Ciprofloxacin Release and Corrosion Behaviour of a Hybrid PEO/PCL Coating on Mg3Zn0.4Ca Alloy

Abstract

In the present work, a hybrid hierarchical coating (HHC) system comprising a plasma electrolytic oxidation (PEO) coating and a homogeneously porous structured polycaprolactone (PCL) top-coat layer, loaded with ciprofloxacin (CIP), was developed on Mg3Zn0.4Ca alloy. According to the findings, the HHC system avoided burst release and ensured gradual drug elution (64% over 240 h). The multi-level protection of the magnesium alloy is achieved through sealing of the PEO coating pores by the polymer layer and the inhibiting effect of CIP (up to 74%). The corrosion inhibition effect of HHC and the eluted drug is associated with the formation of insoluble CIP-Me (Mg/Ca) chelates that repair the defects in the HHC and impede the access of corrosive species as corroborated by FTIR spectra, EIS and SEM images after 24 h of immersion. Therefore, CIP participates in an active protection mechanism by interacting with cations coming through the damaged coating.

Keywords: corrosion inhibitor; drug delivery; hybrid hierarchical coatings; magnesium; orthopaedic; plasma electrolytic oxidation; polycaprolactone.

Figure 1

Secondary electron plan view micrographs of HCC and HCC + CIP specimens.

Figure 2

Comparative of released load fraction from porous PCL films and HHC+CIP systems. The porous PCL films were applied onto a model substrate (glass) with 0.57 g of CIP. The HHC+CIP system was loaded with 1.03 mg of the drug.

Figure 3

Hydrogen evolution for CIP-free and CIP-loaded HHC on Mg3Zn0.4Ca alloy during immersion in inorganic α-MEM at 37 °C under flow of CO₂.

Figure 4

Secondary (a) and backscattered (b,c) electron micrographs of HHC + CIP: (a) view of the PCL layer defects at the specimen edge; (b,c) cross-sectional views following 24 h of immersion in inorganic α-MEM at 37 °C.

Figure 5

TEM micrographs of the (a) barrier layer and (b) detail of discharge channel area of the PEO coating on Mg3Zn0.4Ca alloy.

Figure 6

Optical micrographs and SVET current density distribution on CIP-free and CIP-loaded HCC system after 1, 12 and 24 h of immersion in α-MEM at 37 °C. The red rectangle in each optical micrograph indicates the precise location of local pH and H₂ mapping (3 × 3 mm).

Figure 7

(a,d,g) The visual appearances, (b,c,h) local pH distribution and (c,f,i) local dissolved H₂ concentration for HHC after 1 h, 6 h and 24 h of immersion in inorganic α-MEM at 37 °C. The red rectangle in each optical micrograph indicates the precise location of local pH and H₂ mapping (3 × 3 mm).

Figure 8

(a,d,g) The visual appearances, (b,c,h) distributions of local pH and (c,f,i) H₂ concentration of HHC+CIP in inorganic α-MEM at 37 °C after 1, 6 and 24 h. The red rectangle in each optical micrograph indicates the precise location of local pH and H₂ mapping (3 × 3 mm).

Figure 9

Regulation of pH by CIP zwitterion release.

Figure 10

Backscattered electron cross-sectional images of (a–c) HHC and (d–f) HHC+CIP after local pH and local H₂ concentration measurements in inorganic α-MEM at 37 °C.

Figure 11

FTIR spectra of ciprofloxacin (red), PCL (black) layer in as-received HHC specimen and PCL + CIP layer in HHC+CIP specimens (blue) after 48 h of immersion in inorganic α-MEM solution.

Figure 12

Evolution of (a) total modulus of impedance at low frequencies (0.1 Hz) and (b) pH for HHC and HHC+CIP systems during immersion in inorganic α-MEM at 37 °C.

Figure 13

(a) The evolution of resistance of the layers of CIP-free and CIP-loaded HHC systems during immersion; (b–e) secondary electron images of (b,c) HHC and (d,e) HHC+CIP specimens after EIS measurements at 48 h; (f) local EDS analysis (at. %) of (c,e) images.