Recent Advances in Smart Self-Assembled Bioinspired Hydrogels: A Bridging Weapon for Emerging Health Care Applications from Bench to Bedside

Stimuli-responsive low molecular weight hydrogel interventions for Biomedical challenges are a rapidly evolving paradigm in the bottom-up approach recently. Peptide-based self-assembled nano biomaterials present safer alternatives to their non-degradable counterparts as demanded for today's most urged clinical needs.Although a plethora of work has already been accomplished, programming hydrogelators with appropriate functionalities requires a better understanding as the impact of the macromolecular structure of the peptides and subsequently, their self-assembled nanostructures remain unidentified. Henceforth this review focuses on two aspects: Firstly, the underlying guidelines for building biomimetic strategies to tailor scaffolds leading to hydrogelation along with the role of non-covalent interactions that are the key components of various self-assembly processes. In the second section, it is aimed to bring together the recent achievements with designer assembly concerning their self-aggregation behaviour and applications mainly in the biomedical arena like drug delivery carrier design, antimicrobial, anti-inflammatory as well as wound healing materials. Furthermore, it is anticipated that this article will provide a conceptual demonstration of the different approaches taken towards the construction of these task-specific designer hydrogels. Finally, a collective effort among the material scientists is required to pave the path for the entrance of these intelligent materials into medicine from bench to bedside.

Quinoline-cored Poly(Aryl Ether) Dendritic Organogels with Multiple Stimuli-Responsive and Adsorptive Properties

Functional supramolecular gel materials have potential applications in sensors, optical switches, artificial antennae, drug delivery and so on. In this paper, quinoline-cored poly(aryl ether) dendritic organogelators were designed, synthesized and fully characterized. The gelation behaviour of the dendritic organogelator was tested in organic solvents, mixed solvents and ionic liquids. The dendron Q-G1 was found to be an efficient and versatile organogelator toward various apolar and polar organic solvents with the critical gelation concentrations (CGCs) approaching 1.2×10^-2  mol/L, indicating one dendritic organogelator could immobilize 1.2×10³ solvent molecules in the organogel network. Interestingly, these dendrons exhibited excellent gel formation in ionic liquids. Notably, these dendritic organogels were found to display multiple stimuli-responsive properties toward external stimuli including heat, ultrasound and shear stress, with a reversible sol-gel phase transition. In addition, the dendritic organogel could effectively adsorb heavy metals and organic dyes. The removal rate of Pb²⁺ was up to 20% and the adsorption rate for Rhodamine B was as high as 89%.

Designer Peptide and Protein Dendrimers: A Cross-Sectional Analysis

Dendrimers have attracted immense interest in science and technology due to their unique chemical structure that offers a myriad of opportunities for researchers. Dendritic design allows us to present peptides in a branched three-dimensional fashion that eventually leads to a globular shape, thus mimicking globular proteins. Peptide dendrimers, unlike other classes of dendrimers, have immense applications in biomedical research due to their biological origin. The diversity of potential building blocks and innumerable possibilities for design, along with the fact that the area is relatively underexplored, make peptide dendrimers sought-after candidates for various applications. This review summarizes the stepwise evolution of peptidic dendrimers along with their multifaceted applications in various fields. Further, the introduction of biomacromolecules such as proteins to a dendritic scaffold, resulting in complex macromolecules with discrete molecular weights, is an altogether new addition to the area of organic chemistry. The synthesis of highly complex and fully folded biomacromolecules on a dendritic scaffold requires expertise in synthetic organic chemistry and biology. Presently, there are only a handful of examples of protein dendrimers; we believe that these limited examples will fuel further research in this area.

Stearoyl-appended pendant amino acid-based hyperbranched polymers for selective gelation of oil from oil/water mixtures

Stearic acid-appended pendant amino acid-based poly(methacrylate) hyperbranched polymers were developed for the phase-selective organogelation of crude oil from a binary mixture of oil/water.

A ferrocene–azobenzene derivative showing unprecedented phase transition and better solubility upon UV irradiation

A ferrocene–azobenzene derivative showing unprecedented crystal–liquid phase transition at an elevated temperature and better solubility in organic solvents after UV irradiation has been successfully reported.

Photoresponsive gelators

We review the different approaches that have been used to form low molecular weight gels that respond to light.

Modifying a known gelator scaffold for nitrite detection

The process of selecting and modifying a known gelator scaffold to develop a new nitrite-based sensor is described. Five new azo-sulfonate gelators were discovered and characterized. The most promising scaffold exhibits a stable diazonium intermediate, proceeds in a high yield, and gels nitrite-spiked tap, river, and pond water.

Light induced E–Z isomerization in a multi-responsive organogel: elucidation from 1H NMR spectroscopy

Light induced E–Z isomerization along imine bond in a multiresponsive organogel of anthracene attached 3,4,5-tris(dodecyloxy)benzohydrazide gelator altering morphology, fluorescence and mechanical properties is elucidated from ¹H NMR spectra.

Gelation induced supramolecular chirality: chirality transfer, amplification and application

Supramolecular chirality defines chirality at the supramolecular level, and is generated from the spatial arrangement of component molecules assembling through non-covalent interactions such as hydrogen bonding, van der Waals interactions, π-π stacking, hydrophobic interactions and so on. During the formation of low molecular weight gels (LMWGs), one kind of fascinating soft material, one frequently encounters the phenomenon of chirality as well as chiral nanostructures, either from chiral gelators or even achiral gelators. A view of gelation-induced supramolecular chirality will be very helpful to understand the self-assembly process of the gelator molecules as well as the chiral structures, the regulation of the chirality in the gels and the development of the "smart" chiral materials such as chiroptical devices, catalysts and chiral sensors. It necessitates fundamental understanding of chirality transfer and amplification in these supramolecular systems. In this review, recent progress in gelation-induced supramolecular chirality is discussed.

Supramolecular hydrogels based on short peptides linked with conformational switch

Short peptides appropriately linked with an azobenzene conformational switch were found to be motif and pH dependant supramolecular hydrogelators. The hydrogelation properties of the short peptides linked with the conformational switch were studied in detail with respect to dependence on amino acid residue, pH and salt effect. The presence of amino acids with aromatic side chains such as Phe and Tyr was found to be favorable for the short peptides to gel water at an appropriate pH range. Cationic amino acid residues such as Arg and Lys in the short peptides were found to be unfavorable for hydrogelation. pH and salt effect were also found to be important factors for the hydrogelation properties of the short peptides. A series of short peptides with bioactive sequences were linked with the conformational switch and their hydrogelation properties were investigated. Photoresponsive supramolecular hydrogels were realized based on the E-/Z- transition of the conformational switch upon light irradiation. Proper combination of amino acid residues in the short peptides resulted in smart supramolecular hydrogels with responses to multiple stimuli.

Photoresponsive Organogel Based on Supramolecular Assembly of Tris(phenylisoxazolyl)benzene

Azobenzene-substituted tris(phenylisoxazolyl)benzene 1 was developed as a photoresponsive gelator. A detailed study of the self-assembly of the trans- and cis-isomers in solution revealed that the planar trans-isomer assembled to form molecular stacks along its C 3 axis, whereas the cis-isomer did not owing to steric requirements. Based on diffusion ordered spectroscopy experiments, the size of the aggregates formed from the trans-isomer were roughly four times as large as those of the cis-isomer. The photoinduced gel-to-sol transition was achieved by irradiation with UV light at 360 nm. Solid-state morphologies of the trans- and cis-isomers were quite contrastive; the trans-isomer created fibrous supramolecular networks with a lot of voids in which solvent molecules could be immobilized, whereas the cis-isomer never created such fibrous morphologies. The trans–cis structural change of the azobenzene moieties obviously regulates the gelation ability of 1.

Design and self-assembly of L-glutamate-based aromatic dendrons as ambidextrous gelators of water and organic solvents

A series of L-glutamate-based dendrons containing aromatic cores ranging from phenyl to naphthyl to anthryl were synthesized, and their self-assembly in organic solvents as well as in water was investigated. It was found that all of these dendrons formed organogels with hexane and simultaneously formed the hydrogels with water, thus exhibiting ambidextrous properties. Nanofiber structures are essentially formed in organogels and hydrogels, and some nanostrips are formed in some of the hydrogels. During gel formation, both hydrogen bonds between the amide groups and the pi-pi stacking between the aromatic rings played a predominant role in forming the ID nanostructures. Both the hydrogels and the organogels containing naphthyl and anthryl groups showed fluorescence emission. In comparison with the corresponding compounds in solution, the naphthyl-containing dendrons showed a strong enhancement of fluorescence in the gel. In the case of anthryl-containing dendrons, fluorescence enhancement was observed for the derivative with anthryl substituted at the 9 position, whereas a decrease was observed for the gels of the 2-substituted derivative. Although the chirality of L-glutamate could not be transferred to the aromatic chromophores in solution, it was transferred to the chromophores in gels as a result of the self-assembly and strong pi-pi interaction of the gelators. On the basis of the properties of the organogels and hydrogels, a thermally driven chiroptical switch was proposed. That is, the chirality disappeared when the gel was heated to solution, whereas it returned when cooled to a gel. The process can be repeated many times.

Rapid and reversible gel-sol transition of self-assembled gels induced by photoisomerization of dendritic azobenzenes

An asymmetric bis-dendritic gelator (1) consisting of an azobenzene dendron and an aliphatic amide dendron was synthesized to achieve a photoresponsive self-assembly. The compound gelled in a wide range of organic solvents, even at concentrations as low as 0.02% (w/v) in cyclohexane. The self-assembled fibrillar network structure was confirmed by field-emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), and X-ray diffraction (XRD) analyses. The rapid and reversible gel-sol transition by irradiation with UV and visible light was investigated by UV-vis and Fourier transform infrared (FT-IR) spectroscopy, FE-SEM, and XRD analyses. Upon irradiation of the gel with UV, trans-to-cis isomerization of the azobenzene groups occurred, and the gel turned into a sol state. The gel was recovered immediately by the reverse cis-to-trans isomerization after the exposure to visible light. The trans-to-cis isomerization of the azobenzenes disrupted the hydrogen bonding of azobenzene amide groups, together with the hydrogen bonding in the aliphatic amide dendron. This facile communication between the two amide dendrons leads to the dissociation of the gel fibers and collapse of the gel.