Supramolecular gels - a panorama of low-molecular-weight gelators from ancient origins to next-generation technologies

Supramolecular gels, self-assembled from low-molecular-weight gelators (LMWGs), have a long history and a bright future. This review provides an overview of these materials, from their use in lubrication and personal care in the ancient world, through to next-generation technologies. In academic terms, colloid scientists in the 19th and early 20th centuries first understood such gels as being physically assembled as a result of weak interactions, combining a solid-like network having a degree of crystalline order with a highly mobile liquid-like phase. During the 20th century, industrial scientists began using these materials in new applications in the polymer, oil and food industries. The advent of supramolecular chemistry in the late 20th century, with its focus on non-covalent interactions and controlled self-assembly, saw the horizons for these materials shifted significantly beyond their historic rheological applications, expanding their potential. The ability to tune the LMWG chemical structure, manipulate hierarchical assembly, develop multi-component systems, and introduce new types of responsive and interactive behaviour, has been transformative. Furthermore, the dynamics of these materials are increasingly understood, creating metastable gels and transiently-fueled systems. New approaches to shaping and patterning gels are providing a unique opportunity for more sophisticated uses. These supramolecular advances are increasingly underpinning and informing next-generation applications - from drug delivery and regenerative medicine to environmental remediation and sustainable energy. In summary, this article presents a panorama over the field of supramolecular gels, emphasising how both academic and industrial scientists are building on the past, and engaging new fundamental insights and innovative concepts to open up exciting horizons for their future use.

Recent advances of magnetic chitosan hydrogel: Preparation, properties and applications

Magnetic chitosan hydrogels are organic-inorganic composite material with the characteristics of both magnetic materials and natural polysaccharides. Due to its biocompatibility, low toxicity and biodegradability, chitosan, a natural polymer has been widely used for preparing magnetic hydrogels. The addition of magnetic nanoparticles to chitosan hydrogels not only improves their mechanical strength, but also endows them with magnetic thermal effects, targeting capabilities, magnetically-sensitive release characteristics, easy separation and recovery, thus enabling them to be used in various applications including drug delivery, magnetic resonance imaging, magnetothermal therapy, and adsorption of heavy metals and dyes. In this review, the physical and chemical crosslinking methods of chitosan hydrogels and the methods for binding magnetic nanoparticles in hydrogel networks are first introduced. Subsequently, the properties of magnetic chitosan hydrogels were summarized including mechanical properties, self-healing, pH responsiveness and properties in magnetic fields. Finally, the potential for further technological and applicative advancements of magnetic chitosan hydrogels is discussed.

Rosacea Topical Treatment and Care: From Traditional to New Drug Delivery Systems

Rosacea is a multifactorial chronic inflammatory dermatosis characterized by flushing, nontransient erythema, papules and pustules, telangiectasia, and phymatous alterations accompanied by itching, burning, or stinging, the pathophysiology of which is not yet fully understood. Conventional topical treatments usually show limited efficacy due to the physical barrier property of the skin that hinders skin penetration of the active ingredients, thereby hampering proper drug skin delivery and the respective therapeutic or cosmetic effects. New advances regarding the physiopathological understanding of the disease and the underlying mechanisms suggest the potential of new active ingredients as promising therapeutic and cosmetic approaches to this dermatosis. Additionally, the development of new drug delivery systems for skin delivery, particularly the potential of nanoparticles for the topical treatment and care of rosacea, has been described. Emphasis has been placed on their reduced nanometric size, which contributes to a significant improvement in the attainment of targeted skin drug delivery. In addition to the exposition of the known pathophysiology, epidemiology, diagnosis, and preventive measures, this Review covers the topical approaches used in the control of rosacea, including skin care, cosmetics, and topical therapies, as well as the future perspectives on these strategies.

Alginate Hydrogel Formulation with Ketorolac for the Treatment of Pain Related Sialolithiasis

Sialolithiasis mainly affects the oral salivary glands due to the presence of small stones that obstruct the secretion of saliva. The treatment and control of pain and inflammation during the course of this pathology is essential to guarantee the patient's comfort. For this reason, a ketorolac calcium cross-linked alginate hydrogel was developed, and it was then applied in the area of the buccal cavity. The formulation was characterized (swelling and degradation profile, extrusion, extensibility, surface morphology, viscosity, and drug release). The drug release was studied ex vivo in static Franz cells and with a dynamic ex vivo method under artificial saliva continuous flow. The product exhibits adequate physicochemical properties considering the intended purpose, and the drug concentrations retained in the mucosa were high enough to deliver a therapeutic local concentration able to reduce the pain associated with the patient's conditions. The results confirmed the suitability of the formulation for application in the mouth.

Light-controlled micron-scale molecular motion

The micron-scale movement of biomolecules along supramolecular pathways, mastered by nature, is a remarkable system requiring strong yet reversible interactions between components under the action of a suitable stimulus. Responsive microscopic systems using a variety of stimuli have demonstrated impressive relative molecular motion. However, locating the position of a movable object that travels along self-assembled fibres under an irresistible force has yet to be achieved. Here, we describe a purely supramolecular system where a molecular 'traveller' moves along a 'path' over several microns when irradiated with visible light. Real-time imaging of the motion in the solvated state using total internal reflection fluorescence microscopy shows that anionic porphyrin molecules move along the fibres of a bis-imidazolium gel upon irradiation. Slight solvent changes mean movement and restructuring of the fibres giving microtoroids, indicating control of motion by fibre mechanics with solvent composition. The insight provided here may lead to the development of artificial travellers that can perform catalytic and other functions.

Hybrid Self-Assembled Gel Beads for Tuneable pH-Controlled Rosuvastatin Delivery

This article describes the fabrication of new pH-responsive hybrid gel beads combining the polymer gelator calcium alginate with two different low-molecular-weight gelators (LMWGs) based on 1,3 : 2,4-dibenzylidene-d-sorbitol: pH-responsive DBS-COOH and thermally responsive DBS-CONHNH2 , thus clearly demonstrating that different classes of LMWG can be fabricated into gel beads by using this approach. We also demonstrate that self-assembled multicomponent gel beads can be formed by using different combinations of these gelators. The different gel bead formulations exhibit different responsiveness - the DBS-COOH network can disassemble within those beads in which it is present upon raising the pH. To exemplify preliminary data for a potential application for these hybrid gel beads, we explored aspects of the delivery of the lipid-lowering active pharmaceutical ingredient (API) rosuvastatin. The release profile of this statin from the hybrid gel beads is pH-dependent, with greater release at pH 7.4 than at pH 4.0 - primary control of this process results from the pKa of the API. The extent of pH-mediated API release is also significantly further modified according to gel bead composition. The DBS-COOH/alginate beads show rapid, highly effective drug release at pH 7.4, whereas the three-component DBS-COOH/DBS-CONHNH2 /alginate system shows controlled slow release of the API under the same conditions. These initial results indicate that such gel beads constitute a promising, versatile and easily tuned platform suitable for further development for controlled drug-delivery applications.

Synthesis and validation of DOPY: A new gemini dioleylbispyridinium based amphiphile for nucleic acid transfection

Nucleic acids therapeutics provide a selective and promising alternative to traditional treatments for multiple genetic diseases. A major obstacle is the development of safe and efficient delivery systems. Here, we report the synthesis of the new cationic gemini amphiphile 1,3-bis[(4-oleyl-1-pyridinio)methyl]benzene dibromide (DOPY). Its transfection efficiency was evaluated using PolyPurine Reverse Hoogsteen hairpins (PPRHs), a nucleic acid tool for gene silencing and gene repair developed in our laboratory. The interaction of DOPY with PPRHs was confirmed by gel retardation assays, and it forms complexes of 155 nm. We also demonstrated the prominent internalization of PPRHs using DOPY compared to other chemical vehicles in SH-SY5Y, PC-3 and DF42 cells. Regarding gene silencing, a specific PPRH against the survivin gene delivered with DOPY decreased survivin protein levels and cell viability more effectively than with N-[1-(2,3-Dioleoyloxy)propyl]-N,N,N-trimethylammonium methylsulfate (DOTAP) in both SH-SY5Y and PC-3 cells. We also validated the applicability of DOPY in gene repair approaches by correcting a point mutation in the endogenous locus of the dhfr gene in DF42 cells using repair-PPRHs. All these results indicate both an efficient entry and release of PPRHs at the intracellular level. Therefore, DOPY can be considered as a new lipid-based vehicle for the delivery of therapeutic oligonucleotides.

Topical Mucoadhesive Alginate-Based Hydrogel Loading Ketorolac for Pain Management after Pharmacotherapy, Ablation, or Surgical Removal in Condyloma Acuminata

Condyloma acuminata is an infectious disease caused by the human papilloma virus (HPV) and one of the most common sexually transmitted infections. It is manifested as warts that frequently cause pain, pruritus, burning, and occasional bleeding. Treatment (physical, chemical, or surgical) can result in erosion, scars, or ulcers, implying inflammatory processes causing pain. In this work, a biocompatible topical hydrogel containing 2% ketorolac tromethamine was developed to manage the painful inflammatory processes occurring upon the removal of anogenital condylomas. The hydrogel was physically, mechanically, and morphologically characterized: it showed adequate characteristics for a topical formulation. Up to 73% of ketorolac in the gel can be released following a one-phase exponential model. Upon application on human skin and vaginal mucosa, ketorolac can permeate through both of these and it can be retained within both tissues, particularly on vaginal mucosa. Another advantage is that no systemic side effects should be expected after application of the gel. The hydrogel showed itself to be well tolerated in vivo when applied on humans, and it did not cause any visible irritation. Finally, ketorolac hydrogel showed 53% anti-inflammatory activity, suggesting that it is a stable and suitable formulation for the treatment of inflammatory processes, such as those occurring upon chemical or surgical removal of anogenital warts.

Morphology control in metallosupramolecular assemblies through solvent-induced steric demand

Controlling the supramolecular self-assembly of π-conjugated systems into defined morphologies is a prerequisite for the preparation of functional materials. In recent years, the development of sophisticated sample preparation protocols and modulation of various experimental conditions (solvent, concentration, temperature, etc.) have enabled precise control over aggregation pathways of different types of monomer units. A common method to achieve pathway control consists in the combination of two miscible solvents in defined proportions - a "poor" and "good" solvent. However, the role of solvents of opposed polarity in the self-assembly of a given building block still remains an open question. Herein, we unravel the effect of aggregation-inducing solvent systems of opposed polarity (aqueous vs. non-polar media) on the supramolecular assembly of a new bolaamphiphilic Pt(ii) complex. A number of experimental methods show a comparable molecular packing in both media driven by a synergy of solvophobic, aromatic and weak hydrogen-bonding interactions. However, morphological analysis of the respective aggregates in aqueous and non-polar media reveals a restricted aggregate growth in aqueous media into spherical nanoparticles and a non-restricted 2D-nanosheet formation in non-polar media. These findings are attributed to a considerably more efficient solvation and, in turn, increased steric demand of the hydrophilic chains in aqueous media than in nonpolar media, which can be explained by the entrapment of water molecules in the hydrophilic aggregate shell via hydrogen bonds. Our findings reveal that the different solvation of peripheral solubilizing groups in solvents of opposed polarity is an efficient method for morphology control in self-assembly.

Quantification of energy of activation to supramolecular nanofibre formation reveals enthalpic and entropic effects and morphological consequence

We show a self-assembly process leading to fibres from a system that starts far from equilibrium because of fast solvent - anti-solvent mixing and analyse the activation energies associated with the aggregation. It is in some ways reminiscent of diverse natural fibrous materials that have kinetic behaviour dominated by a rate limiting induction period followed by rapid growth. A full thermodynamic rationale for these systems and related synthetic ones is required for a full understanding of the driving force of their non-equilibrium self-assembly. Here we determine quantitatively the enthalpy and entropy of activation for the processes leading to the growth of fibres of this type, that contrasts with analysis of other systems where final energetic states are analysed. A dramatic effect is revealed whereby comparatively small changes in temperature or solvent composition (the ratio of water to ethanol) lead to alterations in the relative importance of enthalpy and entropy of activation and massive changes in the speed of fibre formation. The characteristics of the kinetic model adopted show a correlation with the fibre morphology of the self-assembled materials, which are isostructural according to diffraction experiments: the control of growth can lead to fibres only two bilayers thick. The crossover in behaviour is characteristic of the solvent mixture and the thermodynamic analysis points to the origins of this effect where different assembly routes are viable under only marginally different conditions.

Nanostructured supramolecular hydrogels: Towards the topical treatment of Psoriasis and other skin diseases

Supramolecular hydrogels were synthesized using a bis-imidazolium based amphiphile, and incorporating chemically diverse drugs, such as the cytostatics gemcitabine hydrochloride and methotrexate sodium salt, the immunosuppressive drug tacrolimus, as well as the corticoid drugs betamethasone 17-valerate and triamcinolone acetonide, and their potential as drug delivery agents in the dermal treatment of Psoriasis was evaluated. The rheological behavior of gels was studied, showing in all cases suitable viscoelastic properties for topical drug delivery. Scanning electron microscopy (SEM) shows that the drugs included have a great influence on the gel morphology at the microscopic level, as the incorporation of gemcitabine hydrochloride leads to slightly thicker fibers, the incorporation of tacrolimus induces flocculation and spherical precipitates, and the incorporation of methotrexate forms curled fibers. ¹H NMR spectroscopy experiments show that these drugs not only remain dissolved at the interstitial space, but up to 72% of either gemcitabine or methotrexate, and up to 38% of tacrolimus, is retained within the gel fibers in gels formed with a 1:1 gelator:drug molar ratio. This unique fiber incorporation not only protects the drug from degradation, but also importantly induces a Two Phase Exponential drug release, where the first phase corresponds to the drug dissolved in the interstitial space, while the second phase corresponds to the drug exiting from the gel fibers, and where the speed in each phase is in accordance with the physicochemical properties of the drugs, opening perspectives for controlled delivery. Skin permeation ex vivo tests show how these gels successfully promote the drug permeation and retention inside the skin for reaching their therapeutic target, while in vivo experiments demonstrate that they decrease the hyperplasia and reduce the macroscopic tissue damage typically observed in psoriatic skin, significantly more than the drugs in solution. All these characteristics, beside the spontaneous and easy preparation (room temperature and soft stirring), make these gels a good alternative to other routes of administration for Psoriasis treatment, increasing the drug concentration at the target tissue, and minimizing side effects.

Enhanced vitamin C skin permeation from supramolecular hydrogels, illustrated using in situ ToF-SIMS 3D chemical profiling

Vitamin C (ascorbic acid) is a naturally occurring, powerful anti-oxidant with the potential to deliver numerous benefits to the skin when applied topically. However, topical use of this compound is currently restricted by an instability in traditional formulations and the delivery and eventual fate of precursor compounds has been largely unexplored. Time of flight secondary ion mass spectrometry (ToF-SIMS) is an emerging technique in the field of skin research and offers detailed chemical analysis, with high mass and spatial resolution, as well as profiling capabilities that allow analysis as a function of sample depth. This work demonstrates the successful use of ToF-SIMS to obtain, in situ, accurate 3D permeation profiles of both ascorbic acid and a popular precursor, ascorbyl glucoside, from ex vivo porcine skin. The significant permeation enhancing effect of a supramolecular hydrogel formulation, produced from an amphiphilic gemini imidazolium-based surfactant, was also demonstrated for both compounds. Using ToF-SIMS, it was also possible to detect and track the breakdown of ascorbyl glucoside into ascorbic acid, elucidating the ability of the hydrogel formulation to preserve this important conversion until the targeted epidermal layer has been reached. This work demonstrates the potential of ToF-SIMS to provide 3D permeation profiles collected in situ from ex vivo tissue samples, offering detailed analysis on compound localisation and degradation. This type of analysis has significant advantages in the area of skin permeation, but can also be readily translated to other tissue types.

Multifunctional Serine Protease Inhibitor-Coated Water-Soluble Gold Nanoparticles as a Novel Targeted Approach for the Treatment of Inflammatory Skin Diseases

The overexpression and increased activity of the serine protease Kallikrein 5 (KLK5) is characteristic of inflammatory skin diseases such as Rosacea. The use of inhibitors of this enzyme-such as 4-(2-aminoethyl)benzenesulfonyl fluoride hydrochloride (AEBSF·HCl) or the anti-human recombinant Kallikrein 5 (anti-KLK5) antibody-in the treatment of the disease has been limited due to their low bioavailability, for which their immobilization in drug delivery agents can contribute to making serine protease inhibitors clinically useful. In this work, we synthesized gold nanoparticles (GNP) coated with a mixture of hydroxyl- and carboxyl-terminated thiolates (GNP.OH/COOH), whose carboxyl groups were used to further functionalize the nanoparticles with the serine protease inhibitor AEBSF·HCl either electrostatically or covalently (GNP.COOH AEBSF and GNP.AEBSF, respectively), or with the anti-KLK5 antibody (GNP.antiKLK5). The synthesized and functionalized GNP were highly water-soluble, and they were extensively characterized using UV-vis absorption spectroscopy, Transmission Electron Microscopy (TEM), Dynamic Light Scattering (DLS), and Thermogravimetric Analysis (TGA). GNP.OH/COOH and their subsequent functionalizations effectively inhibited KLK5 in vitro. Internalization of fluorophore-coated GNP.OH/COOH in human keratinocytes (HaCaT cells) was proven using confocal fluorescence microscopy. Cell viability assays revealed that the cytotoxicity of free AEBSF is importantly decreased when it is incorporated in the nanoparticles, either ionically (GNP.COOH AEBSF) or, most importantly, covalently (GNP.AEBSF). The functionalized nanoparticles GNP.AEBSF and GNP.antiKLK5 inhibited intracellular KLK5 activity in HaCaT cells and diminished secretion of IL-8 under inflammatory conditions triggered by TLR-2 ligands. This study points to the great potential of these GNP as a new intracellular delivery strategy for both small drugs and antibodies in the treatment of skin diseases such as Rosacea.

Release of small bioactive molecules from physical gels

Pharmaceutical drugs with low water solubility have always received great attention within the scientific community. The reduced bioavailability and the need of frequent administrations have motivated the investigation of new drug delivery systems. Within this context, drug carriers that release their payload in a sustained way and hence reduce the administration rate are highly demanded. One interesting strategy to meet these requirements is the entrapment of the drugs into gels. So far, the most investigated materials for such drug-loaded gels are derived from polymers and based on covalent linkages. However, over the last decade the use of physical (or supramolecular) gels derived from low molecular weight compounds has experienced strong growth in this field, mainly due to important properties such as injectability, stimuli responsiveness and ease of synthesis. This review summarizes the use of supramolecular gels for the encapsulation and controlled release of small therapeutic molecules.

Spatially-resolved soft materials for controlled release - hybrid hydrogels combining a robust photo-activated polymer gel with an interactive supramolecular gel

Self-supporting photo-patterned hybrid gels achieve controlled directional release depending on their surrounding environment.

Hybrid hydrogels based on self-assembling low-molecular-weight gelator (LMWG) DBS-CONHNH₂ (DBS = 1,3;2,4-dibenzylidene-d-sorbitol) and crosslinked polymer gelator (PG) PEGDM (poly(ethyleneglycol) dimethacrylate) are reported, and an active pharmaceutical ingredient (naproxen, NPX) is incorporated. The use of PEGDM as PG enhances the mechanical stiffness of the hybrid gel (G′ increases from 400 to 4500 Pa) – the LMWG enhances its stability to very high frequency. Use of DBS-CONHNH₂ as LMWG enables interactions with NPX and hence allows pH-mediated NPX release – the PG network is largely orthogonal and only interferes to a limited extent. Use of photo-activated PEGDM as PG enables spatially-resolved photo-patterning of robust hybrid gel domains within a preformed LMWG network – the presence of the LMWG enhances the spatial resolution. The photo-patterned multi-domain gel retains pH-mediated NPX release properties and directionally releases NPX into a compartment of higher pH. The two components within these hybrid PG/LMWG hydrogels therefore act largely independently of one another, although they do modify each others properties in subtle ways. Hybrid hydrogels capable of spatially controlled unidirectional release have potential applications in tissue engineering and drug-delivery.

Cationic Supramolecular Hydrogels for Overcoming the Skin Barrier in Drug Delivery

A cationic bis-imidazolium-based amphiphile was used to form thermoreversible nanostructured supramolecular hydrogels incorporating neutral and cationic drugs for the topical treatment of rosacea. The concentration of the gelator and the type and concentration of the drug incorporated were found to be factors that strongly influenced the gelling temperature, gel-formation period, and overall stability and morphology. The incorporation of brimonidine tartrate resulted in the formation of the most homogeneous material of the three drugs explored, whereas the incorporation of betamethasone resulted in a gel with a completely different morphology comprising linked particles. NMR spectroscopy studies proved that these gels kept the drug not only at the interstitial space but also within the fibers. Due to the design of the gelator, drug release was up to 10 times faster and retention of the drug within the skin was up to 20 times more effective than that observed for commercial products. Experiments in vivo demonstrated the rapid efficacy of these gels in reducing erythema, especially in the case of the gel with brimonidine. The lack of coulombic attraction between the gelator-host and the guest-drug seemed particularly important in highly effective release, and the intermolecular interactions operating between them were found to lie at the root of the excellent properties of the materials for topical delivery and treatment of rosacea.