Antonia G. Denkova

Non-equilibrium dynamics of block copolymer micelles in solution: recent insights and open questions

The increasing variety and complexity of block copolymers leads to a large morphological diversity of self-assembled structures. This, in turn, opens the way for numerous new applications in fields spanning from drug delivery and templated synthesis of nanoporous materials to oil extraction. However, unlike low-molecular weight surfactants, the dynamics of block copolymer systems can be very slow, which has important consequences for carrying out reliable experiments and for their use in practical applications. This paper highlights the mechanism of non-equilibrium block copolymer dynamics, and discusses micellization and transition kinetics between spherical and rod-like micelles by examining the present knowledge and posing some open questions.

Effects of Salts and Ethanol on the Population and Morphology of Triblock Copolymer Micelles in Solution

The morphological changes of micelles composed of triblock copolymer of ethylene oxide and propylene oxide (EO20PO70EO20) in the presence of different inorganic salts and ethanol have been investigated using dynamic light scattering (DLS), rheometry, and cryogenic transmission electron microscopy (cryo-EM). The following salts were studied: KF, KCl, KI, LiCl, and CsCl. In the presence of KF, KCl, and CsCl, spherical and wormlike micelles coexist. LiCl and KI have little influence on the morphology of the micelles, whereas KF has the most pronounced effect. In agreement with the well-known Hoffmeister anion salt series, F- has the strongest effect of the three anions studied (F-, Cl-, I-). In contrast, the effectiveness of the cation type does not follow the original Hoffmeister cation series. The addition of ethanol to the KCl micellar solutions leads to the formation of more or longer wormlike micelles, which start to interact at certain copolymer concentrations depending on the volume fraction of ethanol added. Both the dilute and the semidilute regimes of the wormlike micelles were studied. The length of the micelles reaches a maximum value at around 8-10 vol % ethanol, after which it decreases again. At higher ethanol concentrations (18 vol %), spherical micelles are formed. Conclusions from this study enhance our understanding of the role played by ethanol and salts in the formation of micelle-templated mesoporous materials, such as SBA-15.

Kinetics and mechanism of the sphere-to-rod transition of triblock copolymer micelles in aqueous solutions

The kinetics of the sphere-to-rod transition of micelles composed of triblock copolymers of ethylene oxide and propylene oxide (EO(20)PO(70)EO(20)) have been investigated using dynamic light scattering (DLS) and cryogenic electron transmission microscopy (Cryo-EM). Sphere-to-rod transition is induced by a solvent jump, initiated by adding KCl and ethanol to an aqueous micellar solution. The growth process of the wormlike micelles depends on the experimental conditions and has two distinct regions that can be described as initiation period and actual growth to equilibrium. All growth curves exhibit a single relaxation time that represents the lifetime of the micelles. The growth curves collapse into a master curve, when shifted by the relaxation time, indicating that the actual growth process of the micelles in all samples occurs through the same mechanism. The relaxation time decreases with increasing surfactant concentration. Additionally, some of the formed micelles exhibit a caterpillarlike shape in which some of the original spherical species can still be detected. These facts suggest that the micelles grow longer predominantly by random coagulation/fragmentation reactions involving micellar species of different sizes. However, the appearance of a unimer peak is detected with DLS during the growth stage. This implies that unimer exchange may also contribute to the elongation of the micelles.

A Critical Review of Alpha Radionuclide Therapy—How to Deal with Recoiling Daughters?

This review presents an overview of the successes and challenges currently faced in alpha radionuclide therapy. Alpha particles have an advantage in killing tumour cells as compared to beta or gamma radiation due to their short penetration depth and high linear energy transfer (LET). Touching briefly on the clinical successes of radionuclides emitting only one alpha particle, the main focus of this article lies on those alpha-emitting radionuclides with multiple alpha-emitting daughters in their decay chain. While having the advantage of longer half-lives, the recoiled daughters of radionuclides like 224Ra (radium), 223Ra, and 225Ac (actinium) can do significant damage to healthy tissue when not retained at the tumour site. Three different approaches to deal with this problem are discussed: encapsulation in a nano-carrier, fast uptake of the alpha emitting radionuclides in tumour cells, and local administration. Each approach has been shown to have its advantages and disadvantages, but when larger activities need to be used clinically, nano-carriers appear to be the most promising solution for reducing toxic effects, provided there is no accumulation in healthy tissue.

Complex morphologies of self-assembled block copolymer micelles in binary solvent mixtures: the role of solvent–solvent correlations

The morphologies and sizes of micellar aggregates, composed of the tri-block copolymer P123 (EO20PO70EO20) in a mixture of the aprotic solvent N,N-dimethylformamide (DMF) and water, were investigated by combining Dynamic Light Scattering (DLS) and Cryogenic Transmission Electron Microscopy (cryo-TEM) experiments. At water concentrations between about 27 and 35 wt% bicontinuous micelles with distinct patterns were formed, in coexistence with very long, non-branched, worm-like micelles. Water concentration affects both the size and the morphology of the micellar aggregates. A careful study of the pure binary solvent mixture revealed the presence of dynamic solvent domains of nanometric size, even in the absence of copolymer. Strikingly, the size of these solvent nano-domains closely matched the size of the bicontinuous micelles in a polymer solution for the same water/DMF ratios. We discuss these findings in terms of spinodal decomposition of the polymer solution, in which two-solvent domains contain solvent quality fluctuations that could determine the decomposition. In addition, we suggest another “soft confinement” mechanism that could be responsible for the formation of bicontinuous micelles. The local excess of one of the solvent species in the nano-domains could entrap a metastable morphology.

Polymersomes as radionuclide carriers loaded via active ion transport through the hydrophobic bilayer

Vesicles composed of amphiphilic block copolymers (i.e. polymersomes) have already been shown to have great potential in drug delivery. Nuclear imaging techniques such as Single Photon Emission Computed Tomography (SPECT) are indispensable in the correct evaluation of biodistribution and pharmacokinetics of newly or not fully investigated polymersome formulations. However, to date, polymer vesicles, in contrast to their lipid counterparts, have not been loaded with radionuclides. In this paper, we have investigated the so-called active loading method to trap radionuclides into preformed polymersomes composed of poly(butadiene-b-ethylene oxide) having variable membrane and brush thickness. We have used tropolone as a lipophilic agent to transport the radioactive isotope of indium, 111In, through the hydrophobic membrane into the aqueous cavity containing the strong hydrophilic chelate diethylene triamine pentaacetic acid (DTPA). The results show that a high loading efficiency of 111In3+ (>85%) can be achieved at short incubation times in polymersomes with membrane thicknesses twice the size of typical lipid bilayers. However, increasing the molecular weight of the block copolymers results in a lower radiolabelling efficiency and a much slower loading rate. In addition, both the DTPA and tropolone concentrations have been found to influence the loading efficiency. Finally, we not only demonstrate that a significant amount of this radioisotope can be successfully encapsulated in the polymersomes, but also report that a negligible loss (<5% in 48 hours) is observed, allowing their safe application in future in vivo studies.

Retention studies of recoiling daughter nuclides of 225Ac in polymer vesicles

Alpha radionuclide therapy is steadily gaining importance and a large number of pre-clinical and clinical studies have been carried out. However, due to the recoil effects the daughter recoil atoms, most of which are alpha emitters as well, receive energies that are much higher than the energies of chemical bonds resulting in decoupling of the radionuclide from common targeting agents. Here, we demonstrate that polymer vesicles (i.e. polymersomes) can retain recoiling daughter nuclei based on an experimental study examining the retention of (221)Fr and (213)Bi when encapsulating (225)Ac.

Interactions of Pluronic nanocarriers with 2D and 3D cell cultures: Effects of PEO block length and aggregation state

This work reveals how the physicochemical properties of Pluronic block copolymers influence significantly their interactions with cancer cells, whether in monolayer or spheroid cultures, and how different clinical applications can be foreseen. Two-dimensional (2D) and three-dimensional (3D) cell culture models were used to investigate the interactions of Pluronic carriers with different PEO block length and aggregation state (unimers versus cross-linked micelles) in HeLa and U87 cancer cells. Stabilized micelles of Pluronic P94 or F127 were obtained by polymerization of a crosslinking agent in the micelles hydrophobic core. Nanocarriers were functionalized with a fluorescent probe for visualization, and with a chelator for radiolabeling with Indium-111 and gamma-quantification. The 2D cell models revealed that the internalization pathways and ultimate cellular localization of the Pluronic nanocarriers depended largely on both the PEO block size and aggregation state of the copolymers. The smaller P94 unimers with an average radius of 2.1nm and the shortest PEO block mass (1100gmol(-1)) displayed the highest cellular uptake and retention. 3D tumor spheroids were used to assess the penetration capacity and toxicity potential of the nanocarriers. Results showed that cross-linked F127 micelles were more efficiently delivered across the tumor spheroids, and the penetration depth depends mostly on the transcellular transport of the carriers. The Pluronic P94-based carriers with the shortest PEO block length induced spheroid toxicity, which was significantly influenced by the spheroid cellular type.

Rheology of worm-like micelles composed of tri-block copolymer in the limit of slow dynamics

We study the influence of micellar kinetics on the rheological behavior of worm-like micelles composed of tri-block copolymers of ethylene oxide and propylene oxide (EO20PO70EO20) in an aqueous solution containing KCl and ethanol. The kinetics of the micelles are adjusted by changing the ethanol concentration, according to a previous study in which the lifetime of the micelles was shown to decrease exponentially with increasing ethanol concentration. At higher ethanol concentrations (15 vol % EtOH), the worm-like micelles behave like Maxwell fluids at low frequencies, but have an upturn at higher frequencies, probably due to Rouse or breathing relaxation modes. At low ethanol concentrations (5 and 8 vol % EtOH) where the lifetime of the micelles is long, the rheological behavior is clearly non-Maxwellian, revealing a spectrum of relaxation times. The slow, block copolymer dependent growth of the micelles leads to scaling of viscosity with surfactant concentration, which varies with time. In this slow brea...

Enhanced retention of encapsulated ions in cross-linked polymersomes

Polymer vesicles (polymersomes) composed of poly(butadiene-b-poly(ethylene oxide)) (PB-b-PEO) are known for their stability and limited permeability. However, when these vesicles are diluted, substances, such as ions, encapsulated in the aqueous cavity can be released due to vesicle disruption. In previous studies, we have shown that these vesicles can be loaded efficiently with sufficient quantities of radionuclides to allow application in radionuclide therapy and pharmacokinetics evaluation, provided that there is no loss of the encapsulated radionuclides when diluted in the bloodstream. In this paper, in order to stabilize the carriers, we propose to cross-link the hydrophobic part of the polymersome membrane and to investigate whether such cross-linking induced by γ radiation can enhance the retention of ions (radionuclides). Retention of ions encapsulated in the lumen in such cross-linked carriers has not been previously quantitatively evaluated, although it is of ultimate importance in any medical application. Here, we also investigate how cross-linking affects the transport of radionuclides (loading) through the membrane of the vesicles. The integrity of the vesicles as a function of the radiation dose is also investigated, including morphological changes. The results show that cross-linking hinders the transport of ions through the membrane, which also leads to higher retention of ions encapsulated prior to cross-linking in the vesicles. Electron micrographs show that the shape of the polymersomes is not greatly affected by γ radiation when left in the original solvent (phosphate buffered saline (PBS) or Milli-Q water), but when diluted in a good solvent for both blocks, i.e., tetrahydrofuran (THF), disintegration of the vesicles and the appearance of droplet-like structures is observed, which had not been reported previously. The results of the present study help to formulate polymersomes as carriers for radionuclide therapy, demonstrating a way to prevent in vivo release of radionuclides, caused by dilution-induced destabilization of the nanocarriers.