## 2 Nanomaterials in Endodontics
### Nanohydroxapatite
The available studies indicate good results when covering the pulp with such preparations. No layer of necrosis was observed in the vicinity of the applied nHAp, but a lower degree of pulp irritation was noted compared to other materials. Interestingly, nHAp stimulates the deposition of larger layers of reparative dentin compared to the calcium hydroxide material. It was found that the addition of 10% nHAp to adhesive resins caused a good pulp response, in the form of a large amount of reparative dentin deposited and the shortest healing period compared to the calcium hydroxide material [50]. Other studies have shown that adding nanohydroxyapatite even in an amount of 5% also causes almost the same phenomena, so this concentration of nHAp also seems to be sufficient [51]. [PERSON] et al. studied the usefulness of the newly synthesized nanohydroxyapatite in direct pulp capping in rat teeth and compared it with Calvital (a material based on calcium hydroxide) and formocresol. After using nanohydroxapsapatite, compared to other preparations, there was a significantly lower level of irritation of the tooth pulp after contact with the material, which was probably related to the pH in the place of the applied compound, which remained at a level close to neutral. Calvital with pH = 12 showed a high level of pulp irritation. Moreover, nHAp caused a much earlier resolution of local inflammation and showed no necrotic layer after its application [52]. [PERSON] et al. evaluated the effectiveness of polyphase calcium phosphate (Poly-CaP) in the direct pulp capping of rats. In this study, the researchers used blocks created by annealing raw nanohydroxyapatite that had two different surfaces--one of soluble calcium phosphate and the other containing insoluble nanohydroxyapatite--combined with a bonding resin that was applied after etching that surface. In this way, polyphase calcium phosphate (Poly-CaP) was obtained, which is a derivative of nanohydroxyapatite The sides prepared in this way were placed on the pulp of rat teeth, and after two and four weeks, the formation of a dentine bridge, the degree of pulp inflammation and the bonding of nohydroxyapatite with adhesive resins were observed. Studies have shown slightly better treatment results when using Poly-CaP blocks compared to nHAp. After 4 weeks, more dentin bridges were observed on the Poly-CaP surface than on the pure nHAp surface. Moreover, moderate pulp inflammation was observed with nHAp and only mild pulp inflammation with Poly-CaP. At the same time, in terms of combining preparations with adhesive resins, a favorable phenomenon of a strong combination of nHAp and resins was observed in all tested samples. Ionic bonds were formed between calcium ions from nanohydroxyapatite and the monomers contained in the adhesive resins [53]. It was found that this phenomenon may be important for increasing the strength of dental fillings by creating a strong chemical bond between resins and tooth tissues. When analyzing the subsequent properties of the substance in question, it is worth mentioning research suggesting that nanohydroxyapatite may have a beneficial effect on nerve regeneration [54]. One of the mechanisms that may be involved in the induction of nerve regeneration by nanohydroxyapatite is the increase in mitochondrial activity in neurons. Mitochondria are cellular organelles responsible for the production of energy in the form of ATP and for the regulation of many cellular processes, including metabolic and signaling processes related to the regeneration of nervous tissue [55; 56; 57]. The interaction between the timing of neuroinflammation and the increase in mitochondrial activity may be crucial for stimulating regenerative processes. Nanohydroxyapatite, through its physico-chemical and biological properties, can influence the metabolism of nerve cells, stimulating their activity and accelerating repair processes [58].