## 4 Discussion  
Most clinical implant failures occur during the early healing phase [58]. Therefore, the development of new bioactive coatings for dental and orthopedic implants, for example, is fundamental to promote biological interfacial responses. In order to produce suitable coatings, it is important to know how the cells interact with the surface properties. Our previous research revealed that osteoblasts are sensitive to the surface charge of a biomaterial and exhibit a specific physiological response, involving individual, phenotypical traits of viability, intracellular calcium ion signaling, and morphology depending on the materials' charge level [33; 34]. However, more detailed investigations on the formation of the cell shape in response to material surface charges are hardly found in the literature. Therefore, in this study, our focus was to elucidate the role of the surface charge on the osteoblastic actin cytoskeleton organization and cell shape. We summarized our previous findings concerning the cell area and circularity during the spreading course of 24 h and could show an overall relation of material surface charge and morphological cell changes. In particular, osteoblasts that were grown on highly positively charged surfaces displayed hampered cell spreading and a maintenance of the round cell shape, which is accompanied by functional alterations as shown by [PERSON] et al. [34]. Cellular spreading is a complex process involving various biological components such as the integrin adhesion receptors and the actin cytoskeleton, which are connected via adapter proteins in functional units called focal adhesions. In cultured cells, beside actin stress fibers throughout the cell, the actin cytoskeleton is presented in lamellipodia, the growth zone of the cell, and filopodia, which protrude from the lamellipocial actin network serving as anchor points for actin stress fibers spanning between two cell-matrix contacts [59; 60; 61; 62]. Through its connection with the focal adhesion complexes of the cell, the actin cytoskeleton mediates the transduction of extracellular signals and mechanical forces into the cell [63; 64]. The multitude of functions renders the actin filament network a highly significant and central key component in cell shape, anchorage, and motility [59; 65]. In consequence, we assumed a rearrangement of the actin cytoskeleton to be the cause of the priorly observed individual phenotypic changes dependent on the \(\zeta\)-potential [33; 34]. As expected, we could find a well-spread cell morphology on negatively charged Ti surfaces, which is dominated by prominent actin stress fibers after 24 h. The more filigree appearance of the actin cytoskeleton with long filopodia on moderately positively charged PPAAm after 1 h can be explained by a higher level of cellular activity [34; 66]. However, after 24 h, cells on PPAAm are also well expanded and display stress fibers. Figure 7: SICM topography images (10 \(\times\) 10 \(\upmu\)m\({}^{2}\)) of fixed MG-63 osteoblast peripheries after 1 h adhesion on glass (**left**) and plasma polymer PPAAm (**right**). The color scale represents the height information. Note that the cells exhibit more and higher peripheral ruffles (bright, leaf-like structures on the cell rim) when cultured on glass compared with flat cell edges on PPAAm. There was no difference in actin filament number between Ti and PPAAm at this time point, as was also indicated by [PERSON] et al. [67] and [PERSON] et al. [45]. In contrast, highly positive surface charges on PDADMA resulted in a drastically diminished filament number and length after 24 h. The remaining high cell circularity and decrease in cell area was accompanied by a clustered actin formation still confined to the outer cell edge.