Individualized Transfer Functions for the Noninvasive Estimation of Central Pressure From Brachial Pressure Readings

A model-based investigation is carried out with the aim of developing an ab-initio methodology for the patient-specific estimation of central pressures from brachial blood pressure readings. The subclavian root-brachial artery segment is modeled as a 1-D tube with all model parameters linked to patient characteristics. A simulation is also run with typical physiological parameters, which gives a “first estimate” of the transfer function (TF). The TF derived using the patient characteristics is studied in detail to investigate the change in the arterial TF occurring with changes in patient characteristics. This TF is compared with the “first estimate” to evaluate the feasibility of using standard arterial properties.

Tissue-Engineered Cartilage in Three-Dimensional Cultured Chondrocytes by Ultrasound Stimulation and Collagen Sponge as Scaffold

This paper describes the effects of ultrasound stimulation on chondrocytes in three-dimensional culture in relation to the production of regenerative cartilage tissue, using collagen sponges as a carrier and supplementation with hyaluronic acid (used in the conservative treatment of osteoarthritis). It has been shown that cell proliferation and matrix production can be facilitated by considering the mechanical environment of the cultured chondrocytes and the mechanical properties of the scaffold structure used in the culture and of the stimulation used.

Electromigration of Charged Polystyrene Beads Through Silicon Nanopores Filled With Low Ionic Strength Solutions

In this work we present preliminary results demonstrating the influence of electrical double layer overlap on the electromigration of polystyrene beads (PSB) through an array of 25 cylindrical nanopores. Each of the cylindrical nanopores of the array used in this study is 360nm long with a diameter of 90nm. We observe frequent Coulter events for solutions of higher ionic strength and absence of Coulter events at low ionic strength solution. At higher ionic strengths, the electric double layers in the nanopore are thin and ion transport through the nanopore follows the bulk behavior of the ionic solution. For solutions of lower ionic strength, the electric double layers are comparable to the nanopore dimensions and start to overlap, suggesting surface charge interaction with the polystyrene beads that pass through the nanopore. The work continues towards detailed statistical analysis of the characteristic events observed for different concentrations.

Patient-Specific 3D Finite-Element Analysis of Miniscrew Implants During Orthodontic Treatment

Miniscrew implants have seen increasing clinical use as orthodontic anchorage devices with demonstrated stability. The focus of this study is to develop and simulate operative factors, such as load magnitudes and anchor locations to achieve desired motions in a patient-specific 3D model undergoing orthodontic treatment with miniscrew implant anchorage. A CT scan of a patient skull was imported into Mimics software (Materialise, 12.1). Segmentation operations were performed on the images to isolate the mandible, filter out noise, then reconstruct a smooth 3D model. A model of the left canine was reconstructed with the PDL modeled as a thin solid layer. A miniscrew was modeled with dimensions based on a clinical implant (BMK OAS-T1207) then inserted into the posterior mandible. All components were volumetrically meshed and optimized in Mimics software. Elements comprising the mandible bone and teeth were assigned a material based on their gray value ranges in HU from the original scan, and meshes were exported into ANSYS software. All materials were defined as linear and isotropic. A nonlinear PDL was also defined for comparison. For transverse forces applied on the miniscrew, maximum stresses increased linearly with loading and appeared at the neck or first thread and in the cortical bone. A distal tipping force was applied on the canine, and maximum stresses appeared in the tooth at the crown and apex and in the bone at the compression surface. Under maximum loading, stresses in bone were sufficient for resorption. The nonlinear PDL exhibited lower stresses and deflections than the linear model due to increasing stiffness. Numerous stress concentrations were seen in all models. Results of this study demonstrate the potential of patient-specific 3D reconstruction from CT scans and finite-element simulation as a versatile and effective pre-operative planning tool for orthodontists.

Computational Fluid Dynamics Analysis of Surgical Adjustment of Ventricular Assist Device Implantation to Minimize Stroke Risk

Presently, mechanical support is the most promising alternative to cardiac transplantation. Ventricular Assist Devices (VADs) were originally used to provide mechanical circulatory support in patients waiting planned heart transplantation (“bridge-to-transplantation” therapy). The success of short-term bridge devices led to clinical trials evaluating the clinical suitability of long-term support (“destination” therapy) with left ventricular assist devices (LVADs). The first larger-scale, randomized trial that tested long-term support with a LVAD reported a 44% reduction in the risk of stroke or death in patients with a LVAD. In spite of the success of LVADs as bridge-to-transplantation and long-term support. Patients carrying these devices are still at risk of several adverse events. The most devastating complication is caused by embolization of thrombi formed within the LVAD or inside the heart into the brain. Prevention of thrombi formation is attempted through anticoagulation management and by improving LVADs design; however there is still significant occurrence of thromboembolic events in patients. Investigators have reported that the incidence of thromboembolic cerebral events ranges from 14% to 47% over a period of 6–12 months. An alternative method to reduce the incidence of cerebral embolization has been proposed by one of the co-authors, namely William DeCampli M.D., Ph.D. The hypothesis is that it is possible to minimize the number of thrombi flowing into the carotid arteries by an optimal placement of the LVAD outflow conduit, and/or the addition of aortic bypass connecting the ascending aorta (AO) and the innominate artery (IA), or left carotid artery (LCA). This paper presents the computational fluid dynamics (CFD) analysis of the aortic arch hemodynamics using a representative geometry of the human aortic arch and an alternative aortic bypass. The alternative aortic bypass is intended to reduce thrombi flow incidence into the carotid arteries in patients with LVAD implants with the aim to reduce thromboembolisms. In order to study the trajectory of the thrombi within the aortic arch, a Lagrangian particle-tracking model is coupled to the CFD model. Results are presented in the form of percentage of thrombi flowing to the carotid arteries as a function of LVAD conduit placement and aortic bypass implantation, revealing promising improvement.

Numerical Simulation of Blood Flow in Patient-Specific Stenotic Carotid Bifurcation Geometries

The hemodynamics and fluid mechanical forces in blood vessels have long been implicated in the deposition and growth of atherosclerotic plaque. Detailed information about the hemodynamics in vessels affected by significant plaque deposits can provide insight into the mechanisms and likelihood of plaque weakening and rupture. In the current study, the governing equations are solved in their finite volume formulation in several patient-specific geometries. Recirculation zones, vortex shedding, and secondary flows are captured. The forces on vessel walls are shown to correlate with unstable plaque deposits. The results of these simulations suggest morphological features that may usefully supplement percent stenosis as a predictor of plaque vulnerability.

Multidisciplinary Design and Optimization for Oscillating Flow Polymerase Chain Reaction Microfluidics Device

A Polymerase Chain Reaction (PCR) process is almost always required prior to DNA (deoxyribonucleic acid) analysis to create multiple copies of DNA fragments. Using microfluidics technology, the PCR process requires much shorter process time and much less DNA samples than conventional PCR systems. Among existing microfluidics-based techniques, the oscillating flow PCR has advantages including faster analysis time than cavity PCR microfluidics, and smaller contact area between the sample and polymer channel wall compared to flow-through PCR. The smaller contact area reduces DNA adsorption and enhances DNA detection accuracy. In the proposed study, new design features of the oscillating flow PCR concept are evaluated including: (1) PDMS (polydimethylsiloxane) and glass are selected as the microfluidics chip material for realizing a disposable chip, (2) water impingement cooling is applied to effectively isolate the temperature zones, and (3) a copper layer is attached outside of the chip to enhance uniform temperature distribution within the temperature zones. When PDMS is used for PCR microfluidics devices, lower efficiency has been a disadvantage. The efficiency is lowered because the DNA fragments are trapped at the PDMS surface. This trapping can be reduced by minimizing the contact area between the sample and the PDMS surface. When the sample contact area is reduced, which can be achieved by increasing the flow channel cross-sectional area, thermal response is degraded. Optimal channel dimensions are determined by considering the trade-off between thermal response and sample contact area with PDMS channel wall. The resulting thermal response of the sample in the temperature zone is comparable to existing studies, which use silicon as the chip material. A transient FEM heat transfer analysis for the temperature zone is performed for more effective thermal design and optimization.

A Modified Method of Arterial Elasticity Index Including Effects of Constraints From Surrounding Tissues

Previously, we proposed an estimation method of arterial elasticity index (EM) independent of geometric factors such as the radius and wall thickness. Since the previous method was based on an equation of a motion that assumed a thin cylindrical model with infinite length, it is thus necessary to account for the effects of dynamic constraints from surrounding tissues. The purpose of this study is to propose a modified method for quantifying arterial elasticity index accounting for the effects of dynamic constraints. We describe the modified method by vibration analysis of a thin cylindrical shell using a natural frequency depending on boundary conditions. To examine the feasibility of the proposed method, we measured the inner pressure, radius and natural frequency of the mock-vessels with dynamic constraints. From these results, the elasticity index (EM) was derived independent of the effects of dynamic constraints. In summary, the proposed method enabled to derive elastic properties of arteries accounting for the effects of dynamic constraints in mock-vessels with both ends restricted.

Experimental Study and Modeling of Equilibrium Point Trajectory Control in Single and Double-Joint Arm Movements

This paper discusses a new model of neuromuscular control of elbow and shoulder joints based on the Equilibrium Point Hypothesis (EPH). The earlier model [1] suggests that the incorporation of relative damping within reflex loops can maintain the dynamic simplicity of the EPH, while being robust over the range of human joint velocities. The model presented here, extends previous work with the use of experimental Electromyography data of 2 muscles to determine the timing parameters of the virtual trajectories and the inclusion of physiological time delays to account for neural transmission and muscle stimulation/activation delays. This model uses delays presented in the literature by other researchers, with a goal of contributing to a resolution of arguments regarding the controversial arguments in the planning sequences. Therefore, this study attempts to demonstrate the possibility for using descending CNS signals to represent relatively simple, monotonic virtual trajectories of the time varying Equilibrium Point for the control of human arm movement. In addition, the study demonstrates that these virtual trajectories were robust enough to control and coordinated movement of elbow and shoulder joints discussed.

Computer Modeling of Fluid-Stress-Induced Blood Damage in a Mechanical Ventricular Assist Device

Congestive heart failure results the heart is unable to pump the required amount of blood to maintain the systemic circulation. World-wide, millions of patients are diagnosed with congestive heart failure every year, many of which ultimately become candidates for heart transplants. The limited number of available donor hearts, however, has resulted in a tremendous demand for alternative, supplemental circulatory support in the form of artificial heart pumps to serve as a “Bridge-to-Transplant”. The prospect of artificial heart pumps used for long-term support of congestive heart failure patients is directly dependent upon excellent blood compatibility. High fluid stress levels may arise due to high rotational speeds and narrow clearances between the stationary and rotating parts of the pump. Thus, fluid stress may result in damage to red blood cells and activation of platelets, contributing to thrombus formation. Therefore, it is essential to evaluate levels of blood trauma for successful design of a mechanical Ventricular Assist Device. Estimating the fluid stress levels that occur in a blood pump during the design phase also provides valuable information for optimization considerations. This study describes the CFD evaluation of blood damage in a magnetically suspended axial pump that occurs due to fluid stress. Using CFD, a blood damage index, reflecting the percentage of damaged red blood cells, was numerically estimated based on the scalar fluid stress values and exposure time to such stresses. A number of particles, with no mass and reactive properties, was injected at the inflow of the computational domain and traveled along their corresponding streamlines. A Lagrangian particle tracking technique was employed to obtain the stress history of each particle along its streamline, making it possible to consider the damage history of each particle. Maximum scalar stresses of approximately 430 Pa were estimated to occur along the tip surface of the impeller blades, more precisely at the leading edge of the impeller blades. The maximum time required for the vast majority of particles to pass through the pump was approximately 0.085sec. A small number of particles (approximately 5%), which traveled through the narrow gap between the stationary and rotating part of the pump, exited the computational domain in approximately 0.2 sec. The mean value of blood damage index was found to be 0.15% with a maximum value of approximately 0.47%. These values are one order of magnitude lower than the approximated damage indices published in the literature for other Ventricular Assist Devices. The low blood damage index indicates that red blood cells traveling along the streamlines considered are not likely to be ruptured, mainly due to the very small time of exposure to high stress.