Human studies

Sensory perception: Knowing which way is ‘up’

Prolonged exposure to microgravity has a long-term effect on the perception of upright. On earth we use visual, body, and gravity cues to help us determine the orientation of ourselves relative to the world which affects many perceptual tasks including reading, recognizing faces, and navigating. Laurence R. Harris and colleagues at York University assessed how seven astronauts who spent 168 days on average on the International Space Station perceived their orientation before, during and after flight. Although no changes were observed during their missions, astronauts’ judgements in the absence of visual cues were worse upon return to earth compared with ground-based controls. Harris and his team found that the effect persisted for up to four months after the astronauts returned to earth. These findings could help develop countermeasures to avoid perceptual mistakes during space travel, and contribute to facilitating safer, long-duration journeys without gravity. 

Vision: Microgravity may alter optic nerve and surrounding sheath

Changes to the optic nerve and surrounding sheath during microgravity could explain why space flight is harmful to an astronaut’s vision. Darius Gerlach from the German Aerospace Center in Cologne and colleagues studied the tissue architecture of the optic nerve and its surrounding sheath in nine healthy men who experienced head-down tilt, a commonly used ground-based model of weightlessness. Using a neuroimaging technique called diffusion tensor imaging, the researchers documented fluid dynamic changes wrought by the microgravity-like conditions that could be due to alterations in the volume and movement of cerebrospinal fluid within and around the optic nerve. The findings may help explain why many astronauts experience poorer vision after long-duration space flights, although more work is needed to explore the effects of true microgravity on the visual system. 

Microbiology: gravity-mimicking suit safe for skin bacteria

A compression garment that applies gravity-like pressure to the skin alters the composition of skin microbes, but not in a dangerous manner. A team led by Peter Taylor from University College London, UK, characterised the bacterial skin communities at dry and moist body sites of five Earth-bound volunteers before and after wearing the Mk VI SkinSuit, which creates a pressure loading system that simulates gravity’s effects. 8 h in the SkinSuit changed the skin microbiota at the genus level but had little to no impact in community structure. The researchers observed more dramatic changes in one astronaut who wore the garment on the International Space Station. However, the microbial makeup reverted back to pre-flight profiles upon the astronaut’s return to Earth. The findings suggest that short-term SkinSuit wear is unlikely to compromise bacterial skin health.

Chemistry, Physics, and Engineering

Cooling microelectronics with nanofibers

As microelectronics get smaller, there is an urgent need to develop efficient methods to keep them cool without extra power input. Under normal gravity, excess heat can be removed by vapor bubbles rising through a coolant. In space however, due to the lack buoyancy force, vapor bubbles remain attached to the submerged heater and prevent heat removal. Prof. Alexander Yarin, at the University of Illinois at Chicago, and his team show that in heaters mimicking high-power microelectronics, the thrust of vapor bubble release (the vapor recoil force, which exists irrespective of gravity) helps shedding merger vapor bubbles by generating a swing-like motion of the heater. Moreover, they demonstrate how nanofiber coatings can increase heat transfer by providing more bubble nucleation sites, and thus enhance the swing-like motion.

Microgravity simulators: Bio-reporters illuminate false signals

Earth-based laboratories can now assess the accuracy of tools used to simulate living organism growth and behaviour in space with bioluminescent assays. Researchers often use rotating machines to minimize gravity effects during the design of extra-terrestrial experiments with plants, cells, and small animals. Jens Hauslage from the DLR German Aerospace Center and colleagues report that device-specific shear forces produced during mechanical movements may cause misinterpretations of initial test data. They developed a biosensor based on marine plankton, known as dinoflagellates, which have cell membranes that naturally emit light when touched by predators. Calibrating this bioluminescence against mechanical stress helped determine the top-like, 2D rotations of ‘‘clinostat’’ devices provided microgravity-like conditions. However, the unexpected 3D movements of Random Positioning Machines generated enough shear force to impact studies of cell signaling pathways or metabolic reactions.

Microgravity simulators: improving oil field assessments

To support oil and gas exploration, researchers sent hydrocarbon mixtures into space to obtain accurate data on how each component behaves. The group—led by Guillaume Galliero from the University of Pau and Pays de l’Adour, France—wanted to study the effect of temperature on the movement of individual hydrocarbons in mixtures under typical reservoir conditions. Eliminating the effects of gravity allowed them to collect more accurate data than has previously been obtained. The team showed that thermodiffusion has a large impact on the distribution of hydrocarbon reservoirs under the ground. They state that thermodiffusion should therefore be considered in computer models that assess analytical data collected at potential underground reservoirs. This would allow oil and gas companies to more accurately predict the suitability of the hydrocarbons at potential drilling sites. 

Microscopy: All shapes and sizes

A way to measure the motion of tiny particles of different sizes on the International Space Station is developed by researchers in the USA. Jacinta Conrad, Peter Vekilov, and their colleagues from the University of Houston demonstrate that this can be achieved using a technique called differential dynamic microscopy. Colloidal systems can contain particles of a wide variety of sizes, all moving at different rates. Conrad and the team demonstrate that differential dynamic microscopy has the ability to resolve the motion of polystyrene particles of two different sizes suspended together in water. The equipment required for their experiment is readily available on the International Space Station, and so the approach could help to better understand how potentially a microgravity environment affects complex dynamics in a variety of systems. 

Fluids: Microgravity releases the tension

Gravity is shown to influence the shape of a small drop of water on a surface in a theoretical model developed by a researcher in Germany. Alfredo Calvimontes from BSH Hausgeräte GmbH demonstrates the importance of the droplet geometry in determining the interfacial energies. Surface tension strongly influences a water drop’s shape, but it was thought that gravity played little role for very small drops. However, recent experimental work has suggested that this might not be true. Calvimontes develops a theoretical model that differs from the conventional approach of the two hundred- years-old Young’s equation in that it assumes a thermodynamic equilibrium of the interfaces, rather than a balance of forces, on the solid-liquid-gas contour line. The model was supported by high-speed-camera images of droplets on various surfaces in free fall using a three-meter drop tower that allows quantifying the change of shape from normal gravity to microgravity.

Granular matter: Coming to rest in low gravity

Microgravity provides a better environment to study how grains move when agitated, scientists in Germany show. Philip Born and co-workers from the Institute of Materials Physics in Space demonstrate that microgravity analogue conditions enable studies on particle dynamics at packing densities not achievable on the ground. Dense granular media under external agitation tends to partially arrest on the ground due to particle collision and settling caused by gravity. This diverseness of the medium makes it hard to develop a theory that can describe the general behavior. Born and colleagues use diffusing-wave spectroscopy to compare the spatial homogeneity and microscopic dynamics of granular media on the ground and in drop towers. They show that unlike on the ground, granular media in microgravity conditions can reach a homogeneous state without partial arrest at high packing densities.

Microbiology

Cavities-causing bacteria altered by space-like conditions

The gene expression patterns, metabolism and physiology of tooth cavities-causing microbes change in a space-like gravity environment. These findings could help explain why astronauts are at a greater risk for dental diseases when in space. Kelly Rice and colleagues from the University of Florida, Gainesville, USA, cultured Streptococcus mutans bacteria under simulated microgravity and normal gravity conditions. The bacteria grown in microgravity were more susceptible to killing with hydrogen peroxide, tended to aggregate in more compact cellular structures, showed changes in their metabolite profile and expressed around 250 genes at levels that were either much higher or lower than normal gravity control cultures. These genes included many involved in carbohydrate metabolism, protein production and stress responses. The observed changes collectively suggest that space flight and microgravity could alter the cavities-causing potential of S. mutans.

Modeling intestinal infection with NASA biotechnology: A new 3-D intestinal co-culture model with macrophages to study enteric infection

Using spaceflight analog bioreactor technology, Cheryl Nickerson at Arizona State University and collaborators developed and validated a new three-dimensional (3-D) intestinal co-culture model containing multiple differentiated epithelial cell types and phagocytic macrophages with antibacterial function to study infection by multiple pathovars of Salmonella. This study is the first to show that these pathovars (known to possess different host adaptations, antibiotic resistance profiles and disease phenotypes),display markedly different colonization and intracellular co-localization patterns using this physiologically relevant new 3-D intestinal co-culture model. This advanced model, that integrates a key immune cell type important for Salmonella infection, offers a powerful new tool in understanding enteric pathogenesis and may lead to unexpected pathogenesis mechanisms and therapeutic targets that have been previously unobserved or unappreciated using other intestinal cell culture models.

Virus reactivation in long-duration spaceflight

Long-duration spaceflight increases the reactivation of latent herpes viruses in astronauts and is accompanied by a rise in stress hormone levels. This study shows that the frequency and viral loads of reactivation of Epstein-Barr virus, varicella-zoster virus, and cytomegalovirus were even greater in blood, urine, and saliva samples from astronauts staying 60 to 180 days onboard the International Space Station than has previously been observed for short-duration (10–16 days) missions. Changes in viral reactivation were also found to be associated with changes in the daily trajectory of salivary cortisol during these long-duration missions. These results indicate that the effects of the microgravity environment on the immune system are increased with prolonged exposure and highlight the potential increased risk of infection among crewmembers. 

Evolution: Bacteria gain advantageous mutations under simulated microgravity

Bacteria grown for an extended period of time under simulated microgravity adopt growth advantages. George Fox and colleagues from the University of Houston, Texas, USA, cultured Escherichia coli bacteria for 1000 generations in a high aspect rotating vessel to simulate the low fluid shear microgravity environment encountered during spaceflight. They then performed growth competition assays and found that the 1000-generation adapted bacteria outcompeted control bacteria grown without simulated microgravity. Genomic sequencing of the adapted bacteria revealed 16 mutations, five of which altered protein sequences. These DNA changes likely explain the growth advantage of the bacteria grown for multiple generations in simulated microgravity. Similar adaptations during prolonged space missions could result in nastier pathogens that might threaten the health of astronauts. Fortunately, the microbes did not appear to acquire antibiotic resistance over the 1000 generation in the modeled microgravity culture.

Cell Biology

Countering Bone Loss in Space

A chemical element naturally found for instance in seafood or grains, could counter bone loss from long-term spaceflight. Alain Guignandon and colleagues from the Université de Lyon à St-Etienne in France exposed multipotent embryonic fibroblasts to microgravity conditions similar to those found in space. They found the balance shifted in these stem cells from differentiating to bone-forming cells (osteoblasts) to differentiating to fatty-tissue forming cells (adipocytes). When the cells were treated with strontium, the shift toward osteoblastogenesis was regained. Strontium achieves this by sustaining the activity of two proteins that play a role in bone development but are suppressed in space. Strontium’s effect on the proteins could happen via release of vascular endothelial growth factor, which, under normal gravity conditions, plays a role in committing the cell to differentiation into osteoblasts rather than adipoyctes. 

Gene expression: potentially low-risk for long-term space missions

Approximately 99% of genes in human lymphocytic cells have the same transcription activity on Earth as they do in microgravity environments. An international team led by Oliver Ullrich and Cora Thiel at the University of Zurich in Switzerland used a combination of parabolic aircraft and suborbital rocket flights and ground-based experiments to show how gene expression is stable through a range of gravity conditions. Screening of microarrays identified several reference genes that may serve as standardized controls for future immune cell trials. Their experiments also revealed that the chromosomal region associated with olfactory and taste receptor genes is particularly robust to altered gravity. Because altered transcripts were associated with fast cellular adaptation, the researchers predict that the risk of unusual gene behavior will be quite low during extended space deployments.

Cartilage: Simulated microgravity promotes cartilage matrix deposition

Co-culture of meniscal cartilage-forming cells with fat-derived stem cells can lead to enhanced cartilage matrix production when cultured under simulated microgravity. Adetola Adesida from the University of Alberta in Edmonton, Canada, and colleagues cultured two types of cells found together in the knee—cartilage-forming chondrocyte cells (taken from the meniscus) and mesenchymal stem cells (isolated from the infrapatellar fat pad)—in a rotary cell culture system designed to model weightlessness on Earth. Simulated microgravity enhanced the synergistic interaction between the two types of cells in culture, resulting in more matrix production, but it also prompted the cartilage-forming cells to differentiate towards bone-forming cells, as evidenced by gene expression analysis. These findings suggest that microgravity and simulated microgravity-based culture technologies could help bioengineers grow knee replacements for people with meniscus tears, but increased bone-directed differentiation could pose a possible problem for astronauts on prolonged missions.