One of the most critical issues concerning orthopedic
implants
is the risk of chronic inflammation, which poses a threat to the bone
healing process. Osteo-immunomodulation plays a pivotal role in implant
technology by influencing proinflammatory and anti-inflammatory responses,
ultimately promoting bone healing. This study aims to investigate
the morphology-dependent osteo-immunomodulatory properties of a hydroxyapatite
(HA)/plasma electrolytic oxidation (PEO)-coated WE43 alloy. In this
context, following the PEO process with various operational parameters
(duty cycles of 50–40, 50–20, 70–40%, and frequencies
of 0.5, 0.8, and 1 kHz), a layer of HA was applied as the top coating
using a straightforward hot-dip process. The results revealed the
formation of the PEO layer with distinct morphologies and pore sizes,
depending on the operational parameters. Specifically, a uniform PEO
coating with small pore sizes (5.2–5.3 μm) led to the
creation of plate-like HA particles, while a random-like HA structure
formed on nonuniform surfaces with large pores (7.0–11.1 μm)
of PEO. Moreover, it was observed that the plate-like HA coating exhibited
higher adhesion strength than the random one (classified as class
2 vs class 3 based on cross-cut standards). Furthermore, electrochemical
impedance spectroscopy (EIS) and polarization studies confirmed a
substantial increase in the polarization resistance (680 kΩ)
and total impedance (48 559.6 Ω) for the plate-like HA/PEO
as compared to the substrate (an increase of 1511-fold and 311-fold,
respectively) and the random HA/PEO samples (an increase of 85-fold
and 18-fold, respectively). In addition, compared to random HA coatings,
there was a significant enhancement in the viability (150% control
vs 96% control), proliferation, and differentiation of MG63 cells
when exposed to plate-like HA coatings. Moreover, surface morphology
and chemistry pronouncedly impacted macrophages’ viability,
morphology, and phenotype. Notably, plate-like HA coatings resulted
in a higher upregulation of BMP-2 and TGF-β than proinflammatory
cytokines (IL-6 and M-CSF), indicating a polarization of macrophage
type 1 (M1) toward type 2 (M2). In summary, the bilayer HA/PEO coating
exhibited remarkable osteo-immunomodulatory activity, making it highly
appealing for use in bone implant applications.