Robust Gels Research Articles

Evolutionary Reinforcement of Polymer Networks: A Stepwise-Enhanced Strategy for Ultrarobust Eutectogels.

Gel materials are appealing due to their diverse applications in biomedicine, soft electronics, sensors, and actuators. Nevertheless, the existing synthetic gels are often plagued by feeble network structures and inherent defects associated with solvents, which compromise their mechanical load-bearing capacity and cast persistent doubts about their reliability. Herein, combined with attractive deep eutectic solvent (DES), a stepwise-enhanced strategy is presented to fabricate ultrarobust eutectogels. It focuses on the continuous modulation and optimization of polymer networks through complementary annealing and solvent exchange processes, which drives a progressive increase in both quantity and mass of the interconnected polymer chains at microscopic scale, hence contributing to the evolutionary enhancement of network structure. The resultant eutectogel exhibits superb mechanical properties, including record-breaking strength (31.8MPa), toughness (76.0MJ m-3 ), and Young's modulus (25.6MPa), together with exceptional resistance ability to tear and crack propagation. Moreover, this eutectogel is able to be further programmed through photolithography to in situ create patterned eutectogel for imparting specific functionalities. Enhanced by its broad applicability to various DES combinations, this stepwise-enhanced strategy is poised to serve as a crucial template and methodology for the future development of robust gels.

Wet spinning imogolite nanotube fibres: an in situ process study.

Imogolite nanotubes (INTs) form transparent aqueous liquid-crystalline solutions, with strong birefringence and X-ray scattering power. They provide an ideal model system for studying the assembly of one-dimensional nanomaterials into fibres, as well as offering interesting properties in their own right. Here, in situ polarised optical microscopy is used to study the wet spinning of pure INTs into fibres, illustrating the influence of process variables during extrusion, coagulation, washing and drying on both structure and mechanical properties. Tapered spinnerets were shown to be significantly more effective than thin cylindrical channels for forming homogeneous fibres; a result related to simple capillary rheology by fitting a shear thinning flow model. The washing step has a strong influence of structure and properties, combining the removal of residual counter-ions and structural relaxation to produce a less aligned, denser and more networked structure; the timescales and scaling behavior of the processes are compared quantitatively. Both strength and stiffness are higher for INT fibres with a higher packing fraction and lower degree of alignment, indicating the importance of forming a rigid jammed network to transfer stress through these porous, rigid rod assemblies. The electrostatically-stabilised, rigid rod INT solutions were successfully cross-linked using multivalent anions, providing robust gels, potentially useful in other contexts.

Synergy between silk fibroin and ionic liquids for active gas-sensing materials.

Silk fibroin is a biobased material with excellent biocompatibility and mechanical properties, but its use in bioelectronics is hampered by the difficult dissolution and low intrinsic conductivity. Some ionic liquids are known to dissolve fibroin but removed after fibroin processing. However, ionic liquids and fibroin can cooperatively give rise to functional materials, and there are untapped opportunities in this combination. The dissolution of fibroin, followed by gelation, in designer ionic liquids from the imidazolium chloride family with varied alkyl chain lengths (2–10 carbons) is shown here. The alkyl chain length of the anion has a large impact on fibroin secondary structure which adopts unconventional arrangements, yielding robust gels with distinct hierarchical organization. Furthermore, and due to their remarkable air-stability and ionic conductivity, fibroin ionogels are exploited as active electrical gas sensors in an electronic nose revealing the unravelled possibilities of fibroin in soft and flexible electronics.

Shear-Thinning and Rapidly Recovering Hydrogels of Polymeric Nanofibers Formed by Supramolecular Self-Assembly

The self-assembly of amphiphilic polymers into worm-like micelles represents a versatile approach to create hydrogels, where interactions and functionalities are widely customizable by the chemistry of the hydrophilic block. However, processing options for such gels remain a bottleneck as fragmentation is often irreversible due to the limited dynamics of the assemblies. We demonstrate here that shear-thinning hydrogels can reversibly be formed by amphiphilic polymers, which assemble into supramolecular polymer nanofibers due to additional directing hydrogen bonds. The addition of bifunctional cross-linkers resulted in robust gels, which feature a surprisingly strong shear-thinning character but recover fully in the absence of shear stress despite the lack of a dynamic exchange of individual building blocks. In addition to increasing the concentration, the strength of the gel can be tuned by varying the content or the length of the bivalent cross-linker. Low viscosities under shear load and the rapid recovery (<5 s) after relief of the strain facilitates an effortless extrusion through even thin needles and subsequent formation of self-supporting structures in a printing process. The polymer covered fiber structure further bestows the gels with an excellent stability in various conditions and good biocompatibility while minimizing cell adhesion. The mesh sizes of the gel allow even large macromolecules to diffuse, but retardation is nevertheless observed for small molecules due to the dense polymer brush structure. This unique set of properties renders these polymer fiber hydrogels a versatile and easily processable scaffold for future applications, for example as an adaptable cell scaffold or injectable drug depots.

Ratiometric Flapping Force Probe That Works in Polymer Gels.

Polymer gels have recently attracted attention for their application in flexible devices, where mechanically robust gels are required. While there are many strategies to produce tough gels by suppressing nanoscale stress concentration on specific polymer chains, it is still challenging to directly verify the toughening mechanism at the molecular level. To solve this problem, the use of the flapping molecular force probe (FLAP) is promising because it can evaluate the nanoscale forces transmitted in the polymer chain network by ratiometric analysis of a stress-dependent dual fluorescence. A flexible conformational change of FLAP enables real-time and reversible responses to the nanoscale forces at the low force threshold, which is suitable for quantifying the percentage of the stressed polymer chains before structural damage. However, the previously reported FLAP only showed a negligible response in solvated environments because undesirable spontaneous planarization occurs in the excited state, even without mechanical force. Here, we have developed a new ratiometric force probe that functions in common organogels. Replacement of the anthraceneimide units in the flapping wings with pyreneimide units largely suppresses the excited-state planarization, leading to the force probe function under wet conditions. The FLAP-doped polyurethane organogel reversibly shows a dual-fluorescence response under sub-MPa compression. Moreover, the structurally modified FLAP is also advantageous in the wide dynamic range of its fluorescence response in solvent-free elastomers, enabling clearer ratiometric fluorescence imaging of the molecular-level stress concentration during crack growth in a stretched polyurethane film.

Making and Breaking of Gels: Stimuli-Responsive Properties of Bis(Pyridyl-N-oxide Urea) Gelators.

The structural modification of existing supramolecular architecture is an efficient strategy to design and synthesize supramolecular gels with tunable and predictable properties. In this work, we have modified bis(pyridyl urea) compounds with different linkers, namely hexylene and butylene, to their corresponding bis(pyridyl-N-oxide urea). The gelation properties of both the parent and the modified compounds were studied, and the results indicated that modification of the 3-pyridyl moieties to the corresponding 3-pyridyl-N-oxides induced hydrogelation. The stability of the parent and modified compounds were evaluated by sol-gel transition temperature (Tgel) and rheological measurements, and single-crystal X-ray diffraction was used to analyze the solid-state interactions of the gelators. The morphologies of the dried gels were analyzed by scanning electron microscopy (SEM), which revealed that the structural modification did not induce any prominent effect on the gel morphology. The stimuli-responsive behavior of these gels in the presence of salts in DMSO/water was evaluated by rheological experiments, which indicated that the modified compounds displayed enhanced gel strength in most cases. However, the gel network collapsed in the presence of the chloride salts of aluminum(III), zinc(II), copper(II), and cadmium(II). The mechanical strength of the parent gels decreased in the presence of salts, indicating that the structural modification resulted in robust gels in most cases. The modified compounds formed gels below minimum gel concentration in the presence of various salts, indicating salt-induced gelation. These results show the making and breaking ability of the gel network in the presence of external stimuli (salts), which explains the potential of using LMWGs based on N-oxide moieties as stimuli-responsive materials.

Chemically Fueled Three-State Chiroptical Switching Supramolecular Gel with Temporal Control.

The recent discovery of temporally controlled gels opens broad perspectives to the field of smart functional materials. However, to obtain fully operative systems, the design of simple and robust gels displaying complex functions is desirable. Herein, we fuel dissipative gelating materials through iterative additions of trichloroacetic acid (TCA). This simple fuel enables to switch over time an acid/base-dependent commercially available amino acid gelator/DBU combination between three distinct states (anionic, cationic, and neutral), while liberating volatile CO2 and CHCl3 upon fuel consumption. Of interest, the anionic resting state of the system is obtained through trapping of 1 equiv of CO2 through the formation of a carbamate. The system is tunable, robust, and resilient over time with over 25 consecutive sol-gel-sol cycles possible without significant loss of properties. Most importantly, because of the chiral nature of the amino acid gelator, the system features chiroptical switching properties moving reversibly between three distinct states as observed by ECD. The described system considerably enhances the potential of smart molecular devices for logic gates or data storage by adding a time dimension based on three states to the gelating materials. It is particularly simple in terms of chemical components involved, but it enables sophisticated functions.

Robust Gels Composed of Self-Assembled Cello-oligosaccharide Networks

Abstract Assembled molecular gels exhibit dynamic properties and have been developed as functional soft materials with self-healing ability, stimuli responsiveness, and other such properties. Nevertheless, dynamicity is not essential to molecular assemblies. We created static and robust hydrogels composed of self-assembled cello-oligosaccharide networks. In fact, the novel gels are solvent-exchangeable from water even to nonpolar organic solvents, reflecting extremely low stimuli responsiveness. This Account summarizes our recent research progress on cello-oligosaccharide gels, from their production to applications that exploit the unique properties of these crystalline oligosaccharide assemblies. Our findings suggest that statically assembled molecular gels have unconventional applications.

Spatial and temporal diffusion-control of dynamic multi-domain self-assembled gels.

The dynamic assembly of a pH-responsive low-molecular-weight gelator (LMWG) within the pre-formed matrix of a second LMWG has been achieved via diffusion of an acid from a reservoir cut into the gel. Self-assembly of the acid-triggered LMWG as it converts from micellar aggregates at basic pH into gel nanofibers at lower pH values can be both spatially and temporally controlled. The pH-responsive LMWG has an impact on the stiffness of the pre-formed gel in the domains in which it assembles. When low acid concentrations are used, LMWG assembly is transient – after the initial proton diffusion phase, the pH rises and disassembly occurs as the system equilibrates. Re-application of additional acid as ‘fuel’ can then re-assemble the LMWG network. Using glucono-δ-lactone (which slowly hydrolyses to gluconic acid) instead of HCl gives slower, more spatially-restricted assembly, and creates longer-lasting pH gradients within the gel. The presence of an agarose polymer gel network improves the mechanical strength of the gels and appears to slightly enhance the rate of proton diffusion. More sophisticated reservoir shapes can be cut into these more mechanically robust gels, enabling the creation of diffusion waves with different geometries, and hence different patterns of LMWG activation. Multiple reservoirs can be used to create overlapping proton diffusion waves, hence achieving differentiated pH patterns in the gel. Using acid diffusion in this way within gels is an intriguing and powerful way of dynamic patterning. The ability to temporally-evolve spatially-resolved patterns using biocompatible weak acids, and the change in rheological performance of the triggered domains, suggest potential future applications of this strategy in tissue engineering.

Hydrogel Synthesis and Stabilization via Tetrazine Click-Induced Secondary Interactions.

The discovery of tetrazine click-induced secondary interactions is reported as a promising new tool for polymeric biomaterial synthesis. This phenomenon is first demonstrated as a tool for poly(ethylene glycol) (PEG) hydrogel assembly via purely non-covalent interactions and is shown to yield robust gels with storage moduli one to two orders of magnitude higher than other non-covalent crosslinking methods. In addition, tetrazine click-induced secondary interactions also enhance the properties of covalently crosslinked hydrogels. A head-to-head comparison of PEG hydrogels crosslinked with tetrazine-norbornene and thiol-norbornene click chemistry reveals an approximately sixfold increase in storage modulus and unprecedented resistance to hydrolytic degradation in tetrazine click-crosslinked gels without substantial differences in gel fraction. Molecular dynamic simulations attribute these differences to the presence of secondary interactions between the tetrazine-norbornene cycloaddition products, which are absent in the thiol-norbornene crosslinked gels.

Enhanced wound healing via collagen-turnover-driven transfer of PDGF-BB gene in a murine wound model.

Wound healing is a complex biological process that requires coordinated cell proliferation, migration, and extracellular matrix production/remodeling, all of which are inhibited/delayed in chronic wounds. In this study, a formulation was developed that marries a fibrin-based, provisional-like matrix with collagen mimetic peptide (CMP)/PDGF gene-modified collagens, leading to the formation of robust gels that supported temporally controlled PDGF expression and facile application within the wound bed. Analysis employing in vitro co-gel scaffolds confirmed sustained and temporally controlled gene release based on matrix metalloproteinase (MMP) activity, with ~30% higher PDGF expression in MMP producing fibroblasts as-compared with non-MMP-expressing cells. The integration of fibrin with the gene-modified collagens resulted in co-gels that strongly supported both fibroblast cell recruitment/invasion as well as multiple aspects of the longer-term healing process. The excisional wound healing studies in mice established faster wound closure using CMP-modified PDGF polyplex-loaded co-gels, which exhibited up to 24% more wound closure (achieved with ~2 orders of magnitude lower growth factor dosing) after 9 days as compared to PDGF-loaded co-gels, and 19% more wound closure after 9 days as compared to CMP-free polyplex loaded co-gels. Moreover, minimal scar formation as well as improved collagen production, myofibroblast activity, and collagen orientation was observed following CMP-modified PDGF polyplex-loaded co-gel application on wounds. Taken together, the combined properties of the co-gels, including their stability and capacity to control both cell recruitment and cell phenotype within the murine wound bed, strongly supports the potential of the co-gel scaffolds for improved treatment of chronic non-healing wounds.

Hydrophobization of Cellulose Nanocrystals for Aqueous Colloidal Suspensions and Gels.

Surface hydrophobization of cellulose nanomaterials has been used in the development of nanofiller-reinforced polymer composites and formulations based on Pickering emulsions. Despite the well-known effect of hydrophobic domains on self-assembly or association of water-soluble polymer amphiphiles, very few studies have addressed the behavior of hydrophobized cellulose nanomaterials in aqueous media. In this study, we investigate the properties of hydrophobized cellulose nanocrystals (CNCs) and their self-assembly and amphiphilic properties in suspensions and gels. CNCs of different hydrophobicity were synthesized from sulfated CNCs by coupling primary alkylamines of different alkyl chain lengths (6, 8, and 12 carbon atoms). The synthetic route permitted the retention of surface charge, ensuring good colloidal stability of hydrophobized CNCs in aqueous suspensions. We compare surface properties (surface charge, ζ potential), hydrophobicity (water contact angle, microenvironment probing using pyrene fluorescence emission), and surface activity (tensiometry) of different hydrophobized CNCs and hydrophilic CNCs. Association of hydrophobized CNCs driven by hydrophobic effects is confirmed by X-ray scattering (SAXS) and autofluorescent spectroscopy experiments. As a result of CNC association, CNC suspensions/gels can be produced with a wide range of rheological properties depending on the hydrophobic/hydrophilic balance. In particular, sol-gel transitions for hydrophobized CNCs occur at lower concentrations than hydrophilic CNCs, and more robust gels are formed by hydrophobized CNCs. Our work illustrates that amphiphilic CNCs can complement associative polymers as modifiers of rheological properties of water-based systems.

The material properties of naked mole-rat hyaluronan

Hyaluronan (HA) is a key component of the extracellular matrix. Given the fundamental role of HA in the cancer resistance of the naked mole-rat (NMR), we undertook to explore the structural and soft matter properties of this species-specific variant, a necessary step for its development as a biomaterial. We examined HA extracted from NMR brain, lung, and skin, as well as that isolated from the medium of immortalised cells. In common with mouse HA, NMR HA forms a range of assemblies corresponding to a wide distribution of molecular weights. However, unique to the NMR, are highly folded structures, whose characteristic morphology is dependent on the tissue type. Skin HA forms tightly packed assemblies that have spring-like mechanical properties in addition to a strong affinity for water. Brain HA forms three dimensional folded structures similar to the macroscopic appearance of the gyri and sulci of the human brain. Lung HA forms an impenetrable mesh of interwoven folds in a morphology that can only be described as resembling a snowman. Unlike HA that is commercially available, NMR HA readily forms robust gels without the need for chemical cross-linking. NMR HA gels sharply transition from viscoelastic to elastic like properties upon dehydration or repeated loading. In addition, NMR HA can form ordered thin films with an underlying semi-crystalline structure. Given the role of HA in maintaining hydration in the skin it is plausible that the folded structures contribute to both the elasticity and youthfulness of NMR skin. It is also possible that such densely folded materials could present a considerable barrier to cell invasion throughout the tissues, a useful characteristic for a biomaterial.

Dicyclohexylurea derivatives of amino acids as dye absorbent organogels and anion sensors.

Dicyclohexyl urea (DCU) derivatives of amino acids Fmoc-Phe-DCU (M1), Fmoc-Phg-DCU (M2) and Fmoc-Gaba-DCU (M3) have been shown to form phase selective, thermoreversible and mechanically robust gels in a large range of organic solvents. This is the first report of low molecular weight gelators (LMWG) from DCU derivatives of amino acids. The self-assembly mechanism of the organogels has been probed using concentration dependent 1H NMR, DMSO titration 1H NMR, fluorescence, FTIR, PXRD and FESEM techniques. Self-assembly leading to gelation process is mainly driven by hydrophobicity and π-π stacking interactions in between Fmoc groups. Interestingly, the gels can absorb several kinds of organic dyes efficiently and can be reused for dye absorption for multiple cycles. Additionally, M1-M3 act as sensors for anions like fluoride, acetate and hydroxide, for which they have specific fluorescence response. Gel formation by M1-M3 is completely arrested in the presence of fluoride. The possible binding mode of fluoride has been delineated using DFT studies. Calculations suggest, involvement of urea NH in a six membered intramolecular hydrogen bond, rendering it unavailable for fluoride binding. Backbone -NH of the amino acids of M1-M3 is responsible for fluoride binding. The reported small, economically viable, synthetically facile molecules not only enrich the repertoire of LMWG molecules, but can have multifaceted applications.

Mechanically Robust Gels Formed from Hydrophobized Cellulose Nanocrystals.

Cellulose nanocrystals (CNCs) that bind to each other through associative hydrophobic interactions have been synthesized by modifying sulfated CNCs (sCNCs) with hydrophobic moieties. These octyl-CNCs form gels at significantly lower concentrations than parent sCNCs, producing extremely strong hydrogels. Unlike sCNCs, these octyl-CNCs do not form ordered liquid crystalline phases indicating a random association into a robust network driven by hydrophobic interactions. Furthermore, involvement of the octyl-CNCs into multicomponent supramolecular assembly was demonstrated in combination with starch. AFM studies confirm favorable interactions between starch and octyl-CNCs, which is thought to be the source of the dramatic increase in gel strength.

Application of Aqueous-Based Covalent Crosslinking Strategies to the Formation of Metal Chalcogenide Gels and Aerogels

Abstract An aqueous-based metal ion crosslinking approach for assembly of metal chalcogenide nanoparticles (NPs) into robust gels is reported. Short chalcogenide ligands (S2−) undergo crosslinking with metal salts (Sn4+) to form a gel [NP/S2−/Sn4+]n (NP=PbTe, PbS, CdS, CdSe). The corresponding aerogel networks retain the crystallinity and quantum confinement effects of the native building blocks while achieving excellent porosity [Brunauer–Emmett–Teller (BET) surface areas of 160–238 m2/g]. Treatment of sulfide-capped PbTe nanoparticles with an excess of Sn4+ leads to ion exchange and formation of an amorphous “SnTe” gel.

Radiation-Responsive Esculin-Derived Molecular Gels as Signal Enhancers for Optical Imaging.

Recent interest in detecting visible photons that emanate from interactions of ionizing radiation (IR) with matter has spurred the development of multifunctional materials that amplify the optical signal from radiotracers. Tailored stimuli-responsive systems may be paired with diagnostic radionuclides to improve surgical guidance and aid in detecting therapeutic radionuclides otherwise difficult to image with conventional nuclear medicine approaches. Because light emanating from these interactions is typically low in intensity and blue-weighted (i.e., greatly scattered and absorbed in vivo), it is imperative to increase or shift the photon flux for improved detection. To address this challenge, a gel that is both scintillating and fluorescent is used to enhance the optical photon output in image mapping for cancer imaging. Tailoring biobased materials to synthesize thixotropic thermoreversible hydrogels (a minimum gelation concentration of 0.12 wt %) offers image-aiding systems which are not only functional but also potentially economical, safe, and environmentally friendly. These robust gels (0.66 wt %, ∼900 Pa) respond predictably to different types of IRs including β- and γ-emitters, resulting in a doubling of the detectable photon flux from these emitters. The synthesis and formulation of such a gel are explored with a focus on its physicochemical and mechanical properties, before being utilized to enhance the visible photon flux from a panel of radionuclides as detected. The possibility of developing a topical cream of this gel makes this system an attractive potential alternative to current techniques, and the multifunctionality of the gelator may serve to inspire future next-generation materials.

In Situ Synthesis of Imidazolium-Crosslinked Ionogels via Debus-Radziszewski Reaction Based on PAMAM Dendrimers in Imidazolium Ionic liquid.

This study reports a remarkably facile method to synthesize novel ionogels with imidazolium cycle crosslinks based on polyamidoamine (PAMAM) dendrimers via one-pot, modified Debus-Radziszewski reaction in ionic liquid 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]). High room temperature ionic conductivity (up to 6.8 mS cm-1 ) is achieved, and more remarkably, it can still exceed 1 mS cm-1 when the dendrimer content reached 70% because PAMAM dendrimers are completely amorphous with many cavities and the newly formed imidazolium crosslinks contains ions. The elastic modulus of these ionogels can exceed 106 Pa due to the newly-formed rigid imidazolium crosslinks. Crucially, these ionogels are robust gels even at temperatures up to 160 °C. Such novel ionogels with high ionic conductivity, tunable modulus, and flexibility are desirable for use in high-temperature flexible electrochemical devices.

Boronic ionogel electrolytes to improve lithium transport for Li-ion batteries

Boron containing ionogels were fabricated through chemical crosslinking of boron allyloxide with polyethylene glycol dimethacrylate in an ionic liquid electrolyte solution to obtain mechanically robust gels. Because of the relatively small concentration of crosslinking agent required to fully solidify the ionic liquid electrolyte, good characters of high ionic conductivity, high thermal stability, and good electrochemical stability were observed. A spectroscopic investigation of the boronic ionogels revealed that the lithium mobility was noticeably enhanced compared with ionogels fabricated without the boronic crosslinker, leading to promising Li-ion battery performance at elevated temperatures.

Applications of liquid crystals in biosensing and organic light-emitting devices: future aspects

ABSTRACTThis article summarises recent advances made in our laboratory towards the development of new technological applications, such as biosensors and organic light-emitting diodes (OLEDs) based on liquid crystals (LCs) other than LC displays. The study of biomolecular interaction using LC material relies on the specific interaction between the LC and the biomolecule of interest at interfaces that permit the biomolecular events to be amplified into easily measured signals for various sensing applications. In the first part, we emphases recent studies in the design and modulation of LC-based interfaces based on robust colloidal LC gels for biological amplification, qualitative and quantitative understanding of important biomolecular interactions at LC–aqueous interfaces for diagnostic and laboratory applications and design of LC droplets that hold promise to act as a marker for cells and cell-based interactions. In the second part, we described design of organic materials for application in OLEDs on various discotic monomers, dimers and oligomers. These molecules have the ability to transport charges, holes and electrons. In addition, because of the high conductivity and π–π stacking, they are considered as the advanced materials for practical applications. The technological advances in our laboratory using discotic LCs will be briefly presented in this article.