Effect of inorganic salts on the micellar behaviour of ethylene oxide-propylene oxide block copolymers in aqueous solution

Surfactant-rescued gelation: Stabilizing P123-chitosan hydrogels in blood-isotonic aqueous solutions for controlled drug delivery

The loss of gelation in Chitosan-Pluronic composite hydrogels under physiological saline conditions restricts their potential for drug delivery applications. To this effect, we present a dynamic and composite hydrogel system based on Pluronic P123 and chitosan, which restores gelation upon the addition of sodium dodecyl sulfate (SDS), even in the presence of NaCl at blood-isotonic concentrations. The entry of SDS induces electrostatic interactions and hydrophobic associations, enabling robust gel formation under physiological conditions. The gelation behavior can be desirably modified by varying chitosan concentration and temperature, allowing tunable mechanical properties suitable for drug delivery applications. Rheological analysis confirmed enhanced mechanical stability in saline environments, while FTIR spectroscopy elucidated molecular interactions within the hydrogel. Quinine sulfate release studies showed sustained drug release without burst effects. Kinetic modeling indicated a predominantly non-Fickian transport mechanism, governed by both diffusion and polymer relaxation. This surfactant-rescued gelation strategy stabilizes chitosan-based hydrogels in physiological conditions, making them promising candidates for controlled drug delivery.

Mechanistic Investigation into the Phase Separation Behavior of Soluplus in the Presence of Biorelevant Media

More than a decade since its introduction, the polymeric excipient Soluplus continues to receive considerable attention for its application in the development of amorphous solid dispersions (ASDs) and its utility as a solubilizer for drugs exhibiting solubility limited absorption. While it is well-recognized that Soluplus forms micelles, the impact of its lower critical solution temperature of approximately 40 °C remains an underexplored aspect. This study investigated the phase behavior of Soluplus in fasted-state simulated intestinal fluid (FaSSIF-V1). It was demonstrated that Soluplus forms a dispersed polymer-rich coacervate phase, which coexists with Soluplus micelles at 37 °C. This behavior was confirmed by cloud point measurements, visually discernible phases after centrifugation, as well as multi-angle dynamic light scattering (MADLS) measurements, and quantitative 1H-nuclear magnetic resonance (NMR) spectroscopy of Soluplus concentrations in the supernatant pre- and post-centrifugation. The practical relevance of these findings was contextualized by solvent shift experiments and dissolution testing of spray-dried ASD. The results demonstrated that the poorly water-soluble drug RO6897779 resided in a polymer-rich coacervate phase and was spun down during centrifugation, which resulted in an amorphous pellet exhibiting the characteristics of a viscous liquid. The entrapment of the drug within the polymer-rich phase was further analyzed by temperature- and time-dependent MADLS experiments. The findings of this study are of particular relevance for a mechanistic understanding, relevant to comprehending in vitro-in vivo relationships of Soluplus-based ASDs. Low sampled drug concentrations in FaSSIF-V1 at 37 °C may originate not only from limited drug release and precipitation but also from the formation of a drug-containing, polymer-rich Soluplus phase. Therefore, a liquid-liquid phase separation occurring from Soluplus-based formulations in a biorelevant medium can be excipient-driven, which is different from the common perception that phase separation in the solution state is triggered primarily by high drug concentrations exceeding their amorphous solubility.

Molecular insights into the aggregation and solubilizing behavior of biocompatible amphiphiles Gelucire® 48/16 and Tetronics® 1304 in aqueous media

A comparative analysis of the micellar and solubilizing properties of two polyethylene glycol (PEG)-based amphiphilic biocompatible excipients: Gelucire® 48/16 (Ge 48/16) and Tetronics® 1304 (T1304), in the presence and absence of salt, was conducted. As there is a dearth of research in this area, the study aims to shed light on the behavior of these two nonionic surfactants and their potential as nanocarriers for solubilizing pharmaceuticals. Various techniques such as cloud point (CP), dynamic light scattering (DLS), small-angle neutron scattering (SANS), Fourier transform infrared spectroscopy (FT-IR), UV spectrophotometry, and high-performance liquid chromatography (HPLC) were employed. The solubility of quercetin (QCT), a flavonoid with anti-inflammatory, antioxidant, and anti-cancer properties, was evaluated and the interaction between QCT and the micellar system was examined. The analysis revealed the occurrence of strong interactions between QCT and surfactant molecules, resulting in enhanced solubility. It was observed that the micellar size and solubilizing ability were significantly improved in the presence of salt, while the CP decreased. Ge 48/16 exhibited superior performance, with a remarkable increase in the solubility of QCT in the presence of salt, suggesting its potential as an effective nanocarrier for a range of pharmaceutics, and yielding better therapeutic outcomes.

Effect of small amounts of akaganeite (β-FeOOH) nanorods on the gelation, phase behaviour and injectability of thermoresponsive Pluronic F127

Pluronic F127 (PF127) is a copolymer with an amphiphilic nature and can self-assemble to form micelles and, beyond 20% (w/v), form a thermoresponsive physical gel state. However, they are mechanically weak and easily dissolve in physiological environments, which limits their use in load-bearing in specific biomedical applications. Therefore, we propose a pluronic-based hydrogel with enhanced stability by incorporating small amounts of paramagnetic nanorods, akaganeite (β-FeOOH) nanorods (NRs) of aspect ratio ∼7, with PF127. Due to their weak magnetic properties, β-FeOOH NRs have been used as a precursor for preparing stable iron-oxide states (e.g., hematite and magnetite), and the studies on β-FeOOH NRs to be used as a primary component in hydrogels are at the nascent stage. Here we report a method to synthesize β-FeOOH NRs on a gram scale using a simple sol-gel process and characterize the NRs with various techniques. A phase diagram and thermoresponsive behaviour based on rheological experiments and visual observations are proposed for 20% (w/v) PF127 with low concentrations (0.1-1.0% (w/v)) of β-FeOOH NRs. We observe a unique non-monotonous behaviour in the gel network represented by various rheological parameters like storage modulus, yield stress, fragility, high-frequency modulus plateau, and characteristic relaxation time as a function of nanorod concentration. A plausible physical mechanism is proposed to fundamentally understand the observed phase behaviour in the composite gels. These gels show thermoresponsiveness and enhanced injectability, and could find applications in tissue engineering and drug delivery.

Evaluation of bioadhesive gels for local action in the esophagus

Direct drug administration to the esophagus faces several obstacles, including continuous salivary dilution and removal of the dosage form from the tissue surface due to esophageal peristalsis. These actions often result in short exposure times and reduced concentrations of drug at the esophageal surface, providing limited opportunities for drug absorption into or across the esophageal mucosa. A variety of bioadhesive polymers were investigated for their ability to resist removal by salivary washings using an ex vivo porcine esophageal tissue model. Hydroxypropylmethylcellulose and carboxymethylcellulose both have reported bioadhesive properties, but neither was able to withstand repeated exposure to saliva, and the gels formulated with these polymers were quickly removed from the esophageal surface. Two polyacrylic polymers, carbomer and polycarbophil, also showed limited esophageal surface retention when exposed to salivary washing, likely due to the ionic composition of saliva affecting the inter-polymer interactions necessary for these polymers to maintain their increased viscosities. In situ gel forming polysaccharide gels (ion-triggered), including xanthan gum, gellan gum, and sodium alginate, showed superior tissue surface retention, and formulations containing these bioadhesive polymers along with ciclesonide, an anti-inflammatory soft prodrug, were investigated as potential, locally-acting esophageal delivery systems. Exposure of a segment of esophagus to the ciclesonide-containing gels resulted in therapeutic concentrations of des-ciclesonide, the active drug metabolite, in the tissues within 30 min. Increasing des-CIC concentrations were also observed over a 3-hour exposure interval suggesting continued release and absorption of ciclesonide into the esophageal tissues. These results demonstrate the ability to achieve therapeutic drug concentrations in the esophageal tissues using in situ gel-forming bioadhesive polymer delivery systems, and these systems provide promising opportunities for the local treatment of esophageal disease.

Numerical investigation of moving gel wall formation in a Y-shaped microchannel

Molecular diffusive membranes play crucial roles in the field of microfluidics for biological applications e.g., 3D cell culture and biosensors. Hydrogels provide a range of benefits such as free diffusion of small molecules, cost-effectiveness, and the ability to be produced in bulk. Among various hydrogels, Pluronic F127 can be used for cell culture purposes due to its biocompatibility and flexible characteristics regarding its environment. Aqueous solutions of Pluronic F127 shows a reversible thermo-thickening property, which can be manipulated by introduction of ions. As a result, controlled diffusion of ions into the solution of Pluronic F127 can result in a controlled gel formation. In this study, the flow of immiscible solutions of Pluronic and sodium phosphate inside a Y-shaped microchannel is simulated using the level set method, and the effects of volume flow rates and temperature on the gel formation are investigated. It is indicated that the gel wall thickness can decrease by either increasing the Pluronic volume flow rate or increasing both volume flow rates while increasing the saline volume flow rate enhances the gel wall thickness. Below a critical temperature value, no gel wall is formed, and above that, a gel wall is constructed, with a thickness that increases with temperature. This setup can be used for drug screening, where gel wall provides an environment for drug-cell interactions.

Article Highlights

Graphical abstract

Direct and Reverse Pluronic Micelles: Design and Characterization of Promising Drug Delivery Nanosystems

Pluronics are a family of amphiphilic block copolymers broadly explored in the pharmaceutical field. Under certain conditions, Pluronics self-assemble in different structures including nanosized direct and reverse micelles. This review provides an overview about the main parameters affecting the micellization process of Pluronics, such as polymer length, fragments distribution within the chain, solvents, additives and loading of cargo. Furthermore, it offers a guide about the most common techniques used to characterize the structure and properties of the micelles. Finally, it presents up-to-date approaches to improve the stability and drug loading of Pluronic micelles. Special attention is paid to reverse Pluronics and reverse micelles, currently underexplored in the literature. Pluronic micelles present a bright future as drug delivery agents. A smart design and thorough characterization will improve the transfer to clinical applications.

Impact of small molecule and reverse poloxamer addition on the micellization and gelation mechanisms of poloxamer hydrogels

Poloxamer 407 (P407) is widely used for targeted drug-delivery because it exhibits thermoresponsive gelation behavior near body temperature, stemming from a disorder-to-order transition. Hydrophobic small molecules can be encapsulated within P407; however, these additives often negatively impact the rheological properties and lower the gelation temperatures of the hydrogels, limiting their clinical utility. Here we investigate the impact of adding two BAB reverse poloxamers (RPs), 25R4 and 31R1, on the thermal transitions, rheological properties, and assembled structures of P407 both with and without incorporated small molecules. By employing a combination of differential scanning calorimetry (DSC), rheology, and small-angle x-ray scattering (SAXS), we determine distinct mechanisms for RP incorporation. While 25R4 addition promotes inter-micelle bridge formation, the highly hydrophobic 31R1 co-micellizes with P407. Small molecule addition lowers thermal transition temperatures and increases the micelle size, while RP addition mitigates the decreases in modulus traditionally associated with small molecule incorporation. This fundamental understanding yields new strategies for tuning the mechanical and structural properties of the hydrogels, enabling design of drug-loaded formulations with ideal thermal transitions for a range of clinical applications.

Small-volume extensional rheology of concentrated protein and protein-excipient solutions

Limited studies measure extensional rheology in protein solutions due to volume constraints and measurement challenges. We developed a small-volume, dripping-onto-substrate (DoS) extensional rheology device to measure the capillary thinning of protein and protein-excipient solutions via DoS for the first time. Ovalbumin (OVA) was used as a model system, examined via DoS both with and without excipient poloxamer 188 (P188). Water and dilute OVA break apart rapidly and demonstrate inertiocapillary (IC) thinning behavior, where longer breakup times in OVA can be attributed to lower surface tension. Further increasing OVA content leads to longer breakup times and deviations from IC thinning at the start of thinning, however, no evidence of elastic behavior is observed. P188 more effectively lowers the droplet surface tension than OVA, transitioning from IC behavior in dilute solution to weakly elastic behavior at higher concentrations. Combined protein/excipient formulations act synergistically at low concentrations, where breakup times are identical to those of the individual components despite the higher total concentration. However concentrated protein/excipient formulations exhibit elasticity, where extensional rheology parameters depend on P188 content and total concentration. These findings imply that excipients intended to stabilize proteins in shear flow can cause undesirable behavior in extensional flows like injection.

Criteria Governing Rod Formation and Growth in Nonionic Polymer Micelles

Self-assembled wormlike micelles (WLMs) are widely studied in small-molecule surfactants due to their unique ability to break and recombine; however, less is known about the structure and dynamics of nonionic polymer WLMs. Here, solutions of seven triblock poloxamers, composed of poly(propylene oxide) (PPO) midblocks and poly(ethylene oxide) (PEO) end blocks, are comprehensively examined to determine the role of poloxamer composition, temperature, and inorganic salt type and concentration on rod formation and subsequent elongation into WLMs. Phase separation and sphere-to-rod transition temperatures were quantified via cloud point measurements and shear rheology, respectively, and corroborated with small-angle neutron scattering (SANS). The local microstructure of resulting rodlike micelles is remarkably similar across poloxamer type and sodium fluoride (NaF) or sodium chloride (NaCl) content. Salt addition reduces transition temperatures, with the most pronounced effects for poloxamers with high PEO molecular weights and PEO fractions. Between these two temperatures, several poloxamers elongate into WLMs, where shear rheology detects increases in viscosity up to 6 orders of magnitude. Despite similar local microstructures, poloxamer identity and salt content impact micelle growth substantially, where large poloxamers with lower PEO fractions exhibit the highest viscosities and longest relaxation times. While sodium fluoride has little impact on micelle growth, increasing NaCl concentration dramatically increases the WLM viscosity and relaxation time. This result is explained by different interactions of each salt with the micelle: whereas NaF interacts primarily with PEO chains, NaCl may also partition to the PPO/PEO interface in low levels, increasing micelle surface tension, scission energy, and contour length.

Molecular Dynamics Simulations of Aqueous Nonionic Surfactants on a Carbonate Surface

The interactions and structure of secondary alcohol ethoxylates with 15 and 40 ethoxylate units in water near a calcite surface are studied. It is found that water binds preferentially to the calcite surface. Prediction of the free-energy landscape for surfactant molecules shows that single-surfactant molecules do not adsorb because they cannot get close enough to the surface because of the water layer for attractive ethoxylate-calcite or dispersion interactions to be significant. Micelles can adsorb onto the surface even with the intervening water layer because of the integrative effect of the attractive interactions of all the surfactant molecules. Adsorption is found to increase because of the closer proximity of the micelles to the surface due to a weakened water layer at higher temperatures. The free-energy well and barrier values are used to estimate surface to bulk partition coefficients for different surfactants and temperatures, and qualitative agreement is found with experimental observations. The combined effect of surfactant-water and surfactant-solid interactions is found to be responsible for an increased adsorption for nonionic surfactants as the system approaches the cloud point.

Pluronic®-bile salt mixed micelles

The present study was aimed to examine the interaction of two bile salts viz. sodium cholate (NaC) and sodium deoxycholate (NaDC) with three ethylene polyoxide-polypropylene polyoxide (PEO-PPO-PEO) triblock copolymers with similar PPO but varying PEO micelles with a focus on the effect of pH on mixed micelles. Mixed micelles of moderately hydrophobic Pluronic^® P123 were examined in the presence of two bile salts and compared with those from very hydrophobic L121 and very hydrophilic F127. Both the bile salts increase the cloud point (CP) of copolymer solution and decreased apparent micelle hydrodynamic diameter (D_h). SANS study revealed that P123 forms small spherical micelles showing a decrease in size on progressive addition of bile salts. The negatively charged mixed micelles contained fewer P123 molecules but progressively rich in bile salt. NaDC being more hydrophobic displays more pronounced effect than NaC. Interestingly, NaC shows micellar growth in acidic media which has been attributed to the formation of bile acids by protonation of carboxylate ion and subsequent solubilization. In contrast, NaDC showed phase separation at higher concentration. Nuclear Overhauser effect spectroscopy (NOESY) experiments provided information on interaction and location of bile salts in micelles. Results are discussed in terms of hydrophobicity of bile salts and Pluronics^® and the site of bile salt in polymer micelles. Proposed molecular interactions are useful to understand more about bile salts which play important role in physiological processes.

Clouding phenomenon in amphiphilic systems: A review of five decades

Phase separation in amphiphilic systems is an important phenomenon. The temperature at which an amphiphilic solution phase separates is known as Cloud Point (CP). This article reviews in detail the process of phase separation in various amphiphiles (surfactants, polymers and drugs) and effect of different classes of additives on the CP of these amphiphilic systems. Ions affect the CP of drugs in a different way: kosmotropes and hard bases decrease while chaotropes and soft bases increase the CP of nonionic and cationic surfactants. Anionic surfactants show CP in presence of quaternary salts only. Thus, depending upon the nature and concentration of additive, the CP of an amphiphilic system gets increased or decreased and, hence, properties of the system may be tuned as per the need and use. A system with CP at high concentration can be made to phase separate at lower concentration by simply introducing an appropriate additive in it. This makes the system cost effective. On the other hand, if not required, a low CP can be enhanced with the help of another type of a suitable additive.

Influence of additives on thermoresponsive polymers in aqueous media: a case study of poly(N-isopropylacrylamide)

Thermoresponsive polymers (TRPs) in different solvent media have been studied over a long period and are important from both scientific and technical points of view.

Structural insights into the effect of cholinium-based ionic liquids on the critical micellization temperature of aqueous triblock copolymers

A symmetrical PEG–PPG–PEG triblock copolymer with 82.5% PEG as the hydrophilic end blocks, and PPG as the hydrophobic middle block, was chosen to study the effect of ionic liquids on the critical micellization temperature of block copolymers in aqueous solution.

An investigation of Pluronic P123–sodium cholate mixed system: micellization, gelation and encapsulation behavior

We report impact of sodium cholate on micellization and gelation of P123.

Vesicular hydrogen silsesquioxane-mediated synthesis of nanocrystalline silicon dispersed in a mesoporous silica/suboxide matrix, with potential for electrochemical applications

Silicon nanoparticles embedded in a mesoporous SiO₂/SiO_x matrix have been prepared by pyrolysis of condensed hydrogen silsesquioxane vesicles.

Nonionic block copolymers assemble on the surface of protein bionanoparticle

Efficient delivery of therapeutic proteins to a target site remains a challenge due to rapid clearance from the body. Here, we selected tobacco mosaic virus (TMV) as a model protein system to investigate the interactions between the protein and a nonionic block copolymer as a possible protecting agent for the protein. By varying the temperature, we were able to obtain core-shell structures based on hydrophobic interactions among PO blocks and noncovalent interactions between TMV and EO blocks. The protein-polymer interactions were characterized by dynamic light scattering and isothermal titration calorimetry. This study establishes principles for the possible design of clinically useful protein delivery systems.