Zn-Mg and Zn-Cu alloys for stenting applications: From nanoscale mechanical characterization to in vitro degradation and biocompatibility

Abstract

In the recent decades, zinc (Zn) and its alloys have been drawing attention as promising candidates for bioresorbable cardiovascular stents due to its degradation rate more suitable than magnesium (Mg) and iron (Fe) alloys. However, its mechanical properties need to be improved in order to meet the criteria for vascular stents. This work investigates the mechanical properties, biodegradability and biocompatibility of Zn-Mg and Zn-Cu alloys in order to determine a proper alloy composition for optimal stent performance. Nanoindentation measurements are performed to characterize the mechanical properties at the nanoscale as a function of the Zn microstructure variations induced by alloying. The biodegradation mechanisms are discussed and correlated to microstructure, mechanical performance and bacterial/cell response. Addition of Mg or Cu alloying elements refined the microstructure of Zn and enhanced yield strength (YS) and ultimate tensile strength (UTS) proportional to the volume fraction of secondary phases. Zn-1Mg showed the higher YS and UTS and better performance in terms of degradation stability in Hanks' solution. Zn-Cu alloys presented an antibacterial effect for S. aureus controlled by diffusion mechanisms and by contact. Biocompatibility was dependent on the degradation rate and the nature of the corrosion products.

Keywords: Biocompatibility; Bioresorbable metals; Galvanic corrosion; Nanoindentation; Zinc alloys.

Graphical abstract

Fig. 1

Microstructure of the cross-section (left) and longitudinal section (right) of: extruded Zn (a, b), and cold rolled Zn-0.5Mg (c, d), Zn-1Mg (e, f), Zn-3Cu (g, h), and Zn-5Cu (i, j) alloys.

Fig. 2

Tensile test of pure Zn and Zn-Mg and Zn-Cu alloys: a) mechanical parameters obtained after tensile and micro-hardness testing. ^{a, b,} symbols join groups with non-statistically significant differences. b) SEM morphology of the tensile fracture surfaces.

Fig. 3

SEM image of nanoindented Zn-0.5Mg alloy with the matrix of 10 by 10 nanoindentations and the phases composition. Scale bar: 10 μm.

Fig. 4

Potentiodynamic polarization curves of Zn, Zn-0.5Mg, Zn-1Mg, Zn-3Cu, and Zn-5Cu in Hanks' solution at 37 ± 1 °C.

Fig. 5

Immersion evaluation of studied Zn and Zn-based surfaces after 1, 3, 7, and 10 days in Hanks' solution at 37 °C: (a) Values of released Zn²⁺ expressed as (μg/dL), (b) pH, and (c) redox potential.

Fig. 6

SEM images of the corroded samples after 10 days of immersion in Hanks' solution at 37 °C, before (left column) and after (right column) the removal of the corrosion products: Zn (a, b), Zn-0.5Mg (c, d), Zn-1Mg (e, f), Zn-3Cu (g, h), and Zn-5Cu (i, j).

Fig. 7

Electrochemical impedance spectroscopy (EIS) evaluation in Hanks' solution of Zn and Zn-based alloys: (a) Equivalent circuits of (i) passivation layer, (ii) porous layer, and (iii) bi-layer model. EIS Nyquist plots: (b) Zn, (c) Zn-0.5Mg, (d) Zn-1Mg, (e) Zn-3Cu, and (f) Zn-5Cu. Time after immersion: -2h, -4h, -8h, -26h, -2d, -3d, and -7d.

Fig. 8

SEM images of bacterial morphology for both S. aureus (left) and P. aeruginosa (right) after culturing for 2 h on pure Zn (a, b), Zn-0.5Mg (c, d), Zn-1Mg (e, f), Zn-3Cu (g, h), and Zn-5Cu (i, j). Scale bar: 20 μm. Yellow arrows indicate P. aeruginosa bacteria.

Fig. 9

Indirect HAoEC viability after 24 h of culture in supernatants prepared by immersion of the alloys in Growth Medium for 3 days. Aliquots of supernatants at decreased concentrations were added to adhered cells.

Fig. 10

Cell viability of HAoEC seeded on the alloys measured by LDH test. (*) Statistically significant differences in comparison to Zn control group, P < 0.05.

Fig. 11

SEM (scale bar 10 μm) and confocal (scale bar 100 μm) images of HAoEC morphology and distribution after 1 day (left hand column), 3 days (2nd column) and 7 days (right hand 2 columns) of culturing on Zn (a)–(d), Zn-0.5Mg (e)–(h), Zn-1Mg (i)–(l), Zn-3Cu (m)–(p), and Zn-5Cu (q)–(t).