Shrinking Phase Research Articles

Volume change of microcapsules induced by solvent exchange

We have measured volume change of microcapsules composed of dioctylphthalate inner core and polyethyleneglycol-grafted poly(ureaurethane) (PUU) wall membrane induced by solvent exchange in dispersing medium from water to ethanol. The time course of the process consisted of a lag phase, a swelling phase and a shrinking phase. The temperature dependence of the duration of the lag phase t1 was roughly described by the Arrhenius plot, and the slope for the logarithm of the time constant vs. the reciprocal of the absolute temperature was coincident with that of the viscosity coefficient of ethanol, suggesting that the permeation rate of ethanol into the core mainly determines the duration of the lag phase.

Volume phase transitions of poly(acryloyl- l-proline methyl ester) gels in response to water–alcohol composition

The volume phase transitions of poly(acryloyl- l-proline methyl ester (A-ProOMe)) gels in mixtures of water and various alcohols (methyl, ethyl, propyl, and t-buthyl alcohols) were investigated. The poly(A-ProOMe) gels showed two swelling phases (first swell, 0–10 vol%; second swell, 50–80 vol%) and two shrinking phases (first shrink, 10–50 vol%; second shrink, 80–100 vol%) in the presence of each aqueous alcohol under alcohol concentrations ranging from 0 to 100 vol%. The swelling profiles at second swell and second shrink varied in terms of the solvent alcohols; this difference was quantitatively elucidated by taking into account the hydrophobicity of the solvents using dynamic hydration numbers as a parameter of the alcohol's hydrophobicity. The gels showed a shrinking phase in the region of high alcohol concentration (second shrink), which had not been observed in other hydrogels such as NIPAAm. Infrared spectra of the poly(A-ProOMe) revealed that the hydrogen bonding of the amide moiety of poly(A-ProOMe) plays a key role in the gel's sensitivity to the solvent composition, resulting in the sudden shrinking.

Shrinking patterns and phase coexistence of polymer gels

We have investigated the shrinking phase transition of slightly ionized poly( N-isopropylacrylamide) (NIPA) gels. The macroscopic conformation change was observed on the heating process in two different methods; a continuous heating process and an isothermal process. It was found that the macroscopic behavior can be characterized by several conformation changes, the phase coexistence (or ‘linked-dumplings’), the grain pattern, the bubble pattern, and the opaque phase. Those have correlations with the phase transition velocity. The heating conditions to determine the characteristic conformations and the stability of the phase coexistence were qualitatively discussed in terms of the classical phase separation model of nucleation and spinodal decomposition.

Phase separation of weakly ionized polymer gels during shrinking phase transition

We have investigated the shrinking phase transition of weakly ionized poly(N-isopropylacrylamide) gels prepared in a cylindrical shape with submillimeter diameter. The macroscopic conformation changes were obtained on heating processes in two different methods. One is a continuous heating process with a constant temperature drift rate, and the other is an isothermal process after a steplike temperature increase beyond the transition point. It was found that the macroscopic behavior can be characterized by several conformation changes; the phase coexistence, the grain pattern, the bubble pattern, and the opaque phase. On a continuous heating process, the phase transition can be characterized by the phase coexistence of completely collapsed and swollen states for the smaller temperature drift rates; the selected portions on the surface can start to collapse at the transition point, which develops with time and finally becomes a completely collapsed phase. For the larger temperature drift rates, the phase transition starts many places on the surface, and the whole gel with surface bubble pattern gradually shrinks with time. These different processes can be clearly observed in the latter measurements, which depend on the degree of the super-heating (quench); for the shallow quenches, the number of the completely collapsed states correspondingly increased with increasing the super-heating. For the deep quenches, the gel becomes opaque, and the transparent surface skin (collapsed phase) develops with time. The stability of the phase coexistence and the relationship with the transition velocity were qualitatively discussed in terms of the classical phase-separation model based on the nucleation and the spinodal decomposition.

Shrinking pattern and phase transition velocity of poly( N -isopropylacrylamide) gel

We have studied the shrinking phase transition of cylindrical poly(N-isopropylacrylamide) gels with submillimeter diameter. The macroscopic conformation change and the phase transition velocity were obtained during the heating process by two different methods. One is a continuous heating process with a constant temperature drift rate, and the other is an isothermal process after a steplike temperature increase beyond the transition point. In the former measurement, the phase transition can be controlled by the nucleation mechanism in the smaller temperature drift rates; at the transition point, after the fine pattern appears and disappears on the surface, for instance, the gel gradually and uniformly shrinks while keeping a smooth surface. On the other hand, at the larger temperature drift rates, the phase transition comes into the unstable region before being completed; after the fine pattern disappears, a coarse pattern appears on the surface, and the entire gel becomes opaque. The gel gradually becomes transparent with time from the surface layer to the core portion. These two processes, characterized by two types of surface pattern as well as the growth of a collapsed surface skin layer, can be clearly observed in the latter measurements, which depend on the degree of super-heating (how far the final temperature is from the transition point). The results are discussed qualitatively on the basis of the classical phase separation model of nucleation and spinodal decomposition, as well as the phase diagram of the present gel system.

The Caspian Lake: History, biota, structure, and function

The elongate, endorheic Caspian Lake has a north‐south orientation and its main freshwater inflow, the Volga River, enters at the shallow north end. Two deep basins occupy its central and southern regions. These facts lead to horizontal differences in temperature, salinity, and ecology. Nutrient levels and primary production are low. Historically, lake level has fluctuated by ~6 m, but on the geological timescale, fluctuations of >200 m have occurred. The lake formed in the late Miocene, first went through a long shrinking phase, expanded to three times its present size in the late Pliocene, and repeatedly rose and fell throughout the Pleistocene, with corresponding freshenings and salinizations. The lake surface has remained below sea level since the last pleniglacial. This dynamic history led to the assemblage of a set of euryhaline biota of Tethyan and freshwater origin, besides Baltic elements, that invaded with glacial meltwater. Endemism is as high as in Lake Baikal, but typical marine groups are absent. Because the lake is oxygenated to the bottom, its biota show a vertical zonation, but without a true abyssal community, suggesting catastrophic episodes of deep anoxia.

Matter production in the early universe

The coupled Dirac-Einstein equations with a negative cosmological constant for an open FRW universe are studied in detail. The corresponding solutions admit bounces (⇝ minimal radius) of the universe such that the matter energy in any comoving 3-volume is either increased or decreased during the bounce according to whether the bounce pressure of the spinor field is appropriately negative or not. If matter is generated (annihilated) during a bounce, the universe subsequently becomes larger (smaller) than before the bounce. Therefore matter can be generated only during the growth of the universe, but it is annihilated again during the subsequent shrinking phase, which together with the growing phase forms a cosmic supercycle.

Structural changes accompanying GTP hydrolysis in microtubules: information from a slowly hydrolyzable analogue guanylyl-(alpha,beta)-methylene-diphosphonate.

We have used cryoelectron microscopy to try to understand the structural basis for the role of GTP hydrolysis in destabilizing the microtubule lattice. We have measured a structural difference introduced into microtubules by replacing GTP with guanylyl-(alpha,beta)-methylene-diphosphonate (GMPCPP). In a stable GMPCPP microtubule lattice, the moiré patterns change and the tubulin subunits increase in size by 1.5 A. This information provides a clue to the role of hydrolysis in inducing the structural change at the end of a microtubule during the transition from a growing to a shrinking phase.

Evaluation of the Mechanical Properties of a Hydrogel Fiber in the Development of a Polymeric Actuator

The ability of polymer gels to undergo reversible conformational variations when some particular environmental parameter is changed, such as temperature, pH, salt concentration, etc., can be used in the development of microactuators which are characterized by high power-to- weight ratio and built-in compliance. Such actuators could fit the requirements of some advanced bioengineering and robotics applications. Hydrogel fibers, derived from poly(acrylonitrile), show remarkable shrinking and swelling capabilities when the surrounding solution becomes acid or basic, respectively, as well as good resistance to mechanical stresses (Umemoto et al., 1991). An experimental protocol for evaluation of the electro-mechano-chemical properties of this material has been formulated, with the aim of extensively evaluating its performance for possible use as a microactuator. Tests have been grouped into two classes: mechano-chemical tests, in order to evaluate the concentration of the charged sites on the polymeric chain which are responsible for the shrinking and swelling of the gel fiber and to estimate the response of the fiber in terms of produced force density, speed of contraction, resistance to fatigue, maximum borne stress, etc., (when work ing in solutions of different pH values); and electro-mechano-chemical tests, in order to estimate the response of the fiber when the shrinking and swelling are caused by local changes of the pH induced, applying an electric field through the solution in which the hydrogel is immersed. The results show that the hydrogel fiber can generate a force density of 15 kg/cm2 during the shrinking phase, and 60 kg/cm2 in resistance to mechanical stresses. From the rest length, the maximum force exerted can be reached in about 5 seconds. The power-to-weight ratio is found to be about 5 10 4J/kg. A major shortcoming is chemical hysteresis, which could theoretically be eliminated by modifying the initial treatment used to ionize the polymer. The methods used for the characterization of the fiber and the results are presented and discussed.

Modulation of the dynamic instability of tubulin assembly by the microtubule-associated protein tau.

Microtubule-associated proteins (MAP), such as tau, modulate the extent and rate of microtubule assembly and play an essential role in morphogenetic processes, such as axonal growth. We have examined the mechanism by which tau affects microtubule polymerization by examining the kinetics of microtubule assembly and disassembly through direct observation of microtubules using dark-field microscopy. Tau increases the rate of polymerization, decreases the rate of transit into the shrinking phase (catastrophe), and inhibits the rate of depolymerization. Tau strongly suppresses the catastrophe rate, and its ability to do so is independent of its ability to increase the elongation rate. Thus, tau generates a partially stable but still dynamic state in microtubules. This state is perturbed by phosphorylation by MAP2 kinase, which affects all three activities by lowering the affinity of tau for the microtubule lattice.

Some thoughts on the partitioning of tubulin between monomer and polymer under conditions of dynamic instability

We have considered the partitioning of tubulin between monomer and polymer in the cell under conditions of dynamic instability. Dynamic instability adds to the on and off rate constant of steady-state dynamics' new parameters: (1) the rate at which growing microtubules transit to a shrinking phase; and (2) the rate at which shrinking microtubules transit to the growing phase. Under these conditions the free-monomer concentration in the cell increases with total tubulin if the number of nucleating sites is fixed. If the number of nucleating sites increases at fixed total tubulin, subunits shift from the monomer to the polymer phase. These important properties deviate from the traditional equilibrium and steady-state theories and have important implications for the biosynthetic regulation of tubulin.

An approach to radially symmetrical solutions of the sine-Gordon equation

The solution φ(r, t) of the radially symmetric sine-Gordon equation is considered in three and two spatial dimensions for initial curves, analogous to a 2 π- kink, in the expanding and in the shrinking phase, for R( t) j⩾ R(0). It is shown that the parameterization φ( r, t) = 4 arcian exp[ γ( r− R(0)] + x( r, t), where R( t) describes the exact propagation of the maximum of φ,( r, t), is suitable. Using an appoximate differential equation, recently given for the propagation of the solitary ring wave, a rough analytic approximation for the correction function x( r = R( t), t) is found and tested numerically. A relationship between the fluctuations in x( r = R( t), t) and those in R ̈ (t), t) and R(t) explains why the solitary wave is almost stable. From x( r = R( t), t) and the supposition x(1, t) ≈ x(∞, t) ≈ 0 an assymetry in φ r ( r, t) with respect to r = R( t) is predicted. It also exhibits fluctuations corresponding to those in x( r = R( t), t). The condition for validity of this approximation apparently is also a limit for the stability of the solitary ring wave.

Ionic basis of volume regulation in mammalian cells following osmotic shock.

AbstractPrevious studies with mammalian cultured cells have shown that volume regulation in hypotonic medium requires active Na transport. In the present study, determinations of intracellular Na and K content were made in cultured mouse lymphoblasts during the process of swelling and subsequent shrinking (volume regulation) in hypotonic medium. Na and K content were measured in cells in which the shrinking phase was inhibited by the cardiac glycoside, ouabain. In osmotically‐shocked cells, an initial permeability increase to K, and not Na, was observed, which allowed K to diffuse out rapidly, down its gradient. Na, meanwhile, rapidly flowed inward with water entry during the swelling process, and was later lost with the same kinetics as the cell shrinkage. This loss of Na was prevented in the presence of ouabain. The results imply that volume regulation is achieved by pumping Na gained during swelling out of the cells, while any K taken up by the pump is rapidly lost through a more permeable membrane. The loss of osmotically active Na, presumably with accompanying anions, allows water to passively diffuse down its osmotic gradient, reducing cell volume subsequent to the initial passive swelling, during which K was rapidly lost.

Adaptation of mouse leukemic cells (L5178Y) to anisotonic media: I. Cell volume regulation

Mouse lymphoma cells (L5178Y) exposed to hypertonic media for 1 h behave as osmometers, but in hypotonic media, after initial swelling, they shrink back to normal volume and maintain it for long periods of time. The lower limit of osmolarity at which this “volume adaptation” will occur lies between 140 and 185 mosM. The “volume adaptation” is associated with a loss of cellular K + probably due to a transient increase in K + permeability and to loss of associated anions and osmotically obliged water. Partial dissipation of the large gradient of K + between cells and medium by pre-exposure to ouabain or to K +-free medium results in a diminished capacity to adapt. After the shrinking phase is completed, a new steady state is established with a reduced cellular K + content, normal Na +, normal K +-permeability, and a reduced activity of the Na + − K + transport system. When adapted cells are returned to normal medium, an initial shrinking is followed by a re-swelling to normal size, associated with a gain in K + content, presumably due to the return to normal activity of the Na + − K + transport system.

Volume changes of mammalian cells subjected to hypotonic solutions in vitro: evidence for the requirement of a sodium pump for the shrinking phase.

AbstractA Coulter‐orifice pulse‐height analyzer system was used to measure volume spectra of mammalian cells in suspension at different times after the addition of an equal volume of water. In appropriate hypotonic medium, cultured mammalian cells rapidly increase in volume and then shrink, more slowly, approaching their initial volumes within 20 to 30 minutes at 37.5°C. The shrinking phase was found to be reversibly inhibited by ouabain and inhibited in both K+‐free and Na+‐free solutions; neither choline+ nor Li+ could substitute for extracellular Na+ in supporting the shrinking phenomenon but Rb+ and Cs+ were fairly good substitutes for K+.Under conditions similar to those with which the shrinking phenomenon was observed with cultured cells, it was not found with either human or mouse red blood cells.Two methods were used to determine intracellular Na+ and K+ content in osmotically shocked cells and in unshocked controls. An isotope equilibration method was employed with L5178‐Y mouse lymphoblasts and a chemical determination by flame photometry was used with Ehrlich ascites tumor cells. The K+ content was significantly reduced and the Na+ content was unchanged or somewhat increased in cells which had returned to their original volumes in hypotonic medium. The K+ content was even more reduced but the Na+ content was greatly increased in cells which were osmotically shocked in the presence of ouabain.