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At least 19 recordsLinked to original sources
PubMed2003

Limitations of the rheological mucoadhesion method: the effect of the choice of conditions and the rheological synergism parameter.

This work demonstrates several limitations of the simple rheological method that is widely used to investigate mucoadhesion of polymer gels. We establish the importance of the choice of conditions and the synergism parameter for the results obtained in comparative studies. Dynamic rheological measurements were performed on gels based on four slightly different poly(acrylic acid) (Carbopol) polymers and their corresponding mixtures with porcine stomach mucin and bovine submaxillary gland mucin. The rationale for the comparison of the polymers had a large influence on the results obtained. The method does not give the same ranking order when two different comparison strategies are used. Moreover, we show that the results obtained are also sensitive to where in the 'rheological range' the comparison is made, e.g., at which value of G'. Positive values of the synergism parameters are, for example, only seen with weak gels. The choice of synergism parameter also has a bearing on the results obtained, and here we suggest a new refined relative parameter, the log ratio (log(G'(mix)/G'(p))). We also investigated the adhesion of the gel preparations to porcine nasal mucosa, using tensile strength measurements. Increased gel strength resulted in stronger adhesion, which is in contrast to the results from the rheological method, where the positive values of the synergism parameters were seen only with weak gels. On the basis of the limitations demonstrated and discussed, we recommend that the rheological method should not be used as a stand-alone method for the studying of mucoadhesive properties of polymer gels.

Acrylic Resins↗
PubMed2002

Examination of the flow rheological and textural properties of polymer gels composed of poly(methylvinylether-co-maleic anhydride) and poly(vinylpyrrolidone): rheological and mathematical interpretation of textural parameters.

The purpose of this study was to mathematically characterize the effects of defined experimental parameters (probe speed and the ratio of the probe diameter to the diameter of sample container) on the textural/mechanical properties of model gel systems. In addition, this study examined the applicability of dimensional analysis for the rheological interpretation of textural data in terms of shear stress and rate of shear. Aqueous gels (pH 7) were prepared containing 15% w/w poly(methylvinylether-co-maleic anhydride) and poly(vinylpyrrolidone) (PVP) (0, 3, 6, or 9% w/w). Texture profile analysis (TPA) was performed using a Stable Micro Systems texture analyzer (model TA-XT 2; Surrey, UK) in which an analytical probe was twice compressed into each formulation to a defined depth (15 mm) and at defined rates (1, 3, 5, 8, and 10 mm s(-1)), allowing a delay period (15 s) between the end of the first and beginning of the second compressions. Flow rheograms were performed using a Carri-Med CSL(2)-100 rheometer (TA Instruments, Surrey, UK) with parallel plate geometry under controlled shearing stresses at 20.0 degrees +/- 0.1 degrees C. All formulations exhibited pseudoplastic flow with no thixotropy. Increasing concentrations of PVP significantly increased formulation hardness, compressibility, adhesiveness, and consistency. Increased hardness, compressibility, and consistency were ascribed to enhanced polymeric entanglements, thereby increasing the resistance to deformation. Increasing probe speed increased formulation hardness in a linear manner, because of the effects of probe speed on probe displacement and surface area. The relationship between formulation hardness and probe displacement was linear and was dependent on probe speed. Furthermore, the proportionality constant (gel strength) increased as a function of PVP concentration. The relationship between formulation hardness and diameter ratio was biphasic and was statistically defined by two linear relationships relating to diameter ratios from 0 to 0.4 and from 0.4 to 0.563. The dramatically increased hardness, associated with diameter ratios in excess of 0.4, was accredited to boundary effects, that is, the effect of the container wall on product flow. Using dimensional analysis, the hardness and probe displacement in TPA were mathematically transformed into corresponding rheological parameters, namely shearing stress and rate of shear, thereby allowing the application of the power law (eta = kgamma(n)) to textural data. Importantly, the consistencies (k) of the formulations, calculated using transformed textural data, were statistically similar to those obtained using flow rheometry. In conclusion, this study has, firstly, characterized the relationships between textural data and two key instrumental parameters in TPA and, secondly, described a method by which rheological information may be derived using this technique. This will enable a greater application of TPA for the rheological characterization of pharmaceutical gels and, in addition, will enable efficient interpretation of textural data under different experimental parameters.

Adhesiveness↗
PubMed1973

A three-dimensional analytical (rheological) model of the human left ventricle in passive-active states. Nontraumatic determination of the in vivo values of the rheological parameters.

In this paper a three-dimensional continuum model of a mammalian left ventricle is formulated. The stresses in the model satisfy the conditions of zero stress on the outer (epicardial surface-representing) boundary. The strains of the model are obtained from the actual dynamic geometry measurements (obtained from cineangiocardiography). Since the left ventricular muscle is incompressible, the dilatational strain is zero and hence the (three-dimensional) deviatric stress components are related to the corresponding strain components by Maxwell and Voigt rheological model analogues of one-dimensional systems; the parameters of the model are series and parallel elastic (SE, PE) elements and the contractile element (CE) (representing the sarcomere). The incorporation of the rheological features of the cardiac muscle into the three-dimensional constitutive equations (for the three-dimensional continuum model of the left ventricle) is a feature of this paper. A procedure is presented to determine the parameters of the constitutive equations (i.e., the SE, PE, and the parameters of the force-velocity relation for the CE) for the left ventricle of a subject from data on the dimensions and chamber pressure of the left ventricle. The values of these parameters characterize the rheology of the left ventricular muscle of the subject. In order to demonstrate clinical application of the analyses, in vivo data of the subjects' left ventricular pressure and dimensions are obtained, and the analyses are applied to the data to determine (for each subject) the values and characteristics of the elastic elements and CEs.

Angiography↗
PubMed1994

Rheological studies of creams. I. Rheological functions and structure of creams.

Large number of washable (o/w type) creams were prepared for rheological investigation. The rheological functions known from the literature were determined in our studies. Rheological constants were determined by measurements and calculations. From these, we selected those ones which were applicable to characterize the energy status of the coherent structure and which gave the most information for practical work, elaboration of composition and evaluation of stability. These functions and parameters are the following: flow curves, viscosity vs shear time and viscosity vs temperature functions, Bingham-type yield value, plastic viscosity, structure breakdown rate constant, activation energy.

Ointments↗
PubMed1977

Rheological studies of matrix--special reference on the mobility of leucocytes in relation to the rheological characters of experimental matrix.

The rheological characteristics of a model matrix, consisting of Bacto-agar, were correlated with the ability of the matrix to permit cellular penetration, using a rheolometer and the agar bullet method. The most important rheologic characteristic that affected cellular mobility was found to be the viscosity rate. A viscosity rate under 1,000 X 10(4) dyne sec/cm2 was needed to permit cellular mobility.

Adhesiveness↗
PubMed1993

[Formulation of diazepam suppositories, results of rheological and biopharmaceutical studies. 1. Rheological study of the used suppository bases].

The rheological properties of four lipophilic and three hydrophilic suppository bases were studied. The flow and viscosity curves, the initial, equilibrium and plastic viscosity as well as the Bingham's yield value of the systems were determined. It was found that the viscosity curves had two different parts in the examined shearing rate range: the part of greater rise described a definitive destruction of the system, while the part of smaller rise represented an equilibrium state. The values of the rheological parameters of the lipophilic and hydrophilic systems showed a definite difference, which was explained by the greater association of the hydrophilic bases.

Diazepam↗
PubMed1981

Comparative rheology of nucleated and non-nucleated red blood cells. II. Rheological properties of avian red cells suspensions in narrow capillaries.

The flow properties of nucleated avian (duck) red cell suspensions were determined in narrow glass capillaries (internal diameter 5- 11 micrometer) perfused at constant pressure. Measurements were carried out of cellular and suspending fluid flow velocity of dynamic (tube) hematocrit. Comparison with previous measurements on human erythrocyte suspensions showed that the dynamic hematocrit reduction (Fahraeus effect) of avian cell suspensions was significantly more sensitive to changes of hematocrit than observed on the human cells. It was in addition found that the Fahraeus effect is the dominating phenomenon determining the viscosity of the nucleated cell suspensions, in contrast to suspensions of non-nucleated human red cells. Furthermore, the apparent viscosity of the nucleated cell suspensions is significantly higher than that of human blood at equal capillary diameter and tube hematocrit. In the living capillary bed of birds, the unfavourable rheological properties of the nucleated red cells could be compensated for by a low capillary hematocrit and/or a higher capillary density compared to mammalian species.

Animals↗
PubMed1988

Cutaneous microcirculation and blood rheology following cardiopulmonary bypass. Laser Doppler flowmetric and blood cell rheologic studies.

In 23 patients undergoing coronary artery bypass grafting, measurements of cutaneous blood flow were made with laser doppler flowmetry. Simultaneously blood was sampled for measurement of red cell filtration rate (RFR) and plasma-white cell filtration rate (P-WFR). The cutaneous blood flow showed significant overall reduction postoperatively. When the saphenous vein or internal mammary artery was used as bypass graft, the reduction in skin blood flow at the sites from which the vessels were taken was significantly greater than in contralateral, undisturbed sites. RFR and P-WFR were also significantly reduced postoperatively, and these changes showed significant concomitance with the fall in laser doppler flow (LDF%). On postoperative day 6 there was some improvement in LDF% and RFR but further slight deterioration in P-WFR. The study indicated that surgical trauma locally reduces cutaneous blood flow and that trauma to blood cells following cardiopulmonary bypass can contribute to this reduction.

Cardiopulmonary Bypass↗