PMCC PMCC

Search tips
Search criteria

Advanced
Results 1-12 (12)
 

Clipboard (0)
None

Select a Filter Below

Journals
Authors
more »
Year of Publication
Document Types
1.  Passive Pressure–Diameter Relationship and Structural Composition of Rat Mesenteric Lymphangions 
Lymphatic Research and Biology  2012;10(4):152-163.
Abstract
Background
Lymph flow depends on both the rate of lymph production by tissues and the extent of passive and active pumping. Here we aim to characterize the passive mechanical properties of a lymphangion in both mid-lymphangion and valve segments to assess regional differences along a lymphangion, as well as evaluating its structural composition.
Methods and Results
Mesenteric lymphatic vessels were isolated and cannulated in a microchamber for pressure–diameter (P-D) testing. Vessels were inflated from 0 to 20 cmH2O at a rate of 4 cmH2O/min, and vessel diameter was continuously tracked, using an inverted microscope, video camera, and custom LabVIEW program, at both mid-lymphangion and valve segments. Isolated lymphatic vessels were also pressure-fixed at 2 and 7 cmH2O and imaged using a nonlinear optical microscope (NLOM) to obtain collagen and elastin structural information. We observed a highly nonlinear P-D response at low pressures (3–5 cmH2O), which was modeled using a three-parameter constitutive equation. No significant difference in the passive P-D response was observed between mid-lymphangion and valve regions. NLOM imaging revealed an inner elastin layer and outer collagen layer at all locations. Lymphatic valve leaflets were predominantly elastin with thick axially oriented collagen bands at the insertion points.
Conclusions
We observed a highly nonlinear P-D response at low pressures (3–5 cmH2O) and developed the first constitutive equation to describe the passive P-D response for a lymphangion. The passive P-D response did not vary among regions, in agreement with the composition of elastin and collagen in the lymphatic wall.
doi:10.1089/lrb.2011.0015
PMCID: PMC3525898  PMID: 23145980
2.  INCREASED ARTERY WALL STRESS POST-STENTING LEADS TO GREATER INTIMAL THICKENING 
Since the first human procedure in the late 1980s, vascular stent implantation has been accepted as a standard form of treatment of atherosclerosis. Despite their tremendous success, these medical devices are not without their problems, as excessive neointimal hyperplasia can result in the formation of a new blockage (restenosis). Clinical data suggest that stent design is a key factor in the development of restenosis. Additionally, computational studies indicate that the biomechanical environment is strongly dependent on the geometrical configuration of the stent, and therefore possibly involved in the development of restenosis. We hypothesize that stents that induce higher stresses on the artery wall lead to a more aggressive pathobiologic response, as determined by the amount of neointimal hyperplasia. The aim of this investigation was to examine the role of solid biomechanics in the development of restenosis. A combination of computational modeling techniques and in vivo analysis were employed to investigate the pathobiologic response to two stent designs that impose greater or lesser levels of stress on the artery wall. Stent designs were implanted in a porcine model (pigs) for approximately 28 days and novel integrative pathology techniques (quantitative micro-computed tomography, histomorphometry) were utilized to quantify the pathobiologic response. Concomitantly, computational methods were used to quantify the mechanical loads that the two stents place on the artery. Results reveal a strong correlation between the computed stress values induced on the artery wall and the pathobiologic response; the stent that subjected the artery to the higher stresses had significantly more neointimal thickening at stent struts (high stress stent: 0.197 ± 0.020 mm vs. low-stress stent: 0.071 ± 0.016 mm). Therefore, we conclude that the pathobiologic differences are a direct result of the solid biomechanical environment, confirming the hypothesis that stents that impose higher wall stresses will provoke a more aggressive pathobiological response.
doi:10.1038/labinvest.2011.57
PMCID: PMC3103652  PMID: 21445059
biomechanics; finite element method; pathobiology; restenosis; vascular stent; vascular pathology
3.  Comparison of Near-wall Hemodynamic Parameters in Stented Artery Models 
Background
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 (CFD) simulations under pulsatile flow conditions.
Method of Approach
The flow disturbance was characterized by the distributions of wall shear stress (WSS), WSS gradients, and flow separation.
Results
Normalized time-averaged effective WSS during the flow cycle was the smallest for the Wallstent compared to others. Regions of low mean WSS (<5 dynes/cm2) and elevated WSS gradients (>20 dynes/cm3) were also the largest for the Wallstent compared to others. WSS gradients were the largest near the struts and remained distinctly non-zero for most of the region between the struts for all stent designs.
Conclusions
The most hemodynamically favorable stents from our computational analysis were Bx Velocity and NIR stents, which were slotted tube, balloon-expandable designs. Since clinical data indicates lower restenosis rates for the Bx Velocity and NIR stents compared to 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.
doi:10.1115/1.3118764
PMCID: PMC2767376  PMID: 19449960
Stent; Wallstent; Aurora stent; NIR stent; Bx Velocity stent; CFD; Strut; Wall shear stress; Wall shear stress gradients; Restenosis
4.  Mechanical Modeling of Stents Deployed in Tapered Arteries 
Annals of biomedical engineering  2008;36(12):2042-2050.
The biomechanical interaction of stents and the arteries into which they are deployed is a potentially important consideration for long-term success. Adverse arterial reactions to excessive stress and the resulting damage have been linked to the development of restenosis. Complex geometric features often encountered in these procedures can confound treatment. In some cases, it is desirable to deploy a stent across a region in which the diameter decreases significantly over the length of the stent. This study aimed to assess the final arterial diameter and circumferential stress in tapered arteries into which two different stents were deployed (one stiff and one less stiff). The artery wall was assumed to be made of a strain stiffening material subjected to large deformations, with a 10% decrease in diameter over the length of the stent. A commercially available finite element code was employed to solve the contact problem between the two elastic bodies. The stiffer stent dominated over arterial taper, resulting in a nearly constant final diameter along the length of the stent, and very high stresses, particularly at the distal end. The less stiff stent followed more closely the tapered contour of the artery, resulting in lower artery wall stresses. More compliant stents should be considered for tapered artery applications, perhaps even to the exclusion of tapered stents.
doi:10.1007/s10439-008-9582-0
PMCID: PMC2739058  PMID: 18846425
Stress; Restenosis; Vascular Interventions; Finite Element Analysis
5.  Biomechanical Issues in Endovascular Device Design 
Journal of Endovascular Therapy  2009;16(Suppl 1):I1-I11.
The biomechanical nature of the arterial system and its major disease states provides a series of challenges to treatment strategies. Endovascular device design objectives have mostly centered on short-term challenges, such as deployability and immediate restoration of reliable flow channels. The resulting design features may be at odds with long-term clinical success. In-stent restenosis, endoleaks, and loss of device structural integrity (e.g., strut fractures) are all manifestations of a lack of compatibility between the host vessel biomechanical environment and the implant design. Initial attempts to adapt device designs for increased compatibility, including drug-eluting and bioabsorbable stents, barely begin to explore the ways in which implant design can be modulated in time to minimize risk of failure. Biomechanical modeling has the potential to provide a virtual vascular environment in which new designs can be tested for their implications on long-term tissue reaction. These models will be based on high quality, highly resolved imaging information, as well as mechanobiology experiments from the cellular to the whole tissue level. These models can then be extended to incorporate biodegradation mechanics, facilitating the next generations of devices whose designs (including drug delivery profiles) change with time to enhance healing. The possibility of initiating changes in device design or drug release according to information on vascular healing (through clinical intervention or automated methods) provides the opportunity for truly individualized dynamic device design optimization.
doi:10.1583/08-2605.1
PMCID: PMC2747241  PMID: 19317580
endograft; stent; stent-graft; hemodynamics; stress; modeling; biomechanics; stent design; stenosis; aneurysm; imaging
6.  Effects of Stent Design and Atherosclerotic Plaque Composition on Arterial Wall Biomechanics 
Journal of Endovascular Therapy  2008;15(6):643-654.
Purpose: To examine the solid mechanical effects of varying stent design and atherosclerotic plaque stiffness on the biomechanical environment induced in a diseased artery wall model.
Methods: Computational modeling techniques were employed to investigate the final radius of the lumen and artery wall stresses after stent implantation. Two stent designs were studied (one stiff and one less stiff). The stenotic artery was modeled as an axisymmetrical diseased vessel with a 20% stenosis by diameter. The material properties of the diseased tissue in the artery models varied. Atherosclerotic plaques half as stiff (0.5×), of equal stiffness (1.0×), or twice as stiff (2.0×) as the artery wall were investigated.
Results: Final lumen radius was dependent on stent design, and the stiffer stent deformed the artery to an approximately 10% greater radius than the more compliant design. Alternatively, circumferential stress levels were dependent on both stent design and plaque material properties. Overall, the stiffer stent subjected the artery wall to much higher stress values than the more compliant design, with differences in peak values of 0.50, 0.31, and 0.09 MPa for the 2.0×, 1.0×, and 0.5× stiff plaques, respectively.
Conclusion: Evidence suggests that a judicious choice of stent design can minimize stress while maintaining a patent lumen in stenotic arteries. If confronted with a rigid, calcified plaque, stent design is more important, as design differences can impose dramatically different stress fields, while still providing arterial patency. Alternatively, stent design is not as much of an issue when treating a soft, lipid-laden plaque, as stress fields do not vary significantly among stent designs.
doi:10.1583/08-2443.1
PMCID: PMC2793418  PMID: 19090628
experimental model; artery wall model; stent; stent design; stress; restenosis; atherosclerotic plaque; finite element analysis
7.  A model of a radially expanding and contracting lymphangion 
Journal of biomechanics  2011;44(6):1001-1007.
The lymphatic system is an extensive vascular network featuring valves and contractile walls that pump interstitial fluid and plasma proteins back to the main circulation. Immune function also relies on the lymphatic system’s ability to transport white blood cells. Failure to drain and pump this excess fluid results in edema characterized by fluid retention and swelling of limbs. It is, therefore, important to understand the mechanisms of fluid transport and pumping of lymphatic vessels. Unfortunately, there are very few studies in this area, most of which assume Poiseuille flow conditions. In vivo observations reveal that these vessels contract strongly, with diameter changes of the order of magnitude of the diameter itself over a cycle that lasts typically 2–3 seconds. The radial velocity of the contracting vessel is on the order of the axial fluid velocity, suggesting that modeling flow in these vessels with a Poiseuille model is inappropriate. In this paper, we describe a model of a radially expanding and contracting lymphatic vessel and investigate the validity of assuming Poiseuille flow to estimate wall shear stress, which is presumably important for lymphatic endothelial cell mechanotransduction. Three different wall motions; periodic sinusoidal, skewed sinusoidal and physiologic wall motions, were investigated with steady and unsteady parabolic inlet velocities. Despite high radial velocities resulting from the wall motion, wall shear stress values were within 4% of quasi-static Poiseuille values. Therefore, Poiseuille flow is valid for the estimation of wall shear stress for the majority of the lymphangion contractile cycle.
doi:10.1016/j.jbiomech.2011.02.018
PMCID: PMC3086717  PMID: 21377158
computational fluid dynamics; lymph flow; contractility; shear stress; Poiseuille
8.  Long-term population demography of Trillium recurvatum on loess bluffs in western Tennessee, USA 
AoB Plants  2012;2012:pls015.
The paper uses modified population viability models and spatial structure via block analysis to assess population demography of Trillium recurvatum a clonal understory plant. The population is expanding, a likely outcome of the relatively high proportion of juvenile and non-flowering adult ramets and fast-replicating non-flowering adults. Further work is needed to elucidate the relative contributions of clonal vs seed recruitment to genetic structure and demography.
Background and aims
Understanding the demography of long-lived clonal herbs, with their extreme modularity, requires knowledge of both their short- and long-term survival and ramet growth patterns. The primary objective of this study was to understand the dynamics of a clonal forest herb, Trillium recurvatum, by examining temporal and small-scale demographic patterns. We hypothesized: (i) there would be more variability in the juvenile age class compared with non-flowering adult and flowering adult classes due to year-to-year fluctuations in recruitment; (ii) rates of population growth (λ) and increase (r) would be highest in non-flowering ramets due to a combination of transitions from the juvenile stage and reversions from flowering adults; and (iii) inter-ramet distances would be most variable between flowering and juvenile ramets due to a combination of clonal growth, seed dispersal by ants and ramet death over time.
Methodology
Census data were collected on the total number of stems in the population from 1990 to 2007, and placed within one of three life stages (juvenile, three-leaf non-flowering and three-leaf flowering). Modified population viability equations were used to assess temporal population viability, and spatial structure was assessed using block krigging. Correlations were performed using current and prior season weather to current population demography.
Principal results
The first hypothesis was rejected. The second hypothesis was supported: population increase (r) and growth rate (λ) were highest in non-flowering ramets. Finally, the third hypothesis was rejected: there was no apparent density dependence within this population of Trillium and no apparent spatial structure among life stages.
Conclusions
Overall population density fluctuated over time, possibly due to storms that move soil, and prior year's temperature and precipitation. However, density remained at some dynamic stable level. The juvenile age class had greater variability for the duration of this study and population growth rate was greatest for non-flowering ramets.
doi:10.1093/aobpla/pls015
PMCID: PMC3357055  PMID: 22616024
9.  Deformation-induced hydrolysis of a degradable polymeric cylindrical annulus 
A thermodynamically consistent framework for describing the response of materials undergoing deformation-induced degradation is developed and applied to a particular biodegradable polymer system. In the current case, energy is dissipated through the mechanism of hydrolytic degradation and its effects are incorporated in the constitutive model by appropriately stipulating the forms for the rate of dissipation and for the degradation-dependent Helmholtz potential which changes with the extent of the degradation of the material. When degradation does not occur, the response of the material follows the response of a power-law generalized neo-Hookean material that fits the response of the non-degraded poly(L-lactic acid) under uniaxial extension. We study the inflation and extension of a degrading cylindrical annulus and the influence of the deformation on the mechanism of degradation and its consequent mechanical response. Depreciation of mechanical properties due to degradation confers time-dependent characteristics to the response of the biodegradable material: the material creeps when subjected to constant loads and stresses necessary to keep a fixed deformation relax.
doi:10.1007/s10237-009-0168-z
PMCID: PMC2837768  PMID: 19680702
degradation; scission; strain-softening; damage; internal variable; poly(lactic acid)
10.  BIOFLUID MECHANICS OF SPECIAL ORGANS AND THE ISSUE OF SYSTEM CONTROL 
Annals of biomedical engineering  2010;38(3):1204-1215.
In the field of fluid flow within the human body, blood flow in the systemic circulation has been the main focus since the recognition by William Harvey that blood was in fact in continuous circulation and carried by a network of blood vessels. But beyond the systemic circulation, other fluids and other fluid flow phenomena pervade the body so totally that it would be hard to imagine a bodily function of any kind that does not involve fluids or fluid flow. In fact, the study of the systemic circulation is limited to not only the type of fluid involved but also to the type of service which the systemic circulation provides - the first being blood, of course, the second is transport - the principal function of the systemic circulation is to provide a means of transporting blood continuously from the heart to tissue cells and back again. Some of the most fascinating fluid flow phenomena within the human body involve fluids other than blood and a service other than transport- the lymphatic and pulmonary systems provide two striking examples. In this paper we outline the special fluid mechanics of these two systems. While transport is still involved in both cases, this is not the only service which they provide and blood is not the only fluid involved. In both systems, filtration, extraction, enrichment, and in general some “treatment” of the fluid itself is the primary function. In the pulmonary system, the liquid lining of the lungs plays a pivotal role, and the mechanical interaction between tissue and liquid is of key importance to lung viability. In disease states such as respiratory distress syndrome, the lining fluid can become dysfunctional, leading to reduced gas exchange and damage to sensitive pulmonary tissues. The study of the systemic circulation has also been conventionally limited to treating the system as if it were an open-loop system in which the main hemodynamic variables such as pressure and flow are governed by the laws of fluid mechanics independently from the physiological controls and regulations that govern these same variables. This implies that any failure of the system can be fully explained in terms of the laws of fluid mechanics, which of course is not the case. While a system failure due to a physical obstruction in a blood vessel can be readily explained in terms of the laws of fluid mechanics, a system failure due to arrhythmia cannot. In this paper we examine the clinical implications of these issues and of the special biofluid mechanics issues that arise in the lymphatic and pulmonary systems.
PMCID: PMC2917121  PMID: 20336840
lymph; endothelium; edema; ventilator-induced lung injury; respiratory distress syndrome; surfactant; neurovascular control; sudden cardiac death; broken heart syndrome
11.  Biomechanical Issues in Endovascular Device Design 
The biomechanical nature of the arterial system and its major disease states provides a series of challenges to treatment strategies. Endovascular device design objectives have mostly centered around short term challenges such as deployability and immediate restoration of reliable flow channels. The resulting design features may be at odds with long term clinical success. In-stent restenosis, endoleaks and loss of device structural integrity (e.g., strut fractures) are all manifestations of a lack of compatibility between the host vessel biomechanical environment and implant design. Initial attempts to adapt device designs for increased compatibility, including drug eluting and bioabsorbable stents, barely begin to explore the ways in which implant design can be modulated in time to minimize risk of failure. Biomechanical modeling has the potential to provide a virtual vascular environment in which new designs can be tested for their implications on long term tissue reaction. These models will be based on high quality, highly resolved imaging information, as well as mechanobiology experiments from the cellular to the whole tissue level. These models can then be extended to incorporate biodegradation mechanics, facilitating the design of the next generations of devices whose designs (including drug delivery profiles) change with time to enhance healing. The possibility of initiating changes in device design or drug release according to information on vascular healing (through clinical intervention or automated methods) provides the opportunity for truly individualized dynamic device design optimization.
doi:10.1583/08-2605.1
PMCID: PMC2747241  PMID: 19317580
Endografts; stents; hemodynamics; stress; modeling
12.  Image Correlation Algorithm for Measuring Lymphocyte Velocity and Diameter Changes in Contracting Microlymphatics 
Annals of biomedical engineering  2006;35(3):387-396.
Efforts have recently been made to estimate wall shear stress throughout the contractile cycle of mesenteric rat lymphatics with a high speed video microscopy system. This was prompted by reports in the literature that lymphatic pumping is related to wall shear stress. While one can estimate wall shear stress by tracking lymphocyte velocity, it is prohibitively tedious to manually track particles over a reasonable time frame for a good number of experiments. To overcome this, an image correlation method similar to digital particle imaging velocimetry was developed and tested on contracting lymphatics to measure both vessel diameter and fluid velocity. The program tracked temporal fluctuations in spatially averaged velocity with a standard error of prediction of 0.4 mm/s. From these studies we have measured velocities ranging from −2 to 4 mm/s. Diameter changes were also measured with a standard error of 7 μm. These algorithms and techniques could be beneficial for investigating various changes in contractile behavior as a function of changes in velocity and wall shear stress.
doi:10.1007/s10439-006-9225-2
PMCID: PMC1989687  PMID: 17151922
Lymphatics; Flow; Image correlation; Vessel contraction; Lymph velocity

Results 1-12 (12)