Redox modulated hydrogelation of a self-assembling short peptide amphiphile

Designed peptide amphiphiles as scaffolds for tissue engineering

Peptide amphiphiles (PAs) are peptide-based molecules that contain a peptide sequence as a head group covalently conjugated to a hydrophobic segment, such as lipid tails. They can self-assemble into well-ordered supramolecular nanostructures such as micelles, vesicles, twisted ribbons and nanofibers. In addition, the diversity of natural amino acids gives the possibility to produce PAs with different sequences. These properties along with their biocompatibility, biodegradability and a high resemblance to native extracellular matrix (ECM) have resulted in PAs being considered as ideal scaffold materials for tissue engineering (TE) applications. This review introduces the 20 natural canonical amino acids as building blocks followed by highlighting the three categories of PAs: amphiphilic peptides, lipidated peptide amphiphiles and supramolecular peptide amphiphile conjugates, as well as their design rules that dictate the peptide self-assembly process. Furthermore, 3D bio-fabrication strategies of PAs hydrogels are discussed and the recent advances of PA-based scaffolds in TE with the emphasis on bone, cartilage and neural tissue regeneration both in vitro and in vivo are considered. Finally, future prospects and challenges are discussed.

Mediating Oxidation of Thioethers with Iodine—A Mild and Versatile Pathway to Trigger the Formation of Peptide Hydrogels

The development of redox‐triggerable peptide hydrogels poses fundamental challenges, since the highly specific peptide architectures required inevitably limit the versatility of such materials. A powerful, yet rarely applied approach to bypass those barriers is the application of a mediating redox reaction to gradually decrease the pH during hydrogel formation. We report a versatile strategy to trigger the formation of peptide hydrogels from readily accessible acid‐triggerable gelators by generating protons by oxidation of thioethers with triiodide. Adding thiodiglycol as a readily available thioether auxiliary to the basic precursor solution of a peptide gelator efficiently yielded hydrogels after mixing with triiodide, as studied in detail for Nap‐FF and demonstrated for other peptides. Furthermore, incorporation of the thioether moiety in the gelator backbone via the amino acid methionine, as shown for the tailormade Nap‐FMDM peptide, reduces the number of required additives.

Peptide hydrogel with self-healing and redox-responsive properties

We have rationally designed a peptide that assembles into a redox-responsive, antimicrobial metallohydrogel. The resulting self-healing material can be rapidly reduced by ascorbate under physiological conditions and demonstrates a remarkable 160-fold change in hydrogel stiffness upon reduction. We provide a computational model of the hydrogel, explaining why position of nitrogen in non-natural amino acid pyridyl-alanine results in drastically different gelation properties of peptides with metal ions. Given its antimicrobial and rheological properties, the newly designed hydrogel can be used for removable wound dressing application, addressing a major unmet need in clinical care.

Peptide Self-assembly into stable Capsid-Like nanospheres and Co-assembly with DNA to produce smart artificial viruses

Smart artificial viruses have been successfully developed by co-assembly of de novo designed peptides with DNA, which achieved stimuli-responsibility and efficient gene transfection in cancer cells. The peptides were designed to incorporate several functional segments, including a hydrophobic aromatic segment to drive self-assembly, two or more cysteines to regulate the assemblage shape and stabilize the assembled nanostructures via forming disulfide bonds, several lysines to facilitate co-assembly with DNA and binding to cell membranes, and an enzyme-cleavable segment to introduce cancer sensitivity. The rationally designed peptides self-assembled into stable nanospheres with a uniform diameter of < 10 nm, which worked as capsid-like subunits to further interact with DNA to produce hierarchical virus-mimicking structures by encapsulating DNA in the interior. Such artificial viruses can effectively protect DNA from nuclease digestion and achieve efficient genome release by enzyme-triggered structure disassembly, which ensured a high level of gene transfection in tumor cells. The system emulates very well the structural and functional properties of natural viruses from the aspects of capsid formation, genome package and gene transfection, which is highly promising for application as efficient gene vectors.

Stimuli-Responsive Supramolecular Hydrogels and Their Applications in Regenerative Medicine

Supramolecular hydrogels are a class of self-assembled network structures formed via non-covalent interactions of the hydrogelators. These hydrogels capable of responding to external stimuli are considered to be smart materials due to their ability to undergo sol-gel and/or gel-sol transition upon subtle changes in their surroundings. Such stimuli-responsive hydrogels are intriguing biomaterials with applications in tissue engineering, delivery of cells and drugs, modulating tissue environment to promote innate tissue repair, and imaging for medical diagnostics among others. This review summarizes the recent developments in stimuli-responsive supramolecular hydrogels and their potential applications in regenerative medicine. Specifically, various structural aspects of supramolecular hydrogelators involved in self-assembly, the role of external stimuli in tuning/controlling their phase transitions, and how these functions could be harnessed to advance applications in regenerative medicine are focused on. Finally, the key challenges and future prospects for these versatile materials are briefly described.

Short Oligopeptides for Biocompatible and Biodegradable Supramolecular Hydrogels

Short Phe-rich oligopeptides, consisting of only four and five amino acids, worked as effective supramolecular hydrogelators for buffer solutions at low gelator concentrations (0.5-1.5 wt %). Among 10 different oligopeptides synthesized, peptide P1 (Ac-Phe-Phe-Phe-Gly-Lys) showed high gelation ability. Transmission electron microscopy observations suggested that the peptide molecules self-assembled into nanofibrous networks, which turned into gels. The hydrogel of peptide P1 showed reversible thermal gel-sol transition and viscoelastic properties typical of a gel. Circular dichroism spectra revealed that peptide P1 formed a β-sheetlike structure, which decreased with increasing temperature. The self-assembly of peptide P1 occurred even in the presence of nutrients in culture media and common surfactants. Escherichia coli and yeast successfully grew on the hydrogel. The hydrogel exhibited low cytotoxicity to animal cells. Finally, we demonstrated that functional compounds can be released from the hydrogel in different manners based on the interaction between the compounds and the hydrogel.

Supramolecular Hydrogelators and Hydrogels: From Soft Matter to Molecular Biomaterials

In this review we intend to provide a relatively comprehensive summary of the work of supramolecular hydrogelators after 2004 and to put emphasis particularly on the applications of supramolecular hydrogels/hydrogelators as molecular biomaterials. After a brief introduction of methods for generating supramolecular hydrogels, we discuss supramolecular hydrogelators on the basis of their categories, such as small organic molecules, coordination complexes, peptides, nucleobases, and saccharides. Following molecular design, we focus on various potential applications of supramolecular hydrogels as molecular biomaterials, classified by their applications in cell cultures, tissue engineering, cell behavior, imaging, and unique applications of hydrogelators. Particularly, we discuss the applications of supramolecular hydrogelators after they form supramolecular assemblies but prior to reaching the critical gelation concentration because this subject is less explored but may hold equally great promise for helping address fundamental questions about the mechanisms or the consequences of the self-assembly of molecules, including low molecular weight ones. Finally, we provide a perspective on supramolecular hydrogelators. We hope that this review will serve as an updated introduction and reference for researchers who are interested in exploring supramolecular hydrogelators as molecular biomaterials for addressing the societal needs at various frontiers.

Solvent effects on kinetic mechanisms of self-assembly by peptide amphiphiles via molecular dynamics simulations

Peptide amphiphiles are known to form a variety of distinctive self-assembled nanostructures (including cylindrical nanofibers in hydrogels) dependent upon the solvent conditions. Using a novel coarse-grained model, large-scale molecular dynamics simulations are performed on a system of 800 peptide amphiphiles (sequence, palmitoyl-Val3Ala3Glu3) to elucidate kinetic mechanisms of molecular assembly as a function of the solvent conditions. The assembly process is found to occur via a multistep process with transient intermediates that ultimately leads to the stabilized nanostructures including open networks of β-sheets, cylindrical nanofibers, and elongated micelles. Different kinetic mechanisms are compared in terms of peptide secondary structures, solvent-accessible surface area, radius of gyration, relative shape anisotropy, intra/intermolecular interactions, and aggregate size dynamics to provide insightful information for the design of functional biomaterials.

Tuning gelation kinetics and mechanical rigidity of β-hairpin peptide hydrogels via hydrophobic amino acid substitutions

Self-assembling peptide hydrogels with faster gelation kinetics and higher mechanical rigidity are favorable for their practical applications. A design strategy to control the folding, self-assembly, and hydrogelation of β-hairpin peptides via hydrophobic amino acid substitutions has been explored in this study. Isoleucine has higher hydrophobicity and stronger propensity for β-sheet hydrogen bonding than valine. After the valine residues of MAX1 (VKVKVKVKV(D)PPTKVKVKVKV-NH2) were replaced with isoleucines, oscillatory rheometry and circular dichroism (CD) spectroscopy characterizations indicated that the variants had clearly faster self-assembly and hydrogelation rates and that the resulting gels displayed higher mechanical stiffness. Transmission electron microscopy (TEM) indicated the parent MAX1 and its variants all formed networks of long and entangled fibrils with the similar diameters of ∼3 nm, suggesting little effect of hydrophobic substitutions on the self-assembled morphology. The MAX1I8 (IKIKIKIKV(D)PPTKIKIKIKI-NH2) hydrogel showed the fastest gelation rate (within 5 min) and the highest gel rigidity with the series, supporting the homogeneous cell distribution within its 3D scaffold. In addition, the MAX1I8 hydrogel showed quick shear-thinning and rapid recovery upon cessation of shear strain, and the MTT and immunological assays indicated its low cytotoxicity and good biocompatibility. These features are highly attractive for its widespread use in 3D cell culturing and regenerative medical treatments.