INTRODUCTION

One out of four people above 65 years of age in the US suffer

from heart valve diseases making prosthetic valvesrsquo structural and fluid

mechanics a great interest among the researchers [1] The prosthetic

valves have been studied since 1950 and despite of significant

supporting evidences that bioprosthetic valves are biocompatible and

have similar hemodynamics to native valves the material of the

xenograft leaflets was adapted but not specifically designed or optimized for

prosthetic use Our research group has developed a novel polymeric

prosthetic valve which was optimized to reduce stresses on the leaflets

improve hemodynamics and has thrombogenic performance similar to

that of a tissue valve [2]

There are currently two types of prosthetic aortic valve procedures

for end stage patients with calcific aortic valve surgical aortic valve

replacement (SAVR) and transcatheter aortic valve replacement

(TAVR) SAVR requires open heart surgery while TAVR is a

minimally invasive procedure for patients who cannot undergo open

heart surgery The mechanics of these devices was previously studied

with numerical tools However to the best of our knowledge no such

study compared the combined structural and fluid mechanics of TAVR

and SAVR valves under identical conditions

The aims of this study are to compare our polymeric TAVR and

SAVR valves using a fluid-structure interaction (FSI) simulation with

Arbitrary Lagrangian-Eulerian (ALE) method and to evaluate

experimentally the TAVR valversquos hemodynamics Specifically the

model compares valvesrsquo hemodynamics parameters and mechanical

stresses The FSI simulation combines structural and fluid dynamics and

can accurately capture the valve kinematics by transferring momentum

between leaflets and blood flow [5 6] The experimental bench tests are

used for thorough hemodynamic evaluation of the fabricated valves

according to ISO 5840 and for validating the FSI simulation

METHODS

The valves are made of cross-linked Styrene-block-IsoButylene-

block-Styrene (xSIBS Innovia LLC Miami FL) polymer and the

geometries were obtained from Polynova cardiovascular Inc (Stony

Brook NY) [2] The initial leaflets design was utilized in the SAVR

valve and later optimized and adapted for a TAVR valve by adding a

sleeve sutured to a self-expandable nitinol stent and by offsetting the

leaflets nominal position to be semi-open for reduction of structural

stress accumulation over the cardiac cycle The stent was drawn using

SolidWorks 2015 (Dassault Systemes Concord MA USA) The fluid

domain was extracted from the ViVitro Pulse Duplicatorrsquos aortic root

geometry (Vivitro System Inc Victoria BC CA) [6]

The fluid governing equations were solved using finite volume

discretizing method while the structural equations were solved using an

implicit displacement based finite element approach The flow solution

employed a coupled pressure-velocity method to solve continuity and

momentum equations An unsteady Reynold averaged Navier-Stokes

(URANS) with k-ω turbulent model was implemented A diffusion

based dynamic mesh was used for remeshing highly skewed fluid

domain cells In order to keep the fluid domain continues and to prevent

small-volume cells virtual walls were added between the leaflets to

maintaining a microscopic gap between the leaflets on the structural

domain Zero penetration was enforced by a Normal Lagrange contact

formulation between the leaflets and the virtual wall surfaces

The fluid and structural solvers were coupled using ANSYS

system coupling where the time step size varied from 01 to 10 ms

during the cardiac cycle The simulations were ran for a cardiac cycle

with a duration of 0854 s The blood was modeled as Newtonian fluid

with a dynamic viscosity of 00035 Pas and a density of 1060 kgm3

The leaflets material property was obtained via a uniaxial tensile

machine and fitted to Mooney-Rivlin isotropic hyperplastic model The

SB3C2017 Summer Biomechanics Bioengineering and Biotransport Conference

June 21 ndash 24 Tucson AZ USA

EVALUATION OF NOVEL POLYMERIC TRANSCATHETER AND SURGICAL AORTIC

VALVES WITH FLUID-STRUCTURE INTERACTION MODELS AND EXPERIMENTAL

ANALYSIS

Ram P Ghosh (1) Gil Marom (1) Oren M Rotman (1) Matteo Bianchi (1)

Saurabh Prabhakar (2) Marc Horner (3) Marvin J Slepian (1 4) Danny Bluestein (1)

(1) Department of Biomedical Engineering Stony Brook University Stony Brook NY USA

(2) ANSYS Fluent India Pvt Ltd

Pune India

(3) ANSYS Inc

Evanston IL USA

(4) Sarver Heart Center

University of Arizona

Tucson AZ USA

Technical Presentation 104 Copyright 2017 The Organizing Committee for the 2017 Summer Biomechanics Bioengineering and Biotransport Conference

ALE type FSI models were used because of their accurate boundary

condition implementation wall shear stress (WSS) calculation and

incorporation of valvesrsquo leaflet thickness hence ensuring accurate

calculation of flowrates orifice areas and mechanical stresses The

2-way iteratively implicit FSI simulation was solved separately for the

fluid and solid equations in transient ANSYS Fluent and Structural 171

(ANSYS Inc Canonsburg PA) respectively

RESULTS

The TAVR and SAVR valves show similar qualitative flow

features and their flow rates orifice areas and WSS during systole and

mechanical stress magnitudes during diastole can be quantitatively

compared The flow acceleration during systole initiates with a central

jet flow and symmetrically forming vortex rings near the sinuses (Fig

1 A D) Then the sinusesrsquo central flow aid in the velocity elevation with

the central orifice jet The vortex rings become weaker during peak

systole and travels upwards towards the aorta (Fig 1 B E) During peak

flow phase the TAVR valversquos central jet flow was expanding more

extensively than in the SAVR valve This is followed by flow

deceleration phase and the beginning of diastole During diastole the

leaflets closure was aided by the flow from the sinuses The leaflets

closure causes a flow depression and is responsible for a fluid suction

The flow from the upper region then travels to the sinuses and creates a

counter rotating vortex to fill the sinuses

The valves show the largest geometric orifice area (GOA Fig

2 A) effective orifice area (EOA) flow rates (Fig 2 B) and maximum

WSS on the leaflets during their systolic phase The TAVR valve was

found to have higher GOA EOA and flowrate but lower WSS than the

SAVR valve The TAVR and SAVR valves calculated EOAs were 233

and 185 cm2 respectively The TAVR valves experienced lower WSS

(7225 Pa) compare to the SAVR valve (9206 Pa) during peak systole

The valves experienced the highest mechanical stresses during peak

diastole (Fig 1 C F) The peak equivalent stress magnitudes for the

TAVR and SAVR valves are 884 and 444 MPa respectively In both

cases the highest stress magnitudes were observed near the leaflets

commissural region where the leaflets are attached

DISCUSSION

This study compared numerically polymeric TAVR and SAVR

valves leaflets kinematics TAVR valve had higher GOA EOA and

flowrate and lower WSS than SAVR valve suggesting that TAVR valve

is an improvement of the exiting SAVR valve The TAVR valve

however experienced higher mechanical stresses than the SAVR valve

The leaflets attachment in the commissural region can explain these

higher stresses The influence of these results on the durability of the

SAVR and TAVR valves will be experimentally validated as described

below

The hemodynamics during diastole was not compared because the

flow blockage model in the virtual gap has not been implemented yet

resulting in unphysical leakage This study assumes an ideal circular

configuration of the valves however deployed TAVR valve will be

either under-expanded or in elliptical shape [7] This interrupts the valve

coaptation and valve functionality which in turns have an impact on

leaflets stresses

The polymeric TAVR valves are now being fabricated and bench

tested in our facility The fabrication is done by compression molding

using an EDM-machined S7 tool steel mold (Fig 3) Vacuum is applied

to the mold throughout the molding process for efficient removal of air

bubbles and for maximizing the molded valve quality The valves are

then sutured to a nitinol stent for completion of the process The valvesrsquo

durability is now being assessed using a Vivitro Hi-Cycle System

(Vivitro Labs Inc Victoria BC) according to ISO 5840 Hydrodynamic

performance is being evaluated using the Vivitro left heart simulator

(Vivitro Labs Inc Victoria BC) This is also used for validation of the

FSI simulation results in terms of the valvesrsquo EOA GOA and flow

rates Flow-induced platelet activation is measured in-vitro in

comparison to a gold standard Carpentier-Edwards Perimount Magna

Ease aortic valve

Figure 3 The polymeric TAVR valve mold and the stented valve

ACKNOWLEDGEMENTS

ANSYS Inc is in an academic partnership with Dr Bluestein

REFERENCES

[1] Bavo AM et al PloS one 11e0154517 2016

[2] Claiborne TE et al ASAIO J 59275-83 2013

[3] Villablanca PA et al Int J Cardiol 225234-243 2016

[4] Sedrakyan A et al JAMA Intern Med 2016

[5] Marom G Arch Computat Methods Eng 1-26 2014

[6] Piatti F et al J Biomech 2015

[7] Martin C and Sun W J Biomech 483026-3034 2015

Figure 1 The TAVR and SAVR valves colored by von Mises

stress and velocity streamline on a middle cross-section in various

instances during the cardiac cycle beginning of systole (left) peak

systole (center) and peak diastole (right)

TA

VR

(A) (B) (C)

SA

VR

(D) (E) (F)

von Mises Stress [MPa]

000 015 030 045 060

von Mises Stress [MPa]

000 010 020 030 040

Velocity Vector [ms]

000 075 150 225 300

Velocity Vector [ms]

000 0875 175 2625 350

Figure 2 The valvesrsquo GOA (A) and area weighted flow rate (B)

during systolic phase

(B)(A)

Technical Presentation 104 Copyright 2017 The Organizing Committee for the 2017 Summer Biomechanics Bioengineering and Biotransport Conference

ALE type FSI models were used because of their accurate boundary

condition implementation wall shear stress (WSS) calculation and

incorporation of valvesrsquo leaflet thickness hence ensuring accurate

calculation of flowrates orifice areas and mechanical stresses The

2-way iteratively implicit FSI simulation was solved separately for the

fluid and solid equations in transient ANSYS Fluent and Structural 171

(ANSYS Inc Canonsburg PA) respectively

RESULTS

The TAVR and SAVR valves show similar qualitative flow

features and their flow rates orifice areas and WSS during systole and

mechanical stress magnitudes during diastole can be quantitatively

compared The flow acceleration during systole initiates with a central

jet flow and symmetrically forming vortex rings near the sinuses (Fig

1 A D) Then the sinusesrsquo central flow aid in the velocity elevation with

the central orifice jet The vortex rings become weaker during peak

systole and travels upwards towards the aorta (Fig 1 B E) During peak

flow phase the TAVR valversquos central jet flow was expanding more

extensively than in the SAVR valve This is followed by flow

deceleration phase and the beginning of diastole During diastole the

leaflets closure was aided by the flow from the sinuses The leaflets

closure causes a flow depression and is responsible for a fluid suction

The flow from the upper region then travels to the sinuses and creates a

counter rotating vortex to fill the sinuses

The valves show the largest geometric orifice area (GOA Fig

2 A) effective orifice area (EOA) flow rates (Fig 2 B) and maximum

WSS on the leaflets during their systolic phase The TAVR valve was

found to have higher GOA EOA and flowrate but lower WSS than the

SAVR valve The TAVR and SAVR valves calculated EOAs were 233

and 185 cm2 respectively The TAVR valves experienced lower WSS

(7225 Pa) compare to the SAVR valve (9206 Pa) during peak systole

The valves experienced the highest mechanical stresses during peak

diastole (Fig 1 C F) The peak equivalent stress magnitudes for the

TAVR and SAVR valves are 884 and 444 MPa respectively In both

cases the highest stress magnitudes were observed near the leaflets

commissural region where the leaflets are attached

DISCUSSION

This study compared numerically polymeric TAVR and SAVR

valves leaflets kinematics TAVR valve had higher GOA EOA and

flowrate and lower WSS than SAVR valve suggesting that TAVR valve

is an improvement of the exiting SAVR valve The TAVR valve

however experienced higher mechanical stresses than the SAVR valve

The leaflets attachment in the commissural region can explain these

higher stresses The influence of these results on the durability of the

SAVR and TAVR valves will be experimentally validated as described

below

The hemodynamics during diastole was not compared because the

flow blockage model in the virtual gap has not been implemented yet

resulting in unphysical leakage This study assumes an ideal circular

configuration of the valves however deployed TAVR valve will be

either under-expanded or in elliptical shape [7] This interrupts the valve

coaptation and valve functionality which in turns have an impact on

leaflets stresses

The polymeric TAVR valves are now being fabricated and bench

tested in our facility The fabrication is done by compression molding

using an EDM-machined S7 tool steel mold (Fig 3) Vacuum is applied

to the mold throughout the molding process for efficient removal of air

bubbles and for maximizing the molded valve quality The valves are

then sutured to a nitinol stent for completion of the process The valvesrsquo

durability is now being assessed using a Vivitro Hi-Cycle System

(Vivitro Labs Inc Victoria BC) according to ISO 5840 Hydrodynamic

performance is being evaluated using the Vivitro left heart simulator

(Vivitro Labs Inc Victoria BC) This is also used for validation of the

FSI simulation results in terms of the valvesrsquo EOA GOA and flow

rates Flow-induced platelet activation is measured in-vitro in

comparison to a gold standard Carpentier-Edwards Perimount Magna

Ease aortic valve

Figure 3 The polymeric TAVR valve mold and the stented valve

ACKNOWLEDGEMENTS

ANSYS Inc is in an academic partnership with Dr Bluestein

REFERENCES

[1] Bavo AM et al PloS one 11e0154517 2016

[2] Claiborne TE et al ASAIO J 59275-83 2013

[3] Villablanca PA et al Int J Cardiol 225234-243 2016

[4] Sedrakyan A et al JAMA Intern Med 2016

[5] Marom G Arch Computat Methods Eng 1-26 2014

[6] Piatti F et al J Biomech 2015

[7] Martin C and Sun W J Biomech 483026-3034 2015

Figure 1 The TAVR and SAVR valves colored by von Mises

stress and velocity streamline on a middle cross-section in various

instances during the cardiac cycle beginning of systole (left) peak

systole (center) and peak diastole (right)

TA

VR

(A) (B) (C)

SA

VR

(D) (E) (F)

von Mises Stress [MPa]

000 015 030 045 060

von Mises Stress [MPa]

000 010 020 030 040

Velocity Vector [ms]

000 075 150 225 300

Velocity Vector [ms]

000 0875 175 2625 350

Figure 2 The valvesrsquo GOA (A) and area weighted flow rate (B)

during systolic phase

(B)(A)

Technical Presentation 104 Copyright 2017 The Organizing Committee for the 2017 Summer Biomechanics Bioengineering and Biotransport Conference