Post-self-assembly cross-linking of molecular nanofibers for oscillatory hydrogels

Self-Assembly, Bioactivity, and Nanomaterials Applications of Peptide Conjugates with Bulky Aromatic Terminal Groups

The self-assembly and structural and functional properties of peptide conjugates containing bulky terminal aromatic substituents are reviewed with a particular focus on bioactivity. Terminal moieties include Fmoc [fluorenylmethyloxycarbonyl], naphthalene, pyrene, naproxen, diimides of naphthalene or pyrene, and others. These provide a driving force for self-assembly due to π-stacking and hydrophobic interactions, in addition to the hydrogen bonding, electrostatic, and other forces between short peptides. The balance of these interactions leads to a propensity to self-assembly, even for conjugates to single amino acids. The hybrid molecules often form hydrogels built from a network of β-sheet fibrils. The properties of these as biomaterials to support cell culture, or in the development of molecules that can assemble in cells (in response to cellular enzymes, or otherwise) with a range of fascinating bioactivities such as anticancer or antimicrobial activity, are highlighted. In addition, applications of hydrogels as slow-release drug delivery systems and in catalysis and other applications are discussed. The aromatic nature of the substituents also provides a diversity of interesting optoelectronic properties that have been demonstrated in the literature, and an overview of this is also provided. Also discussed are coassembly and enzyme-instructed self-assembly which enable precise tuning and (stimulus-responsive) functionalization of peptide nanostructures.

Utilizing Frémy's Salt to Increase the Mechanical Rigidity of Supramolecular Peptide-Based Gel Networks

Peptide-based supramolecular gels are an important class of biomaterials that can be used for biomedical applications ranging from drug delivery to tissue engineering. Methodology that allows one to readily modulate the mechanical properties of these gels will allow yet even a broader range of applications. Frémy's salt is an inorganic salt and long-lived free radical that is known to oxidize phenols. Herein, we show that Frémy's salt can be used to dramatically increase the mechanical rigidity of hydrogels formed by tyrosine-containing self-assembling β-hairpin peptides. When Frémy's salt is added to pre-formed gels, it converts tyrosine residues to o-quinones that can subsequently react with amines present within the lysine side chains of the assembled peptide. This results in the installation of chemical crosslinks that reinforce the gel matrix. We characterized the unoxidized and oxidized gel systems using UV-Vis, transmission electron microscopy and rheological measurements and show that Frémy's salt increases the gel rigidity by nearly one order of magnitude, while retaining the gel's shear-thin/recovery behavior. Thus, Frémy's salt represents an on-demand method to modulate the mechanical rigidity of peptide-based self-assembled gels.

Chemical Oscillation and Morphological Oscillation in Catalyst-Embedded Lyotropic Liquid Crystalline Gels

Liquid crystalline gels offer promising means in generating smart materials due to programmable mechanics and reversible shape changes in response to external stimuli. We demonstrate a simple and convenient method of constructing catalyst-embedded lyotropic liquid crystalline (LLC) gels and achieve chemomechanical oscillator by converting chemical waves in Belousov–Zhabotinsky (BZ) reaction. We observe the directed chemical oscillations on LLC sticks accompanied by small-scale oscillatory swellings–shrinkages that are synchronized with the chemical waves of an LLC stick. To amplify the mechanical oscillations, we further fabricate small LLC fibers and achieve macroscopically oscillatory bending–unbending transition of the LLC fiber driven by a BZ reaction.

Covalent Post-Assembly Modification: A Synthetic Multipurpose Tool in Supramolecular Chemistry

The use of covalent post-assembly modification (PAM) in supramolecular chemistry has grown significantly in recent years, to the point where PAM is now a versatile synthesis tool for tuning, modulating, and expanding the functionality of self-assembled complexes and materials. PAM underpins supramolecular template-synthesis strategies, enables modular derivatization of supramolecular assemblies, permits the covalent 'locking' of unstable structures, and can trigger controlled structural transformations between different assembled morphologies. This Review discusses key examples of PAM spanning a range of material classes, including discrete supramolecular complexes, self-assembled soft nanostructures and hierarchically ordered polymeric and framework materials. As such, we hope to highlight how PAM has continued to evolve as a creative and functional addition to the synthetic chemist's toolbox for constructing bespoke self-assembled complexes and materials.

Oxidoreductase-Initiated Radical Polymerizations to Design Hydrogels and Micro/Nanogels: Mechanism, Molding, and Applications

Due to their 3D cross-linked networks and tunable physicochemical properties, polymer hydrogels with different sizes are applied widely in tissue engineering, drug-delivery systems, pollution regulation, ionic conducting electrolytes, agricultural drought-resistance, cosmetics, and the food industry. Novel, environmentally friendly, and efficient oxidoreductase-initiated radical polymerizations to design hydrogels and micro/nanogels have gained increasing attention. Herein, the recent advances on the use of novel enzyme-initiated systems for hydrogel polymerization, including the mechanisms, and molding of polymeric and hybrid-polymeric networks are reviewed. Preliminary progress related to interfacial enzymatic polymerization for the generation of hybrid micro/nanogels is introduced as an emerging initiating approach. In addition, certain biological applications in tissue engineering, bioimaging, and therapy are demonstrated step by step. Finally, some perspectives on the safety profile of enzymatic formed hydrogels, new enzymatic systems, and potential theranostic applications are discussed.

Enzyme-Instructed Self-Assembly of Small D-Peptides as a Multiple-Step Process for Selectively Killing Cancer Cells

Selective inhibition of cancer cells remains a challenge in chemotherapy. Here we report the molecular and cellular validation of enzyme-instructed self-assembly (EISA) as a multiple step process for selectively killing cancer cells that overexpress alkaline phosphatases (ALPs). We design and synthesize two kinds of d-tetrapeptide containing one or two phosphotyrosine residues and with the N-terminal capped by a naphthyl group. Upon enzymatic dephosphorylation, these d-tetrapeptides turn into self-assembling molecules to form nanofibers in water. Incubating these d-tetrapeptides with several cancer cell lines and one normal cell line, the unphosphorylated d-tetrapeptides are innocuous to all the cell lines, the mono- and diphosphorylated d-tetrapeptides selectively inhibit the cancer cells, but not the normal cell. The monophosphorylated d-tetrapeptides exhibit more potent inhibitory activity than the diphosphorylated d-tetrapeptides do; the cancer cell lines express higher level of ALPs are more susceptible to inhibition by the phosphorylated d-tetrapeptides; the precursors of d-tetrapeptides that possess higher self-assembling abilities exhibit higher inhibitory activities. These results confirm the important role of enzymatic reaction and self-assembly. Using uncompetitive inhibitors of ALPs and fluorescent d-tetrapeptides, we delineate that the enzyme catalyzed dephosphorylation and the self-assembly steps, together, result in the localization of the nanofibers of d-tetrapeptides for killing the cancer cells. We find that the cell death modality likely associates with the cell type and prove the interactions between nanofibers and the death receptors. This work illustrates a paradigm-shifting and biomimetic approach and contributes useful molecular insights for the development of spatiotemporal defined supramolecular processes/assemblies as potential anticancer therapeutics.

Printable hybrid hydrogel by dual enzymatic polymerization with superactivity

Hybrid hydrogels were fabricated via a new approach employing a dual enzyme-mediated redox initiation reaction and their applications for 3D printing and biocatalysis.

A new approach has been developed to fabricate tough hybrid hydrogels by employing dual enzyme-mediated redox initiation to achieve post-self-assembly cross-linking polymerization. The resulting hydrogel combines the merits of supramolecular hydrogels with polymeric hydrogels to achieve higher mechanical strength and porous networks. Designed 3D constructs were fabricated via in situ 3D printing. The in situ immobilized GOx/HRP in Gel II exhibited superactivity compared to free enzymes, which might be attributed to the synergistic effect of co-localized GOx and HRP minimizing the distances for mass transport between the gel and the bulk solution. This mechanically strong hybrid hydrogel maintained high reusability and thermal stability as well. In addition, in situ 3D cell culture was demonstrated, thus indicating that this biodegradable hybrid hydrogel is biocompatible with cells. The subsequent 3D cell printing further indicates that the hybrid hydrogel is a promising scaffold for bio-related applications such as biocatalysis and tissue engineering.

Viscosity-controlled printing of supramolecular-polymeric hydrogels via dual-enzyme catalysis

Hybrid hydrogels based on a guanidinium-containing oligopeptide are prepared via dual-enzyme-triggered reactions. An extended time window is available for in situ viscosity-controlled 3D printing.

Supramolecular Hydrogelators and Hydrogels: From Soft Matter to Molecular Biomaterials

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

Recent advances in the construction of fluorescent metallocycles and metallocages via coordination-driven self-assembly

During the last few years, the construction of fluorescent metallocycles and metallocages has attracted considerable attention because of their wide applications in fluorescence detection of metal ions, anions, or small molecules, mimicking complicated natural photo-processes, and preparing photoelectric devices, etc. This perspective focuses on the recent advances in the construction of a variety of fluorescent metallocycles and metallocages via coordination-driven self-assembly. In addition, the fluorescence properties and the applications of these organometallic architectures have also been discussed.

A modular approach to self-oscillating polymer systems driven by the Belousov–Zhabotinsky reaction

This review explores the principle, modular construction, integral control and engineering aspects of self-oscillating polymer systems driven by the Belousov–Zhabotinsky reaction.

Improved mechanical properties of polyacrylamide hydrogels created in the presence of low-molecular-weight hydrogelators

It was found that the crushing stress of the obtained polyacrylamide hydrogels was enhanced by using the molecular gel as removable templates.

Rational design of MMP degradable peptide-based supramolecular filaments

One-dimensional nanostructures formed by self-assembly of small molecule peptides have been extensively explored for use as biomaterials in various biomedical contexts. However, unlike individual peptides that can be designed to be specifically degradable by enzymes/proteases of interest, their self-assembled nanostructures, particularly those rich in β-sheets, are generally resistant to enzymatic degradation because the specific cleavage sites are often embedded inside the nanostructures. We report here on the rational design of β-sheet rich supramolecular filaments that can specifically dissociate into less stable micellar assemblies and monomers upon treatment with matrix metalloproteases-2 (MMP-2). Through linkage of an oligoproline segment to an amyloid-derived peptide sequence, we first synthesized an amphiphilic peptide that can undergo a rapid morphological transition in response to pH variations. We then used MMP-2 specific peptide substrates as multivalent cross-linkers to covalently fix the amyloid-like filaments in the self-assembled state at pH 4.5. Our results show that the cross-linked filaments are stable at pH 7.5 but gradually break down into much shorter filaments upon cleavage of the peptidic cross-linkers by MMP-2. We believe that the reported work presents a new design platform for the creation of amyloid-like supramolecular filaments responsive to enzymatic degradation.

A self-oscillating gel actuator driven by ferroin

In the wake of the Belousov–Zhabotinsky reaction catalyzed by ferroin, the swelling–deswelling oscillating soft actuator exhibits 7 min period of self-oscillation for the first time.

Coupled chemical oscillators and emergent system properties

We review recent work on a variety of systems, from the nanometre to the centimetre scale, including microemulsions, microfluidic droplet arrays, gels and flow reactors, in which chemical oscillators interact to generate novel spatiotemporal patterns and/or mechanical motion.

Recent aspects of self-oscillating polymeric materials: designing self-oscillating polymers coupled with supramolecular chemistry and ionic liquid science

Herein, we summarise the recent developments in self-oscillating polymeric materials based on the concepts of supramolecular chemistry, where aggregates of molecular building blocks with non-covalent bonds evolve the temporal or spatiotemporal structure.

Nanomaterials design and tests for neural tissue engineering

Nanostructured scaffolds recently showed great promise in tissue engineering: nanomaterials can be tailored at the molecular level and scaffold morphology may more closely resemble features of extracellular matrix components in terms of porosity, framing and biofunctionalities. As a consequence, both biomechanical properties of scaffold microenvironments and biomaterial-protein interactions can be tuned, allowing for improved transplanted cell engraftment and better controlled diffusion of drugs. Easier said than done, a nanotech-based regenerative approach encompasses different fields of know-how, ranging from in silico simulations, nanomaterial synthesis and characterization at the nano-, micro- and mesoscales to random library screening methods (e.g. phage display), in vitro cellular-based experiments and validation in animal models of the target injury. All of these steps of the "assembly line" of nanostructured scaffolds are tightly interconnected both in their standard analysis techniques and in their most recent breakthroughs: indeed their efforts have to jointly provide the deepest possible analyses of the diverse facets of the challenging field of neural tissue engineering. The purpose of this review is therefore to provide a critical overview of the recent advances in and drawbacks and potential of each mentioned field, contributing to the realization of effective nanotech-based therapies for the regeneration of peripheral nerve transections, spinal cord injuries and brain traumatic injuries. Far from being the ultimate overview of such a number of topics, the reader will acknowledge the intrinsic complexity of the goal of nanotech tissue engineering for a conscious approach to the development of a regenerative therapy and, by deciphering the thread connecting all steps of the research, will gain the necessary view of its tremendous potential if each piece of stone is correctly placed to work synergically in this impressive mosaic.