Electrostatically Cross-Linked Reversible Gels—Effects of pH and Ionic Strength

Lowest gelation concentration in a complex-coacervate-driven self-assembly system, achieved by redox-RAFT synthesis of high molecular weight block polyelectrolytes

The objective of this work was to synthesize high molecular weight polyelectrolyte complex (PEC) micelles that are effective in controlling the rheology of aqueous solutions at low concentrations, paving the way for industrial applications of thickeners based on the principle of electrostatic self-assembly. Redox-initiated RAFT (reversible addition-fragmentation chain-transfer) polymerization was used to obtain anionic block polyelectrolytes based on poly(sodium 2-acrylamido-2-methylpropane sulfonate) and poly(acrylamide)-poly(AMPS)-block-poly(AM) (di-block) and poly(AMPS)-block-poly(AM)-block-poly(AMPS) (tri-block), with molecular weights of 237 kDa and 289 kDa and polydispersities of 1.29 and 1.34, respectively. A random poly(AMPS)-co-poly(AM) copolymer was also synthesized for comparison. PEC micelles were obtained upon mixing with cationic poly(N-[3-(dimethylamino)propyl]methacrylamide hydrochloride) - poly(DMAPMA), forming viscoelastic gels at unprecedented low concentrations of <3 wt% for the di-block and <1 wt% for the tri-block, which to date is the lowest demonstrated gelation concentration for a synthetic PEC micelle system. Differences between tri-block and di-block architectures are discussed, with the former being more affected by the addition of salt, which is attributed to percolated network breakdown. The random co-polymer was shown not to be an effective thickener but displayed a surprising lack of phase separation upon coacervation. The assemblies were characterized using dynamic light scattering (DLS) and cryo transmission electron microscopy (cryoTEM), revealing spherical micelles with a diameter of approximately 200 nm for the diblock and a mixture of spherical micelles and network particles for the tri-block PEC micelles. The micelles were not affected by dilution down to a polymer concentration of 7.8 × 10-4% (approx. 0.03 μM). Responsiveness to salinity, pH, and temperature was studied using DLS, revealing a critical NaCl concentration of 1.1 M for the block copolymer micelles.

pyMBE: The Python-based molecule builder for ESPResSo

We present the Python-based Molecule Builder for ESPResSo (pyMBE), an open source software application to design custom coarse-grained (CG) models, as well as pre-defined models of polyelectrolytes, peptides, and globular proteins in the Extensible Simulation Package for Research on Soft Matter (ESPResSo). The Python interface of ESPResSo offers a flexible framework, capable of building custom CG models from scratch. As a downside, building CG models from scratch is prone to mistakes, especially for newcomers in the field of CG modeling, or for molecules with complex architectures. The pyMBE module builds CG models in ESPResSo using a hierarchical bottom-up approach, providing a robust tool to automate the setup of CG models and helping new users prevent common mistakes. ESPResSo features the constant pH (cpH) and grand-reaction (G-RxMC) methods, which have been designed to study chemical reaction equilibria in macromolecular systems with many reactive species. However, setting up these methods for systems, which contain several types of reactive groups, is an error-prone task, especially for beginners. The pyMBE module enables the automatic setup of cpH and G-RxMC simulations in ESPResSo, lowering the barrier for newcomers and opening the door to investigate complex systems not studied with these methods yet. To demonstrate some of the applications of pyMBE, we showcase several case studies where we successfully reproduce previously published simulations of charge-regulating peptides and globular proteins in bulk solution and weak polyelectrolytes in dialysis. The pyMBE module is publicly available as a GitHub repository (https://github.com/pyMBE-dev/pyMBE), which includes its source code and various sample and test scripts, including the ones that we used to generate the data presented in this article.

Charge Regulation Triggers Condensation of Short Oligopeptides to Polyelectrolytes

Electrostatic interactions between charged macromolecules are ubiquitous in biological systems, and they are important also in materials design. Attraction between oppositely charged molecules is often interpreted as if the molecules had a fixed charge, which is not affected by their interaction. Less commonly, charge regulation is invoked to interpret such interactions, i.e., a change of the charge state in response to a change of the local environment. Although some theoretical and simulation studies suggest that charge regulation plays an important role in intermolecular interactions, experimental evidence supporting such a view is very scarce. In the current study, we used a model system, composed of a long polyanion interacting with cationic oligolysines, containing up to 8 lysine residues. We showed using both simulations and experiments that while these lysines are only weakly charged in the absence of the polyanion, they charge up and condense on the polycations if the pH is close to the pKa of the lysine side chains. We show that the lysines coexist in two distinct populations within the same solution: (1) practically nonionized and free in solution; (2) highly ionized and condensed on the polyanion. Using this model system, we demonstrate under what conditions charge regulation plays a significant role in the interactions of oppositely charged macromolecules and generalize our findings beyond the specific system used here.

Sequestration of Small Ions and Weak Acids and Bases by a Polyelectrolyte Complex Studied by Simulation and Experiment

Mixing of oppositely charged polyelectrolytes can result in phase separation into a polymer-poor supernatant and a polymer-rich polyelectrolyte complex (PEC). We present a new coarse-grained model for the Grand-reaction method that enables us to determine the composition of the coexisting phases in a broad range of pH and salt concentrations. We validate the model by comparing it to recent simulations and experimental studies, as well as our own experiments on poly(acrylic acid)/poly(allylamine hydrochloride) complexes. The simulations using our model predict that monovalent ions partition approximately equally between both phases, whereas divalent ones accumulate in the PEC phase. On a semiquantitative level, these results agree with our own experiments, as well as with other experiments and simulations in the literature. In the sequel, we use the model to study the partitioning of a weak diprotic acid at various pH values of the supernatant. Our results show that the ionization of the acid is enhanced in the PEC phase, resulting in its preferential accumulation in this phase, which monotonically increases with the pH. Currently, this effect is still waiting to be confirmed experimentally. We explore how the model parameters (particle size, charge density, permittivity, and solvent quality) affect the measured partition coefficients, showing that fine-tuning of these parameters can make the agreement with the experiments almost quantitative. Nevertheless, our results show that charge regulation in multivalent solutes can potentially be exploited in engineering the partitioning of charged molecules in PEC-based systems at various pH values.

Single-chain simulation of Ising density functional theory for weak polyelectrolytes

Conventional theories of weak polyelectrolytes are either computationally prohibitive to account for the multidimensional inhomogeneity of polymer ionization in a liquid environment or oversimplistic in describing the coupling effects of ion-explicit electrostatic interactions and long-range intrachain correlations. To bridge this gap, we implement the Ising density functional theory (iDFT) for ionizable polymer systems using the single-chain-in-mean-field algorithm. The single-chain-in-iDFT (sc-iDFT) shows significant improvements over conventional mean-field methods in describing segment-level dissociation equilibrium, specific ion effects, and long-range intrachain correlations. With an explicit consideration of the fluctuations of polymer configurations and the position-dependent ionization of individual polymer segments, sc-iDFT provides a faithful description of the structure and thermodynamic properties of inhomogeneous weak polyelectrolyte systems across multiple length scales.

Advances, Applications, and Emerging Opportunities in Electrostatic Hydrogels

Polyelectrolyte complex (PEC) hydrogels, which self-assemble via complexation of oppositely charged block polymers, have recently risen to prominence owing to their unique characteristics such as hierarchical microstructure, tunable bulk properties, and the ability to precisely assimilate charged cargos (i.e., proteins and nucleic acids). Significant foundational research has delineated the structure-property relationship of PEC hydrogels for use in a wide range of applications. In this Perspective, we summarize key findings on the microstructure and bulk properties of PEC hydrogels and discuss how intrinsic and extrinsic factors can be tuned to create specifically tailored PEC hydrogels with desired properties. We highlight successful applications of PEC hydrogels while offering insight into strategies to overcome their shortcomings and elaborate on emerging opportunities in the field of electrostatic self-assemblies.

Ion Partition in Polyelectrolyte Gels and Nanogels

Polyelectrolyte gels provide a load-bearing structural framework for many macroscopic biological tissues, along with the organelles within the cells composing tissues and the extracellular matrices linking the cells at a larger length scale than the cells. In addition, they also provide a medium for the selective transportation and sequestration of ions and molecules necessary for life. Motivated by these diverse problems, we focus on modeling ion partitioning in polyelectrolyte gels immersed in a solution with a single type of ionic valence, i.e., monovalent or divalent salts. Specifically, we investigate the distribution of ions inside the gel structure and compare it with the bulk, i.e., away from the gel structure. In this first exploratory study, we neglect solvation effects in our gel by modeling the gels without an explicit solvent description, with the understanding that such an approach may be inadequate for describing ion partitioning in real polyelectrolyte gels. We see that this type of model is nonetheless a natural reference point for considering gels with solvation. Based on our idealized polymer network model without explicit solvent, we find that the ion partition coefficients scale with the salt concentration, and the ion partition coefficient for divalent ions is higher than for monovalent ions over a wide range of Bjerrum length (lB) values. For gels having both monovalent and divalent salts, we find that divalent ions exhibit higher ion partition coefficients than monovalent salt for low divalent salt concentrations and low lB. However, we also find evidence that the neglect of an explicit solvent, and thus solvation, provides an inadequate description when compared to experimental observations. Thus, in future work, we must consider both ion and polymer solvation to obtain a more realistic description of ion partitioning in polyelectrolyte gels.

pH-Sensitive Poly(acrylic acid)-g-poly(L-lysine) Charge-Driven Self-Assembling Hydrogels with 3D-Printability and Self-Healing Properties

We report the rheological behavior of aqueous solutions of a graft copolymer polyampholyte, constituted of polyacrylic acid (PAA) backbone grafted by Poly(L-lysine) (PAA-b-PLL). The graft copolymer self-assembles in aqueous media, forming a three-dimensional (3D) network through polyelectrolyte complexation of the oppositely charged PAA and PLL segments. Rheological investigations showed that the hydrogel exhibits interesting properties, namely, relatively low critical gel concentration, elastic response with slow dynamics, remarkable extended critical strain to flow, shear responsiveness, injectability, 3D printability and self-healing. Due to the weak nature of the involved polyelectrolyte segments, the hydrogel properties display pH-dependency, and they are affected by the presence of salt. Especially upon varying pH, the PLL secondary structure changes from random coil to α-helix, affecting the crosslinking structural mode and, in turn, the overall network structure as reflected in the rheological properties. Thanks to the biocompatibility of the copolymer constituents and the biodegradability of PLL, the designed gelator seems to exhibit potential for bioapplications.

The promising application of hydrogel microneedles in medical application

OBJECTIVES

Hydrogel microneedles are emerging, and promising microneedles mainly composed of swelling polymers. This review is intended to summarize the preparation materials, formation mechanisms, applications and existing problems of hydrogel microneedles.

METHODS

We collected the literature on the materials, preparation and application of hydrogel microneedles in recent years, and summarized their mechanism and application in drugs delivery.

KEY FINDINGS

Hydrogel microneedles have higher safety and capabilities of controlled drug release, and have been mainly used in tumour and diabetes treatment, as well as clinical monitoring. In recent years, hydrogel microneedles have shown great potential in drug delivery, and have played the role of whitening, anti-inflammatory and promoting healing.

CONCLUSIONS

As an emerging drug delivery idea, hydrogel microneedles for drug delivery has gradually become a research hotspot. This review will provide a systematic vision for the favourable development of hydrogel microneedles and their promising application in medicine, especially drug delivery.

Unusual Aspects of Charge Regulation in Flexible Weak Polyelectrolytes

This article reviews the state of the art of the studies on charge regulation (CR) effects in flexible weak polyelectrolytes (FWPE). The characteristic of FWPE is the strong coupling of ionization and conformational degrees of freedom. After introducing the necessary fundamental concepts, some unconventional aspects of the the physical chemistry of FWPE are discussed. These aspects are: (i) the extension of statistical mechanics techniques to include ionization equilibria and, in particular, the use of the recently proposed Site Binding-Rotational Isomeric State (SBRIS) model, which allows the calculation of ionization and conformational properties on the same foot; (ii) the recent progresses in the inclusion of proton equilibria in computer simulations; (iii) the possibility of mechanically induced CR in the stretching of FWPE; (iv) the non-trivial adsorption of FWPE on ionized surfaces with the same charge sign as the PE (the so-called "wrong side" of the isoelectric point); (v) the influence of macromolecular crowding on CR.

Multicompartment Hydrogels

Hydrogels belong to the most promising materials in polymer and materials science at the moment. As they feature soft and tissue-like character as well as high water-content, a broad range of applications are addressed with hydrogels, e.g. tissue engineering and wound dressings but also soft robotics, drug delivery, actuators and catalysis. Ways to tailor hydrogel properties are crosslinking mechanism, hydrogel shape and reinforcement, but new features can be introduced by variation of hydrogel composition as well, e.g. via monomer choice, functionalization or compartmentalization. Especially, multicompartment hydrogels drive progress towards complex and highly functional soft materials. In the present review the latest developments in multicompartment hydrogels are highlighted with a focus on three types of compartments, i.e. micellar/vesicular, droplets or multi-layers including various sub-categories. Furthermore, several morphologies of compartmentalized hydrogels and applications of multicompartment hydrogels will be discussed as well. Finally, an outlook towards future developments of the field will be given. The further development of multicompartment hydrogels is highly relevant for a broad range of applications and will have a significant impact on biomedicine and organic devices. This article is protected by copyright. All rights reserved.