Pristine Carbon-Nanotube-Included Supramolecular Hydrogels with Tunable Viscoelastic Properties

A repertoire of nanoengineered short peptide-based hydrogels and their applications in biotechnology

Peptide nanotechnology has currently bridged the gap between materials and biological worlds. Bioinspired self-assembly of short-peptide building blocks helps take the leap from molecules to materials by taking inspiration from nature. Owing to their intrinsic biocompatibility, high water content, and extracellular matrix mimicking fibrous morphology, hydrogels engineered from the self-assembly of short peptides exemplify the actualization of peptide nanotechnology into biomedical products. However, the weak mechanical property of these hydrogels jeopardizes their practical applications. Moreover, their functional diversity is limited since they comprise only one building block. Nanoengineering the networks of these hydrogels by incorporating small molecules, polymers, and inorganic/carbon nanomaterials can augment the mechanical properties while retaining their dynamic supramolecular nature. These additives interact with the peptide building blocks supramolecularly and may enhance the branching of the networks via coassembly or crystallographic mismatch. This phenomenon expands the functional diversity of these hydrogels by synergistically combining the attributes of the individual building blocks. This review highlights such nanoengineered peptide hydrogels and their applications in biotechnology. We have included exemplary works on supramolecular modification of the peptide hydrogel networks by integrating other small molecules, synthetic/biopolymers, conductive polymers, and inorganic/carbon nanomaterials and shed light on their various utilities focusing on biotechnology. We finally envision some future prospects in this highly active field of research.

Single-Walled Carbon Nanotubes as Fluorescent Probes for Monitoring the Self-Assembly and Morphology of Peptide/Polymer Hybrid Hydrogels

Hydrogels formed via supramolecular self-assembly of fluorenylmethyloxycarbonyl (Fmoc)-conjugated amino acids provide excellent scaffolds for 3D cell culture, tissue engineering, and tissue recovery matrices. Such hydrogels are usually characterized by rheology or electron microscopy, which are invasive and cannot provide real-time information. Here, we incorporate near-infrared fluorescent single-walled carbon nanotubes (SWCNTs) into Fmoc-diphenylalanine hydrogels as fluorescent probes, reporting in real-time on the morphology and time-dependent structural changes of the self-assembled hydrogels in the transparency window of biological tissue. We further demonstrate that the gelation process and structural changes upon the addition of cross-linking ions are transduced into spectral modulations of the SWCNT-fluorescence. Moreover, morphological differences of the hydrogels induced by polymer additives are manifested in unique features in fluorescence images of the incorporated SWCNTs. SWCNTs can thus serve as optical probes for noninvasive, long-term monitoring of the self-assembly gelation process and the fate of the resulting peptide hydrogel during long-term usage.

Ultralight hybrid silica aerogels derived from supramolecular hydrogels self-assembled from insoluble nano building blocks

Supramolecular hydrogels are a type of hydrogel cross-linked by non-chemical bonds and they have been widely applied in the field of smart systems, sensors, tissue engineering, and controlled drug delivery. Most supramolecular hydrogels are formed by soluble molecules, polymers, and metal ions. In this work, supramolecular hydrogels self-assembled between two insoluble nano building blocks (ISNBBs), graphene oxide (GO) and amino-functionalized silica nanoparticles (SiO₂-NH₂), have been discovered and synthesized. The gelation conditions of the two ISNBBs have been investigated. A step further, ultralight hybrid silica aerogels are obtained by supercritical drying of the physical hydrogels. No visible volume shrinkage is observed, due to the fact that the hydrogel networks are formed by rigid ISNBBs. Thus the hybrid aerogels possess ultralow density (down to 7.5 mg cm^-3), high specific surface areas (178.6 m² g^-1), and extremely high porosity (99.6%). The present work shows an alternative strategy to design and synthesize supramolecular hydrogels and aerogels using predetermined building blocks, together with designable morphology and physical properties for the target aerogels.

Engineering an Injectable Electroactive Nanohybrid Hydrogel for Boosting Peripheral Nerve Growth and Myelination in Combination with Electrical Stimulation

Electrical stimulation (ES) can be used to manipulate recovery after peripheral nerve injuries. Although biomaterial-based strategies have already been implemented to gain momentum for ES and engineer permissive microenvironments for neural regeneration, the development of biomaterials for specific stimuli-responsive modulation of neural cell properties remains a challenge. Herein, we homogeneously incorporate pristine carbon nanotubes into a functional self-assembling peptide to prepare a hybrid hydrogel with good injectability and conductivity. Two-dimensional (on the surface) and three-dimensional (within the hybrid hydrogel) culturing experiments demonstrate that ES promotes axon outgrowth and Schwann cell (SC) migration away from dorsal root ganglia spheres, further revealing that ES-enhanced interactions between SCs and axons result in improved myelination. Thus, our study not only advances the development of tailor-made materials but also provides useful insights into comprehensive approaches for promoting nerve growth and presents a practical strategy of repairing peripheral nerve injuries.

Fabrication of soft-nanocomposites from functional molecules with diversified applications

With the increasing demand for new soft materials having excellent physical and biological characteristics and functionality, the design of hybrid materials offers a simple, yet versatile platform for the development of materials with specific and tunable properties. By definition a "soft-nanocomposite" is the combination of supramolecular self-assemblies with nanomaterials of different origins (inorganic/metallic nanoparticles and carbonaceous allotropes like carbon nanotubes and graphene) through covalent/non-covalent interactions. Dynamic supramolecular self-assemblies can serve as excellent hosts for the incorporation of these dimensionally different nanomaterials. Nanomaterials within the matrix of supramolecular self-assemblies can give rise to new characteristics due to the synergistic contribution of both materials. Although the very initial work intended to use molecular gels as media for the preparation and stabilization of nanoparticles, recent reports have suggested that amalgamation of different supramolecular self-assemblies with nanoparticles is advantageous for both constituents. These newly developed soft-nanocomposites have interesting properties including electrical conductivity, viscoelasticity, thermal robustness, magnetic, phase-selective, redox and near-infrared radiation sensitive properties and so on. This review will focus on some of the most recent advancements in the development of novel soft-nanocomposites. In particular, we intend to correlate various design strategies for synthesis as well as composite preparation from functional molecules with interesting applications in the area of supercapacitors, nanoelectronics, photovoltaic devices, chemical and biosensors, biomedicine and so on. We expect that this article will be a general and conceptual demonstration of various approaches to develop different soft-nanocomposites and will highlight their applications across disciplines.

Protected Amino Acid-Based Hydrogels Incorporating Carbon Nanomaterials for Near-Infrared Irradiation-Triggered Drug Release

Molecular gels formed by the self-assembly of low-molecular-weight gelators have received increasing interest because of their potential applications in drug delivery. In particular, the ability of peptides and amino acids to spontaneously self-assemble into three-dimensional fibrous network has been exploited in the development of hydrogels. In this context, we have investigated the capacity of binary mixtures of aromatic amino acid derivatives to form hydrogels. Carbon nanomaterials, namely oxidized carbon nanotubes or graphene oxide, were incorporated in the two most stable hydrogels, formed by Fmoc-Tyr-OH/Fmoc-Tyr(Bzl)-OH and Fmoc-Phe-OH/Fmoc-Tyr(Bzl)-OH, respectively. The structural and physical properties of these gels were assessed using microscopic techniques and rheology. Circular dichroism and molecular dynamics simulations demonstrated that the hydrogel formation was mainly driven by aromatic interactions. Finally, a model hydrophilic drug (l-ascorbic acid) was loaded into the hybrid hydrogels at a high concentration. Under near-infrared light irradiation, a high amount of drug was released triggered by the heat generated by the carbon nanomaterials, thus offering interesting perspectives for controlled drug delivery.

Tailor-Made Self-Assemblies from Functionalized Amphiphiles: Diversity and Applications

The objective of this feature article is to coalesce our recent advancements on different expressions of tailor-made supramolecular self-assemblies and to explore them as a function of molecular architecture. In the last decade, we have developed a library of elegant and simple functional amphiphilic small molecules, which have very interesting abilities to form diverse manifestations of supramolecular self-assemblies such as micelles, reverse micelles, vesicles, fibers, supramolecular gels, and so on. Each of the expressions of the self-aggregated structures has its individual prominence and finds important applications in the fields of chemistry, physics, biology, and others. In this feature article, the major emphasis is mostly on how to attain precise control over the development of various well-defined supramolecular self-assemblies through the judicious design of low-molecular-weight amphiphiles. By tuning only the functional moieties of the amphiphilic structure, diverse supramolecular architectures can be constructed with task-specific applications. We expect that this article will provide a general and conceptual demonstration of various approaches to the development of different functional supramolecular systems and their prospective applications in numerous domains.

Carbon-Nanotube-Mediated Electrochemical Transition in a Redox-Active Supramolecular Hydrogel Derived from Viologen and an l-Alanine-Based Amphiphile

A two-component hydrogelator (16-A)2 -V(2+) , comprising an l-alanine-based amphiphile (16-A) and a redox-active viologen based partner (V(2+) ), is reported. The formation the hydrogel depended, not only on the acid-to-amine stoichiometric ratio, but on the choice of the l-amino acid group and also on the hydrocarbon chain length of the amphiphilic component. The redox responsive property and the electrochemical behavior of this two-component system were further examined by step-wise chemical and electrochemical reduction of the viologen nucleus (V(2+) /V(+) and V(+) /V(0) ). The half-wave reduction potentials (E1/2 ) associated with the viologen ring shifted to more negative values with increasing amine component. This indicates that higher extent of salt formation hinders reduction of the viologen moiety. Interestingly, the incorporation of single-walled carbon nanotubes in the electrochemically irreversible hydrogel (16-A)2 -V(2+) transformed it into a quasi-reversible electrochemical system.

First preparation of a triterpenoid-based supramolecular hydrogel in physiological phosphate buffered saline

A glycyrrhetinic acid-based supramolecular hydrogel was attained in physiological phosphate buffered saline for the first time.

Supramolecularly knitted tethered oligopeptide/single-walled carbon nanotube organogels

A facile polymerization of an allyl-functionalized N-carboxyanhydride (NCA) monomer is utilized to construct an A-B-A-type triblock structure containing β-sheet-rich oligomeric peptide segments tethered by a poly(ethylene oxide) chain, which are capable of dispersing and gelating single-walled carbon nanotubes (SWCNTs) noncovalently in organic solvents, resulting in significant enhancement of the mechanical properties of polypeptide-based organogels.

Exceptionally strong hydrogels through self-assembly of an indole-capped dipeptide

A dipeptide bearing an indole capping group forms exceptionally strong, hydrogels, with a storage modulus of ∼0.3 MPa. these hydrogels exhibit a minimal fibre-branching, with strong lateral association of fibrils.