Tunable biohybrid hydrogels from coacervation of hyaluronic acid and PEO-based block copolymers

Accurately tuning the macroscopic properties of biopolymer-based hydrogels remains challenging due to the ill-defined molecular architecture of the natural building blocks. Here, we report a biohybrid coacervate hydrogel, combining the biocompatibility and biodegradability of naturally occurring hyaluronic acid (HA) with the tunability of a synthetic polyethylene oxide (PEO) -based ABA-triblock copolymer. Coacervation of the cationic ammonium or guanidinium-functionalized copolymer A-blocks with the anionic HA leads to hydrogel formation. Both mechanical properties and water content of the self-healing hydrogels can be controlled independently by altering the copolymer structure. By controlling the strength of the interaction between the polymer network and small-molecule cargo, both release rate and maximum release are controlled. Finally, we show that coacervation of HA and the triblock copolymer leads to increased biostability upon exposure to hyaluronidase. We envision that noncovalent crosslinking of HA hydrogels through coacervation is an attractive strategy for the facile synthesis of tunable hydrogels for biomedical applications.     

Phase Behavior of Bis(Quaternary Ammonium Bromide)/Sodium Cholate/H2O System

Interactions in an oppositely charged surfactant mixture composed of a gemini surfactant (bis(quaternary ammonium bromide)) and a bile salt (sodium cholate) in water were studied at 30°C. A combination of techniques was used including surface tension, conductometry, light scattering, light microscopy, and microelectrophoretic measurements. A strong dependence of the phase behavior on the molar ratio and actual concentration of surfactants was found. The interplay between electrostatic effects, geometry of molecules, and dissimilar separation of the hydrophobic and hydrophilic moieties in the surfactants dictate the interaction mode and the microstructures formed. Instead of precipitation, in the equivalent mixtures formation of complexes, mixed micelles, vesicles, coacervates, and solid crystalline phases have been observed. The extent of interacting forces in mixed micelles formed in equivalent mixtures was evaluated by regular solution theory. A relatively high negative value of interaction parameter indicated a strong attractive interaction between surfactants. The compositions of both mixed micelles and mixed monolayer are found to be almost equimolar.     

Sequestration of Proteins by Fatty Acid Coacervates for Their Encapsulation within Vesicles

Encapsulating biological materials in lipid vesicles is of interest for mimicking cells; however, except in some particular cases, such processes do not occur spontaneously. Herein, we developed a simple and robust method for encapsulating proteins in fatty acid vesicles in high yields. Fatty acid based, membrane‐free coacervates spontaneously sequester proteins and can reversibly form membranous vesicles upon varying the pH value, the precrowding feature in coacervates allowing for protein encapsulation within vesicles. We then produced enzyme‐enriched vesicles and show that enzymatic reactions can occur in these micrometric capsules. This work could be of interest in the field of synthetic biology for building microreactors.     

Analytical characterisation of glutardialdehyde cross-linking products in gelatine-gum arabic complex coacervates

Encapsulates having shells of cross-linked mixtures of proteins and polysaccharides are widely used in the food and pharmaceutical industry for controlled release of actives and flavour compounds. In order to be able to predict the behaviour and the release characteristics of the microcapsules, a better understanding of the nature and extent of the cross-linking reaction is needed. Several analytical techniques were applied for the characterisation of glutardialdehyde (GDA) cross-linked encapsulates made of gelatine and gum arabic. To allow the use of sensitive, high-resolution methods such as chromatography and mass spectrometry, the sample first had to be hydrolysed. In this way, a mixture of amino acids, small peptides and the cross-link moieties was obtained. High-resolution liquid chromatography coupled to high-resolution mass spectrometry (HPLC-MS) was applied to detect possible cross-link markers through a comparison of HPLC-MS mass-chromatograms obtained for cross-linked and non-cross-linked coacervates. HPLC-MS/MS was used to identify the species responsible for the differences. Cross-linking occurred between GDA molecules and lysine and hydroxylysine epsilon-amino groups, and up to eight cross-link products of different nature could be identified. They included pyridinium ions and Schiff bases, and also unreacted GDA condensation products. Next, based on the insight gained in the possible chemical structures present in the cross-link markers, methods for selective labelling of these functionalities were employed to allow easier detection of related reaction products. Both liquid chromatography (LC) and gas chromatography (GC) were used in these experiments. Unfortunately, these approaches failed to detect new cross-link markers, most likely as a result of the low levels at which these are present.     

Complex dynamics of multicomponent biological coacervates

Phase transitions and coacervates play key roles in natural and synthetic soft matter. In particular, the past few years have seen a rapid expansion in studies of these phenomena in the context of dynamic cellular compartmentalization. In this brief review, we mainly focus on a few concepts and selected in vitro and cellular examples of recent developments in the areas of dynamics and multicomponent systems. Topics covered include the flexibility and conformational dynamics of polymeric species involved in phase separation, valence and non-monotonic effects, noise modulation and feedback loops, and multicomponent systems and substructure. The fundamental concepts discussed in this review are widely applicable, including in the context of cellular function and the development of materials with novel properties.     

Polyelectrolyte Coacervates Deposited as High Gas Barrier Thin Films

Multilayer coatings consisting of oppositely charged polyelectrolytes have proven to be extraordinarily effective oxygen barriers but require many processing steps to fabricate. In an effort to prepare high oxygen barrier thin films more quickly, a polyelectrolyte complex coacervate composed of polyethylenimine and polyacrylic acid is prepared. The coacervate fluid is applied as a thin film using a rod coating process. With humidity and thermal post‐treatment, a 2 µm thin film reduces the oxygen transmission rate of 0.127 mm poly(ethylene terephthalate) by two orders of magnitude, rivalling conventional oxygen barrier technologies. These films are fabricated in ambient conditions using low‐cost, water‐based solutions, providing a tremendous opportunity for single‐step deposition of polymeric high barrier thin films.