Graft-artery junctions: design optimization and CAD development.

Designing and manufacturing of vascular prosthesis for arterial bypass grafts is a very complex problem. The process involves the selection of suitable geometry, materials of appropriate characteristics, and manufacturing technique capable of constructing prosthesis in a cost-effective manner. In this chapter, all engineering aspects related to the design and optimization of an artificial graft are presented and discussed. These aspects include CAD design of the graft, in vitro hemodynamic analysis to ensure good mechanical integrity and functionality, and optimization of the manufacturing techniques. Brief discussion is also given on the endothelization and vascularization of the artificial vessels and the future directions of the development of synthetic vessels for human implementation.

Numerical analysis of coronary artery bypass grafts: an over view.

Arterial bypass grafts tend to fail after some years due to the development of intimal thickening (restenosis). Non-uniform hemodynamics following a bypass operation contributes to restenosis and bypass failure can occur due to the focal development of anastomotic intimal hyperplasia. Additionally, surgical injury aggravated by compliance mismatch between the graft and artery has been suggested as an initiating factor for progress of wall thickening along the suture line Vascular grafts that are small in diameter tend to occlude rapidly. Computational fluid dynamics (CFD) methods have been effectively used to simulate the physical and geometrical parameters characterizing the hemodynamics of various arteries and bypass configurations. The effects of such changes on the pressure and flow characteristics as well as the wall shear stress during a cardiac cycle can be simulated. Recently, utilization of fluid and structure interactions have been used to determine fluid flow parameters and structure forces including stress and strains relationships under steady and transient conditions. In parallel to this, experimental diagnostics techniques such as Laser Doppler Anemometry, Particle Image Velocimetry, Doppler Guide wire and Magnetic Resonance Imaging have been used to provide essential information and to validate the numerical results. Moreover, clinical imaging techniques such as magnetic resonance or computed tomography have assisted considerably in gaining a detailed patient-specific picture of the blood flow and structure dynamics. This paper gives a review of recent numerical investigations of various configurations of coronary artery bypass grafts (CABG). In addition, the paper ends with a summary of the findings and the future directions.

Microwave sterilization of bovine pericardium for heart valve applications.

It is widely recognised that the bioprosthetic valves widely used for heart valve replacements have some drawbacks, for example tearing and occurrence of infections, which can be attributed to the fixation and sterilization techniques currently available. These techniques adversely affect the physical properties, functionality, and lifespan of the leaflets. In the work discussed in this paper we examined a novel procedure of using high-frequency microwaves to fix and disinfect the pericardium, without causing any harmful affects. The test bacteria used were Escherichia coli and Staphylococcus aureus. The pericardium was exposed to microwaves at a frequency of 18 GHz for three consecutive replicates. The findings indicated that there was almost complete inactivation of the bacteria on the biomaterial without compromising the biocompatibility, which was studied using ovine fibroblasts. An effective fixation and sterilization procedure, that is quick and has no adverse effects is presented and discussed.

Gentamicin-impregnated chitosan/nanohydroxyapatite/ethyl cellulose microspheres granules for chronic osteomyelitis therapy.

In this article gentamicin (GM) impregnated microspheres were used to extend the drug release time for the treatment of chronic osteomyelitis. The granules were prepared in solution and consisted of nanohydroxyapatite (nHA), chitosan (CS) and GM loaded ethyl cellulose (EC) microspheres. A rabbit model with chronic osteomyelitis was made by using staphylococcus aureus and morrhuate sodium and special inspection methods were used to test the curative effects of the granules, such as microbiological investigations, tissue, and X-ray observations. The granules were provided with excellent drug release properties, 49 days in vitro and 45 days in vivo, moreover, they showed almost no cytotoxic for fibroblast and osteoblast. The findings indicated that the GM-impregnated CS/nHA/EC microspheres granules showed outstanding curative effect. Generally, it can be concluded that the granules containing GM impregnated microspheres may be used effectively in the treatment of the chronic osteomyelitis.

Improved properties of incorporated chitosan film with ethyl cellulose microspheres for controlled release.

In this article, to discover an innovative drug release system, ciprofloxacin hydrochloride-loaded blending films of chitosan (CS)/ethyl cellulose (EC) microspheres were prepared. Two steps were adopted in the film forming process. The first was formation of the drug-loaded EC microspheres in CS solution by solvent remove/solvent evaporation methods; then, the composite films were made by casting and solvent evaporation. The results were that the drug-loaded round EC microspheres dispersed asymmetrically in the CS films and largely improved the release time. Moreover, the drug-loaded blending film containing 0.5 g EC microspheres prepared at 90 degrees C showed highlighted extended release property. The drug was stable in the blending films, which expressed good cytocompatibility proved by MTT test. The film should be a promising carrier for controlled and extended drug release system in pharmaceutical applications.

A reinforced sternal wiring technique for transverse thoracosternotomy closure in bilateral lung transplantation: from biomechanical test to clinical application.

OBJECTIVES: A high incidence of failure of transverse thoracosternotomy closure, involving the loops of wire cutting through the sternum, remains a significant morbidity after bilateral lung transplantation. We postulated that placing peristernal wires inside the usual longitudinal wires could prevent the longitudinal wires from cutting through the sternum. The aims of this study were to investigate the biomechanical and clinical efficacy of the proposed reinforced sternal closure technique. METHODS: In vitro, 24 artificial sternal models were wired with the reinforced or conventional wiring techniques and were tested either by means of longitudinal distraction or anterior-posterior shear (n = 6 per group). In vivo, the 6-month outcomes of 70 bilateral lung transplantations, including 27 reinforced and 43 conventional wiring techniques, were assessed. RESULTS: Reinforced wiring was stronger than conventional wiring for both longitudinal distraction (yield load: 585 +/- 60 vs 334 +/- 21 N [P = .03]; maximum load: 807 +/- 60 vs 525 +/- 34 N [P = .03]; postyield stiffness: 91.0 +/- 22.0 vs 32.8 +/- 11.8 N/mm [P = .04]) and anterior-posterior shear (yield load: 405 +/- 9 vs 364 +/- 16 N [P = .03]; postyield stiffness: 47.4 +/- 6.1 vs 27.5 +/- 5.1 N/mm [P = .04]). In multivariate analysis, the use of the conventional wiring technique (odds ratio, 5.38; P = .04) and osteoporosis (odds ratio, 18.31; P = .0005) were significant risk factors associated with sternal dehiscence. In the patients with osteoporosis (n = 25), the incidence of sternal dehiscence in the reinforced wiring group (4/16 [25%]) was significantly lower than that in the conventional wiring group (7/9 [78%], P = .02). CONCLUSION: Osteoporosis is a significant risk factor for sternal dehiscence after bilateral lung transplantation. The new reinforced sternal wiring technique provides biomechanically superior fixation of the sternum and clinically reduces the incidence of sternal dehiscence in high-risk osteoporotic patients undergoing bilateral lung transplantation.

Transient fluid-structure coupling for simulation of a trileaflet heart valve using weak coupling.

In this article, a three-dimensional transient numerical approach coupled with fluid-structure interaction for the modeling of an aortic trileaflet heart valve at the initial opening stage is presented. An arbitrary Lagrangian-Eulerian kinematical description together with an appropriate fluid grid was used for the coupling strategy with the structural domain. The fluid dynamics and the structure aspects of the problem were analyzed for various Reynolds numbers and times. The fluid flow predictions indicated that at the initial leaflet opening stage a circulation zone was formed immediately downstream of the leaflet tip and propagated outward as time increased. Moreover, the maximum wall shear stress in the vertical direction of the leaflet was found to be located near the bottom of the leaflet, and its value decreased sharply toward the tip. In the horizontal cross section of the leaflet, the maximum wall shear stresses were found to be located near the sides of the leaflet.

Development of a novel pulsatile bioreactor for tissue culture.

The construction of tissue-engineered parts such as heart valves and arteries requires more than just the seeding of cells onto a biocompatible/biodegradable polymeric scaffold. It is essential that the functionality and mechanical integrity of the cell-seeded scaffold be investigated in vitro prior to in vivo implantation. The correct hemodynamic conditioning would lead to the development of tissues with enhanced mechanical strength and cell viability. Therefore, a bioreactor that can simulate physiological conditions would play an important role in the preparation of tissue-engineered constructs. In this article, we present and discuss the design concepts and criteria, as well as the development, of a multifunctional bioreactor for tissue culture in vitro. The system developed is compact and easily housed in an incubator to maintain sterility of the construct. Moreover, the proposed bioreactor, in addition to mimicking in vivo conditions, is highly flexible, allowing different types of constructs to be exposed to various physiological flow conditions. Initial verification of the hemodynamic parameters using Laser doppler anemometry indicated that the bioreactor performed well and produced the correct physiological conditions.

Polyethyleneterephthalate provides superior retention of endothelial cells during shear stress compared to polytetrafluoroethylene and pericardium.

BACKGROUND: Polyethyleneterephthalate (PET) and polytetrafluoroethylene (PTFE) are polymers successfully used as large diameter arterial grafts for peripheral vascular surgery. However, these prosthetic grafts are rarely used for coronary bypass surgery because of their low patency rates. Endothelialisation of the lumenal surface of these materials may improve their patency. This study aimed to compare the endothelialisation of PET, PTFE and pericardium by examining their seeding efficiency over time and the effect of various shear stresses on retention of endothelial cells. METHODS: Ovine endothelial cells at 4x10(5)cells/cm(2) were seeded onto PET, PTFE and pericardium, and cultured for 1-168 hours. Cell coverage was determined via en face immunocytochemistry and cell retention was quantified after being subjected to shear stresses ranging from 0.018 to 0.037N/m(2) for 15, 30 and 60 minutes. RESULTS: Endothelial cells adhered to all of the materials one hour post-seeding. PET exhibited better cell retention rate, ranging from 66.9+/-5.6% at 0.018N/m(2) for 15min to 44.7+/-1.9% at 0.037N/m(2) for 60 minutes, when compared to PTFE and pericardium (p<0.0001, three-way ANOVA). CONCLUSION: PET shows superior retention of endothelial cells during shear stress compare to PTFE and pericardium.

Tissue engineering a functional aortic heart valve: an appraisal.

Valvular heart disease is an important cause of morbidity and mortality, and currently available substitutes for failing hearts have serious limitations. A new promising alternative that may overcome these shortcomings is provided by the relatively new field of tissue engineering (TE). TE techniques involve the growth of autologous cells on a 3D matrix that can be a biodegradable polymer scaffold, or an acellular tissue matrix. These approaches provide the potential to create living matrix valve structures with an ability to grow, repair and remodel within the recipient. This article provides an appraisal of artificial heart valves and an overview of developments in TE that includes the current limitations and challenges for creating a fully functional valve. Biomaterials and stem cell technologies are now providing the potential for new avenues of research and if combined with advances in the rapid prototyping of biomaterials, the engineering of personalized, fully functional, and autologous tissue valve replacements, may become a clinical alternative.