Regional and Layer Distribution of Residual Stresses in an Unloaded Aortic Medial Wall

The mechanical and structural characteristics of aortic media have profound effects on the physiology and pathophysiology of an aorta. However, many aspects of the aortic tissue remain poorly understood, partly due to the intrinsic layered wall structure and regionally varying residual stresses. Our recent works have demonstrated that a mechanical interaction between the elastic lamina (EL) and smooth muscle layer in the aortic media can be computationally reproduced using a simplified finite element (FE) model. However, it is questionable whether the simplified FE model we created was representative of the structure of a real medial wall and its modeling technique would be applicable to develop a more sophisticated and structure-based aortic FE model. This study aimed to computationally represent EL buckling in the aortic medial ring at an unloaded state and successfully reproduced transmural variation in EL waviness across the aortic wall. We also aimed at confirming the inner and outer layers of the medial wall are subjected to compressive and tensile residual stresses, respectively, at the unloaded state, implying that the ring model will open spontaneously when it is radially cut. Moreover, the computed residual stresses were found to be within the reasonable range of the predicted values, 1–10 kPa, supporting the validity of our modeling approach. Although further study is required, the information obtained here will greatly help improve the understanding of basic aortic physiology and pathophysiology, while simultaneously providing a basis for more sophisticated computational modeling of the aorta.

Prediction of Blood Flow Distribution in Liver Radioembolization Using Convolutional Neural Networks

We have developed a new dosimetry approach, called CFDose, for liver cancer radioembolization based on computational fluid dynamics (CFD) simulation in the hepatic arterial tree. Although CFDose overcomes some of the limitations of the current dosimetry methods such as the unrealistic assumption of homogeneous distribution of yttrium-90 in the liver, it suffers from the expensive computational cost of CFD simulations. To accelerate CFDose, we introduce a deep learning model to predict the blood flow distribution between the liver segments in a patient with hepatocellular carcinoma. The model was trained with the results of CFD simulations under different outlet boundary conditions. The model consisted of convolutional, average pooling and transposed convolution layers. A regression layer with a mean-squared-error loss function was utilized at the network output to estimate the arterial outlet blood flow. The mean-squared error and prediction accuracy were calculated to measure model performance. Results showed that the average difference between the CFD results and predicted flow data was less than 2.45% for all the samples in the test dataset. The proposed model thus estimated the blood flow distribution with high accuracy significantly faster than a CFD simulation. The network output can be used to estimate the yttrium-90 dose distribution in the liver in future studies.

Biomechanical Analysis of the Sensitivity of Brain Tissue Responses to FE Head Models in the Study of Impact-Induced TBI

A numerical investigation is conducted on the injury-related biomechanical parameters of the human head under blunt impacts. The objective of this research is twofold; first to understand the role of the employed finite element (FE) head model — with its specific components, shape, size, material properties, and mesh size — in predicting tissue responses of the brain, and second to investigate the fidelity of pressure response in validating FE head models. Accordingly, two independently established and validated FE head models are impacted in two directions under two impact severities and their predicted responses in terms of intracranial pressure (ICP) and shear stress are compared. The coup-counter ICP peak values are less sensitive to head model, mesh size, and the brain material. In all cases, maximum ICPs occur on the outer surface, vanishing linearly toward the center of the brain. Hence, it is concluded that different head models may simply reproduce the results of ICP variations due to impact. Shear stress prediction, however, is mainly affected by the head model, direction and severity of impact, and the brain material.

Improved Vibration-Model-Based Analysis for Estimation of Arterial Parameters From Noninvasively Measured Arterial Pulse Signals

With the goal of achieving consistence in interpretation of an arterial pulse signal between its vibration model and its hemodynamic relations and improving its physiological implications in our previous study, this paper presents an improved vibration-model-based analysis for estimation of arterial parameters: elasticity (E), viscosity (η), and radius (r0) at diastolic blood pressure (DBP) of the arterial wall, from a noninvasively measured arterial pulse signal. The arterial wall is modeled as a unit-mass vibration model, and its spring stiffness (K) and damping coefficient (D) are related to arterial parameters. Key features of a measured pulse signal and its first-order and second-order derivatives are utilized to estimate the values of K and D. These key features are then utilized in hemodynamic relations, where their interpretation is consistent with the vibration model, to estimate the value of r0 from K and D. Consequently, E, η, and pulse wave velocity (PWV) are also estimated from K and D. The improved vibration-model-based analysis was conducted on pulse signals of a few healthy subjects measured under two conditions: at-rest and immediately post-exercise. With E, r0, and PWV at-rest as baseline, their changes immediately post-exercise were found to be consistent with the related findings in the literature. Thus, this improved vibration-model-based analysis is validated and contributes to estimation of arterial parameters with better physiological implications, as compared with its previous counterpart.

The Effect of Micro Grooving on Goat Total Knee Replacement: A Finite Element Study

A method to improve the mechanical fixation of a total knee replacement (TKR) implant is clinically important and is the purpose of this study. More than one million joint replacement procedures are performed in people each year in the United States, and experts predict the number to increase six-fold by the year 2030. Whether cemented or uncemented, joint prostheses may destabilize over time and necessitate revision. Approximately 40,000 hip arthroplasty surgeries have to be revised each year and the rate is expected to increase by approximately 140% (and by 600% for total knee replacement) over the next 25 years. In veterinary surgery, joint replacement has a long history and the phenomenon of surgical revision is also well recognized. For the betterment of both people and animals, improving the longevity of arthroplasty devices is of the utmost clinical importance, and towards that end, several strategies are under investigation. One approach that we explore in the present research is to improve the biomechanical performance of cemented implant systems by altering the implant surface architecture in a way that facilitates its cement bonding capacity. Beginning with the Charnley system, early femoral stems were polished smooth, but a number of subsequent designs have featured a roughened surface — created with bead or grit blasting — to improve cement bonding. Failure at the implant-cement interface remains an issue with these newer designs, leading us to explore in this present research an alternate, novel approach to surface alteration — specifically, laser microgrooving. This study used various microgrooves architectures that is feasible using a laser micromachining process on a tibia tray (TT) for the goat TKR. Developing the laser microgrooving (LM) procedure, we hypothesized feasibility in producing parallel microgrooves of precise dimensions and spacing on both flat and round metallic surfaces. We further hypothesized that laser microgrooving would increase surface area and roughness of the cement interface of test metallic implants and that such would translate into an improved acute mechanical performance as assessed in vitro under both static and cyclic loads. The objective was to develop a computational model to determine the effect of LIM on the tibial tray to the mechanical stimuli distributions from implant to bone using the finite element method. This study designed goat TT 3D solid model from a computer topography (CT) images, out of which three different laser microgrooves were engraved on TT sample by varying depth, height and space between two adjacent grooves. The simulation test results concluded that microgrooves acchitecures positively influence microstrain behavior around the implant/bone interfaces. There is a higher amount of strain observed for microgroove implant/bone samples compared to non-groove implant/bone samples. Thus, the laser-induced microgrooves have the potential to be used clinically in TKR components.

Arm Movement Analysis in Human Daily Life for Improving Durability of Artificial Elbow Joint

Human movement data can contribute to the quality improvement of industrial and medical products affected by such movement. Such data can be used to improve the quality of industrial products as well as in healthcare applications, such as the development of artificial joints. To develop and design artificial joints with enhance durability, it is necessary to set up standards of durability using human movement data in daily life. The aim of this study is to obtain data that contributes to the improvement in durability of artificial elbow joints. We have developed a wearable device that can measure its self-acceleration, angular velocity, and quaternions to collect human movement data continuously for long-term. Additionally, we collected the arm movement data of 30 participants using the developed device. The participants of this study carried on with their normal lives with the measuring device worn on their wrist. This study calculated the posture of the wrist over time using quaternions and mainly analyzed posture changes. We clarified the characteristics and trends of the movement of bending the elbow in daily human life.

Effect of Loading Direction and Bone Density Distribution in Assessing Hip Fracture via Finite Element Analysis

An accurate assessment of hip fracture risk requires a proper consideration of parameters affecting the fracture. In general, hip fracture is affected by bone morphology, bone mineral Density (BMD), and load amount. Hip fracture is an outcome of the interaction of all those parameters including loading directions. Assessing the effect of the parameters individually may not correctly reflect the root cause of the hip fracture. Hence, this research aims at analyzing the significance of parameters and their interaction. A multivariate regression model was used considering bone density (ρash), different loading directions during sideways fall, represented by load angle (α) on the coronal plane and angle (β) on the transverse plane as independent parameters and Fracture Risk Index (FRI) as a dependent parameter. The statistical results showing the significant value of 0.7321 for α, and 0.0001 for β and ρash indicates that the effect of loading direction about femoral shaft on the coronal plane (α) does not have impact on fracture risk while loading direction about femoral neck axis on the transverse plane (β) and ρash have the significant impact. Furthermore, the analysis of the interaction of parameters shows that the impact of β on fracture risk may depends more on bone density as the significance of interaction of β and ρash is 0.0001.

Fabrication of 3D Bone Scaffolds Functionalized With Spatiotemporal Release of BMP-2 Growth Factor via iCVD to Enhance Osteoregeneration

3D scaffolds are known to be used in bone tissue engineering applications due to their great potential of providing multi-functionalized environment for cells. Different production techniques have been used focusing on changing geometrical features or adding biological/chemical compounds to improve the functionality of current 2D/3D scaffolds. A critical component to this functionalization relates to the effect of endogenous and exogeneous growth factors (GF) in the bone regeneration process that could be incorporated to the scaffolds via Initiated Chemical Vapor Deposition (iCVD) which is a solvent free method that requires low energy while also containing a wide variety of monomer choices for the layer by layer coating of polymers with individual functionality choices. However, GFs come with several difficulties such as rapid deactivation, low protein stability profile and little time of half-life, hence ideal environments that can overcome these issues are yet to be defined. Towards that goal, in this study we develop a computational framework based on the implementation of the advection-diffusion-reaction Partial Differential Equations (PDE) in a Finite Element Analysis (FEA) solver in COMSOL Multiphysics software. The goal is to develop a tool and conduct an initial analysis to be utilized for the simulation of multi-layer scaffold functionalized using encapsulation and immobilization of GFs inside nanoparticles possibly via iCVD. In this paper we focus on the analysis of two typical GF (BMP-2 and TGF) release mechanisms based on the effect of key material and geometrical parameters such as thickness of layers, initial GF concentration, diffusion coefficient, release function and uptake rate (absorption coefficient). The ultimate goal is to develop a model that can be used for future bone scaffold design studies when integrated to more advanced optimization methodologies. This model with further integration and updates of chemical and biological parameter measurements and inclusion of presence of antibodies should lay down a valuable basis for directing possible experimental functionalization efforts and their effects on the healing process of bone tissue. Initial results indicate that the proposed computational model can be utilized to predict the response of multi-layered bone scaffolds in terms of the concentration profiles of the GFs. Results of the parametric study presented in this paper prompt for the relative importance of each parameter in tuning the GF release profiles and point towards the need for formal optimization studies to achieve desired GF release responses considering all factors simultaneously. Among them, the diffusion coefficient is a key parameter with both a dominant effect on the GF profile and its ability to characterize different coatings using iCVD methods. As a next step, the developed framework will be updated to incorporate more detailed surface reactions and morphological data to simulate iCVD coated growth factors and verified with possible in-vitro studies before its integration to a formal optimization methodology.

High-Intensity Focused Ultrasound Scattering at Mammal Vertebrae

High-intensity focused ultrasound (HIFU) is used clinically to heat cells therapeutically or to destroy them through heat or cavitation. In homogeneous media, the highest wave amplitudes occur at a predictable focal region. However, HIFU is generally not used in the proximity of bones due to wave absorption and scattering. Ultrasound is passed through the skull in some clinical trials, but the complex geometry of the spine poses a greater targeting challenge and currently prohibits therapeutic ultrasound treatments near the vertebral column. This paper presents a comprehensive experimental study involving shadowgraphy and hydrophone measurements to determine the spatial distribution of pressure amplitudes from induced HIFU waves near vertebrae. First, a bone-like composite plate that is partially obstructing the induced waves is shown to break the conical HIFU form into two regions. Wave images are captured using pulsed laser shadowgraphy, and hydrophone measurements over the same region are compared to the shadowgraphy intensity plots to validate the procedure. Next, shadowgraphy is performed for an individual, clean, ex-vivo feline vertebra. The results indicate that shadowgraphy can be used to determine energy deposition patterns and to determine heating at a specific location. The latter is confirmed through additional temperature measurements. Overall, these laboratory experiments may help determine the efficacy of warming specific nerve cells within mammal vertebrae without causing damage to adjacent tissue.

Study on Natural Vibration Characteristics Based on the Coupled Wave Theory of Spring Supported Elastic Pipes and Fluids

A gradient of a blood flow velocity on the surface of a blood vessel is one of the clinical medicine concerns from the view point of prevention of the arteriosclerosis. In previous study, we formulated a relationship between the pressure and a flow velocity based on the coupled wave theory of elastic pipes and Newtonian fluids [1]. In addition, a flow velocity distribution and a wall shear stress are estimated by using the blood pressure data, which are non-invasively obtained by the tonometry method. This method is quasi-analytical method to apply the coupled wave theory for industrial flow field inside steel pipes proposed by Urata [4] to blood vessel, and has the advantage of systematic estimator compared with the numerical calculation. However, the coupled wave theory has applied to the elastic pipes that were assumed to be infinitely long. In addition, a single wave was assumed to be dominant within the elastic pipes and the Newtonian fluids. Therefore, in order to apply various length vessels in clinical field, the boundary of the blood vessels that varies from site to site, and the natural vibration characteristics that depend on the boundary conditions, could not be reflected in the wall shear stress estimation. In general, in order to solve the forced vibration with the boundary condition, it is necessary to clarify natural frequency and natural mode as natural vibration characteristics of structure. In this study, we introduce the spring supported elastic pipes to the coupled wave theory and formulated a relationship between the natural vibration characteristics and the boundary conditions. In this proposed method, the spring-supported elastic pipe has a feature that can be treated as an arbitrary boundary condition of an artery by giving an appropriate spring coefficients. Therefore, it is easy to apply to various types of blood vessels clinically. By investigating the natural vibration characteristics of blood vessels that varies from site to site, it may be possible to clarify fluctuations of blood flow in response to blood pressure with some frequency-bands. In addition, natural angular frequencies and natural modes of the spring supported elastic pipes and the Newtonian fluids were estimated for general blood vessel based on the coupled wave theory. In the result, the natural angular frequencies and the natural modes that reflect the clinical vibration characteristics to some extent can be estimated. On the other hand, particular modes may not reflect boundary condition, and further examination of the relationship between natural vibration characteristics and boundary condition is needed.