Linear Viscoelasticity of Polyelectrolyte Complex Coacervates

Brittle-to-Ductile Transitions of Polyelectrolyte Complexes: Humidity, Temperature, and Salt

Polyelectrolyte complexation is an entropically driven, associative phase separation that results in a polymer-rich polyelectrolyte complex (PEC) and a polymer-poor supernatant. PECs show promise as a new class of sustainable materials since they can be processed using aqueous solutions rather than organic solvents. Previous reports have looked at the mechanical properties and glass transitions of PECs as a function of temperature, relative humidity (rH), and salt concentration (CS), but establishing a universal understanding of how these parameters affect PEC mechanics has yet to be achieved. We examined the effects of temperature, rH, and CS on the mechanical properties of PECs formed from poly(methacrylic acid) and poly(trimethyl aminoethyl methacrylate) with a goal of establishing design rules for their mechanical response. Relative humidity was shown to have the most dramatic effect on the mechanical properties, with temperature and salt concentration having far less of an impact. Furthermore, we observed that the glass transition of PECs is tied to both temperature and relative humidity, creating a glass transition rHg/T g line that can be modulated by added salt. Finally, we looked at the thermodynamics behind the glass transition of PECs, which yielded similar energies as the condensation of water. We propose the use of water and/or salt as a low energy and efficient method of processing PECs for various applications.

Network-forming phase separation of oppositely charged polyelectrolytes forming coacervates in a solvent

The formation of coacervates through phase separation of oppositely charged polyelectrolytes (PEs) is critical for understanding biological condensates and developing responsive materials. Traditionally, coacervates are viewed as spherical droplets with growth dynamics resembling liquid-liquid phase separation. However, our fluid particle dynamics simulations incorporating hydrodynamic and electrostatic interactions challenge this perspective. Here, we find that oppositely charged PEs form a percolated network even in semi-dilute solutions, coarsening with a unique growth law, ℓ ∝ t1/2. This self-similarity, absent for neutral polymers in poor solvents, arises because PEs in good solvents exhibit weaker, longer-range attractions due to spatial charge inhomogeneity under global charge neutrality. This results in a lower density of the PEs-rich phase and reduced interfacial tension. Increased charge asymmetry further slows network coarsening. Additionally, coacervate droplets initially display irregular shapes due to weak interfacial tension, transitioning slowly to spherical forms. Our research provides new insights into coacervate morphology and coarsening dynamics.

Viscoelasticity of globular protein-based biomolecular condensates

The phase separation of biomolecules into biomolecular condensates has emerged as a ubiquitous cellular process. Understanding how intrinsically disordered protein sequence controls condensate formation and material properties has provided fundamental biological insights and led to the development of functional synthetic condensates. While these studies provide a valuable framework to understand subcellular organization via phase separation they have largely ignored the presence of folded domains and their impact on condensate properties. We set out to determine how the distribution of sticker interactions across a globular protein contributes to rheological properties of condensates and to what extent globular protein-containing condensates differ from those formed from two disordered components. We designed three variants of green fluorescent protein with different charge patterning and used dynamic light scattering microrheology to measure the viscoelastic spectrum of coacervates formed with poly-lysine over a timescale of 10-6 to 10 seconds, elucidating the response of protein condensates in this range for the first time. We further showed that the phase behavior and rheological characteristics of the condensates varied as a function of both protein charge distribution and polymer/protein ratio, behavior that was distinct to condensates formed with folded domains. Together, this work enhances our fundamental understanding of dynamic condensed biomaterials across biologically relevant length- and time-scales.

Diffusive Trends in Concentrated Oppositely-Charged Polyelectrolyte Solutions and Onset of Glassy Dynamics

We utilize single particle tracking studies to investigate the diffusion of polylysine through concentrated matrices of cationic polylysine and anionic polyglutamic acid with no added salts. These studies show that diffusivity has a strong apparently exponential dependence on concentration in crowded systems that does not appear to be a function of the charge sign. These trends are consistent in both single-phase systems prepared at concentrated conditions and polymer-rich coacervate phases formed from dilute phase-separating systems. The likely origin of this behavior is the onset of glassy dynamics spurred by a decrease in plasticization by water and the large excluded volume associated with charge-bearing species. This effect can be contextualized through free volume-based theories such as the Vrentas-Duda model. Overall, we obtain dynamic behavior that is distinctly different from behavior observed in more dilute systems and warrants further investigation.

Impact of a Lightly Branched Star Polyelectrolyte Architecture on Polyelectrolyte Complexes

The effect of charge density in blocky and statistical linear polyelectrolytes on polyelectrolyte complex (PEC) properties has been studied with the finding that increased charge density in a polyelectrolyte tends to increase the salt resistance and modulus of a PEC across various polyelectrolyte pairs. Here, we demonstrate the ability to orthogonally alter PEC salt resistance while maintaining rheological properties and internal structure by going from linear to lightly branched architectures with similar total degrees of polymerization. Using a model system built around glycidyl methacrylate (GMA) and thiol-epoxy "click" functionalization, we create a library of homologous linear, 4-armed, 6-armed, and 8-armed star polyelectrolytes. The PECs formed from these model polyelectrolyte pairs are then characterized via optical microscopy, rheology, and small-angle X-ray scattering to evaluate their salt resistance, mechanical properties, and internal structure. We argue that our results are due to the difference between linear charge density or charge per unit length along backbone segments for each polyelectrolyte and spatial charge density, the number of charges per unit volume of the polyelectrolyte prior to complexation. Our findings suggest that linear charge density is the dominant factor in determining intermolecular interactions of the complex, leading to identical rheological and structural behavior, whereas the spatial charge density primarily influences the stability of the complexes. These distinct mechanisms for altering various sought-after PEC properties offer greater potential applications in precision design of polyelectrolyte complex materials.

Probing the surface charge of condensates using microelectrophoresis

Biomolecular condensates play an important role in cellular organization. Coacervates are commonly used models that mimic the physicochemical properties of biomolecular condensates. The surface of condensates plays a key role in governing molecular exchange between condensates, accumulation of species at the interface, and the stability of condensates against coalescence. However, most important surface properties, including the surface charge and zeta potential, remain poorly characterized and understood. The zeta potential of coacervates is often measured using laser doppler electrophoresis, which assumes a size-independent electrophoretic mobility. Here, we show that this assumption is incorrect for liquid-like condensates and present an alternative method to study the electrophoretic mobility of coacervates and in vitro condensate models by microelectrophoresis and single-particle tracking. Coacervates have a size-dependent electrophoretic mobility, originating from their fluid nature, from which a well-defined zeta potential is calculated. Interestingly, microelectrophoresis measurements reveal that polylysine chains are enriched at the surface of polylysine/polyaspartic acid complex coacervates, which causes the negatively charged protein ɑ-synuclein to adsorb and accumulate at the interface. Addition of ATP inverts the surface charge, displaces ɑ-synuclein from the surface and may help to suppress its interface-catalyzed aggregation. Together, these findings show how condensate surface charge can be measured and altered, making this microelectrophoresis platform combined with automated single-particle tracking a promising characterization technique for both biomolecular condensates and coacervate protocells.

Structure-Property Relationships of Granular Hybrid Hydrogels Formed through Polyelectrolyte Complexation

Hybrid hydrogels are hydrogels that exhibit heterogeneity in the network architecture by means of chemical composition and/or microstructure. The different types of interactions, together with structural heterogeneity, which can be created on different length scales, determine the mechanical properties of the final material to a large extent. In this work, the microstructure-mechanical property relationships for a hybrid hydrogel that contains both electrostatic and covalent interactions are investigated. The hybrid hydrogel is composed of a microphase-separated polyelectrolyte complex network (PEC) made of poly(4-styrenesulfonate) and poly(diallyldimethylammonium chloride) within a soft and elastic polyacrylamide hydrogel network. The system exhibits a granular structure, which is attributed to the liquid-liquid phase separation into complex coacervate droplets induced by the polymerization and the subsequent crowding effect of the polyacrylamide chains. The coacervate droplets are further hardened into PEC granules upon desalting the hydrogel. The structure formation is confirmed by a combination of electron microscopic imaging and molecular dynamics simulations. The interpenetration of both networks is shown to enhance the toughness of the resulting hydrogels due to the dissipative behavior of the PEC through the rupture of electrostatic interactions. Upon cyclic loading-unloading, the hydrogels show recovery of up to 80% of their original dissipative behavior in less than 300 s of rest with limited plasticity. The granular architecture and the tough and self-recoverable properties of the designed hybrid networks make them good candidates for applications, such as shape-memory materials, actuators, biological tissue mimics, and elastic substrates for soft sensors.

Simple and complex coacervation in systems involving plant proteins

Plant-based foods are gaining popularity as alternatives to meat and dairy products due to sustainability and health concerns. As a consequence, there is a renewed interest in the phase behaviour of plant proteins and of mixtures of plant proteins and polysaccharides, in particular in the cases where coacervation is found to occur, i.e., liquid-liquid phase separation (LLPS) into two phases, one of which is rich in biopolymers and one of which is poor in biopolymer. Here we review recent research into both simple and complex coacervation in systems involving plant proteins, and their applications in food- as well as other technologies, such as microencapsulation, microgel production, adhesives, biopolymer films, and more.

Determinants of viscoelasticity and flow activation energy in biomolecular condensates

The form and function of biomolecular condensates are intimately linked to their material properties. Here, we integrate microrheology with molecular simulations to dissect the physical determinants of condensate fluid phase dynamics. By quantifying the timescales and energetics of network relaxation in a series of heterotypic viscoelastic condensates, we uncover distinctive roles of sticker motifs, binding energy, and chain length in dictating condensate dynamical properties. We find that the mechanical relaxation times of condensate-spanning networks are determined by both intermolecular interactions and chain length. We demonstrate, however, that the energy barrier for network reconfiguration, termed flow activation energy, is independent of chain length and only varies with the strengths of intermolecular interactions. Biomolecular diffusion in the dense phase depends on a complex interplay between viscoelasticity and flow activation energy. Our results illuminate distinctive roles of chain length and sequence-specific multivalent interactions underlying the complex material and transport properties of biomolecular condensates.

Design Rules for the Sequestration of Viruses into Polypeptide Complex Coacervates

Encapsulation is a strategy that has been used to facilitate the delivery and increase the stability of proteins and viruses. Here, we investigate the encapsulation of viruses via complex coacervation, which is a liquid-liquid phase separation resulting from the complexation of oppositely charged polymers. In particular, we utilized polypeptide-based coacervates and explored the effects of peptide chemistry, chain length, charge patterning, and hydrophobicity to better understand the effects of the coacervating polypeptides on virus incorporation. Our study utilized two nonenveloped viruses, porcine parvovirus (PPV) and human rhinovirus (HRV). PPV has a higher charge density than HRV, and they both appear to be relatively hydrophobic. These viruses were compared to characterize how the charge, hydrophobicity, and patterning of chemistry on the surface of the virus capsid affects encapsulation. Consistent with the electrostatic nature of complex coacervation, our results suggest that electrostatic effects associated with the net charge of both the virus and polypeptide dominated the potential for incorporating the virus into a coacervate, with clustering of charges also playing a significant role. Additionally, the hydrophobicity of a virus appears to determine the degree to which increasing the hydrophobicity of the coacervating peptides can enhance virus uptake. Nonintuitive trends in uptake were observed with regard to both charge patterning and polypeptide chain length, with these parameters having a significant effect on the range of coacervate compositions over which virus incorporation was observed. These results provide insights into biophysical mechanisms, where sequence effects can control the uptake of proteins or viruses into biological condensates and provide insights for use in formulation strategies.

Effect of Polyelectrolyte Charge Density on the Linear Viscoelastic Behavior and Processing of Complex Coacervate Adhesives

It is well-known that the phase behavior and physicochemical and adhesive properties of complex coacervates are readily tuneable with the salt concentration of the medium. For toxicity reasons, however, the maximum applicable salt concentration in biomedical applications is typically low. Consequently, other strategies must be implemented in order to optimize the properties of the resulting complex coacervates. In this work, the effect of the charge density of a strong polyanion on the properties of complex coacervates was studied. To control this charge density, statistical anionic/charge-neutral hydrophilic copolymers were synthesized by means of an elegant protection/deprotection strategy and subsequently complexed with a strong polycation. The resulting complexes were observed to have an increasing water content as well as faster relaxation dynamics, with either increasing salt concentration or decreasing charge density. Time-salt and time-salt-charge density superpositions could be performed and showed that the relaxation mechanism of the complex coacervates remained unchanged. When the charge density was decreased, lower salt concentration complexes became suitable for viscoelastic adhesion with improved injectability. Such complex coacervates are promising candidates for injectable biomedical adhesives.

Hydrophobically modified complex coacervates for designing aqueous pressure-sensitive adhesives

The rheology of complex coacervates can be elegantly tuned via the design and control of specific non-covalent hydrophobic interactions between the complexed polymer chains. The well-controlled balance between elasticity and energy dissipation makes complex coacervates perfect candidates for pressure-sensitive adhesives (PSAs). In this work, the polyanion poly(3-sulfopropyl methacrylate) (PSPMA) and the polycation quaternized poly(4-vinylpyridine) (QP4VP) were used to prepare complex coacervates in water. Progressive increase of hydrophobicity is introduced to the polyanion via partial deprotection of the protected precursor. Hence, the polymer chains in the complex coacervates can interact via both electrostatic (controlled by the amount of salt) and hydrophobic (controlled by the deprotection degree) interactions. It was observed that: (i) a rheological time-salt-hydrophobicity superposition principle is applicable, and can be used as a predictive tool for rheology, (ii) the slowdown of the stress relaxation dynamics, due to the increase of hydrophobic stickers (lower deprotection degree), can be captured by the sticky-Rouse model, and (iii) the systematic variation of hydrophobic stickers, amount of salt, and molecular weight of the polymers, enables the identification of optimizing parameters to design aqueous PSA systems. The presented results offer new pathways to control the rheology of complex coacervates and their applicability as PSAs.

A Perspective on the Glass Transition and the Dynamics of Polyelectrolyte Multilayers and Complexes

Polyelectrolyte multilayers (PEMs) or polyelectrolyte complexes (PECs), formed by layer-by-layer assembly or the mixing of oppositely charged polyelectrolytes (PEs) in aqueous solution, respectively, have potential applications in health, energy, and the environment. PEMs and PECs are very tunable because their structure and properties are influenced by factors such as pH, ionic strength, salt type, humidity, and temperature. Therefore, it is increasingly important to understand how these factors affect PECs and PEMs on a molecular level. In this Feature Article, we summarize our contributions to the field in the development of approaches to quantify the swelling, thermal properties, and dynamic mechanical properties of PEMs and PECs. First, the role of water as a plasticizer and in the glass-transition temperature (Tg) in both strong poly(diallyldimethylammonium)/poly(sodium 4-styrenesulfonate) (PDADMA/PSS) and weak poly(allylamine hydrochloride)/poly(acrylic acid) (PAH/PAA) systems is presented. Then, factors influencing the dynamics of PECs and PEMs are discussed. We also reflect on the swelling of PEMs in response to different salts and solvent additives. Last, the nature of water's microenvironment in PEMs/PECs is discussed. A special emphasis is placed on experimental techniques, along with molecular simulations. Taken together, this review presents an outlook and offers recommendations for future research directions, such as studying the additional effects of hydrogen-bonding hydrophobic interactions.

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.

Effect of Dynamically Arrested Domains on the Phase Behavior, Linear Viscoelasticity and Microstructure of Hyaluronic Acid - Chitosan Complex Coacervates

Complex coacervates make up a class of versatile materials formed as a result of the electrostatic associations between oppositely charged polyelectrolytes. It is well-known that the viscoelastic properties of these materials can be easily altered with the ionic strength of the medium, resulting in a range of materials from free-flowing liquids to gel-like solids. However, in addition to electrostatics, several other noncovalent interactions could influence the formation of the coacervate phase depending on the chemical nature of the polymers involved. Here, the importance of intermolecular hydrogen bonds on the phase behavior, microstructure, and viscoelasticity of hyaluronic acid (HA)-chitosan (CHI) complex coacervates is revealed. The density of intermolecular hydrogen bonds between CHI units increases with increasing pH of coacervation, which results in dynamically arrested regions within the complex coacervate, leading to elastic gel-like behavior. This pH-dependent behavior may be very relevant for the controlled solidification of complex coacervates and thus for polyelectrolyte material design.

Bulk Biopolyelectrolyte Complexes from Homopolypeptides: Solid "Salt Bridges"

Salt bridges, pairings between oppositely charged amino acids, are dispersed throughout proteins to assist folding and interactions. Biopolyelectrolyte complexes (BioPECs) were made between the homopolypeptides poly-l-arginine (PLR) and poly-l-lysine (PLK) with sodium triphosphate (STPP), as well as from polypeptide-only combinations. Viscoelastic measurements on these high salt bridge density materials showed many were solid, even glassy, in nature. Although the polypeptide-phosphate complexes had similar moduli at room temperature, the PLR-STPP complex displayed an unusual melting event above 70 °C not seen in PLK-STPP. This event was supported with differential scanning calorimetry. Infrared spectroscopy showed the PLK-STPP system contained β-sheets, while PLR-STPP did not. Stoichiometric, macroscopic BioPECs of PLR and PLK with poly-l-aspartic acid (PLD) and poly-l-glutamic acid (PLE) were made. PLR-PLD was found to undergo a melting event similar to that in PLR-STPP. ATR-FTIR studies showed that BioPECs made with PLD do not contain β-sheets, while those composed of PLE do. This work illustrates an expanded palette of unique properties from these biomaterials, such as strong viscoelastic differences between PECs containing PLE and PLD, even though they differ by only one carbon on the side chain.

Polysaccharide-based polyampholyte complex formation: Investigating the role of intra-chain interactions

The difference in inter-chain and intra-chain electrostatic attraction was investigated in polyelectrolyte and polyampholyte electrostatic complex formation. Three polymers with similar backbone molecular structures including chitosan (Ch) polycation, carboxymethyl cellulose (CMCe) polyanion, and carboxymethyl chitosan (CMCh) polyampholyte were used for this purpose. The turbidimetric, water content, and rheological measurements for polyampholyte self-complex showed more dependence on the ionic strength rather than the polyelectrolyte one. The degree of dissociation (α), dissociation constant (pKa), and intrinsic persistence length were calculated by applying the Katchalsky-Lifson model to potentiometric data. We studied the gyration radii as a function of Debye length and observed the polyampholyte chain contractions due to the intra-chain electrostatic attractions, which minimize the entropic gain of the inter-chain complex formation. This is in accordance with the decrease in pKa by α_c for CMCh which is the opposite of that for the Ch and CMCe samples. We also found that the polyampholyte has less intrinsic and electrostatic persistence length compared with both polyanion and polycation with similar chain structures indicating the impact of the inter-chain electrostatic interaction on the complex properties. This study deepens our insight about the behavior of CMCh and the nature of difference between CMCh and Ch/CMCe electrostatic complexes.

Rheology and Gelation of Hyaluronic Acid/Chitosan Coacervates

Hyaluronic acid (HA) and chitosan (CHI) are biopolyelectrolytes which are interesting for both the medical and polymer physics communities due to their biocompatibility and semi-flexibility, respectively. In this work, we demonstrate by rheology experiments that the linear viscoelasticity of HA/CHI coacervates depends strongly on the molecular weight of the polymers. Moduli for coacervates were found significantly higher than those of individual HA and CHI physical gels. A remarkable 1.5-fold increase in moduli was noted when catechol-conjugated HA and CHI were used instead. This was attributed to the conversion of coacervates to chemical gels by oxidation of 3,4-dihydroxyphenylalanine (DOPA) groups in HA and CHI to di-DOPA crosslinks. These rheological results put HA/CHI coacervates in the category of strong candidates as injectable tissue scaffolds or medical adhesives.

Valence-induced jumps in coacervate properties

Spontaneous phase separation, or coacervation, of oppositely charged macromolecules is a powerful and ubiquitous mechanism for the assembly of natural and synthetic materials. Two critical triggering phenomena in coacervation science and technology are highlighted here. The first is the transition from one (mixed) to two (separated) phases of polyelectrolytes coacervated with small molecules upon the addition of one or two charges per molecule. The second is a large jump in coacervate modulus and viscosity mediated by the addition of just one additional charge to a three-charged system. This previously unknown viscoelastic transition is relevant to those aspects of disease states that are characterized by abnormal mechanical properties and irreversible assembly.

High-throughput microscopy to determine morphology, microrheology, and phase boundaries applied to phase separating coacervates

Evolution of composition, rheology, and morphology during phase separation in complex fluids is highly coupled to rheological and mass transport processes within the emerging phases, and understanding this coupling is critical for materials design of multiphase complex fluids. Characterizing these dependencies typically requires careful measurement of a large number of equilibrium and transport properties that are difficult to measure in situ as phase separation proceeds. Here, we propose and demonstrate a high-throughput microscopy platform to achieve simultaneous, in situ mapping of time-evolving morphology and microrheology in phase separating complex fluids over a large compositional space. The method was applied to a canonical example of polyelectrolyte complex coacervation, whereby mixing of oppositely charged species leads to liquid-liquid phase separation into distinct solute-dense and dilute phases. Morphology and rheology were measured simultaneously and kinetically after mixing to track the progression of phase separation. Once equilibrated, the dense phase viscosity was determined to high compositional accuracy using passive probe microrheology, and the results were used to derive empirical relationships between the composition and viscosity. These relationships were inverted to reconstruct the dense phase boundary itself, and further extended to other mixture compositions. The resulting predictions were validated by independent equilibrium compositional measurements. This platform paves the way for rapid screening and formulation of complex fluids and (bio)macromolecular materials, and serves as a critical link between formulation and rheology for multi-phase material discovery.

DNA dynamics in complex coacervate droplets and micelles

Single stranded DNA (ssDNA), or another polyanion, can be mixed with polycations to form liquid-like complex coacervates. When the polycations are replaced by cationic-neutral diblock copolymers, complex coacervate core micelles (C3Ms) can be formed instead. In both complex coacervates and C3Ms, dynamics plays an important role. Yet, to date, the effect of chain length on the dynamics effect is still not fully understood. The DNA complexes provide a versatile platform to further elucidate these chain length effects because the DNA is monodisperse and its length can be easily adapted. Therefore, we study in this paper the dynamics of fluorescently labelled ssDNA in both complex coacervate droplets and micelles. The DNA dynamics in the complex coacervate droplets is probed by fluorescence recovery after photobleaching (FRAP). We observe that the DNA diffusion coefficient depends more strongly on the DNA length than predicted by the sticky Rouse model and we show that this can be partly explained by changes in complex coacervate density, but that also other factors might play a role. We measure the molecular exchange of C3Ms by making use of Förster resonance energy transfer (FRET) and complement these measurements with Langevin dynamics simulations. We conclude that chain length polydispersity is the main cause of a broad distribution of exchange rates. We hypothesise that the different exchange rates that we observe for the monodisperse DNA are mainly caused by differences in dye interactions and show that the dye can indeed have a large effect on the C3M exchange. In addition, we show that a new description of the C3M molecular exchange is required that accounts among others for the effect of the length of the oppositely charged core species. Together our findings can help to better understand the dynamics in both specific DNA systems and in complex coacervate droplets and micelles in general.

Sodium dodecyl sulfate modulates the structure and rheological properties of Pluronic F108-poly(acrylic acid) coacervates)

Micelles formed within coacervate phases can impart functional properties, but it is unclear if this micellization provides mechanical reinforcement of the coacervate whereby the micelles act as high functionality crosslinkers. Here, we examine how sodium dodecyl sulfate (SDS) influences the structure and properties of Pluronic F108-polyacrylic acid (PAA) coacervates as SDS is known to decrease the aggregation number of Pluronic micelles. Increasing the SDS concentration leads to larger water content in the coacervate and an increase in the relative concentration of PAA to the other solids. Rheological characterization with small angle oscillatory shear (SAOS) demonstrates that these coacervates are viscoelastic liquids with the moduli decreasing with the addition of the SDS. The loss factor (tan δ) initially increases linearly with the addition of SDS, but a step function increase in the loss factor occurs near the reported CMC of SDS. However, this change in rheological properties does not appear to be correlated with any large scale structural differences in the coacervate as determined by small angle X-ray scattering (SAXS) with no signature of Pluronic micelles in the coacervate when SDS concentration is >4 mM during formation of the coacervate, which is less than that observed (6 mM SDS) in initial Pluronic F108 solution despite the higher polymer concentration in the coacervate. These results suggest that the mechanical properties of polyelectrolyte-non-ionic surfactant coacervates are driven by the efficicacy of binding between the complexing species driving the coacervate, which can be disrupted by competitive binding of the SDS to the Pluronic.

Programmable viscoelasticity in protein-RNA condensates with disordered sticker-spacer polypeptides

Liquid-liquid phase separation of multivalent proteins and RNAs drives the formation of biomolecular condensates that facilitate membrane-free compartmentalization of subcellular processes. With recent advances, it is becoming increasingly clear that biomolecular condensates are network fluids with time-dependent material properties. Here, employing microrheology with optical tweezers, we reveal molecular determinants that govern the viscoelastic behavior of condensates formed by multivalent Arg/Gly-rich sticker-spacer polypeptides and RNA. These condensates behave as Maxwell fluids with an elastically-dominant rheological response at shorter timescales and a liquid-like behavior at longer timescales. The viscous and elastic regimes of these condensates can be tuned by the polypeptide and RNA sequences as well as their mixture compositions. Our results establish a quantitative link between the sequence- and structure-encoded biomolecular interactions at the microscopic scale and the rheological properties of the resulting condensates at the mesoscale, enabling a route to systematically probe and rationally engineer biomolecular condensates with programmable mechanics.

A simple simulation model for complex coacervates

When oppositely charged polyelectrolytes mix in an aqueous solution, associative phase separation gives rise to coacervates. Experiments reveal the phase diagram for such coacervates, and determine the impact of charge density, chain length and added salt. Simulations often use hybrid MC-MD methods to produce such phase diagrams, in support of experimental observations. We propose an idealized model and a simple simulation technique to investigate coacervate phase behavior. We model coacervate systems by charged bead-spring chains and counterions with short-range repulsions, of size equal to the Bjerrum length. We determine phase behavior by equilibrating a slab of concentrated coacervate with respect to swelling into a dilute phase of counterions. At salt concentrations below the critical point, the counterion concentration in the coacervate and dilute phases are nearly the same. At high salt concentrations, we find a one-phase region. Along the phase boundary, the total concentration of beads in the coacervate phase is nearly constant, corresponding to a "Bjerrum liquid''. This result can be extended to experimental phase diagrams by assigning appropriate volumes to monomers and salts.

Charge Density and Hydrophobicity-Dominated Regimes in the Phase Behavior of Complex Coacervates

The role of hydrophobicity, and particularly of nonionic hydrophobic comonomers, in the phase behavior of polyelectrolyte complex coacervates is not well-understood. Here, we address this problem by synthesizing a library of polymers with a wide range of charge densities and nonionic hydrophobic side chain lengths, and characterizing their phase behavior by optical turbidity. The polymers were prepared by postpolymerization modification of poly(N-acryloxy succinimide), targeting charge densities between 40 and 100% and nonionic aliphatic side chains with lengths from 0 to 12 carbons long. Turbidity measurements on pairs of polycations and polyanions with matched charge densities and nonionic side chain lengths revealed a complex salt response with distinct charge density-dominated and hydrophobicity-dominated regimes. The polymer solubilities were not directly correlated with the phase behavior of the coacervates, indicating the difficulty of understanding the coacervate phase behavior in terms of the polymer-water interaction parameter. This result suggests that there is significant room for further work to understand the mechanisms by which specific molecular-scale interactions moderate the phase behavior of complex coacervates.

Independent and Synergistic Modulations of Viscoelasticity and Stiffness of Dynamically Cross-Linked Cell-Encapsulating ClickGels by Covalently Tethered Polymer Brushes

We report independent and synergistic modulations of the stiffness and viscoelasticity of ClickGels, formed by a combination of the bio-orthogonal covalent and dynamic noncovalent cross-linking, by covalently incorporating nonionic, zwitterionic, or anionic polymer brushes. Tethering nonionic and zwitterionic brushes at the cost of noncovalent cross-linking increased stiffness and slowed stress relaxation, respectively, without altering the other properties. Meanwhile, tethering anionic brushes significantly increased ClickGel stiffness, while also slowing its stress relaxation. ClickGels with faster stress relaxation, not reduced stiffness, promoted short-term (24 h) viability and YAP/TAZ nuclear localization of encapsulated bone marrow-derived stromal cells (BMSCs). In contrast, ClickGel stiffness, not viscoelasticity, inversely correlated to the short-term dehydrogenase activity of encapsulated BMSCs. This work highlights the role of the ionic state of polymer brushes in modulating the hydrogel elastic modulus and viscoelasticity and establishes the brush-modified ClickGel as a facile and reproducible tool for manipulating mechanical cues of the synthetic cellular niche.

Molecular Mass Dependence of Interfacial Tension in Complex Coacervation

The interfacial tension of coacervates, the liquidlike phase composed of oppositely charged polymers that coexists at equilibrium with a supernatant, forms the basis for multiple technologies. Here we present a comprehensive set of experiments and molecular dynamics simulations to probe the effect of molecular mass on interfacial tension γ, far from the critical point, and derive γ=γ_{∞}(1-h/N), where N is the degree of polymerization, γ_{∞} is the infinite molecular mass limit, and h is a constant that physically corresponds to the number of monomers of one chain within the coacervate correlation volume.

Preclinical evaluation of a regenerative immiscible bioglue for vesico-vaginal fistula

Currently, there are no clinically available tissue adhesives that work effectively in the fluid-rich and highly dynamic environments of the human body, such as the urinary system. This is especially relevant to the management of vesico-vaginal fistula, and developing a high-performance tissue adhesive for this purpose could vastly expand urologists' surgical repertoire and dramatically reduce patient discomfort. Herein, we developed a water-immiscible mussel protein-based bioadhesive (imWIMBA) with significantly improved properties in all clinical respects, allowing it to achieve rapid and strong underwater adhesion with tunable rheological properties. We evaluated in vivo potential of imWIMBA for sealing thermally injured fistula tracts between the bladder and vagina. Importantly, the use of imWIMBA in the presence of prolonged bladder drainage resulted in perfect closure of the vesico-vaginal fistula in operated pigs. Thus, imWIMBA could be successfully used for many surgical applications and improve treatment efficacy when combined with conventional surgical methods. STATEMENT OF SIGNIFICANCE: Vesico-vaginal fistula (VVF) is an abnormal opening between the bladder and the vagina, which is a stigmatized disease in many developing countries. Leakage of urine into internal organs can induce serious complications and delay wound repair. Conventional VVF treatment requires skillful suturing to provide a tension-free and watertight closure. In addition, there is no clinically approved surgical glue that works in wet and highly dynamic environments such as the urinary system. In this work, for potential clinical VVF closure and regeneration, we developed an advanced immiscible mussel protein-based bioglue with fast, strong, wet adhesion and tunable rheological properties. This regenerative immiscible bioglue could be successfully used for sealing fistulas and further diverse surgical applications as an adjuvant for conventional suture methods.

Quantifying viscosity and surface tension of multicomponent protein-nucleic acid condensates

Living cells organize their internal space into dynamic condensates through liquid-liquid phase separation of multivalent proteins in association with cellular nucleic acids. Here, we study how variations in nucleic acid (NA)-to-protein stoichiometry modulate the condensed phase organization and fluid dynamics in a model system of multicomponent heterotypic condensates. Employing a multiparametric approach comprised of video particle tracking microscopy and optical tweezer-induced droplet fusion, we show that the interfacial tension, but not viscosity, of protein-NA condensates is controlled by the NA/protein ratio across the two-phase regime. In parallel, we utilize fluorescence correlation spectroscopy to quantify protein and NA diffusion in the condensed phase. Fluorescence correlation spectroscopy measurements reveal that the diffusion of the component protein and NA within the condensate core is governed by the viscosity, and hence, also remains insensitive to the changes in NA-to-protein stoichiometry. Collectively, our results provide insights into the regulation of multicomponent heterotypic liquid condensates, reflecting how the bulk mixture composition affects their core versus surface organization and dynamical properties.

Complex coacervation and metal-ligand bonding as synergistic design elements for aqueous viscoelastic materials

The application of complex coacervates in promising areas such as coatings and surgical glues requires a tight control of their viscous and elastic behaviour, and a keen understanding of the corresponding microscopic mechanisms. While the viscous, or dissipative, aspect is crucial at pre-setting times and in preventing detachment, elasticity at long waiting times and low strain rates is crucial to sustain a load-bearing joints. The independent tailoring of dissipative and elastic properties proves to be a major challenge that can not be addressed adequately by the complex coacervate motif by itself. We propose a versatile model of complex coacervates with customizable rheological fates by functionalization of polyelectrolytes with terpyridines, which provide transient crosslinks through complexation with metals. We show that the rheology of the hybrid complexes shows distinct footprints of both metal-ligand and coacervate dynamics, the former as a contribution very close to pure Maxwell viscoelasticity, the latter approaching a sticky Rouse fluid. Strikingly, when the contribution of metal-ligand bonds is dominant at long times, the relaxation of the overall complex is much slower than either the "native" coacervate relaxation time or the dissociation time of a comparable non-coacervate polyelectrolyte-metal-ligand complex. We recognize this slowing-down of transient bonds as a synergistic effect that has important implications for the use of complementary transient bonding in coacervate complexes.