Formation of Block Copolymer and Surfactant Vesicles

In this thesis temperature induced vesicle formation in both a block copolymer system and nonionic surfactant system have been investigated. The block copolymer is a triblock copolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), PEO-PPO-PEO, with the average molecular composition of EO5PO68EO5 and the commercial trade name Pluronic L121 (papers I, II). In the L121 system the formation of amphiphilic particles with internal structure has also been observed (paper III). The nonionic surfactant studied is tetraethyleneglycol dodecyl ether (C12E4) (paper IV). In addition, a study of the miscibility of L121 and soybean phosphatidylcholine also denoted Lecithin, with the commercial trade name Epikuron 200, was performed (paper V). To characterize the systems, static and dynamic light scattering (SLS and DLS), nuclear magnetic resonance (NMR), cryogenic transmission electron microscopy (cryo-TEM) and small angle X-ray scattering (SAXS) have been used. The most important conclusions are: i) The Pluronic L121 and lecithin do not form mixed aggregates (paper V). ii) L121 can form vesicles in water (paper I, II). iii) In the L121 system one can study vesicle self-assembly from a solution of monomers (unimers) (paper II). iv) In the L121 system the block copolymer polydispersity affects the vesicle self-assembly and its temperature dependence (paper II). v) In the C12E4 system, vesicles can form upon heating a micellar solution (paper IV). vi) The vesicle size distribution depends on the heating rate, and can be roughly understood from the diffusion limited aggregation/fusion of micelles (paper IV). vii) In the L121 system nanosized particles with an internal structure can be formed (paper III)

Lipid-Block Copolymer-Immiscibility

We have investigated the binary system of a triblock copolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), EO5PO68EO5, in water. The ternary system with the same polymer, water and soybean phosphatidylcholine (lecithin) has also been studied. Small-angle X-ray scattering (SAXS), 2H NMR and differential scanning calorimetry (DSC) were used to characterize these systems. The phase diagram of the binary system is presented together with the characteristic parameters in the lamellar and hexagonal phases. In the ternary system, it was found that the lecithin and the block copolymer are essentially immiscible, forming separate phases. In a differential scanning calorimetry experiment it was found that the presence of the block copolymer did not affect the melting temperature of dipalmitoyl phosphatidyl choline. Again indicating immiscibility. The alkane hexadecane is a bad solvent for polypropylene oxide at room temperature. We conclude that it is the difference in hydrophobicity (or polarity) of the hydrophobic parts of the lecithin (lipid) and the block copolymer that explains their immiscibilit

Vesicle formation from temperature jumps in a nonionic surfactant system

When heating a dilute sample of the binary system of tetraethyleneglycol dodecyl ether (C12E4) and water from the micellar phase (L-1) into the two-phase region of a lamellar phase (L-alpha) and excess water (W) vesicles are formed. During heating, one passes a region of phase separation in the micellar phase (L-1' + L-1") where the initial micelles rapidly fuse into larger aggregates forming the concentrated L-1 phase (L-1") with a structure of branched cylindrical micelles, a so-called "living network". The static correlation length of the micelles are increasing with increasing concentration, from ca. 10 nm to 80 nm in the concentration range of 0.0001 g/cm(3)-0.0035 g/cm(3). The overlap concentration was determined to 0.0035 g/cm(3). When the temperature reaches the L-1' + L-alpha region the network particles transform into bilayer vesicles with a z-average apparent hydrodynamic radius in the order of 200 nm depending on the composition. The size of the final vesicles depends on the extent of aggregation/fusion in the L-1' + L-1" region and hence on the rate of heating. The aggregation/fusion in the L-1' + L-1" is slower than diffusion-limited aggregation, and it is shown that 1/100 of the collisions are sticky results in the fusion event

Formation of internally nanostructured triblock copolymer particles

Particles with an internal structure have been found in dilute water solutions of a triblock copolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), which has short hydrophilic PEO endblocks compared to the central hydrophobic PPO block (EO5PO68EO5, L121). The properties of the block copolymer particles (i.e., their structure, size, and time stability) have been investigated using cryogenic transmission electron microscopy (cryo-TEM) in combination with dynamic light scattering (DLS) and turbidity measurements. The particles were formed in dilute solutions by quenching the temperature to temperatures where the reversed hexagonal phase is in equilibrium with a solution of unaggregated L121 copolymers (L-1). From the DLS measurements, a mean hydrodynamic radius of 158 mn was extracted. The time-scan turbidity measurements were found to be unchanged for about 46 h. At higher copolymer concentrations, a reversed hexagonal phase (H-2) exists in the L121/water system. SAXS was used to investigate the internal structure of the dispersed L121-based particles containing 15 wt % L121. It was found that the internal structure transforms from H2 to an inverse micellar system (L-2) as the temperature increases from 37 to 70 degrees C

Spontaneous vesicle formation in a block copolymer system

We have investigated the formation of vesicles in the binary system of a triblock copolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) with the average composition EO5PO68EO5 in water. Vesicles are formed when a solution of unimers is heated into a two-phase region where, at equilibrium, a concentrated lamellar phase coexists with a dilute solution of unimers. The vesicles were characterized by cryo-TEM, static and dynamic light scattering, and NMR experiments. The average vesicle radius is approximately 60 nm, with an exponential size distribution, and the concentration of vesicles depends strongly on the temperature. The vesicles remain stationary on the time scale of hours. A striking observation is that, on this time scale, both the vesicle size distribution and the concentration of vesicles are reversible with respect to temperature cycles. However, on the time scale of weeks a sedimentation is observed in the solutions

Apparent exchange rate for breast cancer characterization.

Although diffusion MRI has shown promise for the characterization of breast cancer, it has low specificity to malignant subtypes. Higher specificity might be achieved if the effects of cell morphology and molecular exchange across cell membranes could be disentangled. The quantification of exchange might thus allow the differentiation of different types of breast cancer cells. Based on differences in diffusion rates between the intra- and extracellular compartments, filter exchange spectroscopy/imaging (FEXSY/FEXI) provides non-invasive quantification of the apparent exchange rate (AXR) of water between the two compartments. To test the feasibility of FEXSY for the differentiation of different breast cancer cells, we performed experiments on several breast epithelial cell lines in vitro. Furthermore, we performed the first in vivo FEXI measurement of water exchange in human breast. In cell suspensions, pulsed gradient spin-echo experiments with large b values and variable pulse duration allow the characterization of the intracellular compartment, whereas FEXSY provides a quantification of AXR. These experiments are very sensitive to the physiological state of cells and can be used to establish reliable protocols for the culture and harvesting of cells. Our results suggest that different breast cancer subtypes can be distinguished on the basis of their AXR values in cell suspensions. Time-resolved measurements allow the monitoring of the physiological state of cells in suspensions over the time-scale of hours, and reveal an abrupt disintegration of the intracellular compartment. In vivo, exchange can be detected in a tumor, whereas, in normal tissue, the exchange rate is outside the range experimentally accessible for FEXI. At present, low signal-to-noise ratio and limited scan time allows the quantification of AXR only in a region of interest of relatively large tumors. © 2016 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd

Mapping prostatic microscopic anisotropy using linear and spherical b‐tensor encoding: A preliminary study

Purpose: Tensor-valued diffusion encoding provides more specific information than conventional diffusion-weighted imaging (DWI), but has mainly been applied in neuroimaging studies. This study aimed to assess its potential for the imaging of prostate cancer (PCa). Methods: Seventeen patients with histologically proven PCa were enrolled. DWI of the prostate was performed with linear and spherical tensor encoding using a maximal b-value of 1.5 ms/µm2 and a voxel size of 3 × 3 × 4 mm3. The gamma-distribution model was used to estimate the mean diffusivity (MD), the isotropic kurtosis (MKI), and the anisotropic kurtosis (MKA). Regions of interest were placed in MR-defined cancerous tissues, as well as in apparently healthy tissues in the peripheral and transitional zones (PZs and TZs). Results: DWI with linear and spherical encoding yielded different image contrasts at high b-values, which enabled the estimation of MKA and MKI. Compared with healthy tissue (PZs and TZs combined) the cancers displayed a significantly lower MD (P <.05), higher MKI (P < 10−5), and lower MKA (P <.05). Compared with the TZ, tissue in the PZ showed lower MD (P < 10−3) and higher MKA (P < 10−3). No significant differences were found between cancers of different Gleason scores, possibly because of the limited sample size. Conclusion: Tensor-valued diffusion encoding enabled mapping of MKA and MKI in the prostate. The elevated MKI in PCa compared with normal tissues suggests an elevated heterogeneity in the cancers. Increased in-plane resolution could improve tumor delineation in future studies

Liquid crystal phantom for validation of microscopic diffusion anisotropy measurements on clinical MRI systems

Purpose To develop a phantom for validating MRI pulse sequences and data processing methods to quantify microscopic diffusion anisotropy in the human brain. Methods: Using a liquid crystal consisting of water, detergent, and hydrocarbon, we designed a 0.5‐L spherical phantom showing the theoretically highest possible degree of microscopic anisotropy. Data were acquired on the Connectome scanner using echo‐planar imaging signal readout and diffusion encoding with axisymmetric b‐tensors of varying magnitude, anisotropy, and orientation. The mean diffusivity, fractional anisotropy (FA), and microscopic FA (µFA) parameters were estimated. Results: The phantom was observed to have values of mean diffusivity similar to brain tissue, and relaxation times compatible with echo‐planar imaging echo times on the order of 100 ms. The estimated values of µFA were at the theoretical maximum of 1.0, whereas the values of FA spanned the interval from 0.0 to 0.8 as a result of varying orientational order of the anisotropic domains within each voxel. Conclusions: The proposed phantom can be manufactured by mixing three widely available chemicals in volumes comparable to a human head. The acquired data are in excellent agreement with theoretical predictions, showing that the phantom is ideal for validating methods for measuring microscopic diffusion anisotropy on clinical MRI systems. Magn Reson Med 79:1817–1828, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes