Jennifer Kadlowec

Characterizing the mechanical contribution of fiber angular distribution in connective tissue: comparison of two modeling approaches

Modeling of connective tissues often includes collagen fibers explicitly as one of the components. These fibers can be oriented in many directions; therefore, several studies have considered statistical distributions to describe the fiber arrangement. One approach to formulate a constitutive framework for distributed fibers is to express the mechanical parameters, such as strain energy and stresses, in terms of angular integrals. These integrals represent the addition of the contribution of infinitesimal fractions of fibers oriented in a given direction. This approach leads to accurate results; however, it requires lengthy calculations. Recently, the use of generalized structure tensors has been proposed to represent the angular distribution in the constitutive equations of the fibers. Although this formulation is much simpler and fewer calculations are required, such structure tensors can only be used when all the fibers are in tension and the angular distribution is small. However, the amount of error introduced in these cases of non-tensile fiber loading and large angular distributions have not been quantified. Therefore, the objective of this study is to determine the range of values of angular distribution for which acceptable differences (less than 10%) between these two formulations are obtained. It was found, analytically and numerically, that both formulations are equivalent for planar distributions under equal-biaxial stretch. The comparison also showed, for other loading conditions, that the differences decrease when the fiber distribution is very small. Differences of less than 10% were usually obtained when the fiber distribution was very low (κ ≈ 0.03; κ ranges between 0 and 1/3, for aligned and isotropic distributed fibers, respectively). This range of angular distribution greatly limits the types of tissue that can be accurately analyzed using generalized structure tensors. It is expected that the results from this study guide the selection of a proper approach to analyze a particular tissue under a particular loading condition.

Biaxial Tensile Testing and Constitutive Modeling of Human Supraspinatus Tendon

The heterogeneous composition, collagen fiber organization and mechanical properties of the supraspinatus tendon (SST) offer an opportunity for studying the structure-function relationships of fibrous musculoskeletal connective tissues. The objective of this study was to evaluate the contribution of collagen fiber organization to the planar tensile mechanics of the human SST. This was accomplished by fitting biaxial tensile data with a structural constitutive model that incorporates a sample-specific angular distribution of nonlinear fibers. Biaxial testing was employed to avoid the limitation of non-physiologic traction-free boundary conditions present during uniaxial testing. Samples were tested under a range of boundary conditions with simultaneous monitoring of collagen fiber orientation via polarized light imaging. The experimental data were input into a hyperelastic constitutive model incorporating the contributions of the uncrimped fibers. The model fit the longitudinal stresses well and was successfully validated. The transverse stresses were fit less well with greater errors observed for less aligned samples. Additional strain energy terms representing fiber-fiber interactions are likely necessary to provide closer approximation of the transverse stresses. This approach demonstrated that the longitudinal tensile mechanics of the SST are primarily dependent on the moduli, crimp, and angular distribution of its collagen fibers.

Passive cervical spine flexion: The effect of age and gender

BACKGROUND Previous studies reported passive cervical range of motion under unknown loading conditions or with minimal detail of subject positioning. Additionally, such studies have not quantitatively ensured the absence of active muscle during passive measurements. For the purpose of validating biomechanical models the loading condition, initial position, and muscle activation must be clearly defined. A method is needed to quantify the passive range of motion properties of the cervical spine under controlled loading conditions, particularly in the pediatric population where normative clinical and model validation data is limited. METHODS Healthy female pediatric (6-12years; n=10), male pediatric (6-12years; n=9), female adult (21-40years; n=10), and male adult (20-36years; n=9) volunteers were enrolled. Subjects with restrained torsos and lower extremities were exposed to a maximum 1g inertial load in the posterior-anterior direction, such that the head-neck complex flexed when subjects relaxed their neck musculature. Surface electromyography monitored the level of muscle relaxation. A multi-camera 3-D target tracking system captured passive neck flexion angle of the head relative to the thoracic spine. General estimating equations detected statistical differences across age and gender. FINDINGS Passive cervical spine flexion equaled 111.0° (SD 8.0°) for pediatric females, 102.8° (SD 7.8°) for adult females, 103.8° (SD 12.7°) for pediatric males, and 93.7° (SD 9.9°) for adult males. Passive neck flexion significantly decreased with age in both genders (P<0.01). Females exhibited significantly greater flexion than males (P<0.01). INTERPRETATION This study contributes normative data for clinical use, biomechanical modeling, and injury prevention tool development.

Particle mixtures in magnetorheological elastomers (MREs)

Magnetorheological elastomers (MREs) are state-of-the-art elastomagnetic composites comprised of magnetic particles embedded in an elastomer matrix. MREs offer enormous flexibility given that elastomers are easily molded, provide good durability, exhibit hyperelastic behavior, and can be tailored to provide desired mechanical and thermal characteristics. MRE composites combine the capabilities of traditional magnetostrictive materials with the properties of elastomers, creating a novel material capable of both highly responsive sensing and controlled actuation in real-time. This work investigates the response of MRE materials comprised of varying mixtures of 40 and 10 micron iron particles. Samples are tested in compression yielding a compressive modulus and measure of the shear stiffness via Mooney plots. Samples are also tested using a tunable vibration absorber (TVA) designed specifically for this experiment. The TVA loads the samples in oscillatory shear (10-100Hz) under the influence of a magnetic field. In all samples, results show increases in the materials stiffness under the application of a magnetic field as evidenced by the frequency response function of the TVA system. Increases in stiffness of 50% at 0.15T were achieved with samples containing 30%-40 micron particles and 30%-40micron + 2%-10 micron particles. This yields a ratio of over 300%/T. The two-particle MRE appeared not to have reached saturation suggesting further stiffness enhancement was possible beyond the saturated single-particle 40 micron sample. However, this may be a result of the larger iron content. Results also suggest variation in the behavior of two-versus single-particle MRE behavior as evidenced by the shear modulus found in compression, but results are inconclusive. MRE materials made with nanoparticles of hard magnetic barium ferrite show stiffness increases of 70%/T which is comparable to MREs having larger iron particles.

A Hyperelastic Model With Distributed Fibers to Describe Human Supraspinatus Tendon Tensile Mechanics

Tendon tissue is composed of collagen fibers in a hydrated proteoglycan matrix. Although many tendons have fibers that are highly aligned (e.g. flexor tendon), the supraspinatus tendon (SST) of the shoulder has significant distribution of fiber alignment [1]. The alignment and distribution of the fibers likely contributes to the nonlinear and anisotropic mechanical behavior, however this has not been demonstrated. Understanding the role of fiber structure on tendon mechanical behavior, that is, characterizing the structure-function relationships, is critical to evaluate the function of injured, degenerated, or healing tendons and would be invaluable in the design and assessment of tissue engineered tendon replacements. While a structurally based hyperelastic model has been developed for tendon [2], this model contained only a single fiber orientation, which is not adequate for the more distributed fiber structure in the SST. We have recently applied a hyperelastic model formulation that has distributed collagen fiber orientation developed by Gasser and colleagues for the arterial wall [3] to model a tendon analog made from nanofibrous scaffolds [4]. The objective of this study was to build on previous work to apply a hyperelastic fiber-reinforced constitutive model that includes a specific term for fiber distribution to the tensile mechanics of human SST and to evaluate the site-specific model material properties.© 2009 ASME

Visual beams: tools for statics and solid mechanics

A team of faculty and students in the College of Engineering at Rowan University are developing hands-on and visualization tools for use in mechanics courses. The developed tools consist of physical simply-supported and cantilever beams that are instrumented with load cells to which students can apply various loading conditions. Measurements from the load cells and displacement transducers are used in a Labview graphical user interface allows the user to find reaction loads and plot deflections, stresses, and shear and bending diagrams. The tools are designed to help students overcome difficulties in working with forces, moments, displacements and stresses in courses the mechanical and civil engineering courses as well as benefit students with various learning styles.

Coupled response model for elastomeric bushings

Abstract Elastomeric bushings are essential components in tuning vehicle suspension systems. A nonlinear viscoelastic force—deformation relation, which is computationally convenient for use in multibody dynamics simulations of automotive suspensions, is presented. This force—deformation relation extends a single mode force—deformation relation to include coupling effects due to combined radial—torsional deformations. The relation contains a force relaxation function, which is dependent upon the radial displacements and angular rotations imposed on the bushing. The relaxation function is constructed using experimental results from bushing tests under radial, torsional and coupled radial—torsional deformation modes. It is shown that the predictions of the proposed relation are in good agreement with experimental results.

Work in Progress - A Statics Skills Inventory

This paper focuses on assessment of student skills in statics and provides details of development of a statics skills assessment tool. The use of only concept inventories to provide proof of student learning is an incomplete assessment as important engineering knowledge consists of both conceptual knowledge and skill intertwined. A multi-step Delphi process involving a group of engineering educators was used to reach consensus on the important skills of statics. These skills are currently grouped into 10 categories. The Delphi rankings included both the average importance of the skill as judged by the Delphi participants and their judgment of the average proportion of their students whom can perform the skill. Skill-based questions are being developed to probe these areas

Katrina Recovery Clinic Project: an Engineering Service Learning Experience

This paper presents the experience of service learning in the engineering design curriculum through recovery effort projects after Hurricane Katrina. A brief context for Rowans Engineering Clinics and the mechanics and logistics of defining and executing such a project will be discussed. This type of project offers far more than technical aspects of design and fabrication; engineering management, writing, public speaking, and ethics are additional parts of the puzzle and contribute to making this a valuable experience. A range of pros, cons, and recommendations are presented as well as a new model engaging other disciplines on our campus in this endeavor. We hope that our experiences will encourage others to include this type of project as one component of the engineering curricula and service learning opportunities provided for students

Biomimetic hydrogels for tissue engineering of the intervertebral disc

Tissue engineering is a multidisciplinary field that aims to repair or regenerate lost or damaged tissues and organs in the body. One such area with significant medical applications is the degeneration of the intervertebral disc (IVD). The objective of this work is to generate a bioadhesive polymer that, in addition to bonding with tissue, can support and cell survival post-adhesion. A thermosensitive poly(N-isopropylacrylamide) (PNIPAAm) and chondroitin sulfate (CS) scaffold with aldehyde-modified CS adhesive and extracellular matrix (ECM) loaded lipid vesicles was examined as a potential minimally invasive method for repair and regeneration of degenerated IVD tissue. Samples containing varying percentages of aldehyde-modified CS and presence or absence of ECM loaded lipid vesicles were evaluated for physiological relevant performance with porcine skin. Maximum stress and work of adhesion were calculated for each polymer formulation based on force-distance data. Currently, work is being done to investigate the biocompatibility of various polymer compositions to optimize polymer blends for maximum work, stress and biocompatibility for use in vivo.