ionic strength and complex coacervate stability are nonmonotonic as a function of the network modulus and that the local gel concentration can be used to control phase behavior and complex coacervate droplet size scale. By understanding how elastic environments influence simple electrostatic assembly, we can further understand how biomacromolecules exist in complex, crowded, and elastic cellular environments.

Coacervate Droplets for Synthetic Cells

The design and construction of synthetic cells - human-made microcompartments that mimic features of living cells - have experienced a real boom in the past decade. While many efforts have been geared toward assembling membrane-bounded compartments, coacervate droplets produced by liquid-liquid phase separation have emerged as an alternative membrane-free compartmentalization paradigm. Here, the dual role of coacervate droplets in synthetic cell research is discussed: encapsulated within membrane-enclosed compartments, coacervates act as surrogates of membraneless organelles ubiquitously found in living cells; alternatively, they can be viewed as crowded cytosol-like chassis for constructing integrated synthetic cells. After introducing key concepts of coacervation and illustrating the chemical diversity of coacervate systems, their physicochemical properties and resulting bioinspired functions are emphasized. Moving from suspensions of free floating coacervates, the two nascent roles of these droplets in synthetic cell research are highlighted: organelle-like modules and cytosol-like templates. Building the discussion on recent studies from the literature, the potential of coacervate droplets to assemble integrated synthetic cells capable of multiple life-inspired functions is showcased. Future challenges that are still to be tackled in the field are finally discussed.

Understanding the Impacts of Molecular and Macromolecular Crowding Agents on Protein-Polymer Complex Coacervates

Complex coacervation refers to the liquid-liquid phase separation (LLPS) process occurring between charged macromolecules. The study of complex coacervation is of great interest due to its implications in the formation of membraneless organelles (MLOs) in living cells. However, the impacts of the crowded intracellular environment on the behavior and interactions of biomolecules involved in MLO formation are not fully understood. To address this knowledge gap, we investigated the effects of crowding on a model protein-polymer complex coacervate system. Specifically, we examined the influence of sucrose as a molecular crowder and polyethylene glycol (PEG) as a macromolecular crowder. Our results reveal that the presence of crowders led to the formation of larger coacervate droplets that remained stable over a 25-day period. While sucrose had a minimal effect on the physical properties of the coacervates, PEG led to the formation of coacervates with distinct characteristics, including higher density, increased protein and polymer content, and a more compact internal structure. These differences in coacervate properties can be attributed to the effects of crowders on individual macromolecules, such as the conformation of model polymers, and nonspecific interactions among model protein molecules. Moreover, our results show that sucrose and PEG have different partition behaviors: sucrose was present in both the coacervate and dilute phases, while PEG was observed to be excluded from the coacervate phase. Collectively, our findings provide insights into the understanding of crowding effects on complex coacervation, shedding light on the formation and properties of coacervates in the context of MLOs.

Multiphasic Coacervates Assembled by Hydrogen Bonding and Hydrophobic Interactions

Coacervation has emerged as a prevalent mechanism to compartmentalize biomolecules in living cells. Synthetic coacervates help in understanding the assembly process and mimic the functions of biological coacervates as simplified artificial systems. Though the molecular mechanism and mesoscopic properties of coacervates formed from charged coacervates have been well investigated, the details of the assembly and stabilization of nonionic coacervates remain largely unknown. Here, we describe a library of coacervate-forming polyesteramides and show that the water-tertiary amide bridging hydrogen bonds and hydrophobic interactions stabilize these nonionic, single-component coacervates. Analogous to intracellular biological coacervates, these coacervates exhibit "liquid-like" features with low viscosity and low interfacial energy, and form coacervates with as few as five repeating units. By controlling the temperature and engineering the molar ratio between hydrophobic interaction sites and bridging hydrogen bonding sites, we demonstrate the tuneability of the viscosity and interfacial tension of polyesteramide-based coacervates. Taking advantage of the differences in the mesoscopic properties of these nonionic coacervates, we engineered multiphasic coacervates with core-shell architectures similar to those of intracellular biological coacervates, such as nucleoli and stress granule-p-body complexes. The multiphasic structures produced from these synthetic nonionic polyesteramide coacervates may serve as a valuable tool for investigating physicochemical principles deployed by living cells to spatiotemporally control cargo partitioning, biochemical reaction rates, and interorganellar signal transport.

Phase Behavior and Electrochemical Properties of Highly Asymmetric Redox Coacervates

This work reports the phase behavior and electrochemical properties of liquid coacervates made of ferricyanide and poly(ethylenimine). In contrast to the typical polyanion/polycation pairs used in liquid coacervates, the ferricyanide/poly(ethylenimine) system is highly asymmetric because poly(ethylenimine) has approximately 170 charges per molecule, while ferricyanide has only 3. Two types of phase diagrams were measured and fitted with a theoretical model. In the first type of diagram, the stability of the coacervate was studied in the plane given by the concentration of poly(ethylenimine) versus the concentration of ferricyanide for a fixed concentration of added monovalent salt (NaCl). The second type of diagram involved the plane given by the concentration of poly(ethylenimine) vs the concentration of the added monovalent salt for a fixed poly(ethyleneimine)/ferricyanide ratio. Interestingly, these phase diagrams displayed qualitative similarities to those of symmetric polyanion/polycation systems, suggesting that coacervates formed by a polyelectrolyte and a small multivalent ion can be treated as a specific case of polyelectrolyte coacervate. The characterization of the electrochemical properties of the coacervate revealed that the addition of monovalent salt greatly enhances charge transport, presumably by breaking ion pairs between ferricyanide and poly(ethylenimine). This finding highlights the significant influence of added salt on the transport properties of coacervates. This study provides the first comprehensive characterization of the phase behavior and transport properties of asymmetric coacervates and places these results within the broader context of the better-known symmetric polyelectrolyte coacervates.

Visualizing Formation and Dynamics of a Three-Dimensional Sponge-like Network of a Coacervate in Real Time

Coacervates, which are formed by liquid-liquid phase separation, have been extensively explored as models for synthetic cells and membraneless organelles, so their in-depth structural analysis is crucial. However, both the inner structure dynamics and formation mechanism of coacervates remain elusive. Herein, we demonstrate real-time confocal observation of a three-dimensional sponge-like network in a dipeptide-based coacervate. In situ generation of the dipeptide allowed us to capture the emergence of the sponge-like network via unprecedented membrane folding of vesicle-shaped intermediates. We also visualized dynamic fluctuation of the network, including reversible engagement/disengagement of cross-links and a stochastic network kissing event. Photoinduced transient formation of a multiphase coacervate was achieved with a thermally responsive phase transition. Our findings expand the fundamental understanding of synthetic coacervates and provide opportunities to manipulate their physicochemical properties by engineering the inner network for potential applications in development of artificial cells and life-like material fabrication.

Dynamic frustrated charge hotspots created by charge density modulation sequester globular proteins into complex coacervates

This study presents a simple strategy for the sequestration of globular proteins as clients into synthetic polypeptide-based complex coacervates as a scaffold, thereby recapitulating the scaffold-client interaction found in biological condensates. Considering the low net charges of scaffold proteins participating in biological condensates, the linear charge density (σ) on the polyanion, polyethylene glycol-b-poly(aspartic acids), was reduced by introducing hydroxypropyl or butyl moieties as a charge-neutral pendant group. Complex coacervate prepared from the series of reduced-σ polyanions and the polycation, homo-poly-l-lysine, could act as a scaffold that sequestered various globular proteins with high encapsulation efficiency (>80%), which sometimes involved further agglomerations in the coacervates. The sequestration of proteins was basically driven by electrostatic interaction, and therefore depended on the ionic strength and charges of the proteins. However, based on the results of polymer partitioning in the coacervate in the presence or absence of proteins, charge ratios between cationic and anionic polymers were maintained at the charge ratio of unity. Therefore, the origin of the electrostatic interaction with proteins is considered to be dynamic frustrated charges in the complex coacervates created by non-neutralized charges on polymer chains. Furthermore, fluorescence recovery after photobleaching (FRAP) measurements showed that the interaction of side-chains and proteins changed the dynamic property of coacervates. It also suggested that the physical properties of the condensate are tunable before and after the sequestration of globular proteins. The present rational design approach of the scaffold-client interaction is helpful for basic life-science research and the applied frontier of artificial organelles.

Polyelectrolyte-multivalent molecule complexes: physicochemical properties and applications

The complexation of polyelectrolytes with other oppositely charged structures gives rise to a great variety of functional materials with potential applications in a wide spectrum of technological fields. Depending on the assembly conditions, polyelectrolyte complexes can acquire different macroscopic configurations such as dense precipitates, nanosized colloids and liquid coacervates. In the past 50 years, much progress has been achieved to understand the principles behind the phase separation induced by the interaction of two oppositely charged polyelectrolytes in aqueous solutions, especially for symmetric systems (systems in which both polyions have similar molecular weight and concentration). However, in recent years, the complexation of polyelectrolytes with alternative building blocks such as small charged molecules (multivalent inorganic species, oligopeptides, and oligoamines, among others) has gained attention in different areas. In this review, we discuss the physicochemical characteristics of the complexes formed by polyelectrolytes and multivalent small molecules, putting a special emphasis on their similarities with the well-known polycation-polyanion complexes. In addition, we analyze the potential of these complexes to act as versatile functional platforms in various technological fields, such as biomedicine and advanced materials engineering.

Ionic Strength Dependence of the Complex Coacervation between Lactoferrin and β-Lactoglobulin

Heteroprotein complex coacervation is an assembly formed by oppositely charged proteins in aqueous solution that leads to liquid-liquid phase separation. The ability of lactoferrin and β-lactoglobulin to form complex coacervates at pH 5.5 under optimal protein stoichiometry has been studied in a previous work. The goal of the current study is to determine the influence of ionic strength on the complex coacervation between these two proteins using direct mixing and desalting protocols. The initial interaction between lactoferrin and β-lactoglobulin and subsequent coacervation process were highly sensitive to the ionic strength. No microscopic phase separation was observed beyond a salt concentration of 20 mM. The coacervate yield decreased drastically with increasing added NaCl from 0 to 60 mM. The charge-screening effect induced by increasing the ionic strength is attributed to a decrease of interaction between the two oppositely charged proteins throughout a decrease in Debye length. Interestingly, as shown by isothermal titration calorimetry, a small concentration of NaCl around 2.5 mM promoted the binding energy between the two proteins. These results shed new light on the electrostatically driven mechanism governing the complex coacervation in heteroprotein systems.

On shape forming by contractile filaments in the surface of growing tissues

Growing tissues are highly dynamic, and flow on sufficiently long timescales due to cell proliferation, migration, and tissue remodeling. As a consequence, growing tissues can often be approximated as viscous fluids. This means that the shape of microtissues growing in vitro is governed by their surface stress state, as in fluid droplets. Recent work showed that cells in the near-surface region of fibroblastic or osteoblastic microtissues contract with highly oriented actin filaments, thus making the surface properties highly anisotropic, in contrast to what is expected for an isotropic fluid. Here, we develop a model that includes mechanical anisotropy of the surface generated by contractile fibers and we show that mechanical equilibrium requires contractile filaments to follow geodesic lines on the surface. Constant pressure in the fluid forces these contractile filaments to be along geodesics with a constant normal curvature. We then take this into account to determine equilibrium shapes of rotationally symmetric bodies subjected to anisotropic surface stress states and derive a family of surfaces of revolution. A comparison with recently published shapes of microtissues shows that this theory accurately predicts both the surface shape and the direction of the actin filaments on the surface.

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.

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.

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.

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.

Satellite repeat transcripts modulate heterochromatin condensates and safeguard chromosome stability in mouse embryonic stem cells

Heterochromatin maintains genome integrity and function, and is organised into distinct nuclear domains. Some of these domains are proposed to form by phase separation through the accumulation of HP1ɑ. Mouse heterochromatin contains noncoding major satellite repeats (MSR), which are highly transcribed in mouse embryonic stem cells (ESCs). Here, we report that MSR transcripts can drive the formation of HP1ɑ droplets in vitro, and modulate heterochromatin into dynamic condensates in ESCs, contributing to the formation of large nuclear domains that are characteristic of pluripotent cells. Depleting MSR transcripts causes heterochromatin to transition into a more compact and static state. Unexpectedly, changing heterochromatin's biophysical properties has severe consequences for ESCs, including chromosome instability and mitotic defects. These findings uncover an essential role for MSR transcripts in modulating the organisation and properties of heterochromatin to preserve genome stability. They also provide insights into the processes that could regulate phase separation and the functional consequences of disrupting the properties of heterochromatin condensates.

Coacervation between two positively charged poly(ionic liquid)s

Complex coacervates are usually formed through electrostatic attraction between oppositely charged polyelectrolytes, with a few of exceptions such as coacervates of like-charge proteins and polyelectrolytes, both in vivo and in vitro. Understanding of the preparation and mechanism of these coacervates is limited. Here we design a positively charged poly(ionic liquid) poly(1-vinyl-3-benzylimidazolium chloride) (PILben) that bears benzene rings in repeating units. Fluidic coacervates were prepared by mixing the PILben aqueous solution with a like-charge poly(ionic liquid) named poly(dimethyl diallyl ammonium chloride) (PDDA). The effects of polymer concentration, temperature and ionic strength in the PILben-PDDA coacervate were studied. Raman spectroscopy and two-dimensional ¹ H-¹³ C heteronuclear single quantum coherence (¹ H-¹³ C HSQC) characterizations verify that the coacervate formation benefits from the cation-π interaction between PILben and PDDA. This work provides principles and understandings of designing coacervates derived from like-charge poly(ionic liquids) with high charge density. This article is protected by copyright. All rights reserved.

Metal Ion-Directed Functional Metal-Phenolic Materials

Metal ions are ubiquitous in nature and play significant roles in assembling functional materials in fields spanning chemistry, biology, and materials science. Metal-phenolic materials are assembled from phenolic components in the presence of metal ions through the formation of metal-organic complexes. Alkali, alkali-earth, transition, and noble metal ions as well as metalloids interacting with phenolic building blocks have been widely exploited to generate diverse hybrid materials. Despite extensive studies on the synthesis of metal-phenolic materials, a comprehensive summary of how metal ions guide the assembly of phenolic compounds is lacking. A fundamental understanding of the roles of metal ions in metal-phenolic materials engineering will facilitate the assembly of materials with specific and functional properties. In this review, we focus on the diversity and function of metal ions in metal-phenolic material engineering and emerging applications. Specifically, we discuss the range of underlying interactions, including (i) cation-π, (ii) coordination, (iii) redox, and (iv) dynamic covalent interactions, and highlight the wide range of material properties resulting from these interactions. Applications (e.g., biological, catalytic, and environmental) and perspectives of metal-phenolic materials are also highlighted.

Adhesive Materials Inspired by Barnacle Underwater Adhesion: Biological Principles and Biomimetic Designs

Wet adhesion technology has potential applications in various fields, especially in the biomedical field, yet it has not been completely mastered by humans. Many aquatic organisms (e.g., mussels, sandcastle worms, and barnacles) have evolved into wet adhesion specialists with excellent underwater adhesion abilities, and mimicking their adhesion principles to engineer artificial adhesive materials offers an important avenue to address the wet adhesion issue. The crustacean barnacle secretes a proteinaceous adhesive called barnacle cement, with which they firmly attach their bodies to almost any substrate underwater. Owing to the unique chemical composition, structural property, and adhesion mechanism, barnacle cement has attracted widespread research interest as a novel model for designing biomimetic adhesive materials, with significant progress being made. To further boost the development of barnacle cement–inspired adhesive materials (BCIAMs), it is necessary to systematically summarize their design strategies and research advances. However, no relevant reviews have been published yet. In this context, we presented a systematic review for the first time. First, we introduced the underwater adhesion principles of natural barnacle cement, which lay the basis for the design of BCIAMs. Subsequently, we classified the BCIAMs into three major categories according to the different design strategies and summarized their research advances in great detail. Finally, we discussed the research challenge and future trends of this field. We believe that this review can not only improve our understanding of the molecular mechanism of barnacle underwater adhesion but also accelerate the development of barnacle-inspired wet adhesion technology.

pH-Dependent complexation and polyelectrolyte chain conformation of polyzwitterion-polycation coacervates in salted water

The phase behavior and chain conformational structure of biphasic polyzwitterion-polyelectrolyte coacervates in salted aqueous solution are investigated with a model weak cationic polyelectrolyte, poly(2-vinylpyridine) (P2VP), whose charge fraction can be effectively tuned by pH. It is observed that increasing the pH leads to the increase of the yielding volume fraction and the water content of dense coacervates formed between net neutral polybetaine and cationic P2VP in contrast to the decrease of critical salt concentration for the onset of coacervation, where the P2VP charge fraction is reduced correspondingly. Surprisingly, a single-molecule fluorescence spectroscopic study suggests that P2VP chains upon coacervation seem to adopt a swollen or an even more expanded conformational structure at higher pH. As the hydrophobicity of P2VP chains is accompanied by a reduced charge fraction by increasing the pH, a strong pH-dependent phase and conformational behaviors suggest the shift of entropic and enthalpic contribution to the underlying thermodynamic energy landscape and chain structural dynamics of polyelectrolyte coacervation involving weak polyelectrolytes in aqueous solution.

Impact of wet-dry cycling on the phase behavior and compartmentalization properties of complex coacervates

Wet-dry cycling on the early Earth is thought to have facilitated production of molecular building blocks of life, but its impact on self-assembly and compartmentalization remains largely unexplored. Here, we investigate dehydration/rehydration of complex coacervates, which are membraneless compartments formed by phase separation of polyelectrolyte solutions. Solution compositions are identified for which tenfold water loss results in maintenance, disappearance, or appearance of coacervate droplets. Systems maintaining coacervates throughout the dehydration process are further evaluated to understand how their compartmentalization properties change with drying. Although added total RNA concentrations increase tenfold, RNA concentration within coacervates remains steady. Exterior RNA concentrations rise, and exchange rates for encapsulated versus free RNAs increase with dehydration. We explain these results in light of the phase diagram, with dehydration-driven ionic strength increase being particularly important in determining coacervate properties. This work shows that wet-dry cycling can alter the phase behavior and protocell-relevant functions of complex coacervates.

Wet-dry cycling is thought to have enabled the production of molecular building blocks of life. Here, the authors investigate the impact of dehydration/rehydration on RNA-containing complex coacervates, which are membraneless compartments formed by phase separation of polyelectrolyte solutions.

Tau Condensates

Many proteins, particularly those that are intrinsically disordered and carry charges have a tendency to undergo liquid liquid phase separation (LLPS). Phase separation is a widespread mechanism by which cells concentrate a set of proteins to perform molecular reactions, and appear to compartmentalize molecular functions. Among the intrinsically disordered proteins are a subset that tend to form solid inclusions in cells and contribute to the pathology of several neurodegenerative diseases. Among this subset is the tau protein, a critically important inclusion in a class of conditions known as the tauopathies, which include Alzheimer's disease. Tau in neurons strongly and selectively associates with RNA species, most notably tRNA with a nanomolar dissociation constant. Furthermore, tau and RNA, under charge matching conditions, undergo LLPS in a process known as complex coacervation. Tau-RNA LLPS is reversible, and can persist for more than 15 h without subsequent fibrilization, although after longer time periods β-sheet content can be detected by thioflavin T. These findings suggest that LLPS tau droplets or condensates can be placed on a pathway to fibrillization and be arrested by solidification or dissolve into a soluble state, depending on the condition at hand, suggesting a regulatory and physiological role for the phase separated state of tau.

Stabilizing Coacervate by Microfluidic Engulfment Induced by Controlled Interfacial Energy

Low interfacial energy, an intrinsic property of complex coacervate, enables the complex coacervate to easily encapsulate desired cargo substances, making it widely used in encapsulation applications. Despite this advantage, the low interfacial energy of the complex coacervate makes it unstable against mechanical mixing, and changes in pH and salt concentration. Hence, a chemical cross-linker is usually added to enhance the stability of the complex coacervate at the expense of sacrificing all intrinsic properties of the coacervate, including phase transition of the coacervate from liquid to solid. In this study, we observed an abrupt increase in the interfacial energy of the coacervate phase in mineral oil. By controlling the interfacial energy of the coacervate phase using a microfluidic device, we successfully created double engulfed PEG-diacrylate (PEGDA) coacervate microparticles, named DEPOT, in which the coacervate is engulfed in a cross-linked PEGDA shell. The engulfed coacervate remained as a liquid phase, retained its original low interfacial energy property to encapsulate the desired cargo substances, and infiltrated into the target site by a simple solvent exchange from oil to water.

A shear-thinning electrostatic hydrogel with antibacterial activity by nanoengineering of polyelectrolytes

Injectable shear-thinning hydrogels can be prepared by the non-covalent interactions between hydrophilic polymers. Although electrostatic force is a typical non-covalent interaction, direct mixing of two oppositely charged polyelectrolytes usually leads to a complex coacervate rather than an injectable hydrogel. Herein, a facile approach is proposed to prepare a shear-thinning hydrogel by nanoengineering of polyelectrolytes. Nanosized cationic micelles with electroneutral shells were prepared by mixing methoxyl poly(ethylene glycol)-block-poly(ε-caprolactone) and poly(ε-caprolactone)-block-poly(hexamethylene guanidine) hydrochloride-block-poly(ε-caprolactone) in an aqueous solution. When sodium carboxymethyl cellulose was added into the micellar solution, the outer poly(ethylene glycol) shell of mixed micelles prevented the instant electrostatic interaction between poly(hexamethylene guanidine) hydrochloride segments and sodium carboxymethyl cellulose, resulting in a homogenous shear-thinning electrostatic (STES) hydrogel. Because of the cationic poly(hexamethylene guanidine) hydrochloride segments, this hydrogel exhibits strong antibacterial activity against both Gram-positive and Gram-negative bacteria. Furthermore, the poly(ε-caprolactone) core of the mixed micelles can efficiently encapsulate a hydrophobic drug. In this work, curcumin-loaded STES hydrogel prepared by this method was used as wound dressing material that can promote wound healing even in infected wounds by further reducing bacterial infection via releasing curcumin. The present study provides a facile strategy to prepare shear-thinning antibacterial hydrogels from polyelectrolytes, which has great potential in biomedical application.

Cation-π Interactions and Their Contribution to Mussel Underwater Adhesion Studied Using a Surface Forces Apparatus: A Mini-Review

Mussel underwater adhesion is a model phenomenon important for the understanding of broader biological adhesion and the development of biomimetic wet adhesives. The catechol moiety of 3,4-dihydroxyphenyl-l-alanine (DOPA) is known to be actively involved in the mechanism of mussel underwater adhesion; however, other underwater adhesion mechanisms are also crucial. The surface forces apparatus (SFA) has often been used to explore the contributions of other mechanisms to mussel underwater adhesion; e.g., recent SFA-based nanomechanical studies have revealed that cation-π interactions, one of the strongest intermolecular interactions in water, are the pivotal interactions of adhesive proteins involved in underwater mussel adhesion. This mini-review surveys recent research on cation-π interactions and their contributions to strong mussel underwater adhesion, shedding light on some biological processes and facilitating the development of biomedical adhesives.

Dynamic Synthetic Cells Based on Liquid-Liquid Phase Separation

Living cells have long been a source of inspiration for chemists. Their capacity of performing complex tasks relies on the spatiotemporal coordination of matter and energy fluxes. Recent years have witnessed growing interest in the bottom-up construction of cell-like models capable of reproducing aspects of such dynamic organisation. Liquid-liquid phase-separation (LLPS) processes in water are increasingly recognised as representing a viable compartmentalisation strategy through which to produce dynamic synthetic cells. Herein, we highlight examples of the dynamic properties of LLPS used to assemble synthetic cells, including their biocatalytic activity, reversible condensation and dissolution, growth and division, and recent directions towards the design of higher-order structures and behaviour.

Upper Critical Solution Temperature (UCST) Behavior of Coacervate of Cationic Protamine and Multivalent Anions

Complex coacervation is an emerging liquid/liquid phase separation (LLPS) phenomenon that behaves as a membrane-less organelle in living cells. Yet while one of the critical factors for complex coacervation is temperature, little analysis and research has been devoted to the temperature effect on complex coacervation. Here, we performed a complex coacervation of cationic protamine and multivalent anions (citrate and tripolyphosphate (TPP)). Both mixtures (i.e., protamine/citrate and protamine/TPP) underwent coacervation in an aqueous solution, while a mixture of protamine and sodium chloride did not. Interestingly, the complex coacervation of protamine and multivalent anions showed upper critical solution temperature (UCST) behavior, and the coacervation of protamine and multivalent anions was reversible with solution temperature changes. The large asymmetry in molecular weight between positively charged protamine (~4 kDa) and the multivalent anions (<0.4 kDa) and strong electrostatic interactions between positively charged guanidine residues in protamine and multivalent anions were likely to contribute to UCST behavior in this coacervation system.

Electrostatically Interactive Injectable Hydrogels for Drug Delivery

BACKGROUND

Several injectable hydrogels have been developed extensively for a broad range of biomedical applications. Injectable hydrogels forming in situ through the change in external stimuli have the distinct properties of easy management and minimal invasiveness, and thus provide the advantage of bypassing surgical procedures for administration resulting in better patient compliance.

METHODS

The injectable in situ-forming hydrogels can be formed irreversibly or reversibly under physiological stimuli. Among several external stimuli that induce formation of hydrogels in situ, in this review, we focused on the electrostatic interactions as the most simple and interesting stimulus.

RESULTS

Currently, numerous polyelectrolytes have been reported as potential electrostatically interactive in situ-forming hydrogels. In this review, a comprehensive overview of the rapidly developing electrostatically interactive in situ-forming hydrogels, which are produced by various anionic and cationic polyelectrolytes such as chitosan, celluloses, and alginates, has been outlined and summarized. Further, their biomedical applications have also been discussed.

CONCLUSION

The review concludes with perspectives on the future of electrostatically interactive in situ-forming hydrogels.

Reversible generation of coacervate droplets in an enzymatic network

Cells can control the assembly and disassembly of membraneless organelles by enzymatic processes, but similar control has not been achieved in vitro yet. Here we develop ATP-based coacervate droplets as artificial membraneless organelles that can be fully controlled by two cooperating enzymes. Droplets can be generated within a minute following the addition of phosphoenolpyruvate as a substrate, and they can be dissolved within tens of seconds by adding glucose as the second substrate. We show how the rates of droplet generation and dissolution can be tuned by varying the enzyme and substrate concentrations, and we support our findings with a kinetic model of the underlying enzymatic reaction network. As all steps of the coacervate droplet life cycle, including nucleation, coarsening, and dissolution, occur under the same reaction conditions, the cycle can be repeated multiple times. In addition, by carefully balancing the rates of both enzymatic reactions, our system can be programmed to either form or dissolve droplets at specified times, acting as a chemical timer.

Simple peptide coacervates adapted for rapid pressure-sensitive wet adhesion

We report here that a dense liquid formed by spontaneous condensation, also known as simple coacervation, of a single mussel foot protein-3S-mimicking peptide exhibits properties critical for underwater adhesion. A structurally homogeneous coacervate is deposited on underwater surfaces as micrometer-thick layers, and, after compression, displays orders of magnitude higher underwater adhesion at 2 N m^-1 than that reported from thin films of the most adhesive mussel-foot-derived peptides or their synthetic mimics. The increase in adhesion efficiency does not require nor rely on post-deposition curing or chemical processing, but rather represents an intrinsic physical property of the single-component coacervate. Its wet adhesive and rheological properties correlate with significant dehydration, tight peptide packing and restriction in peptide mobility. We suggest that such dense coacervate liquids represent an essential adaptation for the initial priming stages of mussel adhesive deposition, and provide a hitherto untapped design principle for synthetic underwater adhesives.

Phototransformative Supramolecular Assembly of Amphiphilic Diarylethenes Realized by a Combination of Photochromism and Lower Critical Solution Temperature Behavior

Amphiphilic diarylethenes bearing octyloxycarbonyl and N-octylcarbamoyl groups have been designed and synthesized. These ester- and amide-linked compounds form micrometer-sized supramolecular assemblies in water, and these assemblies exhibit photoinduced macroscopic morphological transformations upon alternate irradiation with UV and visible light. The ester-linked diarylethene showed a transformation between colorless spheres and a red-purple hazy fringe, whereas the microspheres of the amide-linked diarylethene showed changes in color, size, and shape, but the spheres did not show division. TEM images revealed that the spheres of the open-ring isomers have coacervate structures, with bicontinuous aqueous and organic phases. The closed-ring isomers of the ester- and amide-linked compounds were found to form nanofibers and thin layers, respectively. These compounds showed absorption spectral shifts at temperatures corresponding to the lower critical solution temperature (LCST) transition. This morphological transformation can be rationalized as the photoinduced phase transition between the high- and low-temperature phases of the LCST transition. These results open up a new avenue for the design of phototransformative supramolecular assemblies based on a combination of photochromism and LCST behavior.

RNA stores tau reversibly in complex coacervates

Nonmembrane-bound organelles that behave like liquid droplets are widespread among eukaryotic cells. Their dysregulation appears to be a critical step in several neurodegenerative conditions. Here, we report that tau protein, the primary constituent of Alzheimer neurofibrillary tangles, can form liquid droplets and therefore has the necessary biophysical properties to undergo liquid-liquid phase separation (LLPS) in cells. Consonant with the factors that induce LLPS, tau is an intrinsically disordered protein that complexes with RNA to form droplets. Uniquely, the pool of RNAs to which tau binds in living cells are tRNAs. This phase state of tau is held in an approximately 1:1 charge balance across the protein and the nucleic acid constituents, and can thus be maximal at different RNA:tau mass ratios, depending on the biopolymer constituents involved. This feature is characteristic of complex coacervation. We furthermore show that the LLPS process is directly and sensitively tuned by salt concentration and temperature, implying it is modulated by both electrostatic interactions between the involved protein and nucleic acid constituents, as well as net changes in entropy. Despite the high protein concentration within the complex coacervate phase, tau is locally freely tumbling and capable of diffusing through the droplet interior. In fact, tau in the condensed phase state does not reveal any immediate changes in local protein packing, local conformations and local protein dynamics from that of tau in the dilute solution state. In contrast, the population of aggregation-prone tau as induced by the complexation with heparin is accompanied by large changes in local tau conformations and irreversible aggregation. However, prolonged residency within the droplet state eventually results in the emergence of detectable β-sheet structures according to thioflavin-T assay. These findings suggest that the droplet state can incubate tau and predispose the protein toward the formation of insoluble fibrils.

Tau is a common neuronal protein that, under circumstances and conditions not well understood to date, self-assembles into intracellular aggregates in several neurodegenerative diseases including Alzheimer disease. These aggregates are formed of fibrous polymers. The mechanism by which this critical transition from a soluble protein to insoluble fibrous material occurs is unknown. We have discovered a novel state in which many tau molecules become compacted into a protein-rich droplet while maintaining their solubility and native-like protein conformations. Chemists refer to this dense liquid droplet state as a complex coacervate phase, and it is held together by the opposite charges of their constituents, ions, and water. In the case of the tau protein, the oppositely charged constituent is RNA. Indeed, we found that in human neuronal cell culture, tau selectively binds to a category of RNA known as tRNA. Interestingly, tau and RNA favorably condense to a complex coacervate phase when the charges between them are matched and at elevated temperatures, such that tau-RNA droplets could be observed at physiologically viable protein concentrations simply by increasing the temperature from room to physiological temperatures. When the tau-RNA–dense droplets are incubated together over time, tau transitions to a conformation similar to that found in pathological fibers. Our experiments therefore demonstrate physicochemical properties of tau that may predispose it to undergo changes associated with neurodegenerative disease.