Evaluation of the interface of metallic-coated biodegradable polymeric stents with prokaryotic and eukaryotic cells

Abstract

Polymeric coronary stents, like the ABSORB™, are commonly used to treat atherosclerosis due to their bioresorbable and cell-compatible polymer structure. However, they face challenges such as high strut thickness, high elastic recoil, and lack of radiopacity. This study aims to address these limitations by modifying degradable stents produced by additive manufacturing with poly(lactic acid) (PLA) and poly(ε-caprolactone) (PCL) with degradable metallic coatings, specifically zinc (Zn) and magnesium (Mg), deposited via radiofrequency (rf) magnetron sputtering. The characterisation included the evaluation of the degradation of the coatings, antibacterial, anti-thrombogenicity, radiopacity, and mechanical properties. The results showed that the metallic coatings inhibited bacterial growth, though Mg exhibited a high degradation rate. Thrombogenicity studies showed that Zn-coated stents had anticoagulant properties, while Mg-coated and controls were thrombogenic. Zn coatings significantly improved radiopacity, enhancing contrast by 43 %. Mechanical testing revealed that metallic coatings reduced yield strength and, thus, diminished elastic recoil after stent expansion. Zn-coated stents improved cyclic compression resistance by 270 % for PCL stents, with PLA-based stents showing smaller improvements. The coatings also enhanced crush resistance, particularly for Zn-coated PCL stents. Overall, Zn-coated polymers have emerged as the premier prototype due to their superior biological and mechanical performance, appropriate degradation during the stent life, and ability to provide the appropriate radiopacity to medical devices.

Keywords: Biological performance; Mechanical properties; Polymeric bioresorbable stents; Radiopacity; Sputtered metallic coatings.

Graphical abstract

Fig. 1

Schematic representation of the building orientation of 3D-printed coronary stents (supports are not visible).

Fig. 2

Schematic representation of the quantitative analysis of antibacterial activity: A) UV-C sterilisation; B) representation of the steps of serial dilution and plating to analyse cell number by colony forming unit (CFU).

Fig. 3

Schematic representation of the blood clotting assays.

Fig. 4

Schematic representation of the tensile specimens (a) and macrographs of the equipment (b).

Fig. 5

Cross-section schematic diagram for measuring the electrical resistance of the coated polymers.

Fig. 6

Schematic representation of cyclic compression test and crush resistance test.

Fig. 7

Surface micrographs of Mg and Zn coatings through the degradation process in SBP (Secondary electrons – SE).

Fig. 8

EDS spectra for Zn surfaces after 48h, 96h and 15 days immersion in SBP and Milli-Q.

Fig. 9

EDS spectra for Mg coatings after an immersion of 48h in Milli-Q and SBP.

Fig. 10

Raman spectra of Mg and Zn coatings after the immersion test in SBP for 96h at 37 °C.

Fig. 11

Mg²⁺ and Zn²⁺ concentration in the immersion fluid during the degradation of the coatings in SBP at 37 °C.

Fig. 12

pH values of the SBP solution during the degradation of the magnesium and zinc coatings at 37 °C (15 days–360h).

Fig. 13

Quantitative analysis of viable bacteria on the surface of the polymers, coated and uncoated, after 24 h of incubation at 37^oC, determined by the spread plate method.

Fig. 14

Schematic representation of possible antibacterial mechanisms for Zn and Mg ions, according to the literature descriptions (Created in BioRender.com).

Fig. 15

Schematic representation of thrombus formation: A simplified diagram of the blood coagulation cascade and thrombus formation resulting from the activation of the intrinsic, extrinsic, and common pathways according to literature information (Created in BioRender.com).

Fig. 16

Thrombus formation during the static in vitro blood clotting assays: A) Representative images of the blood clot after incubation; B) Thrombogenicity percentage of the tested samples.

Fig. 17

Blood Clotting Index of all tested surfaces.

Fig. 18

A simplified schematic representation that illustrates a possible explanation of the thrombus formation occurring on the material surfaces through the activation of coagulation factors (such as, fibrinogen, FXIIa) adsorbed to the materials (Created in BioRender.com).

Fig. 19

Representative Images of the micro-CT analysis of the coated polymers, displaying the thickness of the coatings, the atomic number of the metallic elements and % of contrast increase in relation with the polymer.

Fig. 20

Quantitative analysis of radiopacity.

Fig. 21

Representative stress-strain curves of the uncoated and coated stents.

Fig. 22

Yield strength (mean ± SD) of the uncoated and coated stents.

Fig. 23

Representative stress-strain curves and normalised electrical conductance of the coated stents during the tensile tests.

Fig. 24

Representative optical images of stents after a tensile test (inset following the tensile test, an induced section was fractured to observe if the coating remained attached to the polymeric substrate) (bar = 2 mm).

Fig. 25

Values of the maximum compressive normalised force (mean ± SD) during the cyclic tests.

Fig. 26

Maximum pressure (mean ± SD) suffered by the stents during the first cycle (BP is the average blood pressure in a patient with hypertension).

Fig. 27

Representative curves of the normalised compressive force vs. strain during the cyclic tests.

Fig. 28

Diagram of the hysteresis loop in the stress-strain curve (${\mathrm{\sigma }}_{\text{dmax}}$ and ${\mathrm{\sigma }}_{\text{dmin}}$ are the maximum and minimum stress in a hysteresis loop.

Fig. 29

Variation of the dynamic elastic modulus (mean ± SD) during the cyclic tests.

Fig. 30

Variation of the damping ratio (mean ± SD) during the cyclic tests.

Fig. 31

Representative curves of crush resistance of the uncoated and coated stents: normalised compressive force vs. strain.

Fig. 32

Maximum compressive strength (crush resistance) (mean ± SD) for the coated and uncoated stents.

Fig. 33

Compressive modulus (mean ± SD) for the coated and uncoated stents.