BEE 4530 - 2019 Student Papers

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https://hdl.handle.net/1813/66928

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Finite Element Model of Human Thermoregulation in Cold Conditions
Berman, Aaron; Khalatyan, Yekaterina; Umeki, Chris; Zhou, Max (2019-05)
The human body can only function properly within a narrow range of temperatures. Therefore, the regulation of body temperature is a critical part of survival. Thermal homeostasis is maintained through complex feedback loops, with the body reacting to local temperature changes. The critical components of this feedback loop are linked to the skin temperature and the hypothalamus temperature. the skin is responsible for heat sensing, while the hypothalamus is the body’s temperature control center. In this study the effect of thermoregulation on the temperature of multiple domains throughout the body was considered, and an attempt to create a 3-dimensional model that accurately displays its effect on body temperature was attempted. To investigate the effect that thermoregulation has on maintaining temperature and to investigate the accurate temperature profile of the human body, COMSOL, a finite element software capable of running physics simulations, was utilized. There had been previous research on the topic that used only a few hundred nodes or used lumped parameters to simulate thermal regulation. Our model’s geometry was 3-dimensional and inspired by the dimensions used in a previous model by Dr. Dusan Fiala [1]. To implement thermoregulatory effects, empirical formulas were found in previous literature for shivering, vasodilation and vasoconstriction among others. All empirical formulas were temperature sensitive and were functions of the skin temperature and/or the hypothalamus temperature. These effectors change the local heat generation terms and the local blood perfusion terms, both crucial components of the heat equation in biological systems. An initial simulation of the body temperature was run at cool environment with some air flow. The results showed a sharp decline in average skin and fat temperatures initially due to their proximity to the surface, while the average brain temperatures declined gradually and then flattened off within the first two hours. Simulations excluding different components of thermoregulation were also ran to portray the importance of thermoregulation in the body. The simulations without thermoregulatory components resulted in much lower brain temperatures, as expected . Furthermore, the magnitude of different effectors were measured, allowing for analysis on the importance of different ways body’s thermoregulate. To validate the model, experimental data was compared to our simulated result. Experimental data regarding skin temperature over time, and shivering magnitude over time matched the overall trend of our data. Similarly, research on what external temperatures induce shivering were in line with the simulated results. This model provides insight into the distribution of temperature across the human body and will provide the basis of a comprehensive human model applicable to a range of topics, such as clothing insulation and frostbite analysis. By incorporating empirical equations that are functions of temperature and having a 3D model that is heavily discretized, the model can accurately portray the body’s temperature profile in cold climates.
Modelling two-stage antibiotic release from orthopedic fixation pins to prevent post-op osteomyelitis
Bhatta, Asmita; Kim, Matthew Jundong; Lim, Melanie; Sheng, Rory (2019-05)
The first six hours following orthopedic implantation is a decisive period for preventing bacterial adhesion to ensure an implant’s long-term success. If bacterial adhesion is not adequately impeded, a biofilm will form, acting as a diffusion barrier to slow down the implant integration process. Current therapies to treat osteomyelitis and other forms of implant-related infections include physical removal of the infected device, revision surgery, and prolonged antibiotic therapy. However, osteomyelitis still occurs at significant rates, and affected patients often require surgical adjustment or systemic antibiotic dosages. This project considers a cylindrical drug-eluting pin, comprised of a reservoir of packed mesoporous silica MCM-48 microparticles, where the antibiotic (linezolid) is adsorbed. As simulated body fluid flows into the pin, the drug is desorbed from the microparticles and diffuses down its concentration gradient to be released in a sustained manner. A one dimensional diffusive mass transport simulation in COMSOL 5.3 Multiphysics was used to quantitatively simulate this process with the objective of optimizing design options such as porosity and pin geometry with respect to drug delivery, specifically the concentration of the drug on the surface of the pin where bacterial adhesion occurs. We modeled a pin with 17% wall porosity packed with 440 nm silica beads. These dimensions can be altered to improve current fixation pin designs which can facilitate treatment procedures. Our model exhibited higher rates of release when compared to the experimental data, with 50% of total drug release achieved at approximately 8.5 and 40 hours respectively. This can be explained by the limited volume of SBF used in the experiment in contrast to the “infinite” amount of SBF that was assumed in our model, leading to higher rates of diffusion and release in our computational model. Optimization of porosity revealed that 20% porosity leads to a drug release profile that maximizes the amount of time above the minimal bactericidal concentration (MBC) and minimal inhibitory concentration (MIC). In the design of future pins, since increases in porosity are associated with decreases in mechanical strength and increases in manufacturing costs, the resulting changes in the mechanical properties and manufacturing process are significant factors that must be taken into account when improving the existing design of orthopedic devices.
Don’t Breathe on Me
Brigham, Rae; Machireddy, Meghana; Sequeira, Yohan (2019-05)
The contamination of surfaces in public spaces is of great importance to minimizing disease spread. When a large number of people share public spaces in close proximity, aerial disease transmission becomes common, especially in enclosed spaces such as small rooms and elevators. An understanding of disease-carrying particle deposition from breathing and airflow is a vital step in determining how to prevent the spread of disease in confined public spaces. Previously, studies have modelled the airflow in larger structures such as airplanes and rooms where the airflow is predetermined due to the presence of central ventilation. These models combine the airflow from vents and human sources to study the deposition of particles on surfaces (Tang et al., 2013; Yan, Li, Shang, & Tu, 2017; Zhao, Zhang, & Li, 2005). Similarly, this model incorporates ventilation and human breathing inside of an elevator to determine final particle spread and deposition. This model will determine what factors determine the spread and deposition of particles in an elevator. For example, this model will test whether the flow of air caused by the vents in the elevator is the dominant factor determining the dispersion of particles, or whether breathing air velocity is more important to particle spread. To test these parameters, a 3D model was developed of a standard elevator with a vent input two feet to the left of the center of the elevator and a vent exhaust two feet to the right of the center which follows the model found in most elevators. Using the ASME standard for elevator ventilation it was determined that the flow of air into and out of the elevator occurs at an equal rate of 5 L/(s*m2) (2105 code). Finally, a sine function was created with an amplitude of 1.3 m/s that modeled the breathing velocity of a human subject inside the elevator based on average breathing velocities found in literature (Tang et al., 2013; Yan et al., 2017). After running the model to solve for the laminar flow fluid dynamics, Lagrangian particle tracking (using Stokes drag law) was simulated for a maximum time period of 500 seconds. Using this initial model, a sensitivity analysis was done by varying the breathing function to simulate shallow and deep breaths as well as varying the location of the person around the elevator. The solution was validated by comparing with data from a study done by Tang. et. al in 2013, as well as with experimental data derived from thermal images of breath propagation. The results indicate that the patterns of propagation throughout time were turbulent in the normal solution, and involved the particles moving toward the vent and then being caught in an eddy-like backflow. The particle trajectories varied significantly from this trajectory as parameters such as the breathing velocity, source position and inlet velocity were changed. This indicates that the location of body within the elevator as well as the velocity of breath have significant impacts on the fluid dynamics within the elevator, as well as the amount of time that the disease particles circulate before coming to rest or flowing out through the vents. When the body position is close to the vent, or the vent velocities are higher, the particles are sucked into the outlet vent faster compared to the control. These results indicate that minimizing particle spread and disease transmission in elevators can be accomplished by designing elevators with higher numbers of vents and higher inlet vent velocities.
Shear Stress Induced Thrombogenicity of a Trileaflet Mechanical Heart Valve
Gregan, Chris; Moy, David; Newberger, Nicole; Siomos, Matthew (2019-05)
Trileaflet mechanical valves are a popular topic in industry R&D due to their potential improvements to hemodynamic performance relative to industry-standard bileaflet mechanical valves and their mechanical durability relative to tissue valves. Novostia is currently attempting to bring their Lapeyre-Triflo trileaflet valve through FDA approvals, demonstrating a clear need and viability for the design concept. While some analysis of this new design has been done, there is a lack of research into the effect of the new leaflet geometry on peak shear stresses in the flow, which impact the thrombogenicity of the valve. Thrombosis is one of the leading causes of complications associated with mechanical heart valves on the market today. While many different factors contribute to the thrombogenicity of a heart valve, high shear stresses in the flow are classically considered to be a significant contributing factor due to the platelet damage that occurs in high shear stress regions. Specifically, our team examined whether shear stresses and resultant platelet damage are increased relative to classic bileaflet valve designs when the valve is in the open position at peak systole. The triangular leaflet geometry contains a sharp trailing edge, which could increase the shear stresses, and the design also introduces an additional region of flow (four flow regions rather than three), which brings potential for further impacts to shear stress downstream of the leaflets.
Optimization of Laser Ablation Parameters for Lumbar Discectomy
Ashraf, Shaumik; Chan, William; Delgado, Robert; Hassan, Mohamed (2019-05)
Lower back pain, or lumbar pain, is a potentially debilitating chronic condition that will affect an estimated 80% of individuals in their lifetime. Lumbar pain can be caused by herniated lumbar discs, which is a painful ailment that occurs when the nucleolus pulposus or annulus fibrosus of an intervertebral lumbar disc is displaced beyond the intervertebral space and pinches the spinal nerve. The resulting nervous stimulation produces tremendous localized pain in the lower back and, in some cases, referredpain in the legs or arms. Current methods that can treat herniated discs include physical therapy, epidural steroid injections, and surgical removal. The former two solutions only treat mild cases of lumbar disc herniation, and the latter solution has along recovery time with postoperative complications. The application of laser discectomy to the treatment of lumbar pain associated with herniated discs is not novel; lasers have been used as a minimally-invasive treatment of moderately severe cases of herniated discs since the late 20thcentury. This study aims to analyze and improve minimally invasive laser ablation to remove lumbar disc tissue that has been displaced from its proper position. Concentrating an infrared laser beam on the herniated part of the intervertebral disc causes vaporization of the tissue with unparalleled precision. This results in relieved pressure on the spinal nerve and alleviation of pain. Two parameters in laser surgery are power density and separation between pulses, which must be manipulated to minimize surgery time and reduce collateral thermal affliction.Using COMSOL MultiphysicsⓇa herniated intervertebral disc and adjacent vertebrae was designed for laser ablation and transient heating analysis. A 3-dimensional Cartesian geometry was adopted and modeled with conductive heat transfer coupled with volumetric laser-heat generation determined by optical diffusion. The laser ablation physics was modeled by implementing a velocity at nodes that have reached the ablation temperature.The method of evaluating the effectiveness of the simulation is through determination of the mass loss via ablation due to the total energy transferred. This would ensure safe ablation of tissue during a laser discectomy surgery and lower the risk of protein denaturation, excessive water loss (reducing intervertebral disc shock absorption), and irreversible changes to tissue functionality as a result of changes in material properties. As a result, post-operative complications can be evaded and overall treatment of lumbar disc herniation (LDH) will become more efficient. Furthermore, this computational modeling approach was chosen in order to study the effect and impact of parameter variation on the laser ablation procedure, and to computationally optimize the length of the procedure to allowfor mass loss with minimal thermal damage to healthy tissue.Results of this study indicate that mass loss begins at approximately 6.2 seconds, after the temperature of the tissue has been raised to the ablation temperature. Sensitivity analysis of the model revealed that the mass loss is most dependent on the density of the intervertebral tissue and the power of the laser applied. Optimization of the laser ablation surgery occurs at 10.9 seconds, when the difference between the amount of herniated intervertebral damaged and the amount of healthy intervertebral disc and vertebral bone tissue damaged is maximized. More anatomically accurate computational models can be generated by using meshes of CT scans of the intervertebral disc, manually building various types of hernias and then running the same analysis of finding optimal laser parameters. Non-negligible vapor flow resulting from laser ablation can also be incorporated into future models.
The Effect of Face Topography on Frostbite
Chen, Fang; Huan, Zhongling; Kendl, Sarah; Cambonchi, Alejandra (2019-05)
Winters in most countries within the Northern hemisphere, which include heavily populated regions of Russia, Canada, China, and the United States, are commonly known to be harsh, unforgiving, and unpredictable. Cold weather injuries such as frostbite can occur within only a few minutes of exposure to extremely cold temperatures and high wind chill. We seek to provide a quantitative model of the effects of extremely cold and freezing temperature on the face and the extent of damage to tissue over time. In this study, our model will take into account the airflow of cold temperature on the face and the convective heat given off by the face. Using the duo model, we will be able to show the severity of tissue damaged. In this study, we consider both the tissue temperature on the face and the temperature of the airflow. To investigate the mechanism of forced convection heat extraction in the face, we will use COMSOL, a multiphysics finite element analysis and simulation software, to develop a simple geometry of the face and replicate the heat exchanging properties when exposed to extreme temperature conditions. Our model will be a 3D simulation of the face with boundary conditions a close distance away from the face. We are primarily focused on simulating that the airflow is coming directly in front of the face and that is where the primary damage will occur. We will use the data provided by the National Weather Service that demonstrates how quickly hypothermia and frostbite can occur depending on the windchill and temperature. The model will demonstrate the extent of damage that can occur in varied temperature settings. It will simulate how long the body can retain thermal energy while convective heat loss is simultaneously occurring. This model will allow us to demonstrate the importance of preventative care during extreme temperature conditions to avoid frostbite. It will allow us to quantitatively demonstrate how much tissue is damaged to help diagnose and treat frostbite cases.

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