## 2 Nanomaterials in Endodontics
### Nanohydroxapatite
Currently, many studies are focusing on the potential applications of nanohydroxyapatite in the regeneration of nervous tissue due to its properties that stimulate cell growth and its ability to interact with nerve cells. There are studies suggesting that nanohydroxyapatite can also be used to improve the bioactive properties of polymeric materials and strengthen their structure. The addition of nanohydroxyapatite to polymeric materials can affect their physical, chemical and biological properties, leading to improved usefulness in various applications. The increased abrasion resistance, mechanical strength and thermal stability of these polymeric materials have been proven, which makes them more durable and resistant to operating conditions [58]. In addition, it can increase their ability to stimulate bone tissue regeneration and integration with biological tissues [59]. Another example of the interesting properties of nanohydroxyapatite is stimulating the expression of genes related to the process of bone tissue regeneration. [PERSON] et al. conducted research using a combination of chitosan (polysaccharide obtained from chitin) with nanohydroxyapatite, Runx2 subunits (a transcription factor regulating osteoblast differentiation), ALP (alkaline phosphatase), Smad1 (a transcription factor activated by BMP), BMP-2/4 (bone morphogenetic protein), collagen I and integrin, creating scaffolds that imitated the bone tissue microenvironment and provided appropriate signals to cells to stimulate their differentiation and proliferation towards osteoblasts and the production of new bone tissue and the production and mineralization of bone matrix [60; 61; 62]. Although hydroxyapatite has many advantages, its poor mechanical properties may limit its applications in load-bearing applications. To improve the mechanical properties of nanophase hydroxyapatite and increase its durability, various modification methods are used, including doping with metal ions [63; 64; 65; 66]. The addition of elements such as magnesium, strontium or zinc can significantly affect the physical and chemical properties of hydroxyapatite. It was confirmed that doping with metal ions can lead to a reduction in the biodegradation rate, which is important for long-term applications, and chemical modification through surface functionalization can be used to improve its mechanical properties, stability and functionality in various biomedical applications [67; 68]. Scientific research has shown that adding nanohydroxyapatite to the sealing materials used to fill root canals can provide benefits by improving their properties and effectiveness. [PERSON] et al. tested the cytotoxicity of biomaterials based on calcium phosphates: laboratory-synthesized nanohydroxyapatite and two commercially available ones--Apatite Root Sealer Type I and II (Sankin Industry, Osaka, Japan) [69]. A good response and biological tolerance of cells was demonstrated in samples with pure materials--CHA and Apatite Root Sealer Type I--while Apatite Root Sealer Type II, probably due to iodoform content in its composition, showed increased cytotoxicity. nHA-based materials, in addition to biocompatibility, also have sealing properties comparable to other materials available on the market. Nanohydroxyapatite mixed with epoxy resin showed a higher tightness than preparations based on zinc oxide and calcium hydroxide [70]. Due to the very promising research results, a new sealing material, BioAggregate, containing hydroxyapatite [71] deserves special attention. An important feature of this preparation is the very high tightness of the fillings and much better biocompatibility compared to other available materials. Moreover, research conducted by [PERSON] et al. on the use of chitosan-hydroxyapatite (CS-HA) nanocomposites in endodontics suggests that they may be an effective method of modifying the dentin substrate before filling the root canal [72].