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PubMedDate not supplied

Preparation and release properties of biodegradable chitin microcapsules: II. Sustained release of 6-mercaptopurine from chitin microcapsules.

Chitin microcapsules are prepared using a simple desolvation or nonsolvent addition phase separation method with 6-mercaptopurine (6-MP) as a reference core. Chitin with a molecular weight about 400,000 is used to prepare different core loaded microcapsules. The drug release rates of chitin microcapsules prepared by simple desolvation or nonsolvent addition method have different release profiles which are related to the rate of phase separation. With respect to the solubility parameter difference (delta delta) value between solvent and nonsolvent, the release rate of 6-MP from microcapsules decreases with increasing delta delta of the preparative system. The chitin beads show poor swelling properties and their release rates are pH-dependent. Sustained release of 6-MP from chitin microcapsules in low pH and neutral medium can be accomplished. To determine if the drug release from the polymer matrix is via a diffusion controlled or by an erosion controlled process, 6-MP release profiles of various chitin microcapsules degraded by lysozyme are investigated. The drug-release patterns of the chitin microcapsules prepared by nonsolvent addition (acetone, n-propanol, n-butanol) and simple desolvation in acetone are not only diffusion but also lysozyme digestion influenced. Whereas, by using water or ethanol as nonsolvent or desolvating agent, release profiles of the microcapsules prepared by nonsolvent addition and the simple desolvation method seem to be little affected by enzyme degradation. These results indicate that chitin might prove useful as a polymer carrier for the sustained release of drugs.

Antimetabolites, Antineoplastic↗

PubMed1989

Intermixing of dipalmitoylphosphatidylcholine with phospho- and sphingolipids bearing highly asymmetric hydrocarbon chains.

We have used high-sensitivity differential scanning calorimetry to investigate the mixing of dipalmitoylphosphatidylcholine (DPPC) with N-lignoceroylgalactocerebroside, N-lignoceroylsulfogalactocerebroside and 1-lauroyl-2-lignoceroylphosphatidylcholine. These three lignoceroyl species, whose two hydrocarbon chains are quite discrepant in length, are completely miscible with DPPC in the liquid-crystalline state. Mixtures of all three lignoceroyl lipids with DPPC show phase separation in the gel state, which is observed over a limited range of compositions (from less than 10 mol% to just over 40 mol% sulfatide) in the case of N-lignoceroylsulfatide and over a wide range of compositions in the cases of N-lignoceroylcerebroside (less than 10 mol% to greater than 90 mol% cerebroside) and 1-lauroyl-2-lignoceroyl-PC (roughly 10 mol% to 90 mol% lauroyl/lignoceroyl PC). The extensive solid-solid phase separation observed in mixtures of DPPC and 1-lauroyl-2-lignoceroyl-PC, which show eutectic behavior, is somewhat unexpected given the similar transition temperatures of the two components but appears to reflect the ability of the lignoceroyl species to form an interdigitated gel phase. However, we find no evidence that the N-lignoceroylsphingolipids are markedly more prone to segregate laterally in PC-rich bilayers than are previously studied sphingolipid species with shorter N-acyl chains. We suggest on the basis of these results that the primary biological importance of the very long N-acyl chains found in many sphingolipids may lie in some function other than the promotion of lateral segregation of sphingolipid-enriched domains in biological membranes.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed2003

Isotropic-nematic phase equilibria in the Onsager theory of hard rods with length polydispersity.

We analyze the effect of a continuous spread of particle lengths on the phase behavior of rodlike particles, using the Onsager theory of hard rods. Our aim is to establish whether "unusual" effects such as isotropic-nematic-nematic (I-N-N) phase separation can occur even for length distributions with a single peak. We focus on the onset of I-N coexistence. For a log-normal distribution, we find that a finite upper cutoff on rod lengths is required to make this problem well posed. The cloud curve, which tracks the density at the onset of I-N coexistence as a function of the width of the length distribution, exhibits a kink; this demonstrates that the phase diagram must contain a three-phase I-N-N region. Theoretical analysis shows that in the limit of large cutoff, the cloud point density actually converges to zero, so that phase separation results at any nonzero density; this conclusion applies to all length distributions with fatter-than-exponentail tails. Finally, we consider the case of a Schulz distribution, with its exponential tail. Surprisingly, even here the long rods (and hence the cutoff) can dominate the phase behavior, and a kink in the cloud curve and I-N-N coexistence again result. Theory establishes that there is a nonzero threshold for the width of the length distribution above which these long-rod effects occur, and shows that the cloud and shadow curves approach nonzero limits for a large cutoff, both in good agreement with the numerical results.

Journal Article↗

PubMed1998

Structure and technofunctional properties of protein-polysaccharide complexes: a review.

Food proteins and polysaccharides are the two key structural entities in food materials. Generally, interactions between proteins and polysaccharides in aqueous media can lead to one- or two-phase systems, the latter being generally observed. In some cases of protein-polysaccharide net attraction, mainly mediated through electrostatic interactions, complex coacervation or associative phase separation occurs, giving rise to the formation of protein-polysaccharide complexes. Physicochemical factors such as pH, ionic strength, ratio of protein to polysaccharide, polysaccharide and protein charge, and molecular weight affect the formation and stability of such complexes. Additionally, the temperature and mechanical factors (pressure, shearing rate, and time) have an influence on phase separation and time stability of the system. The protein-polysaccharide complexes exhibit better functional properties than that of the proteins and polysaccharides alone. This improvement could be attributed to the simultaneous presence of the two biopolymers, as well as the structure of the complexes. Consequently, the interesting hydration (solubility, viscosity), structuration (aggregation, gelation) and surface (foaming, emulsifying) properties of these complexes can be used in a number of domains. Among others, these could be macromolecular purification, microencapsulation, food formulation (fat replacers, texturing agents), and synthesis of biomaterials (edible films, artificial grafts).

Biopolymers↗

PubMedDate not supplied

Preparation of microcapsules masking the bitter taste of enoxacin by using one continuous process technique of agglomeration and microencapsulation.

In order to mask the bitter taste of drugs, a novel microencapsulation process combined with the wet spherical agglomeration (WSA) technique was developed by using a modified phase separation method. The spherical agglomerates of enoxacin (ENX) with various additives including disintegrants were successfully produced in the system of acetone-n-hexane-ammonia water or acetone-n-hexane-distilled water by the WSA, using flocculation phenomena of particles in liquid. Resultant agglomerates could be microencapsulated continuously with Eudragit RS utilizing the phase separation method in the same system as agglomeration under stirring. 'Explosible' microcapsules which were free from the bitter taste could be produced in formulating finer particle size of ENX and 50 per cent of Primojel in core agglomerates, using distilled water as a bridging liquid, and treating with 20 per cent polymer coating level. These microcapsules were bioequivalent to the commercial ENX 100 mg tablets in beagle dogs. One continuous process technique of agglomeration and microencapsulation was useful for the design of ENX powders which masked the bitter taste and controlled the drug release rate.

Animals↗

PubMed2006

Macroporous and nanofibrous polymer scaffolds and polymer/bone-like apatite composite scaffolds generated by sugar spheres.

Scaffolds are crucial to tissue engineering/regeneration. In this work, a technique combining a unique phase-separation process with a novel sugar sphere template leaching process has been developed to produce three-dimensional scaffolds. The resulting scaffolds possess high porosities, well connected macropores, and nanofibrous pore walls. The technique advantageously controls macropore shape and size by sugar spheres, interpore opening size by assembly conditions (time and temperature of heat treatment), and pore wall morphology by phase-separation parameters. The bioactivity of a macroporous and nanofibrous poly(L-lactic acid) (PLLA) scaffold was demonstrated by the bone-like apatite deposition throughout the scaffold in a simulated body fluid (SBF). Preincorporation of nanosized hydroxyapatite eliminated the induction period and facilitated the apatite growth in the SBF. Interestingly, the apatite growth primarily occurred on the surface of the pores (internal and external) but not the interior of the nanofibrous network away from the pore surface. It was also noticed that the macropore size did not affect the apatite growth rate, while the interpore opening size did. The compressive modulus also increased substantially when a continuous apatite layer was formed on the pore walls of the scaffold. The resulting composite scaffold mimics natural bone matrix with the combination of an organic phase (a polymer such as PLLA) and an inorganic apatite phase. The demonstrated bioactivity of apatite layer, together with well-controlled macroporous and nanofibrous structures, makes the novel nanocomposite scaffolds desirable for bone tissue engineering.

Apatites↗

PubMed2004

Grand canonical Monte Carlo simulation of a model colloid-polymer mixture: coexistence line, critical behavior, and interfacial tension.

Grand canonical Monte Carlo simulations are used to study phase separation in a simple colloid-polymer model, the so-called Asakura-Oosawa model. To overcome the problem of small acceptance rates of the grand-canonical moves, cluster moves are introduced. Successive umbrella sampling, recently introduced by Virnau and Muller [J. Chem. Phys. 120, 10925 (2004)], is used to access the phase-separated regime. The unmixing binodal and the interfacial tension are measured and compared to theoretical predictions. By means of finite-size scaling, the behavior close to the critical point is also investigated. Close to criticality, we observe substantial deviations from mean-field behavior.

Journal Article↗

PubMed2005

Insulin particle formation in supersaturated aqueous solutions of poly(ethylene glycol).

Protein microspheres are of particular utility in the field of drug delivery. A novel, completely aqueous, process of microsphere fabrication has been devised based on controlled phase separation of protein from water-soluble polymers such as polyethylene glycols. The fabrication process results in the formation of spherical microparticles with narrow particle size distributions. Cooling of preheated human insulin-poly(ethylene glycol)-water solutions results in the facile formation of insulin particles. To map out the supersaturation conditions conducive to particle nucleation and growth, we determined the temperature- and concentration-dependent boundaries of an equilibrium liquid-solid phase separation. The kinetics of formation of microspheres were followed by dynamic and continuous-angle static light scattering techniques. The presence of PEG at a pH that was close to the protein's isoelectric point resulted in rapid nucleation and growth. The time elapsed from the moment of creation of a supersaturated solution and the detection of a solid phase in the system (the induction period, t(ind)) ranged from tens to several hundreds of seconds. The dependence of t(ind) on supersaturation could be described within the framework of classical nucleation theory, with the time needed for the formation of a critical nucleus (size <10 nm) being much longer than the time of the onset of particle growth. The growth was limited by cluster diffusion kinetics. The interfacial energies of the insulin particles were determined to be 3.2-3.4 and 2.2 mJ/m(2) at equilibrium temperatures of 25 and 37 degrees C, respectively. The insulin particles formed as a result of the process were monodisperse and uniformly spherical, in clear distinction to previously reported processes of microcrystalline insulin particle formation.

Biocompatible Materials↗

PubMed2006

Fuel-grade ethanol transport and impacts to groundwater in a pilot-scale aquifer tank.

Fuel-grade ethanol (76L of E95, 95%v/v ethanol, 5%v/v hydrocarbon mixture as a denaturant) was released at the water table in an 8150-L continuous-flow tank packed with fine-grain masonry sand. Ethanol, which is buoyant and hygroscopic, quickly migrated upwards and spread laterally in the capillary zone. Horizontal migration of ethanol occurred through a shallow thin layer with minimal vertical dispersion, and was one order of magnitude slower than the preceding bromide tracer. Dyes, one hydrophobic (Sudan-IV) and one hydrophilic (Fluorescein) provided evidence that the fuel hydrocarbons phase separated from the E95 mixture as ethanol was diluted by pore water and its cosolvent effect was diminished. Most of the added ethanol (98%) was recovered in the effluent wells that captured the flow through the high water content regions above the water table. Complementary bench-scale 2-D visualization experiments with E95 confirmed hydrocarbon phase separation, residual NAPL formation and migration within the capillary fringe. These results corroborate previous bench-scale studies showing that ethanol has high affinity for vadose-zone pore water and can migrate through the capillary zone. The pilot-scale tank experiment provides the first hydrocarbon and ethanol concentration measurements (and thus, quantification of impacts to groundwater quality) from a subsurface spill of E95 in a well-characterized system with a well-defined source. It also provides the first quantitative near-field-scale evidence that capillarity can significantly retard the vertical dispersion and horizontal advection of ethanol. Such effects could be important determinants of the extent of ethanol migration and longevity as well as groundwater impacts.

Environmental Monitoring↗

PubMed2006

A lattice model of vitrification and gelation.

In this work we introduce a simple lattice model with T-shaped molecules in two dimensions that exhibits a rich range of morphological behaviors. Depending on the volume fraction and quench path, this system can adopt uniform liquid, solution, and phase-separated states, as well as inhomogeneous glass or gel-like states, as revealed by dynamic mean-field simulations. An important characteristic of this system is the existence of a large number of degenerate low-energy states with small barriers that leads to a broad, kinetically explored landscape. The mean-field stability and phase diagram of this model is constructed and provides a useful guide for understanding the complex behaviors of the system. One striking feature is that there is a cascade of instabilities that converge to mark the onset of what we identify as the glass transition. Both dynamic mean-field and Monte Carlo simulations reveal glass-like relaxation dynamics. Our results lead to a picture of gelation as a continuation of the glass transition into the two-phase region, or equivalently, as an incomplete phase separation arrested by the onset of the glass transition.

Journal Article↗

PubMed2006

A coherence-based technique for separating phase-locked from non-phase-locked power spectrum estimates during intermittent stimulation.

In addition to evoked responses, which are phase-locked to the stimuli, the stimulation may also change the ongoing EEG in a time-locked manner. This change has been investigated in event-related synchronization/desynchronization (ERS/ERD) studies by comparing the spectra before and during stimulation or alternatively by using the intertrial variance method (IVM). In the present work, a technique based on the coherence estimate (kappa(y)(2)(f)) between the stimulation signal and the EEG is proposed for separating the ongoing EEG activity spectrum from that of the evoked responses. Furthermore, a statistical criterion is applied to reduce spurious spectral peaks. The performance of this procedure was assessed through simulation and illustrated with EEG during photic stimulation. For simulated data (signal-to-noise-ratio of 0.995 within 10-12.5 Hz) kappa(y)(2)(f) led to a non-phase-locked spectrum estimate with an average normalized error of 12.4%, which is reduced to only 0.2% after applying the statistical criterion. The methodology proposed is asymptotically equivalent to the IVM but it does not require previous filtering the EEG data. Kappa(y)(2)(f) together with the statistical correction criterion allows investigating the entrainment within a narrow-band range, particularly in frequencies close to that of the alpha peak. Hence it is useful in ERS/ERD studies. Moreover, it can be also used for characterizing frequencies within the gamma band.

Adult↗

PubMed1983

Lipid domain structures in biological membranes.

Phase separation represents a possibility for segregation of lipidic membrane components into structurally distinct domains. Freeze-fracture electronmicroscopy is a useful method for detection of lipid domains. Indications of a possible domain-nature of structures are a regular pattern within a separated area, a regular outline of such an area and a local modulation of curvature (evagination or invagination). Candidates for domain structures in biological membranes are smooth particle-free areas and arrays of regularly arranged particles. The interpretation of the particle-free areas is more reliable than that of the arrays with regularly arranged particles. Phase separation in biological membranes can be induced experimentally by lowering the temperature, but physiologically the isothermically induced domains are more important. Factors in control of isothermic domain formation are divalent cations, proteins, cholesterol etc. Suggestions on the biological relevance of domain formation concern mainly their role in the mechanism of membrane fusion, but domains in form of transient or stable membrane structures seem to occur also otherwise and disturbances in domain formation or artificially induced domains can be suitable for pathological alterations.

Animals↗

PubMed2003

Two separate growth phases during the development of Leishmania in sand flies: implications for understanding the life cycle.

The life cycle of Leishmania alternates between two main morphological forms: intracellular amastigotes in the mammalian host and motile promastigotes in the sand fly vector. Several different forms of promastigote have been described in sandfly infections, the best known of these being metacyclic promastigotes, the mammal-infective stages. Here we provide evidence that for Leishmania (Leishmania) mexicana and Leishmania (Leishmania) infantum (syn. chagasi) there are two separate, consecutive growth cycles during development in Lutzomyia longipalpis sand flies involving four distinct life cycle stages. The first growth cycle is initiated by procyclic promastigotes, which divide in the bloodmeal in the abdominal midgut and subsequently give rise to non-dividing nectomonad promastigotes. Nectomonad forms are responsible for anterior migration of the infection and in turn transform into leptomonad promastigotes that initiate a second growth cycle in the anterior midgut. Subsequently, leptomonad promastigotes differentiate into non-dividing metacyclic promastigotes in preparation for transmission to a mammalian host. Differences in timing, prevalence and persistence of the four promastigote stages were observed between L. mexicana and L. infantum in vivo, which were reproduced in cultures initiated with lesion amastigotes, indicating that development is to some extent governed by a programmed series of events. A new scheme for the life cycle in the subgenus Leishmania (Leishmania) is proposed that incorporates these findings.

Animals↗

PubMed2005

Zero-range model of traffic flow.

A multicluster model of traffic flow is studied, in which the motion of cars is described by a stochastic master equation. Assuming that the escape rate from a cluster depends only on the cluster size, the dynamics of the model is directly mapped to the mathematically well-studied zero-range process. Knowledge of the asymptotic behavior of the transition rates for large clusters allows us to apply an established criterion for phase separation in one-dimensional driven systems. The distribution over cluster sizes in our zero-range model is given by a one-step master equation in one dimension. It provides an approximate mean-field dynamics, which, however, leads to the exact stationary state. Based on this equation, we have calculated the critical density at which phase separation takes place. We have shown that within a certain range of densities above the critical value a metastable homogeneous state exists before coarsening sets in. Within this approach we have estimated the critical cluster size and the mean nucleation time for a condensate in a large system. The metastablity in the zero-range process is reflected in a metastable branch of the fundamental flux-density diagram of traffic flow. Our work thus provides a possible analytical description of traffic jam formation as well as important insight into condensation in the zero-range process.

Journal Article↗

PubMed1980

Effects of cholesterol on lipid organization in human erythrocyte membrane.

The molar ratio of cholesterol to phospholipid (C/P) in human erythrocyte membrane is modified by incubating the cells with liposomes of various C/P ratios. The observed increase in cell surface area may be accounted for by the addition of cholesterol molecules. Fusion between liposomes and cells or attachment of liposomes to cells is not a significant factor in the alteration of C/P ratio. Onset temperatures for lipid phase separation in modified membranes are measured by electron diffraction. The onset temperature increases with decreasing C/P ration from 2 degrees C at C/P = 0.95 to 20 degrees C at C/P = 0.5. Redistribution of intramembrane particles is observed in membranes freeze-quenched from temperatures below the onset temperature. The heterogeneous distribution of intramembrane particles below the onset temperature suggests phase separation of lipid, with concomitant segregation of intramembrane protein into domains, even in the presence of an intact spectrin network.

Cholesterol↗

PubMed2004

Magnetic filaments in resistive manganites.

The magnetic phase separation in single crystals of the Pr0.67Ca0.33MnO3 manganites is studied using polarized small angle neutron scattering. The measured spectra give a fractal dimension consistent with a configuration in ferromagnetic filaments of nanometric diameter. We argue here that localized charge carriers hop in a random walk fashion mediating a ferromagnetic "hopping exchange" which coexists with superexchange to create the filamentary phase separation. The arguments for this physical picture are validated by Monte Carlo simulations, where magnetism and transport are treated in a self-consistent manner.

Journal Article↗

PubMed1982

Growth Temperature-Induced Alterations in the Thermotropic Properties of Nerium oleander Membrane Lipids.

The temperature boundary for phase separation of membrane lipids extracted from Nerium oleander leaves was determined by analysis of spin label motion using electron spin resonance spectroscopy and by analysis of polarization of fluorescence from the probe, trans-parinaric acid. A discontinuity of the temperature coefficient for spin label motion, and for trans-parinaric acid fluorescence was detected at 7 degrees C and -3 degrees C with membrane lipids from plants grown at 45 degrees C/32 degrees C (day/night) and 20 degrees C/15 degrees C, respectively. This change was associated with a sharp increase in the polarization of fluorescence from trans-parinaric acid indicating that significant domains of solid lipid form below 7 degrees C or -3 degrees C in these preparations but not above these temperatures. In addition, spin label motion indicated that the lipids of plants grown at low temperatures are more fluid than those of plants grown at higher temperatures.A change in the molecular ordering of lipids was also detected by analysis of the separation of the hyperfine extrema of electron spin resonance spectra. This occurred at 2 degrees C and 33 degrees C with lipids from the high and low temperature grown plants, respectively. According to previous interpretation of spin label data the change at 29 degrees C (or 33 degrees C) would have indicated the temperature for the initiation of the phase separation process, and the change at 7 degrees C (or -3 degrees C) its completion. Because of the present results, however, this interpretation needs to be modified.Differences in the physical properties of membrane lipids of plants grown at the hot or cool temperatures correlate with differences in the physiological characteristics of plants and with changes in the fatty acid composition of the corresponding membrane lipids. Environmentally induced modification of membrane lipids could thus account, in part, for the apparently beneficial adjustments of physiological properties of this plant when grown in these regimes.

Journal Article↗

PubMed2001

Identification by integrated computer modeling and light scattering studies of an electrostatic serum albumin-hyaluronic acid binding site.

Dynamic light scattering and turbidimetry, carried out on solutions of hyaluronic acid (HA) and bovine or human serum albumin (SA) at fixed ionic strength (I), revealed a critical pH corresponding to the onset of HA-SA soluble complex formation. Subsequent reduction of pH below pH(c), corresponding to an increase in protein net positive charge, results in phase separation of the complex. The sensitivity of pH(c) to I indicated the primacy of electrostatic interactions in this process. Since pH(c) was always above the pK(a) of HA, these effects could be attributed to the influence of protein charge. The electrostatic potential around HSA was modeled using DelPhi (MSI) under pH, I conditions corresponding to incipient binding, phase separation, and noninteraction. At all incipient binding conditions (i.e., pH(c), at varying I), an identical region of positive potential 5 A from the protein van der Waals surface appeared. This unique domain intensified with a decrease in pH or I (corresponding to stronger binding), and diminished with an increase in pH or I (i.e., at noninteracting conditions). The size and low curvature of this domain could readily accommodate a 12 nm (decamer) sequence of HA. Simple electrostatic considerations indicate an electrostatic binding energy for the formation of this complex of ca. 1 kT, consistent with the condition of incipient complex formation. We suggest that such weak electrostatic binding may characterize nonspecific interactions for other protein-gylcosaminoglycan pairs.

Animals↗