## 4 Discussion  
The development of a biocompatible material for direct additive manufacturing of maxillofacial extraoral prosthesis is still a challenging task. Some attempts were made: [PERSON] et al. developed direct printing materials and technique by customizing a fused deposition modelling printer by replacing the heated nozzle and print bed, and making it suitable for printing two-component customized room temperature vulcanized silicones [26, 27]. [PERSON] et al. presented a complete digital workflow for the manufacture of a nasal prosthesis from a pure solvent-free silicone (ACEO Silicone General Purpose; Wacker Chemie AG, Germany) with a drop-on-demand 3D printer (ACEO; Wacker Chemie AG, Burghausen, Germany) [12]. However, none of the tested materials had the approval for clinical application at the time of the studies. The aim of the present research was to obtain a photocurable PDMS with improved characteristics to be used for soft maxillofacial prosthesis manufacturing using Vat photopolymerization (stereolithography) as 3D printing technique. To the author's knowledge, this is the first attempt for improving a medical silicon by adding TiO\({}_{2}\) nanoparticles. Also, among the proposed 3D printable silicone for extraoral prostheses manufacturing, few studies focused on the new material's biocompatibility. Lately, many applications of photosensitive materials polymerizing under UV radiation have emerged in the areas where 3D printing procedure became important. Commonly, the photosensitive polymers suppose a mixture of monomers or oligomers and a suitable photoinitiator that triggers a radicalic or cationic reticulation upon UV-exposure (Figure 3). The short cure time at room temperature and minimum energy requirements could be mentioned among the UV-curing advantages. The UV-curing process depends on the photoinitiator's behavior and characteristics. Thus, the amount of photosensitive compound conditions the crosslinking rate, while its efficiency and absorption wavelength influence the initiation rate. The free radicals, eventually cations, are generated in a system where exists a photoinitiator compound, following the conversion of the absorbed photons' energy to chemical energy [28]. The successful application of this procedure supposes a sustained rationale behind the right choice of the necessary cosolvents, design of the nanocomposite matrix that could result in a homogeneous polymeric phase, and increased compatibility. For assessing genotoxicity, the sensitivity and specificity of in vitro tests should be a priority to increase predictability and to avoid other unnecessary in vivo tests. In vitro micronucle testing is a complex method because it can be performed on target cells, even in others than those recommended under standard conditions, including human cells [29]. Three type of compounds were comparatively assessed: PDMS, PDMF-BF, and PDMS-BF-TiO\({}_{2}\) and compared with a negative control sample. All three concentrations of PDMS-BF-TiO\({}_{2}\) favor the assessment of the genotoxic capacity. When referring to PDMS-BF-TiO\({}_{2}\) and negative control, low DNA damage was found, with no statistically significant micronuclei incidence in PDMS-BF-TiO\({}_{2}\) compared to the negative control (Table 2), probably due to TiO\({}_{2}\) nanoparticles stabilization. It is well known that metallic nanoparticles have cytotoxic/genotoxic potential, but in this particular case, the combination of the silicon composite and the TiO\({}_{2}\) nanoparticles resulted in a more stable compound, less reactive to the genetic material. Toxicological information on the engineered compounds especially those commonly used in medical practice, is very important in terms of risk assessment and management, as these are either already in use or may be applied in the future in numerous other fields of interest. In this context, the development of new nanomaterials could reduce the risk of interaction with genetic material, thus preventing toxic and allergic effects.