Light-triggered syneresis of a water insoluble peptide-hydrogel effectively removes small molecule waste contaminants

An Amino Acid-Appended Naphthalene Bisimide-Based Bolaamphiphile- Selective Sensing of Chaotropic Anions by Thixotropic Hydrogelations

An amino acid-based fluorophore probe, L-tryptophan appended naphthalene di-imide (NDI-W) forms stable hydrogels selectively in the presence of chaotropic anions (SCN-, ClO4 -, I-, and NO3 -), borderline anion (Br-) and F- in 50 mM phosphate buffer of pH 8 -. NDI-W did not form stable hydrogels in 50 mM phosphate buffer of pH 8 in the presence of kosmotropic anions (S2O3 2-, SO4 2-, CH3COO-, and citrate) at the same conditions. However, gel-like assemblies for NDI-W in presence of S2O3 2-, SO4 2-, CH3COO-, IO3 -, HSO3 -, HCO3 -, and NO2 - anions and precipitate formation in the presence of H2PO4 -, HSO4 -, and citric acid in 50 mM phosphate buffer of pH 8 have been observed. Interestingly, NDI-W did not form hydrogel in 50 mM phosphate buffer of pH 8 at 4 times higher concentration of NDI-W also compared to the concentration used for the anions induced hydrogelations however, forms hydrogel when higher ionic concentration of HPO4 2- is added into it. All these hydrogels are thixotropic in nature. Thus, NDI-W is selectively forming hydrogels with chaotropic and borderline anions which could be used to detect higher concentrations of salts in water and can be useful for corelating supramolecular complex viscoelastic properties at higher salt concentrations of biologically relevant systems.

Luminescent ultrashort peptide hydrogelator with enhanced photophysical implications and biocompatibility

Luminescent peptide hydrogelators have garnered significant attention in biomedical sciences and materials chemistry due to their biological relevance and tunable photophysical features. In this work, we have designed and synthesized a novel ultrashort peptide hydrogelator comprising a tripeptide sequence (FFE) integrated with 1,8-naphthalimide (NI) as an aggregation-induced emissive unit having rich and tuneable photophysical properties. The hydrogelator could self-assemble and form a self-supporting hydrogel having a highly ordered intertwined network structure at pH 5.5 with a minimum gelation concentration of 1 wt/v%. Interestingly, due to the presence of the emissive unit, the assembly could demonstrate strong blue luminescence, which has been thoroughly investigated experimentally. Moreover, spectroscopic investigations and molecular dynamics simulation studies suggest the formation of a β-sheet structure through extended intermolecular H-bonding interactions within the peptide backbones and the strong π-π-stacking interaction among aromatic units, which drive the self-assembly and hydrogelation. The emissive unit of the peptide could arrange in a J-type aggregation pattern and adopt right-handed helical induced chirality in the assembled state. Additionally, the system could exhibit a high safety profile and excellent biocompatibility, when tested in a series of cell lines in vitro. Finally, the intracellular uptake of the system has been exploited, showcasing its luminescence characteristics for potential applications in cellular imaging. The luminescent system holds significant promise for advancing cellular imaging techniques, offering new avenues for research in the future. Briefly, this work highlights the importance of luminescent ultrashort peptide hydrogelators for developing next-generation low-cost functional biomaterials.

Ultrashort Peptide Hydrogels Biomaterials with Potent Antibacterial Activity

For the past few decades, ultrashort peptide hydrogels have been at the forefront of biomaterials due to their unique properties like biocompatibility, tunable mechanical properties, and potent antibacterial activity. These ultrashort peptides self-assemble into a hydrogel matrix with nanofibrous networks. In this minireview, we have explored the design and self-assembly of these ultrashort peptide hydrogels by focusing on their antibacterial properties. Cationic and hydrophobic residues are incorporated to engineer the peptides, facilitating interaction with bacterial membranes and leading to membrane disruption and cell death. The hydrogels exhibit broad-spectrum antibacterial activity against both Gram-positive and Gram-negative bacteria. Overall, this minireview highlights the potential of ultrashort peptide hydrogels as versatile and practical antibacterial biomaterials, providing a novel approach to combating bacterial infections and addressing the growing challenge of antibiotic resistance.

Peptide-based nanomaterials and their diverse applications

The supramolecular self-assembly of peptides offers a promising avenue for both materials science and biological applications. Peptides have garnered significant attention in molecular self-assembly, forming diverse nanostructures with α-helix, β-sheet, and random coil conformations. These self-assembly processes are primarily driven by the amphiphilic nature of peptides and stabilized by non-covalent interactions, leading to complex nanoarchitectures responsive to environmental stimuli. While extensively studied in biomedical applications, including drug delivery and tissue engineering, their potential applications in the fields of piezoresponsive materials, conducting materials, catalysis and energy harvesting remain underexplored. This review comprehensively elucidates the diverse material characteristics and applications of self-assembled peptides. We discuss the multi-stimuli-responsiveness of peptide self-assemblies and their roles as energy harvesters, catalysts, liquid crystalline materials, glass materials and contributors to electrical conductivity. Additionally, we address the challenges and present future perspectives associated with peptide nanomaterials. This review aims to provide insights into the versatile applications of peptide self-assemblies while concisely summarizing their well-established biomedical roles that have previously been extensively reviewed by various research groups, including our group.

Ag+-Induced Supramolecular Polymers of Folic Acid: Reinforced by External Kosmotropic Anions Exhibiting Salting Out

Introducing kosmotropic salts enhances protein stability and reduces solubility by withdrawing water from the protein surface, leading to 'salting out', a phenomenon we have mimicked in supramolecular polymers (SPs). Under the guidance of Ag+, folic acid (FA) self-assembled in water through slipped-stacking and hydrophobic interactions into elongated, robust one-dimensional SPs, resulting in thermo-stable supergels. The SPs exhibited temperature and dilution tolerance, attributed to the stability of the FA-Ag+ complex and its hydrophobic stacking. Importantly, FA-Ag+ SP's stability has been augmented by the kosmotropic anions, such as SO42-, strengthening hydrophobic interactions in the SP, evident from the enhanced J-band, causing improvement of gel's mechanical property. Interestingly, higher kosmotrope concentrations caused a significant decrease in SP's solubility, leading to precipitation of the reinforced SPs─a 'salting out' effect. Conversely, chaotropes like ClO4- slightly destabilized hydrophobic stacking and promoted an extended conformation of individual SP chain with enhanced solubility, resembling a 'salting in' effect.

Mechanism of Peptide Self-assembly and Its Study in Biomedicine

The development of peptide-based materials is one of the most challenging aspects of biomaterials research in recent years. The assembly of peptides is mainly controlled by forces such as hydrogen bonding, hydrophobic interaction, electrostatic interaction, and π-π accumulation. Peptides have unique advantages such as simple structure, easy synthesis, good biocompatibility, non-toxicity, easy modification, etc. These factors make peptides turn into ideal biomedical materials, and they have a broad application prospect in biomedical materials, and thus have received wide attention. In this review, the mechanism and classification of peptide self-assembly and its applications in biomedicine and hydrogels were introduced.

Secondary Chemical Cross-Linking to Improve Mechanical Properties in a Multifaceted Biocompatible Strain Sensor

A new conductive and transparent organohydrogel is developed with high stretchability, excellent mechanical, self-healing, antifreezing, and adhesive properties. A simple one-pot polymerization method is used to create polyacrylamide cross-linked through N,N'-methylenebis(acrylamide) (MBAA) and divinylbenzene (DVB). The dual chemical cross-linked gel network is complemented by several physical cross-links via hydrogen bonding and π-π interaction. Multiple chemical and physical cross-links are used to construct the gel network that allows toughness (171 kPa), low modulus (≈45 kPa), excellent stretchability (>1100%), and self-healing ability. The use of appropriate proportions of the water/glycerol binary solvent system ensures efficient environment tolerance (-20 to 40 °C). Phytic acid is used as a conductive filler that provides excellent conductivity and contributes to the physical cross-linking. Dopamine is incorporated in the gel matrix, which endows excellent adhesive property of the gel. The organohydrogel-based strain sensors are developed with state-independent properties, highly linear dependence, and excellent antifatigue performance (>100 cycles). Moreover, during the practical wearable sensing tests, human motions can be detected, including speaking, smiling, and joint movement. Additionally, the sensor is biocompatible, indicating the potential applications for the next generation of epidermal sensors.

Supramolecular gels - a panorama of low-molecular-weight gelators from ancient origins to next-generation technologies

Supramolecular gels, self-assembled from low-molecular-weight gelators (LMWGs), have a long history and a bright future. This review provides an overview of these materials, from their use in lubrication and personal care in the ancient world, through to next-generation technologies. In academic terms, colloid scientists in the 19th and early 20th centuries first understood such gels as being physically assembled as a result of weak interactions, combining a solid-like network having a degree of crystalline order with a highly mobile liquid-like phase. During the 20th century, industrial scientists began using these materials in new applications in the polymer, oil and food industries. The advent of supramolecular chemistry in the late 20th century, with its focus on non-covalent interactions and controlled self-assembly, saw the horizons for these materials shifted significantly beyond their historic rheological applications, expanding their potential. The ability to tune the LMWG chemical structure, manipulate hierarchical assembly, develop multi-component systems, and introduce new types of responsive and interactive behaviour, has been transformative. Furthermore, the dynamics of these materials are increasingly understood, creating metastable gels and transiently-fueled systems. New approaches to shaping and patterning gels are providing a unique opportunity for more sophisticated uses. These supramolecular advances are increasingly underpinning and informing next-generation applications - from drug delivery and regenerative medicine to environmental remediation and sustainable energy. In summary, this article presents a panorama over the field of supramolecular gels, emphasising how both academic and industrial scientists are building on the past, and engaging new fundamental insights and innovative concepts to open up exciting horizons for their future use.

Rational design of peptide-based implants for corneal bioengineering

Regeneration of damaged cornea can save vision for millions of patients. Most of these patients are waiting for transplantation of a donor cornea or suitable substitute to restore vision. Although donor cornea transplantation is the most clinically accepted treatment, shortage of donor cornea results in almost 69 out of every 70 patients untreated with the waiting list for transplantation drastically increasing every year according to a prepandemic estimation. Therefore, corneal replacements are coming up as a cutting-edge alternative strategy. In view of the peptides, especially collagen-like peptides and peptide amphiphiles with bioactive functional motifs demonstrate promising avenue for the corneal tissue engineering and promoting regeneration, by their hierarchical self-assembling propensity to acquire desired nano- to macroscale 3D architecture. Here, we analyze rational peptide designing, self-assembly, and strategies of peptide/peptide-based nanoscale building blocks to create the extracellular matrix mimetic implants for functional regeneration of the cornea.

Reversible Perspiring Artificial "Fingertips"

Fingertip perspiration is a vital process within human predation, to which the species owes its survival and its biological success. In this paper, the unique human ability of extensive perspiration and controlled friction in self-assembled cholesteric liquid crystals is recreated, mimicking the natural processes that occur in the dermis and epidermis of human skin. This is achieved by inducing porosity in responsive, liquid-bearing material through the controlled-polymerization phase-separation process. The unique topography of human fingerprints is further emulated in the materials by balancing the parallel chirality-induced force and the perpendicular substrate-anchoring force during synthesis. As a result, artificial fingertips are capable of secreting and re-absorbing liquid upon light illumination. By demonstrating the function of the soft material in a tribological aspect, it exhibits a controllable anti-sliding property comparable to human fingertips and subsequently attains a higher degree of biomimicry. This biomimetic fingertip is envisioned being applied in a multitude of fields, ranging from biomedical instruments to interactive, human-like soft robotic devices.

Nucleoside-Based Cross-Linkers for Hydrogels with Tunable Properties

Herein, we report bioderived cross-linkers to create biopolymer-based hydrogels with tunable properties. Nucleosides (inosine and uridine) and ribose (pentose sugar lucking the nitrogenous base) were partially oxidized to yield inosine dialdehyde (IdA), uridine dialdehyde (UdA), and ribose dialdehyde (RdA). The dialdehydes were further used as cross-linkers with polysaccharide chitosan to form hydrogels. Depending on the cross-linker type and concentration, the hydrogels showed tunable rheological, mechanical, and liquid holding properties allowing the preparation of injectable, soft, and moldable hydrogels. Computational modeling and molecular dynamics simulations shed light on hydrogel formation and revealed that, in addition to covalent bonding, noncovalent interactions (π-π stacking, cation-π, and H-bonding) also significantly contributed to the cross-linking process. To demonstrate various application possibilities, the prepared hydrogels were used as a growth platform for plant cells, as injectable inks for layer-by-layer 3D printing applications, and as moldable hydrogels for soft lithography to replicate the microstructure of the plant. These findings suggest that the obtained tunable biocompatible hydrogels have the potential to be good candidates for various biotechnological applications.

Short Peptide-Based Smart Thixotropic Hydrogels †

Thixotropy is a fascinating feature present in many gel systems that has garnered a lot of attention in the medical field in recent decades. When shear stress is applied, the gel transforms into sol and immediately returns to its original state when resting. The thixotropic nature of the hydrogel has inspired scientists to entrap and release enzymes, therapeutics, and other substances inside the human body, where the gel acts as a drug reservoir and can sustainably release therapeutics. Furthermore, thixotropic hydrogels have been widely used in various therapeutic applications, including drug delivery, cornea regeneration and osteogenesis, to name a few. Because of their inherent biocompatibility and structural diversity, peptides are at the forefront of cutting-edge research in this context. This review will discuss the rational design and self-assembly of peptide-based thixotropic hydrogels with some representative examples, followed by their biomedical applications.

Peptide-Based Low Molecular Weight Photosensitive Supramolecular Gelators

Over the last couple of decades, stimuli-responsive supramolecular gels comprising synthetic short peptides as building blocks have been explored for various biological and material applications. Though a wide range of stimuli has been tested depending on the structure of the peptides, light as a stimulus has attracted extensive attention due to its non-invasive, non-contaminant, and remotely controllable nature, precise spatial and temporal resolution, and wavelength tunability. The integration of molecular photo-switch and low-molecular-weight synthetic peptides may thus provide access to supramolecular self-assembled systems, notably supramolecular gels, which may be used to create dynamic, light-responsive "smart" materials with a variety of structures and functions. This short review summarizes the recent advancement in the area of light-sensitive peptide gelation. At first, a glimpse of commonly used molecular photo-switches is given, followed by a detailed description of their incorporation into peptide sequences to design light-responsive peptide gels and the mechanism of their action. Finally, the challenges and future perspectives for developing next-generation photo-responsive gels and materials are outlined.

Azobenzene Based Photo-Responsive Hydrogel: Synthesis, Self-Assembly, and Antimicrobial Activity

A new azobenzene-based symmetric amphiphile was synthesized and characterized using ¹H NMR spectroscopy. Its self-assembly behavior as well as photo-responsive behavior in its solution and gel states were investigated. Such a compound can self-assemble into fiber mesophases in water solvent. After irradiation of the gels with UV light, the trans isomer of the compound rapidly photoisomerized to the cis isomer, which resulted in a rapid destruction of the gel. High temperature also caused a rapid drop in viscosity. To verify the antimicrobial activity of the hydrogel, live and death assays of human fibroblasts L929 properties were used for in vitro cell viability studies. The compound was converted to the terminal tertiary amine in a quaternary ammonium salt molecule by using hydrochloric acid. This azobenzene quaternary ammonium salt has a relatively better antimicrobial effect biocidal activity that was demonstrated when challenged against Escherichia coli on in vitro conditions.

Controlling Syneresis of Hydrogels Using Organic Salts

Supramolecular hydrogels can spontaneously undergo syneresis through fibre-fibre interactions and expel significant amounts of water upon aging. In this process, the hydrophobicity of fibres which regulates the 3D-rearrangement of the self-assembled structures during syneresis is important. Here, we show that we can control the hydrophobic microenvironment of gels by incorporating organic salts into the co-assembled gel fibres thereby enabling control of the macroscopic gel volume phase transition.

Photochromic and photothermal hydrogels derived from natural amino acids and heteropoly acids

A new class of supramolecular hydrogels have been designed and synthesized via the co-assembly of basic amino acids (AAs) and heteropoly acids (HPAs) under acidic conditions. The formation of gel-like samples is identified using an inverted tube method, rheology, and scanning and transmission electron microscopy. Fourier transform infrared spectroscopy reveals that the structural integrity of the HPAs is maintained during the gelation process. X-ray photoelectron spectroscopy (XPS) and proton nuclear magnetic resonance spectroscopy demonstrate that the anionic HPAs interact with both the protonated α-NH₂ and the protonated side groups of the basic amino acids, initiating the preferential growth of one-dimensional nanofibers. These nanofibers bundle and entangle with each other to form extended three-dimensional network structures. The resulting AA/HPA supramolecular hydrogels show clear stereoselectivity of the basic amino acids. With the decreasing enantiomeric excess of the basic amino acids, the gelation propensity of the AA/HPA complexes is found to be depressed. The co-assembled hydrogels show the UV-responsive photochromic behaviour because of the presence of HPAs. The corresponding XPS data confirm that the photochromism of the hydrogels is attributed to the intervalence charge-transfer transition resulting from the reduction of HPAs. Interestingly, the reduced HPAs within the hydrogel matrix can absorb the near-infrared (NIR) light and exhibit photo-thermal conversion properties, which elevates the bulk temperature of the AA/HPA hydrogels and induces the gel-to-sol transition. This study unveils that HPAs have unique capacity to promote the gelation of basic amino acids for the construction of supramolecular soft materials with functional features.

Controlling hydrogel properties by tuning non-covalent interactions in a charge complementary multicomponent system

Mixing small molecule gelators is a promising route to prepare useful and exciting materials that cannot be accessed from any of the individual components. Here, we describe pH-triggered hydrogelation by mixing of two non-gelling amphiphiles. The intermolecular interactions among the molecules can be tuned either by controlling the degree of ionization of the components or by a preparative pathway, which enables us to control material properties such as gel strength, gel stiffness, thermal stability, and an unusual shrinking/swelling behaviour.

Intelligent writable material based on a supramolecular self-assembly gel

A simple dual acylhydrazone-functionalized gelator (G1) has been designed and synthesized, and it was found to form a supramolecular organogel (G1-gel) in a mixed solvent of DMF-H2O. The gelator solution shows brilliant blue light upon mixing with Mg2+; this blue light can be erased by saliva or CO32-. Owing to this characteristic, a smart erasable writable material was prepared.

Luminescent Behavior of Gels and Sols Comprised of Molecular Gelators

We present a brief review of some important conceptual and practical aspects for the design and properties of molecular luminescent gelators and their gels. Topics considered include structural and dynamic aspects of the gels, including factors important to their ability to emit radiation from electronically excited states.

Smart Thixotropic Hydrogels by Disulfide-Linked Short Peptides for Effective Three-Dimensional Cell Proliferation

Supramolecular assembly of short peptides is a crucial process and has shown numerous potential applications as biomaterials. In the present work, the hydrogelation process of short peptides containing C-terminal "Lys-Cys" (KC) residues have been studied in detail. The N-terminal capping is found to be essential for effective gelation. Out of 12 peptides we studied, two of them could form hydrogels efficiently: Ac-VVKC-NH₂ and Ac-FFKC-NH₂. In both cases, the monomer-to-dimer formation through disulfide linkages by Cys residues controls the aggregation process. Interestingly, the presence of H₂O₂ facilitated the dimerization and thereby reduced the gelation time but could not impart much effect on the mechanical properties of the gels. Detailed rheological study revealed that both hydrogels are thixotropic in nature. Moreover, they are responsive to glutathione (GSH) due to the presence of disulfide linkages. However, the hydrogel of Ac-FFKC-NH₂ is found to be stronger and more effective for biological applications. The thixotropic nature as well as a model drug release study in response to varying GSH concentration indicates the possible use of the hydrogel as an injectable local drug delivery vehicle. The hydrogel of Ac-FFKC-NH₂ is noncytotoxic in nature. Three-dimensional cell proliferation has been found to be more effective than 2D, as it mimics the in vivo situation more closely if not exactly. In the present study, we have shown that both differentiated RAW macrophages and undifferentiated THP-1 monocytes could proliferate significantly within the 3D matrix of the hydrogel, without depicting any apparent cytotoxicity. Thus, the hydrogel of Ac-FFKC-NH₂ has potential for application in localized drug administration and as a supporting biomaterial to study basic phenomena involving cell behavior.

Heterochirality and Halogenation Control Phe-Phe Hierarchical Assembly

Diphenylalanine is an amyloidogenic building block that can form a versatile array of supramolecular materials. Its shortcomings, however, include the uncontrolled hierarchical assembly into microtubes of heterogeneous size distribution and well-known cytotoxicity. This study rationalized heterochirality as a successful strategy to address both of these pitfalls and it provided an unprotected heterochiral dipeptide that self-organized into a homogeneous and optically clear hydrogel with excellent ability to sustain fibroblast cell proliferation and viability. Substitution of one l-amino acid with its d-enantiomer preserved the ability of the dipeptide to self-organize into nanotubes, as shown by single-crystal XRD analysis, whereby the pattern of electrostatic and hydrogen bonding interactions of the backbone was unaltered. The effect of heterochirality was manifested in subtle changes in the positioning of the aromatic side chains, which resulted in weaker intermolecular interactions between nanotubes. As a result, d-Phe-l-Phe self-organized into homogeneous nanofibrils with a diameter of 4 nm, corresponding to two layers of peptides around a water channel, and yielded a transparent hydrogel. In contrast with homochiral Phe-Phe stereoisomer, it formed stable hydrogels thermoreversibly. d-Phe-l-Phe displayed no amyloid toxicity in cell cultures with fibroblast cells proliferating in high numbers and viability on this biomaterial, marking it as a preferred substrate over tissue-culture plastic. Halogenation also enabled the tailoring of d-Phe-l-Phe self-organization. Fluorination allowed analogous supramolecular packing as confirmed by XRD, thus nanotube formation, and gave intermediate levels of bundling. In contrast, iodination was the most effective strategy to augment the stability of the resulting hydrogel, although at the expense of optical transparency and biocompatibility. Interestingly, iodine presence hindered the supramolecular packing into nanotubes, resulting instead into amphipathic layers of stacked peptides without the occurrence of halogen bonding. By unravelling fine details to control these materials at the meso- and macro-scale, this study significantly advanced our understanding of these systems.

The ratio of hydrogelator to precursor controls the enzymatic hydrogelation of a branched peptide

Here, we report an apparently counterintuitive observation, in which a lower volume fraction of a branched peptide forms a stronger hydrogel after an enterokinase (ENTK) cleaves off the branch from the peptide. By varying the ratios of the branched peptide and ENTK and analysing the ratio of hydrogelator to precursor (H/P) in the enzymatic proteolysis reaction, our study shows that the H/P ratio controls the critical strain of the hydrogel formed, through enzymatic cleavage of the branch from the peptide. This work demonstrates that emergent properties (e.g., hydrogelation) of peptide assemblies, resulting from enzymatic noncovalent synthesis (ENS), are context-dependent, while also providing insights for developing dynamic soft materials via ENS.

Light-responsive self-assembly of a cationic azobenzene surfactant at high concentration

The formation of high-concentration mesophases by a cationic azobenzene photosurfactant is described for the first time. Using a combination of polarised optical microscopy and small-angle X-ray scattering, optically anisotropic, self-assembled structures with long-range order are reported. The mesophases are disrupted or lost upon UV irradiation.