applied sciences  
Article  
For extraoral prosthesis, the material used needs to mimic soft-tissue characteristics in terms of visual and tactile properties, being simultaneously physically and chemically stable, as well as having microbiological resistance and biocompatibility [3].  
Polydimethysiloxane (PDMS) is presently the first option as material for soft maxill prosthesis production due to its versatility, soft-tissue-like properties, ease of manipulation, chemical inertness, durability, and good biocompatibility. Moreover, PDMS presents a few other desirable characteristics such as hydrophobicity, thermal stability, low reactivity, resistance to ozone, and UV radiation due to the particular polymeric backbone with Si-O-Si bonds [4]. Its structural features transform PDMS into a structure with a highly flexible backbone (Figure 1), formed by the large Si atoms and oxygen atoms with smaller size connected through a 1.65 A bound. The PDMS backbone structure is also characterized by two different bond angles: for Si-O-Si--143\({}^{\circ}\) and O-Si-O--110\({}^{\circ}\)[5]. Consequently, the dynamic flexibility of the PDMS chains increase significantly, offering pliability and softness similar to human skin.  
As maxillofacial prosthesis are not worn while sleeping, removal and attachment must be straightforward [2], therefore mechanical properties, such as tensile strength, tear strength, and elongation at break are important, mainly around the thin edges of the prosthetic where adhesive need to be used for bonding with the skin [3]. Regarding this aspect, PDMS has some significant disadvantages as the low mechanical properties [6]. These could be overcome by inserting reinforcing additives, such as silica [7]. Silica particles with a modified surface could interact with Si-O from PDMS, generating an in situ reinforced polymeric matrix with significantly improved mechanical properties.  
Prostheses are often used by patients for many hours each day, so tissue compatibility issues must be considered when selecting a material. Also, the humidity and temperature at the skin-prosthetic interface is a perfect environment for the proliferation of opportunistic bacteria and fungi, leading to a biofilm formation, not only on the epithesis surface but it could also penetrate in the material structure [2; 8]. For good biocompatibility and for its antibacterial properties [9], titanium oxide nanoparticles were extensively studied, as the generated structures and compounds are stable and with photochemical properties [10; 11]. However, to our knowledge, improving PDMS structure with nanotitania was not yet addressed.  
For significant improvements in the capabilities of maxillofacial prostheses, mainly with respect to their esthetics, attachment, function, cost and resistance [2] not only the materials need to be enhanced but also the manufacturing technique. A review of the recent developments in maxillofacial prosthesis manufacturing [1] highlighted the progress registered in patients data acquisition and design software for a simplified and predictable digitalized protocol. Manufacturing prosthesis based on digital designs were carried out directly, by printing the prosthesis itself and indirectly by printing prosthesis prototypes or molds [1]. Most of the epithesis (extraoral maxillofacial prosthesis) were manufactured indirectly using regular silicone in a 3D printed mold, and for the ones obtained  
Figure 1: Polydimethysiloxane (PDMS) backbone.  
directly, the 3D-printed silicones with suitable prosthetic properties are currently under development [12; 13].  
Therefore, the aim of the present study was to obtain a photocurable silicone, with nano TiO\({}_{2}\) inclusions, for directly 3D printing of extraoral, soft, maxillofacial prosthesis. The biocompatibility of the newly obtained nanocomposite was also investigated.