Transforming the Stability, Encapsulation, and Sustained Release Properties of Calcium Alginate Beads through Gel-Confined Coacervation.

Calcium alginate (Ca2+/alginate) gel beads find use in diverse applications, ranging from drug delivery and tissue engineering to bioprocessing, food formulation, and agriculture. Unless modified, however, these gels have limited stability in alkaline media (including phosphate buffers), and their high solute permeability limits their ability to efficiently encapsulate and slowly release water-soluble small molecules. Here, we show how these limitations can be addressed by mixing the alginate solutions used in the bead preparation with the nontoxic anionic polymer polyphosphate (PP). Upon complexing Ca2+ ions, PP undergoes complex coacervation (i.e., liquid/liquid phase separation into a Ca2+/PP-rich coacervate phase and a dilute supernatant phase). At lower PP concentrations, the Ca2+/PP coacervate appears to simply remain dispersed within the beads. Though its presence makes the beads more stable in alkaline media (phosphate-buffered saline and seawater), it has little impact on the bead stiffness, morphology, and (at least in the absence of substantial payload/coacervate association) encapsulation and release properties. When the PP concentrations exceed a critical value, however, Ca2+/PP coacervation within the gelling Ca2+/alginate beads collapses the resulting beads into more compact, interpenetrating polymer networks. Besides their enhanced stability to alkaline environments, these hybrid beads exhibit irregular morphologies with wrinkled and dimpled surface structures and macroscopic (closed) internal pores, and their collapse into these polymer-rich networks also makes them significantly stiffer than their PP-free counterparts. Crucially, these beads also exhibit a much lower solute permeability, which enables highly efficient encapsulation and multiday release of water-soluble small molecules (with the beads encapsulating >90% of the added model payload and sustaining its release over 3-5 d). Collectively, these findings provide a mild and simple (single-step) pathway to generating ionically cross-linked alginate beads with significantly enhanced stability, encapsulation efficiency, and sustained release.

Customizing polyelectrolyte complex shapes through photolithographic directed assembly.

Polyelectrolyte complexes (PECs) form through the association of oppositely charged polymers and, due to their attractive properties, such as their mild/simple preparation and stimulus-sensitivity, attract widespread interest. The diverse applications of these materials often require control over PEC shapes. As a versatile approach to achieving such control, we report a new photolithographic directed assembly method for tailoring their structure. This method uses aqueous solutions of a polyelectrolyte, an oppositely charged monomer and a photoinitiator. Irradiation of these mixtures leads to site-specific polymerization of the ionic monomer into a polymer and, through this localized polyanion/polycation mixture formation, results in the assembly of PECs with 2-D and 3-D shapes that reflect the photoirradiation pattern. In addition to generating macroscopic PECs using photomasks, this photodirected PEC assembly method can be combined with multiphoton lithography, which enables the preparation of custom-shaped PECs with microscopic dimensions. Like other PECs, the custom-shaped structures formed through this photodirected assembly approach are stimulus-responsive, and can be made to switch shape or dissolve in response to changes in their external environments. This control over PEC shape and stimulus-sensitivity suggests the photopolymerization-based directed PEC assembly method as a potentially attractive route to stimulus-responsive soft device fabrication (e.g., preparation of intricately shaped, function-specific PECs through photolithographic 3-D printing).

Ionically cross-linked poly(allylamine) as a stimulus-responsive underwater adhesive: ionic strength and pH effects.

Gel-like coacervates that adhere to both hydrophilic and hydrophobic substrates under water have recently been prepared by ionically cross-linking poly(allylamine) (PAH) with pyrophosphate (PPi) and tripolyphosphate (TPP). Among the many advantages of these underwater adhesives (which include their simple preparation and low cost) is their ability to dissolve on demand when exposed to high or low pH. To further analyze their stimulus-responsive properties, we have investigated the pH and ionic strength effects on the formation, rheology and adhesion of PAH/PPi and PAH/TPP complexes. The ionic cross-linker concentrations needed to form these adhesives decreased with increasing pH and ionic strength (although the complexes ceased to form when the parent solution pH exceeded ca. 8.5; i.e., the effective pKa of PAH). Once formed, their ionic cross-links were most stable (as inferred from their relaxation times) at near-neutral or slightly alkaline pH values (of roughly 6.5-9) and at low ionic strengths. The decrease in ionic cross-link stability within complexes prepared at other pH values and at elevated (150-300 mM) NaCl concentrations diminished both the strength and longevity of adhesion (although, under most conditions tested, the short-term tensile adhesion strengths remained above 10(5) Pa). Additionally, the sensitivity of PAH/PPi and PAH/TPP complexes to ionic strength was demonstrated as a potential route to injectable adhesive design (where spontaneous adhesive formation was triggered via injection of low-viscosity, colloidal PAH/TPP dispersions into phosphate buffered saline). Thus, while the sensitivity of ionically cross-linked PAH networks to pH and ionic strength can weaken their adhesion, it can also impart them with additional functionality, such as minimally invasive, injectable delivery, and ability to form and dissolve their bonds on demand.

Simple preparation of polyelectrolyte complex beads for the long-term release of small molecules.

We report a simple method for preparing solid polyelectrolyte complex (PEC) beads, which provide effective barriers to diffusion and can be used for the multiple-day release of small molecules. Single-phase poly(allylamine) (PAH) and poly(styrenesulfonate) (PSS) mixtures were prepared at pH 11.6 (significantly above the effective pKa of PAH), where the PAH amine groups were deprotonated and therefore neutral. These mixtures were added dropwise into acid baths, whereupon the rapid acid diffusion into the polyelectrolyte droplets led to instant ionization of PAH amine groups and, thus, the formation of PEC beads (i.e., via phase inversion). In stark contrast to the PEC particles prepared through phase inversion in previous studies, which had (solvent-filled) capsule-like morphologies, these beads had solid internal structures. The solute permeabilities of these PEC matrices could be extensively tuned by air drying the beads, which led to the apparently-irreversible closure of pores. Thus, by tuning the drying conditions and polymer compositions used during bead preparation, a model small molecule (Fast Green FCF dye) was released over times ranging between 2 and 18 days.

Self-assembly of stiff, adhesive and self-healing gels from common polyelectrolytes.

Underwater adhesion has numerous potential medical, household, and industrial applications. It is typically achieved through covalent polymerization and cross-linking reactions and/or the use of highly specialized biological or biomimetic polymers. As a simpler alternative to these covalent and biomimetic strategies, this article shows that stiff, gel-like complexes that adhere to various substrates under water can also be prepared through the ionic cross-linking of common, commercial polyelectrolytes. The gels form spontaneously when synthetic polycations, such as poly(allylamine) (PAH), are mixed with strongly binding multivalent anions, pyrophosphate (PPi) and tripolyphosphate (TPP). The PAH/PPi and PAH/TPP gels exhibit very high storage moduli (G∞' ≈ 400 kPa), self-heal when torn, and adhere to both hydrophilic and hydrophobic substrates under water (with short-term tensile adhesion strengths of 350­450 kPa). Furthermore, these gels can be dissolved on demand (if adhesion needs to be reversed) by changing the ambient pH, which controls the ionization state of the polyelectrolyte and ionic cross-linker. These properties suggest that synthetic polycations cross-linked with PPi and TPP ions could provide a simple, inexpensive, and scalable platform for underwater adhesion.

Smart exosomes enhance PDAC targeted therapy.

Exosomes continue to attract interest as a promising nanocarrier drug delivery technology. They are naturally derived nanoscale extracellular vesicles with innate properties well suited to shuttle proteins, lipids, and nucleic acids between cells. Nonetheless, their clinical utility is currently limited by several major challenges, such as their inability to target tumor cells and a high proportion of clearance by the mononuclear phagocyte system (MPS) of the liver and spleen. To overcome these limitations, we developed "Smart Exosomes" that co-display RGD and CD47p110-130 through CD9 engineering (ExoSmart). The resultant ExoSmart demonstrates enhanced binding capacity to αvß3 on pancreatic ductal adenocarcinoma (PDAC) cells, resulting in amplified cellular uptake in in vitro and in vivo models and increased chemotherapeutic efficacies. Simultaneously, ExoSmart significantly reduced liver and spleen clearance of exosomes by inhibiting macrophage phagocytosis via CD47p110-130 interaction with signal regulatory proteins (SIRPα) on macrophages. These studies demonstrate that an engineered exosome drug delivery system increases PDAC therapeutic efficacy by enhancing active PDAC targeting and prolonging circulation times, and their findings hold tremendous translational potential for cancer therapy while providing a concrete foundation for future work utilizing novel peptide-engineered exosome strategies.

Accelerating Payload Release from Complex Coacervates through Mechanical Stimulation.

Complex coacervates formed through the association of charged polymers with oppositely charged species are often investigated for controlled release applications and can provide highly sustained (multi-day, -week or -month) release of both small-molecule and macromolecular actives. This release, however, can sometimes be too slow to deliver the active molecules in the doses needed to achieve the desired effect. Here, we explore how the slow release of small molecules from coacervate matrices can be accelerated through mechanical stimulation. Using coacervates formed through the association of poly(allylamine hydrochloride) (PAH) with pentavalent tripolyphosphate (TPP) ions and Rhodamine B dye as the model coacervate and payload, we demonstrate that slow payload release from complex coacervates can be accelerated severalfold through mechanical stimulation (akin to flavor release from a chewed piece of gum). The stimulation leading to this effect can be readily achieved through either perforation (with needles) or compression of the coacervates and, besides accelerating the release, can result in a deswelling of the coacervate phases. The mechanical activation effect evidently reflects the rupture and collapse of solvent-filled pores, which form due to osmotic swelling of the solute-charged coacervate pellets and is most pronounced in release media that favor swelling. This stimulation effect is therefore strong in deionized water (where the swelling is substantial) and only subtle and shorter-lived in phosphate buffered saline (where the PAH/TPP coacervate swelling is inhibited). Taken together, these findings suggest that mechanical activation could be useful in extending the complex coacervate matrix efficacy in highly sustained release applications where the slowly releasing coacervate-based sustained release vehicles undergo significant osmotic swelling.

Salt-assisted mechanistic analysis of chitosan/tripolyphosphate micro- and nanogel formation.

Self-assembled micro- and nanogels are frequently prepared by mixing tripolyphosphate (TPP) with dilute chitosan solutions. Upon its addition, the TPP ionically cross-links the chitosan molecules into gel-like colloids that range from tens of nanometers to micrometers in diameter. These particles are biocompatible, mucoadhesive and, because they are easy to prepare under very mild conditions, attract widespread interest in the encapsulation of drugs, neutraceuticals, and other bioactive payloads. Despite their broad use, however, their formation mechanism has remained largely obscured by the very fast kinetics of their self-assembly. To this end, we have tuned the TPP and monovalent salt (NaCl) concentrations to dramatically slow down this process (to occur on the time scale of days instead of milliseconds), and then probed the evolution in the size and morphology of micro- and nanogels during their formation. This revealed that the micro- and nanogel formation rates are extremely sensitive to NaCl and TPP concentrations, and that the formation process occurs in two stages: (1) formation of small primary nanoparticles and (2) aggregation of primary particles into larger, higher-order colloids that are obtained at the end of the ionotropic gelation process.

Monovalent salt enhances colloidal stability during the formation of chitosan/tripolyphosphate microgels.

Chitosan micro- and nanoparticles are routinely prepared through ionotropic gelation, where sodium tripolyphosphate (TPP) is added as a cross-linker to dilute chitosan solutions. Despite the wide use of these gel-like particles, their preparation currently relies on trial and error. To address this, we used isothermal titration calorimetry (ITC), dynamic light scattering (DLS), transmission electron microscopy (TEM), and ζ-potential measurements to investigate how the formation, structure, and colloidal stability of chitosan microgels are linked to the molecular interactions that underlie their self-assembly. The strength of the chitosan/TPP interactions was systematically varied through the addition of monovalent salt (NaCl). Remarkably, and contrary to other colloidal systems, this revealed that moderate amounts of NaCl (e.g., 150 mM) enhance the colloidal stability of chitosan/TPP microgels during their formation. This stems from the weakened chitosan/TPP binding, which apparently inhibits the bridging of the newly formed microgels by TPP. The enhanced colloidal stability during the ionic cross-linking process yields microgels with dramatically narrower size distributions, which hitherto have typically required the deacetylation and fractionation of the parent chitosan. Conversely, at high ionic strengths (ca. 500 mM) the chitosan/TPP binding is weakened to the point that the microgels cease to form, thus suggesting the existence of an optimal ionic strength for ionotropic microgel preparation.

Biomolecular uptake effects on chitosan/tripolyphosphate micro- and nanoparticle stability.

Colloidal chitosan/tripolyphosphate (TPP) particles have attracted significant attention as potential delivery vehicles for drugs, genes and vaccines. Yet, there have been several fundamental studies that showed these particles to disintegrate at physiological pH and ionic strength levels. To reconcile these findings with the published drug, gene and vaccine delivery research where chitosan/TPP particle disintegration was not reported, it has been postulated that the particles could be stabilized by their bioactive payloads. To test this hypothesis, here we examine whether the association of chitosan/TPP particles with model anionic proteins, α-lactalbumin (α-LA) and bovine serum albumin (BSA), and polynucleotides (DNA) enhances chitosan/TPP particle stability at physiological ionic strengths, using 150 mM NaCl (pH 5.5) and 1× PBS (pH 6.0) as the dissolution media. Light scattering and UV-vis spectroscopy revealed that anionic protein uptake had no impact on particle stability, likely due to the relatively weak protein/particle binding at near-physiological ionic strengths, which caused the protein to be rapidly released. This result occurred regardless of whether the protein was loaded during or after particle formation. Conversely, DNA uptake (at least at some compositions) increased the chitosan fractions persisting in a complexed/particulate form in model dissolution media, with the DNA remaining largely complexed to the chitosan at all investigated conditions. Collectively, these findings suggest that, while most bioactive payloads do not interact with chitosan strongly enough to stabilize chitosan/TPP particles, these chitosan particles can be stabilized to dissolution through the incorporation of polyanions.

Highly Sustained Release of Bactericides from Complex Coacervates.

Materials for preventing harmful bacterial contamination attract widespread interest in areas that include healthcare, home/personal care products, and crop protection. One approach to achieving this functionality is through the sustained release of antibacterial compounds. To this end, we show how putty-like complex coacervates, formed through the association of poly(allylamine hydrochloride) (PAH) with pentavalent tripolyphosphate (TPP) ions, can provide a sustained antibacterial effect by slowly releasing bactericides. Using triclosan (TC) as a model bactericide, we demonstrate that, through their dispersion in the parent PAH solution with nonionic surfactants, hydrophobic biocides can be efficiently and predictably encapsulated within PAH/TPP coacervates. Once encapsulated, the bactericide can be released over multiple months, and the release rates can be readily tuned by varying the bactericide and surfactant compositions used during encapsulation. Through this release, the PAH/TPP coacervates provide sustained bactericidal activity against model Gram-positive and Gram-negative bacteria (Staphylococcus aureus and Escherichia coli) grown under a nutrient-rich condition over at least two weeks. Thereafter, though some partial activity persists after one month, the release slows down and the bactericide-eluting coacervates lose their efficacy. Overall, we show that bactericide release from easy-to-prepare complex coacervates can provide a pathway to sustained disinfection.

PEGylation and folate conjugation effects on the stability of chitosan-tripolyphosphate nanoparticles.

Chitosan-based nanoparticles (Ch-NPs) prepared via ionotropic gelation of Ch with sodium tripolyphosphate (TPP) have been widely examined as potential drug carriers. Yet, recent studies have shown these particles to be unstable in model (pH 7.2-7.4) physiological media. To this end, here we explored the possibility of improving TPP-crosslinked Ch-NP stability through chemical Ch modification. Specifically, Ch samples with either 76% or 92% degrees of deacetylation (DD) were grafted with either polyethylene glycol (PEG), a hydrophilic molecule, or folic acid (F), a hydrophobic molecule. Limited variation in dispersion light scattering intensity, particle size and apparent ζ-potential, and lack of macroscopic precipitation were chosen as analytical evidence of dispersion stability. TPP titrations were performed to determine the optimal TPP:glucosamine molar ratio for preparing particles with near 200-nm diameters, which are desirable for systemic administration of drugs, cellular uptake, and enhancing NP blood circulation. Both DD and Ch modification influenced the particle formation process and the evolution in NP size and ζ-potential upon 30-day storage in virtually salt-free water at 25 °C and 37 °C, where the NPs underwent partial aggregation (along with possible dissolution and swelling) but remained colloidally dispersed. Under model physiological (pH 7.2; 163 mM ionic strength) conditions, however (where the chitosan amine groups were largely deprotonated), the particles quickly became destabilized, evidently due to particle dissolution followed by Ch precipitation. Overall, within the degrees of substitution used for this work (~1% for PEG, and 3 and 6% for F), neither PEG nor F qualitatively improved Ch-NP stability at physiological pH 7.2 conditions. Thus, application of TPP-crosslinked Ch-NPs in drug delivery (even when Ch is derivatized with PEG or F) should likely be limited to administration routes with acidic pH (at which these NPs remain stable).

Pitfalls in analyzing release from chitosan/tripolyphosphate micro- and nanoparticles.

Submicron particles prepared by complexing chitosan with tripolyphosphate (TPP) attract widespread interest as potential drug, gene and vaccine delivery vehicles, and many published studies examine their release properties. Despite these sustained efforts, however, literature on the release performance of chitosan/TPP micro- and nanoparticles is filled with conflicting results, with some reporting nearly instantaneous release, while others showing the release to be sustained for up to multiple days. To resolve these opposing findings, we recently postulated that the in vitro release profiles obtained from chitosan/TPP particles by the standard "sample and separate" or "solvent replacement" method (where the solvent was periodically replaced with fresh buffer and analyzed for the released bioactive molecule content) may have been subject to strong experimental artifacts and not have reflected their true release behavior. To explore this possibility, here we examine several experimental artifacts that may arise during such in vitro experiments and show that conflicting findings on release from chitosan/TPP particles can arise from: (1) incomplete particle separation from the release media upon centrifugation; (2) irreversible particle coagulation; and (3) failure to maintain sink conditions. Moreover, we show that some of the longer-lasting release profiles may reflect the use of physiologically irrelevant (low-ionic-strength) release media. By analyzing and discussing these effects, this article provides guidelines for obtaining more reliable release profiles for chitosan/TPP micro- and nanoparticles and other/related colloidal carriers.

Supramolecular Strategy Effects on Chitosan Bead Stability in Acidic Media: A Comparative Study.

Chitosan beads attract interest in diverse applications, including drug delivery, biocatalysis and water treatment. They can be formed through several supramolecular pathways, ranging from phase inversion in alkaline solutions, to the ionic crosslinking of chitosan with multivalent anions, to polyelectrolyte or surfactant/polyelectrolyte complexation. Many chitosan bead uses require control over their stability to dissolution. To help elucidate how this stability depends on the choice of supramolecular gelation chemistry, we present a comparative study of chitosan bead stability in acidic aqueous media using three common classes of supramolecular chitosan beads: (1) alkaline solution-derived beads, prepared through simple precipitation in NaOH solution; (2) ionically-crosslinked beads, prepared using tripolyphosphate (TPP); and (3) surfactant-crosslinked beads prepared via surfactant/polyelectrolyte complexation using sodium salts of dodecyl sulfate (SDS), caprate (NaC10) and laurate (NaC12). Highly variable bead stabilities with dissimilar sensitivities to pH were achieved using these methods. At low pH levels (e.g., pH 1.2), chitosan/SDS beads were the most stable, requiring roughly 2 days to dissolve. In weakly acidic media (at pH 3.0⁻5.0), however, chitosan/TPP beads exhibited the highest stability, remaining intact throughout the entire experiment. Beads prepared using only NaOH solution (i.e., without ionic crosslinking or surfactant complexation) were the least stable, except at pH 5.0, where the NaC10 and NaC12-derived beads dissolved slightly faster. Collectively, these findings provide further guidelines for tailoring supramolecular chitosan bead stability in acidic media.

Modular biodegradable biomaterials from surfactant and polyelectrolyte mixtures.

Polymeric assemblies are used in many biomaterials applications, ranging from drug-bearing nanoparticles to macroscopic scaffolds. Control over their biodegradation rates is usually achieved through synthetic modification of their molecular structure. As a simpler alternative, we exploit the associative phase separation in mixtures of bioderived surfactants and polyelectrolytes. The gel fiber scaffolds are formed via phase inversion, using a homologous series of fatty acid salts-sodium caprate (NaC10), laurate (NaC12), and myristate (NaC14), and a water-soluble chitosan derivative, N-[(2-hydroxy-3-trimethylammonium)propyl] chitosan chloride (HTCC). Their dissolution times are modulated through the selection of the fatty acid molecule and vary in a predictable manner from minutes (for NaC10-HTCC), to hours (for NaC12-HTCC), to days (for NaC14-HTCC). These variations are linked to differences in surfactant-polyelectrolyte binding strength and scale with the equilibrium binding constants of their mixtures. These fibers were found to be both cytocompatible and cell-adhesive using neural stem/progenitor cells, suggesting their potential for utility in biomedical applications.

Poly(allylamine)/tripolyphosphate coacervates enable high loading and multiple-month release of weakly amphiphilic anionic drugs: an in vitro study with ibuprofen.

When synthetic polyamines, such poly(allylamine hydrochloride) (PAH), are mixed with crosslink-forming multivalent anions, they can undergo complex coacervation. This phenomenon has recently been exploited in various applications, ranging from inorganic material synthesis, to underwater adhesion, to multiple-month release of small, water-soluble molecules. Here, using ibuprofen as a model drug molecule, we show that these coacervates may be especially effective in the long-term release of weakly amphiphilic anionic drugs. Colloidal amphiphile/polyelectrolyte complex dispersions are first prepared by mixing the amphiphilic drug (ibuprofen) with PAH. Pentavalent tripolyphosphate (TPP) ions are then added to these dispersions to form ibuprofen-loaded PAH/TPP coacervates (where the strongly-binding TPP displaces the weaker-bound ibuprofen from the PAH amine groups). The initial ibuprofen/PAH binding leads to extremely high drug loading capacities (LC-values), where the ibuprofen comprises up to roughly 30% of the coacervate mass. Conversely, the dense ionic crosslinking of PAH by TPP results in very slow release rates, where the release of ibuprofen (a small, water-soluble drug) is extended over timescales that exceed 6 months. When ibuprofen is replaced with strong anionic amphiphiles, however (i.e., sodium dodecyl sulfate and sodium dodecylbenzenesulfonate), the stronger amphiphile/polyelectrolyte binding disrupts PAH/TPP association and sharply increases the coacervate solute permeability. These findings suggest that: (1) as sustained release vehicles, PAH/TPP coacervates might be very attractive for the encapsulation and multiple-month release of weakly amphiphilic anionic payloads; and (2) strong amphiphile incorporation could be useful for tailoring PAH/TPP coacervate properties.

Photolithographically assembled polyelectrolyte complexes as shape-directing templates for thermoreversible gels.

Preparation of soft materials with diverse, customized shapes has been a topic of intense research interest. To this end, we have recently demonstrated photolithographic directed assembly as a strategy for customizing polyelectrolyte complex (PEC) shape. This process uses in situ photopolymerization of an anionic monomer in the presence of a cationic polymer, which drives localized PEC formation at the irradiation sites. Here, we show how such photolithographically assembled PECs can serve as structure-directing templates for tailoring the shapes of other soft materials; namely, thermoreversible gels. These templated hydrogels are prepared by adding a thermogelling polymer (agarose) to the anionic monomer/cationic polymer/photoinitiator precursor solutions so that, upon irradiation, custom-shaped PECs form within agarose gel matrices. Once these PECs are formed, the surrounding agarose gels are melted (through heating) and washed away which, upon returning the samples to room temperature, produces interpenetrating PEC/agarose gel networks with photopatterned shapes and dimensions. Dissolution of these sacrificial PEC templates in concentrated NaCl solutions then generates photolithographically templated agarose gels, whose shapes and dimensions match those of their PEC templates. Besides tuning their shapes and sizes, the mechanical properties of these gels can be easily tailored by varying the initial agarose concentrations used. Moreover, this PEC-templated gel synthesis appears to not adversely affect hydrogel cytocompatibility, suggesting its potential suitability for biological and biomedical applications. Though the present study uses only agarose as the model gel system, this PEC-based strategy for customizing gel shape can likely also be applied to other thermoreversible gel networks (e.g., those based on methylcellulose, poloxamers or thermoresponsive chitosan derivatives) and could have many attractive applications, ranging from drug delivery and tissue engineering, to sensing and soft robotics.

Effect of small molecules on the phase behavior and coacervation of aqueous solutions of poly(diallyldimethylammonium chloride) and poly(sodium 4-styrene sulfonate).

HYPOTHESIS: Complex coacervates are capable of easily partitioning solutes within them based on relative affinities of solute-water and solute-polyelectrolyte pairs, as the coacervate phase has low surface tension with water, facilitating the transport of small molecules into the coacervate phase. The uptake of small molecules is expected to influence the physicochemical properties of the complex coacervate, including the hydrophobicity within coacervate droplets, phase boundaries of coacervation and precipitation, solute uptake capacity, as well as the coacervate rheological properties. EXPERIMENTS: Phase behavior of aqueous solutions of poly(diallyldimethylammonium chloride) (PDAC) and poly(sodium 4-styrene sulfonate) (SPS) was investigated in the presence of various concentrations of two different dyes, positively charged methylene blue (MB) or non-charged bromothymol blue (BtB), using turbidity measurements. These materials were characterized with UV-vis spectroscopy, zeta potential measurements, isothermal titration calorimetry (ITC), fluorescence spectroscopy, and dynamic rheological measurements. FINDINGS: The presence of MB or BtB accelerates the coacervation process due to the increased hydrophobicity within coacervates by the addition of MB or BtB. The encapsulated MB or BtB tends to reduce the ionic crosslink density in the PDAC-SPS coacervates, resulting in a much weaker interconnecting network of the PDAC-SPS coacervates.

Calorimetric determination of surfactant/polyelectrolyte binding isotherms.

Mixing of oppositely charged surfactants and polyelectrolytes in aqueous solutions leads to cooperative surfactant adsorption onto the polyelectrolyte chains. Experimental determination of surfactant/polyelectrolyte binding isotherms is usually done using custom-built surfactant-ion-specific electrodes. As an alternative, we present an indirect isotherm approximation method that uses conventional isothermal titration calorimetry (ITC). The calorimetric data is fitted to the two-binding-state Satake-Yang adsorption model, which quantifies the extent of binding in terms of the binding constant (Ku) and the cooperativity parameter (u). This approach is investigated using two surfactant/polyelectrolyte mixtures: sodium perfluorooctanoate (FC7) and N,N,N-trimethylammonium derivatized hydroxyethyl cellulose (UCARE Polymer JR-400), whose binding behavior follows the Satake-Yang model, and dodecyltrimethylammonium bromide (DTAB) and poly(styrenesulfonate) (NaPSS), whose behavior deviates dramatically from the Satake-Yang model. These studies demonstrate that, in order to apply the indirect ITC method of binding isotherm determination, the surfactant/polyelectrolyte adsorption process must have no more than two dominant binding states. Thus, the technique works well for the FC7/JR-400 mixture. It fails in the case of the DTAB/NaPSS adsorption, but its mode of failure offers insight into the multiple-binding-state adsorption mechanism.

On the kinetics of chitosan/tripolyphosphate micro- and nanogel aggregation and their effects on particle polydispersity.

Submicron chitosan/tripolyphosphate (TPP) particles are widely investigated as nanocarriers for drugs, genes and vaccines. One of the key particle properties that requires control is their size distribution, which depends on the extent of chitosan/TPP primary nanoparticle aggregation into higher-order submicron colloids. To provide a better understanding of this higher-order aggregation process, this study analyzes the factors that control chitosan/TPP particle aggregation kinetics in the presence of free TPP (such as present during particle formation). The aggregation rates exhibit a sharp power-law decrease with the monovalent salt concentration and a power-law increase with the free TPP concentration. Moreover, the aggregation rates increase with the pH and with the chitosan degree of deacetylation (DD). These variations in aggregation rates reflect the effects of monovalent salt, TPP concentration, pH and chitosan DD on particle bridging by the surface-bound TPP. Furthermore, these aggregation rates are much faster than those predicted based on Derjaguin and Landau, Verwey and Overbeek (DLVO) interaction potentials, which might reflect nonuniformities in particle shape and charge, and/or complications caused by particle softness. Finally, implications of the above aggregation kinetics on the uniformity of chitosan/TPP micro- and nanogel size are analyzed, where we: (1) show how particle polydispersity can be diminished by lowering the chitosan DD; and (2) explain the opposing results on how chitosan/TPP particle polydispersity is affected by monovalent salt.