Effect of blood flow patterns on localized platelet adhesion under physiologic flow conditions using two-dimensional and three-dimensional stent models: An experimental and computational approach

This dissertation presents dynamic flow experiments with fluorescently labeled platelets to allow for spatial observation of wall attachment in inter-strut spacings, to investigate their relationship to flow patterns. Human blood with fluorescently labeled platelets was circulated through an in vitro system that produced physiologic pulsatile flow in (1) a parallel plate blow chamber that contained two-dimensional (2D) stents that feature completely recirculating flow, partially recirculating flow, and completely reattached flow, and (2) a three-dimensional (3D) cylindrical tube that contained stents of various geometric designs. Flow detachment and reattachment points exhibited very low platelet deposition. Platelet deposition was very low in the recirculation regions in the 3D stents unlike the 2D stents. Deposition distal to a strut was always high in 2D and 3D stents. Spirally recirculating regions were found in 3D unlike in 2D stents, where the deposition was higher than at well-separated regions of recirculation

Mechanical properties of normotensive and hypertensive carotid and coronary arteries and their quantification

Due to the character of the original source materials and the nature of batch digitization, quality control issues may be present in this document. Please report any quality issues you encounter to [email protected], referencing the URI of the item.Includes bibliographical references (leaves 89-95).Issued also on microfiche from Lange Micrographics.Hypertension is a major risk factor for a variety of cardiovascular diseases like atherosclerosis, stroke, dissecting aortic aneurysms, etc. and is responsible for significant mortality and morbidity. Hypertension results in changes in the arterial wall structure, properties, biology, and function because of the existence of mechanical stress, i.e., increased blood pressure. Experimental and theoretical results are published with different animal models of hypertension, and time course of occurrence of these changes is neglected. Hence, a lack of complete understanding exists. In this paper, in vitro multiaxial (cyclic stretch and inflation) tests were performed on live carotid and coronary arteries from an aortic coarctation hypertension model of pigs, subjected to two, five, and nine weeks of hypertension. We found that increased blood pressure induced changes in the multiaxial stress-stretch response of the tested arteries. Changes were significant both in the axial and circumferential directions. Passive and active residual stress measurements also showed changes in opening angles, radius to thickness ratio, thickness, and area. Experiments revealed considerable activation of the hypertensive specimens as compared to that of normotensive specimens. Finally, multiaxial data was fitted with the Fung's constitutive equation to predict the arterial behavior

Comparison of near-wall hemodynamic parameters in stented artery models

Four commercially available stent designs (two balloon expandable - Bx Velocity and NIR, and two self-expanding - Wallstent and Aurora) were modeled to compare the near-wall flow characteristics of stented arteries using computational fluid dynamics simulations under pulsatile flow conditions. A flat rectangular stented vessel model was constructed and simulations were carried out using rigid walls and sinusoidal velocity input (nominal wall shear stress of 10±5 dyn/cm ). Mesh independence was determined from convergence (less than 10%) of the axial wall shear stress (WSS) along the length of the stented model. The flow disturbance was characterized and quantified by the distributions of axial and transverse WSS, WSS gradients, and flow separation parameters. Normalized time-averaged effective WSS during the flow cycle was the smallest for the Wallstent (2.9 dyn/cm ) compared with the others (5.8 dyn/cm for the Bx Velocity stent, 5.0 dyn/cm for the Aurora stent, and 5.3 dyn/cm for the NIR stent). Regions of low mean WSS (less than 5 dyn/cm ) and elevated WSS gradients (less than 20 dyn/cm ) were also the largest for the Wallstent compared with the others. WSS gradients were the largest near the struts and remained distinctly nonzero for most of the region between the struts for all stent designs. Fully recirculating regions (as determined by separation parameter) were the largest for the Bx Velocity stent compared with the others. The most hemodynamically favorable stents from our computational analysis were the Bx Velocity and NIR stents, which were slotted-tube balloon-expandable designs. Since clinical data indicate lower restenosis rates for the Bx Velocity and NIR stents compared with the Wallstent, our data suggest that near-wall hemodynamics may predict some aspects of in vivo performance. Further consideration of biomechanics, including solid mechanics, in stent design is warranted. Copyright © 2009 by ASME. 2 2 2 2 2 2

A phospholipid-modified polystyrene-polyisobutylene-polystyrene (SIBS) triblock polymer for enhanced hemocompatibility and potential use in artificial heart valves

Poly(styrene-block-isobutylene-block-styrene) (\u27SIBS\u27) is selected for a novel trileaflet heart valve due to its high resistance to oxidation, hydrolysis, and enzyme attack. SIBS is modified using six different phospholipids and its mechanical properties characterized by tensile stress, peel strength, shear strength, contact angle, and surface energy, and then for hemocompatibility by studying the adhesion of fluorescently labeled platelets in a parallel plate chamber under physiological flow conditions. Phospholipid modification decreases SIBS tensile stress (at 45% strain) by 30% and reduces platelet adhesion by a factor of 10, thereby improving its hemocompatibility and its potential use as a synthetic heart valve. © SAGE Publications 2009

Effects of stent geometry on local flow dynamics and resulting platelet deposition in an in vitro model

Platelet deposition has been shown previously to depend on convective transport patterns, visualized by the instantaneous streamlines. Previous attempts to quantify hemodynamic studies of platelet deposition have been limited to 2D geometries. This study provides a physiologic assessment of the effects of stent geometry on platelet deposition by using actual 3D stents. Human blood with fluorescently labeled platelets was circulated through an in vitro system producing physiologic pulsatile flow in a compliant tube in which Bx Velocity, Wallstent and Aurora stents were implanted. Computational fluid dynamic models of the stents provided flow data to aid in explaining localized platelet deposition. Regions of constant flow separation proximal and distal to the strut exhibited very low platelet deposition. Platelet deposition was highest just downstream of flow stagnation regions due to convection towards the wall, then decreased with axial distance from the strut as flow streamlines became locally parallel to the wall. The nearly helically recirculating regions near the Bx Velocity stent connectors exhibited complex fluid dynamics with more platelet deposition, than the smaller separation regions. Localized platelet deposition was heavily dependent on flow convection, suggesting that arterial reaction to stents can be modulated in part by altering the hemodynamics associated with stent design. © 2008 - IOS Press and the authors. All rights reserved

Stented artery flow patterns and their effects on the artery wall

Stent design and geometry influence the fluid mechanical environment in an artery and hence affect clinical outcomes of restenosis. There is clearly a role for biomechanics in improving current stent designs. This review summarizes some of the work that has been done to address the fluid mechanical aspects of stenting. A variety of computational, experimental, and in vivo approaches have been employed, and the results demonstrate a strong dependence on stent design, as well as effects on hemodynamics in locations of the circulatory system quite removed from the stented segment. There are also important solid mechanical aspects that affect clinical failures of stents that are not summarized here. Copyright © 2007 by Annual Reviews. All rights reserved

Mechanical and leakage integrity testing considerations for evaluating the performance of tissue containment systems

Abstract Tissue containment systems (TCS) are medical devices that may be used during morcellation procedures during minimally invasive laparoscopic surgery. TCS are not new devices but their use as a potential mitigation for the spread of occult malignancy during laparoscopic power morcellation of fibroids and/or the uterus has been the subject of interest following reports of upstaging of previously undetected sarcoma in women who underwent a laparoscopic hysterectomy. Development of standardized test methods and acceptance criteria to evaluate the safety and performance of these devices will speed development, allowing for more devices to benefit patients. As a part of this study, a series of preclinical experimental bench test methods were developed to evaluate the mechanical and leakage performance of TCS that may be used in power morcellation procedures. Experimental tests were developed to evaluate mechanical integrity, e.g., tensile, burst, puncture, and penetration strengths for the TCS, and leakage integrity, e.g., dye and microbiological leakage (both acting as surrogates for blood and cancer cells) through the TCS. In addition, to evaluate both mechanical integrity and leakage integrity as a combined methodology, partial puncture and dye leakage was conducted on the TCS to evaluate the potential for leakage due to partial damage caused by surgical tools. Samples from 7 different TCSs were subjected to preclinical bench testing to evaluate leakage and mechanical performance. The performance of the TCSs varied significantly between different brands. The leakage pressure of the TCS varied between 26 and > 1293 mmHg for the 7 TCS brands. Similarly, the tensile force to failure, burst pressure, and puncture force varied between 14 and 80 MPa, 2 and 78 psi, and 2.5 N and 47 N, respectively. The mechanical failure and leakage performance of the TCS were different for homogeneous and composite TCSs. The test methods reported in this study may facilitate the development and regulatory review of these devices, may help compare TCS performance between devices, and increase provider and patient accessibility to improved tissue containment technologies

Spatial distribution of platelet deposition in stented arterial models under physiologic flow

This paper presents dynamic flow experiments with fluorescently labeled platelets to allow for spatial observation of wall attachment in inter-strut spacings, to investigate their relationship to flow patterns. Human blood with fluorescently labeled platelets was circulated through an in vitro system that produced physiologic pulsatile flow in a parallel plate flow chamber that contained three different stent designs that feature completely recirculating flow, partially recirculating flow (intermediate strut spacing), and completely reattached flow. Highly resolved spatial distribution of platelets was obtained by imaging fluorescently labeled platelets between the struts. Platelet deposition was higher in areas where flow is directed towards the wall, and lower in areas where flow is directed away from the wall. Flow detachment and reattachment points exhibited very low platelet deposition. Platelet deposition within intermediate strut spacing continued to increase throughout the experimental period, indicating that the deposition rate had not plateaued unlike other strut spacings. The spatial uniformity and temporal increase in platelet deposition for the intermediate strut spacing confirms and helps explain our previous finding that platelet deposition was highest with this strut spacing. Further experimental investigations will include more complex three-dimensional geometries. © 2005 Biomedical Engineering Society