Cell Physics

Hardware to support living organisms is designed to accommodate the conditions of spaceflight, but microgravity poses special engineering challenges. Plants are usually flown attached to a substrate so that nutrients and water can be provided through the root system. Cultured cells are flown in suspensions of renewable media contained within specialized hardware units. However, fluids behave differently in microgravity. On Earth, fluids are dominated by pressure (i.e., the weight of fluid above) and buoyancy, whereas in microgravity physical properties such as surface tension (Figure 1-09), Marangoni effects (Figure 1-10), and diffusion are dominant. Furthermore, in normal gravity, objects in a fluid sink or float depending on whether they are more or less dense than the fluid. The sinking or rising speed is dependent on the fluid viscosity. In microgravity, density differences do not cause objects to move in a fluid because buoyancy and sedimentation effects do not exist.

Within a single cell on Earth, density-dependent streaming and separation, or at least orientation, occur along the gravitational vector. The cell interior behaves in part like a fluid having only modest viscosity. Therefore active and passive cell deformations can be easily modified or governed by gravity. For cells in suspension, such as in blood where no interstitial meshwork prevents movement, the more dense elements will be pulled "downwards", or sedimented, in the direction of gravity. In fluid-filled compartments, changes in the gravitational force will lead to fluid shift redistribution, varied hydrostatic load across the walls, and passive deformation of the tissues.

Figure 1-09. Astronaut Donald R. Pettit, during his stay on board the ISS, photographed this surface tension demonstration in micro-gravity using food coloring added to the water that is being held in place by a metal loop. Photo courtesy of NASA.

It is important to distinguish between the indirect and the direct effect of microgravity. Among the indirect effects are the absence of sedimentation and the loss of gravity-driven convection. The latter include density-driven phase separation (vinegar goes to bottom and oil goes to top), and thermal convection (heated liquids rise), which are caused by buoyancy. In normal gravity, thermal convection establishes a current that rapidly dissipates heat, renews nutrient supplies, and removes waste materials. This factor is most important when no fluid flow (like blood flow) exists to dissipate metabolic products and exchange nutrients around the cell. Without convection, slow diffusion processes are the only means for heat and nutrient exchange. This factor is likely to be most relevant in plants and single celled microorganisms such as bacteria that have no motile structures like cilia or flagella. Although

Figure 1-09. Astronaut Donald R. Pettit, during his stay on board the ISS, photographed this surface tension demonstration in micro-gravity using food coloring added to the water that is being held in place by a metal loop. Photo courtesy of NASA.

blood flow in animal tissues mostly overcomes this effect, it could still be a factor in cells localized where blood flow is minimal.

Many cell-based systems rely on being anchorage-dependent, or at rest against a surface. By means of a solid-liquid interface they have access to nutrients, metabolize, lay down matrix material, and reject wastes. When the cell's ability to interface with a surface is taken away, such as in microgravity where sedimentation is absent, the role of that particular interaction can be investigated. Interestingly, when cells are prevented from sedimenting against a surface, they make their own surface with which to work.

Cells are also exposed to hydrodynamic shear. Hydrodynamic shear is a force created by fluid moving past a fixed object, objects moving at a faster or slower relative rate, or an object moving in a direction opposing the flow. The red and white blood cells, as well as the endothelial cells that line the blood vessels, are exposed to a fairly violent environment of hydrodynamic shear. In fact, endothelial cells need these hydrodynamic forces to express certain sets of genes that allow them to mature. Hydrodynamic shear facilitates renewal, by removing old cells. We do this every morning when we take a shower. We are shearing cells off of our bodies and sending them back to the recycle system. However, too much shear results in death, or in substantial changes in membrane composition, because cells then produce large amounts of extra-cellular material. The ability of a cell to respond to a specific ligand and transduce a signal to the nucleus may then be substantially affected (Pellis 2005).

On Earth, when maintaining an upright posture, there is a considerable hydrostatic pressure gradient along the body axis. Various cells within the body respond to and rely upon hydrostatic pressure gradients for normal function. For example, bone growth, maintenance, and renewal depends on physical force profiles that include hydrostatic pressure along the body axis. In microgravity, the pressure gradient is redistributed in such a way that it is essentially homogeneous throughout the long body axis. One hypothesis for the bone loss that occurs in microgravity conditions is the absence of stimulation by hydrostatic pressure of the cells responsible for bone formation. By the same principle, it is necessary to put shear back into the process to grow pieces of biological tissues, for example, that have endothelium in them (Pellis 2005).

Cells are also submitted to mechanical forces. Chronic abrasion induces cells to proliferate in those abraded areas. This occurs artificially in cell culture just by the stirring mechanism. A spinner flask used in a lab has a stirring bar suspended in it, which stimulates cell growth. However, the selective role of vibration on cells is largely unknown. Some injuries result from repetitive use, such as tennis elbow. On the other hand, there are also reparative mechanisms that seem to be invoked by certain frequencies of vibration and change in activities within the endothelial cells.

The direct effects of microgravity include no surface attachment, the tendency toward a change to a spherical shape unless previously attached to a surface, and the disorganization of microtubules organizing the skeletal array within the cells. Cells in microgravity have also less of a tendency to apoptose (die), which could potentially lead to them become cancerous and increase the risk for an autoimmune event. Finally, the increase in surface tension forces and Marangoni convection, i.e., the surface tension driven convection induced by a temperature or concentration gradient, presumably tends to favor cell-cell interaction. Indeed, the Marangoni convection is masked by gravity-driven convection on Earth, but becomes the dominant form of convection in microgravity, where it facilitates mixing. Diffusion, i.e., the mixing of liquids by random molecular movement, can also be a dominant force in mixing of microgravity bulk liquids. The pure effects of diffusion are masked by buoyancy convection and sedimentation in Earth's gravity (Pellis 2005).

In summary, microgravity affords a unique environment for cells. It is unlike anything that cells have experienced before. The response of cells to the space environment must undergo a careful analysis to understand its direct and indirect contribution. What sensory device within the cell is triggered for gravity, if there is one? The direct effects of microgravity on cell shape probably reset many of its functions. Inversely, the restoration of mass transfer, shear, and/or vibration could favor cell culture in microgravity, thus offering promising possibilities for space bioprocessing.

Figure 1-10. Marangoni convection can be observed with a bowl shallow filled with silicon oil and graphite and heated from below (left). The reflection of a chessboard, in which the surface of the oil is used as mirror, shows the deformation on the surface. Adapted from Jäger (1996).
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