Shear forces and proper centrol

To draw any conclusions from microgravity, experiments it is important to have a proper control. To define the this we have to understand the artifacts involved in such studies. Based on Einstein's equivalent principal, a Ixg control can either be a sample that remains on Earth or that is put into an on-board centrifuge. It was with facilities like Biorack that one could use an on-board Ixg control. One of the never addressed differences between ground lg and in-flight lg is the shear force generated in on-board centrifuges while this shear force might be one of the main artifacts in spaceflight and on-ground studies involving centrifuges. Shear forces are generated in two ways. Static shear and fluid shear. Fluid shear is best known from the circulatory system where the endothelium is exposed blood flows. Static shear forces are generated in materials exposed to e.g. accelerations. The latter being the main issue of this numerical study. Although differently shear forces have, in vitro, an impact on both attached and free-floating cells. They may even have a significant effect in animal centrifuge studies. We calculated that for some experiment set-ups in the past as well as for some future ISS facilities the level of shear force in on-board centrifuges could be as much as 95% of the total force. Some of the differences reported between ground Ixg and in-flight lxg centrifuge could have been caused by this phenomenon. The artifact should be dealt with for future missions and hardware designs as well as for the interpretation ofprevious data.