Structure and Relaxation Dynamics for Complex Coacervate Hydrogels Formed by ABA Triblock Copolymers

Ionic Strength-Dependent Structure of Complex Coacervate Core Micelles

Salt concentration-dependent structure of complex coacervate core micelles (C3Ms), formed by polyether-based block copolyelectrolytes containing cationic ammonium (A) or anionic sulfonate (S) groups in aqueous media, is investigated by light scattering and small-angle X-ray/neutron scattering (SAX/NS). As the salt concentration increases, both a core radius (Rcore) and an aggregation number (Nagg) significantly decrease, but a corona thickness (Lcorona) is nearly unchanged. Larger salt concentrations can lower the interfacial tension between the coacervate cores and aqueous media, resulting in an increased interfacial area per chain and a more relaxed conformation of the core blocks. Based on the structure characterization, the scaling relationship between structure parameters (i.e., Rcore, Nagg, and Lcorona) and salt concentration is obtained and compared to the theoretical description estimated by the free energy balance between the entropic penalty of core stretching and the interfacial energy. We propose that the free energy contribution of the core block stretching is not negligible in C3Ms because of the highly swollen cores caused by water.

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.

Scaling Relationship of Complex Coacervate Core Micelles: Role of Core Block Stretching

The scaling relationship of complex coacervate core micelles (C3Ms) has been investigated experimentally and theoretically. The C3Ms are formed by mixing two oppositely charged block copolyelectrolyte solutions (i.e., AB + AC system) and are characterized by small-angle neutron (SANS) and X-ray scattering (SAXS). Scaling relationships for micellar structure parameters, including core radius, total radius, corona thickness, and aggregation number, all with respect to the core block length, are determined. A scaling theory is also proposed by minimizing the free energy per chain, leading to four regimes depending on the core and corona chain conformations. Although the corona block is significantly longer than the core block, the structure of our C3Ms is consistent with that of the crew-cut I regime. A highly swollen core by water enables the core blocks to be stretched significantly and corona chains to be minimally overlapped.

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.

Strong and Tough Nanostructured Hydrogels and Organogels Prepared by Polymerization-Induced Self-Assembly

In nature, the hierarchical structure of biological tissues endows them with outstanding mechanics and elaborated functions. However, it remains a great challenge to construct biomimetic hydrogels with well-defined nanostructures and good mechanical properties. Herein, polymerization-induced self-assembly (PISA) is for the first time exploited as a general strategy for nanostructured hydrogels and organogels with tailored nanodomains and outstanding mechanical properties. As a proof-of-concept, PISA of BAB triblock copolymer is used to fabricate hydrogels with precisely regulated spherical nanodomains. These nanostructured hydrogels are strong, tough, stretchable, and recoverable, with mechanical properties correlating to their nanostructure. The outstanding mechanical properties are ascribed to the unique network architecture, where the entanglements of the hydrophilic chains act as slip links that transmit the tension to the micellar crosslinkers, while the micellar crosslinkers dissipate the energy via reversible deformation and irreversible detachment of the constituting polymers. The general feasibility of the PISA strategy toward nanostructured gels is confirmed by the successful fabrication of nanostructured hydrogels, alcogels, poly(ethylene glycol) gels, and ionogels with various PISA formulations. This work has provided a general platform for the design and fabrication of biomimetic hydrogels and organogels with tailorable nanostructures and mechanics and will inspire the design of functional nanostructured gels.

Chemical signal regulated injectable coacervate hydrogels

In the quest for stimuli-responsive materials with specific, controllable functions, coacervate hydrogels have become a promising candidate, featuring sensitive responsiveness to environmental signals enabling control over sol-gel transitions. However, conventional coacervation-based materials are regulated by relatively non-specific signals, such as temperature, pH or salt concentration, which limits their possible applications. In this work, we constructed a coacervate hydrogel with a Michael addition-based chemical reaction network (CRN) as a platform, where the state of coacervate materials can be easily tuned by specific chemical signals. We designed a pyridine-based ABA triblock copolymer, whose quaternization can be regulated by an allyl acetate electrophile and an amine nucleophile, leading to gel construction and collapse in the presence of polyanions. Our coacervate gels showed not only highly tunable stiffness and gelation times, but excellent self-healing ability and injectability with different sized needles, and accelerated degradation resulting from chemical signal-induced coacervation disruption. This work is expected to be a first step in the realization of a new class of signal-responsive injectable materials.

Fabrication and Bioactivity of Peptide-Conjugated Biomaterial Tissue Engineering Constructs

Tissue engineering combines materials engineering, cells and biochemical factors to improve, restore or replace various types of biological tissues. A nearly limitless combination of these strategies can be combined, providing a means to augment the function of a number of biological tissues such as skin tissue, neural tissue, bones, and cartilage. Compounds such as small molecule therapeutics, proteins, and even living cells have been incorporated into tissue engineering constructs to influence biological processes at the site of implantation. Peptides have been conjugated to tissue engineering constructs to circumvent limitations associated with conjugation of proteins or incorporation of cells. This review highlights various contemporary examples in which peptide conjugation is used to overcome the disadvantages associated with the inclusion of other bioactive compounds. This review covers several peptides that are commonly used in the literature as well as those that do not appear as frequently to provide a broad scope of the utility of the peptide conjugation technique for designing constructs capable of influencing the repair and regeneration of various bodily tissues. Additionally, a brief description of the construct fabrication techniques encountered in the covered examples and their advantages in various tissue engineering applications is provided.

Self-assembly of globular proteins with intrinsically disordered protein polyelectrolytes and block copolymers

Intrinsically disordered polypeptides are a versatile class of materials, combining the biocompatibility of peptides with the disordered structure and diverse phase behaviors of synthetic polymers. Synthetic polyelectrolytes are capable of complex phase behavior when mixed with oppositely charged polyelectrolytes, facilitating nanoparticle formation and bulk phase separation. However, there has been limited exploration of intrinsically disordered protein polyelectrolytes as potential bio-based replacements for synthetic polyelectrolytes. Here, we produce negatively charged, intrinsically disordered polypeptides, capable of high-yield expression in E. coli and use this intrinsically disordered peptide to produce entirely protein-based polyelectrolyte complexes. The complexes display rich phase behavior, showing sensitivity to charge density, salt concentration, temperature, and charge fraction. We characterize this behavior through a combination of turbidity assays, dynamic light scattering, and transmission electron microscopy. The robust expression profile and stimuli-responsive phase behavior of the intrinsically disordered peptides demonstrates their potential as easily producible, biocompatible substitutes for synthetic polyelectrolytes.

Ionic group-dependent structure of complex coacervate hydrogels formed by ABA triblock copolymers

This study investigates the nanostructure of complex coacervate core hydrogels (C3Gs) with varying compositions of cationic charged groups (i.e., ammonium and guanidinium) using small-angle X-ray/neutron scattering (SAX/NS). C3Gs were prepared by stoichiometric mixing of two oppositely charged ABA triblock copolymers in aqueous solvents, in which A end-blocks were functionalized with either sulfonate groups or a mixture of ammonium and guanidinium groups. Comprehensive small-angle X-ray/neutron scattering (SAX/NS) analysis elucidated the dependence of C3Gs structures on the fraction of guanidinium groups in the cationic end-block (x) and salt concentration (c_s). As x increases, the polymer volume fraction in the cores, and interfacial tension (γ_core) and salt resistance (c*) of the coacervate cores increase, which is attributed to the greater hydrophobicity and non-electrostatic association. Furthermore, we observed that the salt dependence of the interfacial tension follows γ_core ∼ (1 - c_s/c*)^3/2 in all series of x. The results show that the variation of the ionic group provides a powerful method to control the salt-responsiveness of C3Gs as stimuli-responsive materials.

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.

Molecular Exchange Kinetics in Complex Coacervate Core Micelles: Role of Associative Interaction

Molecular exchange dynamics between spherical complex coacervate core micelles (C3Ms) are documented using time-resolved small-angle neutron scattering measurements (TR-SANS), and the effects of salt concentration, type of charges, and core block polydispersity to the chain exchange are quantified. Isotopically labeled block copolyelectrolytes were prepared by postpolymerization modification of two nearly identical poly(ethylene oxide-b-allyl glycidyl ether), one with normal and the other with deuterated PEO blocks (i.e., hPEO-PAGE and dPEO-PAGE). The observed rates at multiple salt concentrations are consolidated using time-salt superposition shift factors representing chain exchange rates and analyzed. Our comprehensive analytical relaxation function based on the sticky-Rouse model and the thermodynamic barrier for core block extraction successfully describes the molecular exchange kinetics between the isotopically labeled C3Ms. We believe this work provides fundamental design criteria for C3Ms with engineered chain exchange dynamics.

Self-Assembly Behavior of Elastin-like Polypeptide Diblock Copolymers Containing a Charged Moiety

Elastin-like polypeptides (ELPs) are stimulus-responsive protein-based biopolymers, and some ELP block copolymers can assemble into spherical nanoparticles with thermosensitivity. In this study, two different ELP diblock copolymers, each composed of a hydrophobic and a charged moiety, were synthesized, and the dependence of their physical properties on pH, temperature, and salt concentration was investigated. A series of analyses revealed that hydrophobic core micelles could be generated in response to a change in their surroundings and that micelles did not self-aggregate, a phenomenon due to the repulsive forces between like-charged molecules on the surface. We also demonstrated that self-assembly behavior was closely dependent on the character of the charged amino acid and the specific anion in solution.

The effect of ion pairs on coacervate-driven self-assembly of block polyelectrolytes

The incorporation of oppositely charged polyelectrolytes into a block copolymer system can lead to formation of microphase separated nanostructures driven by the electrostatic complex between two oppositely charged blocks. It is a theoretical challenge to build an appropriate model to handle such coacervate-driven self-assembly, which should capture the strong electrostatic correlations for highly charged polymers. In this paper, we develop the self-consistent field theory considering the ion paring effect to predict the phase behavior of block polyelectrolytes. In our model, two types of ion pairs, the binding between two oppositely charged monomers and the binding between charged monomers and counterions, are included. Their strength of formation is controlled by two parameters K_aa and K_ac, respectively. We give a detailed analysis about how the binding strength K_ac and K_aa and salt concentration affect the self-assembled nanostructure of diblock polyelectrolyte systems. The results show that the binding between two oppositely charged blocks provides driven force for microphase separation, while the binding between charged monomers and counterions competes with the polyion pairing and thus suppresses the microphase separation. The addition of salt has a shielding effect on the charges of polymers, which is a disadvantage to microphase separation. The phase diagrams as a function of polymer concentration and salt concentration at different situations are constructed, and the influence of K_aa, K_ac, and charged block composition f_a is analyzed in depth. The obtained phase diagrams are in good agreement with currently existing experimental and theoretical results.