Introduction: Corrosion in ceramics occurs preferentially at high energy sites such as grain boundaries (GB) and GB corrosion is a well-known phenomenon for polycrystalline oxide ceramics.[1] When highly porous ceramic materials such as bone scaffolds with large surface-to-volume ratios are exposed to corrosive environment, GB corrosion can have a particularly detrimental impact on the mechanical properties of the scaffold. This study aims to investigate the suitability of atomic layer deposition (ALD) of TiO2 thin films to protect a macro-porous TiO2 scaffold from GB corrosion and maintain its mechanical properties in physiologically low pH conditions. Two different deposition temperatures were chosen to assess the effect of film structure on the barrier properties of the deposited layers.
Materials and Methods: TiO2 thin films were deposited at 150°C and 250 °C on the surface of highly porous TiO2 scaffolds using the chloride-water ALD process. The ALD films were characterised by scanning electron and atomic force microscopy, x-ray diffraction, and ellipsometry. The compressive strength of the ALD-coated scaffolds was tested following up to 28 d exposure to corrosive medium (1mM HCl).
Results and Discussion: Although the ALD coatings deposited at 150 °C were mostly characterised as having amorphous structure, some randomly formed anatase crystallites were observed to protrude through the amorphous film as shown in Figure 1A. Films deposited at 250 °C had anatase crystal structure and were composed of densely packed grains with an average diameter of 31 ± 6 nm was (Figure 1B). The thicknesses of the amorphous and anatase coatings were 26 ± 1 and 22 ± 2 nm, respectively, and these thicknesses were not altered by 28 d exposure to 1mM HCl as measured by ellipsometry. In addition, strong adhesion between the coating and substrate was observed with no cracking or delamination upon fracture of the scaffold struts.
Figure 1: Atomic force micrographs merged from height and amplitude scans (10×10 µm2) and height profiles in cross-section (below) for TiO2 deposited at 150 °C (A) and 250 °C (B) with high resolution scans (300×300 nm2) as 3D illustrations.
After 7 and 28 days immersion into 1 mM HCl, a significant reduction in the compressive strength of the uncoated scaffolds was observed due to dissolution of siliceous GB phase, whereas both ALD coated scaffolds maintained the initial strength (Figure 2). This indicates that GB corrosion in highly porous scaffolds is mitigated under acidic conditions up to an exposure period of 28 days. In contrast to the unchanged strength of amorphous coated scaffolds, a slight decrease for the anatase coated scaffolds to 94 % of the initial strength after 28 days immersion was observed.
Figure 2: Compressive strength of non-coated and ALD coated scaffolds prior and after immersion into 1 mM HCl for 7 and 28 days (n=10).
Conclusions: GB corrosion in highly porous TiO2 scaffolds was successfully diminished by atomic layer deposition of both amorphous and anatase TiO2. The compressive strength of ALD coated scaffolds was maintained in physiologically low pH conditions up to 28 days. Amorphous TiO2 films offered excellent protection against GB corrosion, whereas anatase coated scaffolds with nano-sized grains may allow for improved osteoconduction in vitro and in vivo.
This study was supported by Research Council of Norway grant 228415.; Jonas Wengenroth is acknowledged for his help with the AFM imaging of the surfaces.
References:
[1] T. Hollstein, Corrosion of Advanced Ceramics by Liquid Media: A Review, Adv. Sci. Technol., (1999) A433-A444