1

Introduction

Edentulism, with its severe consequences such as lower intakes of nutritious foods, unpleasant appearance and lacks ability to engage in social and interpersonal activities, has a high prevalence in many countries as demonstrate by a survey from the World Health Organization cited by Petersen at al. . The percentage of edentulous people among European adults aged 65 and over was reported to be in 2004 between 13% (for Italy) and 78% (for Bosnia and Herzegovina) .

Despite the estimations of reductions in tooth lost, the age groups where edentulism will still be highly prevalent are getting larger , hence, the need for denture treatment in public health will increase as the population ages .

Nowadays, for elderly edentulous, a conventional complete denture is still the elective treatment for effectively improving functional performances, and aesthetics.

Polymer poly(methyl methacrylate) (PMMA) has long being used in dentistry for complete dentures manufacturing due to its composition and qualities such as: relative ease of processing and repair , low cost and great acceptability by the patients, biocompatibility, tasteless and odorless properties, transparency and also excellent esthetic characteristics .

Regardless its incontestable qualities, PMMA used as denture base material has a great number of issues such as: polymerization shrinkage, low mechanical resistance , great susceptibility to microbial colonization from the highly contaminated oral environment , lack of radio-opacity, allergic reactions mostly due to leaching of the monomer , degradation of the mechanical properties and resistance to wear in aqueous environment, especially in human saliva .

Additionally, microbe adhesion to the denture surface and biofilm formation, due to persistence in the oral cavity can cause local (denture stomatitis) and systemic infections (e.g. aspiration pneumonitis) .

Olsen & Singharo highlights a possible correlation between late-onset Alzheimer’s disease and patients wearing dentures heavily contaminated with Candida albicans which can be a source of systemic mycosis.

Oral hygiene is difficult to manage due to limited dexterity (e.g. rheumatoid arthritis, Parkinson disease) of geriatric patients . Therefore, hygienic denture materials would be of great help in improving oral and systemic health in denture wearers . Moreover, it is well established that conventional antibiotics, in safe doses, does not work in attacking mature biofilms . In such cases associated bacteria are 100–1000 times less susceptible to antibiotics than regular . The use of nanomaterials may be advantageous as efficient antibacterial groups since their active surface area is larger comparing with their size, as result may provide higher activity in a small dose . In the context of the previously mentioned failures of current dentures, the use of nanotechnology to develop new materials/composites became necessary. PMMA is still considered the best material for wide dental application, therefore, a handy solution for obtaining better material would be the improvement of this PMMA polymer by including an appropriate additive .

Antimicrobial nanocomposites for dental use have been extensively investigated lately . Candida Albicans formation and adherence was diminished by adding silver (Ag) − colloidal nanoparticles and Ag nanoparticles solution . Recent studies using silver containing nanomaterials have suggested that the bacterial toxicity of these materials partially originates from membrane damage and disruption of ion homeostasis, with unpredictable effects on human health .

As revealed by several studies, PMMA nanocomposite based on functionalized TiO ₂ nanoparticles demonstrated better both mechanical and antibacterial characteristics and also high physicochemical stability, low cost, together with proved nontoxicity and bio-compatibility . Titanium dioxide nanoparticles have a large spectrum of activity against microorganisms, including Gram-negative and positive-bacteria and fungi. More important, titania-polymer nanocomposites are intrinsically environmentally friendly and exert a non-contact biocidal action . Therefore, TiO ₂ is recommended by itself as the best alternative as polymeric filler when seeking for a material with antimicrobial activity.

In recent years, advances in miniaturized electronic manufacturing and computer technology lead to the developed modeling techniques, also known as rapid prototyping systems (RPS). Stereolithography (3D Printing), a relatively new technology, is a RPS designed to create solid and detailed three-dimensional (3D) physical models that can accurately replicate complex anatomical structures directly from computer data . Combining the scanned information of dental structures and virtual design of final prosthetic reconstructions (CAD − computer aided design) with an Ultraviolet (UV) laser beam sequentially passed over a photosensitive resin, it is possible to produce an accurate 3D dental prosthesis, as a complete replica of the external surface and internal structures in a layer-by-layer fashion .

The main advantages of stereolithography (SLA) are the ability to create complex shapes with internal architecture, ease of removal of un-polymerized resin (ready to be reused), economy of material with cost reduction, and an extremely high feature resolution (∼1.2 μm) . The state of art 3D printing, especially for the production of dental devices, is severely limited by printable materials photo-curable characteristics. Stereolitographic technique could be used for complete denture manufacturing with successful results, but photocatalytic polymers with improved mechanical resistance and antibacterial characteristics need to be developed. From our knowledge, PMMA-TiO ₂ nanocomposite for high resolution 3D printing complex structures was not used and, consequently, was not yet investigated.

Therefore, the aim of this study was to obtain a PMMA-TiO ₂ nanocomposite material with improved qualities, especially antimicrobial ones, suitable for manufacturing 3D printed dental prosthesis.