Synergistic Coassembly of Two Structurally Different Molecular Gelators

Manipulating supramolecular gels with surfactants: Interfacial and non-interfacial mechanisms

Gel is a class of self-supporting soft materials with applications in many fields. Fast, controllable gelation, micro/nano structure and suitable rheological properties are essential considerations for the design of gels for specific applications. Many methods can be used to control these parameters, among which the additive approach is convenient as it is a simple physical mixing process with significant advantages, such as avoidance of pH change and external energy fields (ultrasound, UV light and others). Although surfactants are widely used to control the formation of many materials, particularly nanomaterials, their effects on gelation are less known. This review summarizes the studies that utilized different surfactants to control the formation, structure, and properties of molecular and silk fibroin gels. The mechanisms of surfactants, which are interfacial and non-interfacial effects, are classified and discussed. Knowledge and technical gaps are identified, and perspectives for further research are outlined. This review is expected to inspire increasing research interest in using surfactants for designing/fabricating gels with desirable formation kinetics, structure, properties and functionalities.

Antimicrobial and Bioactive Silk Peptide Hybrid Hydrogel with a Heterogeneous Double Network Formed by Orthogonal Assembly

Hydrogels mimic the natural extracellular matrix in terms of their nanofibrous structure and large water content. However, the lack of a combination of properties including sufficient heterogeneity in the gel structure, intrinsic antimicrobial activity, and bioactivity limits the efficiency of hydrogels for tissue engineering applications. In this work, a hydrogel with a combination of these properties was fabricated by hybridizing silk fibroin with a low-molecular-weight peptide gelator. It was observed that silk fibroin and the peptide gelator assembled orthogonally in sequence. While the morphology of silk fibroin nanofibrils was not affected by the peptide gelator, silk fibroin promoted the formation of wider nanoribbons of the peptide gelator by modulating its nucleation and growth. Orthogonal assembly maintained the antimicrobial activity of the peptide gelator and the excellent biocompatibility of silk fibroin in the hybrid gel. The hybrid gel also demonstrated improved interactions with cells, an indicator of a higher bioactivity, possibly due to the heterogeneous double network structure.

Synergistic Effect of Two Organogelators for the Creation of Bio-Based, Shape-Stable Phase-Change Materials

Two organogelators of different chemistry (a fatty acid derivative and a bis-urea derivative), as well as their blends, were used to impart shape stability to a bio-based phase-change material (PCM) bearing a near-ambient phase-transition temperature. Characterization of the individual gelators and their blends revealed their ability to immobilize the PCM by forming a continuous fibrillar network. The fibrils formed by the fatty acid derivative were helical, while the bis-urea derivative formed smooth fibrils. Also, the bis-urea derivative formed a continuous network at a lower critical concentration than the fatty acid derivative. At each fixed concentration, the bis-urea derivative yielded gels with higher thermal stability than the fatty acid derivative. The two gelators blended in certain ratios demonstrated a strong synergistic effect, providing gels with a significantly higher modulus (∼20-fold) and yield stress (∼1.5-fold) than each gelator individually. PCM gelation did not significantly affect its thermal behavior, however, affected its crystalline morphology. The gelled PCM displayed stacked structures, consisting of alternating pure PCM layers separated by layers formed by gelator fibrils. The phase diagram of the triple system comprising both gelators and PCM demonstrated either single or double gelation behavior depending on the composition. These findings may provide guidelines for the development of novel, shape-stable PCMs, which could be of potential use in various thermal energy storage applications.

Supramolecular Self-Assembly To Control Structural and Biological Properties of Multicomponent Hydrogels

Self-assembled nanofibers are ubiquitous in nature and serve as inspiration for the design of supramolecular hydrogels. A multicomponent approach offers the possibility of enhancing the tunability and functionality of this class of materials. We report on the synergistic multicomponent self-assembly involving a peptide amphiphile (PA) and a 1,3:2,4-dibenzylidene-d-sorbitol (DBS) gelator to generate hydrogels with tunable nanoscale morphology, improved stiffness, enhanced self-healing, and stability to enzymatic degradation. Using induced circular dichroism of Thioflavin T (ThT), electron microscopy, small-angle neutron scattering, and molecular dynamics approaches, we confirm that the PA undergoes self-sorting, while the DBS gelator acts as an additive modifier for the PA nanofibers. The supramolecular interactions between the PA and DBS gelators result in improved bulk properties and cytocompatibility of the two-component hydrogels as compared to those of the single-component systems. The tunable mechanical properties, self-healing ability, resistance to proteolysis, and biocompatibility of the hydrogels suggest future opportunities for the hydrogels as scaffolds for tissue engineering and drug delivery vehicles.

Hierarchical Hybrid Nanostructures Constructed by Fullerene and Molecular Tweezer

For the construction of well-defined hierarchical superstructures of pristine [60]fullerene (C₆₀) arrays, pyrene-based molecular tweezers (PT) were used as host molecules for catching and arranging C₆₀ guest molecules. The formation of host-guest complexes was systematically studied in solution as well as in the solid state. Two-dimensional proton nuclear magnetic resonance spectroscopic studies revealed that PT-host and C₆₀-guest complexes were closely related to the molecular self-assembly of PT. Ultraviolet and fluorescence spectroscopic titrations indicated the formation of stable 1:1 and 2:1 (PT/C₆₀) complexes. From the nonlinear curve-fitting analysis, equilibrium constants for the 1:1 (log K₁) and 2:1 (log K₂) complexes were estimated to be 4.96 and 5.01, respectively. X-ray diffraction results combined with transmission electron microscopy observations clearly exhibited the construction of well-defined layered superstructures of the PT-host and C₆₀-guest complexes. From electron mobility measurements, it was demonstrated that the well-defined hierarchical hybrid nanostructure incorporating a C₆₀ array exhibited a high electron mobility of 1.7 × 10^-2 cm² V^-1 s^-1. This study can provide a guideline for the hierarchical hybrid nanostructures of host-guest complex and its applications.

Targetable Mechanical Properties by Switching between Self-Sorting and Co-assembly with In Situ Formed Tripodal Ketoenamine Supramolecular Hydrogels

A new family of supramolecular hydrogelators are introduced in which self-sorting and co-assembly can be utilised in the tuneability of the mechanical properties of the materials, a property closely tied to the nanostructure of the gel network. The in situ reactivity of the components of the gelators allows for system chemistry concepts to be applied to the formation of the gels and shows that molecular properties, and not necessarily the chemical identity, determines some gel properties in these family of gels.

Stereochemical effects on dynamics in two-component systems of gelators with perfluoroalkyl and alkyl chains as revealed by vibrational circular dichroism

The VCD method was applied to the gelation processes of chiral two-component gel systems.

Cellulose Nanofibril Aerogels: Synergistic Improvement of Hydrophobicity, Strength, and Thermal Stability via Cross-Linking with Diisocyanate

A facile gelation cross-linking approach was devised to fabricate meso- and macroporous cellulose nanofibril (CNF) aerogels with multiple improved properties. CNF hydrogels made using a freezing-thawing method with a 94 kPa modulus were solvent exchanged with acetone and then cross-linked with methylene diphenyl diisocyanate (MDI) to produce aerogels with significantly improved compressive properties that follow a power law increment against aerogel density with impressive 1.69, 2.49, and 1.43 scaling factors for Young's modulus, yield stress, and ultimate stress, respectively. The optimally cross-linked aerogels had nearly tripled specific surface area (228 m²/g) and doubled pore volume (1 m³/g) from numerous new 9-12 nm wide mesopores as well as significantly improved thermal stability (43% char residue at 500 C vs 9.1% for un-cross-linked aerogel). Cross-linking also made the amphiphilic CNF aerogel highly hydrophobic and capable of completely separating chloroform from water via simple filtration. These nanocellulose aerogels show great promise for efficient and continuous separation of oils and hydrophobic liquids from water.