## 4 Discussion
Future works will need to investigate in more depth the origin of the secreted proteins and the exact role that human MSCs play in this process. The positive interactions of biomaterials and cells are crucial for successful bone regeneration. Research have shown that the surface characteristics of calcium phosphates, like the chemical composition, molecular weight, and surface properties, including the hydrophilicity and topography, can positively affect stem cell functions such as the adhesion, proliferation and differentiation [36; 37]. The power deposited in the CAP used for the treatment of the \(\beta\)-TCP samples was kept at the level of 1 W, which means that most likely there was no erosion of the treated material. The CAPs have an important characteristic in that they operate at room temperature while having a rich gas-phase chemistry due to the electrons with much higher energy than the neutral gas, ions, and other species. This leads to the production of reactive oxygen and nitrogen species (RONS): superoxide anion (O\({}_{2}^{-}\)), hydroxyl radicals (OH), oxygen atoms (O), hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)), ozone (O\({}_{3}\)), nitrite (NO\({}_{2}\)), nitric oxide (NO), nitrate (NO\({}_{3}\)), peroxonitrile (ONOO-), etc. [38; 39; 40]. Broadly speaking, the production rates and amounts of RONS are connected to the deposited power in the plasma, to the presence of surrounding gas that is diffused in the plasma working gas, the geometry of the discharge and the type of target. In our case, the RONS created in the plasma gas phase are in contact with the surface of the \(\beta\)-TCP samples during the treatments, and their mutual interaction leads to changes in the material surface chemistry. One can expect that the wettability is increased as well as the surface energy of the plasma treated samples of \(\beta\)-TCP [24; 41]. This can lead to better adhesion of the cells, with the result of better bone regeneration. The influence of CAP treatment on materials is limited to the surface and surface chemistry. During plasma treatment, C-C and C-H bonds are usually broken and carbon content decreases with the increase in oxygen content. [PERSON] et al. confirmed the increase in the oxygen and calcium percentages and decrease in the carbon presence in the surface chemistry of \(\beta\)-TCP scaffolds when treated by argon CAP [24]. [PERSON] et al. have demonstrated that the protein adsorption and murine osteoblasts adhesion are increased by argon plasma surface modification of hydroxyapatite (HA) and calcium phosphate ceramics [35]. [PERSON] et al. also showed that following the treatment of HA/\(\beta\)-TCP with nitrogen and air CAP, the attachment and proliferation of MC3T3 mouse osteoblasts were increased [15]. This can be achieved by increasing the hydrophilicity of the scaffold surface. It has been reported that when a surface is treated with CAP, the carbon levels decrease, whereas the levels of oxygen functional groups increase, resulting in a more polar and hydrophilic surface [13; 14; 15]. The hydrophilicity of a surface is essential for the adsorption of ECM proteins and hence the adhesion of cells via cell-ECM interactions [42]. In addition, it has been reported that hydrophilic surfaces contribute to the adhesion and proliferation of osteoblasts while simultaneously promoting the maturation and differentiation of bone precursor cells, which leads to the creation of a pro-osteogenic microenvironment [43]. On the other hand, macrophage activity is also affected by the hydrophilicity and microporosity of surfaces [44; 45]. The increased surface energy of hydrophilic surfaces enhances the release of anti-inflammatory markers by macrophages and modulates their polarization to anti-inflammatory phenotypes [46; 47]. It has also been shown that the surface topography influences the gene expression and cytokine secretion profile of MSCs and enhances their osteogenic differentiation capacity [48; 49].