Rheological study of genipin cross-linked chitosan hydrogels

This paper reports the rheological behavior of chitosan solutions that have been cross-linked with different amounts of genipin, at body temperature and physiological pH. The effect of the cross-linker loading on the rheological properties of hydrogels has been evaluated. The oscillatory time sweep method was used to monitor the dynamic viscoelastic parameters during in situ (i.e., in the rheometer) gelation experiments, enabling the determination of the gelation time. The stress and frequency sweeps were employed to measure G' of the cured hydrogels. It was found that the solutions of chitosan cross-linked with genipin, under physiological conditions, could form relatively strong elastic gels when compared to those of pure chitosan. Moreover, the gelation time obtained from the crossover of G' and G' was in excellent agreement with the value obtained from the Winter-Chambon criterion. A significant reduction on this parameter was achieved even at low genipin concentrations. This behavior suggests that these formulations are able to be produced in situ and thus constitute promising matrices for cells and bioactive molecule encapsulations.     

Rheological properties of cysteine-containing elastin-like polypeptide solutions and hydrogels

The rheological properties of cysteine-containing elastin-like polypeptide (Cys-ELP) solutions and Cys-ELP hydrogels are reported. The Cys-ELP solutions exhibit a surprisingly high apparent viscosity at low shear rate. The high viscosity is attributed to the formation of an interfacial cross-linked "skin" at the sample surface, rather than the bulk of the Cys-ELP solution. At higher shear rate, the interfacial cross-linked film breaks, and its influence on the viscosity of the Cys-ELP solution can be ignored. Cys-ELP hydrogels are formed by mixing Cys-ELP and hydrogen peroxide (H(2)O(2)). At fixed concentration of Cys-ELP, the gelation time can be tuned by the concentration of H(2)O(2). Cys-ELP hydrogels have the typical characteristics of covalent cross-linked networks, as the storage moduli are larger than the loss moduli and are independent of frequency in dynamic oscillatory frequency sweep experiments. The plateau moduli obtained from linear frequency sweep experiments are much lower than those estimated from the number of thiol groups along the Cys-ELP chain, indicating that only a small fraction of thiols form elastically active cross-links. From the small value of the fraction of elastically active cross-links, the Cys-ELP hydrogel is concluded to be an inhomogenous network. Under steady shear, a 2.5 wt % Cys-ELP hydrogel shear thickens at shear rates lower than that necessary for fracture.     

Development of a biostable replacement for PEGDA hydrogels

The exceptional tunability of poly(ethylene glycol) (PEG) hydrogel chemical, mechanical, and biological properties enables their successful use in a wide range of biomedical applications. Although PEG diacrylate (PEGDA) hydrogels are often used as nondegradable controls in short-term in vitro studies, it is widely acknowledged that the hydrolytically labile esters formed upon acrylation of the PEG diol make them susceptible to slow degradation in vivo. A PEG hydrogel system that maintains the desirable properties of PEGDA while improving biostability would be valuable in preventing degradation-related failure of gel-based devices in long-term in vivo applications. To this end, PEG diacrylamide (PEGDAA) hydrogels were synthesized and characterized in quantitative comparison to traditional PEGDA hydrogels. It was found that PEGDAA hydrogel modulus and swelling can be tuned over a similar range and to comparable degrees as PEGDA hydrogels with changes in macromer molecular weight and concentration. Additionally, PEGDAA cytocompatibility, low cell adhesion, and capacity for incorporation of bioactivity were analogous to that of PEGDA. In vitro hydrolytic degradation studies showed that the amide-based PEGDAA had significantly increased biostability relative to PEGDA. Overall, these findings indicate that PEGDAA hydrogels are a suitable replacement for PEGDA hydrogels with enhanced hydrolytic resistance. In addition, these studies provide a quantitative measure of the hydrolytic degradation rate of PEGDA hydrogels which was previously lacking in the literature.     

Viscoelastic properties of dispersed chitosan/xanthan hydrogels

In this work a gel was formed by complexation of two natural polyelectrolytes, chitosan and xanthan. Changes in the hydrogels rheological properties have been studied in terms of hydrogel concentration (7–10% w/w), chemical media used for the hydrogel dispersion, and ‘test lag time’; i.e., the time between hydrogel dispersion in the chemical media and the start of the rheological test (up to 390 min). The viscoelastic properties of this polysaccharide system were characterized by oscillatory shear measurements under small-deformation conditions and the results show that chitosan/xanthan hydrogels behave like weak gels. The shear modulus increased almost linearly with frequency in the range studied (0.1–65 s⁻¹). The effects of hydrogel concentration and dispersion medium have been related to electrostatic equilibrium and by the presence of counter-ions modifying the internal structure of the hydrogel.     

Flow properties of N-(carboxymethyl) chitosan aqueous systems in the sol and gel domains

The flow behaviour and the viscoelastic properties of N-(carboxymethyl) chitosan aqueous systems in the sol and gel domains have been investigated by means of dynamic, steady and transient shear techniques. For polymer concentrations Cp up to 1%, a typical response of moderately concentrated polymer solutions was observed under continuous and oscillatory shear conditions. No time-dependent properties were detected during transient shear experiments. On the other hand, for all the samples with Cp greater than 1%, the rheological properties were more similar to those of a weak gel system. The continuous shear flow behaviour was of the plastic type and the viscoelastic quantities G' and G" were parallel to each other and slightly dependent on the frequency of oscillation omega. Stress overshoots were observed during transient shear experiments, and the kinetics of the structural breakdown and build-up processes were found to be dependent both on the polymer concentration and the applied shear rate.     

Rheology of synovial fluid

After a discussion of the role of synovial fluid as a joint lubricant, rheological measurements are described with both normal (healthy) synovial fluids and pathological ones. Shear stress and first normal stress difference are measured as a function of shear gradient to calculate the apparent shear viscosity eta 1 and the apparent normal viscosity psi 7 as well as an apparent shear modulus G'. It is found, that in case of diseased synoviae all rheological parameters deteriorate. Most significant changes are observed with the zero shear viscosity eta 0, the shear modulus G', and a characteristic time theta 1, which is the reciprocal of the critical shear rate Dc which determines the onset of shear thinning. The rheological deterioration of synovial fluids is explained in terms of solute structure, whereby a molecular mass of the backbone hyaluronic acid of at least 10(7) g.mol-1 is required for satisfactory function. A theory of the rheological performance of normal synovial fluid as well as its pathological deterioration is proposed.     

Biodegradable polymers from renewable sources: rheological characterization of hemicellulose-based hydrogels

Hemicellulose-based hydrogels were prepared by radical polymerization of 2-hydroxyethyl methacrylate or poly(ethylene glycol) dimethacrylate with oligomeric hydrosoluble hemicellulose modified with well-defined amounts of methacrylic functions. The polymerization reaction was carried out in water at 40 degrees C using a redox initiator system. The hydrogels were in general elastic, soft, and easily swellable in water. Their viscoelastic properties were determined by oscillatory shear measurements on 2 mm thick hydrogels under a slight compression to avoid slip, over the frequency range 10(-1) to 10(2). The rheological characterization indicated that the elastic response of the hydrogels was stronger than the viscous response, leading to the conclusion that the hydrogel systems displayed a predominantly solid-like behavior. The curves showed an increase in shear storage modulus with increasing cross-linking density. The nature of the synthetic comonomer in the hemicellulose-based hydrogels also influenced the shear storage modulus. Comparison of hemicellulose-based hydrogels with pure poly(2-hydroxyethyl methacrylate) hydrogels showed that their behaviors were rather similar, demonstrating that the synthetic procedure made it possible to prepare hemicellulose-based hydrogels with properties similar to those of pure poly(2-hydroxyethyl methacrylate) hydrogels.     

Synthesis and evaluation of novel biodegradable hydrogels based on poly(ethylene glycol) and sebacic acid as tissue engineering scaffolds

Novel biodegradable poly(ethylene glycol) (PEG) based hydrogels, namely, PEG sebacate diacrylate (PEGSDA) were synthesized, and their properties were evaluated. Chemical structures of these polymers were confirmed by Fourier transform infrared and proton nuclear magnetic resonance (1H NMR) spectroscopy. After photopolymerization, the dynamic shear modulus of the hydrogels was up to 0.2 MPa for 50% PEGSDA hydrogel, significantly higher than conventional hydrogels such as PEG diacrylate (PEGDA). The swelling ratios of these macromers were significantly lower than PEGDA. The in vitro degradation study demonstrated that these hydrogels were biodegradable with weight losses about 66% and 32% for 25% and 50% PEGSDA after 8 weeks of incubation in phosphate-buffered saline at 37 degrees C. In vitro biocompatibility was assessed using cultured rat bone marrow stromal cells (MSCs) in the presence of unreacted monomers or degradation products. Unlike conventional PEGDA hydrogels, PEGSDA hydrogel without RGD peptide modification induced MSC cell adhesion similar to tissue culture polystyrene. Finally, complex three-dimensional structures of PEGSDA hydrogels using solid free form technique were fabricated and their structure integrity was better maintained than PEGDA hydrogels. These hydrogels may find use as scaffolds for tissue engineering applications.     

Influence of Extraction Method on the Rheological Properties of Jackfruit (<Emphasis Type="Italic">Artocarpus heterophyllus</Emphasis>) Seed Starch Dispersions

Starches are widely used in food processing, and their rheological properties are affected by thermal conditions imposed during the process. The objective of the present study was to assess the rheological behavior of jackfruit seed starch (JSS) dispersions, with a particular interest on the effects of the starch extraction techniques using either water or an alkali solution (0.1 M sodium hydroxide) as solvents. The analyses on the starches were performed using small amplitude oscillatory shear tests, including frequency, temperature, and time sweeps, and determining flow curves at different temperatures (10–40 °C). JSS dispersions were classified as weak-intermediary gels, and those prepared with JSS extracted with water showed higher values of G’ and G” in all of the tested samples. All gelatinized samples exhibited viscoelastic behavior, and with increasing temperatures the storage and loss moduli increased until a maximum value before reaching a plateau. Gelatinization temperatures were slightly lower for dispersions prepared with JSS extracted with water. Gelatinization kinetics showed that increasing temperatures caused a reduction in the time to achieve gelatinization. The JSS dispersions displayed thixotropic behavior, which was influenced by their concentration and the extraction method employed. The flow curves exhibited pseudoplastic behavior. The Power Law model was used to describe the JSS dispersions rheological behavior, which enabled to assess the effects of starch concentration and extraction method on their rheological properties.     

Viscoelastic properties of solutions of ovine submaxillary mucin

The linear viscoelastic and rheological properties of high molecular weight ovine submaxillary mucin (OSM) solution have been investigated in terms of the Newtonian steady-flow viscosity [eta(gamma)], the complex oscillatory viscosity [eta*(omega)], and the storage and loss shear moduli [G'(omega) and G"(omega)]. It was observed that tau(gamma), eta*(omega), and G'(omega) are always higher when OSM is dissolved in 0.1M NaCl than when at the same concentration in 6M GdnHCl. This is consistent with previous observations that submaxillary mucins self-associate in 0.1M NaCl to form large aggregates, which are disrupted in 6M GdnHCl. As the OSM concentration increases, the appearance of a plateau shear modulus indicates the formation of a gel network in both solvents. The results suggest gelation involves specific intermolecular interactions, perhaps due to hydrophobic forces between interdigitated oligosaccharide side chains. The viscoelastic behavior of OSM solution at high concentration is thus similar to that reported in the literature for porcine gastric mucin (PGM). However, the OSM gels are mechanically weaker, having moduli that are an order of magnitude lower than those for PGM gels of comparable concentration. The oligosaccharide side chains of OSM consist of only 1-2 sugar units compared to 10-15 for PGM, but it appears that this is sufficient to allow for intermolecular interaction and the formation of weak gels.     

Clinical assessment of rheumatic diseases using viscoelastic parameters for synovial fluid

For the first time it is clearly exhibited that synovial fluid (SF) is thixotropic. Although no hysteresis loops were observed for SF, not even at high shear rates, thixotropy may be exhibited by measuring the rate of recovery after extensive shearing. The rebuilding of the structure in a small-amplitude oscillatory state following the high-shear-rate state reveals the thixotropic behaviour. Five different viscoelastic parameters for various synovial fluids (SF) were obtained using oscillatory rheometry. It was also shown that for SF in the low frequency range, corresponding to a knee joint almost at rest, the shear loss modulus G" is greater than the shear storage modulus G', since the system is allowed to dissipate energy at rest. However, with movement, G' increases and eventually becomes greater than G" at a characteristic frequency above which the system has insufficient time to dissipate energy and hence responds as an elastic body. This functional behaviour, characteristic for normal SF, broke down in the SF of rheumatoid arthritis. It was also absent in the SF of knee joints with meniscus lesions and ligament defects.     

Viscoelastic evaluation of topical creams containing microcrystalline cellulose/sodium carboxymethyl cellulose as stabilizer

The purpose of this study was to examine the viscoelastic properties of topical creams containing various concentrations of microcrystalline cellulose and sodium carboxymethyl cellulose (Avicel(R) CL-611) as a stabilizer. Avicel CL-611 was used at 4 different levels (1%, 2%, 4%, and 6% dispersion) to prepare topical creams, and hydrocortisone acetate was used as a model drug. The viscoelastic properties such as loss modulus (G"), storage modulus (G'), and loss tangent (tan delta) of these creams were measured using a TA Instruments AR 1000 Rheometer and compared to a commercially available formulation. Continuous flow test to determine the yield stress and thixotropic behavior, and dynamic mechanical tests for determining the linear viscosity time sweep data, were performed. Drug release from the various formulations was studied using an Enhancer TM Cell assembly. Formulations containing 1% and 2% Avicel CL-611 had relative viscosity, yield stress, and thixotropic values that were similar to those of the commercial formulation. The elastic modulus (G') of the 1% and 2% formulation was relatively high and did not cross the loss modulus (G"), indicating that the gels were strong. In the commercial formulation, G' increased after preshearing and broke down after 600 seconds. The strain sweep tests showed that for all formulations containing Avicel CL-611, the G' was above G" with a good distance between them. The gel strength and the predominance of G' can be ranked 6% > 4% > 2%. The strain profiles for the 1% and 2% formulations were similar to those of the commercial formulation. The delta values for the 1% and 2% formulations were similar, and the formulations containing 4% Avicel CL-611 had lower delta values, indicating greater elasticity. Drug release from the commercial preparation was fastest compared to the formulations prepared using Avicel CL-611, a correlation with the viscoelastic properties. It was found that viscoelastic data, especially the strain sweep profiles of products containing Avicel CL-611 1% and 2%, correlated with the commercial formulation. Rheological tests that measure the viscosity, yield stress, thixotropic behavior, other oscillatory parameters such as G' and G" are necessary tools in predicting performance of semisolids.     

Liquid crystalline phase and gel-sol transitions for concentrated microcrystalline cellulose (MCC)/1-ethyl-3-methylimidazolium acetate (EMIMAc) solutions

Liquid crystalline (LC) phase transition and gel-sol transition in the solutions of microcrystalline cellulose (MCC) and ionic liquid (1-ethyl-3-methylimidazolium acetate, EMIMAc) have been investigated through a combination of polarized optical microscope (POM) observation and rheological measurements. Molecular LC phase forms at the 10 wt % cellulose concentration, as observed by POM, whereas the critical gel point is 12.5 wt % by rheological measurements according to the Winter and Chambon theory, for which the loss tangent, tan δ, shows frequency independence. Dramatic decreases of G' and G' in the phase transition temperature range during temperature sweep are observed due to disassembling of the LC domain junctions. The phase diagram describing the LC phase and gel-sol transitions is obtained and the associated mechanisms are elucidated. A significant feature shown in the phase diagram is the presence of a narrow lyotropic LC solution region, which potentially has a great importance for the cellulose fiber wet spinning.     

Oscillatory deformation of human erythrocytes in sinusoidally modulated shear flow

The red cell deformation under oscillatory shear stress was studied. Shear stress was sinusoidally modulated between 8 and 32 dyn/cm2, thus, the extent of cellular deformation altered sinusoidally. At a low modulation frequency (less than 1.8 Hz), intact red cells perfectly responded to the shear stress applied on cells, and they could deform as much as the deformation in stationary shear flow. Above 2 Hz, the cellular deformation could not follow changes in shear stress along up-phase in the shear stress cycle. As decreasing the intracellular hemoglobin concentration, the cellular response to oscillatory shear stress became better. Treatment of cells with low concentrations of diamide impaired the response of intact cells to oscillatory shear stress, but unaffected the response of partially hemolyzed cells. These data suggest that the cellular response to oscillatory shear stress is determined by the cytoskeletal structure and the intracellular viscosity.     

Biomimetic poly(ethylene glycol)-based hydrogels as scaffolds for inducing endothelial adhesion and capillary-like network formation

The extracellular matrix (ECM) is an attractive model for designing synthetic scaffolds with a desirable environment for tissue engineering. Here, we report on the synthesis of ECM-mimetic poly(ethylene glycol) (PEG) hydrogels for inducing endothelial cell (EC) adhesion and capillary-like network formation. A collagen type I-derived peptide GPQGIAGQ (GIA)-containing PEGDA (GIA-PEGDA) was synthesized with the collagenase-sensitive GIA sequence attached in the middle of the PEGDA chain, which was then copolymerized with RGD capped-PEG monoacrylate (RGD-PEGMA) to form biomimetic hydrogels. The hydrogels degraded in vitro with the rate dependent on the concentration of collagenase and also supported the adhesion of human umbilical vein ECs (HUVECs). Biomimetic RGD/GIA-PEGDA hydrogels with incorporation of 1% RGD-PEGDA into GIA-PEGDA hydrogels induced capillary-like organization when HUVECs were seeded on the hydrogel surface, while RGD/PEGDA and GIA-PEGDA hydrogels did not. These results indicate that both cell adhesion and biodegradability of scaffolds play important roles in the formation of capillary-like networks.     

Diepoxide-triggered conformational transition of silk fibroin: formation of hydrogels

Silk fibroin hydrogels with tunable properties could be obtained from aqueous fibroin solutions (4.2 w/v %) in a short period of time. This was achieved by the addition of ethylene glycol diglycidyl ether (EGDE) into the reaction solution. Introduction of EGDE cross-links between the fibroin molecules decreases the mobility of the chains, which triggers the conformational transition from random-coil to β-sheet structure and hence fibroin gelation. Dynamic rheological measurements conducted at 50 °C show the formation of strong to weak hydrogels depending on the pH of the reaction solution. Although EGDE attacks the amino groups of fibroin and forms interstrand cross-links, β-sheets acting as physical cross-links dominate the elasticity of the hydrogels. Mechanical response of low-modulus fibroin hydrogels formed above pH 9.7 is highly nonlinear with strong strain hardening behavior (700%) arising from the alignment of the crystallizable amino acid segments.     

Rheological Studies of PMMA–PVC Based Polymer Blend Electrolytes with LiTFSI as Doping Salt

In this research, two systems are studied. In the first system, the ratio of poly (methyl methacrylate) (PMMA) and poly (vinyl chloride) (PVC) is varied, whereas in the second system, the composition of PMMA–PVC polymer blends is varied with dopant salt, lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) with a fixed ratio of 70 wt% of PMMA to 30 wt% of PVC. Oscillation tests such as amplitude sweep and frequency sweep are discussed in order to study the viscoelastic properties of samples. Elastic properties are much higher than viscous properties within the range in the amplitude sweep and oscillatory shear sweep studies. The crossover of and is absent. Linear viscoelastic (LVE) range was further determined in order to perform the frequency sweep. However, the absence of viscous behavior in the frequency sweep indicates the solid-like characteristic within the frequency regime. The viscosity of all samples is found to decrease as shear rate increases.     

Comparison of curdlan and its carboxymethylated derivative by means of Rheology, DSC, and AFM

Curdlan was carboxymethylated in an aqueous alkaline medium using monochloroacetic acid as the etherifying agent. The structure of carboxymethylated curdlan (CMc) was analyzed by FT-IR and NMR spectroscopy, which revealed that the carboxymethyl group was introduced mainly at the C-6 position as well as at the C-2 and C-4 positions. Furthermore, CMc was compared with the native curdlan by using rheology and DSC methods. It was found that in water, both polysaccharides behaved as pseudoplastic fluids and fit the power law and Herschel-Bulkley rheological models well. Both the storage shear modulus G' and the loss shear modulus G' of CMc aqueous solutions decreased and became more frequency dependent with decreasing concentration in comparison with the curdlan aqueous suspensions. The modulus-temperature curve also suggested that the gel characteristic of curdlan has been lost after chemical modification, which is consistent with the DSC results. AFM images revealed differences in the conformation of native and carboxymethylated curdlan, which changed from the aggregation of macromolecules to triple helices. All the experimental results suggest that the hydrogen bonds that bind curdlan with interstitial water to form the micelles have been destroyed completely and that the hydrophobic interactions related to the methylene groups at C-6 formed above 55 degrees C disappeared due to the introduction of the hydrophilic carboxymethyl group.     

Methylation of poloxamine for enhanced cell adhesion

Aiming at producing a synthetic collagen-mimetic material that is stiffer than collagen but that like collagen allows both cell encapsulation and cell growth on the surface, a positively charged poloxamine hydrogel was prepared by methylating the tertiary amine groups of a four-arm poly(ethylene oxide)-poly(propylene oxide) block copolymer derivative (Tetronic 1107). This derivative was subsequently reacted with methacryloyl isocyanate, rendering positively charged materials that are further cross-linkable by a photointiated free radical polymerization. Different hydrogels containing methylated poloxamine methacrylate concentrations between 6% and 18% were produced and characterized by means of water uptake and viscoelastic properties. A sharp increase in water content was observed in distilled water during the first week; some of the gels showed water uptakes as high as 2 times the initial wet weight. In PBS, this effect was less prominent due to the decrease in the osmotic gradient. Also, a gradual increase of both the storage modulus (G') and the loss modulus (G") resulted from increasing the polymer concentration: for example, G' values ranged between 70 and 23000 Pa for 6% and 18% methylated poloxamine methacrylate hydrogels (at 1 Hz, 100 Pa of oscillatory stress). HepG2 cells embedded in different compositions and exposed to UV light displayed good viability levels after the cross-linking, unlike a previously reported attempt at creating a synthetic collagen-mimetic material. A well-spread endothelial cell morphology was apparent on methylated poloxamine films after preincubation in serum-containing medium, while on unmodified poloxamine methacrylate hydrogels cells attached poorly. However, EC did not attach well to the same material when fabricated not as films but as cylindrical modules as needed for the modular construct for which this material was intended. Thus, for this apparently more challenging geometry, it was necessary to combine collagen with the methylated poloxamine to have good attachment of EC on the surface of modules as well as films. While the challenge of creating a synthetic alternative to collagen as a stiffer cell-compatible substrate remains, methylated poloxamine displays many of the attributes that make it a useful material for tissue engineering.     

Rheological evidence of the gelation behavior of hyaluronan-gellan mixtures

It was found that solutions of calcium hyaluronate (CaHA) (0.1 to approximately 0.5 wt%) could form a gel by mixing with solutions of sodium type gellan (0.1 to approximately 0.5 wt%), although neither polymer by itself forms a gel at low concentrations (0.1 to approximately 0.5 wt% in this experiment). The rheological properties of CaHA-gellan mixtures were investigated by dynamic and steady shear measurements. Both storage shear modulus G' and loss shear modulus G' for CaHA-gellan mixtures increased with increasing time, and tended to an equilibrium value after 1 h. After reaching steady values of G' and G", the frequency dependence of G' and G' was observed. G' was always larger than G' in the accessible frequency range from 10(-2) to 10(2) rad/s. The effects of pH and calcium ions were examined. Gel formation of the mixtures was promoted by decreasing pH and adding from 0.01 to 0.1 M calcium ions, but excessive calcium ions weakened the gel.