Regarding the upper compartment, peripheral turbulent flow along the outer edges was similar to the fluid movements within the area in between the plates. At the top, the turbulent flow directed from the centre to the periphery whereas the turbulences at the bottom were orientated in reverse to that. Moreover, the effect of the shear forces on the osteoblast cells was also influenced by the centrifugal force. This force can be calculated using formula 3, in which ρ = density, h = height, ω = angular velocity and r = radius.

Figure 3 shows the respective physical force and its dependence on a bigger radius and higher rotational speed. The results of this study indicate that the centrifugal force represents only a little proportion of effective forces. Hence, the centrifugal forces’ impacts on the tested cells are considered to be insignificant.

Morphological changes of osteoblast cell clusters and individual cells

Cells were subjected to 24 h of fluid flow rotations at a speed level of 200 rpm. The exposed test cells within the new FSS chamber changed their orientation in accordance with the flow direction. Microscopic evaluation was conducted via a random screening at the peripheral site (2.5-cm radial distance) within a predefined area of interest (0.2 cm × 0.2 cm) through cell counting (at least n = 40 cells/region of interest) and morphological cell characterisation. The FSS-triggered effect was demonstrated as cells realigned themselves towards the flow direction, whereby only cells with an aspect ratio greater than 2:1 were included. To assess the alterations taking place inside the cell body, osteoblast cells were treated with a fluorescence stain to visualise actin fibres. In addition, the cells were split into two groups; the first group (n = 5) remained untreated, without any impact of shear stress (Fig. 4) while the cells in the second group (n = 5) underwent a 24-h rotational impact with 8.35 dyn/cm² shear force. All tests and analysis were repeated at least six times. The cells showed a reproducible (n > 6) realignment of the actin cytoskeleton towards the fluid flow direction (Fig. 5), whereas the actin fibres of the untreated group showed random orientations. Findings were termed a trend if more than 50% of all screened cells (n > 21 cells) underwent reorientation.