## 3 Hydroxyapatite Particles in Scaffold Manufacturing and Cell Signaling
### Porous HA-Based Scaffolds  
In 2018, [PERSON] et al. created HA scaffolds with micropores that had microchannels on the surface. The aim of these so-called HA scaffolds with grooved structure was to increase the adhesion of proteins at the surface and to induce bone formation and vascularization. The scaffolds were placed in contact with human placenta-derived mesenchymal stem cells, exhibiting high osteoinductive performance by promoting osteoblastic differentiation and proliferation. Moreover, tests conducted on in vivo models established that four weeks after implantation, a thin calcified bone layer was formed, and the appearance of connective tissues and blood vessels was observed. Nevertheless, various methods have been explored in order to obtain a proper porosity and a suitable surface chemistry, such as the addition of inorganic, organic, or porogen agents into the scaffold structure [43]. Furthermore, [PERSON] et al. stated that a combination between chitosan, gelatin, and HA should induce the osteogenic differentiation due to two major aspects, namely the high osteoconduction and osteoinduction capacities of HA and the structure of chitosan, which is a polysaccharide derived from chitin that closely mimics the structure of human body polysaccharides, leading to an increased biocompatibility, antigenic and antimicrobial properties, and a suitable degradation rate [46]. Moreover, the reason for adding gelatin is based on the ability to form an RGD-like structure (Arginylglyglyaspartic acid) that is very close in structure with the native bone ECM [47]. This, together with the addition of synthetic HA particles should be able to form a biomimetic scaffold. Therefore, the research group created electrospun nanofibers of chitosan and gelatin that were crosslinked with glutaraldehyde and then deposited the HA particles by a wet-chemical method, obtaining HA-chitosan-gelatin porous scaffolds. The scaffolds obtained were tested in contact with MG-63 cells and several parameters were evaluated, such as the biocompatibility and influence on cellular attachment and the proliferation at different time points. The importance of inorganic particles in scaffold composition was clearly remarked as they made a parallel testing on scaffolds with HA particles and the results confirmed the fact that the number of cells on the HA-chitosan-gelatin scaffold was significantly higher when compared to chitosan-gelatin scaffold. Additionally, after 5 days, there was observed a connection between cells, while on chitosan-gelatin the spreading rate was considerably low. Moreover, after 7 days, the cells covered the HA-chitosan-gelatin scaffold. Therefore, it was concluded that the biomimetic scaffolds which present porosity and imitate the bone ECM have promising results due to the existence of inorganic particles in their composition. HA particles were responsible for osteoconductivity, cell ingrowth, and bioactivity, which sustains the idea of guiding the cellular response by using proper materials for scaffold fabrication [46]. The theory of recreating the fibrous structure of ECM was also exploited by [PERSON] et al., who used carbonated-HA, chitosan, and polyvinyl alcohol to create a fibrous scaffold. Carbonated-HA was obtained by co-precipitation and subsequently added into the chitosan/polyvinyl alcohol solution. The resulted mixture was processed by the electrospinning equipment until the final carbonated-HA-chitosan-polyvinyl alcohol nanofibrous composite scaffolds were obtained. As expected, the protein adsorption was increased in the case of carbonated-HA scaffold, as well as its bioactivity after 7 days. Moreover, the inorganic content of the scaffold influenced, in a good manner, the cellular viability of the MC3T3E1 cell line and it enhanced osteoconduction when compared to chitosan-polyvinyl alcohol scaffolds [48]. Chitosan is not the only natural polysaccharide investigated together with HA, bacterial cellulose is gaining a lot of attention as well.