Mesh Size Of Network Research Articles

Vimentin Affects Actin Network Percolation and Mechanics

The mechanics of the cytoskeleton are determined largely by actin, microtubules, and intermediate filaments. In vitro, composite actin/microtubule networks and actin/myosin networks have revealed unexpected emergent features, suggesting that cytoskeletal mechanical behavior is greater than the sum of its parts.Here, we construct in vitro composite networks of actin and the intermediate filament vimentin, and study the composite network mechanics using bulk rheology. When using biotin-neutravidin as an actin crosslinker, we find that vimentin can have opposing effects on network mechanics, depending on the actin and crosslinker concentrations. Vimentin strengthens actin networks with abundant biotinylation, and weakens composite networks when actin biotinylation is sparse, suggesting that vimentin can either add to or subtract from the number of effective crosslinks in the composite system. The crossover between these two effects occurs when the actin crosslinker spacing is roughly comparable to the actin network mesh size, pointing to a reduction of actin crosslinking through steric constraints by vimentin as the origin of the mechanical changes.When actin networks are crosslinked using Filamin-A or alpha-actinin, vimentin causes a mechanical weakening of the composite network, suggesting that the effective degree of crosslinking is reduced. This is confirmed by confocal fluorescence imaging, which shows actin networks reaching an earlier state of dynamic arrest in the presence of vimentin.Together, these data suggest that vimentin affects the formation of an interpenetrating actin network through steric interactions between the polymers. These indirect interactions affect both the final percolation state and the mechanical strength of the composite network, indicating that the polymers must be considered together as a composite system in determining the properties of the network.Support: NIH (P01GM096971)

Spatially resolved microrheology of heterogeneous biopolymer hydrogels using covalently bound microspheres

Characterization of the rheological properties of heterogeneous biopolymers is important not only to understand the effect of substrate elasticity on cell behaviors, but also to provide insights into mechanical changes during cellular remodeling of the environment. Conventional particle-tracking microrheology (PTM) techniques are compromised by probe-network slippage and cage-hopping problems, and require a priori knowledge of network mesh size in order to determine a suitable probe size. We demonstrated here the usefulness of covalently bound probes for PTM of biopolymers to overcome the above limitations. We showed that, in a well-defined system like polyacrylamide gels, surface-modified probe particles using a zero-length crosslinker provided more reliable measurements of network mechanics as compared to standard carboxylated probes. We further demonstrated that appropriate surface modification of microspheres for PTM circumvented the requirement of using microspheres larger than the network mesh, an approach typically considered to be ideal. Using the method presented in this study, we found the local network at the leading edge of a typical C6 glioma cell to be stiffer as compared to the side. Our findings established that permanent interaction between the probe and network is crucial to reliably measure the local network mechanics in reconstituted, heterogeneous networks using PTM.

Life Cycle-Dependent Cytoskeletal Modifications in Plasmodium falciparum Infected Erythrocytes

Plasmodium falciparum infection of human erythrocytes is known to result in the modification of the host cell cytoskeleton by parasite-coded proteins. However, such modifications and corresponding implications in malaria pathogenesis have not been fully explored. Here, we probed the gradual modification of infected erythrocyte cytoskeleton with advancing stages of infection using atomic force microscopy (AFM). We reported a novel strategy to derive accurate and quantitative information on the knob structures and their connections with the spectrin network by performing AFM–based imaging analysis of the cytoplasmic surface of infected erythrocytes. Significant changes on the red cell cytoskeleton were observed from the expansion of spectrin network mesh size, extension of spectrin tetramers and the decrease of spectrin abundance with advancing stages of infection. The spectrin network appeared to aggregate around knobs but also appeared sparser at non-knob areas as the parasite matured. This dramatic modification of the erythrocyte skeleton during the advancing stage of malaria infection could contribute to the loss of deformability of the infected erythrocyte.

Effect of nanotube size on the mechanical properties of elastomeric composites

Using a mesoscale coarse-grained model and dissipative particle dynamics, the mechanical properties of a cross-linked elastomer filled with surface-functionalized carbon nanotubes are investigated under uniaxial stretching, depending on nanotube length, nanotube bulk density, and crosslink density. Importantly, the system is deformed at equilibrium, allowing the cross-linked chains to be fully relaxed. Our results suggest that for a composite with chemical couplings between polymer and fillers, there actually exist different regimes of elastomer reinforcement, manifesting themselves in the stress–strain response, which is found to be dramatically dependent on the nanotube length L and the characteristic network mesh size : while the effect of the filler particles is relatively small at L −1 ∼ 1, there is a sharp increase in the mechanical modulus when L −1 ≫ 1.

Investigation of the Diels-Alder reaction as a cross-linking mechanism for degradable poly(ethylene glycol) based hydrogels.

The Diels-Alder (DA) reaction was investigated as a cross-linking mechanism for poly(ethylene glycol) (PEG) based hydrogels. Two complementary macromonomers were synthesized by functionalizing star-shaped PEG with furyl and maleimide groups. Gel formation occurred in water at 37 °C; the gelation time ranged between 171 ± 25 min and 14 ± 1 min depending on the used hydrogel formulation. The complex shear modulus was dependent on the concentration, branching factor and molecular weight of the macromonomers; values between 2821 ± 1479 Pa and 37097 ± 6698 Pa were observed. Hydrogel swelling and degradation were influenced by the same parameters; the degradation time varied between a few days and several weeks. Gel dissolution was found to occur by retro-DA reaction and subsequent hydrolysis of maleimide groups. Calculations of the network mesh size revealed that the prepared hydrogels would be suitable for the controlled release of therapeutic proteins.

Analysis of the mesh size in a supramolecular hydrogel by PFG-NMR spectroscopy

Pulsed field gradient NMR (PFG-NMR) spectroscopy has been used to determine the network mesh size in stable hydrogels formed upon addition of Ca2+ to solutions of naphthalene diphenylalanine (2FF). At pH 12, the solutions at 0.55 wt% 2FF comprise worm-like micelles. Addition of Ca2+ results in cross-linking of these micelles. The self-diffusion of dextran guests of nominal 6, 40, 70, 100, 500, 670, 1400 and 2000 kDa, possessing hydrodynamic diameters (2Rh) similar to the expected pore sizes in these systems, was studied both in the precursor micellar solutions and in the hydrogels. The diffusivity of probes with 2Rh < 40 nm is restricted to a similar extent in both types of network with diffusion coefficients scaling as ∼Mr−0.5, where Mr is the nominal mass of the probe, consistent with relatively unrestricted diffusion. Diffusion coefficients fit well the equation Dn/Do = exp(−Rh/ξ), where Dn and Do are the diffusion coefficients in the presence and absence of network respectively and ξ is the mesh size, giving a mesh size of approximately 40 nm. The heaviest ca. 10% of the nominal 2000 kDa dextran fraction having approximate mass and hydrodynamic diameter 3300 kDa and 84 nm respectively was almost immobilised by the gel, consistent with this estimate of the mesh size. The restriction was much weaker in the micellar solution, which is attributed to the transient nature of this micellar network in the absence of Ca2+. Finally, the mesh size for micellar solutions prepared at 1.1 wt% 2FF is smaller than that of micellar solutions prepared at lower concentrations of 2FF. However, the corresponding gels have a larger mesh size than those prepared at lower concentrations of 2FF. We attribute this to increased fibre aggregation at the higher 2FF concentration. This correlates with lower rheological moduli at higher 2FF concentrations.

Measurement of the effective γ/γ′ lattice mismatch during high temperature creep of Ni-based single crystal superalloy

The high temperature effective (constrained) gamma/gamma' lattice mismatch of a Ni-based single crystal superalloy has been measured in a wide temperature range [930 degrees C-1125 degrees C] using three different techniques: in-situ X-ray diffraction under synchrotron radiation, and post-mortem gamma/gamma' interfacial dislocation network mesh size measurements using Transmission Electron Microscopy (TEM) or high resolution Scanning Electron Microscopy (SEM). It is shown that all three techniques are complementary considering precision and spatial distribution of lattice mismatches in the analyzed volume and that they provide a very similar average value of the gamma/gamma' lattice mismatch. Since isothermal creep experiments were performed under variable applied stress, it is deduced that the post-mortem lattice misfit measurements using both TEM and SEM are representative of the very last loading step of the experiments, even if dislocation networks are out of an equilibrium configuration. Contrary to the unconstrained gamma/gamma' lattice mismatch, it is shown that the effective lattice mismatch is a function of the temperature, applied stress and accumulated plastic strain, i.e. of the thermomechanical histories of the samples.

Synthesis and Characterization of Poly(ethylene glycol) Methacrylate Based Hydrogel Networks for Anti‐Biofouling Applications

AbstractA series of hydrogels based on poly(ethylenglycol) methyl ether methacrylate (PEGMEMA) is synthesized using macromonomers of three different molecular weights, in combination with varied degrees of chemical crosslinking. The effects of PEGMEMA, initiator, and crosslinker concentrations on gel yield and swelling properties are studied. In addition, the chemical structure of the gels is characterized by FTIR and solid‐state NMR spectra. The swelling and rheological behaviors of hydrogels as well as protein partitioning into the gels are discussed in terms of the network mesh size. Low protein sorption and bacteria deposition tendencies indicate that PEGMEMA‐based hydrogels could be highly beneficial for uses as fouling‐resistant materials, for instance, as protective coatings for desalination membranes. magnified image

Temperature Controlled Activity of Lipase B from Candida Antarctica after Immobilization within p-NIPAM Microgel Particles

Abstract The immobilization of lipase B from Candida antarctica (CalB) within micronsized poly-N-Isopropylacrylamide (p-NIPAM) microgel particles with a crosslinker content of 5% is reported. The immobilization of the enzyme was reached by an exchange from polar to organic solvents. After determining the embedded amount of CalB within the polymer network, an enhanced specific activity in n-hexane was obtained. Due to the thermoresponsibility of the polymer particles, the activity reaction was done at 25 ºC and 50 ºC. The results presented show that the reversible collapse of the microgel leads to a decreased activity with increasing temperature. Hence, p-NIPAM microgels display a good opportunity to tailor the activity of CalB. An interesting side effect is that CalB presents a suitable probe to estimate the mesh size of the polymer network, since it penetrates in the unlabeled form but not after labeling with FITC.

Crowding effect on helix-coil transition: Beyond entropic stabilization

We report circular dichroism measurements on the helix-coil transition of poly(L-glutamic acid) in solution with polyethylene glycol (PEG) as a crowding agent. The PEG solutions have been characterized by small angle neutron scattering and are well described by the picture of a network of mesh size ξ, usual for semi-dilute chains in good solvent. We show that the increase of PEG concentration stabilizes the helices and increases the transition temperature. But more unexpectedly, we also notice that the increase of concentration of crowding agent reduces the mean helix extent at the transition, or in other words reduces its cooperativity. This result cannot be taken into account for by an entropic stabilization mechanism. Comparing the mean length of helices at the transition and the mesh size of the PEG network, our results strongly suggest two regimes: helices shorter or longer than the mesh size.

Rheo-optics of cationic surfactant micellar solutions with mixed aromatic counterions

The effects of different ratios of mixed aromatic counterions of sodium salicylate (NaSal) and sodium 4-ethylbenzenesulfonate (NaEBS) at a total counterion concentration of 0.125 M on the rheo-optical behaviors of 0.05 M cetyltrimethylammonium chloride (CTAC) cationic surfactant solutions were studied. The introduction of NaEBS into CTAC–NaSal solution led to a significant deviation from the Cox–Merz rule. Mixing of the two aromatic counterions lowered the CTAC solutions’ zero shear viscosities, increased their plateau shear modulus, and decreased their relaxation time. In addition, the persistence lengths of the threadlike micelles and the mesh size of the micellar network were also significantly reduced. These effects are believed to be due to increased degree of branching and denser meshes of the micellar networks in the mixed counterion systems.

Studies of network organization and dynamics of e-beam crosslinked PVPs: From macro to nano

In this work the influence of poly(N-vinyl pyrrolidone) (PVP) concentration in water on the organization and dynamics of the corresponding macro-/nanogel networks has been systematically investigated. Irradiation has been performed at the same irradiation dose (within the sterilization dose range) and dose rate. In the selected irradiation conditions, the transition between macroscopic gelation and micro-/nanogels formation is observed just below the critical overlap concentration (∼1wt%), whereas the net prevalence of intra-molecular over inter-molecular crosslinking occurs at a lower polymer concentration (below 0.25wt%). Dynamic–mechanical spectroscopy has been applied as a classical methodology to estimate the network mesh size for macrogels in their swollen state, while 13C NMR spin–lattice relaxation spectroscopy has been applied on both the macrogel and nanogel freeze dried residues to withdraw interesting information of the network spatial organization in the passage of scale from macro to nano.

Angiogenesis inhibition by the maleimide-based small molecule GNX-686

We investigated the anti-angiogenic properties of GNX-686, a newly identified maleimide-based small molecule. In vitro studies on HUVEC showed that GNX-686 inhibited cell growth with an ED50 of 20–25μM, while human HeLa tumor cells and non-transformed embryonic mouse fibroblasts were less sensitive for the drug. More importantly, at 4μM, a concentration that was non-toxic to any cell in culture, GNX-686 showed a significant inhibitory effect on tube formation by HUVEC, indicating a profound anti-angiogenic activity. Angiogenesis inhibition was subsequenly tested in the chorioallantoic membrane (CAM) of the chicken embryo. A significant angiostatic activity was observed in the CAM model, and results were compared with the effect of bevacizumab, a well known and clinically used VEGF inhibitor. Under our experimental conditions, GNX-686 was found to be as effective as bevacizumab, significantly changing the morphology of the vascular network, as illustrated and quantified by the relative number of branching points and the relative mean mesh size of the vascular network. In another in vivo model of neovascularization, the mouse retinopathy of prematurity (ROP), the vascular network of GNX-686-treated mice was significantly altered, reducing the density of the retinal microvasculature, as compared to the control retinas. Immunohistochemical processing of the GNX-686 treated (4μM) eyes showed over 50% reduction of the number of cell nuclei associated with neovasculature, as compared to the control-treated eye. Taken together these results demonstrate that GNX-686 is a promising anti-angiogenic compound that could be developed for the treatment of diseases characterized by aberrant angiogenesis such as ocular pathologies and cancer.

Swelling behavior and structural characteristics of polyvinyl alcohol/montmorillonite nanocomposite hydrogels

AbstractA series of nanocomposite hydrogels based on polyvinyl alcohol containing 0–10 wt % of the organically modified montmorillonite clay were prepared by freezing‐thawing cyclic method. The morphology of the nanocomposite hydrogels was observed by the scanning electron microscopy technique. The structural properties were determined by measuring the network mesh size, crosslinking density, and average molecular weight of polymer chains between crosslinks. The swelling behavior and the effect of swelling medium temperature on the swelling kinetics and characteristics of the nanocomposite hydrogels were also investigated. The results showed that two structural characteristics i.e., network mesh size and average molecular weight of polymer chains between crosslinks have inverse dependence on the clay loading level in the nanocomposite hydrogel, while crosslinking density shows completely direct dependence. Swelling measurements demonstrated a linear relation between the degree of swelling and the square root of immersion time at all swelling medium temperatures. The results indicated that the swelling characteristics of the nanocomposite hydrogels including the equilibrium degree of weight and volume swelling and the equilibrium water content were decreased by increasing the quantity of the clay incorporated into the hydrogel as well as by decreasing the temperature of swelling medium. While, the time required to reach to the equilibrium condition, as another swelling characteristic of the hydrogels, exhibited a completely opposite behavior. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.

Light triggered release of solutes from covalent DNA gels

DNA gels were prepared by cross-linking and were examined with respect to the release of DNA as well as macromolecular cosolutes introduced into the gels. In particular the effect of exposure to ultraviolet light was examined for these photodegradable gels. Different cross-linker densities (0.5%, 1%, 3% and 5% ethylene glycol diglycidyl ether) of the DNA gels were used. Network mesh size and the extent of swelling during degradation have been determined to characterize the effect of radiation on hydrogel degradation kinetics and simultaneous changes in gel structure on solute release profile. Modelling release kinetics, using the Weibull function, can describe the DNA release pattern, which is dependent on both radiation wavelength and cross-linker concentration; the mechanism of desorption is found to be complex. The kinetics release of cosolutes (lysozyme, BSA and FITC-dextran), initially loaded on DNA gels, were studied and clear differences on the release pattern were obtained. From the cosolute mechanism release it can be inferred that not only effective size plays an important role, but also interaction with the DNA network.

Mechanical Spectroscopy and Relaxometry on Alginate Hydrogels: A Comparative Analysis for Structural Characterization and Network Mesh Size Determination

The structure of calcium-saturated alginate hydrogels has been studied by combining rheological determinations and relaxometry measurements. The mechanical spectroscopy analyses performed on alginate gel cylinders at different polysaccharide concentration allowed estimating their main structural features such as the average mesh size. The calculation was based on the introduction of a front factor in the classical rubber elasticity approach which was correlated to the average length of the Guluronic acid blocks along the polysaccharide chain. Transverse relaxation time (T(2)) determinations performed on the cylinders revealed the presence of two relaxation rates of the water entrapped within the hydrogel network. The cross-correlation of the latter data with the rheological measurements allowed estimating the mesh size distribution of the hydrogel network. The results obtained for the hydrogel cylinders were found to be consistent with the relaxometric analysis performed on the alginate microbeads where, however, only one type of water bound into the network structure was detected. A good correlation was found in the average mesh size determined by means of relaxometric measurements on alginate microbeads and by a statistical analysis performed on TEM micrographs. Finally, the addition of a solution containing calcium ions allowed investigating further the different water relaxation modes within alginate hydrogels.

Self-Standing Three-Dimensional Networks of Nanoparticles With Controllable Morphology by Dynamic Templating of Surfactant Hexagonal Domains

Assembly of nanoparticles into free-standing three-dimensional networks has implications for a wide range of applications. We show that dynamic templating of surfactant hexagonal domains is a facile technique to organize nanoparticles into a network of particulate strands. Dispersed particles (>10 nm), independent of particle chemistry, assemble into networks, when the surfactant matrix cools into the hexagonal phase. We demonstrate assembly of inorganic, polymeric, and protein nanoparticles into networks. Where particle assembly is reversible, particles are coated with polymers that are subsequently cross-linked to stabilize the networks after surfactant removal. This technique involves near ambient temperatures and a benign water wash for template removal. The network mesh size can be varied from submicrometers to tens of micrometers by controlling the cooling rate. Particle networks can be flow-oriented prior to cross-linking, and interpenetrating networks can also be formed.

Mesh-size control and functionalization of reorganizable chemical gels by monomer insertion into their cross-linking points

Mesh-size control of network structures of chemical gels, i.e. mesh expansion by insertion of styrene derivatives and mesh shrinking by insertion of divinylbenzene in the gels, was carried out. Chemical gels with alkoxyamine at their cross-linking points were synthesized by radical copolymerization of methyl methacrylate and divinyl monomer with alkoxyamine units. The monomers were inserted by heating the gels with each monomer separately, and the network size was evaluated by small-angle X-ray scattering (SAXS) measurements. Since a living polymerization procedure was adopted for the reactions, network mesh sizes were controlled precisely by controlling the reaction time and selecting an appropriate monomer. In addition, gel properties such as swelling degree and phase separation could also be controlled by employing functional monomers such as hydrophilic and fluorinated monomers. These reorganizable chemical gels combine the advantages of high stability of chemical gels with ease of structural changeability of physical gels.

Engineering oligo(ethylene glycol)-based thermosensitive microgels for drug delivery applications

Novel oligo(ethylene glycol)-based thermosensitive microgels with well engineered core-shell structures were developed for storage and delivery of chemotherapeutic agents. The core is consisted of hydrophobic poly[2-(2-methoxyethoxy)ethyl methacrylate], while the shell is consisted of hydrophilic copolymer of 2-(2-methoxyethoxy)ethyl methacrylate with oligo(ethylene glycol) methyl ether methacrylates. These core-shell microgels exhibit tunable volume phase transition temperature and excellent colloidal stability across the physiologically important temperature range. The thickness of the hydrophilic shell can control the collapsing degree (or mesh size) of the hydrophobic core network, which can be utilized to significantly increase the loading capacity of the model hydrophobic drugs dipyridamole by tailoring the shell thickness of microgels. While the microgels are nontoxic, the drug molecules released from the microgels remain active to kill the cancer cells. The presented results provide important guidelines for the rational design of core-shell structured polymeric microgels for drug uptake and release applications.

STRUCTURE AND RHEOLOGICAL PROPERTIES OF POLY(VINYL ALCOHOL) HYDROGELS OBTAINED BY FREEZING/THAWING CYCLES

The relationships between the structure and the rheological behavior of poly(vinyl alcohol) (PVA) hydrogels prepared by freeze /thaw cycles were investigated as a function of polymer concentration (5 wt% ～ 25 wt% ). The degree of crystallinity and apparent dimensions of crystallite were determined by XRD. Rheological properties of PVA solutions including modulus and creep properties were characterized by controlled stress rheometer. Measurements of the shear storage and loss modulus were performed at a fixed frequency of 1 Hz and a low stress,under conditions where the deformation imposed on the gel structure is entirely reversible. The storage modulus at low frequency and low strain amplitude increases as the concentration increases,but the increase rate becomes clearly slow when the concentration is higher than 20 wt%. The storage modulus at 1 Hz was determined and related to the degree of crystallinity and concentration. Based on these measurements it was concluded that a minimum crystallinity and concentration were required for these PVA samples to exhibit gel behaviour. A more detailed comparison of XRD and rheological data led to the conclusion that the storage modulus was mainly controlled by the PVA crystallinity while the hydrogen bond interactions had a much smaller contribution,when the concentration was lower than 15 wt%. A structural parameter related to the network mesh size is derived from measurements of the dynamic storage moduli at low values of strain frequency and amplitude. The storage modulus of PVA hydrogels at different temperatures was analyzed on the basis of a theoretical model based on the scaling approach. The creep response of PVA hydrogels was investigated. With increase in concentration,these hydrogels exhibited an evolution from a linear viscoelastic response to a nonlinear viscoelastic response.