Tetrapeptide-Based Hydrogels: for Encapsulation and Slow Release of an Anticancer Drug at Physiological pH

Rapid Semiconducting Supramolecular Mg(II)-Metallohydrogel: Exploring Its Potential in Nonvolatile Resistive Switching Applications and Antiseptic Wound Healing Properties

An effective strategy was employed for the rapid development of a supramolecular metallohydrogel of Mg(II) ion (i.e., Mg@PEHA) using pentaethylenehexamine (PEHA) as a low-molecular-weight gelator in aqueous medium under ambient conditions. The mechanical stability of the synthesized Mg@PEHA metallohydrogel was characterized by using rheological analysis, which showed its robustness across different angular frequencies and oscillator stress levels. The metallohydrogel exhibited excellent thixotropic behavior, which signifies that Mg@PEHA has a self-healing nature. Field emission scanning electron microscopy and transmission electron microscopy images were utilized to explore the rectangular pebble-like hierarchical network of the Mg@PEHA metallohydrogel. Elemental mapping through energy-dispersive X-ray spectroscopy analysis confirmed the presence of primary chemical constituents in the metallohydrogel. Fourier transform infrared spectroscopy spectroscopy provided insights into the possible formation strategy of the metallohydrogel. In this work, Schottky diode structures in a metal-semiconductor-metal geometry based on a magnesium(II) metallohydrogel (Mg@PEHA) were constructed, and the charge transport behavior was observed. Additionally, a resistive random access memory (RRAM) device was developed using Mg@PEHA, which displayed bipolar resistive switching behavior at room temperature. The researchers investigated the switching mechanism, which involved the formation or rupture of conduction filaments, to gain insights into the resistive switching process. The RRAM device demonstrated excellent performance with a high ON/OFF ratio of approximately 100 and remarkable endurance of over 5000 switching cycles. RRAM devices exhibit good endurance, meaning they can endure a large number of read and write cycles without significant degradation in performance. RRAM devices have shown promising reliability in terms of long-term performance and stability, making them suitable for critical applications that require reliable memory solutions. Significant inhibitory activity against the drug-resistant Klebsiella pneumonia strain and its biofilm formation ability was demonstrated by Mg@PEHA. The minimum inhibitory concentration value of the metallohydrogel was determined to be 3 mg/mL when it was dissolved in 1% DMSO. To study the antibiofilm activity, an MTT assay was performed, revealing that biofilm inhibition (60%) commenced at 1 mg/mL of Mg@PEHA when dissolved in 1% DMSO. Moreover, in the mouse excisional wound model, Mg@PEHA played a crucial role in preventing postoperative wound infections and promoting wound healing.

Phenylalanine-based fibrillar systems

Phenylketonuria (PKU) is an inborn metabolic disorder characterized by excess accumulation of phenylalanine (Phe) and its fibril formation, resulting in progressive intellectual disability. Several research groups have approached from various directions to understand the formation of toxic amyloid fibrils from the essential amino acid Phe. Different parameters like the nature of the solvent, pH, Phe concentration, temperature, etc. influence the fibril formation kinetics. In this article, we have summarized all major findings regarding the formation of Phe-based fibrils in aqueous and organic media and discussed how non-covalent interactions are involved in the self-assembly process using spectroscopic and microscopic techniques. The toxicity of Phe-based fibrils is compared with other neurodegenerative peptides. It is noted that the Phe-based fibrils can also induce various globular proteins into toxic fibrils. Later, we discuss the different approaches to inhibit fibril formation and reduce its toxicity. The presence of polyphenolic compounds, drugs, amino acids, nanoparticles, metal ions, crown ethers, and others showed a remarkable inhibitory effect on fibril formation. To the best of our knowledge, this is the first-ever etymological analysis of the Phe-fibrillar system and its inhibition to create a strong database against PKU.

Peptide- and Metabolite-Based Hydrogels: Minimalistic Approach for the Identification and Characterization of Gelating Building Blocks

Minimalistic peptide- and metabolite-based supramolecular hydrogels have great potential relative to traditional polymeric hydrogels in various biomedical and technological applications. Advantages such as remarkable biodegradability, high water content, favorable mechanical properties, biocompatibility, self-healing, synthetic feasibility, low cost, easy design, biological function, remarkable injectability, and multi-responsiveness to external stimuli make supramolecular hydrogels promising candidates for drug delivery, tissue engineering, tissue regeneration, and wound healing. Non-covalent interactions such as hydrogen bonding, hydrophobic interactions, electrostatic interactions, and π-π stacking interactions play key roles in the formation of peptide- and metabolite-containing low-molecular-weight hydrogels. Peptide- and metabolite-based hydrogels display shear-thinning and immediate recovery behavior due to the involvement of weak non-covalent interactions, making them supreme models for the delivery of drug molecules. In the areas of regenerative medicine, tissue engineering, pre-clinical evaluation, and numerous other biomedical applications, peptide- and metabolite-based hydrogelators with rationally designed architectures have intriguing uses. In this review, we summarize the recent advancements in the field of peptide- and metabolite-based hydrogels, including their modifications using a minimalistic building-blocks approach for various applications.

A Light Scattering Investigation of Enzymatic Gelation in Self-Assembling Peptides

Self-assembling peptides (SAPs) have been increasingly studied as hydrogel-former gelators because they can create biocompatible environments. A common strategy to trigger gelation, is to use a pH variation, but most methods result in a change in pH that is too rapid, leading to gels with hardly reproducible properties. Here, we use the urea-urease reaction to tune gel properties, by a slow and uniform pH increase. We were able to produce very homogeneous and transparent gels at several SAP concentrations, ranging from c=1g/L to c=10g/L. In addition, by exploiting such a pH control strategy, and combining photon correlation imaging with dynamic light scattering measurements, we managed to unravel the mechanism by which gelation occurs in solutions of (LDLK)3-based SAPs. We found that, in diluted and concentrated solutions, gelation follows different pathways. This leads to gels with different microscopic dynamics and capability of trapping nanoparticles. At high concentrations, a strong gel is formed, made of relatively thick and rigid branches that firmly entrap nanoparticles. By contrast, the gel formed in dilute conditions is weaker, characterized by entanglements and crosslinks of very thin and flexible filaments. The gel is still able to entrap nanoparticles, but their motion is not completely arrested. These different gel morphologies can potentially be exploited for controlled multiple drug release.

π-System Functionalization Transforms Amyloidogenic Peptide Fragment of Human Islet Amyloid Polypeptide into a Super Hydrogelator

While a considerable number of ultra-short/short amyloid peptides have been reported to form 3D supramolecular hydrogels, they all possess high minimum gelation concentration (MGC) (≥1 wt%), which preclude their applications. In this context, we demonstrate that functionalisation of a well-known amyloidogenic ultra-short peptide fragment NFGAIL (IAPf) of human Islet amyloid polypeptide with a π-system (Fluorenyl, Fm) at the N-terminus of the peptide (Fm-IAPf) yield not only highly thermostable hydrogel at physiological pH but also exhibited super gelator nature as the MGC (0.08 wt%) falls below 0.1 wt%. Various experimental results confirmed that aromatic π-π interactions from fluorenyl moieties and hydrogen bonding interactions between the IAPf drive the self-assembly/fibril formation. Fm-IAPf is the first super hydrogelator derived from amyloid-based ultra-short peptides, to the best of our knowledge. We strongly believe that this report, i. e., functionalization of an amyloid peptide with π-system, provides a lead to develop super hydrogelators from other amyloid-forming peptide fragments for their potential applications.

Supramolecular Loading of DNA Hydrogels with Dye-Drug Conjugates for Real-Time Photoacoustic Monitoring of Chemotherapy

A longstanding problem with conventional cancer therapy is the nonspecific distribution of chemotherapeutics. Monitoring drug release in vivo via noninvasive bioimaging can thus have value, but it is difficult to distinguish loaded from released drug in live tissue. Here, this work describes an injectable supramolecular hydrogel that allows slow and trackable release of doxorubicin (Dox) via photoacoustic (PA) tomography. Dox is covalently linked with photoacoustic methylene blue (MB) to monitor Dox before, during, and after release from the hydrogel carrier. The conjugate (MB-Dox) possesses an IC50 of 161.4 × 10-9  m against human ovarian carcinoma (SKOV3) cells and loads into a DNA-clad hydrogel with 91.3% loading efficiency due to MB-Dox's inherent intramolecular affinity to DNA. The hydrogel is biodegradable by nuclease digestion, which causes gradual release of MB-Dox. This release rate is tunable based on the wt% of the hydrogel. This hydrogel maintains distinct PA contrast on the order of days when injected in vivo and demonstrates activatable PA spectral shifts   during hydrogel degradation. The released and loaded payload can be imaged relative to live tissue via PA and ultrasound signal being overlaid in real-time. The hydrogel slowed the rate of the murine intraperitoneal tumor growth 72.2% more than free Dox.

Designer peptides as versatile building blocks for functional materials

Peptides and pseudopeptides show distinct self-assembled nanostructures such as fibers, nanotubes, vesicles, micelles, toroids, helices and rods. The formation of such molecular communities through the collective behavior of molecules is not fully understood at a molecular level. All these self-assembled nanostructured materials have a wide range of applications such as drug delivery, gene delivery, biosensing, bioimaging, catalysis, tissue engineering, nano-electronics and sensing. Self-assembly is one of the most efficient and a simple strategy to generate complex functional materials. Owing to its significance, the last few decades witnessed a remarkable advancement in the field of self-assembling peptides with a plethora of new designer synthetic systems being discovered. These systems range from amphiphilic, cyclic, linear and polymeric peptides. This article presents only selected examples of such self-assembling peptides and pseudopeptides.

Supramolecular Shear-Thinning Glycopeptide Hydrogels for Injectable Enzyme Prodrug Therapy Applications

Transplantable catalytic reactors have attracted considerable attention as therapeutic biomedical materials. However, existing transplantable reactors such as biocatalytic films are limited by their invasiveness. Here, we report the fabrication of biocatalytic supramolecular hydrogels via self-assembly of amphiphilic glycopeptides. We show that the hydrogels have shear-thinning properties, demonstrating their potential to be administered using a syringe. Enzymes can be loaded into the hydrogels by simply adding enzyme solution, and the enzyme-loaded hydrogels can transform a prodrug into an anticancer drug that inhibits tumor cell growth. This study demonstrates the potential of these biocatalytic hydrogels as injectable therapeutic reactors for enzyme prodrug therapy.

Controlled Hydrolysis of Odorants Schiff Bases in Low-Molecular-Weight Gels

Imines or Schiff bases (SB) are formed by the condensation of an aldehyde or a ketone with a primary amine, with the removal of a water molecule. Schiff bases are central molecules in several biological processes for their ability to form and cleave by small variation of the medium. We report here the controlled hydrolysis of four SBs that may be applied in the fragrance industry, as they are profragrances all containing odorant molecules: methyl anthranilate as primary amine, and four aldehydes (cyclamal, helional, hydroxycitronellal and triplal) that are very volatile odorants. The SB stability was assessed over time by HPLC-MS in neutral or acidic conditions, both in solution and when trapped in low molecular weight gels. Our results demonstrate that it is possible to control the hydrolysis of the Schiff bases in the gel environment, thus tuning the quantity of aldehyde released and the persistency of the fragrance.

S-Benzyl Cysteine Based Cyclic Dipeptide Super Hydrogelator: Enhancing Efficacy of an Anticancer Drug via Sustainable Release

Peptide based low molecular weight supramolecular hydrogels hold promising aspects in various fields of application especially in biomaterial and biomedical sciences such as drug delivery, wound healing, tissue engineering, cell proliferation, etc due to their extreme biocompatibility. Unlike linear peptides, cyclic peptides have more structural rigidity and tolerance to enzymatic degradation and high environmental stability which make them even better candidates for the above said applications. Herein, a new small cyclic dipeptide (CDP) cyclo-(Leu-S-Bzl-Cys) (P1) consisting of L-leucine and S-benzyl protected L-cysteine was reported which formed hydrogel at physiological conditions (at 37^o C and pH=7.46). The hydrogel formed from the cyclic dipeptide P1 showed very good tolerance towards environmental parameters such as pH, temperature and was seen to be stable for more than a year without any deformation. The hydrogel was thermoreversible and stable in the pH range 6-12. Mechanical strength of P1 hydrogel was measured by rheology experiment. AFM and FE-SEM images revealed that in aqueous solvents P1 self-assembled into a highly cross-linked nanofibrillar network which immobilized water molecules inside the cages and formed the hydrogel. The self-assembled cyclic dipeptide acquired antiparallel β-sheet secondary structure which was evident from CD and FT-IR studies. The β-sheet arrangement and formation of amyloid fibrils were further established by ThT binding assay. Furthermore, P1 was able to form hydrogel in presence of anticancer drug 5-fluorouracil (5FU) and sustainable release of the drug from the hydrogel was measured in-vitro. The hydrogelator P1 showed almost no cytotoxicity towards human colorectal cancer cell line HCT116 up to a considerable high concentration and showed potential application in sustainable drug delivery. The co-assembly of 5FU and P1 hydrogel exhibited much better anticancer activity towards HCT116 cancer cell line than 5-fluorouracil alone and decreased the IC₅₀ dose of 5-fluorouracil to a much lower value.

A Close Look at Molecular Self-Assembly with the Transmission Electron Microscope

Molecular self-assembly is pervasive in the formation of living and synthetic materials. Knowledge gained from research into the principles of molecular self-assembly drives innovation in the biological, chemical, and materials sciences. Self-assembly processes span a wide range of temporal and spatial domains and are often unintuitive and complex. Studying such complex processes requires an arsenal of analytical and computational tools. Within this arsenal, the transmission electron microscope stands out for its unique ability to visualize and quantify self-assembly structures and processes. This review describes the contribution that the transmission electron microscope has made to the field of molecular self-assembly. An emphasis is placed on which TEM methods are applicable to different structures and processes and how TEM can be used in combination with other experimental or computational methods. Finally, we provide an outlook on the current challenges to, and opportunities for, increasing the impact that the transmission electron microscope can have on molecular self-assembly.

Metal Cation Triggered Peptide Hydrogels and Their Application in Food Freshness Monitoring and Dye Adsorption

Metal-ligand interactions have emerged as an important tool to trigger and modulate self-assembly, and to tune the properties of the final supramolecular materials. Herein, we report the metal-cation induced self-assembly of a pyrene-peptide conjugate to form hydrogels. The peptide has been rationally designed to favor the formation of β-sheet 1D assemblies and metal coordination through the Glu side chains. We studied in detail the self-assembly process in the presence of H+, Li+, Na+, K+, Ca2+, Ni2+, Cu2+, Zn2+, Cd2+, Co2+, Fe3+, and Cr3+ and found that the morphology and mechanical properties of the hydrogels are ion-dependent. Moreover, thanks to the presence of the metal, new applications could be explored. Cu2+ metallogels could be used for amine sensing and meat freshness monitoring, while Zn2+ metallogels showed good selectivity for cationic dye adsorption and separation.

Supramolecular Antiparallel β-Sheet Formation by Tetrapeptides Based on Amyloid Sequence

Self-assembly of short peptides has emerged as an interesting research field for a wide range of applications. Recently, several truncated fragments of long-chain peptides or proteins responsible for different neurodegenerative diseases were studied to understand whether they can mimic the property and function of native peptides or not. It was reported that such a kind of peptide adopts a β-sheet structure in the disease state. It was observed that aromatic amino acid-rich peptide fragments possess a high tendency to adopt a β-sheet conformation. In this article, we are first time reporting the crystal structure of two tetrapeptides: Boc-GAII-OMe (Peptide 1) and Boc-GGVV-OMe (Peptide 2), composed of aliphatic amino acids, and the sequences are similar to the Aβ-peptide fragments Aβ_29-32 and Aβ_37-40 , respectively. In the solid-state, they are self-assembled in an antiparallel β-sheet fashion. The peptide units are connected by the strong amide hydrogen-bonding (N-H···O) interactions. Apart from that, other noncovalent interactions are also present, which help to stabilize the cross-β-sheet arrangement. Interestingly, in the crystal structure of Peptide 1, noncovalent C···C interaction between the electron-deficient carbonyl carbon, and the electron-rich sp³-carbon atom is observed, which is quite rare in the literature. The calculated torsion angles for these peptides are lying in the β-sheet region of the Ramachandran plot. FT-IR studies also indicate the formation of an antiparallel β-sheet structure in the solid-state. Circular dichroism of the peptides in the aqueous solution also suggests the presence of predominantly β-sheet-like conformation in the aqueous solution. Under cross-polarized light, Congo Red stained both peptides showed green-gold color due to birefringence indicating their amyloidogenic nature. This result indicates that the short peptide composed of aliphatic amino acid is capable of forming a β-sheet structure in the absence of aromatic amino acid and also can mimic the function of the native amyloid peptide.

The impact of thermal history on the structure of glycylalanylglycine ethanol/water gels

This work revisits several open questions regarding the mechanisms of GAG fibril formation and structure as a function of temperature. The authors recently hypothesized that there is a solubility limit of GAG in ethanol/water that induces self-assembly. In other words, not all peptides can participate in fibrillization and some fraction is still soluble in solution. We show via FTIR spectroscopy that, indeed, free peptides are still present in solution after fibril formation, strongly supporting the solubility model. Furthermore, previous work showed GAG self-assembled into right-handed (phase I) or left-handed (phase II) chiral structures depending on temperature. In this study, we analyze the crystalline structure of phase I and II gels via WAXS and SAXS to compare their crystalline structures and order. Rheological measurements were used to investigate the response of the fibrillar network to temperature. They reveal that the ability of the peptide to self-assemble depends on the solubility at a given temperature and not on thermal history. Furthermore, the gel softening point, the linear viscoelastic gel microstructure, and relaxation spectrum are very similar between phase I and phase II. Overall, the temperature only affects the chirality of the fibrils and the formation kinetics.

Advances in engineering of low molecular weight hydrogels for chemotherapeutic applications

Chemotherapy is the primary option for the treatment of cancer, inflammation, and infectious diseases. Conventional drug delivery poses solubility and bioavailability challenges, systemic toxicity, non-specific targeting, and poor accumulation of chemotherapeutic drugs at the desired site. Nanotechnology has led to the development of various nanomaterials that have decreased the toxicity and increased the accumulation of drugs at the target site. Systemic administration of nanomaterials causes burst release and non-specific targeting of chemotherapeutics, leading to off-target organ toxicity. Drug delivery based on low molecular weight hydrogels (LMWHs) provides a suitable alternative for drug delivery due to their ability to entrap chemotherapeutic drugs. Injectable and biodegradable LMWHs allow the administration of chemotherapeutics with minimal invasion, allow the sustained release of chemotherapeutic drugs for long periods, and reduce the challenges of immunogenicity and low drug entrapment efficiency. Herein, we summarize the advances in the engineering of LMWHs for controlled and prolonged delivery of chemotherapeutics for cancer, infectious diseases, and inflammatory disorders.

Advances in self-assembled injectable hydrogels for cancer therapy

Non-specific toxicity of chemotherapeutics and evolution of malignant tumors against them are major challenges for existing cancer chemotherapeutic regimens. Engineering of nanomaterials has attempted to minimize the toxicity of anticancer drugs, but systemic delivery of these nanomaterials still imposes many hurdles in their clinical use like burst release of chemotherapeutics and toxicity and immunogenicity associated with excipients of nanomaterials. However, there has been a surge in the development of natural and synthetic nanomaterials to deliver anticancer agents to the diseased (tumor) site as it can minimize the systemic circulation of anticancer drugs and reduce the toxicity-related challenges. Therefore, localized drug delivery is considered as the most effective way to deliver therapeutics but is further challenged by poor biodegradability, high immunogenicity, poor drug entrapment efficacy and inability to maintain sustained release of anticancer agents at the tumor site. This review maps out recent advancements in engineering of low molecular weight hydrogels derived from amino acid, fatty acyl, steroidal lipid and drug conjugated amphiphilic scaffolds. We have summarized the efforts for the development of molecular hydrogels in terms of biocompatibility, therapeutic potential and challenges associated with existing molecular hydrogels for cancer therapy.

The aging effect on the enhancement of thermal stability, mechanical stiffness and fluorescence properties of histidine-appended naphthalenediimide based two-component hydrogels

A histidine attached naphthalenediimide (NDI)-containing amphiphilic molecule (NDIP) self-assembles into nanotubes in aqueous solution at pH 6.6 as revealed by high-resolution transmission electron microscopy studies. This histidine-appended NDI forms a two-component hydrogel in the presence of tartaric acid at a molar ratio of 1 : 2. A morphological transformation was observed from a nanotube structure in the non-gel aggregated state of histidine appended NDI to interconnected cross-linked nanofibers of the two-component hydrogel in the presence of tartaric acid. Interestingly, the gel exhibits an unusual behavior upon aging compared to the fresh gel. It is found that the thermal stability and gel stiffness increase very significantly upon aging. Another important feature noted is that the very weak fluorescence of the fresh gel is transformed into bright greenish fluorescence upon aging. These results suggest that intermolecular interactions among the gelator molecules and tartaric acid in the gel phase slowly increase with time to form a mechanically very stiff and thermally robust gel.

Low molecular weight self-assembling peptide-based materials for cell culture, antimicrobial, anti-inflammatory, wound healing, anticancer, drug delivery, bioimaging and 3D bioprinting applications

In this review, we have focused on the design and development of low molecular weight self-assembling peptide-based materials for various applications including cell proliferation, tissue engineering, antibacterial, antifungal, anti-inflammatory, anticancer, wound healing, drug delivery, bioimaging and 3D bioprinting. The first part of the review describes about stimuli and various noncovalent interactions, which are the key components of various self-assembly processes for the construction of organized structures. Subsequently, the chemical functionalization of the peptides has been discussed, which is required for the designing of self-assembling peptide-based soft materials. Various low molecular weight self-assembling peptides have been discussed to explain the important structural features for the construction of defined functional nanostructures. Finally, we have discussed various examples of low molecular weight self-assembling peptide-based materials for cell culture, antimicrobial, anti-inflammatory, anticancer, wound healing, drug delivery, bioimaging and 3D bioprinting applications.

Amyloids and their untapped potential as hydrogelators

Amyloid fibrils are cross-β-sheet-rich fibrous aggregates. They were originally identified as disease-associated protein/peptide deposits. The cross-β motif was consequently labelled as an alien and pathogenic fold. Subsequent research revealed that the fibrillar aggregates were benign, and the cytotoxicity in the amyloid diseases was attributed to the pre-fibrillar structures. Research in the past two decades has identified the native functional amyloids in organisms ranging from bacteria to human. The amyloid-like fibrils, therefore, are not necessarily pathogenic, and the cross-β motif is very much native. This premise makes way for the amyloids to be used as biocompatible materials. Many naturally occurring amyloidogenic proteins/peptides or their fragments have been reported in the literature to form hydrogels. Hydrogels constitute one of the most interesting classes of soft materials that find application in diverse fields such as environmental, electronic, and biomedical engineering. Applications of hydrogels in medicine are particularly extensive. Among various classes of peptides that form hydrogels, the potential of amyloids is largely untapped. In this review, we have attempted to compile the literature on amyloid hydrogels and discuss their potential applications.

Nuclear delivery of dual anticancer drug-based nanomedicine constructed by cisplatinum-induced peptide self-assembly

Nuclear delivery of anticancer drugs, particularly dual complementary anticancer drugs, can significantly improve chemotherapy efficacy. However, successful examples are rare. We reported a novel dual anticancer drug-based nanomedicine with nuclear accumulation properties. The nanomedicine was formed by chelation between a drug peptide amphiphile Rh-GFFYERGD (Rh represents Rhein, 1,8-dihydroxy-3-carboxy anthraquinonea) and cisplatinum (Pt). A single molecule of the drug peptide amphiphile could chelate up to 8 equiv. of cisplatinum in the resulting nanofibers. The nanofibers with a 1 : 4 ratio of Rh-GFFYERGD to cisplatinum demonstrated remarkable cellular uptake, and more significantly, superior nuclear accumulation properties. Additionally, the nanofibers could also bind to the DNA molecule more efficiently than those formed by the drug peptide amphiphile. Thus the nanofibers exhibited excellent anticancer properties both in vitro and in vivo. We envision a significant therapeutic potential of the dual anticancer drug-based nanomedicine with cisplatinum in cancer.

The Overview of Porous, Bioactive Scaffolds as Instructive Biomaterials for Tissue Regeneration and Their Clinical Translation

Porous scaffolds have been employed for decades in the biomedical field where researchers have been seeking to produce an environment which could approach one of the extracellular matrixes supporting cells in natural tissues. Such three-dimensional systems offer many degrees of freedom to modulate cell activity, ranging from the chemistry of the structure and the architectural properties such as the porosity, the pore, and interconnection size. All these features can be exploited synergistically to tailor the cell-material interactions, and further, the tissue growth within the voids of the scaffold. Herein, an overview of the materials employed to generate porous scaffolds as well as the various techniques that are used to process them is supplied. Furthermore, scaffold parameters which modulate cell behavior are identified under distinct aspects: the architecture of inert scaffolds (i.e., pore and interconnection size, porosity, mechanical properties, etc.) alone on cell functions followed by comparison with bioactive scaffolds to grasp the most relevant features driving tissue regeneration. Finally, in vivo outcomes are highlighted comparing the accordance between in vitro and in vivo results in order to tackle the future translational challenges in tissue repair and regeneration.

A Self-Assembled Peptide-Appended Naphthalene Diimide: A Fluorescent Switch for Sensing Acid and Base Vapors

A histidine-containing bola-amphiphilic molecule (NDIP) containing a peptide-appended naphthalenediimide (NDI) forms fluorescent hydrogels in phosphate buffer and organogels with benzenoid solvents. These gels were characterized by several spectroscopic and microscopic techniques including FT-IR, HR-TEM, powder X-ray diffraction and small-angle X-ray scattering, UV-Vis and fluorescence studies. The gelator molecule exhibits no significant fluorescence in the xerogel state, while it shows a significant fluorescence (bright cyan) in the presence of volatile organic/inorganic acid vapors; this cyan color vanishes in presence of base (ammonia vapors). A reusable paper-strip-based method based on this self-assembled fluorescent material can be used to easily detect hazardous volatile acid and base vapors with the naked eye.

Coassembly Generates Peptide Hydrogel with Wound Dressing Material Properties

Multicomponent self-assembly of peptides is a powerful strategy to fabricate novel functional materials with synergetic properties that can be used for several nanobiotechnological applications. In the present study, we used a coassembly strategy to generate an injectable ultrashort bioactive peptide hydrogel formed by mixing a dipeptide hydrogelator with a macrophage attracting short chemotactic peptide ligand. Coassembly does not impede hydrogelation as shown by cryo-transmission electron microscopy (cryo-TEM), scanning electron microscopy, and rheology. Biocompatibility was shown by cytotoxicity assays and confocal microscopy. The hydrogels release the entrapped skin antibiotic ciprofloxacin, among others, in a slow and continuous manner. Such bioinspired advanced functional materials can find applications as wound dressing materials to treat chronic wound conditions like diabetic foot ulcer.

Engineered Dynamic Boronate Ester-Mediated Self-Healable Biocompatible G-Quadruplex Hydrogels for Sustained Release of Vitamins

Injectable, self-healable, and biocompatible dynamic hydrogels prepared from the molecular self-assembly and reversible covalent bond formation of low-molecular-weight hydrogelators are increasing in the field of drug delivery. Herein, we report the formation of G-quadruplex hydrogels via the multicomponent self-assembly and reversible bond formation between guanosine (G) and 1-naphthaleneboronic acid in the presence of the monovalent cation K⁺. The cation-templated stacking interaction of G4 quartets and the formation of dynamic cyclic boronate esters are responsible for the construction of dynamic G-quadruplex assembly. The in situ-synthesized dynamic cyclic boronate esters are well characterized by ¹¹B nuclear magnetic resonance and Fourier transform infrared spectroscopy methods. The formation and morphology of the G-quadruplex hydrogel are well supported by several spectroscopic and microscopic techniques. The injectability and self-healing ability of the G-quadruplex hydrogel are also investigated. The in vivo cytotoxicity of the G-quadruplex hydrogel is extensively evaluated over different cell lines (HeLa, MCF-7, and HEK293) to observe the biostability and broad-spectrum biocompatibility of the hydrogel. Further, this injectable, biocompatible G-quadruplex hydrogel has been used for encapsulation and sustained release of two important vitamins (B₂ and B₁₂) over 40 h at physiological pH (7.46) and temperature (37 °C) without the influence of any external stimuli.

Targeting CAMKII to reprogram tumor-associated macrophages and inhibit tumor cells for cancer immunotherapy with an injectable hybrid peptide hydrogel

Simultaneously targeted treatment of tumor cells and their surrounding growth-supporting immune cells is a promising strategy to reshape immunosuppressive tumor microenvironment (TME) and potentiate host innate and adaptive antitumor immune responses. Methods: We designed a series of melittin-(RADA)_n hybrid peptide sequences with varying self-assembling motifs of RADA and screened out a melittin-(RADA)₆ peptide that has an optimal gel-formation ability and in vitro antitumor activity. Results: The formed melittin-(RADA)₆ (MR₅₂) hydrogel scaffold could be loaded with a specific Ca²⁺/calmodulin-dependent protein kinase II (CAMKII) inhibitor, KN93, originally found to have both direct tumoricidal activity and macrophages-reprogramming ability, for potent immunotherapy against melanoma and hepatoma ascites in mice models. Our MR₅₂ hydrogel has an interweaving nanofiber-like structure, possesses direct antitumor and controlled drug release properties, and promotes the enhanced intracellular uptake of loaded cargo. Compared to free KN93, the MR₅₂-KN93 hydrogel (MRK) improved the killing effects and levels of immunogenic cell death (ICD) on tumor cells significantly. Due to the dual role of KN93, the injection of the MRK hydrogel retarded the growth of subcutaneous melanoma tumors dramatically and resulted in a high number of mature dendritic cells of draining lymph nodes, significantly enhancing the portion of cytotoxic T cells and reduced number of M2-like tumor-associated macrophages (TAMs) in tumors. Using a mouse model of malignant ascites (MAs), where traditional therapy was ineffective, we demonstrated that the MRK hydrogel treatment offered a significantly prolonged survival compared to controls. Following treatment with the MRK hydrogel, macrophages had elevated programmed cell death protein ligand-1 (PD-L1) expression, promising follow-up combined anti-PD-1 therapy that confers a cure rate of approximately 30% against MAs in mice models. Conclusion: Thus, the MRK hydrogel may serve as a prospective platform for antitumor applications.

A redox-responsive organogel based on a selenium-containing low molecular mass gelator

A redox-responsive low molecular mass gelator (Chol-Se) with selenium as its redox-active center was designed and synthesized. Responsive transformation of Chol-Se in ethyl acetate between the gelation and solution was achieved.

Short to ultrashort peptide-based hydrogels as a platform for biomedical applications

Short peptides have attracted significant attention from researchers in the past few years due to their easy design, synthesis and characterization, diverse functionalisation possibilities, low cost, possibility to make a large range of hierarchical nanostructures and most importantly their high biocompatibility and biodegradability. Generally, short peptides are also relatively more stable than their longer variants, non-immunogenic in nature and many of them self-assemble to provide an exciting range of nanostructures, including hydrogels. Thus, the development of short peptide-based hydrogels has become an area of intense investigation. Although these hydrogels have a water content of greater than 90%, they are surprisingly highly stable structures, and thus have been used for various biomedical applications, including cell therapeutics, drug delivery, tissue engineering and regeneration, contact lenses, biosensors, and wound healing, by different researchers. Herein, we review the progress of research in the rapidly expanding field of short to ultrashort peptide-based hydrogels and their possible applications. Special attention is paid to address and review this field with regard to the stability of peptide-based hydrogels, particularly to enzymatic degradation.

Low Molecular Weight Gelators Based on Functionalized l-Dopa Promote Organogels Formation

We prepared the small pseudopeptide Lau-l-Dopa(OBn)2-d-Oxd-OBn (Lau = lauric acid; l-Dopa = l-3,4-dihydroxyphenylalanine; d-Oxd = (4R,5S)-4-methyl-5-carboxyl-oxazolidin-2-one; Bn = benzyl) through a number of coupling reactions between lauric acid, protected l-Dopa and d-Oxd with an excellent overall yield. The ability of the product to form supramolecular organogels has been tested with different organic solvents of increasing polarity and compared with the results obtained with the small pseudopeptide Fmoc-l-Dopa(OBn)2-d-Oxd-OBn. The mechanical and rheological properties of the organogels demonstrated solvent-dependent properties, with a storage modulus of 82 kPa for the ethanol organogel. Finally, to have a preliminary test of the organogels' ability to adsorb pollutants, we treated a sample of the ethanol organogel with an aqueous solution of Rhodamine B (RhB) for 24 h. The water solution slowly lost its pink color, which became trapped in the organogel.

Amino Acid-Modified Conjugated Oligomer Self-Assembly Hydrogel for Efficient Capture and Specific Killing of Antibiotic-Resistant Bacteria

Bacterial infection is one of main causes that threaten global human health. Especially, antibiotic-resistant bacteria like methicillin-resistant Staphylococcus aureus (MRSA) lead to high mortality rate and more expensive treatment cost. Here, a novel amino-acid-modified conjugated oligomer OTE-d-Phe was synthesized by modifying the side chain of conjugated oligo(thiophene ethynylene) with d-phenylalanine. By mixing 9-fluorenylmethyloxycarbonyl-l-phenylalanin (Fmoc-l-Phe) with OTE-d-Phe, a new and biocompatible low-molecular weight hydrogel (HG-2) was prepared through self-assembly. In solution, HG-2 can effectively capture bacteria spontaneously, such as Escherichia coli and MRSA. Most importantly, the hydrogel has specific and strong antibacterial activity against MRSA over methicillin-susceptible S. aureus, Staphylococcus epidermidis, and E. coli. Interestingly, when the hydrogel was put on a model surface, a piece of cloth, it also is able to selectively kill MRSA with low cell cytotoxicity. The antibacterial mechanism was investigated, and it demonstrated that the HG-2 interacts with and physically breaks the cell wall and membrane, which leads to MRSA death. Therefore, this new conjugated oligomer-based hydrogel provides promising applications in disinfection and therapy of MRSA in hospital and in community.

Unraveling the Design Rules in Ultrashort Amyloid-Based Peptide Assemblies toward Shape-Controlled Synthesis of Gold Nanoparticles

The fundamental understanding of the detailed relationship between molecular structure and material function remains a challenging task, until now. In order to understand the relative contribution of aromatic moieties and hydrophobicity of amino acid chains, we designed a library of ultrashort amyloid-like peptides based on Ar-Phe-X (where "Ar" represents different aromatic moieties and "X" represents amino acids having varied side-chain functionalities). Our research clearly indicated that the alteration in the size and hydrophobicity of the aromatic capping play a crucial role compared to the subtle change in the amino acid sequence of the dipeptide in dictating the final self-assembled structure and properties of these short peptide amphiphiles. Further, we explored our detailed understanding toward the controlled synthesis of bioinspired organic-inorganic hybrids. For the first time, we established the differential role of aliphatic and aromatic hydroxyl moieties toward the in situ shape-controlled synthesis of gold nanoparticles in three-dimensional nanostructures of hydrogels. To the best of our knowledge, it is the first report which demonstrated the formation of rectangular platonic gold nanoparticles using simple dipeptide hydrogels, exhibiting pH-dependent size control. Our study shows promising implications in bottom-up nanofabrication of next-generation nanomaterials with emergent properties.

A novel low molecular weight nanocomposite hydrogel formulation for intra-tumoural delivery of anti-cancer drugs

Herein, an injectable formulation composed of a low molecular weight gelator (LMWG) based hydrogel and drug-loaded polymeric nanocapsules (NCs) is described. The NCs, made of hyaluronic acid and polyglutamic acid and loaded with C14-Gemcitabine (GEM C14), showed a size of 40 and 80 nm and a encapsulation efficiency >90%. These NCs exhibited a capacity to control the release of the encapsulated drug for >1 month. GEM C14-loaded NCs showed activity against various cancer cell lines in vitro; cell growth inhibition by 50% (GI₅₀) values of 15 ± 6, 10 ± 9, 13 ± 3 and 410 ± 463 nM were obtained in HCT 116, MIA PaCa-2, Panc-1 and Panc-1 GEM resistant cell lines respectively. Nanocomposite hydrogels were prepared using the LMWG - N4-octanoyl-2'-deoxycytidine and loaded for the first time with polymeric NCs. 2% and 4% w/v nanocapsule concentrations as compared to 8% w/v NC concentrations with 2% and 3% w/v gelator concentrations gave mechanically stronger gels as determined by oscillatory rheology. Most importantly, the nanocomposite formulation reformed instantly into a gel after injection through a needle. Based on these properties, the nanocomposite gel formulation has potential for the intratumoural delivery of anticancer drugs.

Tuning the Handedness: Role of Chiral Component in Peptide-Appended Bolaamphiphile-Based Coassembled Hydrogels

Chirality is the intrinsic property of a molecule that can be tuned by the change in chirality of a molecule or by the addition of a chiral component as an external stimulus. An l-leucine-based dipeptide-appended succinic acid-based bolaamphiphile coassembled with d-tartaric acid to form supramolecular right-handed nanostructured hydrogel, whereas l-tartaric acid coassembled to form supramolecular left-handed nanostructured hydrogel. Scanning electron microscopy and transmission electron microscopy experiments revealed the right- and left-handed helical nanofibers that are responsible for the formation of supramolecular nanostructured hydrogels. The synergistic chiral effect of l-leucine in peptide bolaamphiphile and d/l-tartaric acid plays a significant role in bicomponent gelation with helical nanofibers. The first two amino acids attached to both sides of succinic acid moiety act as a tuning button for supramolecular chirality of amino acids/peptides attached with succinic acid-based bolaamphiphiles. The second amino acid plays the role of modulating supramolecular chirality if the first two amino acids act neutrally to the chirality of bolaamphiphiles, which was confirmed by circular dichroism spectroscopy.

In search of bioinspired hydrogels from amphiphilic peptides: a template for nanoparticle stabilization for the sustained release of anticancer drugs

This report presents the efficiency of palmitic acid-based proteolytically stable, biocompatible hydrogelators for the sustained release of anticancer drugs.

Applications of self-assembling ultrashort peptides in bionanotechnology

Peptides are intriguing building blocks for a variety of applications in bionanotechnology. Peptides can self-assemble into well-ordered nanostructures. Among the various nanomorphology forms, peptide nanofibers and nanotubes are relevant in biomedical applications. In this review, their applications as tissue engineering scaffolds, drug delivery vehicles, three-dimensional printing bioinks and bioimaging nanoprobes will be illustrated. This review article describes di-, tri-, tetra-peptides because they are cost-effective, simple to prepare, and amenable to production on a large scale.

Ultrashort peptide applications in bionanotechnology.

A temperature-responsive supramolecular hydrogel: preparation, gel-gel transition and molecular aggregation

In this study, a novel supramolecular hydrogel, abbreviated as AGC16/NTS, was designed and constructed by the molecular self-assembly of a cationic gemini surfactant, 1,3-bis(N,N-dimethyl-N-cetylammonium)-2-propylacrylatedibromide (AGC16), and an anionic aromatic gelator, trisodium 1,3,6-naphthalenetrisulfonate (NTS). The AGC16/NTS hydrogel was able to form in a mass ratio range of AGC16 and NTS from 20 : 1 to 10 : 1. It was interestingly found that AGC16/NTS exhibited two phase transitions (gel-to-gel and gel-to-sol) observed by visual and rheological measurements during the heating process, which is rarely reported in the previous literature reports of hydrogels prepared using low molecular weight gelators. Cryogenic scanning electron microscopy (cryo-SEM), fluorescence emission spectroscopy and X-ray diffraction (XRD) were used to investigate the temperature-responsive properties and molecular self-assembly mechanism of the hydrogel AGC16/NTS. During the gel-to-gel transition process, the temperature-responsive changes in the visual appearance of AGC16/NTS (turbid to transparent) were clearly observed. Compared with the transparent gel, the turbid gel possesses higher mechanical strength and a much more compact network mophology due to stronger intermolecular hydrophobic association beetween gelators. The molecular self-assembly modes for the two different hydrogel states (turbid and transparent gel) were proposed, helping to further understand the hydrogel transition mechanisms at a molecular level.

Tumor Ablation and Therapeutic Immunity Induction by an Injectable Peptide Hydrogel

Immunosuppressive tumor microenvironments (TMEs) create tremendous obstacles for an effective cancer therapy. Herein, we developed a melittin-RADA₃₂ hybrid peptide hydrogel loaded with doxorubicin (DOX) for a potent chemoimmunotherapy against melanoma through the active regulation of TMEs. The formed melittin-RADA₃₂-DOX (MRD) hydrogel has an interweaving nanofiber structure and exhibits excellent biocompatibility, controlled drug release properties both in vitro and in vivo, and an enhanced killing effect to melanoma cells. A single-dose injection of MRD hydrogel retarded the growth of primary melanoma tumors by more than 95% due to loaded melittin and DOX, with concomitant recruitment of activated natural killer cells in the tumors. Furthermore, MRD hydrogel can activate dendritic cells of draining lymph nodes, specifically deplete M2-like tumor-associated macrophages (TAMs), and produce active, cytotoxic T cells to further defend the cells against remaining tumors, providing potent anticancer efficacy against subcutaneous and metastatic tumors in vivo. Multidose injection of MRD hydrogel eliminated 50% of the primary tumors and provided a strong immunological memory effect against tumor rechallenge after eradication of the initial tumors. Owing to its abilities to perform controlled drug release, regulate innate immune cells, deplete M2-like TAMs, direct anticancer and immune-stimulating capabilities, and reshape immunosuppressive TMEs, MRD hydrogel may serve as a powerful tool for anticancer applications.

Controlled drug delivery vehicles for cancer treatment and their performance

Although conventional chemotherapy has been successful to some extent, the main drawbacks of chemotherapy are its poor bioavailability, high-dose requirements, adverse side effects, low therapeutic indices, development of multiple drug resistance, and non-specific targeting. The main aim in the development of drug delivery vehicles is to successfully address these delivery-related problems and carry drugs to the desired sites of therapeutic action while reducing adverse side effects. In this review, we will discuss the different types of materials used as delivery vehicles for chemotherapeutic agents and their structural characteristics that improve the therapeutic efficacy of their drugs and will describe recent scientific advances in the area of chemotherapy, emphasizing challenges in cancer treatments.

Improving the delivery of cancer therapies to tumor sites is crucial to reduce unwanted side effects and patient mortality rates. Pralay Maiti and colleagues at the Indian Institute of Technology in Varanasi, India, review the latest developments in drug delivery vehicles and treatment approaches designed to enhance the effectiveness of current cancer therapies. New nanoparticle-based carriers, hydrogels and hybrid materials that offer controlled and sustained drug release are showing great promise in animal models. Furthermore, materials that respond to stimuli such as heat, light, magnetic or electric fields are also being tested to aid target-specific drug delivery and, thus, avoid damage to healthy tissues. Although there are some challenges in translating these findings to the clinic, there is no doubt that technological advances are shaping better and safer treatment options.

Release of small bioactive molecules from physical gels

Pharmaceutical drugs with low water solubility have always received great attention within the scientific community. The reduced bioavailability and the need of frequent administrations have motivated the investigation of new drug delivery systems. Within this context, drug carriers that release their payload in a sustained way and hence reduce the administration rate are highly demanded. One interesting strategy to meet these requirements is the entrapment of the drugs into gels. So far, the most investigated materials for such drug-loaded gels are derived from polymers and based on covalent linkages. However, over the last decade the use of physical (or supramolecular) gels derived from low molecular weight compounds has experienced strong growth in this field, mainly due to important properties such as injectability, stimuli responsiveness and ease of synthesis. This review summarizes the use of supramolecular gels for the encapsulation and controlled release of small therapeutic molecules.

An auxin–tyrosine derivative based biocompatible supergelator: a template for fabrication of nanoparticles for sustained release of model drugs

This report reveals the supergelating ability of an auxin-tyrosine derivative based biocompatible hydrogel in drug delivery.

l-Carnosine-Derived Fmoc-Tripeptides Forming pH-Sensitive and Proteolytically Stable Supramolecular Hydrogels

A series of β-amino acid containing tripeptides has been designed and synthesized in order to develop oligopeptide-based, thermoreversible, pH-sensitive, and proteolytically stable hydrogels. The Fmoc [N-(fluorenyl-9-methoxycarbonyl)]-protected tripeptides were found to produce hydrogels in both pH 7 and 2 buffers at a very low concentration (<0.2% w/v). It has been shown that the Fmoc group plays an important role in the gelation process. Also a dependence of gelation ability on hydrophobicity of the side chain of the Fmoc-protected α-amino acid was observed. The effect of the addition of inorganic salts on the gelation process was investigated as well. Spectroscopic studies indicated formation of J-aggregates through π-π stacking interactions between Fmoc groups in solution as well as in the gel state. In the gel phase, these self-assembling tripeptides form long interconnected nanofibrils leading to the formation of 3-dimensional network structure. The hydrogels were characterized by various techniques, including field emission electron microscopy, transmission electron microscopy, atomic force microscopy, rheology, Fourier transform IR, circular dichroism (CD), and wide-angle X-ray diffraction (WAXD) spectroscopy. The CD studies and WAXD analyses show an antiparallel β-sheet structure in the gel state. l-Phenylalanine and l-tyrosine containing tripeptides formed helical aggregates with handedness opposite to those containing l-valine and l-leucine residues. The mechanical stability of the hydrogels was found to depend on the hydrophobicity of the side chain of the tripeptide as well as on the pH of the solution. Also, the tripeptides exhibit in vitro proteolytic stability against proteinase K enzyme.

Amphiphilic Peptide-Based Supramolecular, Noncytotoxic, Stimuli-Responsive Hydrogels with Antibacterial Activity

A series of peptides with a long fatty acyl chain covalently attached to the C-terminal part and a free amine (-NH₂) group at the N-terminus have been designed so that these molecules can be assembled in aqueous medium by using various noncovalent interactions. Five different peptide amphiphiles with a general chemical formula [H₂N-(CH₂)_nCONH-Phe-CONHC₁₂ (n = 1-5, C₁₂ = dodecylamine)] have been synthesized, characterized, and examined for self-assembly and hydrogelation. All of these molecules [P1 (n = 1), P2 (n = 2), P3 (n = 3), P4 (n = 4), P5 (n = 5)] form thermoresponsive hydrogels in water (pH 6.6) with a nanofibrillar network structure. Interestingly, the hydrogels obtained from compounds P4 and P5 exhibit potential antimicrobial activity against Gram-positive bacteria (Staphylococcus aureus, Bacillus subtilis) and Gram-negative bacteria (Escherichia coli). Dose-dependent cell-viability studies using MTT assay (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) by taking human lung carcinoma (A549) cells vividly demonstrates the noncytotoxic nature of these gelator molecules in vitro. Hemolytic studies show nonsignificant or little hemolysis of human erythrocyte cells at the minimum inhibitory concentration (MIC) of these tested bacteria. Interestingly, it has been found that these antibacterial noncytotoxic hydrogels exhibit proteolytic resistance toward the enzymes proteinase K and chymotrypsin. Moreover, the gel strength and gel recovery time have been successfully modulated by varying the alkyl chain length of the N-terminally located amino acid residues. Similarly, the thermal stability of these hydrogels has been nicely tuned by altering the alkyl chain length of the N-terminally located amino acid residues. In the era of antibiotic-resistant strains of bacteria, the discovery of this new class of peptide-based antibacterial, proteolytically stable, injectable, and noncytotoxic soft materials holds future promise for the development of new antibiotics.