The use of dilute solution viscometry to characterize the network properties of carbopol microgels

We used dilute solution viscometry, specifically, measurements of intrinsic viscosity and overlap concentration to characterize two members of the carbopol family, carbopols 940 and 941. These measured quantities were then used to calculate the swollen-to-dry volume ratios for both resins over a range of ionic strengths. The variation in this ratio is representative of the swelling equilibria of the micronetwork and is modeled using standard network theory with modifications for non-Gaussian chain statistics and for fixed charges on the chains. By fitting to experimental data, the cross-link density is determined as 1450 monomer units between cross-link sites for carbopol 940 and 3300 units for carbopol 941. The shear modulus of the microgel domain is then predicted from the swelling data and found to be an order of magnitude greater than the elastic modulus measured on concentrated carbopol dispersions. We argue that this discrepancy is due to a large number of defects which reduce the cycle rank of the network. Current microgel technology specifies that their rheological behavior can be predicted once the domain cross-link density is known. Simple dilute solution viscometry appears capable of providing this prerequisite.     

Host-guest interactions of 4-carboxyphenoxy phthalocyanines and beta-cyclodextrins in aqueous media

Beta-cyclodextrin and its permethylated derivatives form 2:1 inclusion complexes with tetrakis- and octakis(4-carboxyphenoxy)phthalocyanines 1-4, reducing their aggregation tendency and promoting their sensitization of singlet oxygen formation in aqueous media.     

Multiplexing interactions to control antibiotic release from cyclodextrin hydrogels

A new strategy for affinity-based drug delivery by modification of the drug rather than modification of the device is presented. Rifampin is modified to contain either one or two PEG-adamantane arms, and the drug release properties of dimeric coumermycin are compared to novobiocin with only one biding domain. The drugs are loaded into affinity-based and diffusion-only delivery platforms, the loading efficiency is calculated, and the release kinetics is determined in vitro. The presence of additional binding domains prolongs the release of antibiotics. Release rates differ little between modified and unmodified drug from the diffusion-only system. The results demonstrate the feasibility of custom-tuning drug delivery by multiplexing interactions with an affinity-based polymer platform.     

Particle interactions in electrophoresis due to inertia

The inertial forces acting on two cylinders and two spheres have been calculated. The cylinders or spheres are of the same radius and zeta potential, and arbitrarily oriented in an electric field. It is found that when two particles are aligned perpendicular to the direction of the electric field, the force between them along the line of the centers is attractive. When they are along the direction of the electric field, the force between them along the line of the centers is repulsive. On a pair of arbitrarily oriented particles, the force perpendicular to the line of the centers tends to rotate the particles around the midpoint between the particle centers such that the pair is aligned normal to the applied electric field. The stable equilibrium orientation of a pair of particles and the attractive interaction forces between the particles when they are stably orientated may give rise to aggregation of particles during electrophoretic motion of a suspension.     

Rheological properties of polyacrylamide-based hydrogels

Rheological properties of hydrogels of polyacrylamide-based polyelectrolytes were studied. The dependence of the rheological characteristics of the gels on the content of an organic modifier and parameters of a mechanical action was disclosed. The range of parameters where the hydrogels of modified polyacrylamide demonstrated the highest mechanical strength was determined.Translated from Kolloidnyi Zhurnal, Vol. 66, No. 6, 2004, pp. 840–843.Original Russian Text Copyright © 2004 by Shevchenko, Ulrikh, Yakovchenko, Pirogov, Smirnov.     

Study of the interactions between protein-imprinted hydrogels and their templates

The interactions between lysozyme-imprinted hydrogel and their template protein were studied using adsorption measurements, competitive adsorption experiments, and isothermal titration calorimetry (ITC). The results were compared to the interactions between the imprinted polymer and a reference protein, cytochrome c. Experimental adsorption isotherms and competitive adsorption studies detected better affinity and higher capacity of the imprinted polymer toward the template protein. Moreover, analysis of ITC data identified major differences in the binding enthalpy of lysozyme when the imprinted and the non-imprinted polymers were compared. On the other hand, cytochrome C did not exhibit any major changes in the adsorption enthalpy when comparing the imprinted and the non-imprinted polymers. This is the first thermodynamic evidence for the creation of new binding sites in the process of protein imprinting.     

Physical properties of glycosaminoglycan hydrogels

The chemical composition of glycosaminoglycan (GAG) hydrogels was found to have a profound effect on the physical properties of gels. Hyaluronan (HA) and chondroitin sulfate (CS) were each modified with adipic dihydrazide (ADH) with carbodiimide chemistry. The resulting polymer was crosslinked with various concentrations of poly(ethylene glycol) dialdehyde (PEG‐diald) to produce a series of hydrogels. The physical properties of these GAG hydrogels varied in a concentration‐dependent fashion. Maximal crosslinking was observed at a theoretical crosslinking of 50% for the HA‐ADH‐PEG‐diald hydrogels and 75% for the CS‐ADH‐PEG‐diald hydrogels. Adding PEG‐diald beyond the optimum for crosslinking prolonged the in vitro enzymatic degradation time of the hydrogels. The swelling of the crosslinked GAG hydrogels was correlated with the amount of PEG‐diald used rather than with the crosslinking density. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4344–4356, 2004     

Using supramolecular hydrogels to discover the interactions between proteins and molecular nanofibers of small molecules

Here we report the first example of the use of supramolecular hydrogels to discover the protein targets of aggregates of small molecules.     

Swelling properties of aspartic acid-based hydrogels

Chemically crosslinked poly(aspartic acid) (PASP) gels were prepared by the hydrolysis of polysuccinimide (PSI). This latter was prepared by thermal polycondensation of aspartic acid. The PSI chains were crosslinked by 1,4-diaminobutane. The consecutive reactions of hydrolysis and swelling kinetics of PSI- and PASP-based gels were studied at different pH values. Two distinct swelling mechanisms were proposed. The cooperative diffusion coefficient has been found to be three orders of magnitude higher in pH 14 solution than at pH 8.     

Chemical shifts due to long range dispersion interactions

Dispersion chemical shifts, σ_w, for a variety of solute-solvent systems have been obtained by three different methods. The value of σ_w for a given system differs with the method employed for evaluating the characteristic frequency.     

An attempt to evaluate the vibrational intensity changes due to long-range interactions

The effects of intermolecular interactions within the framework of long-range interactions on IR and Raman intensities are investigated.     

New optical properties of polyelectrolyte hydrogels

A new optically active medium for lasers — a polyelectrolyte hydrogen swollen in a mixture of water + alcohol + dye — is presented. An anomalous dependence of the absorbance on the composition of the mixture is observed. It shows a maximum which corresponds to the composition when the hydrogel starts to swell as a hydrophilic network.     

Rheological properties of a telechelic associative polymer in the presence of alpha- and methylated beta-cyclodextrins

The viscosity of hydrophobic ethoxylated urethane (HEUR) solution decreased in the presence of alpha-CD or m-beta-CD; however their interactions were quite different. When the alpha-CD/hydrophobe molar ratio exceeded 5.0, the viscosity was close to that of a PEO solution of similar molecular weight. Oscillatory shear indicated that the mechanically active chains in HEUR solution decreased with the addition of alpha-CD. This agreed with the hypothesis that alpha-CD formed an inclusion complex with the hydrophobic moiety of the HEUR polymer, thereby destroying the transient hydrophobic associative network. The viscosity/temperature relationship of the alpha-CD/HEUR system (for HEUR with 70% of the PEO chains capped at both ends) did not obey the Arrhenius relationship for alpha-CD/hydrophobe molar ratio in the range 0.8-5.0. The low shear viscosity increased with increasing temperature at molar ratio of 1.0, and this was attributed to the competitive complexation of the alpha-CD/hydrophobe and the alpha-CD/PEO chain. Increasing temperature favored alpha-CD/PEO complexation. Comparison between the behavior of alpha-CD/HEUR and m-beta-CD/HEUR resulting from the different binding characteristics was discussed.     

Surfactant-induced healing of tough hydrogels formed via hydrophobic interactions

Different reversible molecular interactions have been used in the past few years to generate self-healing in synthetic hydrogels. However, self-healing hydrogels synthesized so far suffer from low mechanical strength which may limit their use in any stress-bearing applications. Here, we present a simple technique to heal mechanically strong polyacrylamide hydrogels formed via hydrophobic interactions between stearyl groups. A complete healing in the hydrogels was achieved by the treatment of the damaged areas with an aqueous solution of wormlike sodium dodecyl sulfate micelles. The micelles in the healing agent solubilize the hydrophobes in the cut surfaces, so that they easily find their partners in the other cut surface due to the hydrophobic interactions. Surfactant-induced healing produces high toughness (～1 MPa) gels withstanding 150 kPa of stress at a deformation ratio of 1,100 %. The healing technique developed here is generally applicable to the physical gels formed by hydrophobic associations.     

Thermal and rheological properties of gelatin-dextran hydrogels

Hydrogels prepared by crosslinkage of gelatin with dextran dialdehyde have been characterized by dynamic shear oscillation measurements at small strain. Isothermal as well as temperature scan measurements were performed. The results obtained demonstrated that the final polymer network is a result of a chemical gelatin-dextran dialdehyde interaction as well as a gelatin-gelatin (physical association) and a polymer-solvent interaction. This balance is strongly dependent on the composition of the system, the polymer concentration, the storage temperature and the storage time.We found that a short cryogenic treatment at –20°C of physically structured gels, significantly increases the chemical crosslinkage. DSC measurements at low cooling rate confirm these results and demonstrate a chemical reaction enthalpy contribution.This work was supported by the Flemish Institute for Science and Technology (IWT) and the Belgian Government (PAI-III-40).The authors also express their gratitude to Systèmes Bio Industries Benelux for providing free samples of well characterized gelatin.     

Copolymerization and properties of multicomponent crosslinked hydrogels

A series of multicomponent hydrogels were prepared by the copolymerization of hydrophobic silicon-containing monomer 3-bis(trimethylsilyloxy) methylsilylpropyl glycerol methacrylate(Si MA) with the solvent-responsive monomers 2-hydroxyethyl methacrylate(HEMA) and N-vinyl pyrrolidone(NVP) and thermosensitive monomer N,N-dimethyl acrylamide(DMA). 2-Hydroxy-2-methyl phenyl acetone(D-1173) was chosen as UV initiator and five different dienes/triene monomers were selected as crosslinking agent in order to select the best crosslinker. The ethanol extraction experiments as well as the FTIR, DSC and TG results showed that the copolymerization was effective. The optical, permeability, and mechanical analysis results demonstrated that the obtained hydrogels were highly transparent with good oxygen permeability and mechanical properties. And the impact of crosslinker on the mechanical properties of the hydrogels was also discussed in detail. The basic results demonstrated that the obtained hydrogels had good stimuli-responsive effects to both p H value and solvent.     

Use of organic solvent to prevent protein adsorption in CE-MS experiments

Protein adsorption onto capillary wall often hampers CE separations, particularly in the CZE mode. Electrostatic interactions are not the only factors affecting adsorption, as hydrophobic interactions and/or protein conformational changes are also involved in the adsorption phenomenon. Numerous methods can be used to reduce or avoid adsorption, such as (i) addition of low molecular weight molecules in the BGE, (ii) use of surfactants, or (iii) capillary coatings. However, most of these methods are not MS-compatible. In this study, we evaluated the addition of organic solvent as an alternative MS-compatible method to decrease protein adsorption. The effect of the solvent addition was emphasized using classical methods for estimating reversible and irreversible adsorption. In many cases, organic solvents were effective at decreasing adsorption. However, the influence of the organic solvent on protein adsorption should be evaluated case-by-case in CE method development.     

Noncovalent binding interactions of polyacrylamide and clay in nanocomposite hydrogels

The interactions between organic and inorganic components in pregel solution for polyacrylamide (PAAm)/clay nanocomposite hydrogels (NC gels) and in prepared NC gels are investigated. Besides, a kind of self‐crosslinked PAAm gels with excellent mechanical properties is fabricated in the absence of any cross‐linking agents, the hydrogen bonding interactions among PAAm chains are acted as the cross‐linking force. It is revealed that the binding interactions of PAAm and clay in NC gels are owing to the noncovalent interactions between amide groups on PAAm chains and clay platelets, which afford the cross‐linking force for NC gels network formation. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Use of styrene radical cations as probes for the complexation dynamics of charged guests with alpha- and beta-cyclodextrins

The complexation dynamics of radical cations with cyclodextrins (CD) was studied using photophysical techniques. Radical cations of 4-vinylanisole and trans-anethole were formed within alpha- and beta-CD cavities by two-photon photolysis of the respective styrene precursors. Exit of the radical cations from alpha-CD complexes with 1: 1 and 1:2 (guest: CD) stoichiometries and beta-CD complexes with 1:1 stoichiometries occurred with lifetimes shorter than 100 ns. Most of the radical cations formed escape from the CD cavities, but a small portion do react with alpha-CD when this host is present in high concentrations.     

A facile way to tune mechanical properties of artificial elastomeric proteins-based hydrogels

Protein-based hydrogels have attracted considerable interests due to their potential applications in biomedical engineering and material sciences. Using a tandem modular protein (GB1)(8) as building blocks, we have engineered chemically cross-linked hydrogels via a photochemical cross-linking strategy, which is based on the cross-linking of two adjacent tyrosine residues into dityrosine adducts. However, because of the relatively low reactivity of tyrosine residues in GB1, (GB1)(8)-based hydrogels exhibit poor mechanical properties. Here, we report a Bolton-Hunter reagent-based, facile method to improve and tune the mechanical properties of such protein-based hydrogels. Using Bolton-Hunter reagent, we can derivatize lysine residues with phenolic functional groups to modulate the phenolic (tyrosine-like) content of (GB1)(8). We show that hydrogels made from derivatized (GB1)(8) with increased phenolic content show significantly improved mechanical properties, including improved Young's modulus, breaking modulus as well as reduced swelling. These results demonstrate the great potential of this derivatization method in constructing protein-based biomaterials with desired macroscopic mechanical properties.