Luis Dorfmann

Silk-Based Electrospun Tubular Scaffolds for Tissue-Engineered Vascular Grafts

Electrospinning was used to fabricate non-woven nanofibrous tubular scaffolds from Bombyx mori silk fibroin using an all aqueous process. Cell studies and mechanical characterization tests were performed on the electrospun silk tubes to assess the viability of their usage in bioengineering small-diameter vascular grafts. Human endothelial cells and smooth muscle cells were successfully cultured on the electrospun silk. Mechanical characterization tests demonstrated burst strength sufficient to withstand arterial pressures and tensile properties comparable to native vessels. These cellular and mechanics outcomes demonstrate potential utility of these electrospun silk scaffolds for small-diameter vascular grafts.

Nonlinear theory of electroelastic and magnetoelastic interactions

Introduction.- Electromagnetic Theory.- Nonlinear Elasticity Background.- Nonlinear Electroelastic Interactions.- Electroelastic Boundary-Value Problems.- Nonlinear Magnetoelastic Interactions.- Magnetoelastic Boundary-Value Problems.- Variational Formulations in Electroelasticity and Magnetoelasticity.- Incremental Equations.- Electroelastic Stability.- Magnetoelastic Wave Propagation.- Basic Vector and Tensor Operations.- Index.

Directional Differences in the Biaxial Material Properties of Fascia Lata and the Implications for Fascia Function

Fascia is a highly organized collagenous tissue that is ubiquitous in the body, but whose function is not well understood. Because fascia has a sheet-like structure attaching to muscles and bones at multiple sites, it is exposed to different states of multi- or biaxial strain. In order to measure how biaxial strain affects fascia material behavior, planar biaxial tests with strain control were performed on longitudinal and transversely oriented samples of goat fascia lata (FL). Cruciform samples were cycled to multiple strain levels while the perpendicular direction was held at a constant strain. Structural differences among FL layers were examined using histology and SEM. Results show that FL stiffness, hysteresis, and strain energy density are greater in the longitudinal vs. transverse direction. Increased stiffness in the longitudinal layer is likely due to its greater thickness and greater average fibril diameter compared to the transverse layer(s). Perpendicular strain did not affect FL material behavior. Differential loading in the longitudinal vs. transverse directions may lead to structural changes, enhancing the ability of the longitudinal FL to transmit force, store energy, or stabilize the limb during locomotion. The relative compliance of the transverse fibers may allow expansion of underlying muscles when they contract.

A constitutive description of the anisotropic response of the fascia lata.

In this paper we propose a constitutive model to analyze in-plane extension of goat fascia lata. We first perform a histological analysis of the fascia that shows a well-organized bi-layered arrangement of undulated collagen fascicles oriented along two well defined directions. To develop a model consistent with the tissue structure we identify the absolute and relative thickness of each layer and the orientation of the preferred directions. New data are presented showing the mechanical response in uniaxial and planar biaxial extension. The paper proposes a constitutive relation to describe the mechanical response. We provide a summary of the main ingredients of the nonlinear theory of elasticity and introduce a suitable strain-energy function to describe the anisotropic response of the fascia. We validate the model by showing good fit of the numerical results and the experimental data. Comments are included about differences and analogies between goat fascia lata and the human iliotibial band.

Nonlinear electroelasticity: material properties, continuum theory and applications

In the last few years, it has been recognized that the large deformation capacity of elastomeric materials that are sensitive to electric fields can be harnessed for use in transducer devices such as actuators and sensors. This has led to the reassessment of the mathematical theory that is needed for the description of the electromechanical (in particular, electroelastic) interactions for purposes of material characterization and prediction. After a review of the key experiments concerned with determining the nature of the electromechanical interactions and a discussion of the range of applications to devices, we provide a short account of the history of developments in the nonlinear theory. This is followed by a succinct modern treatment of electroelastic theory, including the governing equations and constitutive laws needed for both material characterization and the analysis of general electroelastic coupling problems. For illustration, the theory is then applied to two simple representative boundary-value problems that are relevant to the geometries of activation devices; in particular, (a) a rectangular plate and (b) a circular cylindrical tube, in each case with compliant electrodes on the major surfaces and a potential difference between them. In (a), an electric field is generated normal to the major surfaces and in (b), a radial electric field is present. This is followed by a short section in which other problems addressed on the basis of the general theory are described briefly.

Modelling of residually stressed materials with application to AAA

Residual stresses are generated in living tissues by processes of growth and adaptation and they significantly influence the mechanical behaviour of the tissues. Thus, to effectively model the elastic response of the tissues relative to a residually stressed configuration the residual stresses need to be incorporated into the constitutive equations. The purposes of this paper are (a) to summarise a general elastic constitutive formulation that includes residual stress, (b) to specify the tensors needed for the three-dimensional implementation of the theory in a nonlinear finite element code, and (c) to use the theory and its implementation to evaluate the wall stress distribution in an abdominal aortic aneurysm (AAA) using patient specific geometry and material model parameters. The considered material is anisotropic with two preferred directions indicating the orientation of the collagen fibres in the aortic tissue. The method described in this paper is general and can be used, by specifying appropriate energy functions, to investigate other residually stressed biological systems.

Gas Storage and Operations in Single-Bedded Salt Caverns: Stability Analyses

Bedded salt formations are located throughout the United States, providing valuable storage capacity for natural gas and other hydrocarbons. In order to increase gas storage capabilities and provide operators with improved geotechnical design and operating guidelines for these caverns, stability analyses of single bedded salt cavern have been completed and are described in this paper. This work is a part of integrated

The elastic secrets of the chameleon tongue

The ballistic projection of the chameleon tongue is an extreme example of quick energy release in the animal kingdom. It relies on a complicated physiological structure and an elaborate balance between tissue elasticity, collagen fibre anisotropy, active muscular contraction, stress release and geometry. A general biophysical model for the dynamics of the chameleon tongue based on large deformation elasticity is proposed. The model involves three distinct coupled subsystems: the energetics of the intralingual sheaths, the mechanics of the activating accelerator muscle and the dynamics of tongue extension. Together, these three systems elucidate the key physical principles of prey-catching among chameleonides.

Nonlinear mechanics of soft fibrous materials

Nonlinear elasticity with applications to soft fibre-reinforced materials.- Porous materials with statistically oriented reinforcing fibres.- Nonlinear elasticity for soft fibrous materials.- Modeling of bioactive materials.- Incremental equations for soft fibrous materials.- Effects of fibre bending and twisting resistance on the mechanics of fibre-reinforced elastomers.

Modeling Dynamic Site Response Using the Overlay Concept

Finite element programs allow for an enhancement in the computational capabilities of earthquake site response models. However, many finite element programs involve complicated constitutive models that are difficult for the end user to implement. We present a methodology for modeling earthquake site response within a general finite element framework, using an overlay model to represent nonlinear soil behavior. Using parallel load-carrying elements with varying stiffness and yield stress, the behavior of any given backbone stress-strain relation can be replicated, along with hysteretic unloading-reloading (Masing) behavior. Our finite element modeling methodology makes use of existing conventional elastoplastic material models available in any general finite element program, without requiring the specification of any complicated constitutive models. To represent overlay elements in a finite element model, the user defines a number of finite elements and assigns each of them identical node numbers. The only necessary input parameters are density, elastic constants, and the backbone curve of a stress-strain relation. This paper focuses on the application of one-dimensional total-stress site response, but the framework could be easily extended to model cyclic hardening and softening, three- dimensional wave propagation, and soil-structure interaction.