Multi-Scale Modelling of Textile Reinforced Tissue Engineered Heart Valves
Transcatheter aortic valve implantation of fibrin-based tissue engineered heart valves with a tubular leaflet construct have been developed as an alternative to invasive traditional surgical heart valve implantation, but currently need reinforcement to withstand pressures found in the aortic position. To increase valve strength, a PET textile reinforcement has been introduced to the fibrin scaffold. However, care must be taken when using a textile reinforcement. Increasing reinforcement may increase the strength, but also increases stiffness which can interfere with the functionality of the valve, prohibiting full valve closure. In order to predict the behavior of the valve, improve design, and eventually optimize valve performance by tailoring the textile reinforcement, a 4-tiered hierarchical multi-scale modelling framework has been created. The model includes an individual fiber scale model, two textile-level models, and a full heart valve model. Geometry generation based on microscopic images of the composite will be discussed, along with application of boundary conditions and loading application. Fitting the various material models based on homogenization of the smaller scale will be demonstrated. Finally, experimental validation and characterization at various scales will be discussed.
A temporarily stented, autologous pericardial aortic valve prosthesis.
BACKGROUND AND AIM OF THE STUDY
A new bioprosthesis has been developed that is: (i) constructed from glutaraldehyde-treated autologous pericardium due to its lack of antigenicity and low cost; (ii) easily constructed in the operating room; (iii) stentless but easy to implant by using a temporary stent that is removed once its function is no longer necessary; and (iv) implanted with a single proximal suture and three commissural stitches.


METHODS
This prosthesis was implanted in the subcoronary aortic position of six sheep. The mean prosthesis size was 19.7 +/- 3.5 mm, and manufacturing time 16 +/- 3 min. Cardiopulmonary bypass and cross-clamp times were 142.5 +/- 26.7 min and 100.2 +/- 28.8 min, respectively. Three sheep were euthanized at three, seven, or 27 days, and the remainder at five months postoperatively.


RESULTS
In all cases the valve was competent and the single commissural stitches were well-anchored. The pericardium was pliable in all cases, but one animal had obvious endocarditis (at 27 days). Small calcific nodules were found at the commissures in valves explanted at five months. Histology showed an intact collagen and elastin structure, and neointimal growth was seen covering the basal quarter of the leaflets and commissures.


CONCLUSION
This new bioprosthesis might offer a particularly attractive alternative for the young patient population of the developing world. The simplicity of construction, low cost, and absence of any need for anticoagulation makes this prosthesis close to the ideal aortic valve substitute.
Aortic valve mechanics. Part II: a stress analysis of the porcine aortic valve leaflets in diastole.
The stress analysis of the porcine aortic valve leaflets in diastole at 80 mm Hg pressure in-vitro is presented. Incorporation of local geometrical asymmetry, material inhomogeneity, anistropy and non-linearity are applied. The stress theory used is a modified form of the thin membrane stress theory for a homogeneous linearly elastic and orthotropic lamina. Modifications are made so that the Hooke's law constitutive equations of stress may be applied to the inhomogeneous, non-lineary elastic and orthotropic thin (membrane) aortic valve leaflets. Stress calculations are made on the premise that the valve is in pre-transition (i.e. low elastic modulus) in the circumferential direction and post-transition (i.e. high elastic modulus) in the radial direction. It is shown that sigmaCIR less than 1 gm/mm2, and for most of the noncoronary leaflet, 0 less than sigmaRAD less than 30 gm/mm2. The areas of highest stress concentrations are in the areas of mutual leaflet coaptation near the Node of Arantii. A progressive increase of radial stresses from the sinus-annulus edge toward the node is observed.