Modification of Anti-Bacterial Activity and Bone Cell Proliferation by Surface Engineering of Ga- or Mn-Doped Ceria-Coated Biomedical Titanium Alloy

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Copyright: Khosravanihaghighi, Ayda
Abstract
The two leading causes of failure of orthopaedic implants are aseptic loosening and periprosthetic joint infection. Since the numbers of primary and revision joint replacement surgeries are increasing, strategies to mitigate these failure modes have become increasingly important. However, most recent work has focused on the design of coatings to prevent infection or to enhance bone mineralisation. However, long-term success of the implants is contingent on addressing both of these issues. Consequently, the present work focussed on multifunctional orthopaedic coatings that inhibit microbial cells while still promoting osseointegration. Nanoceria has considerable potential to be used in biomedical applications owing to its unique bio-responsive redox switching and its capacity to be doped with different therapeutic ions of varying functionalities. Therefore, the effect of different cations incorporated in ceria on cellular behaviour in vitro as well as the anti-bacterial performance were investigated. The two main foci were: (1) characterisation of the bioceramic materials and (2) biological response to undoped and doped ceria ceramics in vitro using bacteria colonies forming unit (CFU) and cytotoxicity Ceria (CeO2) thin films (~820 nm thickness) doped with 0-9 mol% Ga or Mn were fabricated by spin coating on 3D-printed Ti6Al4V followed by heat treatment at 650°C for 2 h, and these were characterised by transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM) (microstructure), 3D laser scanning confocal microscopy (topography), glancing angle X-ray diffraction (GAXRD) (structure and mineralogy), and X-ray photoelectron spectroscopy (XPS) (surface chemistry). In vitro testing was conducted, including inhibition of bacterial growth, simulated body fluid (SBF) testing, and cell attachment and proliferation studies. The data are interpreted in terms of the following: (1) The roles of the sol-gel precursor viscosity, which affected pore filling and surface coverage, (2) Lattice contraction, which contradicted the XPS data, (3) Intervalence charge transfer, which increased the Ce3+ concentration but was a minor effect, (4) Substitutional solid solubility, which is consistent with Hume-Rothery’s rules and the GAXRD data, (5) Redox charge compensation, where the defect equilibria highlight the key role of this mechanism, which decreased the Ce3+ concentration and provided the majority effect, (6) Electronegativity, which plays a small, if any, role in affecting the ion valences but is important in initiating intervalence charge transfer, (7) Multivalence charge transfer, which combined the electron exchanges between film matrix, dopants, and Ti substrate. The most significant outcome was that the bioactivity of ceria derives directly from the Ce3+ concentration, which itself results from solid solubility (substitutional and interstitial) and charge compensation and redox. This challenges the common assumption of the dominance of oxygen vacancies in the performance of ceria. The antibacterial activity was dependent on the type, amount, and valence of the dopant, where opposite trends were observed for gram-positive S. aureus and gram-negative E. coli bacteria. All of the doped samples resulted in enhanced cell proliferation, although this was greatest at the lowest dopant concentration. Surface hydroxyapatite formation on the samples was achieved by soaking in SBF at 2 weeks and 1 month.
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Publication Year
2022
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Thesis
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PhD Doctorate
UNSW Faculty