Biological Performance of Duplex PEO + CNT/PCL Coating on AZ31B Mg Alloy for Orthopedic and Dental Applications

Abstract

To regulate the degradation rate and improve the surface biocompatibility of the AZ31B magnesium alloy, three different coating systems were produced via plasma electrolytic oxidation (PEO): simple PEO, PEO incorporating multi-walled carbon nanotubes (PEO + CNT), and a duplex coating that included a polycaprolactone top layer (PEO + CNT/PCL). Surfaces were characterized by chemical content, roughness, topography, and wettability. Biological properties analysis included cell metabolism and adhesion. PEO ± CNT resulted in an augmented surface roughness compared with the base material (BM), while PCL deposition produced the smoothest surface. All surfaces had a contact angle below 90°. The exposure of gFib-TERT and bmMSC to culture media collected after 3 or 24 h did not affect their metabolism. A decrease in metabolic activity of 9% and 14% for bmMSC and of 14% and 29% for gFib-TERT was observed after 3 and 7 days, respectively. All cells died after 7 days of exposure to BM and after 15 days of exposure to coated surfaces. Saos-2 and gFib-TERT adhered poorly to BM, in contrast to bmMSC. All cells on PEO anchored into the pores with filopodia, exhibited tiny adhesion protrusions on PEO + CNT, and presented a web-like spreading with lamellipodia on PEO + CNT/PCL. The smooth and homogenous surface of the duplex PEO + CNT/PCL coating decreased magnesium corrosion and led to better biological functionality.

Keywords: biodegradable Mg-based implant; cell adhesion; cell metabolism; multi-walled carbon nanotubes (MWCNT); plasma electrolyte oxidation (PEO); polycaprolactone (PCL).

Figure 1

Schematic summary of the coating development procedures.

Figure 2

Characterization of Mg alloy surfaces. The SEM micrograph depicting the surface of BM (a), EDS point analysis of BM (b), SEM micrograph of PEO surface (c), EDS analysis of the selected PEO area (d), PEO cross-section EDS-Line (e), SEM micrograph PEO + CNT surface (f), EDS analysis of the selected PEO + CNT area (g), PEO + CNT cross-section EDS-Line (h), SEM micrograph of PEO + CNT/PCL surface (i), EDS analysis of the selected PEO + CNT/PCL area (j), PEO + CNT/PCL cross-section EDS-Line (k). The percentages of elements based on EDS analysis are depicted. The semi-quantitative EDS analysis results are given in %.

Figure 3

Evaluation of surface topography. Two-dimensional optical images showing the topography of the BM (a) PEO, (b) PEO + CNT (c), and PEO + CNT/PCL (d) coatings. Please note the differences in the sidebar reference scale.

Figure 4

Contact angles for different magnesium surfaces. Contact angles were measured on (a) BM, (b) PEO, (c) PEO + CNT, and (d) PEO + CNT/PCL surfaces.

Figure 5

Cell metabolism on different surfaces. Bone marrow MSCs (a) and gFib-TERTs (b) metabolism expressed as an absorbance ratio measured at 570 nm and 630 nm. Cells were exposed for 24 h to the culture media collected upon incubation with BM, PEO, PEO + CNT, and PEO + CNT/PCL for 3 h, 1, 3, and 7 days. Controls comprised exposure to 5% DMSO (positive control) and fresh cell culture media (negative control). Significant differences (p < 0.05) are indicated: * for p < 0.05, and ** for p < 0.01.

Figure 6

Cell adhesion and spreading on different surfaces. Phenotypic appearance of osteoblastic cell line Saos-2, oral gingival fibroblasts gFib-TERTs, and bone marrow MSCs on tissue culture plate (a), base material (b), PEO (c), PEO + CNT (d), and PEO + CNT/PCL (e) after 24 h. Filopodia are indicated with arrows.