Biomedical subjects

Stanislav Rangelov

Publications and source records attributed to Stanislav Rangelov.

4 recordsLinked to original sources

PubMed2006

Light scattering and cryogenic transmission electron microscopy of vesicles and other structures formed in water by mixtures of copolymers bearing lipid-mimetic units.

The phase behavior of mixtures of a vesicle-forming copolymer and related to it copolymers of different phase propensities was studied by light scattering and cryogenic transmission electron microscopy. The vesicle-forming copolymer and the copolymeric additives are based on poly(ethylene glycol) (PEG) and comprise from 1 to 8 lipophilic lipid-mimetic anchors which form hydrophobic moieties. The ability of the additives to control the vesicle size and particle morphology was examined as a function of their content, composition, and phase propensity. At the lower guest to host molar ratios, the bilayer organization is preserved and changes in the vesicle dimensions are not observed. At a certain ratio which is dependent on the composition and phase propensity of the guest copolymer the particle dimensions start to change, and typically, transitions from low- to high-curvature structures are observed. The critical value of the mean PEG interfacial area at which the particles undergo size, size distribution, and phase transitions slightly depends on the composition of the guest copolymer and is found to be roughly 3 nm2.

Cryoelectron Microscopy↗

PubMed2005

Particulate and bulk bicontinuous cubic phases obtained from mixtures of glyceryl monooleate and copolymers bearing blocks of lipid-mimetic anchors in water.

Copolymers based on poly(ethylene glycol) bearing one or more lipid-mimetic anchors were mixed with glycerylmonooleate (GMO)-a lipid with nonlamellar propensity-to form bulk and particulate bicontinuous cubic phases in water. The particulate phase was obtained via a liquid precursor method. Three forms of copolymer/GMO mixtures were investigated-precursor dispersions in glycerol and bulk and particulate phases in water-by visual observations, dynamic light scattering (DLS), and cryogenic transmission electron microscopy (cryo-TEM). The bulk phases were found to very slowly develop a macroscopic appearance that can be associated with the bicontinuous cubic phase. They were prepared in a slight excess of water, which became opalescent in some of the preparations. Cryo-TEM investigation of the excess showed that vesicles and particles with a dense interior coexisted. The precursors were prepared as solutions in glycerol. The viscous liquid material was investigated by DLS. Diffusion coefficients and the corresponding hydrodynamic radii, ranging from about 10 to 30 nm, were calculated. The particles are presumably of a structure similar to that of conventional emulsion droplets with GMO in the interior and copolymer molecules in the outer regions. The particulate phase in water was obtained upon hydration of the liquid precursors. The dispersions were investigated by DLS and cryo-TEM. DLS revealed the formation of nanosized particles. The size was found to increase with increasing copolymer content for copolymers with only one lipid-mimetic anchor, whereas the opposite trend was observed for the formulations with copolymers bearing more than one lipid-mimetic anchor. The shape and interior of the particles were studied by cryo-TEM. It was found that most particles were globular. For some of the compositions, particles with a dense internal structure dominated. The texture of the internal structures was assigned to dispersed bicontinuous cubic or L3 phases. In other compositions, the interior seemingly consists of arrays of interlamellar attachments.

Cryoelectron Microscopy↗

PubMed2005

Mixed block copolymer aggregates with tunable temperature behavior.

A block copolymer of propylene oxide (PO) and ethoxyethyl glycidyl ether (EEGE), (PO)(2)(EEGE)(6)(PO)(2), that has been found to possess lower critical solution temperature properties in water in the temperature range below 20 degrees C was mixed at 1:0.1, 1:1, and 1:10 weight ratios with commercially available Pluronic (L64 or P85) block copolymers. The cooperative association of the copolymers in aqueous solution was studied by dynamic light scattering over a wide temperature range (5-60 degrees C). At lower temperatures, the systems containing either L64 or P85 behave similarly irrespective of the composition: three species corresponding to (PO)(2)(EEGE)(6)(PO)(2) unimers, Pluronic-dominated mixed micelles, and large (50-60 nm in radius) composite (PO)(2)(EEGE)(6)(PO)(2)/Pluronic aggregates were identified. At a certain temperature, which is composition-dependent, the systems phase-separate [(PO)(2)(EEGE)(6)(PO)(2)/L64 1:0.1], enter an interval of instability [(PO)(2)(EEGE)(6)(PO)(2)/L64 1:1 and 1:10], or rearrange by dissociation of the large composite particles [(PO)(2)(EEGE)(6)(PO)(2)/P85]. The presence of a Pluronic micellar peak in the relaxation time distribution at lower temperatures, the dimensions of the composite particles, and the different behavior of the systems at elevated temperatures are discussed. A possible application of the thermosensitive mixtures in delivery/release of active substances is suggested.

Ethers↗

PubMed2004

Spontaneously formed nonequilibrium vesicles of cetyltrimethylammonium bromide and sodium octyl sulfate in aqueous dispersions.

It is well-known that vesicles form in mixtures of cationic and anionic surfactants. We have investigated mixtures of cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS) with the latter in excess over a long time, about 500 days. We have followed the growth of the aggregates by light scattering and checked the morphologies by cryogenic transmission electron microscopy (cryoTEM). All samples showed a monotonic growth with decreasing rate (the change of size was about linear on a logarithmic time scale). In series of samples with weight ratio 30:70 of CTAB/SOS and total surfactant concentration between 0.5 and 3 wt %, the size increased with the surfactant concentration up to 2 wt % and decreased thereafter; cryoTEM examination revealed that the samples contained a majority of open bilayer structures at the highest concentrations. Part of the sample at 2 wt % was diluted to 0.5 wt % after 60 days. The size measured after dilution was slightly smaller than before but well above that found in the directly prepared 0.5 wt % sample, and the particle size in the three samples continued to grow in parallel. Structures other than unilamellar vesicles were observed also in samples at 2 wt % total surfactant concentration at CTAB/SOS ratios close to the borders of the vesicle lobe in the (quasi) ternary phase diagram as published (Yatcilla, M. T.; Herrington, K. L.; Brasher, L. L.; Kaler, E. W.; Chiruvolu, S.; Zasadzinski, J. A. J. Phys. Chem. 1996, 100, 5874). The results clearly show that the spontaneous vesicle populations do not represent equilibrium populations. They also suggest that the vesicle lobes in the phase diagram mainly represent areas where a lamellar phase is easily dispersed in the form of vesicles in an aqueous solution.

Journal Article↗