Mitchell Litt

Rheology of the vitreous body. Part I: Viscoelasticity of human vitreous.

The rheological properties of the vitreous body of the eye are believed to be a function of composition and to differ among species, as well as to vary regionally within the vitreous. These properties are essential to the mechanical functioning of the eye. Although there are gross, qualitative data on vitreous rheology available in the literature, quantitative rheological measurements on human vitreous and on eyes of other species are sparse and incomplete. The aim of the research reported in this series of papers is to study the rheological behavior of human, bovine, and porcine vitreous, to measure the macromolecular and electrolyte content of these samples, and to correlate and compare these values for the different species as a function of location in the vitreous. In this paper, the rheological model used to correlate viscoelasticity of the vitreous is presented, and a detailed description of the rheological instrumentation and methods of analysis used is given. Data on the rheological properties of human vitreous, as a function of location within the eye, are presented. The results show that in the human eye there are significant differences in a number of the viscoelastic parameters as a function of location within the vitreous body.

Mass transport from a rotating disk into power-law liquids

Abstract Both theoretical and experimental studies on the solid-liquid mass transfer from a rotating disk to non-Newtonian solutions are reported. The rates of dissolution of benzoic acid and β-naphthol into aqueous solutions of carboxymethylcellulose, and of benzoic acid into polyethylene oxide, have been determined. An expression for the mass transfer has been derived in terms of the system parameters and the power-law constants of the liquid. At high rotational speeds the slope of the experimental mass transfer curve is predicted by the theory. At lower rotational speeds, secondary flows were observed about the rotating disk producing anomalous mass transfer effects. The nature of these flows, together with their relation to the mass transfer in non-Newtonian liquids, is discussed.

Human cervical mucus. I. Rheologic characteristics.

The viscoelastic properties of human cervical mucus were subjected to detailed characterization by microrheometry. An introduced to the range of parameters that can be measured and the sensitivity of microroheometry to variations in mucus viscoelasticity was presented, along with an interpretation of results obtained by this approach. Fresh cervical mucus samples were shown to be rheologically heterogeneous, reflecting compositional differences in situ. Mucas viscoelasticity was stable for several hours when samples were stored at ambient temperature in the microrheometer sample holder. Mucus viscoelasticity was strongly dependent upon nondialyzable solids (NDS) concentration. In order to eliminate (NDS) as a variable in comparative studies of mucus, procedures were developed for reconstitution of mucus at identical (NDS) or for normalization of viscoelasticity data derived from fresh samples to 2.5% NDS. The validity of this approach was then examined by applying these procedures to mucus collected during the follicular, ovulatory, and luteal phases of the normal menstrual cycle.

Human Cervical Mucus. II. Changes in Viscoelasticity During the Ovulatory Menstrual Cycle

Quantitative viscoelasticity measurements were made on individual human cervical mucus samples by microrheometry. Increases in mean values for mucus spinnbarkeit, ferning, and wet weight were associated with the ovulatory phase of the menstrual cycle, while no significant differences versus time were noted for mean values for sample pH or nondialyzable dry weight. A nadir in mucus nondialyzable solids (NDS) concentration and in visoelasticity was seen at or near midcycle. Substantial, highly reproducible, variations in mucus viscoelasticity were observed when mucus from different donors was compared. When the contribution of (NDS) to viscoelasticity was minimized by data normalization or by sample reconstitution, a significant increase in viscoelasticity was associated with the ovulatory phase of the cycle, suggesting the occurrence of a relative increase in mucin concentration or a compositional change in the mucus.

An electrochemical study of flow instability on a rotating disk

Data for the fluctuating mass-transfer coefficient to point electrodes on the surface of a rotating disk are presented for transition and turbulent flow, along with analysis of the energy spectrum. The results corroborate previous evidence of standing vortices on the surface in the transition region. It is shown that the electrochemical method gives a more sensitive probe of flow instabilities than has been possible previously; the transition region is found to be wider than has been previously thought to be and lies between roughly Re = 1·7 × 10 5 and 3·5 × 10 5 , where Re = r 2 ω/ v is the Reynolds number, ω being the angular velocity of the disk (in rad/s) and v the kinematic viscosity. As the Reynolds number is increased, the stationary vortices in the transition region propagate primarily with a frequency matching that of the disk speed. After a peak energy has been reached at Re = 2·6 × 10 5 , the vortices break down into a fully developed turbulent flow.

Chemical reactions on a rotating disk

Abstract The theoretical solution for the rate of a rapid, irreversible two-component chemical reaction is extended to the case of laminar flow on a rotating disk. The rate is expressed in terms of a reaction factor, that is, the ratio of mass transfer with reaction to that with no reaction. The equations of momentum and mass transport have been programmed for an analogue computer so that the reaction factor may be calculated for any combination of reactant Schmidt numbers, concentrations, and inter-facial velocity. The model is easily extended to include rapid catalytic surface reactions and combustion. An experimental system for investigating the rate of a simple reaction of this type, an acid-base neutralization, is described. Data were obtained for dissolution of benzoic acid in water and in sodium hydroxide solutions. Mass transfer data without reaction were also obtained for cinnamic acid and for beta-naphthol in water and for naphthalene in air. The benzoic acid and the naphthalene data agree well with the theoretical solution for no reaction. The reaction data also agree very well with the predicted rates. However, the cinnamic acid and beta-naphthol fail to agree with theory even for simple dissolution.

Effect of mucolytic agents on the rheological properties of tracheal mucos

Canine tracheal mucus was dissolved by a number of mucolytic agents, including disulfide bond reducing agents, hydrogen bond breaking agents, and chaotropic ions, and their effect on rheological properties was assessed. Sodium thiocyanate led to 85-100% dissolution with the maximum retention of elasticity. Thiocyanate exposure did not result in demonstrable alterations in the size or shape of the mucus glycoproteins. Sodium thiocyanate is therefore recommended as a suitable dispersing agent for physiochemical studies of glycoprotein secretions.

Functional aspects of mucus rheology.

Mucosal epithelia from all sources are always coated with mucus. Although specific properties of such secretions vary among organ tracts, there are overall chemical and physical similarities which, presumably, reflect a common function. The four types of mucus secretions that have been studied extensively — salivary, gastric, respiratory and cervical — serve a protective function. To this end, the mucus must be dispersed in a thin layer. In addition, specific roles for mucus include particulate clearance and maintenance of proper water balance in the respiratory system, and acting as a barrier and/or reservoir for sperm in the cervix. In order to carry out these functions, the mucus must have particular physical properties that are maintained within suitable bounds. Discussions of the source of mucus secretions and their control are presented in other papers of this symposium.

Educational Methods and Best Practices in BME Laboratories 1

Biomedical engineering (BME) is a practical discipline and laboratory courses that teach the practice of this discipline are an integral component of an effective undergraduate curriculum. Laboratory courses provide students with the opportunity to observe how the physical world compares to the quantitative descriptions of that world taught in the classroom. They also should provide students with the skills necessary for enhancing our descriptions of the physical world, for developing the tools required to interact with that world on a variety of scales, for designing experiments to accomplish these goals and for effectively communicating the results of these experiments. The purpose of this document is to outline the critical components of an effective BME laboratory curriculum and teaching methodologies most appropriate for delivering the content of that curriculum. The content is based upon a white paper submitted in advance of the second Biomedical Engineering Education Summit (BEES II) sponsored by the Whitaker Foundation, and on the discussions that occurred at that summit. BME is a broad discipline that integrates knowledge from the physical, chemical, and engineering sciences for application to the study of biology and medicine. The challenge in developing an undergraduate BME curriculum is determining how to provide students with the breadth required to understand the interdependence of these disciplines as well as the depth necessary to apply the acquired knowledge throughout their careers. Laboratory courses can provide hands-on exposure to the practice of BME. However, because of limited time and resources, it is not possible to cover the tools and techniques spanning the entire field in any undergraduate curriculum. Hence, it is imperative that laboratory courses deliver a foundation to support the skills

Human cervical mucus v. oral contraceptives and mucus rheologic properties.

Mucus viscoelasticity on individual samples obtained from patients using combination oral contraceptives was quantitated by microrheometry. These results, in conjunction with mucus chemical characterization, indicate that combination oral contraceptive use eliminates the cyclic variations in mucus chemical, physicochemical, and rheologic properties associated with the ovulatory menstrual cycle. A correlation was demonstrated between the mucus elastic modulus and mucus nondialyzable dry weight, and the mucins produced during oral contraceptive therapy were shown to be similar to those recovered from ovulatory donors. Differences in mucus properties were noted when donors using estrogenic contraceptives were contrasted with those using androgenic contraceptives. On the basis of established relationships between sperm penetrability and mucus solids content, it was concluded that the use of contraceptives, as examined in this study, provided a secondary degree of fertility control at the cervical level.