The International Space Station is the world's most extreme and expensive scientific laboratory. In its more than 20 years of operations it has housed thousands of experiments, providing fascinating insights into the effects of microgravity on the human body, cultured cells or various materials and chemical processes. Here are the most interesting findings that the space station delivered in 2021.

Stem cells are sometimes seen as the holy grail of future medicine. Capable of almost endlessly regenerating and turning themselves into all sorts of cells, stem cells are abundant in young bodies but lose their vigor as we age. There are various types of stem cells. Those found in embryos, also called the pluripotent stem cells, can give rise to all kinds of cells in the human body. But stem cells exist in adult bodies too, ensuring the ability of various organs to repair themselves.

A recent experiment flown on the International Space Station found that in the weightless environment, stem cells from the human heart improved their ability to regenerate, survive and proliferate.

The effects were observed in both adult and neonatal stem cells. The discovery, part of NASA's Cardiac Stem Cells research project, is good news for the future of regenerative medicine as it shows that it is possible to kick adult heart stem cells into better action. That is to increase their 'stemness', their ability to regenerate, proliferate and create new types of cells that a damaged organ might need. Regenerative medicine hopes to one day be able to engineer tissue to repair and replace failing organs and cells. The study was published in the International Journal of Molecular Sciences.

Related: What does space do to the human body? 29 studies investigate the effects of exploration

Microgravity is bad news for bones. The lack of mechanical loading tells the body to stop maintaining these important support structures since they don't seem to be needed. When astronauts return to Earth, they suffer from serious bone loss.

The good news is, that just like on Earth, exercising in space seems to keep the body fit, including the bones. A new study published in the British Journal of Sports Medicine revealed that crew members who increased their resistance training during their space missions were more likely to preserve their bone strength.

The study, part of NASA's Biochem Profile and the Canadian Space Agency's TBone investigations, also found that bone loss in some astronauts could be predicted by the elevation of certain biomarkers before their flight. These biomarkers, found in the astronauts' blood and urine, together with the astronauts' exercise history could help space surgeons identify astronauts at greater risk for bone loss.

Microbes can efficiently extract valuable metals from lunar and martian rocks in space, a recent experiment by the European Space Agency (ESA) revealed. The experiment, called Biorock, used microorganisms to extract the metal element vanadium from basalt, which can be commonly found on the moon and Mars.

The microbes extracted 283% more vanadium while on the space station than on Earth. Biomining is a cheaper and more environmentally friendly alternative to chemical extraction of important materials from ores, a process that usually relies on harsh chemicals and requires a lot of energy. Using biomining in space will surely come handy to future Mars and moon colonists as they will be able to get raw materials for making tools, spacecraft parts and other equipment.

A European instrument called the Atmosphere-Space Interactions Monitor (ASIM) has provided new insights into the genesis of some little understood phenomena in Earth's atmosphere. Used to study severe thunderstorms and their atmospheric effects, ASIM previously helped shed light on the generation of high-energy terrestrial gamma-ray flashes (TGFs), the most energetic natural phenomena on Earth that accompany lightings during thunderstorms.

But more recently, the instrument studied the so-called blue jets, which are essentially upward shooting bursts of lighting generated by disturbances of positively and negatively charged regions in the tops of the clouds. Blue jets, which get their characteristic blue color from nitrogen ions, can shoot up to altitudes of 30 miles (50 kilometers) in less than a second.

Scientists found that the blue jets are generated by "blue bangs," short discharges in the upper layers of storm clouds. The mechanism behind these blue jets appears to be somewhat different from that behind normal lightning that we can observe on the ground.

Astronauts on the International Space Station experimented with making cement in space and found that although it creates somewhat different microstructures than on Earth, it works. The experiment, called Microgravity Investigation of Cement Solidification (MICS), involved mixing cement powders with various additives and different amounts of water.

In the latest round of experiments, a mixture of tricalcium aluminate and gypsum showed interesting results.

In the future, these "made in space" cement blends could be used to build stations on Mars or the moon. Cement is used to make concrete, which has excellent shielding properties against cosmic radiation. It is also strong enough to protect against impacting meteorites.

And to make things easy, future Mars and moon colonists could actually 3D-print structures from concrete made from lunar and martian soils in a 3D printer similar to the Additive Manufacturing Facility that is currently on the space station.

New space station research has shown that the technology used to shield astronauts from dangerous space radiation can be made even more efficient in the future using a mineral called colemanite. This boron-rich mineral is a type of borax that forms as a deposit during evaporation of alkaline water.

An experiment by the Japan Aerospace Exploration Agency (JAXA) exposed several pieces of a polymer material to space conditions outside the International Space Station. The polymer sample treated with colemanite suffered almost no radiation damage and looked nearly indistinguishable from a sample that was not exposed to space. The researchers published their results in the Journal of Applied Polymer Science in July.

In the future, colemanite could be used to treat satellites, space station exteriors or even high altitude planes, NASA said in a statement.

Astronauts and cosmonauts in space frequently suffer from changes to the structure of their veins, especially in their legs. A study by the Russian space agency Roscosmos, however, found that these changes can be somewhat prevented by exercise and can be reversed post-flight if the space travellers have enough time off between missions.

The veins of the 11 cosmonauts that participated in this study, published in the journal Experimental and Theoretical Research, didn't show worse damage after the second flight compared to the first. The spacefarers had breaks of about 4 years between their missions.

Follow Tereza Pultarova on Twitter @TerezaPultarova. Follow us on Twitter @Spacedotcom and on Facebook.

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Here's 7 things the International Space Station taught us in 2021 - Space.com