Tubular Nitinol Knit Meshes for External Reinforcement of Saphenous Vein Grafts: A Numerical Design Study

The difference in mechanical properties between grafts and host arteries is a complicating factor for vascular bypass surgery and can cause patho-physiological problems after implantation [1–7]. Diffuse and focal intimal hyperplasia, one of the key factors of vein graft failure, has been attributed to over-distension and diametric irregularities of the veins when exposed to the arterial circulation [8]. The external reinforcement of saphenous vein grafts with open-mesh knitted Nitinol structures is suggested to prevent over-distension, smooth the luminal diameter, and address the mismatch in mechanical properties of vein graft and host vessel. The objectives of this work were: 1) development of Finite Element (FE) models of knitted Nitinol structures to assess mechanical behaviour and structural properties, e.g. vascular compliance, and 2) proof of feasibility of the FE method developed for structural design optimisation of the Nitinol mesh.

Simulated Compensatory Motion of Transradial Prostheses

The prostheses used by the majority of persons with hand/arm amputations today have a very limited range of motion. Transradial (below the elbow) amputees lose the three degrees of freedom provided by the wrist and forearm. Some myoeletric prostheses currently allow for forearm pronation and supination (rotation about an axis parallel to the forearm) and the operation of a powered prosthetic hand. Older body-powered prostheses, incorporating hooks and other cable driven terminal devices, have even fewer degrees of freedom. In order to perform activities of daily living (ADL), a person with amputation(s) must use a greater than normal range of movement from other body joints to compensate for the loss of movement caused by the amputation. By studying the compensatory motion of prosthetic users we can understand the mechanics of how they adapt to the loss of range of motion in a given limb for select tasks. The purpose of this study is to create a biomechanical model that can predict the compensatory motion using given subject data. The simulation can then be used to select the best prosthesis for a given user, or to design prostheses that are more effective at selected tasks, once enough data has been analyzed. Joint locations necessary to accomplish the task with a given configuration are calculated by the simulation for a set of prostheses and tasks. The simulation contains a set of prosthetic configurations that are represented by parameters that consist of the degrees of freedom provided by the selected prosthesis. The simulation also contains a set of task information that includes joint constraints, and trajectories which the hand or prosthesis follows to perform the task. The simulation allows for movement in the wrist and forearm, which is dependent on the prosthetic configuration, elbow flexion, three degrees of rotation at the shoulder joint, movement of the shoulder joint about the sternoclavicular joint, and translation and rotation of the torso. All joints have definable restrictions determined by the prosthesis, and task.

A Mathematical Model of Parathyroid Hormone Response to Acute Changes in Plasma Ionized Calcium Concentration in Humans

A complex bio-mechanism, referred to as calcium homeostasis, regulates plasma ionized calcium (Ca++) concentration in the human body to within a narrow physiologic range which is crucial for maintaining normal physiology and metabolism. In this paper we present a qualitative model of the calcium homeostatic system and then focus on a particular sub-system, termed Ca-PTH axis. We consider the dynamics of the axis involving the response of the parathyroid glands to acute changes in plasma Ca++ concentration. We use a two-pool, linear time-varying model to describe the Ca-PTH axis. We show that this model, parameterized using a guided iterative parametrization scheme and induced hypocalcemic clamp test data, successfully predicts dynamics observed in clinical tests of induced hypercalcemia in normal humans.

Effect of Inhaling Patterns on Aerosol Drug Delivery: CFD Simulation

Inhaled Pharmaceutical Aerosols (IPAs) delivery has great potential in treatment of a variety of respiratory diseases, including asthma, pulmonary diseases, and allergies. Aerosol delivery has many advantages. It delivers medication directly to where it is needed and it is effective in much lower doses than required for oral administration. Currently, there are several types of IPA delivery systems, including pressurized metered dose inhaler (pMDI), the dry powder inhaler (DPI), and the medical nebulizer. IPAs should be delivered deep into the respiratory system where the drug substance can be absorbed into blood through the capillaries via the alveoli. Researchers have proved that most aerosol particles with aerodynamic diameter of about 1–5 μm, if slowly and deeply inhaled, could be deposited in the peripheral regions that are rich in alveoli [1–3]. The purpose of this study is to investigate the effects of various inhaling rates with breath-holding pause on the aerosol deposition (Dp = 0.5–5 μm) in a human upper airway model extending from mouth to 3rd generation of trachea. The oral airway model is three dimensional and non-planar configurations. The dimensions of the model are adapted from a human cast. The air flow is assumed to be unsteady, laminar, and incompressible. The investigation is carried out by Computational Fluid Dynamics (CFD) using the software Fluent 6.2. The user-defined function (UDF) is employed to simulate the cyclic inspiratory flows for different IPA inhalation patterns. When an aerosol particle enters the mouth respiratory tract, its particles experience abrupt changes in direction. The secondary flow changes its direction as the airflow passes curvature. Intensity of the secondary flow is strong after first bend at pharynx and becomes weaker after larynx. In flow separation, a particle can be trapped and follow the eddy and deposit on the surface. Particle deposition fraction generally increases as particle size and inhaling airflow velocity increase.

Cerebral Blood Vessel Rupture During Head Impacts: A Parametric Study on Properties of SAS Trabeculae and Pia Mater

Blunt head impacts cause relative motion between the brain and skull. This increases the normal and shear stresses in the (skull/brain) interface region, which leads to the rupture of cerebral blood vessels and in particular bridging veins. Mechanical properties of meningeal layers, in particular, subarachnoid space (SAS) trabeculae and the pia mater are not well established in the literature and could have a wide range depending on an individual. In our previous studies, knowing that SAS trabeculae and pia mater are collagen-based structures, these mechanical properties have been estimated using the properties of similar collagen based tissues. However, recent study Xin Jin et al. (2008), suggests that the mechanical properties of trabeculae and the pia matter are significantly less than a collagen-based tissue. Therefore, the goal of this study is to investigate the effect of the mechanical properties of these tissue on the stress and strain of the neighboring tissues when the head is subjected to a blunt impact. Specifically, the objectives of this study is to determine the stress/strain changes of the cerebral blood vessels as a function of the mechanical properties of the SAS trabeculae and pia mater, when the loading and the boundary conditions of the local model are kept the same. Note that the variation of the properties of these tissues affects the failure of cerebral blood vessels which leads to traumatic brain injury.

Development and Validation of a High Fidelity 3-D Computational Spine Model That Incorporates Spatial Material Density Variations

Anatomically accurate high fidelity computational model of human spine is developed from Computed Tomography scans of a healthy subject and validated against experimental and other computational results. Procedures developed in this work will serve useful in noninvasive evaluation of patients with back problems. The advantage of this approach is that the model can be tailored to specific individual or specimen.

Changes in the Dynamic Stability of Walking in Active Healthy Older Adults Independent of Changes in Walking Speed

Older adults commonly walk slower, which many believe helps improve their walking stability. However, they remain at increased risk of falls. We investigated how differences in age and walking speed independently affect dynamic stability during walking, and how age-related changes in leg strength and ROM affected this relationship. Eighteen active healthy older and 17 younger adults walked on a treadmill for 5 minutes each at each of 5 speeds (80–120% of preferred). Local divergence exponents and maximum Floquet multipliers (FM) were calculated to quantify each subject’s responses to small inherent perturbations during walking. These older adults exhibited the same preferred walking speeds as the younger subjects (p = 0.860). However, these older adults still exhibited greater local divergence exponents (p<0.0001) and higher maximum FM (p<0.007) than young adults at all walking speeds. These older adults remained more unstable (p<0.04) even after adjusting for declines in both strength and ROM. In both age groups, local divergence exponents decreased at slower speeds and increased at faster speeds (p<0.0001). Maximum FM showed similar changes with speed (p<0.02). The older adults in this study were healthy enough to walk at normal speeds. However, these adults were still more unstable than the young adults, independent of walking speed. This greater instability was not explained by loss of leg strength and ROM. Slower speeds led to decreased instability in both groups.

Quantification of Arterial Stiffness Reduced Effect of Vessel Geometry

Arteriosclerosis is such as phenomena hardening of arteries, with thickening and loss of elasticity. Previous indexes include effect of geometric and mechanical factors as the radius, the wall thickness and mechanical properties of arteries. In this study, we proposed viscoelasticity indexes formulated by thin cylindrical shell theory estimated dynamic strain, and this index was independent of wall thickness and radius of arterial vessels. To confirm the validity of these indexes, we evaluated the parameters of viscoelasticity using the latex tube with different wall thickness of blood vessel model. We measured a radius of the latex tube and an inner pressure maintained by a pulsatile pump in a mock circuit filled with the water. Estimating the parameters of elasticity using these measured values, we concluded that a proposal index was independent of the wall thickness of the artery.

Dynamics and Control of Electroporation

Electroporation has become an important tool for drug delivery such as gene therapy. The technique uses electric pulses to create transient pores in the cell membrane. To ensure proper uptake of targeted molecules, it is essential to create sufficiently large pores, which remain open long enough. In this work, we explore evolution of the pores using dynamical analysis and control of electroporation based on a simplified two-dimensional model. A detailed bifurcation analysis reveals the existence of saddle-node bifurcations, which induce hysteresis into the system dynamics. The bifurcation analysis also sheds light on the relation between the applied voltage and the pore radius. Based on the dynamics and bifurcation analysis, we design a feedback control algorithm that is able to achieve any desired pore size. Numerical examples demonstrate the control strategy is robust. The control algorithm will improve the operation of electroporation in drug delivery.

A Multi-Scale Modeling Based Theory for Mechanical Load Induced Cell Damage in a 3D Cell-Encapsulated Alginate Tissue Construct

Bio-fabrication methods utilize mechanical means to manufacture products with living cells incorporated. During the fabrication process, cells are involuntarily manipulated and/or exposed to mechanical disturbances that may not be present in the normal physiological environment. One of fundamental questions that need to be answered is whether cells remain viable and/or functional when subjected to mechanical disturbances. This paper presents the development of a theory that can address relation between applied mechanical forces and cellular response from a mechanics point of view. Specifically, a 3D multi-scale numerical model is developed and applied to determine the stress and deformation fields at the cellular level when the tissue construct is subjected to macro-level loads. Based on the detailed information rendered for the micro stress and deformation fields, a general theory is then formulated. A simulation for a 3D alginate tissue construct with encapsulated cells under uniform compression is conducted to illustrate the solution technique. Comparison between the predicted cell viability and experimental data demonstrates that the proposed theory is capable of capturing the experimental trend.