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Research Papers

A Computational Tool for the Microstructure Optimization of a Polymeric Heart Valve Prosthesis

[+] Author and Article Information
M. Serrani

Department of Chemical Engineering and Biotechnology,
University of Cambridge,
Pembroke Street,
Cambridge CB23RA, UK
e-mail: ms2214@cam.ac.uk

J. Brubert, J. Stasiak, G. D. Moggridge

Department of Chemical Engineering and
Biotechnology,
University of Cambridge,
Pembroke Street,
Cambridge CB23RA, UK

F. De Gaetano, A. Zaffora, M. L. Costantino

Department of Chemistry Materials and
Chemical Engineering “Giulio Natta,”
Politecnico di Milano,
Piazza Leonardo da Vinci 32,
Milan 20133, Italy

1Corresponding author.

Manuscript received October 21, 2015; final manuscript received March 22, 2016; published online April 11, 2016. Assoc. Editor: Kristen Billiar.

J Biomech Eng 138(6), 061001 (Apr 11, 2016) (8 pages) Paper No: BIO-15-1528; doi: 10.1115/1.4033178 History: Received October 21, 2015; Revised March 22, 2016

Styrene-based block copolymers are promising materials for the development of a polymeric heart valve prosthesis (PHV), and the mechanical properties of these polymers can be tuned via the manufacturing process, orienting the cylindrical domains to achieve material anisotropy. The aim of this work is the development of a computational tool for the optimization of the material microstructure in a new PHV intended for aortic valve replacement to enhance the mechanical performance of the device. An iterative procedure was implemented to orient the cylinders along the maximum principal stress direction of the leaflet. A numerical model of the leaflet was developed, and the polymer mechanical behavior was described by a hyperelastic anisotropic constitutive law. A custom routine was implemented to align the cylinders with the maximum principal stress direction in the leaflet for each iteration. The study was focused on valve closure, since during this phase the fibrous structure of the leaflets must bear the greatest load. The optimal microstructure obtained by our procedure is characterized by mainly circumferential orientation of the cylinders within the valve leaflet. An increase in the radial strain and a decrease in the circumferential strain due to the microstructure optimization were observed. Also, a decrease in the maximum value of the strain energy density was found in the case of optimized orientation; since the strain energy density is a widely used criterion to predict elastomer's lifetime, this result suggests a possible increase of the device durability if the polymer microstructure is optimized. The present method represents a valuable tool for the design of a new anisotropic PHV, allowing the investigation of different designs, materials, and loading conditions.

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Figures

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Fig. 1

CAD design of the valve leaflet; the central spherical region is highlighted. Geometrical parameters: valve diameter = 23 mm; leaflet height = 11 mm; and leaflet thickness = 0.3 mm.

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Fig. 2

Experimental and modeled stress–strain relationship for the isotropic and the anisotropic case. For the anisotropic case, both the direction parallel and perpendicular to the principle strain direction are presented.

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Fig. 3

Schematic representation of the cylinders' reorientation procedure: for each element, the first axis of the reference system in the (n − 1)th iteration is rotated via the matrix R0, which superimposes this axis with the maximum principal stress direction of iteration n − 1. Thus, the new reference system to be used in the nth iteration is obtained.

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Fig. 4

Cylinder orientation within the leaflet at the beginning (baseline, left) and at the end (optimized, middle) of the optimization process. Right: collagen fiber architecture in a native porcine aortic valve [16].

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Fig. 5

Percentage of rotated local systems in the model for each iteration. After three iterations, the cylinder orientation changes in fewer than 1% of the total elements of the leaflet.

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Fig. 6

Maximum principal stress (top) and maximum principal logarithmic strain (bottom) distributions in the leaflet in the case of isotropic and anisotropic material

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Fig. 7

Circumferential (top) and radial (bottom) logarithmic strain in the isotropic and optimized cases

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Fig. 8

Strain energy density calculated for all the leaflet elements for isotropic and anisotropic cases. For the anisotropic material, both the baseline and the optimized configuration are presented.

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