REPOSITORY > RESULTS

Abstract

One of the most commonly used metal materials for the manufacture of biomedical implants are stainless steel and titanium-based alloys. Due to the specific conditions and complex environment in the human body, different strategies have been employed to protect them and improve their functionality in vivo. In this thesis, a novel method of atomic layer deposition (ALD) was used for the deposition of ultrathin protective barrier oxide films on commercially pure titanium (cp-Ti) and stainless steel 316L (SS316L). The aim was to study the composition, thickness, structure, morphology, corrosion properties, biocompatibility and antibacterials properties of ALD oxide films.
Alumina (Al2O3) and hafnia (HfO2) ALD thin films of different thicknesses were applied on differently prepared surfaces of cp-Ti and SS316L. In addition to single layers, the combination of alumina and hafnia in a multilayer configuration was studied. Electrochemical techniques, potentiodynamic polarization and electrochemical impedance spectroscopy were used as the main methods of testing the corrosion properties of bare and ALD-coated substrates. All measurements were carried out in simulated physiological solution at 37 °C. The composition, structure and morphology of deposited films were studied using SEM/EDS, ToF-SIMS, XPS, AFM-SKPFM, TEM, XRD and ellipsometry.
The method of metal surface preparation significantly affects the protective properties of ALD alumina and hafnia thin films, even when deposited as relatively thick layers of ca. 150 nm. There is a critical deposition thickness that provides barrier corrosion protection of ALD films on cp-Ti and SS316L. Very thin ALD alumina film of 20 nm proved to be unprotective, while 60 nm thin film showed good protection during 30 days of immersion in simulated physiological solution. Multilayer ALD thin film, consisting of hafnia interlayer between alumina layers, showed no significant improvement compared to single-layer alumina thin film of the same thickness.
Both ALD thin films, alumina and hafnia, did not show significantly altered biocompatible nor antibacterial properties compared to bare cp-Ti and SS316L substrates. However, the surface roughness of ALD thin films, primarily the presence of agglomerates, can significantly affect the attachment and development of cells on the surface. On the other hand, the development of bacteria on the surface of ALD thin films is not related to the presence of agglomerate but rather to surface chemical properties.
We concluded that ALD thin films of alumina and hafnia, prepared under optimized conditions of substrate preparation and process parameters, assure excellent corrosion protection of cp-titanium and stainless steel in simulated biomedical applications and are biocompatible.