A Hemodynamic Comparison of Myocardial Bridging and Coronary Atherosclerotic Stenosis: A Computational Model With Experimental Evaluation.

Myocardial bridging (MB) and coronary atherosclerotic stenosis can impair coronary blood flow and may cause myocardial ischemia or even heart attack. It remains unclear how MB and stenosis are similar or different regarding their impacts on coronary hemodynamics. The purpose of this study was to compare the hemodynamic effects of coronary stenosis and MB using experimental and computational fluid dynamics (CFD) approaches. For CFD modeling, three MB patients with different levels of lumen obstruction, mild, moderate, and severe were selected. Patient-specific left anterior descending (LAD) coronary artery models were reconstructed from biplane angiograms. For each MB patient, the virtually healthy and stenotic models were also simulated for comparison. In addition, an in vitro flow-loop was developed, and the pressure drop was measured for comparison. The CFD simulations results demonstrated that the difference between MB and stenosis increased with increasing MB/stenosis severity and flowrate. Experimental results showed that increasing the MB length (by 140%) only had significant impact on the pressure drop in the severe MB (39% increase at the exercise), but increasing the stenosis length dramatically increased the pressure drop in both moderate and severe stenoses at all flow rates (31% and 93% increase at the exercise, respectively). Both CFD and experimental results confirmed that the MB had a higher maximum and a lower mean pressure drop in comparison with the stenosis, regardless of the degree of lumen obstruction. A better understanding of MB and atherosclerotic stenosis may improve the therapeutic strategies in coronary disease patients and prevent acute coronary syndromes.

Biomechanical Restoration Potential of Pentagalloyl Glucose after Arterial Extracellular Matrix Degeneration.

The objective of this study was to quantify pentagalloyl glucose (PGG) mediated biomechanical restoration of degenerated extracellular matrix (ECM). Planar biaxial tensile testing was performed for native (N), enzyme-treated (collagenase and elastase) (E), and PGG (P) treated porcine abdominal aorta specimens (n = 6 per group). An Ogden material model was fitted to the stress-strain data and finite element computational analyses of simulated native aorta and aneurysmal abdominal aorta were performed. The maximum tensile stress of the N group was higher than that in both E and P groups for both circumferential (43.78 ± 14.18 kPa vs. 10.03 ± 2.68 kPa vs. 13.85 ± 3.02 kPa; p = 0.0226) and longitudinal directions (33.89 ± 8.98 kPa vs. 9.04 ± 2.68 kPa vs. 14.69 ± 5.88 kPa; p = 0.0441). Tensile moduli in the circumferential direction was found to be in descending order as N > P > E (195.6 ± 58.72 kPa > 81.8 ± 22.76 kPa > 46.51 ± 15.04 kPa; p = 0.0314), whereas no significant differences were found in the longitudinal direction (p = 0.1607). PGG binds to the hydrophobic core of arterial tissues and the crosslinking of ECM fibers is one of the possible explanations for the recovery of biomechanical properties observed in this study. PGG is a beneficial polyphenol that can be potentially translated to clinical practice for preventing rupture of the aneurysmal arterial wall.

Static and Flow Conditions: Endothelial Cell Migration onto Metal Stent Surfaces.

Restenosis associated with intimal hyperplasia and thrombosis at sites of balloon angioplasty or stent placement remains an important clinical problem. It is likely that loss or damage to the arterial endothelium associated with these interventional procedures as well as the rate of its restoration plays a critical role in the extent of restenosis. Migration of arterial endothelial cells from adjacent intact endothelium is the predominant source of cells involved in re-endothelialization of the injured site. In this paper, we review the influence of hemodynamics on endothelial cell migration, both in vivo and in vitro. In addition, we present recent in vitro studies demonstrating the importance of the nature of metal substrates in modulating endothelial cell migration rate. Finally, we review the cellular and molecular mechanisms likely involved in governing endothelial cell migration, and relate them to a possible scenario of endothelial response to injury at sites of arterial intervention. Understanding the important factors regulating endothelial migration may provide insights that will ultimately lead to methods to accelerate endothelial healing and reduce the occurrence of arterial restenosis.

Hemodynamic Flow-Induced Mechanotransduction Signaling Influences the Radiation Response of the Vascular Endothelium.

Hemodynamic shear stress is defined as the physical force exerted by the continuous flow of blood in the vascular system. Endothelial cells, which line the inner layer of blood vessels, sense this physiological force through mechanotransduction signaling and adapt to maintain structural and functional homeostasis. Hemodynamic flow, shear stress and mechanotransduction signaling are, therefore, an integral part of endothelial pathophysiology. Although this is a well-established concept in the cardiovascular field, it is largely dismissed in studies aimed at understanding radiation injury to the endothelium and subsequent cardiovascular complications. We and others have reported on the differential response of the endothelium when the cells are under hemodynamic flow shear compared with static culture. Further, we have demonstrated significant differences in the gene expression of static versus shear-stressed irradiated cells in four key pathways, reinforcing the importance of shear stress in understanding radiation injury of the endothelium. This article further emphasizes the influence of hemodynamic shear stress and the associated mechanotransduction signaling on physiological functioning of the vascular endothelium and underscores its significance in understanding radiation injury to the vasculature and associated cardiac complications. Studies of radiation effect on endothelial biology and its implication on cardiotoxicity and vascular complications thus far have failed to highlight the significance of these factors. Factoring in these integral parts of the endothelium will enhance our understanding of the contribution of the endothelium to radiation biology. Without such information, the current approaches to studying radiation-induced injury to the endothelium and its consequences in health and disease are limited.

Impact of parallel micro-engineered stent grooves on endothelial cell migration, proliferation, and function: an in vivo correlation study of the healing response in the coronary swine model.

BACKGROUND: Stent luminal surface characteristics influence surface endothelialization. We hypothesize that luminal stent microgrooves created in the direction of coronary flow accelerate endothelial cell migration, resulting in lower levels of neointimal formation. METHODS AND RESULTS: Surface coverage efficiency was evaluated in vitro by allowing human aortic endothelial cells (HAEC) to migrate onto microgrooved (G) or smooth (NG) surfaces. HAEC functionality was assessed by proliferation rate, apoptosis rate, nitric oxide production, and inflammatory markers TNF-α and VCAM-1 expression. Early endothelialization and restenosis studies were performed using the porcine coronary injury model. Stainless steel stents of identical design with (GS) and without (NGS) luminal microgrooves were used. The commercially available Multi-Link Vision (MLVS) stent of identical design was used as a control. The degree of GS and NGS surface endothelialization was compared at 3 days. Biocompatibility and tissue response outcomes were evaluated at 28 days. The in vitro study demonstrated that at 7 days the presence of surface microgrooves increased HAEC migration distance >2-fold. Cell proliferation rate and nitric oxide production were increased and apoptosis rate was decreased. There was no difference in inflammatory marker expression. At 3 days, coronary artery stent endothelialization was significantly increased in GS compared with NGS (81.3% versus 67.5%, P=0.0002). At 28 days, GS exhibited lower neointimal thickness compared with either NGS (21.1%, P=0.011) or MLVS (40.8%, P=0.014). CONCLUSION: Parallel microgrooves on coronary stent luminal surfaces promote endothelial cell migration and positively influence endothelial cell function, resulting in decreased neointimal formation in the porcine coronary injury model.

Mechanical stress-activated integrin α5ß1 induces opening of connexin 43 hemichannels.

The connexin 43 (Cx43) hemichannel (HC) in the mechanosensory osteocytes is a major portal for the release of factors responsible for the anabolic effects of mechanical loading on bone formation and remodeling. However, little is known about how the Cx43 molecule responds to mechanical stimulation leading to the opening of the HC. Here, we demonstrate that integrin α5ß1 interacts directly with Cx43 and that this interaction is required for mechanical stimulation-induced opening of the Cx43 HC. Direct mechanical perturbation via magnetic beads or conformational activation of integrin α5ß1 leads to the opening of the Cx43 HC, and this role of the integrin is independent of its association with an extracellular fibronectin substrate. PI3K signaling is responsible for the shear stress-induced conformational activation of integrin α5ß1 leading to the opening of the HC. These results identify an unconventional function of integrin that acts as a mechanical tether to induce opening of the HC and provide a mechanism connecting the effect of mechanical forces directly to anabolic function of the bone.

Effects of Axial Stretch on Cell Proliferation and Intimal Thickness in Arteries in Organ Culture.

Intimal hyperplasia (IH) remains the major cause of intermediate and long-term failure of vascular grafts and endovascular interventions. Arteries are subjected to a significant longitudinal stress in addition to the shear stress and tensile stress from the blood flow. The aim of this study was to determine the effect of axial stretch on cell proliferation and IH in arteries. Porcine carotid arteries, intact or endothelial cell (EC) denudated, were maintained ex vivo at different stretch ratios (1.3, 1.5, and 1.8) and flow rates (16 or 160 mL/min) while remaining at physiologic pressure for 7 days. The viability of the arteries was verified with norepinephrine, carbachol, and sodium nitroprusside stimulations, and the cell proliferation was detected using bromodeoxyuridine labeling and immunostaining. Our results showed that the axial stretch ratio did not significantly affect intimal thickness and cell proliferation in normal arteries. However, axial stretch increased the neointimal thickness in EC denudated arteries cultured under low flow conditions. The cell proliferation increased significantly in the intima and inner half of the media of the EC denudated arteries under normal or elevated axial stretch in comparison to intact arteries at the same stretch ratio. These results demonstrated that axial stretch with EC denudation and low flow increases neointimal formation and cell proliferation in the arteries.

A novel wounding device suitable for quantitative biochemical analysis of wound healing and regeneration of cultured epithelium.

We describe the fabrication and use of an in vitro wounding device that denudes cultured epithelium in patterns designed to leave behind strips or islands of cells sufficiently narrow or small to ensure that all the remaining cells become rapidly activated and then migrate, dedifferentiate, and proliferate in near synchrony. The design ensures that signals specific to regenerating cells do not become diluted by quiescent differentiated cells that are not affected by wound-induced activation. The device consists of a flat circular disk of rubber, engraved to produce alternating ridges and grooves in patterns of concentric circles or parallel lines. The disk is mounted at the end of a pneumatically controlled piston assembly. Application of controlled pressure and circular or linear movement of the disk on cultures produced highly reproducible wounding patterns. The near-synchronous regenerative activity of cell bands or islands allowed the collection of samples large enough for biochemical studies to sensitively detect alterations involving mRNA for several early response genes and protein phosphorylation in major signaling pathways. The method is versatile, easy to use and reproducible, and should facilitate biochemical, proteomic, and genomic studies of wound-induced regeneration of cultured epithelium.

Prostaglandin promotion of osteocyte gap junction function through transcriptional regulation of connexin 43 by glycogen synthase kinase 3/beta-catenin signaling.

Gap junction intercellular communication in osteocytes plays an important role in bone remodeling in response to mechanical loading; however, the responsible molecular mechanisms remain largely unknown. Here, we show that phosphoinositide-3 kinase (PI3K)/Akt signaling activated by fluid flow shear stress and prostaglandin E(2) (PGE(2)) had a stimulatory effect on both connexin 43 (Cx43) mRNA and protein expression. PGE(2) inactivated glycogen synthase kinase 3 (GSK-3) and promoted nuclear localization and accumulation of beta-catenin. Knockdown of beta-catenin expression resulted in a reduction in Cx43 protein. Furthermore, the chromatin immunoprecipitation (ChIP) assay demonstrated an association of beta-catenin with the Cx43 promoter, suggesting that beta-catenin could regulate Cx43 expression at the level of gene transcription. We have previously reported that PGE(2) activates cyclic AMP (cAMP)-protein kinase A (PKA) signaling and increases Cx43 and gap junctions. Interestingly, the activation of PI3K/Akt appeared to be independent of the activation of PKA, whereas both PI3K/Akt and PKA signaling inactivated GSK-3 and increased beta-catenin translocation. Together, these results suggest that shear stress, through PGE(2) release, activates both PI3K/Akt and cAMP-PKA signaling, which converge through the inactivation of GSK-3, leading to the increase in nuclear accumulation of beta-catenin. beta-Catenin binds to the Cx43 promoter, stimulating Cx43 expression and functional gap junctions between osteocytes.

Comparison of artery organ culture and co-culture models for studying endothelial cell migration and its effect on smooth muscle cell proliferation and migration.

Arterial restenosis associated with intimal hyperplasia is the major cause of long-term failure of vascular interventions. Endothelium injury and the proliferation and migration of smooth muscle cells (SMC) are key events in the development of intimal hyperplasia. The objectives of this study were to develop an ex vivo artery injury model for studying endothelial cell (EC) migration and to compare it with an in vitro co-culture arterial wall injury model in terms of the effect of flow on EC migration and its effect on SMC migration and proliferation. Our results demonstrated that shear flow improves reendothelialization in the injured area by promoting EC migration. The migration distance of ECs is much smaller in the arteries than in an in vitro cell culture model (3.57+/-1.29 mm vs. 5.2+/-1.4 cm, p<0.001). SMC proliferation was significantly less in the EC intact and reendothelialization areas than in the EC denuded areas indicating that reendothelialization suppresses SMC proliferation. Our models provide a new approach to study techniques to enhance endothelium healing.

Human aortic endothelial cell response to 316L stainless steel material microstructure.

The role of metal microstructure (e.g. grain sizes) in modulating cell adherence behavior is not well understood. This study investigates the effect of varying grain sizes of 316L stainless steel (SS) on the attachment and spreading of human aortic endothelial cells (HAECs). Four different grain size samples; from 16 to 66 microm (ASTM 9.0-4.9) were sectioned from sheets. Grain structure was revealed by polishing and etching with glycergia. Contact angle measurement was done to assess the hydrophilicity of the specimens. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the roughness and surface chemistry of the specimens. Cells were seeded on mechanically polished and chemically etched specimens followed by identification of activated focal adhesion sites using fluorescently tagged anti-pFAK (phosphorylated focal adhesion kinase). The 16 microm grain size etched specimens had significantly (P < 0.01) higher number of cells attached per cm(2) than other specimens, which may be attributed to the greater grain boundary area and associated higher surface free energy. This study shows that the underlying material microstructure may influence the HAEC behavior and may have important implications in endothelialization.

Adaptation of connexin 43-hemichannel prostaglandin release to mechanical loading.

Bone tissues respond to mechanical loading/unloading regimens to accommodate (re)modeling requirements; however, the underlying molecular mechanism responsible for these responses is largely unknown. Previously, we reported that connexin (Cx) 43 hemichannels in mechanosensing osteocytes mediate the release of prostaglandin, PGE(2), a crucial factor for bone formation in response to anabolic loading. We show here that the opening of hemichannels and release of PGE(2) by shear stress were significantly inhibited by a potent antibody we developed that specifically blocks Cx43-hemichannels, but not gap junctions or other channels. The opening of hemichannels and release of PGE(2) are magnitude-dependent on the level of shear stress. Insertion of a rest period between stress enhances this response. Hemichannels gradually close after 24 h of continuous shear stress corresponding with reduced Cx43 expression on the cell surface, thereby reducing any potential negative effects of channels staying open for extended periods. These data suggest that Cx43-hemichannel activity associated with PGE(2) release is adaptively regulated by mechanical loading to provide an effective means of regulating levels of extracellular signaling molecules responsible for initiation of bone (re)modeling.

In vivo cardiovascular assays for drug discovery: evolution of the drug-eluting stent.

Percutaneous intervention using balloon angioplasty accompanied by stent implantation has become the predominant procedure to treat occlusive coronary and peripheral vascular disease. Unfortunately, restenosis associated with intimal hyperplasia and arterial remodeling at the stented site occurs within 6 months in 20 to 30% of cases. To address this problem, the concept of utilizing a stent as the vehicle to deliver agents locally and limit the overexuberant tissue response related to its placement has been developed. Targeting excess arterial wall smooth muscle cell proliferation, preclinical studies have demonstrated the efficacy of two drugs, paclitaxel and rapamycin, in both in vitro and in vivo animal studies. Early, as well as large, randomized clinical studies using polymer-coated, drug-eluting stents have clearly demonstrated a significant and dramatic efficacy in reducing restenosis rates and improving clinical outcomes compared with the use of the bare stent for revascularization procedures. Despite the low incidence of late thrombosis associated with the rapamycin- and paclitaxel-eluting stents, some concerns remain (such as the need for sustained anticoagulant therapy), providing the impetus for developing coated stents that promote rather than inhibit endothelial healing in order to limit the restenotic response.

Polyunsaturated fatty acids mobilize intracellular Ca2+ in NT2 human teratocarcinoma cells by causing release of Ca2+ from mitochondria.

In a variety of disorders, overaccumulation of lipid in nonadipose tissues, including the heart, skeletal muscle, kidney, and liver, is associated with deterioration of normal organ function, and is accompanied by excessive plasma and cellular levels of free fatty acids (FA). Increased concentrations of FA may lead to defects in mitochondrial function found in diverse diseases. One of the most important regulators of mitochondrial function is mitochondrial Ca(2+) ([Ca(2+)](m)), which fluctuates in coordination with intracellular Ca(2+) ([Ca(2+)](i)). Polyunsaturated FA (PUFA) have been shown to cause [Ca(2+)](i) mobilization albeit by unknown mechanisms. We have found that PUFA but not monounsaturated or saturated FA cause [Ca(2+)](i) mobilization in NT2 human teratocarcinoma cells. Unlike the [Ca(2+)](i) response to the muscarinic G protein-coupled receptor agonist carbachol, PUFA-mediated [Ca(2+)](i) mobilization in NT2 cells is independent of phospholipase C and inositol-1,4,5-trisphospate (IP(3)) receptor activation, as well as IP(3)-sensitive internal Ca(2+) stores. Furthermore, PUFA-mediated [Ca(2+)](i) mobilization is inhibited by the mitochondria uncoupler carboxyl cyanide m-chlorophenylhydrozone. Direct measurements of [Ca(2+)](m) with X-rhod-1 and (45)Ca(2+) indicate that PUFA induce Ca(2+) efflux from mitochondria. Further studies show that ruthenium red, an inhibitor of the mitochondrial Ca(2+) uniporter, blocks PUFA-induced Ca(2+) efflux from mitochondria, whereas inhibitors of the mitochondrial permeability transition pore cyclosporin A and bongkrekic acid have no effect. Thus PUFA-gated Ca(2+) release from mitochondria, possibly via the Ca(2+) uniporter, appears to be the underlying mechanism for PUFA-induced [Ca(2+)](i) mobilization in NT2 cells.

A model system to assess key vascular responses to biomaterials.

PURPOSE: To establish a reproducible laboratory test to evaluate prospective vascular biomaterials with respect to their thromboinflammatory properties by examining fibrinogen, platelet, and monocyte binding. Endothelial migration onto these surfaces was used as an index of vascular healing. METHODS: To evaluate biomaterials for potential thrombogenicity and inflammation, binding assays of radiolabeled human fibrinogen, platelets, and monocytes were performed on standard pieces of vascular biomaterials, including metals and polymeric and ceramic-coated materials. Using an established in vitro endothelial cell migration model, the relative migration rate of cultured human aortic endothelial cells onto these vascular biomaterials was measured and compared. The fibrinogen, platelet, and monocyte binding results were combined along with the migration results to create an overall score of biocompatibility. RESULTS: A significant direct relation of platelet and monocyte binding to the amount of adsorbed fibrinogen was observed. In contrast, migration rates of cultured human aortic endothelial cells onto the same biomaterial surfaces were found to be inversely related the amount of bound fibrinogen. Among the materials tested, stainless steel received the highest score of biocompatibility, while turbostratic carbon scored the lowest. CONCLUSIONS: Fibrinogen, platelet, and monocyte binding levels, as well as endothelial migration rates onto vascular material surfaces, provide a basis for evaluating thrombogenicity, inflammatory potential, and endothelialization in the laboratory prior to in vivo testing.

Mechanical strain opens connexin 43 hemichannels in osteocytes: a novel mechanism for the release of prostaglandin.

Mechanosensing bone osteocytes express large amounts of connexin (Cx)43, the component of gap junctions; yet, gap junctions are only active at the small tips of their dendritic processes, suggesting another function for Cx43. Both primary osteocytes and the osteocyte-like MLO-Y4 cells respond to fluid flow shear stress by releasing intracellular prostaglandin E2 (PGE2). Cells plated at lower densities release more PGE2 than cells plated at higher densities. This response was significantly reduced by antisense to Cx43 and by the gap junction and hemichannel inhibitors 18 beta-glycyrrhetinic acid and carbenoxolone, even in cells without physical contact, suggesting the involvement of Cx43-hemichannels. Inhibitors of other channels, such as the purinergic receptor P2X7 and the prostaglandin transporter PGT, had no effect on PGE2 release. Cell surface biotinylation analysis showed that surface expression of Cx43 was increased by shear stress. Together, these results suggest fluid flow shear stress induces the translocation of Cx43 to the membrane surface and that unapposed hemichannels formed by Cx43 serve as a novel portal for the release of PGE2 in response to mechanical strain.

Effects of mechanical strain on the function of Gap junctions in osteocytes are mediated through the prostaglandin EP2 receptor.

Osteocytes embedded in the matrix of bone are thought to be mechanosensory cells that translate mechanical strain into biochemical signals that regulate bone modeling and remodeling. We have shown previously that fluid flow shear stress dramatically induces prostaglandin release and COX-2 mRNA expression in osteocyte-like MLO-Y4 cells, and that prostaglandin E2 (PGE2) released by these cells functions in an autocrine manner to regulate gap junction function and connexin 43 (Cx43) expression. Here we show that fluid flow regulates gap junctions through the PGE2 receptor EP2 activation of cAMP-dependent protein kinase A (PKA) signaling. The expression of the EP2 receptor, but not the subtypes EP1,EP3, and EP4, increased in response to fluid flow. Application of PGE2 or conditioned medium from fluid flow-treated cells to non-stressed MLO-Y4 cells increased expression of the EP2 receptor. The EP2 receptor antagonist, AH6809, suppressed the stimulatory effects of PGE2 and fluid flow-conditioned medium on the expression of the EP2 receptor, on Cx43 protein expression, and on gap junction-mediated intercellular coupling. In contrast, the EP2 receptor agonist butaprost, not the E1/E3 receptor agonist sulprostone, stimulated the expression of Cx43 and gap junction function. Fluid flow conditioned medium and PGE2 stimulated cAMP production and PKA activity suggesting that PGE2 released by mechanically stimulated cells is responsible for the activation of cAMP and PKA. The adenylate cyclase activators, forskolin and 8-bromo-cAMP, enhanced intercellular connectivity, the number of functional gap junctions, and Cx43 protein expression, whereas the PKA inhibitor, H89, inhibited the stimulatory effect of PGE2 on gap junctions. These studies suggest that the EP2 receptor mediates the effects of autocrine PGE2 on the osteocyte gap junction in response to fluid flow-induced shear stress. These data support the hypothesis that the EP2 receptor, cAMP, and PKA are critical components of the signaling cascade between mechanical strain and gap junction-mediated communication between osteocytes.

IkappaBalpha-dependent regulation of low-shear flow-induced NF-kappa B activity: role of nitric oxide.

We have investigated the role of inhibitor kappaBalpha (IkappaBalpha) in the activation of nuclear factor kappaB (NF-kappaB) observed in human aortic endothelial cells (HAEC) undergoing a low shear stress of 2 dynes/cm(2). Low shear for 6 h resulted in a reduction of IkappaBalpha levels, an activation of NF-kappaB, and an increase in kappaB-dependent vascular cell adhesion molecule 1 (VCAM-1) mRNA expression and endothelial-monocyte adhesion. Overexpression of IkappaBalpha in HAEC attenuated all of these shear-induced responses. These results suggest that downregulation of IkappaBalpha is the major factor in the low shear-induced activation of NF-kappaB in HAEC. We then investigated the role of nitric oxide (NO) in the regulation of IkappaBalpha/NF-kappaB. Overexpression of endothelial nitric oxide synthase (eNOS) inhibited NF-kappaB activation in HAEC exposed to 6 h of low shear stress. Addition of the structurally unrelated NO donors S-nitrosoglutathione (300 microM) or sodium nitroprusside (1 mM) before low shear stress significantly increased cytoplasmic IkappaBalpha and concomitantly reduced NF-kappaB binding activity and kappaB-dependent VCAM-1 promoter activity. Together, these data suggest that NO may play a major role in the regulation of IkappaBalpha levels in HAEC and that the application of low shear flow increases NF-kappaB activity by attenuating NO generation and thus IkappaBalpha levels.

Influence of stent design and material composition on procedure outcome.

Although intravascular stents have received widespread application, significant limitations remain. In stent restenosis, the most pervasive problem affecting stents, is related in part to technical aspects of the device. Design features of the stent that influence outcome have been identified and optimized for improved performance. The influence of stent materials on critical aspects of healing, such as thrombotic, inflammatory, and hyperplastic responses, are less well understood. For this reason, significant progress in this area is lacking. Current stents have significant contamination with industrial impurities on the surface and in the bulk. This fact adds to the difficulties in interpreting the biologic reaction of the host to the device. Better understanding of the basic biologic interactions is the path to significant improvement.