Evaluating Polylactic Acid and Basalt Fibre Composites as a Potential Bioabsorbable Stent Material

Abstract

Bioabsorbable polymer stents (BPSs) were developed to address the long-term clinical drawbacks associated with permanent metallic stents by gradually dissolving over time before these drawbacks have time to develop. However, the polymers used in BPSs, such as polylactic acid (PLA), have lower mechanical properties than metals, often requiring larger struts to provide the necessary structural support. These larger struts have been linked to delayed endothelialisation and an increased risk of stent thrombosis. To address this limitation, this study investigated the incorporation of high-strength basalt fibres into PLA to enhance its mechanical performance, with an emphasis on optimising the processing conditions to achieve notable improvements at minimal fibre loadings. In this regard, PLA/basalt fibre composites were prepared via twin-screw extrusion at screw speeds of 50, 200, and 350 RPM. The effects were assessed through ash content testing, tensile testing, SEM, and rheometry. The results showed that lower screw speeds achieved adequate fibre dispersion while minimising the molecular weight reduction, leading to the most substantial improvement in the mechanical properties. To examine whether a second extrusion run could enhance the fibre dispersion, improving the composite's uniformity and, therefore, mechanical enhancement, all the batches underwent a second extrusion run. This run improved the dispersion, leading to increased strength and an increased modulus; however, it also reduced the fibre-matrix adhesion and resulted in a notable reduction in the molecular weight. The highest mechanical performance was observed at 10% fibre loading and 50 RPM following a second extrusion run, with the tensile strength increasing by 20.23% and the modulus by 27.52%. This study demonstrates that the processing conditions can influence the fibres' effectiveness, impacting dispersion, adhesion, and molecular weight retention, all of which affect this composite's mechanical performance.

Keywords: basalt fibre; bioabsorbable polymer stents; polylactic acid; twin-screw extrusion.

Figure 1

Crossover point of storage modulus (G′) and loss modulus (G″) as indicator of molecular weight. Shift to higher frequencies signifies molecular weight reduction. Adapted from [26].

Figure 2

Actual fibre content (%) measured via ash content analysis for the PLA/basalt fibre composites under different fibre loadings and processing conditions. Fibre loadings of 5%, 7.5%, and 10% are represented in red, blue, and beige, respectively. The grey dashed lines represent the target fibre loadings for each condition. Solid bars indicate a single extrusion run, while hatched bars indicate a second extrusion run.

Figure 3

SEM images of fractured tensile samples at 5% (a), 7.5% (b), and 10% (c) basalt fibre loadings, processed at 200 RPM, showing fibre dispersion and fibre–matrix adhesion at increasing magnifications.

Figure 4

SEM images of fractured tensile samples at 7.5% basalt fibre loading, processed at 50 RPM (a), 200 RPM (b), and 350 RPM (c), showing effect of screw speed on fibre dispersion and fibre–matrix adhesion at increasing magnifications.

Figure 5

SEM image at 50× magnification of the 7.5% basalt fibre composite processed at 50 RPM. The red arrow highlights a fibre-free region, indicating a less effective fibre distribution compared to that at higher screw speeds.

Figure 6

SEM images comparing the effects of a single extrusion run (a) and a second extrusion run (b) on 7.5% basalt fibre composites processed at 200 RPM.

Figure 7

SEM images comparing the fibre dispersion in 7.5% basalt fibre composites processed at 200 RPM after a single extrusion run (left) and a second extrusion run (right). Red arrows in the single extrusion run highlight large fibre-free regions, indicative of poor dispersion. The composites subjected to a second extrusion run showed improved fibre dispersion with fewer fibre-free regions.

Figure 8

Crossover frequency (G′ = G″) of PLA/basalt fibre composites at varying screw speeds (50, 200, and 350 RPM) and after different extrusion runs (R1, R2). Virgin PLA is shown in green, 5% fibre loading in red, 7.5% in blue, and 10% in beige.

Figure 9

Zero-shear viscosity of PLA/basalt fibre composites at varying screw speeds (50, 200, and 350 RPM) and a single extrusion run Bar colours correspond to fibre loadings as defined in Figure 8.

Figure 10

Comparison of crossover frequency (G′ = G″) across different basalt fibre loadings (5%, 7.5%, and 10%) to assess effect of fibre content. Virgin PLA is shown in green, 5% fibre loading in red, 7.5% in blue, and 10% in beige.

Figure 11

Comparison of zero-shear viscosity across different basalt fibre loadings (5%, 7.5%, and 10%) to assess effect of fibre content. Bar colours correspond to fibre loadings as defined in Figure 10.

Figure 12

Crossover frequency (G’ = G”) of PLA/basalt fibre composites at varying screw speeds (50, 200, and 350 RPM) and after different extrusion runs (R1, R2). Virgin PLA is shown in green, 5% fibre loading in red, 7.5% in blue, and 10% in beige. Hatched bars represent samples processed using second extrusion run (R2).

Figure 13

Zero-shear viscosity of PLA/basalt fibre composites at varying screw speeds (50, 200, and 350 RPM) and after different extrusion runs (R1, R2). Bar colours correspond to fibre loadings as defined in Figure 8.

Figure 14

Young’s modulus of PLA/basalt fibre composites at 50 RPM for different fibre loadings (5%, 7.5%, and 10%) and extrusion runs (R1, R2). Virgin PLA is shown in green, with fibre loadings represented by red (5%), blue (7.5%), and beige (10%). Hatched bars indicate samples processed using second extrusion run (R2).

Figure 15

Ultimate tensile strength (UTS) of PLA/basalt fibre composites at 50 RPM for different fibre loadings (5%, 7.5%, and 10%) and extrusion runs (R1, R2). Virgin PLA is shown in green, with fibre loadings represented by red (5%), blue (7.5%), and beige (10%). Hatched bars indicate samples processed using second extrusion run (R2).