New synergic deposition route for the osteointegrative functionalisation of titanium substrates for dental implant applications

Nowadays, roughly 70-80% of dental implants are manufactured from metallic biomaterials. Among them, commercial pure titanium (cp-Ti) and its alloys have been widely used for dental implant applications due to their excellent combination of strength-to-weight ratio, corrosion resistance and biocompatibility [1]. However, due to its limited bioactivity, the Ti surface is generally functionalized in order to enhance, accelerate and direct the bone growth. A significant number of methods for the chemical and/or physical surface modification of Ti implants have been proposed [1,2]. In this field, a typical methodology of surface modification is the coating of the Ti substrates with layers of bioactive calcium phosphate ceramics (CPC) [3]. This approach permits a faster bone-to-implant integration with a strong and stable bond. Moreover, the bioactive layer could minimize the direct metal-body fluid contact, so limiting undesired metallic ions release into the body [4]. Conversely, CPC coatings on metal implants usually suffer from poor adhesion. With the aim of improving the adhesion strength between the Ti substrate and the CPC layer, the insertion of dense and compact ceramic inter-layers is reported to be useful [4]. Among various ceramics, crystalline titania (TiO2) has been extensively used as an inter-layer thanks to its well-known biocompatibility, bioactivity and chemical affinity to the materials already present [4]. In this field, this research activity deals with the development of a new synergic deposition route for the functionalisation of Ti dental implants, through their coating with suitable ceramic materials. In detail, a multi-step process is proposed, with the aim of obtaining a final composite material made up of a dense, compact and crystalline titania inter-layer (obtained via Metal Organic Chemical Vapour Deposition technique, MOCVD) and of a homogeneously spread discontinuous calcium phosphate ceramic top-layer, with particular chemical composition, crystallinity and morphology (obtained by means of spray pyrolysis technique). This composite material should both increase the Ti bioactivity (thanks to the presence of the bioactive layers) and ideally ensure the short and long-term success of the implant. The further goal is obtained thanks to the presence of a homogeneously spread discontinuous calcium phosphate layer that permits both the ceramic materials (CPC and TiO2) to simultaneously contribute to the osteointegrative process as two different materials are simultaneously available on the surface: a bioactive, biocompatible but not resorbable titania inter-layer (for the long term integration) and a more bioactive but resorbable CPC top layer (for the short term anchoring). During all the experiments, in order to provide a deeper investigation on the role of the coatings, three kinds of Ti substrates commonly used in dental implant applications and with different surficial morphology (i.e. Ti machined, sandblasted, and sandblasted/acid etched, all of commercial grade IV) were considered and compared. With respect to uncoated Ti substrates, coated surfaces exhibit better electrochemical performances in artificial saliva, and reduced ions (Ti and Al) release in PBS and lactic acid. The mechanical properties at nano-scale level (nanohardness) of the composite materials were improved with the functionalization. The effect of the functionalization on the wettability was also studied (Figure 1): the coatings make the titanium pristine substrates hydrophilic (even super hydrophilic for the only titania coated surfaces). However, in air environments, the wettability decreases with ageing time due to hydrocarbon contamination. Acellular in-vitro bioactivity of all species was tested studying the ability of the materials to form a bone-like apatite layer on the surface after immersion in Dulbecco Phosphate Buffer Saline solution (DPBS) at 37°C (Figure 2). The obtained results show that a bone-like apatite layer was easily formed on the dual functionalized Ti substrates after incubation for only 30 minutes. Differently, un-coated and titania coated surfaces needed longer soaking time in order to be covered with a homogeneous bone-like apatite layer. Finally, higher apatite nucleation rate was detected in TiO2 coated samples with respect to bare Ti, then confirming the improvement of bioactivity of the only titania coated surfaces with respect to the uncoated ones.