Experimental Tests, FEM Constitutive Modeling and Validation of PLGA Bioresorbable Polymer for Stent Applications

Abstract

The use of bioresorbable polymers such as poly(lactic-co-glycolic acid) (PLGA) in coronary stents can hypothetically reduce the risk of complications (e.g., restenosis, thrombosis) after percutaneous coronary intervention. However, there is a need for a constitutive modeling strategy that combines the simplicity of implementation with strain rate dependency. Here, a constitutive modeling methodology for PLGA comprising numerical simulation using a finite element method is presented. First, the methodology and results of PLGA experimental tests are presented, with a focus on tension tests of tubular-type specimens with different strain rates. Subsequently, the constitutive modeling methodology is proposed and described. Material model constants are determined based on the results of the experimental tests. Finally, the developed methodology is validated by experimental and numerical comparisons of stent free compression tests with various compression speeds. The validation results show acceptable correlation in terms of both quality and quantity. The proposed and validated constitutive modeling approach for the bioresorbable polymer provides a useful tool for the design and evaluation of bioresorbable stents.

Keywords: PLGA; bioresorbable; constitutive modeling; experimental tests; polymer; stent.

Figure 1

Chemical formula of the poly(lactic-co-glycolic acid) (PLGA) bioresorbable polymer developed at the Centre of Polymer and Carbon Materials [27].

Figure 2

Schematic overview (cross section) of mounting of tubular specimens in the tensile test of the PLGA bioresorbable polymer (longitudinal tension).

Figure 3

FEM model of a specimen tube in the longitudinal tensile test: symmetry boundary conditions are marked in black, and the displacement load is marked in red.

Figure 4

Crimping machine used in the stent compression tests. The machine was equipped with a radial force measuring head (friction compensated) and a thermal chamber.

Figure 5

Model geometry for validation analyses of the PLGA bioresorbable polymer constitutive model.

Figure 6

Boundary conditions for free stent compression: front view of the displacement of 12 predefined rigid wall structures (shown in black).

Figure 7

Longitudinal tensile force vs machine grip displacement for tubular specimens made of PLGA (assumed strain rate 0.0003 1/s).

Figure 8

Longitudinal tensile force vs machine grip displacement for tubular specimens made of PLGA (assumed strain rate 0.03 1/s).

Figure 9

Comparison of the experimental and corresponding numerical results of tension forces during longitudinal tensile tests of tubes made of PLGA bioresorbable polymer: variant 1:0.72 mm/min.

Figure 10

Comparison of the experimental and corresponding numerical results of tension forces during longitudinal tensile tests of tubes made of PLGA bioresorbable polymer: variant 2:72.0 mm/min.

Figure 11

Effective strain rates for 3 compression rate variants: 1.0 mm/s (blue), 0.1 mm/s (green), and 0.01 mm/s (red).

Figure 12

Comparison of experimental and numerical results for radial forces during free compression of undeformed coronary stents made of PLGA bioresorbable polymer with three compression rate variants: 1.0 mm/s, 0.1 mm/s, and 0.01 mm/s.

Figure 13

Acute outcome after PLGA/PLLA blend polymer stent implantation in coronary angiography (a) and optical coherence tomography (b). Lumen area increase before and after implantation (c). Geometrical view of vessel after stent implantation (d).

Figure 13

Acute outcome after PLGA/PLLA blend polymer stent implantation in coronary angiography (a) and optical coherence tomography (b). Lumen area increase before and after implantation (c). Geometrical view of vessel after stent implantation (d).

Figure 14

Optical coherence tomography at 28 days follow-up show optimal vascular response and stent geometry. Stent struts show blurred image, a sign of early hydrolysis (rectangle).

Figure 15

One of the test variants of a bioresorbable coronary stent developed with the use of the proposed constitutive methodology within the Apollo STRATEGMED2 project: (a) Stent after cutting and free compression; (b) area of clinical test implementation (angiography image); (c) area of clinical test implementation (intravascular ultrasound image).