## 2 Nanomaterials in Endodontics
### Nanocompounds Based on Chitosan  
One of the most frequently chosen substances that complement the properties of bioglass is chitosan--an organic chemical compound from the group of polysaccharides, which is obtained by the deacetylation of chitin in an alkaline environment. Chitosan is a homopolyaccharide, which means it consists of one type of sugar monomer--glucosamine. [PERSON] et al. emphasized that chitosan contains deacetylated and acylated glucosamine units in its structure--D-glucosamine and N-acetyl-D-glucosamine [117]. Its source is the exoskeletons of arthropods, molluscs and insects. Due to its properties, such as biocompatibility, biodegradability and the ability to create matrices, chitosan is widely used as a carrier of biologically active substances, in tissue regeneration and in protection against infections [118]. The solubility of chitosan in an acidic environment plays a key role in its antibacterial effect. When chitosan is exposed to acids, its molecules become protonated, which creates positive charges on the amino groups. These positively charged amino groups are able to interact with cations formed by the protonation of C-2 amino groups [119]. As a result, these positively charged amino groups can disorganize the structures of microbial cell membranes by interacting with the negative charges of these membranes. This, in turn, may lead to damage to the cell membranes, which may result in the death of microorganisms [120]. The antibacterial effect of chitosan, especially in an acidic environment, may contribute to reducing the number of _E. faecalis_ bacteria in root canals, which is crucial for effective endodontic treatment and the prevention of recurrent infections [121; 122]. [PERSON] et al., as part of their research, created a cellular fibrin hydrogel scaffold that allowed odontoblast cells to develop and organize in the dentin pulp. The addition of chitosan to this scaffold contributed to providing additional antibacterial effectiveness, which may be particularly important in the case of infections within the pulp. It is also important that the addition of chitosan did not negatively affect the morphology and viability of the collagen matrix, which is crucial for the proper development of pulp tissue. Therefore, the cellular fibrin hydrogel with chitosan may be a promising tool in the regeneration of tooth tissues and in the fight against infections, contributing to the effective treatment and protection of the dentin pulp [123]. [PERSON] et al. also obtained interesting research results. They developed a chitosan hydrogel that delivered organic amelogenin to enamel defects. Amelogenin is a protein that plays a key role in the enamel mineralization process, and its presence in damagedareas can support natural repair mechanisms, while chitosan, as a biodegradable and biocompatible material, is an excellent scaffold for the transport and release of amelogenin in the place of enamel defects [124]. The goal of endodontic therapy focuses on eliminating the microbiological load in the root canal system. The intracanal medications and root canal rinsing agents used are designed to effectively combat bacteria, fungi and other microorganisms present in the root canal. Research on them often involves testing their effectiveness in reducing single-species and multi-species biofilms, which are the main cause of infection in tooth periapical tissues. This makes it possible to select the best and most effective agents that can be used in clinical practice to improve the results of endodontic therapy [125]. Research on the antibiofilm effect of chitosan nanoparticles against multi-species infections involving bacteria such as _S. mutans_, fungi such as _C. albicans_ and _Enterococcus faecalis_ bacteria was conducted, among others, by [PERSON] et al. and [PERSON] et al.