Real space observation of three-dimensional network structure of hydrated fibrin gel

The three-dimensional (3-d) network structure of the gel composed of rigid rod-shaped protein (fibrin gel) in a hydrated state was elucidated from a real space observation by confocal laser scanning microscopy. It was ascertained that two the length scales that characterize the gel network (diameter of polymer chain and typical mesh size of the gel network) can be determined quantitatively by a 3-d box-counting analysis and a 3-d Fourier transform (FT) analysis to obtain the power spectra. Turbidity measurements were employed for the determination of average fiber diameter. Self-similar structure of the gel network was found to be realized in the range between those two scales. The fibrin gels formed by larger amounts of thrombin showed a smaller fractal dimension that, deduced by the box-counting method, was in good agreement with the result from 3-d FT analysis and with a recent dynamic light scattering study (Kita R. et al. (2002) Biomacromolecules 3:1013).     

Interaction between blood plasma proteins and magnetite nanoparticles

Spin label EPR spectroscopy and dynamic and Rayleigh light scattering are employed to study the interaction between magnetite nanoparticles with a diameter of 17 nm and plasma proteins (fibrinogen and albumin). Protein molecules are shown to be adsorbed on nanoparticle surface with the formation of multilayer shells. When a buffer solution (pH 8.5) contains 0.01 vol % nanoparticles, 90–100 fibrinogen molecules are adsorbed per one particle and the thickness of an adsorbed layer is 30–40 nm. For albumin, the layer thickness is 10–15 nm. In a constant magnetic field, large linear microsized aggregates oriented parallel to field lines are formed in dispersions of nanoparticles covered with adsorbed protein molecules. The study of fibrin gel formation resulting from the action of thrombin enzyme on fibrinogen suggests that, in the presence of nanoparticles, the rate of gelation decreases by a factor of approximately two, while the ratio between the average mass and average length of fibrin polymer fibers rises.     

Coupling between polysaccharide gelation and micro-phase separation of globular protein clusters

The effect of gelation of the polysaccharide phase on the phase separation was investigated for mixtures of anionic polysaccharide (kappa-carrageenan) and globular protein (beta-lactoglobulin) clusters at pH 7 well above the iso-electric point. Gelation of kappa-carrageenan was induced by cooling in the presence of KCl. In the liquid state the protein clusters phase-separate into relatively dense micro-domains. When the polysaccharide phase gelled during cooling, the turbidity of the systems decreased dramatically. Light scattering experiments showed that the density of the micro-domains decreased, while microscopy showed that the number and size was not strongly modified. It is concluded that smaller protein clusters leave the micro-domains when kappa-carrageenan gels. The effect could be reversed by reheating the samples and thus melting the gel and was observed for repeated heating and cooling cycles. The effect of gelation on phase separation decreases with increasing polysaccharide concentration and with ageing of the liquid mixture. The latter is caused by the formation of bonds between the protein clusters in the micro-domains that slowly reinforce with time.     

Optimization of fibrin gelation for enhanced cell seeding and proliferation in regenerative medicine applications

In order to improve the cell seeding efficiency and cell compatibility inside porous tissue scaffolds, a method of fibrin gel‐mediated cell encapsulation inside the scaffold was optimized. Disc‐type poly(d ,l ‐glycolic‐co‐lactic acid) (PLGA) scaffolds without a dense surface skin layer were fabricated using an established solvent casting and particulate leaching method as a model porous scaffold, which showed high porosity ranging from 90 ± 2% to 96 ± 2%. The thrombin and fibrinogen concentration as precursors of fibrin gel was varied to control the gelation kinetics as measured by rheology analysis, and optimized conditions were developed for a uniform fibrin gel formation with the target cells inside the porous PLGA scaffold. The fibroblast cell seeding accompanied by a uniform fibrin gel formation at an optimized gelation condition inside the PLGA scaffold resulted in an increase in cell seeding efficiency, a better cell proliferation, and an increase in final cell density inside the scaffold. Scanning electron microscopy images revealed that cells were better spread and grown by fibrin gel encapsulation inside scaffold compared with the case of bare PLGA scaffold. Copyright © 2016 John Wiley & Sons, Ltd.     

Gelation in the Copolymerization of Diallyldimethylammonium Chloride with Acrylamide

Diallyldimethylammonium chloride (DADMAC) was free-radical copolymerized with acrylamide (AA) in water at a total monomer concentration of 4 mol/L and 40°C with different monomer feed compositions. Gelation occurred only for 20/80 DADMAC/AA monomer feed although crosslinking was observed for all monomer feed compositions. The gel point was at 51% conversion, and the swelling ratios of the resulting gels were quite high, from 1400 to 700. Addition of 2‐propanol as a chain‐transfer reagent reduced crosslinking and prevented gelation. These results are mechanistically discussed in connection with the cyclopolymerizability of DADMAC, and significant allylic hydrogen abstraction by the growing polymer radical characteristic of allyl polymerization is proposed.     

The microfibrillar network in gels of poly(γ-benzyl-L-glutamate) in benzyl alcohol

The gelation and gel-melting phenomena in semidilute isotropic solutions of poly(γ-benzyl-L -glutamate) (PBLG) in benzyl alcohol were studied by small-angle neutron scattering measurements, using a deuterated solvent, and by cryotransmission electron microscopy. The reversible gels are formed when the solution is cooled below the gelation temperature, and the gels melt upon heating. Hysteresis, of about 15°C, is observed between gelation and melting temperatures. In the isotropic solution, PBLG exists as isolated helices. Gelation is apparent as a large increase in the intensity scattered at low angles, signifying the heterogeneous microstructure of the gel. Direct visualization by electron microscopy of vitrified gel samples shows the formation of a microfibrillar network. The dimension of the observed microfibrils is about 10 nm. Upon melting, microstructural changes appear in a temperature range of about 10°C. The unique feature of the gel melting is that initially only the intensity in the mid-angle range decreases. This is interpreted as thickening of the microstructure due to melting of the thinner microfibrils. The final stage marks the melting of the thicker microfibrils. © 1996 John Wiley & Sons, Inc.     

Effect of hydrogen bonds on intermolecular crosslinking reaction by introduction of carboxyl groups in free-radical crosslinking monomethacrylate/dimethacrylate copolymerizations

The effect of physical interaction through hydrogen bonds on the intermolecular crosslinking reaction leading to the promoted gelation in free-radical crosslinking monovinyl/divinyl copolymerizations was discussed from the standpoint of the control of network formation. The solution copolymerizations of benzyl methacrylate (BzMA) with 2 mol% of 1,6-hexanediol dimethacrylate in t-butylbenzene were conducted in the absence and presence of different amounts of mono(2-methacryloyloxyethyl) succinate (MMOES). Gelation was promoted by the addition of MMOES and the ratio of the actual gel point to the theoretical one became smaller; this would be related to the formation of hydrogen bonds between carboxyl groups introduced into prepolymer and growing polymer radical. As an extension of the above discussion, we treated the effect of hydrogen bonds on the gelation in the crosslinking BzMA/triicosaethylene glycol dimethacrylate copolymerization. The addition of MMOES obviously promoted the gelation. The ratio of the actual gel point to the theoretical one calculated according to Stockmayer's equation [J. Chem. Phys. 12 (1944) 125] was obtained as 1.9, very close to unity.     

Interfacial routes to colloidal gelation

Colloidal gelation is a rheological transition from fluid-like to solid-like viscoelasticity in a particulate suspension and is often instigated by causing the net interparticle interaction to be attractive. In this article, three routes to colloidal gelation that have been discovered recently and involve interfacial phenomena at a fluid interface are reviewed. As in conventional systems, gelation is due to a percolating particle network that imparts elasticity to the mixture, but the network formation involves interfacial particle jamming or bridging, or capillary interactions along or across interfaces, in a mixture of immiscible fluids. Gelation imparts mechanical stability to these multiphase mixtures and paves the way for their use as templates for the synthesis of functional, microstructured materials and composites. The gel mechanical properties are mediated by the interfacial forces and the mixture's microstructure, and therefore show different dependencies on particle volume fraction across the three systems.     

Rheological and related study of gelation of xyloglucan in the presence of small molecules and other polysaccharides

Interaction between tamarind seed xyloglucan and the other polysaccharides, gellan gum or xanthan investigated by rheology, differential scanning calorimetry, and related methods was discussed. All these three polysaccharides do not form a gel at lower concentrations by itself at the experimental conditions studied but the gelation of xyloglucan occurs in the presence of gellan or xanthan. Gelation of xyloglucan in the presence of a polyphenol, epigallocatechin gallate, is also discussed. Hence the gelation of these mixtures is caused by the synergistic interaction, and the models for the synergistic interaction were discussed. The gelation of polysaccharides by the synergistic interaction is of great value for food and related industries.     

Comparison of irreversible and reversible cluster formation

Covalent and reversible cluster molecules were synthesized by an A₃B₂ type gelation. Crosslinking of three-arm hydroxyl-terminated star polymers with 2,4-toluenediisocyanate gave branched polymers, while the reversible analogue was made by crosslinking of tertiary amine-terminated star polymers with bis(4-hydroxy-3,5-dinitrophenyl) adipate. Gelation process was followed by static and dynamic light scattering. The extent of reacted groups was measured with UV spectroscopy. Growth of the covalent clusters could be described in terms of percolation scaling laws. The experimental gel point (P_{OH, cr} = 0.70) was shifted significantly from the theoretical predicted gel point (P_{OH, cr} = 0.50), indicating extensive ring formation during the gelation. The reversible endlinking reaction gave no macroscopic gelation, though increase of the cluster dimensions was observed. Ring formation proved to be an important side reaction in both cases; however, the ring formation ability seems to change in a different manner during the course of a gelation.     

Thermoreversible gelation of poly(vinylidene fluoride) in phthalates: the influence of aliphatic chain length of solvents

Thermoreversible gelation of poly(vinylidene fluoride) (PVDF) has been studied in a new series of solvents (phthalates), for example, dimethyl phthalate (DMP), diethyl phthalate (DEP), dibutyl phthalate (DBP), and dihexyl phthalate (DHP) as a function of temperature and polymer concentration, both by test tube tilting and dynamic light scattering (DLS) method. The effect of aliphatic chain length (n) of diesters on the gelation kinetics, structure/microstructure and morphology of PVDF gels has been examined. Gelation rate was found to increase with increasing aliphatic chain length of diester. DLS results indicate that the sol-gel transformation proceeds via two-steps: first, microgel domains were formed, and then the infinite three-dimensional (3D) network is established by connecting microgels through polymer chains. The crystallites are responsible for 3D network for gelation in phthalates, and alpha-polymorph is formed during gelation producing higher amount of crystallinity with increasing aliphatic chain length of diester. Morphology of the networks of dried gels in different phthalates showed that fibril thickness and lateral dimensions decrease with higher homologues of phthalates. The scattering intensity is fitted with Debye-Bueche model in small-angle neutron scattering and suggested that both the correlation length and interlamellar spacing increases with n. A model has been proposed, based on electronic structure calculations, to explain the conformation of PVDF chain in presence of various phthalates and their complexes, which offer the cause of higher gelation rate for longer aliphatic chain length.     

Gelation of covalently edge-modified laponites in aqueous media. 1. rheology and nuclear magnetic resonance

We describe the covalent modification of the edges of laponite with organic groups and the influence of this modification on gelation behavior. We compare three materials: an unmodified laponite, a laponite edge modified with a trimethyl moiety (MLap), and an octyldimethyl moiety (OLap). Gelation is investigated using rheology and NMR T1 relaxation measurements and nuclear Overhauser enhancement spectroscopy (NOESY). MLap and OLap show qualitatively different gelation. Gelation of MLap is very similar to laponite: MLap gels over the same time scale as laponite and has about the same solid modulus, and the MLap gel is almost as transparent as laponite. In contrast, OLap gels rapidly relative to laponite and forms a weak, turbid gel. We believe that gelation in laponite and MLap results from the formation of a network of well-dispersed platelets (or a few platelets), while in OLap, gelation results from a network of stacks of several platelets. NMR relaxation measurements indicate that gelation does not affect the average relaxation of water protons. However, T1 increases marginally for the protons in the organic moieties in MLap and decreases for protons in the organic moieties in OLap. Relaxation measurements, analyses of line width, and NOESY taken together suggest that, in OLap, gelation is a consequence of association of the organic moieties on the laponite edges, and that this association strengthens with time. Thus, the time-dependent changes in NMR suggest a structural origin for the time-dependent changes in the rheological behavior.     

Gelation of PEO-PLGA-PEO triblock copolymers induced by macroscopic phase separation

The gelation behavior of aqueous solutions of poly(ethylene oxide-b-(DL-lactic acid-co-glycolic acid)-b-ethylene oxide) (PEO-PLGA-PEO) triblock copolymer containing short hydrophilic PEO end blocks is investigated using dynamic light scattering, rheology, small-angle neutron scattering (SANS), and differential scanning calorimetry (DSC). For polymer concentrations between 5 and 35 wt %, four distinct regions of the turbidity change depending on temperature were observed. Interestingly, in the turbid solution region, gel phase is formed for polymer concentrations above 14 wt % and an extremely slow relaxation was detected. In fact, a power law, which takes into account the dynamics of percolation clusters, dominates the correlation function. In rheological measurements, the local maximum in G' is observed at around the temperature of maximum turbidity. We further found that G" > G' and G' is highly dependent on frequency at the gel state implying viscoelastic characteristics, which is quite different from general concepts of gels, typically formed by the micellar packing. SANS profiles showing multiple peaks in the sol state rather than in the gel state as well as a DSC exotherm at the temperature of gels can also serve as the evidence of different gel states. Based upon the experimental data obtained in the present study, a new gelation mechanism induced by the macroscopic phase separation of triblock copolymers containing short hydrophilic PEO end blocks such as PEO-PLGA-PEO is proposed. The effect of the type ofhydrophobic middle blocks on the gelation is also discussed.     

Polymer-solvent co-crystallization during thermoreversible gelation in solutions of the helical polypeptide PBLG

Macromolecular aggregation during thermoreversible gelation in solutions of the helical polypeptide poly(γ-benzyl-L-glutamate) [PBLG] in benzyl alcohol [BA] were studied by small angle neutron and small angle X-ray scattering. Gelation is apparent as a large increase in the intensity scattered at low angles, signifying formation of a microfibrillar PBLG network. The aggregated phase in isotropic gels from semidilute solutions contains about 28% solvent. A periodic structure is observed when gelation is induced by rapid cooling to a low temperature, but not by slow cooling or gelation at a higher temperature. In gels from concentrated liquid crystal solutions, two crystalline structures are observed, depending on whether the solution is rapidly quenched and then annealed or slowly gelled at an elevated temperature. A phase diagram for the PBLG/BA system is presented and the observed microstructural transitions are rationalized in terms of a gelation mechanism involving a combination of liquid-liquid phase separation and crystallization in the form of polymer-solvent co-crystals.     

Rheological and DSC study of sol-gel transition in aqueous dispersions of industrially important polymers and colloids

 Gelation kinetics, mechanical spectra, thermal scanning rheology (TSR), and differential scanning calorimetry (DSC) in aqueous solutions of gelling polymers and colloids such as seaweed polysaccharides (agarose, carrageenans), microbial polysaccharides (gellan, curdlan), plant polysaccharides (methylcellulose), globular proteins (casein, glycinin, β-conglycinin), fibrous proteins (gelatin, fibrin), and polyvinyl alcohol, which are related to foods, cosmetics, biomedical and pharmaceutical applications, are described. Some gelation processes at a constant temperature have been treated successfully by an equation of first order kinetics or by other modified equations, and the molecular mechanism of gel formation is discussed briefly. For water-soluble polymers, the criterion of the gel or sol based on the frequency dependence of storage and loss moduli gives valuable informations. TSR and DSC are complementary, and the combination of these methods has been proved to be useful. Received: 17 June 1997 Accepted: 28 August 1997     

Two Gel States of a PEO‐PPO‐PEO Triblock Copolymer Formed by Different Mechanisms

Two gel states of a PEO‐PPO‐PEO (Pluronic P103) triblock copolymer in water are investigated using small‐angle X‐ray scattering, rheology and differential scanning calorimetry. The first gel state turns out to be the hexagonal microphase while the second gel state, showing turbidity change with four distinct regions, is somewhat disrupted. The second gel is moreover not thermoreversible as evidenced by rheology. Based upon the present study, two different gelation mechanisms for aqueous PEO‐PPO‐PEO solutions are proposed.     

Gel formation and photoactive properties of azobenzene-containing polymer in liquid crystal mixture

For the first time, a photochromic azobenzene-containing liquid crystalline (LC) acrylic polymer was used for gelation of low-molar-mass nematic mixture (LMNM). Dissolution of LC polymer in amount of only 2.5 wt.% in LMNM at 120°C (isotropic state) followed by cooling down results in formation of the solid-like photochromic LC gel. Gelation is associated with a phase separation and formation of microsized LC polymer domains, which form a physical “network” containing encapsulated nematic host. Textural changes of mixture during gel formation were analyzed, and absorbance spectra were measured. A special attention was paid to the kinetic study of photoinduced E-Z and Z-E isomerization of azobenzene side groups of polymer in gel. It was shown that ultraviolet (UV)-irradiation and E-Z isomerization processes are accompanied by disruption of H-aggregates of azobenzene moieties and partial dissolution of polymer.     

Diffusion of dextran probes in a self-assembled fibrous gel composed of two-dimensional arborols

Two-dimensional arborols are bolaform amphiphiles in which a central, hydrophobic spacer separates twin hydrophilic ends. Under appropriate conditions, these relatively small molecules assemble into very long fibers; subsequently, the system gels if the arborol concentration is sufficiently high. The diffusion of linear or slightly branched dextran probes in 3 and 6% arborol gels, as determined by fluorescence photobleaching recovery, resembles that of dextrans in water, suggesting a highly open network structure. Melting the gels produces almost no change in diffusion of the dextran probes. Small-angle X-ray scattering (SAXS) of wet arborol gels at different concentrations and temperatures reveals the diameter of the repeating unit of the fibers to be 8.26+/-0.68 nm. This dimension, which is independent of concentration and temperature, exceeds the length of a single arborol molecule by about a factor of 3. Rheological investigation identifies the linear response regime of the gels and permits an examination of the weak correlation between dextran probe diffusion and gel viscoelasticity.     

Polysiloxane network formation observed by time-resolved small-angle X-ray scattering

Time-resolved small-angle X-ray scattering was observed on systems undergoing gelation. A polysiloxane network was used as a model system, where the polysiloxane network was formed by hydrosylation of VT-M with F4-C. The Flory-Stockmayer model for gelation was found to be satisfactory to describe the polysiloxane network formation. No inhomogeneous distribution of densely crosslinked region exists, and in this respect the gel formed by the polysiloxane network is homogeneous. The invariance of the correlation length ξ evaluated from the Lorenzian term suggests that the local network architecture would not change by gelation.     

Effects of poloxamer on the gelation of silk sericin

Effects of the concentration of poloxamer 407, temperature, and the concentration of sericin on the gelation of silk sericin (SS) were studied. Gelation of SS was accelerated with an increase in poloxamer concentration and temperature. The sol‐gel transition of SS became irreversible with respect to the temperature in the presence of poloxamer, whereas the sol‐gel transition of SS itself was reversible. Infrared (IR) and circular dichroism (CD) spectra show that the conformational change of SS in the presence of poloxamer was accelerated from random coil to β‐structure. X‐ray diffraction and differential scanning calorimetry (DSC) show that the crystalline structure of poloxamer in the mixture was affected by the presence of SS and that its melting temperature was shifted to lower temperature with increasing SS content, indicating an interaction between poloxamer and SS through hydrogen and hydrophobic bondings.