Publications by authors named "Igor Dolbnya"

23 Publications

  • Page 1 of 1

Synchrotron X-ray Scattering Analysis of Nylon-12 Crystallisation Variation Depending on 3D Printing Conditions.

Polymers (Basel) 2020 May 20;12(5). Epub 2020 May 20.

MBLEM, Department of Engineering Science, University of Oxford, Parks Road OX1 3PJ, UK.

Nylon-12 is an important structural polymer in wide use in the form of fibres and bulk structures. Fused filament fabrication (FFF) is an extrusion-based additive manufacturing (AM) method for rapid prototyping and final product manufacturing of thermoplastic polymer objects. The resultant microstructure of FFF-produced samples is strongly affected by the cooling rates and thermal gradients experienced across the part. The crystallisation behaviour during cooling and solidification influences the micro- and nano-structure, and deserves detailed investigation. A commercial Nylon-12 filament and FFF-produced Nylon-12 parts were studied by differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS) to examine the effect of cooling rates under non-isothermal crystallisation conditions on the microstructure and properties. Slower cooling rates caused more perfect crystallite formation, as well as alteration to the thermal properties.
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http://dx.doi.org/10.3390/polym12051169DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7285354PMC
May 2020

Investigations into the interface failure of yttria partially stabilised zirconia - porcelain dental prostheses through microscale residual stress and phase quantification.

Dent Mater 2019 11 13;35(11):1576-1593. Epub 2019 Sep 13.

Department of Engineering Science, University of Oxford, Oxford, OX1 3PJ, UK.

Objectives: Yttria Partially Stabilised Zirconia (YPSZ) is a high strength ceramic which has become widely used in porcelain veneered dental copings due to its exceptional toughness. Within these components the residual stress and crystallographic phase of YPSZ close to the interface are highly influential in the primary failure mode; near interface porcelain chipping. In order to improve present understanding of this behaviour, characterisation of these parameters is needed at an improved spatial resolution.

Methods: In this study transmission micro-focus X-ray Diffraction, Raman spectroscopy, and focused ion beam milling residual stress analysis techniques have, for the first time, been used to quantify and cross-validate the microscale spatial variation of phase and residual stress of YPSZ in a prosthesis cross-section.

Results: The results of all techniques were found to be comparable and complementary. Monoclinic YPSZ was observed within the first 10μm of the YPSZ-porcelain interface with a maximum volume fraction of 60%. Tensile stresses were observed within the first 150 μm of the interface with a maximum value of ≈300 MPa at 50 μm from the interface. The remainder of the coping was in mild compression at ≈-30MPa, with shear stresses of a similar magnitude also being induced by the YPSZ phase transformation.

Significance: The analysis indicates that the interaction between phase transformation, residual stress and porcelain creep at YPSZ-porcelain interface results in a localised porcelain fracture toughness reduction. This explains the increased propensity of failure at this location, and can be used as a basis for improving prosthesis design.
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http://dx.doi.org/10.1016/j.dental.2019.08.098DOI Listing
November 2019

Spatially resolved mapping of phase transitions in liquid-crystalline materials by X-ray birefringence imaging.

Chem Sci 2019 Mar 2;10(10):3005-3011. Epub 2019 Jan 2.

School of Chemistry , Cardiff University , Park Place , Cardiff CF10 3AT , Wales , UK . Email:

The X-ray Birefringence Imaging (XBI) technique, first reported in 2014, is a sensitive method for spatially resolved mapping of the local orientational properties of anisotropic materials. We report the first application of the XBI technique to characterize molecular orientational ordering in a liquid crystalline material, demonstrating significant potential for exploiting XBI measurements to advance structural understanding of liquid crystal phases.
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http://dx.doi.org/10.1039/c8sc05285aDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6430193PMC
March 2019

A water-cooled monochromator for the B16 Test beamline at the Diamond Light Source: capabilities and performance characterization.

J Synchrotron Radiat 2019 Jan 1;26(Pt 1):253-262. Epub 2019 Jan 1.

Instrument Design Technology Ltd, Unit 2 Turnstone Business Park, Mulberry Avenue, Widnes WA8 0WN, UK.

Systematic studies of the performance of a water-cooled X-ray monochromator, designed and built for the B16 Test beamline at the Diamond Light Source, UK, are presented. A technical description of the monochromator is given and the results of commissioning measurements are discussed. Overall, the monochromator satisfies the original specifications well and meets all the major requirements of the versatile beamline. Following its successful implementation on B16, the basic monochromator design has been reproduced and adapted on other Diamond Light Source beamlines, including B18 and B21.
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http://dx.doi.org/10.1107/S1600577518014662DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6337885PMC
January 2019

Pulse picker for synchrotron radiation driven by a surface acoustic wave: erratum.

Opt Lett 2018 06;43(11):2490

An erratum is presented to correct the typographical errors concerning the composition of the multilayer used in the experiment in Opt. Lett. 42, 1915.
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http://dx.doi.org/10.1364/OL.43.002490DOI Listing
June 2018

Observation of dose-rate dependence in a Fricke dosimeter irradiated at low dose rates with monoenergetic X-rays.

Sci Rep 2018 03 16;8(1):4735. Epub 2018 Mar 16.

School of Maths & Physics, Queen's University Belfast, University Road, Belfast, BT7 1NN, UK.

Absolute measurements of the radiolytic yield of Fe3+ in a ferrous sulphate dosimeter formulation (6 mM Fe2+), with a 20 keV x-ray monoenergetic beam, are reported. Dose-rate suppression of the radiolytic yield was observed at dose rates lower than and different in nature to those previously reported with x-rays. We present evidence that this effect is most likely to be due to recombination of free radicals radiolytically produced from water. The method used to make these measurements is also new and it provides radiolytic yields which are directly traceable to the SI standards system. The data presented provides new and exacting tests of radiation chemistry codes.
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http://dx.doi.org/10.1038/s41598-018-21813-zDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5856745PMC
March 2018

Pulse picker for synchrotron radiation driven by a surface acoustic wave.

Opt Lett 2017 May;42(10):1915-1918

A functional test for a pulse picker for synchrotron radiation was performed at Diamond Light Source. The purpose of a pulse picker is to select which pulse from the synchrotron hybrid-mode bunch pattern reaches the experiment. In the present work, the Bragg reflection on a Si/B4C multilayer was modified using surface acoustic wave (SAW) trains. Diffraction on the SAW alters the direction of the x rays and it can be used to modulate the intensity of the x rays that reach the experimental chamber. Using electronic modulation of the SAW amplitude, it is possible to obtain different scattering conditions for different x-ray pulses. To isolate the single bunch, the state of the SAW must be changed in the short time gap between the pulses. To achieve the necessary time resolution, the measurements have been performed in conical diffraction geometry. The achieved time resolution was 120 ns.
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http://dx.doi.org/10.1364/OL.42.001915DOI Listing
May 2017

X-ray Birefringence Imaging of Materials with Anisotropic Molecular Dynamics.

J Phys Chem Lett 2015 Feb 27;6(3):561-7. Epub 2015 Jan 27.

§Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, England.

The X-ray birefringence imaging (XBI) technique, reported very recently, is a sensitive tool for spatially resolved mapping of the local orientational properties of anisotropic materials. In this paper, we report the first XBI measurements on materials that undergo anisotropic molecular dynamics. Using incident linearly polarized X-rays with energy close to the Br K-edge, the X-ray birefringence is dictated by the orientational properties of the C-Br bonds in the material. We focus on two materials (urea inclusion compounds containing 1,8-dibromooctane and 1,10-dibromodecane guest molecules) for which the reorientational dynamics of the brominated guest molecules (and hence the reorientational dynamics of the C-Br bonds) are already well characterized by other experimental techniques. The XBI results demonstrate clearly that, for the anisotropic molecular dynamics in these materials, the effective X-ray optic axis for the X-ray birefringence phenomenon is the time-averaged resultant of the orientational distribution of the C-Br bonds.
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http://dx.doi.org/10.1021/jz502652nDOI Listing
February 2015

Multiple-length-scale deformation analysis in a thermoplastic polyurethane.

Nat Commun 2015 Mar 11;6:6583. Epub 2015 Mar 11.

1] Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, UK [2] Diamond Light Source, Harwell Campus, Didcot OX11 0DE, UK.

Thermoplastic polyurethane elastomers enjoy an exceptionally wide range of applications due to their remarkable versatility. These block co-polymers are used here as an example of a structurally inhomogeneous composite containing nano-scale gradients, whose internal strain differs depending on the length scale of consideration. Here we present a combined experimental and modelling approach to the hierarchical characterization of block co-polymer deformation. Synchrotron-based small- and wide-angle X-ray scattering and radiography are used for strain evaluation across the scales. Transmission electron microscopy image-based finite element modelling and fast Fourier transform analysis are used to develop a multi-phase numerical model that achieves agreement with the combined experimental data using a minimal number of adjustable structural parameters. The results highlight the importance of fuzzy interfaces, that is, regions of nanometre-scale structure and property gradients, in determining the mechanical properties of hierarchical composites across the scales.
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http://dx.doi.org/10.1038/ncomms7583DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4382697PMC
March 2015

Structure-delivery relationships of lysine-based gemini surfactants and their lipoplexes.

Soft Matter 2014 Aug 27;10(31):5702-14. Epub 2014 Jun 27.

Radboud University Nijmegen, Institute for Molecules and Materials, Heijendaalseweg 135, 6525 AJ Nijmegen, The Netherlands.

The synthesis and properties of gemini surfactants of the type (R(1)(CO)-Lys(H)-NH)2(CH2)n are reported. For a spacer length of n = 6, the hydrophobic acyl tail was varied in length (R(1) = C8, C10, C12, C14, C16, and C18) and, for R(1) = C18, the degree of unsaturation. For R(1)(CO) = oleoyl (C18:1 Z) the spacer length (n = 2-8) and the stereochemistry of the lysine building block were varied; a 'half-gemini' derivative with a single oleoyl tail and head group was also prepared. The potential of the gemini surfactants to transfer polynucleotides across a cell membrane was investigated by transfection of HeLa cells with beta-galactosidase, both in the presence and absence of the helper lipid DOPE. Oleoyl was found to be by far the best hydrophobic tail for this biological activity, whereas the effect of the lysine stereochemistry was less pronounced. The effect of an optimum spacer length (n = 6) was observed only in the absence of helper lipid. The most active surfactant, i.e. the one with oleoyl chains and n = 6, formed liposomes with sizes in the range of 60-350 nm, and its lipoplex underwent a transition from a lamellar to a hexagonal morphology upon lowering the pH from 7 to 3.
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http://dx.doi.org/10.1039/c4sm00881bDOI Listing
August 2014

X-ray birefringence imaging.

Science 2014 May;344(6187):1013-6

Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, England.

The polarizing optical microscope has been used since the 19th century to study the structural anisotropy of materials, based on the phenomenon of optical birefringence. In contrast, the phenomenon of x-ray birefringence has been demonstrated only recently and has been shown to be a sensitive probe of the orientational properties of individual molecules and/or bonds in anisotropic solids. Here, we report a technique-x-ray birefringence imaging (XBI)-that enables spatially resolved mapping of x-ray birefringence of materials, representing the x-ray analog of the polarizing optical microscope. Our results demonstrate the utility and potential of XBI as a sensitive technique for imaging the local orientational properties of anisotropic materials, including characterization of changes in molecular orientational ordering associated with solid-state phase transitions and identification of the size, spatial distribution, and temperature dependence of domain structures.
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http://dx.doi.org/10.1126/science.1253537DOI Listing
May 2014

Hierarchical modelling of in situ elastic deformation of human enamel based on photoelastic and diffraction analysis of stresses and strains.

Acta Biomater 2014 Jan 9;10(1):343-54. Epub 2013 Oct 9.

Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, UK. Electronic address:

Human enamel is a typical hierarchical mineralized tissue with a two-level composite structure. To date, few studies have focused on how the mechanical behaviour of this tissue is affected by both the rod orientation at the microscale and the preferred orientation of mineral crystallites at the nanoscale. In this study, wide-angle X-ray scattering was used to determine the internal lattice strain response of human enamel samples (with differing rod directions) as a function of in situ uniaxial compressive loading. Quantitative stress distribution evaluation in the birefringent mounting epoxy was performed in parallel using photoelastic techniques. The resulting experimental data was analysed using an advanced multiscale Eshelby inclusion model that takes into account the two-level hierarchical structure of human enamel, and reflects the differing rod directions and orientation distributions of hydroxyapatite crystals. The achieved satisfactory agreement between the model and the experimental data, in terms of the values of multidirectional strain components under the action of differently orientated loads, suggests that the multiscale approach captures reasonably successfully the structure-property relationship between the hierarchical architecture of human enamel and its response to the applied forces. This novel and systematic approach can be used to improve the interpretation of the mechanical properties of enamel, as well as of the textured hierarchical biomaterials in general.
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http://dx.doi.org/10.1016/j.actbio.2013.09.043DOI Listing
January 2014

Hierarchical modelling of elastic behaviour of human enamel based on synchrotron diffraction characterisation.

J Struct Biol 2013 Nov 7;184(2):136-46. Epub 2013 Oct 7.

Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, United Kingdom. Electronic address:

Human enamel is a hierarchical mineralized tissue with a two-level composite structure. Few studies have focused on the structure-mechanical property relationship and its link to the multi-scale architecture of human enamel, whereby the response to mechanical loading is affected not only by the rod distribution at micro-scale, but also strongly influenced by the mineral crystallite shape, and spatial arrangement and orientation. In this study, two complementary synchrotron X-ray diffraction techniques, wide and small angle X-ray scattering (WAXS/SAXS) were used to obtain multi-scale quantitative information about the structure and deformation response of human enamel to in situ uniaxial compressive loading. The apparent modulus was determined linking the external load and the internal strain in hydroxyapatite (HAp) crystallites. An improved multi-scale Eshelby model is proposed taking into account the two-level hierarchical structure of enamel. This framework has been used to analyse the experimental data for the elastic lattice strain evolution within the HAp crystals. The achieved agreement between the model prediction and experiment along the loading direction validates the model and suggests that the new multi-scale approach reasonably captures the structure-property relationship for the human enamel. The ability of the model to predict multi-directional strain components is also evaluated by comparison with the measurements. The results are useful for understanding the intricate relationship between the hierarchical structure and the mechanical properties of enamel, and for making predictions of the effect of structural alterations that may occur due to the disease or treatment on the performance of dental tissues and their artificial replacements.
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http://dx.doi.org/10.1016/j.jsb.2013.09.023DOI Listing
November 2013

Multiscale modelling and diffraction-based characterization of elastic behaviour of human dentine.

Acta Biomater 2013 Aug 18;9(8):7937-47. Epub 2013 Apr 18.

Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, UK.

Human dentine is a hierarchical mineralized tissue with a two-level composite structure, with tubules being the prominent structural feature at a microlevel, and collagen fibres decorated with hydroxyapatite (HAp) crystallite platelets dominating the nanoscale. Few studies have focused on this two-level structure of human dentine, where the response to mechanical loading is thought to be affected not only by the tubule volume fraction at the microscale, but also by the shape and orientation distribution of mineral crystallites, and their nanoscale spatial arrangement and alignment. In this paper, in situ elastic strain evolution within HAp in dentine subjected to uniaxial compressive loading along both longitudinal and transverse directions was characterized simultaneously by two synchrotron X-ray scattering techniques: small- and wide-angle X-ray scattering (SAXS and WAXS, respectively). WAXS allows the evaluation of the apparent modulus linking the external load to the internal HAp crystallite strain, while the nanoscale HAp distribution and arrangement can be quantified by SAXS. We proposed an improved multiscale Eshelby inclusion model that takes into account the two-level hierarchical structure, and validated it with a multidirectional experimental strain evaluation. The agreement between the simulation and measurement indicates that the multiscale hierarchical model developed here accurately reflects the structural arrangement and mechanical response of human dentine. This study benefits the comprehensive understanding of the mechanical behaviour of hierarchical biomaterials. The knowledge of the mechanical properties related to the hierarchical structure is essential for the understanding and predicting the effects of structural alterations that may occur due to disease or treatment on the performance of dental tissues and their artificial replacements.
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http://dx.doi.org/10.1016/j.actbio.2013.04.020DOI Listing
August 2013

High-resolution high-efficiency X-ray imaging system based on the in-line Bragg magnifier and the Medipix detector.

J Synchrotron Radiat 2013 Jan 10;20(Pt 1):153-9. Epub 2012 Nov 10.

ANKA Light Source, Karlsruhe Institute of Technology, Karlsruhe, Germany.

The performance of a recently developed full-field X-ray micro-imaging system based on an in-line Bragg magnifier is reported. The system is composed of quasi-channel-cut crystals in combination with a Medipix single-photon-counting detector. A theoretical and experimental study of the imaging performance of the crystals-detector combination and a comparison with a standard indirect detector typically used in high-resolution X-ray imaging schemes are reported. The spatial resolution attained by our system is about 0.75 µm, limited only by the current magnification. Compared with an indirect detector system, this system features a better efficiency, signal-to-noise ratio and spatial resolution. The optimal working resolution range of this system is between ∼0.4 µm and 1 µm, filling the gap between transmission X-ray microscopes and indirect detectors. Applications for coherent full-field imaging of weakly absorbing samples are shown and discussed.
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http://dx.doi.org/10.1107/S0909049512044366DOI Listing
January 2013

X-ray Birefringence: A New Strategy for Determining Molecular Orientation in Materials.

J Phys Chem Lett 2012 Nov 19;3(21):3216-22. Epub 2012 Oct 19.

‡Harwell Science and Innovation Campus, Diamond Light Source, Didcot, Oxfordshire OX11 0DE, England.

While the phenomenon of birefringence is well-established in the case of visible radiation and is exploited in many fields (e.g., through the use of the polarizing optical microscope), the analogous phenomenon for X-rays has been a virtually neglected topic. Here, we demonstrate the scope and potential for exploiting X-ray birefringence to determine the orientational properties of specific types of bonds in solids. Specifically, orientational characteristics of C-Br bonds in the bromocyclohexane/thiourea inclusion compound are elucidated from X-ray birefringence measurements at energies close to the bromine K-edge, revealing inter alia the changes in the orientational distribution of the C-Br bonds associated with a low-temperature order-disorder phase transition. From fitting a theoretical model to the experimental data, reliable quantitative information on the orientational properties of the C-Br bonds is determined. The experimental strategy reported here represents the basis of a new approach for gaining insights into the orientational properties of molecules in anisotropic materials.
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http://dx.doi.org/10.1021/jz3013547DOI Listing
November 2012

Self-assembly of the perfluoroalkyl-alkane F14H20 in ultrathin films.

Langmuir 2005 Mar;21(6):2308-16

Organische Chemie III, Makromolekulare Chemie, Universität Ulm, Albert-Einstein-Allee 11, 89069 Ulm, Germany.

Scanning force microscopy on monomolecular films of eicosylperfluorotetradecane, F(CF(2))(14)(CH(2))(20)H, on mica, silicon oxide, or water revealed spontaneous organization to well-defined nanoscopic ribbon and spiral or toroidal superstructures. Whether ribbons or nanospirals were formed depended on the solvent from which the molecular monofilm was cast. Ribbons were observed when a hydrocarbon or a perfluorocarbon solvent was used, e.g., decalin or perfluorodecalin. When the compound, however, was deposited from nonselective hexafluoroxylene, the molecules assembled into spirals of defined size. The spirals/toroids transformed to ribbons when exposed either to decalin or perfluorodecalin vapor, and the ribbons transformed to toroids when exposed to hexafluoroxylene vapor. These changes could be observed in situ. Scanning force microscopy yielded an identical height and width for the bands forming the spirals and for the parallel flat ribbons. X-ray reflectivity yielded a height of 3.61 +/- 0.05 nm, again identical for both morphologies. Yet, the length of the extended F(CF(2))(14)(CH(2))(20)H molecule, i.e., 4.65 nm, exceeds the layer thickness obtained from X-ray reflectometry. It is, however, consistent with an arrangement where the fluorinated chains are oriented normal to the surface layer and where the alkyl segments are tilted with a 122 degrees angle between the two segments. Within the plane defined by the tilt, this angle allows a dense packing of the alkyl segments compensating for the larger cross-section of the fluorocarbon segment. The tilt plane defines an "easy" direction along which the monolayer structure can preserve order. In the plane perpendicular to this axis, long-range ordered dense packing of the alkyl chains is not possible. Incommensurable packing can in principle explain the finite and regular width of the ribbons and the stepwise turn in the spirals.
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http://dx.doi.org/10.1021/la048069yDOI Listing
March 2005

Modelling the stratum corneum lipid organisation with synthetic lipid mixtures: the importance of synthetic ceramide composition.

Biochim Biophys Acta 2004 Aug;1664(2):132-40

Leiden/Amsterdam Center for Drug Research, Gorlaeus Laboratories, Department of Drug Delivery Technology, University of Leiden, PO Box 9502, 2300 RA, The Netherlands.

Cholesterol (CHOL), free fatty acids (FFA) and nine classes of ceramides (CER1-CER9) form the main constituents of the intercellular lipid lamellae in stratum corneum (SC), which regulate the skin barrier function. Both the presence of a unique 13-nm lamellar phase, of which the formation depends on the presence of CER1, and its dense lateral packing are characteristic for the SC lipid organisation. The present study focuses on the lipid organisation in mixtures prepared with CHOL, FFA and a limited number of synthetic CER, namely CER1, CER3 and bovine brain CER type IV (SigmaCERIV). The main objective is to determine the optimal molar ratio of CER3 to SigmaCERIV for the formation of the 13-nm lamellar phase. CER3 contains a uniform acyl chain length, whereas SigmaCERIV contains fatty acids with varying chain lengths. Using small angle X-ray diffraction (SAXD), it is demonstrated that the CER3 to SigmaCERIV ratio affects the formation of the 13-nm lamellar phase and that the optimal ratio depends on the presence of FFA. Furthermore, the formation of the 13-nm lamellar phase is not very sensitive to variations in the total CER level, which is similar to the in vivo situation.
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http://dx.doi.org/10.1016/j.bbamem.2004.05.001DOI Listing
August 2004

Novel lipid mixtures based on synthetic ceramides reproduce the unique stratum corneum lipid organization.

J Lipid Res 2004 May 16;45(5):923-32. Epub 2004 Feb 16.

Leiden/Amsterdam Center for Drug Research, Department of Drug Delivery Technology, University of Leiden, Leiden, The Netherlands.

Lipid lamellae present in the outermost layer of the skin protect the body from uncontrolled water loss. In human stratum corneum (SC), two crystalline lamellar phases are present, which contain mostly cholesterol, free fatty acids, and nine types of free ceramides. Previous studies have demonstrated that the SC lipid organization can be mimicked with model mixtures based on isolated SC lipids. However, those studies are hampered by low availability and high interindividual variability of the native tissue. To elucidate the role of each lipid class in the formation of a competent skin barrier, the use of synthetic lipids would offer an alternative. The small- and wide-angle X-ray diffraction results of the present study show for the first time that synthetic lipid mixtures, containing only three synthetic ceramides, reflect to a high extent the SC lipid organization. Both an appropriately chosen preparation method and lipid composition promote the formation of two characteristic lamellar phases with repeat distances similar to those found in native SC. From all synthetic lipid mixtures examined, equimolar mixtures of cholesterol, ceramides, and free fatty acids equilibrated at 80 degrees C resemble to the highest extent the lamellar and lateral SC lipid organization, both at room and increased temperatures.
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http://dx.doi.org/10.1194/jlr.M300484-JLR200DOI Listing
May 2004

Surface and bulk elasticity determined fluctuation regimes in smectic membranes.

Phys Rev Lett 2003 Oct 17;91(16):165504. Epub 2003 Oct 17.

FOM-Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands.

We report combined x-ray photon correlation spectroscopy (XPCS) and neutron spin echo (NSE) measurements of the layer-displacement fluctuations in smectic liquid-crystal membranes in the range from 10 ns to 10 micros. NSE reveals a new regime, determined by bulk elasticity, in which relaxation times decrease with the wave vector of the fluctuations. XPCS probes slower surface-tension-dominated relaxation times, independent of the wave vector. XPCS gives a difference in correlation times at specular and off-specular positions that can be related to different detection schemes.
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http://dx.doi.org/10.1103/PhysRevLett.91.165504DOI Listing
October 2003

Transfection mediated by gemini surfactants: engineered escape from the endosomal compartment.

J Am Chem Soc 2003 Feb;125(6):1551-8

Contribution from the Physical Organic Chemistry Unit, Stratingh Institute, University of Groningen, 4 Nijenborgh, 9747 AG Groningen, The Netherlands.

The structure of the lipoplex formed from DNA and the sugar-based cationic gemini surfactant 1, which exhibits excellent transfection efficiency, has been investigated in the pH range 8.8-3.0 utilizing small-angle X-ray scattering (SAXS) and cryo-electron microscopy (cryo-TEM). Uniquely, three well-defined morphologies of the lipoplex were observed upon gradual acidification: a lamellar phase, a condensed lamellar phase, and an inverted hexagonal (H(II)) columnar phase. Using molecular modeling, we link the observed lipoplex morphologies and physical behavior to specific structural features in the individual surfactant, illuminating key factors in future surfactant design, viz., a spacer of six methylene groups, the presence of two nitrogens that can be protonated in the physiological pH range, two unsaturated alkyl tails, and hydrophilic sugar headgroups. Assuming that the mechanism of transfection by synthetic cationic surfactants involves endocytosis, we contend that the efficacy of gemini surfactant 1 as a gene delivery vehicle can be explained by the unprecedented observation of a pH-induced formation of the inverted hexagonal phase of the lipoplex in the endosomal pH range. This change in morphology leads to destabilization of the endosome through fusion of the lipoplex with the endosomal wall, resulting in release of DNA into the cytoplasm.
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http://dx.doi.org/10.1021/ja020707gDOI Listing
February 2003

Are metastable, precrystallisation, density-fluctuations a universal phenomena?

Faraday Discuss 2003 ;122:343-61; discussion 381-93

Department of Chemistry, University of Sheffield, Sheffield, UK S3 7HF.

In-situ observations of crystallisation in minerals and organic polymers have been made by simultaneous, time-resolved small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) techniques. In isotactic polypropylene slow quiescent crystallisation shows the onset of large scale ordering prior to crystal growth. Rapid crystallisations studied by melt extrusion indicate the development of well resolved oriented SAXS patterns associated with long range order before the development of crystalline peaks in the WAXS region. Block copolymers self-assemble into mesophases in polymer melts above a critical chain length (or above a critical temperature) and this self-assembly process is shown to be susceptible to an incipient crystallisation. Mesophase formation is observed at anomalously high temperatures in ethylene-oxide containing block copolymers below the normal melting point of the polyoxy ethylene chains. Formation of calcium carbonate from aqueous solutions of sodium carbonate and calcium nitrate is observed to be a two-stage process and precipitation proceeds by the production of an amorphous metastable phase. This phase grows until it is volume filling and leads to the formation of the two polymorphs Calcite and Vaterite. These three sets of results suggest pre-nucleation density fluctuations, leading to a metastable phase, play an integral role in all three classes of crystallisation. In due course, this phase undergoes transformation to "normal" crystals.
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http://dx.doi.org/10.1039/b202680eDOI Listing
June 2003

Smectic membranes in motion: approaching the fast limits of x-ray photon correlation spectroscopy.

Phys Rev Lett 2002 Mar 28;88(11):115503. Epub 2002 Feb 28.

FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands.

The dynamics of the layer-displacement fluctuations in smectic membranes have been studied by x-ray photon correlation spectroscopy (XPCS). We report transitions from an oscillatory damping regime to simple exponential decay of the fluctuations, both as a function of membrane thickness and upon changing from specular to off-specular scattering. This behavior is in agreement with recent theories. Employing avalanche photodiode detectors and the uniform filling mode of the synchrotron storage ring, the fast limits of XPCS have been explored down to 50 ns.
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http://dx.doi.org/10.1103/PhysRevLett.88.115503DOI Listing
March 2002