A homogenized constrained mixture model of restenosis and vascular remodelling after balloon angioplasty

Restenosis is one of the main adverse effects of the treatment of atherosclerosis through balloon angioplasty or stenting. During the intervention, the arterial wall is overstretched, causing a cascade of cellular events and subsequent neointima formation. This mechanical stimulus and its mechanobiological effects can be reproduced in biomechanical simulations. The aim of these models is to predict the long-term outcome of these procedures, to help increase the understanding of restenosis formation and to allow for in silico optimization of the treatment. We propose a predictive finite-element model of restenosis, using the homogenized constrained mixture modelling framework designed to model growth and remodelling in soft tissues. We compare the results with clinical observations in human coronary arteries and experimental findings in non-human primate models. We also explore the model’s clinical relevance by testing its response to different balloon loads and to the use of drug-eluting balloons. The comparison of the results with experimental data shows the relevance of the model. We show its ability to predict both inward and outward remodelling as observed in vivo and we show the importance of an improved understanding of restenosis formation from a biomechanical point of view.     

An in silico study on the role of smooth muscle cell migration in neointimal formation after coronary stenting

Excessive migration and proliferation of smooth muscle cells (SMCs) has been observed as a major factor contributing to the development of in-stent restenosis after coronary stenting. Building upon the results from in vivo experiments, we formulated a hypothesis that the speed of the initial tissue re-growth response is determined by the early migration of SMCs from the injured intima. To test this hypothesis, a cellular Potts model of the stented artery is developed where stent struts were deployed at different depths into the tissue. An extreme scenario with a ruptured internal elastic lamina was also considered to study the role of severe injury in tissue re-growth. Based on the outcomes, we hypothesize that a deeper stent deployment results in on average larger fenestrae in the elastic lamina, allowing easier migration of SMCs into the lumen. The data also suggest that growth of the neointimal lesions owing to SMC proliferation is strongly dependent on the initial number of migrated cells, which form an initial condition for the later phase of the vascular repair. This mechanism could explain the in vivo observation that the initial rate of neointima formation and injury score are strongly correlated.     

Effect of stents coated with a combination of sirolimus and alpha-lipoic acid in a porcine coronary restenosis model

The aim of this study was to evaluate antiproliferative sirolimus- and antioxidative alpha-lipoic acid (ALA)-eluting stents using biodegradable polymer [poly-l-lactic acid (PLA)] in a porcine coronary overstretch restenosis model. Forty coronary arteries of 20 pigs were randomized into four groups in which the coronary arteries had a bare metal stent (BMS, n = 10), ALA-eluting stent with PLA (AES, n = 10), sirolimus-eluting stent with PLA (SES, n = 10), or sirolimus- and ALA-eluting stent with PLA (SAS, n = 10). A histopathological analysis was performed 28 days after the stenting. The ALA and sirolimus released slowly over 30 days. There were no significant differences between groups in the injury or inflammation score; however, there were significant differences in the percent area of stenosis (56.2 ± 11.78 % in BMS vs. 51.5 ± 12.20 % in AES vs. 34.7 ± 7.23 % in SES vs. 28.7 ± 7.30 % in SAS, P < 0.0001) and fibrin score [1.0 (range 1.0–1.0) in BMS vs. 1.0 (range 1.0–1.0) in AES vs. 2.0 (range 2.0–2.0) in SES vs. 2.0 (range 2.0–2.0) in SAS, P < 0.0001] between the four groups. The percent area of stenosis based on micro-computed tomography corresponded with the restenosis rates based on histopathological stenosis in different proportions in the four groups (54.8 ± 7.88 % in BMS vs. 50.4 ± 14.87 % in AES vs. 34.5 ± 7.22 % in SES vs. 28.9 ± 7.22 % in SAS, P < 0.05). SAS showed a better neointimal inhibitory effect than BMS, AES, and SES at 1 month after stenting in a porcine coronary restenosis model. Therefore, SAS with PLA can be a useful drug combination for coronary stent coating to suppress neointimal hyperplasia.     

Nickel-free stainless steel avoids neointima formation following coronary stent implantation

SUS316L stainless steel and cobalt–chromium and platinum–chromium alloys are widely used platforms for coronary stents. These alloys also contain nickel (Ni), which reportedly induces allergic reactions in some subjects and is known to have various cellular effects. The effects of Ni on neointima formation after stent implantation remain unknown, however. We developed coronary stents made of Ni-free high-nitrogen austenitic stainless steel prepared using a N₂-gas pressurized electroslag remelting (P-ESR) process. Neointima formation and inflammatory responses following stent implantation in porcine coronary arteries were then compared between the Ni-free and SUS316L stainless steel stents. We found significantly less neointima formation and inflammation in arteries implanted with Ni-free stents, as compared to SUS316L stents. Notably, Ni²⁺ was eluted into the medium from SUS316L but not from Ni-free stainless steel. Mechanistically, Ni²⁺ increased levels of hypoxia inducible factor protein-1α (HIF-1α) and its target genes in cultured smooth muscle cells. HIF-1α and their target gene levels were also increased in the vascular wall at SUS316L stent sites but not at Ni-free stent sites. The Ni-free stainless steel coronary stent reduces neointima formation, in part by avoiding activation of inflammatory processes via the Ni-HIF pathway. The Ni-free-stainless steel stent is a promising new coronary stent platform.     

Evaluation of fluorinated polymers as coronary stent coating

In this report, some fluorinated polyphosphazenes and polymethacrylates were selected for evaluation as coronary stent coating. After applying the polymer film by dipcoating, the stents were implanted in porcine coronary arteries. No acute thrombotic occlusions were observed. The neointimal proliferation was studied by a 6 week follow-up of the minimal lumen stented diameter, using quantitative coronary analysis. All polymers demonstrated a slight hyperplasia, resulting in a 10–20% lumen narrowing at follow-up. Only for one fluorinated polymethacrylate, PFM-P75, a minimal neointimal response (3% lumen narrowing) was found. ©2000 Kluwer Academic Publishers     

Nickel-free stainless steel avoids neointima formation following coronary stent implantation

Abstract

SUS316L stainless steel and cobalt–chromium and platinum–chromium alloys are widely used platforms for coronary stents. These alloys also contain nickel (Ni), which reportedly induces allergic reactions in some subjects and is known to have various cellular effects. The effects of Ni on neointima formation after stent implantation remain unknown, however. We developed coronary stents made of Ni-free high-nitrogen austenitic stainless steel prepared using a N₂-gas pressurized electroslag remelting (P-ESR) process. Neointima formation and inflammatory responses following stent implantation in porcine coronary arteries were then compared between the Ni-free and SUS316L stainless steel stents. We found significantly less neointima formation and inflammation in arteries implanted with Ni-free stents, as compared to SUS316L stents. Notably, Ni²⁺ was eluted into the medium from SUS316L but not from Ni-free stainless steel. Mechanistically, Ni²⁺ increased levels of hypoxia inducible factor protein-1α (HIF-1α) and its target genes in cultured smooth muscle cells. HIF-1α and their target gene levels were also increased in the vascular wall at SUS316L stent sites but not at Ni-free stent sites. The Ni-free stainless steel coronary stent reduces neointima formation, in part by avoiding activation of inflammatory processes via the Ni-HIF pathway. The Ni-free-stainless steel stent is a promising new coronary stent platform.     

Simulation of lung alveolar epithelial wound healing in vitro

The mechanisms that enable and regulate alveolar type II (AT II) epithelial cell wound healing in vitro and in vivo remain largely unknown and need further elucidation. We used an in silico AT II cell-mimetic analogue to explore and better understand plausible wound healing mechanisms for two conditions: cyst repair in three-dimensional cultures and monolayer wound healing. Starting with the analogue that validated for key features of AT II cystogenesis in vitro, we devised an additional cell rearrangement action enabling cyst repair. Monolayer repair was enabled by providing ‘cells’ a control mechanism to switch automatically to a repair mode in the presence of a distress signal. In cyst wound simulations, the revised analogue closed wounds by adhering to essentially the same axioms available for alveolar-like cystogenesis. In silico cell proliferation was not needed. The analogue recovered within a few simulation cycles but required a longer recovery time for larger or multiple wounds. In simulated monolayer wound repair, diffusive factor-mediated ‘cell’ migration led to repair patterns comparable to those of in vitro cultures exposed to different growth factors. Simulations predicted directional cell locomotion to be critical for successful in vitro wound repair. We anticipate that with further use and refinement, the methods used will develop as a rigorous, extensible means of unravelling mechanisms of lung alveolar repair and regeneration.     

Nitinol Carbofilm coated stents for peripheral applications: Study in the porcine model

Purpose: Testing the safety and foreign body reaction (FBR) of a nitinol self-expandable carbon-coated stent system in the porcine animal model at different follow-up (FU) periods. Methods: Fifteen minipigs received 30 carbon-coated self-expandable nitinol stents in iliac arteries. Explants were carried out at 7 (3 animals), 30 (4 animals), 90 (4 animals) and 180 (4 animals) day FU, for evaluation of acute, sub-acute and chronic biological response to the implanted devices. Histological, immunohistochemical, histomorphometric and scanning electron microscopy (SEM) analyses were performed to assess inflammatory reaction, endothelialization process, neointimal growth and cellular composition. Results: Thirty stents were successfully implanted. No mural thrombi were observed at gross examination or by angiography. Histologically no significant inflammatory reaction was detected: the stents appeared covered by a thin monolayer of endothelial cells even at 7 day FU. The neointima presented homogeneous growth and moderate thickness after 30, 90 and 180 days explants (0.38± 0.36 mm, 0.33± 0.30 mm, 0.27± 0.25 mm respectively). Internal and external elastic laminae were intact in 95% of stented arteries. Histological data validations of vessel endothelialization was obtained with SEM for the seven day follow-up group. Conclusions: This study showed good remarkable technical performances, minimal FBR and biocompatibility comparable with other available pre-clinical experimentations.     

A validated predictive model of coronary fractional flow reserve

Myocardial fractional flow reserve (FFR), an important index of coronary stenosis, is measured by a pressure sensor guidewire. The determination of FFR, only based on the dimensions (lumen diameters and length) of stenosis and hyperaemic coronary flow with no other ad hoc parameters, is currently not possible. We propose an analytical model derived from conservation of energy, which considers various energy losses along the length of a stenosis, i.e. convective and diffusive energy losses as well as energy loss due to sudden constriction and expansion in lumen area. In vitro (constrictions were created in isolated arteries using symmetric and asymmetric tubes as well as an inflatable occluder cuff) and in vivo (constrictions were induced in coronary arteries of eight swine by an occluder cuff) experiments were used to validate the proposed analytical model. The proposed model agreed well with the experimental measurements. A least-squares fit showed a linear relation as (Δp or FFR)_experiment = a(Δp or FFR)_theory + b, where a and b were 1.08 and −1.15 mmHg (r² = 0.99) for in vitro Δp, 0.96 and 1.79 mmHg (r² = 0.75) for in vivo Δp, and 0.85 and 0.1 (r² = 0.7) for FFR. Flow pulsatility and stenosis shape (e.g. eccentricity, exit angle divergence, etc.) had a negligible effect on myocardial FFR, while the entrance effect in a coronary stenosis was found to contribute significantly to the pressure drop. We present a physics-based experimentally validated analytical model of coronary stenosis, which allows prediction of FFR based on stenosis dimensions and hyperaemic coronary flow with no empirical parameters.     

Disabling partners in crime: Gold nanoparticles disrupt multicellular communications within the tumor microenvironment to inhibit ovarian tumor aggressiveness

The tumor microenvironment (TME) plays a key role in the poor prognosis of many cancers. However, there is a knowledge gap concerning how multicellular communication among the critical players within the TME contributes to such poor outcomes. Using epithelial ovarian cancer (EOC) as a model, we show how crosstalk among cancer cells (CC), cancer associated fibroblasts (CAF), and endothelial cells (EC) promotes EOC growth. We demonstrate here that co-culturing CC with CAF and EC promotes CC proliferation, migration, and invasion in vitro and that co-implantation of the three cell types facilitates tumor growth in vivo. We further demonstrate that disruption of this multicellular crosstalk using gold nanoparticles (GNP) inhibits these pro-tumorigenic phenotypes in vitro as well as tumor growth in vivo. Mechanistically, GNP treatment reduces expression of several tumor-promoting cytokines and growth factors, resulting in inhibition of MAPK and PI3K-AKT activation and epithelial-mesenchymal transition - three key oncogenic signaling pathways responsible for the aggressiveness of EOC. The current work highlights the importance of multicellular crosstalk within the TME and its role for the aggressive nature of EOC, and demonstrates the disruption of these multicellular communications by self-therapeutic GNP, thus providing new avenues to interrogate the crosstalk and identify key perpetrators responsible for poor prognosis of this intractable malignancy.     

A mesh-updating scheme for hemodynamic simulations in vessels undergoing large deformations

Several arteries, notably the coronary arteries,experience large motions and deformations believed to affect their hemodynamic environment. These arterial motions are 3D and complex, and in vivo data sets usually do not completely describe the resulting arterial wall motions. Here we present a mesh-updating scheme for such situations, thus allowing numerical simulation of arterial hemodynamics in arteries whose motion is described by (necessarily) incomplete in vivo data sets. The scheme works by first coupling information about arterial cross-sectional shape to motion of the artery axis. Motion of a subset of surface nodes (“control nodes”) is then specified, and this motion is used to drive all nodes in the mesh through an extension of a semi-torsional spring-analogy model. Numerical experiments were carried out on unstructured tetrahedral meshes generated in model geometries and in a right coronary artery model undergoing physiologically accurate motion. Results show that the quality of the dynamic mesh, as evaluated through elemental aspect ratios and Jacobian values, is extremely well preserved by this scheme, even during large deformations. Testing indicates the number of control nodes necessary to attain a high level of geometric fidelity of the mesh. We conclude that the algorithm is valuable for computing hemodynamic patterns in moving arteries using fully unstructured meshes.     

Improvement of stenting therapy with a silicon carbide coated tantalum stent

State of the art cardiovascular stent materials are a compromise between bulk properties and surface related properties. As a consequence, deficiencies in both characteristics lead to serious limitations of stenting therapy. Beside a dissatisfying X-ray visibility of current stent materials, which hinders precise angiographic control of the stent during implantation, insufficient hemocompatibility causes subacute vessel occlusions despite stringent anticoagulant medication. Additionally, bleeding complications result which further limit the therapeutical success. Therefore it is essential to develop a new coronary stent with improved material properties for the bulk of the stent and its surface. This is realized by a hybrid concept. The stent is manufactured from tantalum, having a high inherent radio-opacity. The stent is coated with amorphous silicon carbide, optimized for hemocompatibility. An appropriate deposition technology to maximize coating adhesion was developed. Amorphous silicon carbide was investigated in vitro and in vivo to assess its suitability for coronary stents.     

Seeding density matters: extensive intercellular contact masks the surface dependence of endothelial cell-biomaterial interactions

The effects of seeding density have often been overlooked in evaluating endothelial cell-biomaterial interactions. This study compared the cell attachment and proliferation characteristics of endothelial cells on modified poly (l-lactic acid) (PLLA) films conjugated to gelatin and chitosan at low and high seeding densities (5,000 and 50,000 cells/cm²). During the early stage (2 h) of cell-biomaterial interaction, a low seeding density enabled us to observe the intrinsic surface-dependent differences in cell attachment capacity and morphogenesis, whereas extensive intercellular interactions at high seeding density masked differences between substrates and improved cell attachment on low-affinity substrates. During the later stage of cell-biomaterial interaction over 7-days of culture, the proliferation rate was found to be surface-dependent at low seeding density, whereas this surface-dependent difference was not apparent at high seeding density. It is recommended that low seeding density should be utilized for evaluating biomaterial applications where EC density is likely to be low, such as in situ endothelialization of blood-contacting devices.     

Promotion of adhesion and proliferation of endothelial progenitor cells on decellularized valves by covalent incorporation of RGD peptide and VEGF

Tissue engineered heart valve is a promising alternative to current heart valve surgery, for its capability of growth, repair, and remodeling. However, extensive development is needed to ensure tissue compatibility, durability and antithrombotic potential. This study aims to investigate the biological effects of multi-signal composite material of polyethyl glycol-cross-linked decellularized valve on adhesion and proliferation of endothelial progenitor cells. Group A to E was decellularized valve leaflets, composite material of polyethyl glycol-cross-linked decellularized valves leaflets, vascular endothelial growth factor-composite materials, Arg-Gly-Asp peptide-composite materials and multi-signal modified materials of polyethyl glycol-cross-linked decellularized valve leaflets, respectively. The endothelial progenitor cells were seeded for each group, cell adhesion and proliferation were detected and neo-endothelium antithrombotic function of the multi-signal composite materials was evaluated. At 2, 4, and 8?h after the seeding, the cell numbers and 3H-TdR incorporation in group D were the highest. At 2, 4, and 8 days after the seeding, the cell numbers and 3H-TdR incorporation were significantly higher in groups C, D, and E compared with groups A and B (P?<?0.05) and cell numbers and the expression of t-PA and eons in the neo-endothelium were quite similar to those in the human umbilical vein endothelial cells at 2, 4, and 8 days after the seeding. The Arg-Gly-Asp- peptides (a sequential peptide composed of arginine (Arg), glycine (Gly) and aspartic acid (Asp)) and VEGF-conjugated onto the composite material of PEG-crosslinked decellularized valve leaflets synergistically promoted the adhesion and proliferation of endothelial progenitor cells on the composite material, which may help in tissue engineering of heart valves.     

Controlled release of BSA by microsphere-incorporated PLGA scaffolds under cyclic loading

Localized delivery of bioactive molecules from porous biodegradable scaffolds is very important in advanced tissue engineering strategies, and it is necessary to study the delivery under dynamic loading which mimics the in vivo biomechanical environments. In this study, bovine serum albumin (BSA), a model of bioactive proteins, was incorporated into porous poly(l-lactide-co-glycolide) (PLGA) scaffolds by seeding BSA-loaded microspheres onto the scaffold pore wall, where the microspheres of poly(ethylene glycol)-b-poly(l-lactide) (PELA) were prepared by double emulsion technique. The in vitro release behavior of BSA from the scaffold under dynamic cyclic loading was studied in comparison with that under a static condition as well as from PELA microspheres. It was observed that the microsphere-incorporated scaffold prolonged BSA release with respect to the microspheres. The cyclic loading accelerated the release of BSA from the scaffold and the cumulative release on day 10 reached 85% of the totally encapsulated BSA. The delivery under a dynamic condition would be an initial study of in vivo localized delivery of growth factors.     

Endothelial cell cultures as a tool in biomaterial research

Progress in biocompatibility and tissue engineering would today be inconceivable without the aid of in vitro techniques. Endothelial cell cultures represent a valuable tool not just in haemocompatibility testing, but also in the concept of designing hybrid organs. In the past endothelial cells (EC) have frequently been used in cytotoxicity testing of materials, especially polymers, used in blood-contacting implants, as well as for investigating seeding technologies for vascular prostheses. At present the exponential development both in theory and practice of cell and molecular biology of the endothelium offers great promise in the biomaterial field. Up until now this EC research field has mostly been non-biomaterial orientated. Nevertheless, the relevance for biomaterial research is apparent. Four aspects will be concisely reviewed under the headings inflammation, with special reference to cell adhesion molecules (CAMs) and cytokines, angiogenesis, focusing on the healing response, signal transduction, presenting examples from cytokine- and metal ion-induced up-regulation of genes coding for CAMs, and, finally, endothelial functionality, with emphasis on the principal characteristics of the physiological endothelial phenotype. Finally, the application of these fields to three foci of biomaterial research will be discussed, emphasizing the role of EC culture techniques in controlling the host response to biomaterials (microvascular EC), controlling EC functionality (promoting positive effects and down-regulating negative effects), and tissue engineering (integration of EC into hybrid organs/biosensors). The need for more co-culture and three-dimensional models will be stressed and data from the authors’ laboratory presented to illustrate these principles. © 1999 Kluwer Academic Publishers     

Regulation of endothelial MAPK/ERK signalling and capillary morphogenesis by low-amplitude electric field

Low-amplitude electric field (EF) is an important component of wound-healing response and can promote vascular tissue repair; however, the mechanisms of action on endothelium remain unclear. We hypothesized that physiological amplitude EF regulates angiogenic response of microvascular endothelial cells via activation of the mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathway. A custom set-up allowed non-thermal application of EF of high (7.5 GHz) and low (60 Hz) frequency. Cell responses following up to 24 h of EF exposure, including proliferation and apoptosis, capillary morphogenesis, vascular endothelial growth factor (VEGF) expression and MAPK pathways activation were quantified. A db/db mouse model of diabetic wound healing was used for in vivo validation. High-frequency EF enhanced capillary morphogenesis, VEGF release, MEK-cRaf complex formation, MEK and ERK phosphorylation, whereas no MAPK/JNK and MAPK/p38 pathways activation was observed. The endothelial response to EF did not require VEGF binding to VEGFR2 receptor. EF-induced MEK phosphorylation was reversed in the presence of MEK and Ca²⁺ inhibitors, reduced by endothelial nitric oxide synthase inhibition, and did not depend on PI3K pathway activation. The results provide evidence for a novel intracellular mechanism for EF regulation of endothelial angiogenic response via frequency-sensitive MAPK/ERK pathway activation, with important implications for EF-based therapies for vascular tissue regeneration.     

Micromechanical methodology for fatigue in cardiovascular stents

A finite element based micromechanical methodology for cyclic plasticity and fatigue crack initiation in cardiovascular stents is presented. The methodology is based on the combined use of a (global) three-dimensional continuum stent-artery model, a local micromechanical stent model, the development of a combined kinematic–isotropic hardening crystal plasticity constitutive formulation, and the application of microstructure sensitive crack initiation parameters. The methodology is applied to 316L stainless steel stents with random polycrystalline microstructures, based on scanning electron microscopy images of the grain morphology, under realistic elastic–plastic loading histories, including crimp, deployment and in vivo systolic–diastolic cyclic pressurisation. Identification of the micromechanical cyclic plasticity and failure constants is achieved via application of an objective function and a unit cell representative volume element for 316L stainless steel. Cyclic stent deformations are compared with the J₂-predicted response and conventional fatigue life prediction techniques. It is shown that micromechanical fatigue analysis of stents is necessary due to the significant predicted effects of material inhomogeneity on micro-plasticity and micro-crack initiation.     

冠状动脉支架纵向拉伸变形行为有限元分析

纵向强度是冠状动脉支架的一项重要力学性能,因强度不足而导致的支架纵向变形行为已成为经皮冠状动脉支架介入术最严重的并发症之一。运用有限元方法系统地分析了支架的扩张尺度、拉伸力的作用点位置和支撑单元之间的周向连接单元数量与形状对支架纵向拉伸变形行为的影响。结果显示,在支架结构设计参数中,连接单元的数量对支架纵向拉伸变形行为影响最大,增加连接单元的数量可以提高支架抵抗变形能力。连接单元形状对支架纵向拉伸变形行为影响次之,当支架的纵向变形位移为0.5mm且将连接单元形状由S型转换为L型时支架纵向强度可以提高50%。对于同一种连接单元类型的支架,扩张尺度越大的支架越容易发生纵向变形。同时,结果显示支架远端与近端端部比支架中部更容易发生纵向变形。合理调整支架的设计参数可以有效改善支架的纵向拉伸变形行为,所得结论有助于支架设计师优化支架的结构设计以降低支架纵向变形程度。     

Up-regulation of thrombomodulin is inducible on an endothelialized polyester

The concept of endothelial cell seeding of vascular prostheses is designed as a method for improving long-term patency substitutes because endothelium is considered as the haemocompatible surface of reference. The assessment of the functionality of cells lining a biomaterial is thus of crucial importance. We have reported encouraging results concerning the ability of a polyester coated with albumin and chitosan (M 11) to be lined by a confluent monolayer of cultured human endothelial cells (EC). The aim of the present work was to study the expression of thrombomodulin (membrane glycoprotein responsible for anticoagulant activity) in EC lying on M.11 by anticoagulant activity and mRNA level with and without stimulation. Results obtained in basal conditions showed that EC on M.11 have a comparable expression of TM mRNA when compared to control (EC on tissue culture plates) despite a lower TM surface activity for EC on the biomaterial. In terms of ratio (stimulated cells to unstimulated ones) the response in activity for EC on M.11 is comparable to that of the control. These results indicate that cells lying on M.11 are able to respond to physiological-like stimuli, despite a tendency for these cells to express a procoagulant phenotype when compared with control EC.