# Response of Osteoblasts to Electric Field Line Patterns Emerging from Molecule Stripe Landscapes
## 3 Results and Discussion
### Cell Response to Molecular Landscapes
Of the three cells chosen on each molecule species sample (COOH-dendrimer, PSS, and PDADMA), none of them were located on the molecule stripe. Figure 5: MG-63 cells 24 h after seeding onto a BSA \(\mu\)CP glass surface. The scalebars in the bottom right corner indicate 100 \(\mu\)m (**a**) Brightfield image of a representative spot at the transition from glass- to BSA-surface. A homogeneous cell distribution is present. (**b**) Corresponding auto-fluorescence image. The bright area (bottom half) shows the presence of a BSA-layer. A total of 36 cells are adhered on glass while 43 reside on the BSA-layer. Considering the area of the molecule layer vs. glass (\(0.74\) mm\({}^{2}\) and \(0.65\) mm\({}^{2}\), respectively) the cell density is similar. This shows that there is no glass preference in terms of cell adhesion when unstructured BSA-layers are offered. Thus, the orientation of the electric field vector at the glass-molecule interface (COOH and PSS) either might not be the dominating factor that controls the adhesion preference or the lowering of the \(\zeta\) potential was not successful. Since the \(\zeta\) potentials of glass and PSS are very similar, reversal of the electric field vector at the transition lines might not have taken place. In general, the error of \(\zeta\) potentials with molecule coating is \(\approx\)\(\pm\)10 mV. It is much more difficult to increase the negativity of already negative surfaces compared to the charging of negative surfaces towards the positive direction. This is due to the electron spill-out, i.e., the fact that electrons are less localized in atoms than the positive nuclei. Three cells for each molecule-type were chosen and their topography as well as location were checked via SICM. The cell topography at the poles featured membrane protrusions, so called ruffles (see Figure 7). Strikingly, the amount of ruffles is higher than at the rim of cells which did not adopt the spindle shape [49]. Since ruffles act among other functions as membrane storage, the spindle-shaped cells apparently provide additional membrane at the poles. This shows that the apical membrane morphology indicates the spreading direction (along the poles). In all cases, the cell alignment was along the stripe-patterns and the cells chose to adhere at the glass site. Figure 6: Depicted is the cross section of an NH\({}_{2}\)-terminated dendrimer printed glass sample where MG-63 osteoblastic cells are adhered. The difference in \(\zeta\) potential between the molecules and glass (illustrated in green) lead to an alternating lateral electric field (red) at the glass–molecule interface. We put a double-sided cathodic taxis ansatz forward to explain the apparent cell preference on glass. Figure 7: Fixed MG-63 cell after 24 h adhesion time on BSA \(\upmu\)CP glass in PBS. (**a**) Brightfield image of a spindle-shaped cell oriented parallel to the molecule stripes. The blue square at the cell pole shows the area of investigation by SICM. (**b**) SICM-topography image of the cell pole. Bright colors mark high spots. Extensive membrane ruffling is visible. The ruffles (peripheral) at the edges are several \(\upmu\)m higher compared to the surrounding cell membrane, indicating that the cell either folded back its membrane or that the cell attempted to spread further and, therefore, provided membrane at the pole. If we were to assume cathodic taxis of the osteoblasts, the movement could be explained by the push-pull model [50] or the directional sensing model [51]. Both suggest the Ca\({}^{2+}\)-flow being essential for the mechanism. A transcellular Ca\({}^{2+}\) current may pass from the rear to the leading edge, resulting in a Ca\({}^{2+}\) concentration gradient along the cell. The low Ca\({}^{2+}\) concentration towards the anode leads to depolymerization of actin while an increased Ca\({}^{2+}\) level at the cathodic side leads to a promotion of polymerization. The concentration gradient also comes along with actomyosin exertion and thus to a migration towards the cathode [50].