## 1 Introduction  
In modern medicine and, in particular, in modern dentistry, prevention has become fundamental in every private practice. Professional hygiene programs are the basis for teaching adequate at-home oral hygiene, caries interception and caries prevention. Educational programs in schools provide adequate means for children and parents to learn how to take care of their teeth. Nonetheless, primary and secondary caries are of course still present and need to be treated and prevented as much as possible. In this day and age, primary caries treatment involves using materials which guarantee excellent results both mechanically and esthetically over time: composite resins have achieved excellent esthetic and mechanical qualities thanks to the addition of glass nanoparticles, which provide adequate light and force transmission over the whole restoration. Indirect resin restorations, in particular, have been proven to be less prone to biofilm adherence compared to direct resin restorations [1]. The durability of composite resins used for indirect restorations has been observed even with long-term follow-ups [2]. Since these materials are commonly used in lieu of others (e.g., zirconia, metal-ceramic) especially for conservative treatments such as composite onlays for cuspal protection, their performance over time is not only influenced by their mechanical properties, but also by their ability to prevent infiltration and secondary caries, which has been and still is one of the main reasons for the failure of direct restorations, and the most common reason for the replacement of restorations [3]. The prevention of secondary caries, then, needs to be a priority. In order to prevent secondary caries, researchers have focused their work towards incorporating new additives to composite resin, in order to create materials which also have the ability to release substances able to inhibit bacterial activity, or prevent soft tissue inflammation (i.e., bioactive composites) [4]. A material is defined as \"bioactive\" when it has the ability to interact or provoke a response from living tissue. In particular, in dentistry, a bioactive material can either cause remineralization of surrounding tissue, deposition of hydroxyapatite or aid pulp regeneration. Biocative materials acting on remineralization concentrate their action during the multiple cycles of demineralization and remineralization of enamel during the day, especially at the interface between restorations and natural tooth tissue and between biofilm and tooth tissue: the phosphate ion, calcium and fluoride help rebuilding a new, stronger surface on the recently demineralized enamel. It is important to note that natural hydroxyapatite has a critical pH of 5.5, while the new fluorapatite created thanks to these materials has a critical pH of 4.5. Some bioactive materials are capable of actively depositing hydroxyapatite when immersed in body fluids such as saliva: these are calcium aluminates and calcium silicates, which are alkaline materials used for cementation: these materials, when contacting saliva, stimulate hydroxyapatite formation and deposition thanks to their alkaline activity and basically occlude marginal gaps between natural tooth and indirect restoration. Other bioactive materials have the capability of regenerating live tissue, and are widely used in cases of pulp exposure: some examples are calcium hydroxide products, mineral trioxide aggregate (MTA), calcium silicates (Biodentin) and bioceramics. All these materials have the application of nanomaterials technology for their production and their function in common. These are just some of many applications of nanomaterials in dentistry: the small scale of these materials may be able to create tougher, stronger materials or to make them more biocompatible for surrounding tissue and bone, in case of implant fixtures and abutments [5].