Rheology of aqueous dispersions of Laponite and TEMPO-oxidized nanofibrillated cellulose.

The viscoelastic and flow properties of aqueous Laponite/TEMPO-oxidized nanofibrillated cellulose (TEMPO-NFC) systems are investigated with the aim to study the effect of content and proportion of both components on the properties. Both Laponite and TEMPO-NFC aqueous dispersions can find numerous applications in different fields (from industrial to biomedical one) due to their structural and rheological properties. The most important effects of blending are observed at low Laponite fractions, also with qualitative changes in the shape of the flow curves and mechanical spectra. Positive synergistic effects are ascribed to the interpenetration and electrostatic interactions between TEMPO-NFC fibrils and Laponite nanodisks. These mechanisms are more effective in Laponite-poor blends when a minority of clay nanoparticles and its aggregated forms are distributed within the nanocellulose matrix, acting as bridging agents between polymer nanofibrils. The synergistic effects are compared to those observed for Laponite blends with a nonionic biopolymer, scleroglucan.

Rheology of Laponite-scleroglucan hydrogels.

Both Laponite and scleroglucan can find several applications in various fields (from industrial to biomedical one) in virtue of their peculiar features and rheological properties displayed in aqueous phases. Structural states of Laponite dispersions strongly depend on concentration and ionic strength. When attractive and repulsive interparticle interactions are so effective that they lead to arrested states (attractive gel or repulsive glass), the rheological behavior of the dispersion undergoes a sharp transition, from quasi-Newtonian to markedly shear thinning and viscoelastic. Conversely, scleroglucan solutions gradually change to weak gels with increasing polymer concentration. The present work is concerned with aqueous Laponite-scleroglucan mixed systems, obtained according to different preparation modes, and is aimed at examining how much the content and proportion of both components affect the viscoelastic and flow properties of the mixed system.

The complex mechanism of HM pectin self-assembly: A rheological investigation.

Several biopolymers are widely employed in food, pharmaceutical and biomedical sectors by virtue of their ability to generate supramolecular structures, typically physical hydrogels. In the case of high methoxyl pectins (HMP) the gel formation is promoted by the presence of cosolutes (sugars or polyols) and low pH. The present investigation regards the structuring kinetics of aqueous HMP solutions having different polymer concentration and equal sucrose content at 20°C. A sequence of consecutive frequency sweep was applied to each sample immediately after its preparation. The time evolution of the linear viscoelastic behavior is described by the sigmoidal profiles of both moduli at each applied frequency and more thoroughly defined through the change of the mechanical spectrum, i.e. the variation of the parameters of the generalized Maxwell model or the Friedrich-Braun model which are both suitable to provide a satisfactory data fitting.

Topological characterization of a bacterial cellulose-acrylic acid polymeric matrix.

This paper focuses on the micro- and nano-topological organization of a hydrogel, constituted by a mixture of bacterial cellulose and acrylic acid, and intended for biomedical applications. The presence of acrylic acid promotes the formation of two interpenetrated continuous phases: the primary "pores phase" (PP) containing only water and the secondary "polymeric network phase" (PNP) constituted by the polymeric network swollen by the water. Low field Nuclear Magnetic Resonance (LF NMR), rheology, Scanning Electron Microscopy (SEM) and release tests were used to determine the characteristics of the two phases. In particular, we found that this system is a strong hydrogel constituted by 81% (v/v) of PP phase the remaining part being occupied by the PNP phase. Pores diameters span in the range 10-100 µm, the majority of them (85%) falling in the range 30-90 µm. The high PP phase tortuosity indicates that big pores are not directly connected to each other, but their connection is realized by a series of interconnected small pores that rend the drug path tortuous. The PNP is characterized by a polymer volume fraction around 0.73 while mesh size is around 3 nm. The theoretical interpretation of the experimental data coming from the techniques panel adopted, yielded to the micro- and nano-organization of our hydrogel.

Mathematical modelling of NABD release from endoluminal gel paved stent.

Coronary restenosis consists of the partial/total re-occlusion of the artery lumen following percutaneous transluminal angioplasty (PTCA). In order to match this pathology, PTCA is followed by the implantation of rigid scaffolds (stent or coated stent) aimed to contrast the most important mechanical (coronary wall elastic recoil and late remodelling) and biological (smooth muscle cells iper-proliferation) factors leading to restenosis. In the light of the clinical problems recently arisen about the use of traditional coated stents, this paper proposes a theoretical study to comprehend the release kinetics of novel anti-proliferative drugs, i.e. nucleic acid based drugs (NABD), complexed with the proper delivery agent (DA). The release of NABD-DA is supposed to occur from a double gel layer adhering to coronary wall and embedding the stent. The proposed mathematical model assumes that diffusion, convection and cellular internalisation/metabolism are the leading mechanisms ruling drug spreading in the coronary wall. In addition, stent void fraction, positioning (totally embedded or totally out of the coronary wall) and continuous or discontinuous character of the gel layer are other three important model parameters. In order to generalise the results, stent geometry is idealised as a series of not connected, equally spaced, rings positioned in the stented zone. In correspondence of stent strut, drug transport cannot occur. The most important outcomes of this study are that, in the usual void fraction range (0.7-0.9), stent presence does not sensibly affect NABD-DA release kinetics. In addition, whereas stent positioning in the continuous gel configuration (totally embedded or totally out of coronary wall) is not very important, in the discontinuous case, it becomes relevant. Finally, this study evidences that a proper mixture of NABD complexed with different (in dimensions and kind) DA can ensure an almost constant NABD coronary concentration for several months, as requested by clinical observations.

A new heavy internal tamponade in vitreoretinal surgery: an in vitro study.

AimTo study the solubility of perfluorohexyloctane (F6H8) in silicone oil (polydimethylsiloxane (PDMS) 1000) and to measure the viscosity and the specific gravity of the mixture obtained (heavy silicone oil or HSO tamponade) to define the ideal ratio of these components.MethodsThe solubility diagram of the mixture was obtained with the turbidimetric method, indicating the miscibility of F6H8 and silicone oil 1000 at all the useful temperatures. The viscosity was measured in steady shear conditions by using a controlled stress rheometer (Haake RS150) and a double cone/plate (DC 60/4) system, both at 25 and 37 degrees C for different volume per cent compositions of the mixture. The specific gravity was measured at 37 degrees C using a digital densimeter.ResultsA mixture of F6H8 30 v% and PDMS 70 v% was found to be transparent and stable at all the useful temperatures. By combining these proportions of the two substances, a resultant density of 1.06 g/cm(3) was obtained. The viscosity of the 30% F6H8 mixture was 203 mPa.s at 25 degrees C and 163 mPa.s at 37 degrees C respectively.ConclusionsThe ideal F6H8 and silicone oil mixture can be obtained combining 30% of F6H8 with 70% of silicone oil 1000. This mixture seems to have rheological properties useful for its use as an alternative intraocular heavy tamponade.Eye (2008) 22, 1082-1088; doi:10.1038/eye.2008.144; published online 6 June 2008.

Rheological properties of aqueous Pluronic-alginate systems containing liposomes.

Rheological and erosion studies regarding a liposome-containing polymeric blend that is propaedeutic to its use in paving techniques in tubular organs, such as blood vessels, are reported. Attention is focused on an aqueous polymeric blend composed of Pluronic (PF127) and alginate (Protanal LF 10/60) because both polymers, when dissolved in water at a sufficiently high concentration, are subjected to different structural mechanisms, which are driven by temperature increase and addition of bivalent cations, respectively, and both result in marked viscoelastic and plastic properties. After proving the compatibility between PF127 and alginate, we show that the structural transition temperature of the blend, T(ST), can be properly modulated. In particular, we found that T(ST) for an aqueous solution of pure Pluronic 20% w/w is about 21 degrees C and that even slight reductions in polymer concentration result in considerable T(ST) decrease. The addition of salts or alginate (provided as Na-alginate) provokes a substantial decrease of T(ST) and thus the alginate concentration in the blend should not exceed 1% w/w. In addition, liposomes slow down the structural transition but do not substantially affect the rheological properties of the system in the final state at higher temperatures, thus showing that they can be added to the polymeric blend without significant effects. Finally, erosion tests show that after contact with a source of bivalent cations, the polymeric blend containing PF127 and alginate shows an erosion resistance neatly improved with respect to the simple structured Pluronic system having the same polymer concentration. As a whole, all these results constitute the basis for future potential applications of the considered polymeric blend in tubular organs such as blood vessels.

Rheological characterization of polysaccharide-surfactant matrices for cosmetic O/W emulsions.

Rheometrical techniques can be profitably used for polysaccharide matrices in order to evaluate their suitability for the preparation of stable cosmetic O/W emulsions. In particular, the rheological properties of aqueous scleroglucan systems were investigated under continuous and oscillatory shear conditions in a polymer concentration range (0.2-1.2% w/w) embracing the sol/gel transition. The effects due to the addition of two different surfactants (up to 10% w/w) were examined at constant polymer concentration (0.4% w/w). The selected additives are a nonionic polymeric siliconic surfactant (dimethicone copolyol) and a cationic surfactant (tetradecyltrimethylammonium bromide), respectively. Polysaccharide-surfactant interactions leading to complex formation were detected also through rheology. The combined action of both nonionic and cationic surfactants in the polymer solution was examined at two different surfactant concentration levels (5 and 10% w/w), demonstrating the beneficial effects produced on the mechanical properties of the polymer matrix by the coexistence of both surfactants. Such beneficial effects are confirmed by the stability and rheology shown by the emulsions prepared. In this way, the results point out the good agreement between the rheology of the continuous phase and the final characteristics of the emulsion obtained.

Scleroglucan/borax: characterization of a novel hydrogel system suitable for drug delivery.

A new hydrogel, with scleroglucan using borax as a crosslinker, has been prepared. The physical gel has been loaded with a model molecule (theophylline) and the release of the drug from the gel was evaluated. The same system was used to prepare tablets and the delivery of theophylline in different environmental conditions (HCl and SIF) was determined. A recent theoretical approach has been applied to the dissolution profiles obtained from the tablets and a satisfactory agreement has been found with the experimental data. Furthermore, the diffusion coefficient of the model molecule was evaluated according to a suitable strategy that was tested on two set of data obtained with different set-ups (permeation and diffusion experiments). A simplified mathematical approach allows to reduce the two-dimensional problem of the Fick's second law in a one-dimensional system leading to a much easier handling of the data without loosing the accuracy of the original problem in two dimensions. The characterization of the gel has been also carried out following the kinetics of swelling in terms of water uptake.

Experimental determination of the theophylline diffusion coefficient in swollen sodium-alginate membranes.

In this paper attention is focused on the determination of the drug diffusion coefficient in a swollen polymeric membrane referring to a recent mathematical model (linear model). The main advantage deriving from its use is that, despite its analytical nature and its ability to account for the most important aspects characterising a permeation experiment, it can also be applied in the case of thick membranes. To check the model reliability, a comparison is made with a more complex numerical model and with a largely employed model in terms of data fitting quality. To this purpose, particular care is devoted to the experimental and theoretical tools employed to calculate the auxiliary parameters required by the three models, and with the aim of getting a drug diffusion coefficient value as accurate as possible. Theophylline was chosen as model drug owing to its wide employment in the pharmaceutical field. Membranes were prepared with sodium alginates hydrogels at three different polymer concentrations. The present analysis demonstrates the reliability of the linear model and reveals that the theophylline diffusion coefficient is not significantly affected by the polymer concentration. Indeed, such a parameter is reflected in different membrane thicknesses rather than in different mesh sizes of the polymeric network.

Haemoconcentration, shear-stress increase and carotid artery diameter regulation after furosemide administration in older hypertensives.

The aim of the present study was to determine whether changes of carotid wall shear stress induced by changes in blood viscosity after diuretic administration cause carotid arterial dilatation in elderly hypertensives, as reported in the cat. Arterial wall shear rate (ultrasound technique, profilmeter FRP III), the systo-diastolic diameter (echotracking technique) and the mean blood flow velocity and volume of the common carotid artery, the blood viscosity (rotational viscometer) and the finger arterial blood pressure (Finapress Ohmeda) were measured in 12 young volunteers (aged 25+/-2 years) and in 12 elderly hypertensives (aged 80+/-4 years) treated with short-acting calcium antagonists up to 24h before the study, both at baseline and after intravenous furosemide infusion (0.5mg/min), when the haematocrit had increased by at least two percentage points. After furosemide administration the mean arterial blood pressure decreased and blood viscosity and carotid systolic shear stress increased in both groups. However, common carotid artery diameter increased only in the young controls but not in the elderly hypertensives. These data show that an increase in carotid shear stress caused by haemoconcentration induces carotid vasodilatation only in young healthy subjects, and not in elderly hypertensives. This effect may be related to impaired endothelium function and/or arterial wall mechanics.

Drug release from an ensemble of swellable crosslinked polymer particles.

This paper presents a new model suitable to describe the drug release from drug delivery systems constituted by an ensemble of drug loaded crosslinked polymer particles. The model accounts for the main factors affecting the drug release such as the particle size distribution, the physical state and the concentration profile of the drug inside the polymeric particles, the viscoelastic properties of the polymer-penetrant system and the dissolution-diffusion properties of the loaded drug. In order to check the validity of the model, release experiments were performed by using crosslinked polyvinyl-pyrrolidone (PVP) particles and two different model drugs, MAP (medroxyprogesterone acetate) and TEM (Temazepam). MAP and TEM were chosen because of their completely different dissolution behaviours in water. In particular, TEM undergoes a phase transition to the crystalline state upon dissolution when it is loaded in the polymeric network in the amorphous state. The comparison with the experimental results confirms that the most important factors determining the drug release kinetics can be properly accounted for.

Synergistic gelation of xanthan gum with locust bean gum: a rheological investigation.

Many industrial products often include in their formulation more than one polysaccharide to achieve the desired properties during and after processing. Many such mixed systems behave as would be expected from the known properties of the individual polymers. In others, however, their properties are superior to those of either component alone, or may be qualitatively different. In many polysaccharide systems, the combination of a gelling polymer with a nongelling one gives rise to strong synergistic effects, as a consequence of interaction among different chain polymers and formation of mixed junction zones. Probably, the most exploited mixed gels, especially by the food industry, are those involving the microbial polysaccharide xanthan gum (XG) and the plant galactomannans, like locust bean gum (LBG). Concentrated aqueous systems of LBG and XG display quite different rheological properties: the former show the behaviour typical of hyperentangled macromolecular solutions, whereas the flow and viscoelastic properties of XG systems correspond to those of tenuous, weak-gel networks. Interestingly, when mixed together these macromolecules interact to form a firm, thermoreversible gel with synergistic effects. In the present paper we report the results of a thorough investigation of both polymer concentration and temperature effects on the rheological properties of mixed LBG-XG systems in 20 mM KCl under continuous and oscillatory flow conditions. Under continuous shear at 25 degrees C, pure LBG shows the flow properties of a macromolecular solution, with a shear-thinning behaviour and a Newtonian region at low shear rates, whereas the rheological behaviour of XG and all LX mixed systems is that typical of weak-gels. Furthermore, in the mixed systems the viscosity values do not increase monotonically with increasing xanthan concentration, but the synergistic effect has a maximum in accordance with the XG:LBG ratio 1:1. As the temperature is increased from 25 degrees C to 85 degrees C, whilst the LBG system do not show any qualitative change but there is only a parallel, downward shift of viscosity values, in the case of xanthan there is a dramatic change in the corresponding curve profiles, due to the thermally induced helix-coil conformational transition. The differences in the rheological behaviour of the systems examined can be better shown through dynamic tests at 25 degrees C. The strain sweeps performed at constant frequency of oscillation reveal that the mixed systems show higher sensitivity to strain amplitude, and lower strain values must be attained to ensure linear viscoelastic properties. The mechanical spectra clearly show the influence of composition on the viscoelastic properties of these biopolymer systems. All LX systems show the mechanical spectra typical of polysaccharide gels: G' is always much greater than G" and is nearly independent of the applied frequency over a wide frequency range. In addition, the marked gap between the elastic responses of the pure LBG and the LX 1:3 systems demonstrates the strong effect of the initial addition of xanthan to the pure LBG, especially in the low frequency range, whereas the highest synergistic effect is attained for the LX 1:1 system. A comprehensive description of the frequency dependence of both moduli can be suitably obtained through the four-parameter Friedrich model, which belongs to the class of fractional derivative approaches viscoelasticity. The same thermal effect is observed for the XG and all LX mixed systems considered, indicating a progressive change from the behaviour of a typical gel to that of a quasi-solution state, when temperature is increased from 25 degrees C to 85 degrees C. Among all mixed systems, the LX 1:1 has the highest values of the moduli at any temperature considered, and is characterized by the highest gel-sol transition temperature. (ABSTRACT TRUNCATED)

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.