Intracranial Stents Past, Present and the Future Trend: Stents Made with Nano-particle or Nanocomposite Biomaterials

Stroke or cerebral vascular accidents are among the leading causes of death in the world. With the availability of Digital Subtraction Angiography, transluminal angioplasty has become feasible in many situations and the role of intracranial stents is becoming ever more important in the management of cerebral vascular diseases. In current review, we outline the chronological development of various stents namely; balloon expandable stent, self-expandable open cell stent, self-expandable close cell stent and the flow diverting stent. Further we discuss their advantages and limitations in terms of stent migration, thromboemboli, damage to vessels during procedure, in-stent stenosis and hyper-perfusion damage. We also discuss the importance of in-situ endothelialization, controlled expandability and hemodynamic manipulation in stent design. Further, we summarized the role and need for further development in the areas of bio-compatible materials, endothelial progenitor cell capture technique, bio-functionalized-magnetic-nano-particles and nanotechnology which are significant in intracranial stent development

THERMAL BUCKLING ANALYSIS OF FUNCTIONALLY GRADED CIRCULAR PLATE RESTING ON THE PASTERNAK ELASTIC FOUNDATION VIA THE DIFFERENTIAL TRANSFORM METHOD

In this paper, we propose a thermal buckling analysis of a functionally graded (FG) circular plate exhibiting polar orthotropic characteristics and resting on the Pasternak elastic foundation. The plate is assumed to be exposed to two kinds of thermal loads, namely, uniform temperature rise and linear temperature rise through thickness. The FG properties are assumed to vary continuously in the direction of thickness according to the simple power law model in terms of the volume fraction of two constituents. The governing equilibrium equations in buckling are based on the Von-Karman nonlinearity. To obtain the critical buckling temperature, we exploit a semi-numerical technique called differential transform method (DTM). This method provides fast accurate results and has a short computational calculation compared with the Taylor expansion method. Furthermore, some numerical examples are provided to consider the influence of various parameters such as volume fraction index, thickness-to-radius ratio, elastic foundation stiffness, modulus ratio of orthotropic materials and influence of boundary conditions. In order to predict the critical buckling temperature, it is observed that the critical temperature can be easily adjusted by appropriate variation of elastic foundation parameters and gradient index of FG material. Finally, the numerical results are compared with those available in the literature to confirm the accuracy and reliability of the DTM to determine the critical buckling temperature

Development of Coronary Artery Covered Stent Using Nanocomposite Materials

Bare-metal stents (BMS) and drug-eluting stents (DES) are the two main categories of FDA-approved coronary stents in the market for treating atherosclerosis. Problems associated with BMS include in-stent restenosis due to intimal hyperplasia, leading to stent failure, while DES harbours a life-threatening complication called late-stent thrombosis due to drug-polymer hypersensitivity and impaired re-endothelialization. One approach to overcoming the above-mentioned problems could be using covered stents. Covered stents have an additional layer of membrane spanning the stent struts, and can be considered hybrid stent-grafts. Due to the added protection that the membrane affords, covered stents are currently used for vessel perforations and aneurysms. They can act as a physical barrier to inhibit smooth muscle cell in-growth and intimal hyperplasia formation. The most commonly used membrane for covered stents is expanded polytetrafluoethylene (ePTFE). However, its non-compliant and thrombogenic nature prevents it from being suitable for use in small-diameter vessels, resulting in an unmet clinical need for a haemocompatible covered stent for this application. A polyhedral oligomeric silsesquioxane poly(carbonate-urea) urethane (POSS-PCU) nanocomposite polymer was developed by our group, and has already been used in three first-in-man studies as a bypass graft, lacrimal duct, and the world’s first synthetic trachea. Hence we sought to assess the feasibility of using POSS-PCU as a membrane for covered stents. Results indicate that POSS-PCU was haemocompatible, and was able to support the growth and proliferation of endothelial cells, compared to controls. Mechanical tests on membranes revealed that POSS-PCU was superior to ePTFE. Furthermore, it was also found that integration of POSS-PCU membrane onto stents did not adversely affect stent mechanics. In summary, the overall biomechanical performance of POSS-PCU indicates that it has the potential to function as a viable membrane material for covered stents in small diameter vessels

Investigation on Buckling of Orthotropic Circular and Annular Plates of Continuously Variable Thickness by Optimized Ritz Method

This paper investigates symmetrical buckling of orthotropic circular and annular plates of continuous variable thickness. Uniform compression loading is applied at the plate outer boundary. Thickness varies linearly along radial direction. Inner edge is free, while outer edge has different boundary conditions: clamped, simply and elastically restraint against rotation. The optimized Ritz method is applied for buckling analysis. In this method, a polynomial function that is based on static deformation of orthotropic circular plates in bending is used. Also, by employing an exponential parameter in deformation function, eigenvalue is minimized in respect to this parameter. The obtained results show that in plate with identical thickness, increasing of outer radius decreases the buckling load factor

Optimizing the buckling characteristics and weight of functionally graded circular plates using the multi-objective Pareto archived simulated annealing algorithm (PASA)

In this study for the first time, weight and critical buckling load in two kinds of functionally graded (FG) circular plates, namely, aluminum–alumina of (Al/Al2O3) and aluminum–zirconia (Al/ZnO2), are optimized using multi-objective Pareto archived simulated annealing algorithm (PASA). Material properties are assumed to vary with the power law in terms of the volume fractions of the constituent in two forms of symmetric and asymmetric with respect to the middle surface. The plate is subjected to uniform radial load and is considered for two boundary conditions, namely, simply supported and clamped edges. Aim at obtaining the Pareto archive is to achieve simultaneously the maximum buckling and the minimum weight concerning with proposed constraints. The parameters include the radius, thickness and volume fraction that the certain range is intended individually. The constraints are presented in form of the ratio of thickness to radius in category of the thin plates as well as the critical buckling stress being in the elastic range. Proposed simulated annealing algorithm is coded in MATLAB to obtain optimal non-dominated solution

Determination and comparison of mechanical properties of articular cartilage using pro-hyper-viscoelastic models based on an unconfined stress relaxation

Recently, the most common tool to compensate for various organ defects is tissue transplantation with several problems involved. These problems have led to the rapid growth of tissue engineering with a designed tissue approach or organ substitute in the last decade. For this purpose, it is important to determine the tissue mechanical properties. In this study, to obtain the cartilage structural parameters, isotropic Pro-Hyper-Viscoelastic Mooney-Rivlin and Neo-Hooke are used. These model coefficients are obtained by reverse engineering methods and using a coupled finite element-optimization algorithm utilized unconfined stress relaxation tests with root-mean-square error (RMSE)) less than 0.036, 0.033 for Neo-Hooke and Mooney-Rivlin respectively. Using Neo-Hooke and Mooney-Rivlin models, the modulus of elasticity was 0.47 MPa and 0.44 MPa, and the shear modulus was 0.188 MPa and 0.184 MPa, respectively. The predicted tissue mechanical response obtained by the finite element model showed that the Mooney-Rivlin model is more consistent with the stress relaxation experiments than Neo-Hooke one. The results showed that during the stress relaxation test, by applying a compressing load on the sample, initially the fluid pressurization in the matrix pores has the most contribution in the load-bearing (total stress). When time elapses, the fluid contribution in the load-bearing decreases, and the solid matrix contribution increases

Accelerating in Situ Endothelialisation of Cardiovascular Bypass Grafts

The patency of synthetic cardiovascular grafts in the long run is synonymous with their ability to inhibit the processes of intimal hyperplasia, thrombosis and calcification. In the human body, the endothelium of blood vessels exhibits characteristics that inhibit such processes. As such it is not surprising that research in tissue engineering is directed towards replicating the functionality of the natural endothelium in cardiovascular grafts. This can be done either by seeding the endothelium within the lumen of the grafts prior to implantation or by designing the graft such that in situ endothelialisation takes place after implantation. Due to certain difficulties identified with in vitro endothelialisation, in situ endothelialisation, which will be the focus of this article, has garnered interest in the last years. To promote in situ endothelialisation, the following aspects can be taken into account: (1) Endothelial progenital cell mobilization, adhesion and proliferation; (2) Regulating differentiation of progenitor cells to mature endothelium; (3) Preventing thrombogenesis and inflammation during endothelialisation. This article aims to review and compile recent developments to promote the in situ endothelialisation of cardiovascular grafts and subsequently improve their patency, which can also have widespread implications in the field of tissue engineering

An anti-CD34 antibody-functionalized clinical-grade POSS-PCU nanocomposite polymer for cardiovascular stent coating applications : a preliminary assessment of endothelial progenitor cell capture and hemocompatibility

In situ endothelialization of cardiovascular implants has emerged in recent years as an attractive means of targeting the persistent problems of thrombosis and intimal hyperplasia. This study aimed to investigate the efficacy of immobilizing anti-CD34 antibodies onto a POSS-PCU nanocomposite polymer surface to sequester endothelial progenitor cells (EPCs) from human blood, and to characterize the surface properties and hemocompatibility of this surface. Amine-functionalized fumed silica was used to covalently conjugate anti-CD34 to the polymer surface. Water contact angle, fluorescence microscopy, and scanning electron microscopy were used for surface characterization. Peripheral blood mononuclear cells (PBMCs) were seeded on modified and pristine POSS-PCU polymer films. After 7 days, adhered cells were immunostained for the expression of EPC and endothelial cell markers, and assessed for the formation of EPC colonies. Hemocompatibility was assessed by thromboelastography, and platelet activation and adhesion assays. The number of EPC colonies formed on anti-CD34-coated POSS-PCU surfaces was not significantly higher than that of POSS-PCU (5.0±1.0 vs. 1.7±0.6, p>0.05). However, antibody conjugation significantly improved hemocompatibility, as seen from the prolonged reaction and clotting times, decreased angle and maximum amplitude (p<0.05), as well as decreased platelet adhesion (76.8±7.8 vs. 8.4±0.7, p<0.05) and activation. Here, we demonstrate that POSS-PCU surface immobilized anti-CD34 antibodies selectively captured CD34+ cells from peripheral blood, although only a minority of these were EPCs. Nevertheless, antibody conjugation significantly improves the hemocompatibility of POSS-PCU, and should therefore continue to be explored in combination with other strategies to improve the specificity of EPC capture to promote in situ endothelialization.Published versio