In the late 1980's, scientists and engineers at NASA’s Johnson Space Center developed a new type of cell culture system. The system was based on the principle of clinorotation, defined as the nullification of the force of gravity by slow rotation about one or two axes. The clinostat developed at NASA was a single axis device known as the Rotating Wall Vessel (RWV). The original purpose of the RWV was to simulate the effects of microgravity in order to study the effects of weightlessness at a cellular and molecular level in a ground-based system. It was quickly noted that when anchorage-dependent cells were cultured in the RWV, they formed three-dimensional aggregates that resembled in vivo tissue. In subsequent years many investigators also demonstrated that these 3D aggregates were functionally more similar to native tissues than cells cultured in conventional 2D cell culture technology. The basic elements that govern the cell culture environment in the RWV conducive to 3D cell culture are the rotational speed of the vessel and the mass transfer of nutrients in the media.
 
Rotational Speed
As the vessel rotates, the cells and cell aggregates accelerate until they reach a terminal (sedimentation) velocity (Vs), at which the gravitational force is counterbalanced by shear, centrifugal and Coriolis hydrodynamic forces (Figure 1). The size of the cell aggregate is the major determinant of sedimentation velocity; specifically, the sedimentation velocity is proportional to the square of a particle’s radius. Therefore, as cell aggregates grow in size, they sediment more rapidly and it is necessary to increase the rotational speed of the culture vessel for aggregates to remain in suspension. 

Mass Transfer
Nutrient and oxygen delivery to the 3D cell aggregates, and metabolic waste removal from these aggregates, is critically important to their function and survival. In the RWV, these parameters are optimized by the continuous sedimentation of the cells through the media. In contrast to static, 2D cultures, the particles and other molecules are more readily transported through the media, providing more efficient mass transfer. Additionally, oxygen is continuously supplied to the media via a gas permeable silicone membrane.