Mark A. Nicosia

The fluid mechanics of bolus ejection from the oral cavity

The squeezing action of the tongue against the palate provides driving forces to propel swallowed material out of the mouth and through the pharynx. Transport in response to these driving forces, however, is dependent on the material properties of the swallowed bolus. Given the complex geometry of the oral cavity and the unsteady nature of this process, the mechanics governing the oral phase of swallowing are not well understood. In the current work, the squeezing flow between two approaching parallel plates is used as a simplified mathematical model to study the fluid mechanics of bolus ejection from the oral cavity. Driving forces generated by the contraction of intrinsic and extrinsic lingual muscles are modeled as a spatially uniform pressure applied to the tongue. Approximating the tongue as a rigid body, the motion of tongue and fluid are then computed simultaneously as a function of time. Bolus ejection is parameterized by the time taken to clear half the bolus from the oral cavity, t(1/2). We find that t(1/2) increases with increased viscosity and density and decreases with increased applied pressure. In addition, for low viscosity boluses (mu approximately 100 cP), density variations dominate the fluid mechanics while for high viscosity boluses (mu approximately 1000 cP), viscosity dominates. A transition region between these two regimes is found in which both properties affect the solution characteristics. The relationship of these results to the assessment and treatment of swallowing disorders is discussed.

The Relationship of Normal and Abnormal Microstructural Proliferation to the Mitral Valve Closure Sound

BACKGROUND Many diseases that affect the mitral valve are accompanied by the proliferation or degradation of tissue microstructure. The early acoustic detection of these changes may lead to the better management of mitral valve disease. In this study, we examine the nonstationary acoustic effects of perturbing material parameters that characterize mitral valve tissue in terms of its microstructural components. Specifically, we examine the influence of the volume fraction, stiffness and splay of collagen fibers as well as the stiffness of the nonlinear matrix in which they are embedded. METHODS AND RESULTS To model the transient vibrations of the mitral valve apparatus bathed in a blood medium, we have constructed a dynamic nonlinear fluid-coupled finite element model of the valve leaflets and chordae tendinae. The material behavior for the leaflets is based on an experimentally derived structural constitutive equation. The gross movement and small-scale acoustic vibrations of the valvular structures result from the application of physiologic pressure loads. Material changes that preserved the anisotropy of the valve leaflets were found to preserve valvular function. By contrast, material changes that altered the anisotropy of the valve were found to profoundly alter valvular function. These changes were manifest in the acoustic signatures of the valve closure sounds. Abnormally, stiffened valves closed more slowly and were accompanied by lower peak frequencies. CONCLUSION The relationship between stiffness and frequency, though never documented in a native mitral valve, has been an axiom of heart sounds research. We find that the relationship is more subtle and that increases in stiffness may lead to either increases or decreases in peak frequency depending on their relationship to valvular function.

Dynamic Finite Element Implementation of Nonlinear, Anisotropic Hyperelastic Biological Membranes

We present a novel method for the implementation of hyperelastic finite strain, non-linear strain-energy functions for biological membranes in an explicit finite element environment. The technique is implemented in LS-DYNA but may also be implemented in any suitable non-linear explicit code. The constitutive equations are implemented on the foundation of a co-rotational uniformly reduced Hughes-Liu shell. This shell is based on an updated-Lagrangian formulation suitable for relating Cauchy stress to the rate-of-deformation, i.e. hypo -elasticity. To accommodate finite deformation hyper -elastic formulations, a co-rotational deformation gradient is assembled over time, resulting in a formulation suitable for pseudo-hyperelastic constitutive equations that are standard assumptions in biomechanics. Our method was validated by comparison with (1) an analytic solution to a spherically-symmetric dynamic membrane inflation problem, incorporating a Mooney-Rivlin hyperelastic equation and (2) with previously published finite element solutions to a non-linear transversely isotropic inflation problem. Finally, we implemented a transversely isotropic strain-energy function for mitral valve tissue. The method is simple and accurate and is believed to be generally useful for anyone who wishes to model biologic membranes with an experimentally driven strain-energy function.

The Usefulness of the Line Spread Test as a Measure of Liquid Consistency

Although dietary modification is a common treatment strategy used to manage dysphagic patients who aspirate thin liquids, there are no standard definitions for thickened liquid preparation. This lack of standardization leads to variability in practice and points to the need for a simple tool for clinicians to assess thickened liquid consistency. The current study analyzed the utility of the Line Spread Test (LST) in this regard. Twenty-six liquids (10 powder-thickened “nectar” juices, 10 powder-thickened “honey” juices, and 6 barium mixtures) were assessed using both a viscometer for objective measurement of viscosity and the LST. Whereas the LST was able to separate the juices into nectar and honey categories, it was not able to separate barium mixtures into these categories nor compare barium to juices. Furthermore, the LST was not predictive of viscosity. Thus, the results of the current study suggest that the LST may be useful in the broad categorization of fluids into therapeutically significant groupings but that it cannot be used more specifically to measure fluid viscosity. Further studies of this and other tools are necessary to identify inexpensive practical tools for quantification of thickened liquid consistency.

The Effects of Intraoral Pressure Sensors on Normal Young and Old Swallowing Patterns

Lingual pressure generation plays a crucial role in oropharyngeal swallowing. To more discretely study the dynamic oropharyngeal system, a 3-bulb array of pressure sensors was designed with the Kay Elemetrics Corporation (Lincoln Park, NJ). The influence of the device upon normal swallowing mechanics and boluses representative of flow relative to age and bolus condition was the focus of this study. Twelve healthy adults in two age groups (31 ± 5 years, 2 males and 4 females, and 78 ± 7 years, 2 males and 4 females) participated. Each subject was instructed to swallow four boluses representative of conditions with and without three pressure sensors affixed to the hard palate. Postswallow residue at four locations, Penetration/Aspiration Scale scores, and three bolus flow timing measures were assessed videofluoroscopically with respect to age and bolus condition. The only statistically significant influences attributable to the presence of the pressure sensors were slight increases in residue in the oral cavity and upper esophageal sphincter with some bolus consistencies, 8% more frequent trace penetration of the laryngeal vestibule predominantly with effortful swallowing, and variances in oral clearance duration. We conclude that the presence of the pressure sensors does not significantly alter normal swallowing patterns of healthy individuals.

A design backbone for the biomedical engineering curriculum

In this paper, we summarize our experiences as advisors supervising biomedical engineering design projects in the design backbone of our curriculum, the six-semester design course sequence required for all biomedical engineering majors at the University of Wisconsin-Madison.

A theoretical framework to analyze bend testing of soft tissue.

It has been hypothesized that repetitive flexural stresses contribute to the fatigue-induced failure of bioprosthetic heart valves. Although experimental apparatuses capable of measuring the bending properties of biomaterials have been described, a theoretical framework to analyze the resulting data is lacking. Given the large displacements present in these bending experiments and the nonlinear constitutive behavior of most biomaterials, such a formulation must be based on finite elasticity theory. We present such a theory in this work, which is capable of fitting bending moment versus radius of curvature experimental data to an arbitrary strain energy function. A simple finite element model was constructed to study the validity of the proposed method. To demonstrate the application of the proposed approach, bend testing data from the literature for gluteraldehyde-fixed bovine pericardium were fit to a nonlinear strain energy function, which showed good agreement to the data. This method may be used to integrate bending behavior in constitutive models for soft tissue.

Coupled fluid-structure finite element modeling of the aortic valve and root

The objective was to develop a three-dimensional coupled fluid-structure computational model of the aortic valve and root. The long-term goal is to develop a framework for quantitative noninvasive evaluation, and design of new therapeutic treatment options for aortic valve and root disease. A 3-D geometric model was developed based on MR images of normal human valves, including normal variations in collagen fiber orientation and leaflet thickness. The aortic root and leaflets were modeled with linear elastic material properties. A dynamic analysis was performed using the LS-DYNA explicit finite element package. A complete opening/closing cycle of the aortic valve was successfully simulated. Leaflet motion was consistent with that seen in intact hearts. Thus, we have developed the most advanced 3-D coupled fluid-structure finite element model of the aortic valve and root to date. Our ultimate goal is to apply such models to evaluate disease severity, determine optimal timing for intervention, and guide surgical techniques.