Air-Water Interfacial Properties of Chloroform-Spread versus Water-Spread Poly((d,l-lactic acid- co-glycolic acid)- block-ethylene glycol) (PLGA-PEG) Polymers

Overcoming limitations of conventional solvent exchange methods: Achieving monodisperse non-equilibrium polymer micelles through equilibration-nanoprecipitation (ENP)

Hypothesis Conventional solvent exchange formulation methods face limitations when trying to control the final non-equilibrium size properties of block copolymer micelles containing a strongly hydrophobicity and a rigid block because the solvent conditions are not well controlled during micelle formation. Therefore, using an alternative formulation method, named Equilibration-Nanoprecipitation (ENP), in which micelles are formed under uniform solvent conditions, will significantly reduce the final dispersity compared a conventional solvent exchange method. EXPERIMENTAL: Size properties of the final aqueous micelle dispersions formed from the ENP method and a conventional solvent exchange are measured using DLS. Also, a parallel modelling study is completed to predict the final size distributions using both methods. Findings The experimental results demonstrate the ENP method is effective producing non-equilibrium micelles with low dispersity below the monodisperse polydispersity index (PDI) cutoff for DLS while the conventional solvent exchange method leads to significantly greater dispersity. Also the experimental results highlight ENP can be used to tune the final size properties which cannot be done using methods which do not properly control the micelle formation conditions. Additionally, the modelling study supports the utility of the ENP approach for producing monodisperse dispersions of nonequilibrium polymer micelles.

Effect of temperature on the air-water surface mechanical behavior of water-spread block copolymer micelles

In the pursuit of the development of a first-in-kind polymer lung surfactant (PLS) therapeutic whose effects are biophysical in nature, a comprehensive understanding of the factors affecting the air-water surface mechanical behavior of water-spread block copolymer micelles is desired. To this end, we explore the effect of temperature on the surface mechanical behavior of two different micelle core chemistries, poly(styrene) (PS) and poly(tert-butyl methacrylate) (PtBMA), each having poly(ethylene glycol) (PEG) as the hydrophilic block. The behavior is characterized using surface pressure-area isotherms and quantitative Brewster angle microscopy. The results indicate that the temperature has a significant effect on the micelle structure at the interface and this effect is related to the core Tg as well as the core interfacial tension properties. When temperature is higher than the core Tg for PS-PEG, the spherical micelle core rearranges to form an oblate-like structure which increases its interfacial area. The structural rearrangement changes the mechanism by which the film produces high surface pressure. For PtBMA-PEG, which has a lower interfacial tension with water and air compared to PS, the core domains spread at the interface when the mobility is sufficiently high such that a PtBMA film is formed under high compression. The implications of these changes on PLS efficacy are discussed highlighting the importance of core Tg characterization for polymer nanoparticle applications.

Surface Pressure-Area Mechanics of Water-Spread Poly(ethylene glycol)-Based Block Copolymer Micelle Monolayers at the Air-Water Interface: Effect of Hydrophobic Block Chemistry

Amphiphilic block copolymer micelles can mimic the ability of natural lung surfactant to reduce the air-water interfacial tension close to zero and prevent the Laplace pressure-induced alveolar collapse. In this work, we investigated the air-water interfacial behaviors of polymer micelles derived from eight different poly(ethylene glycol) (PEG)-based block copolymers having different hydrophobic block chemistries to elucidate the effect of the core block chemistry on the surface mechanics of the block copolymer micelles. Aqueous micelles of about 30 nm in hydrodynamic diameter were prepared from the PEG-based block copolymers via equilibration-nanoprecipitation (ENP) and spread on the water surface using water as the spreading medium. Surface pressure-area isotherm and quantitative Brewster angle microscopy (QBAM) measurements were performed to investigate how the micelle/monolayer structures change during lateral compression of the monolayer; widely varying structural behaviors were observed, including the wrinkling/collapse of micelle monolayers and deformation and/or the desorption of individual micelles. By bivariate correlation regression analysis of surface pressure-area isotherm data, it was found that the rigidity and hydrophobicity of the hydrophobic core domain, which are quantified by glass-transition temperature (Tg) and water contact angle (θ) measurements, respectively, are coupled factors that need to be taken into account concurrently in order to control the surface mechanical properties of polymer micelle monolayers; micelles having rigid and strongly hydrophobic cores exhibited high surface pressure and a high compressibility modulus under high compression. High surface pressure and a high compressibility modulus were also found to be correlated with the formation of wrinkles in the micelle monolayer (visualized by Brewster angle microscopy (BAM)). From this study, we conclude that polymer micelles based on hydrophobic block materials having higher Tg and θ are more suitable for surfactant replacement therapy applications that require the therapeutic surfactant to produce a high surface pressure and modulus at the alveolar air-water interface.

Surface mechanical behavior of water-spread poly(styrene)-poly(ethylene glycol) (PS-PEG) micelles at the air-water interface: Effect of micelle size and polymer end/linking group chemistry

HYPOTHESIS

The surface mechanical properties of poly(styrene)-poly(ethylene glycol) (PS-PEG) micelles are influenced by the PEG corona structure. Changes in micelle aggregation number as well as changes in the PEG end group and linking group chemistry of the PS-PEG block copolymer are expected to alter PEG corona characteristics and therefore affect surface mechanical properties of the resulting micelle film.

EXPERIMENTS

Different sized micelles comprised of PS-PEG block copolymer chains were formulated by equilibrating micelles in different ratios of acetone/water mixtures and subsequently removing acetone using dialysis. Additionally, micelles of a similar size and PS-PEG molecular weight but slightly different chemistry were formulated. The micelles were characterized using dynamic light scattering (DLS), transmission electron microscopy (TEM), ¹H NMR, surface pressure-area isotherms and Brewster angle microscopy (BAM).

FINDINGS

The reduction in micelle aggregation number results in the subsequent monolayer having higher compressibility moduli and bending stiffnesses and collapsing at lower surface pressures. Micelle hydrophobicity was shown to improve readsorption of micelles to interface after collapse. Analysis of Brewster angle microscopy images of out-of-plane wrinkle structures which formed upon monolayer collapse indicates the presence of continuous 1 nm thick PEG layer which allows micelle monolayers to bend under high compression.

Polymer Lung Surfactants

Animal-derived lung surfactants annually save 40 000 infants with neonatal respiratory distress syndrome (NRDS) in the United States. Lung surfactants have further potential for treating about 190 000 adult patients with acute respiratory distress syndrome (ARDS) each year. To this end, the properties of current therapeutics need to be modified. Although the limitations of current therapeutics have been recognized since the 1990s, there has been little improvement. To address this gap, our laboratory has been exploring a radically different approach in which, instead of lipids, proteins, or peptides, synthetic polymers are used as the active ingredient. This endeavor has led to an identification of a promising polymer-based lung surfactant candidate, poly(styrene-b-ethylene glycol) (PS-PEG) polymer nanomicelles. PS-PEG micelles produce extremely low surface tension under high compression because PS-PEG micelles have a strong affinity to the air-water interface. NMR measurements support that PS-PEG micelles are less hydrated than ordinary polymer micelles. Studies using mouse models of acid aspiration confirm that PS-PEG lung surfactant is safe and efficacious.