Low Molecular Weight Hydrogelators Research Articles

Mechanical reinforcement of C2-phenyl-derived hydrogels for controlled cell adhesion

Many low molecular weight (LMWG) hydrogels have been widely used as scaffolds and substrates due to their particular structures and properties. However, LMWG hydrogels generally show a weak mechanical performance which confines their applications in the field of tissue engineering. Here, we report a new kind of hydrogel derived from the combination of a C2-phenyl-derived gelator and a polysaccharide (alginate). Rheology testing showed that the elastic modulus of C2-phenyl-derived hydrogels could be increased by nearly one order of magnitude by interpenetrating them with an alginate–calcium network. Increasing the concentrations of the gelator and calcium ions or decreasing the concentration of alginate will lead to an increase of the elastic modulus of the hybrid hydrogels. Imaging and spectroscopic analysis confirmed that the surface roughness and morphology of the hybrid hydrogels were almost the same with that of a pure C2 hydrogel. Significant improvements in cell adhesion and spreading were observed on the reinforced hydrogels. The new hybrid hydrogels have great potential for tissue engineering applications in vivo.

The importance of the self-assembly process to control mechanical properties of low molecular weight hydrogels

Hydrogels can be formed by the self-assembly of certain small molecules in water. Self-assembly occurs via non-covalent interactions. The self-assembly leads to the formation of fibrous structures which form the matrix of the gel. The mechanical properties of the gels arise from the properties of the fibres themselves (thickness, persistence length etc.), the number and type of cross-links and also how the fibres are distributed in space (the microstructure). We discuss here the effect of assembling the molecules under different conditions, i.e. the self-assembly process. There is sufficient literature showing that how the molecules are assembled can have a significant effect on the properties of the resulting gels.

Phenylalanine-containing cyclic dipeptides – the lowest molecular weight hydrogelators based on unmodified proteinogenic amino acids

Cyclic dipeptides (diketopiperazines - DKPs) that are based on the proteinogenic amino acid phenylalanine in combination with serine, cysteine, glutamate, histidine and lysine are described as simple and remarkable low molecular weight hydrogelators. Blends of selected DKPs show remarkable pH-dependent properties and can be applied as easy to tune materials in drug delivery.

Incorporation of graphene into photopolymerizable hydrogels of N-acyl glutanamides: Rheological and swelling behavior study of soft nanocomposite materials

Two different respectively N-stearoyl and diacetylenic fatty acyl glutanamide were prepared and employed as both potent low molecular weight hydrogelators (LMWGs). These compounds were able to gelate water at 1.0 wt% of concentration independently of the pH. Due to the presence of two free amino groups on the extremities of the gelator, an aqueous colloidal dispersion of graphene oxide (GO) could be stabilized into the hydrogel matrix simply by ionic bonds. The inclusion of a slightly amount of GO inside the gel networks increased its natural rigidity, confirmed by rheology study. An in-situ reduction of GO afforded after treatment, a semi-transparent hydrogel containing reduced graphene (rGO) that could be easily manipulated without apparent syneresis phenomenon. When the diacetylenic type hydrogel containing GO was irradiated upon UV light, a deep blue coloration change was observed; evidence of an effective photopolymerization. In case of thermal stimulation of the resulted blue polydiacetylene (PDA) hydrogel, the material turned in a red solution precursor of a robust red (PDA) hydrogel at room temperature. This red hydrogel never displayed reversible color change and its swelling behavior was studied at different temperatures. All hydrogels were characterized by different methods such as Field-Emission Scanning Electron Microscopy (FE-SEM) for determining the homogeneity of the gel networks, but also by (FT-IR) Fourier Transform-Infrared spectroscopy and UV–visible measurement.

A thermosensitive low molecular weight hydrogel as scaffold for tissue engineering

Hydrogels that are non-toxic, easy to use, cytocompatible, injectable and degradable are valuable biomaterials for tissue engineering and tissue repair. However, few compounds currently fulfil these requirements. In this study, we describe the biological properties of a new type of thermosensitive hydrogel based on low-molecular weight glycosyl-nucleosyl-fluorinated (GNF) compound. This gel forms within 25 min by self-assembly of monomers as temperature decreases. It degrades slowly in vitro and in vivo. It induces moderate chronic inflammation and is progressively invaded by host cells and vessels, suggesting good integration to the host environment. Although human adult mesenchymal stem cells derived from adipose tissue (ASC) cannot adhere on the gel surface or within a 3D gel scaffold, cell aggregates grow and differentiate normally when entrapped in the GNF-based gel. Moreover, this hydrogel stimulates osteoblast differentiation of ASC in the absence of osteogenic factors. When implanted in mice, gel-entrapped cell aggregates survive for several weeks in contrast with gel-free spheroids. They are maintained in their original site of implantation where they interact with the host tissue and adhere on the extracellular matrix. They can differentiate in situ into alkaline phosphatase positive osteoblasts, which deposit a calcium phosphate-rich matrix. When injected into subcutaneous sites, gel-encapsulated cells show similar biological properties as implanted gel-cells complexes. These data point GNF-based gels as a novel class of hydrogels with original properties, in particular osteogenic potential, susceptible of providing new therapeutic solutions especially for bone tissue engineering applications.

Helical Assembly of Azobenzene-Conjugated Carbohydrate Hydrogelators with Specific Affinity for Lectins

Carbohydrate-mediated interactions are involved in various biological processes via specific molecular assembly and recognition. Such interactions are enhanced by multivalent effects of the sugar moieties, and thus supramolecular sugar-assembly, i.e., spontaneous association of glycoamphiphiles, is a promising approach to tailor glycocluster formation. In this study, novel sugar-decorated nanofibers were successfully prepared by self-assembly of low molecular weight hydrogelators composed of azobenzene and disaccharide lactones. Circular dichroism measurement of the as-prepared hydrogels indicated that the azobenzene amphiphile containing a lactose moiety possessed (R)-chirality, while the maltose-azobenzene conjugate exhibited (S)-chirality, even though the cellobiose-conjugated azobenzene existed in an achiral form. This suggests that the chiral orientation of the chromophoric azobenzene depended on both the glycosidic linkages and the steric arrangement of hydroxyl groups in the conjugated carbohydrates. Lectin-binding and cell adhesion assays revealed that the nonreducing ends of the conjugated sugar moieties were exposed on the surfaces of self-assembled nanofibrous hydrogels, allowing them to be effectively recognized by the corresponding lectins. In addition, photoisomerization of azobenzene under ultraviolet irradiation induced the sol-gel transitions of the hydrogels. These results demonstrate that the reversibly transformed fibrous glycohydrogels show potential for application as carbohydrate-decorated scaffolds for cell culture engineering.

Versatile low-molecular-weight hydrogelators: achieving multiresponsiveness through a modular design.

Multiresponsive low-molecular-weight hydrogelators (LMWHs) are ideal candidates for the development of smart, soft, nanotechnology materials. The synthesis is however very challenging. On the one hand, de novo design is hampered by our limited ability to predict the assembly of small molecules in water. On the other hand, modification of pre-existing LMWHs is limited by the number of different stimuli-sensitive chemical moieties that can be introduced into a small molecule without seriously disrupting the ability to gelate water. Herein we report the synthesis and characterization of multistimuli LMWHs, based on a modular design, composed of a hydrophobic, disulfide, aromatic moiety, a maleimide linker, and a hydrophilic section based on an amino acid, here N-acetyl-L-cysteine (NAC). As most LMWHs, these gelators experience reversible gel-to-sol transition following temperature changes. Additionally, the NAC moiety allows reversible control of the assembly of the gel by pH changes. The reduction of the aromatic disulfide triggers a gel-to-sol transition that, depending on the design of the particular LMWH, can be reverted by reoxidation of the resulting thiol. Finally, the hydrolysis of the cyclic imide moieties provides an additional trigger for the gel-to-sol transition with a timescale that is appropriate for use in drug-delivery applications. The efficient response to the multiple external stimuli, coupled to the modular design makes these LMWHs an excellent starting point for the development of smart nanomaterials with applications that include controlled drug release. These hydrogelators, which were discovered by serendipity rather than design, suggest nonetheless a general strategy for the introduction of multiple stimuli-sensitive chemical moieties, to offset the introduction of hydrophilic moieties with additional hydrophobic ones, in order to minimize the upsetting of the critical hydrophobic-hydrophilic balance of the LMWH.

Spontaneous Gelation of a Novel Histamine H4 Receptor Antagonist in Aqueous Solution

Low molecular weight hydrogelators typically require a stimulus such as heat, antisolvent, or pH adjustment to produce a gel. This study examines gelation of a novel histamine H4 receptor antagonist that forms hydrogels spontaneously at room temperature. To elucidate the mechanism and structural moieties responsible for this unusual gelation, hydrogels were characterized by rheology, optical microscopy, and XRD. SEM was performed on xerogels; NMR measurements were conducted in gelator solutions in the presence of a gel-breaker. The influence of temperature, concentration, pH, and ionic strength on elastic and viscous moduli of the hydrogels was evaluated; gel points were established via thorough rheological criteria. The observed are "true" gels with a fibrillar texture and lamellar microstructure. On a molecular level, the gels are composed of aggregates of partially ionized species stabilized by hydrophobic interactions of aromatic moieties. The gel-to-sol transition occurs at physiologically relevant temperatures and is concentration-, pH-, and ionic strength-dependent. We hypothesize that this spontaneous gelation is due to the so-called "spring" effect, a high energy salt form that transiently increases aqueous solubility above its equilibrium limit. Upon equilibration, this supersaturated system undergoes aggregation that avoids crystallization and produces a hydrogel.

Effect of C-Terminal Modification on the Self-Assembly and Hydrogelation of Fluorinated Fmoc-Phe Derivatives

The development of hydrogels resulting from the self-assembly of low molecular weight (LMW) hydrogelators is a rapidly expanding area of study. Fluorenylmethoxycarbonyl (Fmoc) protected aromatic amino acids derived from phenylalanine (Phe) have been shown to be highly effective LMW hydrogelators. It has been found that side chain functionalization of Fmoc-Phe exerts a significant effect on the self-assembly and hydrogelation behavior of these molecules; fluorinated derivatives, including pentafluorophenylalanine (F(5)-Phe) and 3-F-phenylalanine (3-F-Phe), spontaneously self-assemble into fibrils that form a hydrogel network upon dissolution into water. In this study, Fmoc-F(5)-Phe-OH and Fmoc-3-F-Phe-OH were used to characterize the role of the C-terminal carboxylic acid on the self-assembly and hydrogelation of these derivatives. The C-terminal carboxylic acid moieties of Fmoc-F(5)-Phe-OH and Fmoc-3-F-Phe-OH were converted to C-terminal amide and methyl ester groups in order to perturb the hydrophobicity and hydrogen bond capacity of the C-terminus. Self-assembly and hydrogelation of these derivatives was investigated in comparison to the parent carboxylic acid compounds at neutral and acidic pH. It was found that hydrogelation of the C-terminal acids was highly sensitive to solvent pH, which influences the charge state of the terminal group. Rigid hydrogels form at pH 3.5, but at pH 7 hydrogel rigidity is dramatically weakened. C-terminal esters self-assembled into fibrils only slowly and failed to form hydrogels due to the higher hydrophobicity of these derivatives. C-terminal amide derivatives assembled much more rapidly than the parent carboxylic acids at both acidic and neutral pH, but the resultant hydrogels were unstable to shear stress as a function of the lower water solubility of the amide functionality. Co-assembly of acid and amide functionalized monomers was also explored in order to characterize the properties of hybrid hydrogels; these gels were rigid in unbuffered water but significantly weaker in phosphate buffered saline. These results highlight the complex nature of monomer/solvent interactions and their ultimate influence on self-assembly and hydrogelation, and provide insight that will facilitate the development of optimal amino acid LMW hydrogelators for gelation of complex buffered media.

Supramolecular binary hydrogels from calixarenes and amino acids and their entrapment–release of model dye molecules

A series of supramolecular binary hydrogels based on tetra-proline modified calix[4]arenes and basic amino acids (arginine, histidine and lysine) were constructed in acidic condition. The obtained results show that different amino acids lead to distinguishable backbones of hydrogels, which were identified by the combination of atomic force microscopy, transmission electron microscopy and scanning electron microscopy. Moreover, all the prepared low molecular weight hydrogels are thermoreversible and the gel-to-sol transition temperature (Tgel) depends on the concentration of calixarene, which endows these hydrogels with the capability of entrapment–release of model dye molecules.

Effect of Molecular Structure on the Properties of Naphthalene−Dipeptide Hydrogelators

Dipeptide-conjugates can be efficient low molecular weight hydrogelators. However, the effective design of a gelator for a specific application is compromised by the lack of a clear understanding of the design rules that govern assembly and hence gelation. Here, we report a library of naphthalene-dipeptides, their physical chemical properties, and gelation ability. We have varied both the amino acids in the dipeptides and the substitution on the naphthalene ring to allow variation of the structure throughout the molecule. We have examined the effects of these permutations on the critical micelle concentration and air-water partition coefficient at high pH and the apparent pK(a). We show that there is a clear link between these properties and the predicted hydrophobicity of the overall conjugates, rather than the properties varying with, for example, the dipeptide sequence. The majority of these dipeptide-conjugates are effective hydrogelators, although there is no apparent link between the solution properties and whether or not a conjugate is a hydrogelator. Nevertheless, where gelation occurs, the link between hydrophobicity and apparent pK(a) allows the prediction of the pH at which a gel will be formed and hence informed choice of gelator for specific applications.

Synthesis and characterization of d-glucosamine-derived low molecular weight gelators

Carbohydrate-based low molecular weight gelators are an interesting class of molecules with many potential applications. Previously, we have found that certain esters and carbamates of 4,6- O-benzylidene-α- d-methyl-glucopyranoside are low molecular weight gelators for a variety of solvents, including water. In order to obtain effective and robust sugar-based organogelators and understand the structure and gelation relationship, we extended our studies using 4,6- O-benzylidene-α- d-methyl-2-deoxy-2-amino-glucopyranoside as the headgroup. A series of amides and ureas were prepared from the protected d-glucosamine and the corresponding isocyanates or acid chlorides, in good yields. The self-assembling properties of these compounds were studied in several solvents, including water and aqueous solutions. Comparing to the ester and carbamate derivatives previously prepared from d-glucose, the amides and urea derivatives afforded more robust gels at lower concentrations typically. Most of these compounds were found to be efficient low molecular weight hydrogelators (LMHGs) for aqueous solutions at concentrations lower than 0.5 wt %. The preparation and characterization of these compounds are reported here.

Self-assembly and hydrogelation promoted by F5-phenylalanine

Phenylalanine (Phe)-derived molecules have been exploited as low molecular weight hydrogelators. Perturbing the hydrophobic and π–π interactions that promote self-assembly and hydrogelation of these derivatives will facilitate improved understanding of hydrogelation phenomena and the design of small molecule hydrogelators with novel properties. The efficient self-assembly and hydrogelation of Fmoc-protected pentafluorophenylalanine (Fmoc-F5-Phe) are reported herein. Suspensions of Fmoc-F5-Phe in water undergo rapid self-assembly to entangled fibrillar structures within minutes, giving rise to rigid supramolecular gels. Self-assembly occurs at concentrations as low as 2 mM (0.1 wt%). Variation of the fluorinated aromatic side chain or N-terminal functionalization perturbs hydrogelation, implicating fluorous and π–π interactions as the primary determinants for molecular recognition and self-assembly. The hydrophobic and electronic properties of F5-Phe provide remarkable potential for functional self-assembly in a minimal amino acid scaffold.

A new method for maintaining homogeneity during liquid–hydrogel transitions using low molecular weight hydrogelators

We demonstrate a generic new approach to produce homogeneous and reproducible hydrogels from low molecular weight hydrogelators using the controlled hydrolysis of glucono-δ-lactone (GdL). GdL slowly hydrolyses in water to give gluconic acid, which controllably lowers the pH. This hydrolysis is slower than the rate of dissolution; hence uniform pH change throughout the sample is possible. This results in homogeneous hydrogels that are unaffected by their shear or mixing history. A further advantage of this method is that it allows the gelation process to be monitored, giving further insight into the mechanism by which gelation occurs.

Counterion dependent hydrogelation of amino acid based amphiphiles: switching from non-gelators to gelators and facile synthesis of silver nanoparticles

With the growing importance of hydrogels in scientific applications, the search for low molecular weight hydrogelators (LMWH), with simultaneous logical structural correlation is continuously increasing. In the present work, counterion variation of amino acid based amphiphiles was done to qualitatively evaluate its contribution towards hydrogelation. Further changes in the molecular skeleton of the amino acid amphiphile were done along with counterion variation to establish the importance of π–π interactions of aromatic planar ring in hydrogelation. An efficient conversion of a non-gelator to gelator molecule was achieved simply by changing the counterions to aromatic carboxylates. Role of the counterion in the mechanism of gelation process through the self-assembly of amino acid based amphiphiles has been discussed. The formation of supramolecular structures during hydrogelation was investigated by FESEM, CD, FT-IR, luminescence, 2D-NOESY and rheological studies. Interestingly, the L-tryptophan containing amphiphile hydrogelators were further utilized for synthesis of Ag nanoparticles under mild conditions without any need for high temperature, alkaline medium and external reducing agent. The nanoparticles obtained were characterized by UV-Vis, TEM, AFM and XRD experiments.

Supramolecular Self-organization of Potential Hydrogelators on Attracting Surfaces

This article highlights the aggregation behaviour of potential low molecular weight hydrogelators on attracting surfaces. Our goal was the development of a method, which enables the finding of new hydrogelators that are not easily recognizable as such because they only form instable or no hydrogels in aqueous solution. To this end, a series of negatively charges azo-dyes was synthesized and positive charged glass slides were immersed into their aqueous solutions. All dyes showed supramolecular organization and significant concentration on the attracting glass surface. Microscopic investigations mostly revealed the formation of crystals. However, one compound, (1-(2-n-octylphenylazo)-2-hydroxy-6-naphthalenesulphonate, selectively formed a hydrogel on the surface whereas it does not gel in aqueous solution. This reveals the hydrogel as the stable form of this compound under equilibrium conditions. This method of surface-induced hydrogelation might facilitate the identification of new hydrogelators. Further more, it might also allow the mimicking of surface gelation as a process of biological relevance.

A low molecular weight hydrogel which exhibits electroosmotic flow and its use as a bioreactor and for electrochromatography of neutral species

A low molecular weight hydrogel which exhibits electroosmotic flow is described, and its use for separation and biocatalytic applications that require passage of a solvent stream through the gel is demonstrated.

Structure and Properties of Low Molecular Weight Amphiphilic Peptide Hydrogelators

The search for low molecular weight hydrogelators (LMWH) with varying structural motif is getting intense because of its potential application in the field of biomedicines as well as the diversified area of nano-biotechnology. In this paper, we have developed hydrogels of simple cationic dipeptide amphiphiles that have a wide range of minimum gelation concentration (MGC), 12-0.25% (w/v) in plain water. The self-aggregation behavior of these thermoreversible hydrogelators has been investigated through different spectroscopic and microscopic techniques. A balanced participation of hydrophilicity and hydrophobicity is the major driving force for gelation, which could be modulated by a minute change in the architecture of the amphiphile head. The prospective use of this material in controlled release suggests that this system could also be applied as the drug delivery vehicle. Moreover, the presence of a biodegradable amide linkage susceptible to base or enzyme-catalyzed hydrolysis increases its probable applications as biomaterials.

Structure and Properties of an Exceptional Low Molecular Weight Hydrogelator

Research for low molecular weight (LMW) hydrogelators has become an area of increasing interest due to many possible new applications for such compounds. They might serve as templates for nanostructures or in biomedical applications and even in drug discovery. In this investigation, we explored the structure-gelling relations of a LMW hydrogelator that can form a hydrogel in water at room temperature. To this end, various analytical techniques such as nuclear magnetic resonance, rheology, X-ray scattering, birefringence, and microscopy were applied to gain better insight on the structure of the gel and to provide an explanation for this unique gelation behavior.

Alkyl Chain Length Dependent Hydrogelation of l-Tryptophan-Based Amphiphile

The search for low molecular weight hydrogelators (LMWHs) with varying structural motif is getting intense because of its potential application in biomedicines as well as the diversified area of nanobiotechnology. Hydrophobic interaction is one of the most crucial parameters in the design and development of such LMWHs. To this notion, a methodical investigation was carried out to find the influence of varying alkyl chain length of amphiphile on water gelation efficacy, which has been only marginally addressed in the literature to date. We have synthesized a series of low molecular weight L-tryptophan-based gelators, some of which are excellent gelator for plain water, an essential criterion for biological use. The alkyl chain induced hydrophobicity at the molecular level has remarkable influence in modulating water immobilization. Water gelation efficiency was improved more than 100 times on moving from 10 to 18 carbon atoms. The self-aggregation behavior of these thermoreversible hydrogelators investigated through different spectroscopic and microscopic techniques showed that an optimum balance between hydrophilicity and hydrophobicity is indeed essential, which can be largely regulated by varying the alkyl chain length. Thus, the study offers better understanding toward tailoring the properties of gel in plain water and thereby paving the way for potential applications.