Development and properties of duplex MgF2/PCL coatings on biodegradable magnesium alloy for biomedical applications

Abstract

The present work addresses the performance of polycaprolactone (PCL) coating on fluoride treated (MgF2) biodegradable ZK60 magnesium alloy (Mg) for biomedical application. MgF2 conversion layer was first produced by immersing Mg alloy substrate in hydrofluoric acid solution. The outer PCL coating was then prepared using dip coating technique. Morphology, elements profile, phase structure, roughness, mechanical properties, invitro corrosion, and biocompatibility of duplex MgF2/PCL coating were then characterized and compared to those of fluoride coated and uncoated Mg samples. The invivo degradation behavior and biocompatibility of duplex MgF2/PCL coating with respect to ZK60 Mg alloy were also studied using rabbit model for 2 weeks. SEM and TEM analysis showed that the duplex coating was uniform and comprised of porous PCL film (~3.3 μm) as upper layer with compact MgF2 (~2.2 μm) as inner layer. No significant change in microhardness was found on duplex coating compared with uncoated ZK60 Mg alloy. The duplex coating showed improved invitro corrosion resistance than single layered MgF2 or uncoated alloy samples. The duplex coating also resulted in better cell viability, cell adhesion, and cell proliferation compared to fluoride coated or uncoated alloy. Preliminary invivo studies indicated that duplex MgF2/PCL coating reduced the degradation rate of ZK60 Mg alloy and exhibited good biocompatibility. These results suggested that duplex MgF2/PCL coating on magnesium alloy might have great potential for orthopedic applications.

Fig 1. Optical micrograph of as-received ZK60 Mg alloy.

Fig 2. SEM images of (a) ZK60 Mg alloy, (b) MgF₂ and (c) duplex MgF₂/PCL coatings along with EDX (d-f).

Fig 3. Dark field TEM image showing cross-section of duplex MgF₂/PCL coatings along with corresponding SAD patterns and mapping profile.

Fig 4. ZK60 Mg alloy, MgF₂, and MgF₂/PCL coatings (a) XRD, and (b) XPS analysis.

Fig 5. ZK60 Mg alloy, MgF₂ and MgF₂/PCL coatings (a) AFM, and (b) Wettability studies.

Fig 6. (a) Adhesion strength of MgF₂, PCL and MgF₂/PCL coatings, and (b) Hardness studies of ZK60 Mg alloy and duplex MgF₂/PCL coatings.

Fig 7

(a) pH change, (b) Volume of hydrogen released, (c) Weight loss as a function of immersion time in PBS solution for 14 days for ZK60 Mg alloy, MgF₂, and MgF₂/PCL coating samples. (d) Schematic illustration of the degradation mechanism of MgF₂/PCL coated alloy after immersion in PBS solution.

Fig 8. SEM images of ZK60 Mg alloy, MgF₂, and MgF₂/PCL coatings.

(a) After immersion in PBS solution for 14 days along with EDS; (b) After washing of corroded products.

Fig 9. Confocal images of adhesion and attachment of MC3T3-E1 cells after 4 hr seeding on MgF₂ and MgF₂/PCL coatings.

Phalloidin, Vinculin and nucleus were labelled with green, red and blue respectively.

Fig 10

(a) Cytotoxicity of MgF₂ and duplex MgF₂/PCL against MC3T3-E1 cells. MTT assay was performed using indirect assay after 1day incubation with increasing extract concentrations. (b) Cell proliferation behaviour of MgF₂ and MgF₂/PCL coating after incubation of 1, 3, and 5 days.

Fig 11. Control (defect), ZK60 Mg alloy and duplex MgF₂/PCL coating (a) Photographs at the time of implantation and extraction, (b) Degradation rate and Bone volume at the interface, (c) 3-D MicroCT scans of the implanted zone and (d) Histological analysis after 2 weeks implantation.