## 2 Nanomaterials in Endodontics
### Nanohydroxapatite  
Hydroxapatite (HA) in the form of calcium phosphate is a substance that occurs naturally in the human body. It is the main building component of enamel, dentin and bones [37]. Due to its biocompatible properties, it can be used as a drug carrier for bone tissue, e.g., to treat inflammation. Thanks to its structure and properties, hydroxyapatite releases drugs gradually, ensuring long-lasting therapeutic effects. One of the advantages of using hydroxyapatite as a drug carrier is its ability to mimic natural biological processes in the body, which minimizes the risk of adverse reactions and improves the body's tolerance to therapy. Moreover, hydroxyapatite has the ability to interact with bone cells, which can support the healing and regeneration processes of bone tissue [38; 39]. Due to the origin of HA, three types can be distinguished: mineralogical, biological and synthetic [40]. Mineralogical hydroxyapatite are a natural form of hydroxyapatite that occur in living organisms. This type of hydroxyapatite is the main mineral component of tooth enamel, dentin and bones. Biological hydroxyapatite occur in pathologically calcified tissues, forming kidney or tonsil stones [41]. Synthetic hydroxyapatite can be obtained by various chemical reactions, hydrolysis, pyrolysis or the sol-gel method. Obtaining nanohydroxapatite is possible using the same technologies, which, however, require several modifications [42]. In order for a substance synthesized in this way to be effectively used in medical applications, the critical factors--the size and shape of the particles--must be as close as possible to the crystals that build natural bone. Nanoparticles of appropriate sizes and  
Figure 2: The effect of nanoparticles on the impairment of biofilm-forming bacteria (BioRender.com, accessed on 25 July 2024). morphology can easily penetrate cells and influence their metabolic functions [43], and the molar ratio of calcium to phosphorus (Ca/P) in the structure of nanohydroxyapatite is important for its biological properties, including its stability, biological activity and ability to interact with tissues [44]. Nanohydroxyapatite is biocompatible with human tissues, which is why it is used in implantology, conservative dentistry, dental prophylaxis and endodontics. The high bioactivity of nanohydroxyapatite is due to many factors, including its similarity to natural bone apatite (it has a structure and chemical composition similar to natural bone apatite, which makes it well recognized by the body and it can easily integrate with bone tissues) and strong ion exchange affinity (this enables interaction with ions in the surrounding tissue, which is crucial for the processes of the regeneration and remineralization of bone tissue) [45; 46]. It is well tolerated by the human body and does not cause undesirable immunological reactions, it does not create a contact layer of pulp necrosis, there are little or no inflammatory reactions in the pulp after using this preparation, it affects the regeneration of dentin and bone stimulation in the healing process--this means that research is constantly being conducted and new endodontic materials containing nHAp are being created [47]. They are mainly used for biological pulp treatment in direct and indirect covering or as sealers for root canal fillings [48]. Biological methods of treating pulp diseases include direct and indirect pulp capping. The goal of these procedures is to keep some or all of the pulp viable. Living pulp is the best protective barrier against local and general complications resulting from the presence of dead tissue in the body. As a result of the removal of pathogenic stimuli and the use of odontropic preparations, defensive dentin is formed [49]. Currently, many materials are used to cover the pulp, but they do not always meet the expectations of clinicians. Therefore, in recent years, there have been many reports on the possibility of using bonding systems in preparations for direct pulp capping. nHAp is experimentally added to adhesive resins.