Multi-step control over self-assembled hydrogels of peptide-derived building blocks and a polymeric cross-linker

Attenuation of Pathway Complexity in Arginine-Rich Peptide with Polydopamine

Dynamic peptide networks represent an attractive structural space of supramolecular polymers in the realm of emergent complexity. Point mutations in the peptide sequence exert profound effects over the landscapes of self-assembly with an intricate interplay among the structure-function relationships. Herein, the pathway complexity of an arginine-rich peptide is studied, FmocVFFARR derived by the mutation of minimalist amyloid-inspired peptide amphiphile FmocVFFAKK, thereby focusing on its pathway-dependent self-assembly behavior. Interestingly, an interplay of competing primary and secondary nucleation in this minimalist model presumably due to the sticky interactions of the di-arginine motifs is encountered. This furnishes transient nanosheets from on-pathway metastable nanoparticles upon pH trigger, eventually leading to nanofibers. Moreover, external cues, e.g., pH, and temperature convert the nanofibers in off-pathway nanoparticles. For the first time, polydopamine-based surface engineering strategy to mask the arginines is demonstrated to render permanent arrest of the dynamic, transient peptide nanostructures. Finally, such polydopamine layer over the peptide nanostructures furnishes resilience against environmental stress, while also imparting mechanical robustness to the composites. The dynamic peptide nanostructures exhibited adaptive systems capable of processing chemical information while the surface coated nanostructures open wide avenues for designing stress-resilient biomaterials.

Real-Space Visualization of Charged Polymer Network of Hydrogel by Double Network Strategy and Mineral Staining

Hydrogels consist of three-dimensional (3D) and complicated polymer networks that determine their physical properties. Among the methods for structural analyses of hydrogels, the real-space imaging of a polymer network of hydrogels on a nanometer scale is one of the optimal methods; however, it is highly challenging. In this study, we propose a direct observation method for cationic polymer networks using transmission electron microscopy (TEM). By combining the double network strategy and the mineral staining technique, we overcame the challenges of polymer aggregation and the low electron density of the polymer. An objective cationic network was incorporated into a neutral skeleton network to suppress shrinkage during subsequent staining. Titania mineralization along the cationic polymer strands provided sufficient electron density for the objective polymer network for TEM observation. This observation method enables the visualization of local structures in real space and plays a complementary role to scattering methods for soft matter structure analysis.

A hydrogel based on Fe(II)-GMP demonstrates tunable emission, self-healing mechanical strength and Fenton chemistry-mediated notable antibacterial properties

Supramolecular hydrogels serve as an excellent platform to enable in situ reactive oxygen species (ROS) generation while maintaining controlled localized conditions, thereby mitigating cytotoxicity. Herein, we demonstrate hydrogel formation using guanosine-5'-monophosphate (GMP) with tetra(4-carboxylphenyl) ethylene (1) to exhibit aggregation-induced emission (AIE) and tunable mechanical strength in the presence of divalent metal ions such as Ca2+, Mg2+, and Fe2+. The addition of divalent metal ions leads to structural transformation in the metallogels (M-1GMP). Furthermore, the incorporation of Fe2+ ions into the hydrogel (Fe-1GMP) promotes the Fenton reaction that could be upregulated upon adding ascorbic acid (AA), demonstrating antibacterial efficacy via ROS generation. In vitro studies on AA-loaded Fe-1GMP demonstrate excellent bacterial killing efficacy against E. coli, S. aureus and vancomycin-resistant enterococci (VRE) strains. Finally, in vivo studies involving topical administration of Fe-1GMP to Balb/c mice with skin infections further suggest the potential antibacterial efficacy of the hydrogel. Taken together, the hydrogel with its unique combination of mechanical tunability, ROS generation capability and antibacterial efficacy can be used for biomedical applications, particularly in wound healing and infection control.

Thermoresponsive Injectable Hydrogel To Mimic the Heat- and Strain-Stiffening Behavior of Biopolymers toward Muscle Cell Proliferation

Injectable hydrogels with nonlinear mechanical attributes to emulate natural biopolymers hold paramount significance in tissue engineering, offering the potential to create scaffolds that seamlessly mimic the biomechanical intricacies of living tissues. Herein, we unveil a synthetic design strategy employing Schiff base chemistry to furnish a peptide-polymer hierarchical contractile injectable hydrogel network. This innovative design demonstrates cross-linking of supramolecular peptide nanostructures such as nanofibers, 1NF, and twisted bundles, 1TB, with a thermosensitive aldehyde-functionalized polymer, PCHO. These networks exhibit interesting nonlinear mechanical stiffening responses to temperature and external stress. Furthermore, the hydrogels transform into a gel state at physiological temperature to exhibit injectable behavior and demonstrate compression load-bearing capabilities. Finally, the hydrogel network exhibits excellent biocompatibility and cell proliferation toward fibroblast, L929, and myoblast, C2C12, to validate their use as potential extracellular matrix mimetic injectable scaffolds.

Stimuli-Responsive Self-Assembly Disassembly in Peptide Amphiphiles to Endow Block-co-Fibers and Tunable Piezoelectric Response

Supramolecular assemblies with well-defined structural attenuation toward varied functional implications are an emerging area in mimicking natural biomaterials. In that regard, the redox stimuli-responsive ferrocene moiety can reversibly change between a nonpolar ferrocenyl and polar ferrocenium cation that endows interesting modular features to the building blocks with respect to self-assembly/disassembly. We design a series of ferrocene anchored peptide fragment ^NVFFAKK^C using hydrophobic alkyl spacers of different chain lengths. Increasing the spacer length between the redox-responsive and self-assembling motifs increases the propensity to form robust nanofibers, which can be physically cross-linked to form hydrogels. The controlled redox response of the ferrocene moiety tandem with pH control provides access to structural control over the peptide nanostructures and tunable mechanical strengths. Further, such redox-sequestered dormant states hinder the spontaneous nucleation process that we exploit toward seeded supramolecular polymerization to form block cofibers composed of redox-responsive periphery and nonresponsive cores. Finally, such redox sequestration of peptide self-assembly renders an on-off piezoelectric response for potential utilization in peptide bioelectronics.

Tailorable and Biocompatible Supramolecular-Based Hydrogels Featuring two Dynamic Covalent Chemistries

Dynamic covalent chemistry (DCC) has proven to be a valuable tool in creating fascinating molecules, structures, and emergent properties in fully synthetic systems. Here we report a system that uses two dynamic covalent bonds in tandem, namely disulfides and hydrazones, for the formation of hydrogels containing biologically relevant ligands. The reversibility of disulfide bonds allows fiber formation upon oxidation of dithiol-peptide building block, while the reaction between NH-NH₂ functionalized C-terminus and aldehyde cross-linkers results in a gel. The same bond-forming reaction was exploited for the "decoration" of the supramolecular assemblies by cell-adhesion-promoting sequences (RGD and LDV). Fast triggered gelation, cytocompatibility and ability to "on-demand" chemically customize fibrillar scaffold offer potential for applying these systems as a bioactive platform for cell culture and tissue engineering.

Nonaffine Deformation of Semiflexible Polymer and Fiber Networks

Networks of semiflexible or stiff polymers such as most biopolymers are known to deform inhomogeneously when sheared. The effects of such nonaffine deformation have been shown to be much stronger than for flexible polymers. To date, our understanding of nonaffinity in such systems is limited to simulations or specific 2D models of athermal fibers. Here, we present an effective medium theory for nonaffine deformation of semiflexible polymer and fiber networks, which is general to both 2D and 3D and in both thermal and athermal limits. The predictions of this model are in good agreement with both prior computational and experimental results for linear elasticity. Moreover, the framework we introduce can be extended to address nonlinear elasticity and network dynamics.

Anion-responsive self-assembled hydrogels of a phenylalanine-TREN conjugate allow sequential release of propranolol and doxorubicin

Stimuli-responsive self-assembled and supramolecular hydrogels derived from peptide amphiphiles have opened exciting new avenues in biomedicine and drug delivery. Herein, we screened a series of phenylalanine-amphiphiles possessing polyamine and oxyethylene appendages for their self-assembly and anion-responsiveness and found that the tris(aminoethyl)amine (TREN) containing amphiphile NapF-TREN formed injectable hydrogels that could be disrupted upon the addition of stoichiometric amounts of tetrahedral monovalent anions such as H₂PO₄^- and HSO₄^-, while the addition of other anions such as Cl^-, HPO₄^2-, CO₃^2-, HCO₃^- or SO₄^2- did not affect the gel stability. The anion-gelator interaction was investigated by ¹H and ³¹P NMR spectroscopy as well as by Isothermal Titration Calorimetry (ITC). These studies confirmed a 1 : 1 stoichiometry and revealed negative enthalpy and negative entropy for the binding of H₂PO₄^- with NapF-TREN. Microscopic investigations by TEM, AFM, and SAXS revealed that H₂PO₄^- anions induced a nanofiber-to-nanoglobule morphological change in the aqueous self-assemblies of NapF-TREN. However, upon ageing the samples, slow reformation of the nanofibers was also observed, reflecting the reversibility of the anion-gelator interaction. The anion- and pH-responsive nature of the NapF-TREN hydrogels was exploited to program sequential release of entrapped drugs propranolol and doxorubicin.

Constructing ECM-like Structure on the Plasma Membrane via Peptide Assembly to Regulate the Cellular Response

This feature article introduces the design of self-assembling peptides that serve as the basic building blocks for the construction of extracellular matrix (ECM)-like structure in the vicinity of the plasma membrane. By covalently conjugating a bioactive motif, such as membrane protein binding ligand or enzymatic responsive building block, with a self-assembling motif, especially the aromatic peptide, a self-assembling peptide that retains bioactivity is obtained. Instructed by the target membrane protein or enzyme, the bioactive peptides self-assemble into ECM-like structure exerting various stimuli to regulate the cellular response via intracellular signaling, especially mechanotransduction. By briefly summarizing the properties and applications (e.g., wound healing, controlling cell motility and cell fate) of these peptides, we intend to illustrate the basic requirements and promises of the peptide assembly as a true bottom-up approach in the construction of artificial ECM.

Tandem Interplay of the Host-Guest Interaction and Photoresponsive Supramolecular Polymerization to 1D and 2D Functional Peptide Materials

Peptide 1 with an Aβ42 amyloid nucleating core and a photodimerizable 4-methylcoumarin moiety at its N terminus demonstrates the step-wise self-assembly in water to form nanoparticles, with eventual transformation into 1D nanofibers. Addition of γ-cyclodextrin to 1 with subsequent irradiation with UV light at 320 nm resulted in morphological conversion to free-standing 2D nanosheets mediated by the host-guest interaction. Mechanical agitation of the 1D and 2D nanostructures led to seeds with narrow polydispersity indices, which by mediation of seeded supramolecular polymerization found seamless control over the dimensions of the nanostructures. Such structural and temporal control to differentiate the pathway was exploited to tune the mechanical strength of hierarchical hydrogel materials. Finally, the dimensional characteristics of the positively charged peptide fibers and sheets were envisaged as excellent exfoliating agents for inorganic hybrid materials, for example, MoS₂.

Supramolecular Gel Based on Crown-Ether-Appended Dynamic Covalent Macrocycles

A new type of dynamic covalent macrocycle with self-promoted supramolecular gelation behavior is developed. Under oxidative conditions, the dithiol compound containing a diamide alkyl linker with an odd number (7) of carbon chain and an appended crown ether shows a remarkable gelation ability in acetonitrile, without any template molecules. Due to the existence of crown ethers and disulfide bonds, the obtained gel shows a multiple stimuli-responsiveness behavior. The mechanical properties and reversibility of the gel are investigated. Computational modeling suggests that the peripheral chain for diamide hydrogen bonding is responsible for the gelation process.

Pathway driven self-assembly and living supramolecular polymerization in an amyloid-inspired peptide amphiphile

Temperature dependent stepwise self-assembly and seeded supramolecular polymerization of a peptide amphiphile form metastable nanoparticles to single nanofibers or twisted bundles, to render a mechanically tunable hydrogel.

Modulating Supramolecular Peptide Hydrogel Viscoelasticity Using Biomolecular Recognition

Self-assembled peptide-based hydrogels are emerging materials that have been exploited for wound healing, drug delivery, tissue engineering, and other applications. In comparison to synthetic polymer hydrogels, supramolecular peptide-based gels have advantages in biocompatibility, biodegradability, and ease of synthesis and modification. Modification of the emergent viscoelasticity of peptide hydrogels in a stimulus responsive fashion is a longstanding goal in the development of next-generation materials. In an effort to selectively modulate hydrogel viscoelasticity, we report herein a method to enhance the elasticity of β-sheet peptide hydrogels using specific molecular recognition events between functionalized hydrogel fibrils and biomolecules. Two distinct biomolecular recognition strategies are demonstrated: oligonucleotide Watson-Crick duplex formation between peptide nucleic acid (PNA) modified fibrils with a bridging oligonucleotide and protein-ligand recognition between mannose modified fibrils with concanavalin A. These methods to modulate hydrogel elasticity should be broadly adaptable in the context of these materials to a wide variety of molecular recognition partners.

Crosslinker-Induced Effects on the Gelation Pathway of a Low Molecular Weight Hydrogel

The use of polymeric crosslinkers is an attractive method to modify the mechanical properties of supramolecular materials, but their effects on the self-assembly of the underlying supramolecular polymer networks are poorly understood. Modulation of the gelation pathway of a reaction-coupled low molecular weight hydrogelator is demonstrated using (bio)polymeric crosslinkers of disparate physicochemical identities, providing a handle for control over materials properties.