Improvement of dental implants properties by surface dual-step functionalisation

The dental implant related technologies are under constant implementation for improving the device performance. About 70-80% of dental implants are made of metallic biomaterials and commercial pure titanium (cp-Ti) is widely used for dental applications thanks to the optimum combination of strength-to-weight ratio, corrosion resistance and biocompatibility [1]. Nevertheless, the reduced titanium bioactivity suggests the device surface functionalisation to enhance, accelerate and direct the bone growth. In this way, several methods have been proposed but only a restricted number of them have been actually scaled-up to industrial level [1]. The deposition of calcium phosphate ceramics layers (CPCs) on the implant surface is a typical modification methodology [2]. The CPC presence permits a faster bone-to-implant integration with a strong and stable bond. On the other hand, the CPC layer usually suffers of poor adhesion on Titanium substrates. Moreover, CPCs are reabsorbed by dissolution in body fluids and osteoclasts action. The biological removal of the CPCs layer, obviously, involves the loosening of the implant stability and can lead to its failure. Literature suggests that CPCs adhesion can be improved by the insertion of a dense and compact ceramic inter-layer [3], but the CPCs biological reabsorption phenomena cannot be avoided. The developed research deals with an original synergic deposition route for the functionalisation of titanium dental implants by a multi-step process. The final composite device is designed with a bioactive, dense, compact and crystalline TiO2 inter-layer, obtained by Metal Organic Chemical Vapour Deposition (MOCVD) and an homogeneously spread discontinuous calcium phosphate ceramic top-layer, with particular chemical composition, crystallinity and morphology, obtained by means of spray pyrolysis. In detail, the CPC deposition step leads to the presence of hydroxyapatite (HA) islands on the TiO2 compact layer. The unique architecture of the device is planned for improving different characteristics that, joined together, define the overall implant performance. In detail, the TiO2 layer is conceived to act as a protective and functional coating. Indeed, it reduces the titanium ions release into body fluids, improves the tribo-corrosion, mechanical and wettability properties. The HA spots on the surface, on the other hand, are imagined for the improvement of the early stage of the osseointegration process. Beside the above mentioned properties, the planned architecture takes its advantage in the bioactive synergic role of both TiO2 and HA. The novelty, indeed, lies in their simultaneous exposure to the body fluids and then their contribution to the healing process: a bioactive, biocompatible and not resorbable TiO2 inter-layer is available for the long-term integration, whereas a more bioactive and resorbable HA top-layer permits the short-term anchoring. In the here proposed composite device, new bone simultaneously starts to anchor on HA (quickly) and on TiO2 (slowly) and when the HA is completely reabsorbed the implant is still steadily bound. During all the experiments, in order to provide a deeper investigation on the role of the coatings, three kinds of titanium substrates commonly used in dental implant applications and with different surficial morphology (i.e. titanium machined, sandblasted, and sandblasted/acid etched, all of commercial grade IV) were considered and compared. Coated surfaces exhibit better electrochemical performances in artificial saliva, and reduced ions release in lactic acid. The mechanical properties at nano-scale level of the composite materials were improved with the functionalization. The coatings make the composite hydrophilic or even super hydrophilic. 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