# Antibacterial Ceramics for Dental Applications
## 3 Results and Discussion
### Antibacterial Properties of Zinc Oxide  
Zinc is the second-most abundant trace element after iron and plays an important role in various physiological functions [80; 81]. Thirty percent of the zinc accumulated in the  
Figure 9: Possible antibacterial mechanisms on the surface of NAS/Cu ceramic dental restorative materials. Released Cu ions accumulated on the membranes, affecting membrane permeability, disrupting respiratory chain activity, and interfering with gene replication in _S. mutans_[35]. Reprinted with permission from Elsevier. body is found in the bone tissue [81; 82]. Zinc plays a key role in inhibiting bone resorption as well as in bone formation, development, mineralization, and maintenance. Zinc can be used to prevent bone diseases caused by oxidative stress, such as osteoporosis [81; 82; 83]. Zinc oxide nanoparticles (ZnO-NPs) are one of the most commonly used inorganic materials with antibacterial activity. Due to their high application safety, they are used in disinfectants, dental materials, cosmetics, and pharmaceuticals [82]. They have been shown to exhibit selective toxicity against both Gram-positive and Gram-negative bacteria, while the effect on human cells is negligible [24; 84; 82]. A controlled release rate of Zn\({}^{+}\) from ceramics plays a crucial role in antibacterial activity. Zn is vital for the survival and development of bacteria. Within an expected concentration range, bacteria can regulate their internal Zn concentrations for their physiological metabolism. However, excess zinc ions lead to some toxic effects, while zinc deficiency leads to the disruption of normal biological activities [84]. This means that the concentration of zinc ions in the cell medium is important for the antibacterial activity of zinc ions. Zn\({}^{2+}\) ions interact with the negative charges of the bacteria membrane and cause physical damage to the cell membrane, leading to cell death [24; 70]. The antimicrobial effect of ZnO-NPs depends on the particle size and concentration. Smaller particles are more active, but the effect may vary from microorganism to microorganism [70; 82]. It has been confirmed that a sufficient number of zinc ions stimulates bone cells and induces a series of in vivo responses, including adhesion, spreading, proliferation, osteogenic processes, differentiation, osteogenesis, and mineralization [21; 85]. [PERSON] et al. [21] used ZnO-NPs to impart antibacterial activity to the barium titanate)/hydroxyapatite (BT/HA) piezoelectric ceramic system. BT/HA piezoceramics were used because natural bone also exhibits piezoelectric behavior and generates an induced potential under mechanical loading [21]. The concentration of ZnO-NPs used in this study was 1%. At a concentration below 1%, ZnO-NPs exhibit no pronounced antibacterial activity, while at a concentration of 2%, a detrimental effect on cell survival was observed. The composition of ZnO@BT/HA ceramic was 1% ZnO-NPs, 70% BT, and 30% HA. [PERSON] et al. used _S. aureus_ as Gram-positive bacteria and _E. coli_ as Gram-negative bacteria to evaluate the antibacterial activity of these ceramics. As shown in Figure 10A, ZnO@BT/HA exhibited high antibacterial activity against _S. aureus_ and _E. coli_[21]. The counting of viable and dead bacteria data (Figure 10B,C) quantitatively confirmed the antibacterial activity of ZnO-NPs incorporated into BT/HA ceramics. The authors used a CCK-8 assay to evaluate the toxicity profile of ZnO-NPs toward DPSCs (dental pulp stem cells). A significant decline in the cell survival rate was observed at concentrations exceeding 1 wt%, whereas concentrations below 1 wt% showed no detrimental effects on DPSC viability. The authors investigated the bioactivity of these ceramics. ZnO-NP-doped BT/HA ceramic samples exhibited significantly enhanced proliferation capabilities compared to those on HA and BT/HA surfaces. An optimal concentration of ZnO-NPs also promoted MSC (mesenchymal stem cell) osteogenic differentiation through various pathways, which is a powerful therapeutic tool for bone defects.