An experimental rheological phase diagram of a tri-block co-polymer in water validated against dissipative particle dynamics simulations

Pluronic F68 Micelles as Carriers for an Anti-Inflammatory Drug: A Rheological and Scattering Investigation

Age-long ambition of medical scientists has always been advancement in healthcare and therapeutic medicine. Biomedical research indeed claims paramount importance in nanomedicine and drug delivery, and the development of biocompatible storage structures for delivering drugs stands at the heart of emerging scientific works. The delivery of drugs into the human body is nevertheless a nontrivial and challenging task, and it is often addressed by using amphiphilic compounds as nanosized delivery vehicles. Pluronics belong to a peculiar class of biocompatible and thermosensitive nonionic amphiphilic copolymers, and their self-assemblies are employed as drug delivery excipients because of their unique properties. We herein report on the encapsulation of diclofenac sodium within Pluronic F68 self-assemblies in water, underpinning the impact of the drug on the rheological and microstructural evolution of pluronic-based systems. The self-assembly and thermoresponsive micellization were studied through isothermal steady rheological experiments at different temperatures on samples containing 45 wt % Pluronic F68 and different amounts of diclofenac sodium. The adoption of scattering techniques, small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS), allowed for the description of the system features at the nanometer length scale, providing information about the characteristic size of each part of the micellar structures as a function of temperature and drug concentration. Diclofenac sodium is not a good fellow for Pluronic F68. The triblock copolymer aids the encapsulation of the drug, highly improving its water solubility, whereas diclofenac sodium somehow hinders Pluronic self-assembly. By using a simple empirical model and no fitting parameters, the steady viscosity can be predicted, although qualitatively, through the volume fraction of the micelles extracted through scattering techniques and compared to the rheological one. A tunable control of the viscous behavior of such biomedical systems may be achieved through the suitable choice of their composition.

Phase transitions of aqueous solutions of Pluronic F68 in the presence of Diclofenac Sodium

In recent years, advancements in bioengineering and materials science have witnessed increasing interest in synthetic polymers capable of fulfilling various applications. Owing to their distinctive properties, Pluronics can be used as nano-drug carriers, to deliver poorly water-soluble drugs, and as model systems to study colloidal science by tuning amphiphilic properties. In this work, we investigated the effect of diclofenac sodium on the self-assembly and thermoresponsive crystallization of Pluronic F68 in water solutions, by employing experimental rheology and Nuclear Magnetic Resonance (NMR). We built a complete phase diagram as a function of temperature and concentration for 45 wt% Pluronic F68 with various amounts of diclofenac sodium in water. The morphological transitions were followed as a function of temperature via linear rheology. We extrapolated the transition temperatures - identifying distinct phases - as a function of the drug concentration and proposed an empirical model for their prediction. NMR analysis provided further information on the structural characteristics of the systems, shedding light on the interactions between F68 and diclofenac sodium. Although dealing with a pharmaceutical salt, the study is focused on a colloidal system and its interaction with a binding molecule, that is of general interest for colloidal science.

Effect of 3-Carene and the Micellar Formulation on Leishmania (Leishmania) amazonensis

Leishmaniases are neglected tropical diseases caused by obligate intracellular protozoa of the genus Leishmania. The drugs used in treatment have a high financial cost, a long treatment time, high toxicity, and variable efficacy. 3-Carene (3CR) is a hydrocarbon monoterpene that has shown in vitro activity against some Leishmania species; however, it has low water solubility and high volatility. This study aimed to develop Poloxamer 407 micelles capable of delivering 3CR (P407-3CR) to improve antileishmanial activity. The micelles formulated presented nanometric size, medium or low polydispersity, and Newtonian fluid rheological behavior. 3CR and P407-3CR inhibited the growth of L. (L.) amazonensis promastigote with IC₅₀/48h of 488.1 ± 3.7 and 419.9 ±1.5 mM, respectively. Transmission electron microscopy analysis showed that 3CR induces multiple nuclei and kinetoplast phenotypes and the formation of numerous cytosolic invaginations. Additionally, the micelles were not cytotoxic to L929 cells or murine peritoneal macrophages, presenting activity on intracellular amastigotes. P407-3CR micelles (IC₅₀/72 h = 0.7 ± 0.1 mM) increased the monoterpene activity by at least twice (3CR: IC₅₀/72 h >1.5 mM). These results showed that P407 micelles are an effective nanosystem for delivering 3CR and potentiating antileishmanial activity. More studies are needed to evaluate this system as a potential therapeutic option for leishmaniases.

Phase behavior of medium-length hydrophobically associating PEO-PPO multiblock copolymers in aqueous media

HYPOTHESIS

The micellization of block copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) is driven by the dehydration of PPO at elevated temperatures. At low concentrations, a viscous solution of isolated micelles is obtained, whereas at higher concentrations, crowding of micelles results in an elastic gel. Alternating PEO-PPO multiblock copolymers are expected to exhibit different phase behavior, with altered phase boundaries and thermodynamics, as compared to PEO-PPO-PEO triblock copolymers (Pluronics®) with equal hydrophobicity, thereby proving the pivotal role of copolymer architecture and molecular weight.

EXPERIMENTS

Multiple characterization techniques were used to map the phase behavior as a function of temperature and concentration of PEO-PPO multiblock copolymers (ExpertGel®) in aqueous solution. These techniques include shear rheology, differential and adiabatic scanning calorimetry, isothermal titration calorimetry and light transmittance. The micellar size and topology were studied by dynamic light scattering.

FINDINGS

Multiblocks have lower transition temperatures and higher thermodynamic driving forces for micellization as compared to triblocks due to the presence of more than one PPO block per chain. With increasing concentration, the multiblock copolymers in solution gradually evolve into a viscoelastic network formed by soluble bridges in between micellar nodes, whereas hairy triblock micelles jam into liquid crystalline phases resembling an elastic colloidal crystal.

Thermo-responsive nonionic amphiphilic copolymers as draw solutes in forward osmosis process for high-salinity water reclamation

Recently, thermo-responsive nonionic amphiphilic copolymers have shown a great potential as forward osmosis (FO) draw solutes for high-salinity water desalination and zero-liquid discharge (ZLD). However, the relationship between the copolymer structural properties and key characteristics as draw solutes, as well as copolymer's chemical stability after regeneration have not been much studied. In this work, we systematically investigated poly (ethylene oxide)-block-poly (propylene oxide)-block-poly (ethylene oxide) (PEO-PPO-PEO) copolymers as draw solute. The results showed that the PEO segments significantly influenced the viscosity, osmotic pressure and lowest phase separation temperature of the copolymer aqueous solutions. Among four commercial copolymers studied, Pluronic® L35 with moderate molecular weight (Mn 1,900 Da), 50% PEO, and relatively high hydrophilic-lipophilic balance (HLB) showed the best draw solution (DS) performance. It also showed great stability in physiochemical properties and draw capacity after more than ten cycles of regeneration. On the other hand, despite the fact that membrane fouling was observed due to the use of copolymer DS, the FO flux (∼1.2 L m^‒2 h^‒1, as similar with the virgin membrane) was not affected when high-salinity feedwater such as seawater RO brine was applied. Overall, our study has provided a more comprehensive understanding on the characteristics of nonionic amphiphilic copolymer DS and showcased the promise of copolymer-driven FO process in high-salinity water desalination and ZLD.

Insights into the Phase Diagram of Pluronic L64 Using Coarse-Grained Molecular Dynamics Simulations

We investigate the concentration-dependent phase diagram of pluronic L64 in aqueous media at 300 and 320 K using coarse-grained (CG) molecular dynamics (MD) simulations. The CG model is derived by adapting the Martini model for nonbonded interactions along with the Boltzmann inversion (BI) of bonded interactions from all-atom (AA) simulations. Our derived CG model successfully captures the experimentally observed micellar-, hexagonal-, lamellar-, and polymer-rich solution phase. The end-to-end distance reveals the conformational change from an open-chain structure in the micellar phase to a folded-chain structure in the lamellar phase, increasing the orientational order. An increase in temperature leads to expulsion of water molecules from the L64 moiety, suggesting an increase in L64 hydrophobicity. Thermodynamic analysis using the two-phase thermodynamics (2PT) method suggests the entropy of the system decreases with increasing L64 concentration and the decrease in free energy (F) with temperature is mainly driven by the entropic factor (-TS). Further, the increase in aggregation number at lower concentrations and self-assembly at very high concentrations is energetically driven, whereas the change from the micellar phase to the lamellar phase with increasing L64 concentration is entropically driven. Our model provides molecular insights into L64 phases which can be further explored to design functionality-based suprastructures for drug delivery and tissue engineering applications.

Comparison of equilibrium techniques for the viscosity calculation from DPD simulations

Dissipative Particle Dynamics (DPD) is a powerful mesoscopic modelling technique that is routinely used to predict complex fluid morphology and structural properties. While its ability to quickly scan the conformational space is well known, it is unclear if DPD can correctly calculate the viscosity of complex fluids. In this work, we estimate the viscosity of several unentangled polymer solutions using both the Einstein and Green-Kubo formulas. For this purpose, an Einstein relation is derived analogous to the revised Green-Kubo formula suggested by Jung and Schmid, J. Chem. Phys., 2016, 144, 204104. We show that the DPD simulations reproduce the dynamical behaviour predicted by the theory irrespectively of the values of the conservative and friction parameters used and estimate a Schmidt number compatible to that of a fluid system. Moreover, we observe that the Einstein method requires shorter trajectories to achieve the same statistical accuracy as the Green-Kubo formula. This work shows that DPD can confidently be used to calculate the viscosity of complex fluids and that the statistical accuracy of short trajectories can be improved by using our revised Einstein formula.

In Situ Visualization of the Structural Evolution and Alignment of Lyotropic Liquid Crystals in Confined Flow

Self-assembled materials such as lyotropic liquid crystals offer a wide variety of structures and applications by tuning the composition. Understanding materials behavior under flow and the induced alignment is wanted in order to tailor structure related properties. A method to visualize the structure and anisotropy of ordered systems in situ under dynamic conditions is presented where flow-induced nanostructural alignment in microfluidic channels is observed by scanning small angle X-ray scattering in hexagonal and lamellar self-assembled phases. In the hexagonal phase, the material in regions with high extensional flow exhibits orientation perpendicular to the flow and is oriented in the flow direction only in regions with a high enough shear rate. For the lamellar phase, a flow-induced morphological transition occurs from aligned lamellae toward multilamellar vesicles. However, the vesicles do not withstand the mechanical forces and break in extended lamellae in regions with high shear rates. This evolution of nanostructure with different shear rates can be correlated with a shear thinning viscosity curve with different slopes. The results demonstrate new fundamental knowledge about the structuring of liquid crystals under flow. The methodology widens the quantitative investigation of complex structures and identifies important mechanisms of reorientation and structural changes.

Effect of the Janus Amphiphilic Wall on the Viscosity Behavior of Aqueous Surfactant Solutions

The effects of the chemical nature of an interface are one of the key parameters which can affect self-assembly and rheological behavior. To date, several studies have reported self-assembled structures and rheological behaviors in the development of various functional materials. In this study, we investigated the self-assembly and viscosity behavior of aqueous surfactant solutions confined in three types of Janus amphiphilic nanotubes (JANTs), which have two, four, and eight sequential domains, respectively, using molecular simulation. We found that the viscosity behavior depends on the surfactant concentration and the chemical nature of the wall surface. For instance, although the concentration levels of the surfactants are the same (c = 10%), completely different viscosity behaviors were observed in the two sequential domains (Newtonian-like) and the four and eight sequential domains (strong shear-thinning) of the JANTs. Our simulations demonstrated how the rheological properties of aqueous surfactant solutions, including viscosity and velocity profiles, can be controlled by the chemical nature of the JANT wall surface, effect of confinement, and their self-assembly structures. Considering the foregoing, we hope that our study offers new knowledge on nanofluid systems.