Masayuki Imai

Tohoku University, Japan

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Publications (116)210.22 Total impact

  • Ai Sakashita, Masayuki Imai, Hiroshi Noguchi
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    ABSTRACT: Morphologies of genus-1 and 2 toroidal vesicles are studied numerically by dynamically triangulated membrane models and experimentally by confocal laser microscopy. Our simulation results reproduce shape transformations observed in our experiments well. At large reduced volumes of the genus-1 vesicles, obtained vesicle shapes agree with the previous theoretical results for axisymmetric shapes: double-necked stomatocyte, discoidal toroid, and circular toroid. However, for small reduced volumes, it is found that a non-axisymmetric discoidal toroid and handled discocyte exist in thermal equilibrium in the parameter range, in which the previous theory predicts axisymmetric shapes. The entropy caused by shape fluctuations slightly modifies the stability of the vesicle shapes.
    Soft Matter 05/2014; · 4.15 Impact Factor
  • Ai Sakashita, Masayuki Imai, Hiroshi Noguchi
    [Show abstract] [Hide abstract]
    ABSTRACT: Morphologies of genus-1 and 2 toroidal vesicles are studied numerically by dynamically triangulated membrane models and experimentally by confocal laser microscopy. Our simulation results reproduce shape transformations observed in our experiments well. At large reduced volumes of the genus-1 vesicles, obtained vesicle shapes agree with the previous theoretical results for axisymmetric shapes: double-necked stomatocyte, discoidal toroid, and circular toroid. However, for small reduced volumes, it is found that a non-axisymmetric discoidal toroid and handled discocyte exist in thermal equilibrium in the parameter range, in which the previous theory predicts axisymmetric shapes. The entropy caused by shape fluctuations slightly modifies the stability of the vesicle shapes.
    04/2014;
  • Ai Sakashita, Masayuki Imai, Hiroshi Noguchi
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    ABSTRACT: Morphologies of a double-bilayer vesicle were explored experimentally by fast confocal laser microscopy and numerically by a dynamically triangulated membrane model with area-difference elasticity. The confinement was found to induce several shapes of the inner vesicles that had not been observed in unilamellar vesicles: double and quadruple stomatocytes, slit vesicle, and vesicles of two or three compartments with various shapes. The simulations reproduced the experimental results very well and some of the shape transitions can be understood by a simple theoretical model for axisymmetric shapes.
    Physical Review E 04/2014; 89(4-1):040701. · 2.31 Impact Factor
  • Ai Sakashita, Masayuki Imai, Hiroshi Noguchi
    [Show abstract] [Hide abstract]
    ABSTRACT: Morphologies of a double-bilayer vesicle were explored experimentally by fast confocal laser microscopy and numerically by a dynamically triangulated membrane model with area-difference elasticity. The confinement was found to induce several shapes of the inner vesicles that had not been observed in unilamellar vesicles: double and quadruple stomatocytes, slit vesicle, and vesicles of two or three compartments with various shapes. The simulations reproduced the experimental results very well and some of the shape transitions can be understood by a simple theoretical model for axisymmetric shapes.
    03/2014; 89(4).
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    ABSTRACT: We have succeeded in controlling tubular membrane formations in binary giant unilamellar vesicles (GUVs) using a simple temperature changing between the homogeneous one-phase region and the two-phase coexistence region. The binary GUV is composed of inverse-cone (bulky hydrocarbon chains and a small headgroup) and cylinder-shaped lipids. When the temperature was set in the two-phase coexistence region, the binary GUV had a spherical shape with solidlike domains. By increasing the temperature to the homogeneous one-phase region, the excess area created by the chain melting of the lipid produced tubes inside the GUV. The tubes had a radius on the micrometer scale and were stable in the one-phase region. When we again decreased the temperature to the two-phase coexisting region, the tubes regressed and the GUVs recovered their phase-separated spherical shape. We infer that the tubular formation was based on the mechanical balance of the vesicle membrane (spontaneous tension) coupled with the asymmetric distribution of the inverse-cone-shaped lipids between the inner and outer leaflets of the vesicle (lipid sorting).
    Biophysical Journal 11/2013; 105(9):2074-81. · 3.67 Impact Factor
  • Source
    Ai Sakashita, Masayuki Imai, Hiroshi Noguchi
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    ABSTRACT: Morphologies of a vesicle confined in a spherical vesicle were explored experimentally by fast confocal laser microscopy and numerically by a dynamically-triangulated membrane model with area-difference elasticity. The confinement was found to induce several novel shapes of the inner vesicles, that had been never observed in unilamellar vesicles: double and quadruple stomatocytes, slit vesicle, and vesicles of two or three compartments with various shapes. The simulations reproduced the experimental results very well and some of the shape transitions can be understood by a simple theoretical model for axisymmetric shapes.
    09/2013;
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    ABSTRACT: We have investigated diffusion of spherical surfactant micelles in fluid and crystal phases. The short-time and the long-time collective diffusions of spherical micelles were addressed by a neutron spin echo (NSE) and a dynamical light scattering (DLS) technique. In the fluid phase, the first-order virial coefficients of the normalized short-time collective diffusion coefficients agreed with those of the hard spheres, whereas the long-time collective diffusion coefficient exhibited the characteristic slowing down toward the fluid-HCP transition. At the fluid-HCP transition, the short-time diffusion coefficient increased abruptly, indicating increase of the effective free volume available for the micelles. Simultaneously, we observed a very slow mode characteristic of the crystal phases, suggesting hopping of micelles among the lattice sites.
    02/2013;
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    ABSTRACT: We have investigated diffusion of spherical surfactant micelles having a hard-core and weak repulsive potential in fluid, hexagonally close-packed (HCP), and body-centered cubic (BCC) phases by means of neutron spin echo (NSE) and dynamical light scattering (DLS) techniques. The intermediate scattering functions give short-time and long-time diffusion coefficients. In the fluid phase, the first-order virial coefficients of the normalized short-time self- and collective diffusion coefficients agreed with those of the hard spheres. The long-time collective diffusion coefficient exhibited the characteristic slowing down toward the fluid–HCP transition obeying a power law. At the fluid–HCP transition, the short-time diffusion coefficient increased abruptly, indicating increase of the effective free volume available for the micelles. Simultaneously, the long-time diffusion mode disappeared and a very slow mode characteristic of the crystal phase appeared, which might be related to the viscoelastic nature of the micelle crystals.
    Soft Matter 09/2012; 8(38):9892-9905. · 4.15 Impact Factor
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    ABSTRACT: We use fast confocal laser microscopy to quantitatively study axisymmetric and nonaxisymmetric shapes of lipid vesicles undergoing spontaneous transformations. To characterize the observed three-dimensional shapes, we compute their respective reduced monolayer area difference and reduced volume. The transformations allow us to analyze a broad range of vesicle shapes including stomatocytes, elliptocytes, discocytes, cigars, necklaces, and many nonaxisymmetric shapes. The transformations are marked by a step-like time dependence of the reduced monolayer area difference, which is explained in terms of the area-difference-elasticity model. The simplest mechanism consistent with the observed shape transformations are small folded multilamellar patches on either monolayer. The experimental methodology validated by the reported results can be used to monitor and analyze more complex vesicle shapes and their evolution.
    Soft Matter 08/2012; 8(33):8569-8581. · 4.15 Impact Factor
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    ABSTRACT: We study the deformation of a lipid membrane in response to a local pH modification. Experimentally, a basic solution is microinjected close to a giant unilamellar vesicle. A local deformation appears in the zone of the membrane that is closest to the micropipette, and relaxes when the injection is stopped. A theoretical description of this phenomenon is provided. It takes fully into account the spatiotemporal evolution of the concentration of hydroxide ions during and after the microinjection, as well as the linear dynamics of the membrane. This description applies to a local injection of any substance that reacts reversibly with the membrane lipids. We compare experimental data obtained in the domain of small deformations to the results of our linear description, and we obtain a good agreement between theory and experiments. In addition, we present direct experimental observations of the pH profile on the membrane during and after the microinjection, using pH-sensitive fluorescent lipids.
    Biophysical Journal 05/2012; 102(3). · 3.67 Impact Factor
  • Yuka Sakuma, Masayuki Imai
    Biophysical Journal 01/2012; 102(3):33-. · 3.67 Impact Factor
  • Yuka Sakuma, Masayuki Imai
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    ABSTRACT: Development of self-reproducing vesicle systems is the first step for autopoietic cycles. We established a model self-reproducing vesicle system without the membrane molecule synthesis route. The model vesicle composed of cylinder- and inverse-cone-shaped lipids formed inclusion vesicles inside the mother vesicle, and the inclusion vesicles were then expelled by a temperature cycling. By changing the vesicle composition, the mother vesicles showed a budding-type self-reproduction pathway. A key concept of this system is the coupling of the main-chain transition and the shape of lipids.
    Physical Review Letters 11/2011; 107(19):198101. · 7.73 Impact Factor
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    ABSTRACT: We carried out molecular dynamics simulations to investigate the morphological change of water-in-oil microemulsion (ME) droplets by addition of polymer chains. Several experiments have shown an increase in the polydispersity of ME droplet size and the morphological transition of spherical ME droplets to a rod-like shape by encapsulating the polymer chain in ME droplets. In order to understand these changes in the ME droplets, we studied the effects of the chain concentration and the interaction strength between chains and surfactant. When the interaction strength was weak, we obtained spherical droplets rather than rod-like droplets. Two types of spherical ME droplets were observed: large spherical droplets containing polymer chains and small spherical droplets without chains, which resulted in a large size polydispersity. However, when the interaction strength was great, a coalescence of ME droplets containing polymer chains was observed and the morphology of the droplets finally changed to a rod-like shape with an increase in the chain concentration. The morphological change can be explained by the competition between the chain–surfactant interaction energy and the surfactant monolayer bending energy, thus the interaction energy prefers a rod-like shape to gain a large contact area between the chains and the surfactant monolayer.
    Soft Matter 08/2011; 7(16):7504-7510. · 4.15 Impact Factor
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    ABSTRACT: We have investigated the lipid sorting in a binary small unilamellar vesicle (SUV) composed of cone-shaped (1,2-dihexanoyl-sn-glycero-3-phosphocholine: DHPC) and cylinder-shaped (1,2-dipalmitoyl-sn-glycero-3-phosphocholine: DPPC) lipids. In order to reveal the lipid sorting we adopted a contrast matching technique of small angle neutron scattering (SANS), which extracts the distribution of deuterated lipids in the bilayer quantitatively without steric modification of lipids as in fluorescence probe techniques. First the SANS profile of protonated SUVs at a film contrast condition showed that SUVs have a spherical shape with an inner radius of 190 Å and a bilayer thickness of 40 Å. The SANS profile of deuterated SUVs at a contrast matching condition showed a characteristic scattering profile, indicating an asymmetric distribution of cone-shaped lipids in the bilayer. The characteristic profile was described well by a spherical bilayer model. The fitting revealed that most DHPC molecules are localized in the outer leaflet. Thus the shape of the lipid is strongly coupled with the membrane curvature. We compared the obtained asymmetric distribution of the cone-shaped lipids in the bilayer with the theoretical prediction based on the curvature energy model.
    Journal of Physics Condensed Matter 07/2011; 23(28):284104. · 2.22 Impact Factor
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    ABSTRACT: We examined the dynamics of the deformation and phase separation of two-component vesicles. First, we numerically investigated the effects of (i) thermal noise, (ii) hydrodynamic flow induced by the line tension of the domain boundary and (iii) composition-dependent bending rigidity on the coarsening dynamics of a phase-separated pattern on the surfaces of vesicles with fixed shapes. The dynamical exponent z (N(DB) ∼ t(-z), the total length of the domain boundaries) of the coarsening of the phase-separated pattern was found to decrease from z = 1/3 under no thermal noise to 1/5 < z < 1/4 when including the effects of thermal noise. We also found that the hydrodynamic effect enhances the coarsening in a bicontinuous phase separation for a spherical vesicle. In phase separations of a shape-fixed tubular vesicle, a band-like phase separation with periodicity along the longer axis of the tube occurs because of the composition-dependent bending rigidity and the higher curvatures at the tube end-caps. Second, we also explored the dynamics of shape deformation coupled with phase separation through the bending rigidity of the membrane which depends on the local composition in lipids and found that the composition-dependent bending rigidity crucially influences the phase separation and deformation of the vesicle. The results of simulations are in good agreement with experimentally observed behavior known as 'shape convergence' (Yanagisawa et al 2008 Phys. Rev. Lett. 100 148102).
    Journal of Physics Condensed Matter 07/2011; 23(28):284103. · 2.22 Impact Factor
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    ABSTRACT: We have investigated the diffusion coefficient of nano-meter-sized domains on a vesicle composed of saturated phospholipids, unsaturated phospholipids and cholesterols by means of neutron spin echo spectroscopy. The obtained diffusion coefficient was examined from a viewpoint of the hydrodynamic model of Brownian objects in a fluid membrane.
    Journal of Physics Conference Series 12/2010; 251(1):012036.
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    ABSTRACT: We investigate the hydrodynamic effects on the dynamics of critical concentration fluctuations in multicomponent fluid membranes. Two geometrical cases are considered; (i) confined membrane case and (ii) supported membrane case. We numerically calculate the wavenumber dependence of the effective diffusion coefficient by changing the temperature and/or the thickness of the bulk fluid. For some limiting cases, the result is compared with the previously obtained analytical expression. An analogy of the multicomponent membrane to 2D microemulsion is explored for the confined membrane geometry. Comment: 10 pages, 7 figures. Accepted for publication in Soft Matter
    Soft Matter 11/2010; · 4.15 Impact Factor
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    ABSTRACT: We investigated changes in the shape of tubular ternary vesicles induced by phase separation into liquid-ordered and liquid-disordered phases. Tubular vesicles transform into two types of periodically modulated vesicles depending on the area fraction of the liquid-ordered phase. One type is a necklace of oblate subunits with two circular domains of the liquid-order phase, and the other is a periodically modulated tube with stripes of the liquid-order phase. The transition between the circular and striped domains is governed by the domain boundary energy, whereas the periodicity of modulated vesicles is determined geometrically based on the fixed volume and area constraints. The observed multidomain vesicles are kinetically trapped in metastable states, and all domains show budding to reduce the boundary energy.
    Physical Review E 11/2010; 82(5 Pt 1):051928. · 2.31 Impact Factor
  • Soft Matter 09/2010; 6(21):5359-5366. · 4.15 Impact Factor
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    ABSTRACT: We have investigated shape deformations of giant vesicles induced by the confinement of charged colloids. By confining charged colloids densely, the non-spherical vesicles with tube, discocyte, and stomatocyte shapes transformed to multi-bead vesicles composed of n spheres. The size and number of spheres in the multi-bead vesicle are determined by a conservation law of the total volume and area. The morphology transition of the vesicle induced by the charged colloid confinement is caused by the optimization of the free volume of the confined charged colloids in a vesicle.
    Soft Matter 09/2010; 6(21):5359-5366. · 4.15 Impact Factor

Publication Stats

607 Citations
210.22 Total Impact Points

Institutions

  • 2013–2014
    • Tohoku University
      • Department of Physics
      Japan
  • 1999–2012
    • Ochanomizu University
      • • Department of Physics
      • • Faculty of Science
      Tōkyō, Japan
  • 1992–2011
    • Kyoto University
      • • Department of Chemical Engineering
      • • Department of Physics II
      • • Institute for Chemical Research
      Kyoto, Kyoto-fu, Japan
  • 2007
    • National Institute of Advanced Industrial Science and Technology
      Tsukuba, Ibaraki, Japan
  • 1994–2007
    • The University of Tokyo
      • Institute for Solid State Physics
      Edo, Tōkyō, Japan
  • 2001
    • Tokyo Metropolitan University
      • Department of Chemistry
      Edo, Tōkyō, Japan
    • University of Tsukuba
      • Institute of Applied Biochemistry
      Tsukuba, Ibaraki, Japan
  • 1996
    • Hiroshima University
      • Faculty of Integrated Arts and Sciences
      Hirosima, Hiroshima, Japan
  • 1982
    • Nagoya University
      • Division of General Medicine
      Nagoya-shi, Aichi-ken, Japan