Why our body runs like clockwork

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Microscope image on which individual atoms are visible.
Microscope image on which individual atoms are visible.
The friction in our joints is extremely low - how is that physically possible at all? Measurements at TU Wien provide explanations and ideas for new treatment methods.

Friction and wear as an eternal nuisance - this is as familiar in technology as it is in medicine. Whether it’s a manual transmission or a knee joint, you always want moving parts to slide over each other with as little friction as possible, so that energy expenditure and wear are kept to a minimum.

Nature solves this problem in an admirably effective way: The friction in a healthy joint is orders of magnitude smaller than in moving parts of a machine. How nature manages this is difficult to explain: It requires an understanding of the complicated interplay of different molecules at the boundary layers. An important discovery has now been made at the Vienna University of Technology, with support from Canada and China. The decisive key to joints that move almost frictionlessly probably lies in ions that are dissolved in water. This also raises hopes for the targeted development of improved treatment methods for joint diseases.

A liquid layer for low friction

Prof. Markus Valtiner from the Institute of Applied Physics at the Vienna University of Technology is a specialist in interfacial physics - with his research team, he studies effects that take place at the boundary between two different aggregate states, for example between a solid and a liquid.

It is precisely such interface effects that are crucial for the functioning of our joints: -If a bone were to rub directly against a bone, or a cartilage against a cartilage, the friction would be very high and the joint would quickly break,- says Markus Valtiner. -It is important that there is a fluid layer in between that ensures the lowest possible friction.

However, water as a lubricant alone is not enough. The decisive factor is that the water molecules also remain permanently in place. -The question is how the body manages to keep such a liquid film stable even under load - this is called superlubricity," says Matteo Olgiati, who is currently working on his dissertation in Markus Valtiner’s team. This can only be answered by analyzing the surface on an atomic scale. Olgiati therefore created microscope images with atomic resolution to be able to explain the surface processes.

Positive charge helps retain water

-The exact mechanisms have been hotly debated for years, but there was already a suspicion that positively charged cations could play a crucial role in this process-, says Markus Valtiner. Biological tissue itself is often negatively charged at its surface. Positively charged particles are attracted and fixed by it. And these positively charged particles, in turn, are then excellently suited to hold water molecules - because water molecules have a positively and a negatively charged side, and therefore attach themselves strongly to locally bound ions.

The TU Vienna research group collaborated on the project with Xavier Banquy from the University of Montral, who also conducted experiments at TU Vienna during a guest stay, and with Jianbin Luo from Tsinghua University in Beijing, who developed computer simulations for it. Together, they tested the hypothesis of water-fixing positive ions by using triple positively charged lanthanum atoms.

In fact, it was possible to see in the microscope images that the positively charged lanthanum atoms settled on the substrate and then accumulated water molecules all around them. Exactly where there were particularly many lanthanum particles, the water film all around was also most pronounced.

In a sense, this creates a mountain-and-valley landscape on a molecular scale of liquid water. If this water film is smooth and uniform, then the friction is minimal. If it is irregular, the friction is somewhat greater - this is shown by the measurements as well as the computer simulations.

In biological joints, of course, no lanthanum atoms are responsible for this effect - but the same mechanism is likely to occur there through so-called lubricin molecules. -This is a molecule that also has positively charged sites at its two ends," explains Valtiner. These two ends can cling to the tissue, while the middle of the molecule forms a kind of loop in which the water is held by molecular interaction. When stress is applied, this water can then be released.

Movement is good for the joint

-When you analyze these molecules, you also find out: it is important to squeeze this loop in order to maintain the water film-, says Markus Valtiner. -This also explains why it is important to move regularly, especially in the case of joint problems: In immobile joints, friction increases again over time.-.

The physical principles that have now been discovered will now be studied in more detail in order to be able to use them to develop future treatment methods for joint problems. -We are very confident that the targeted use of molecular charge effects in interaction with water accumulation can play an important role for therapeutic measures-, says Markus Valtiner.

Original publication

T. Han et al, Hydration Layer Structure Modulates Superlubrication by Trivalent La3+ Electrolytes, Science Advances (2023), DOI: 10.1126/sciadv.adf3902.