A new chapter

The institute's scientists succeeded in creating a new lubrication system, which reduces friction to levels similar to those that exist in natural systems

In the technological competition that takes place between man and nature, nature enjoys a significant seniority advantage - 3.5 billion years of experience, during which its capabilities in the fields of design, planning and execution have been highly refined. This is how sophisticated natural systems were created that maximize their potential, and allow the animals to function very efficiently. One of the examples of an effective technological system, made by nature, is the shielding system that is activated at the points of friction in the human body - the joints. Nature has succeeded in developing methods and materials that reduce friction to extremely low levels, even in joints that operate under high pressures - such as the hips and knees. The low friction in these joints is equivalent, for illustration, to applying a force of one kilogram - a force easily applied with the finger - to displace a weight of a ton.

But, unfortunately, and despite careful planning, natural systems do not work forever. Years of strenuous use take a heavy toll on the joints. The pin starts to get damaged, and as a result the cartilage wears away and arthritis is caused. The constant increase in life expectancy exacerbates the wear and tear damage, so that more than half of the population is expected to suffer from damage to the joints. Over the years, scientists have succeeded in developing various artificial cushioning systems, however

Even the most efficient systems did not come close to the low levels of friction - which withstand high pressures - that characterize the joints in the human body. The difficulty in developing suitable cushioning systems was one of the main factors that limited the use of artificial hip and knee joints. Research carried out by Weizmann Institute of Science scientists may help develop an advanced way to overcome this difficulty.

A team of scientists, led by Prof. Jacob Klein from the Department of Materials and Surfaces at the Institute, succeeded in creating a new shielding system, which reduces friction to levels similar to those that exist in natural systems. The findings of the study, in which (then) research student Meng Chen and scientists from England participated, were recently published in the scientific journal Science. The structure of the molecules of the new shielding system is similar to a brush: the "bristles" of the brush are made of polymer - that is, long molecular chains consisting of a sequence of "beads" of smaller molecules, and connected on one side to the surface. When two such brushes move relative to each other, the bristles of the brushes come into contact with each other (instead of the surfaces of the surface), thus reducing the friction between the two bodies.

The uniqueness of this shielding system stems from the type of molecules chosen as well as the way the scientists grew the bristles of the brush. the scientists
Use surfaces with a very smooth surface, coated with a special coating - which is responsible for the beginning of the creation of the polymer. In this way, the bristles of the brush emerged straight from the surface, when they were firmly attached to it. The researchers then extended the bristles that were created, by adding additional molecular units and tying them together with strong chemical bonds (similar to how you extend a chain by threading beads on a string). The uniqueness of the molecules that make up the polymer is in their "thirst" for water: although these are electrically neutral molecules, different areas of them are charged with a relative, positive and negative charge. Thanks to this, they attract water molecules - which are also charged in the same way. The water molecules surround the polymer units and bind to them tightly. The resulting structure, of polymer chains wrapped in water molecules, acts like a tiny ball bearing.

"Water-related molecules - such as these - provide an extremely high level of lubrication, and the tightly held chains are very resistant to friction and high pressures. This way they do not wear out," says Prof. Klein. "The new combination we created, between various beneficial properties, allowed us to reduce the friction levels, even at higher pressures than anything previously tried in artificial lubrication systems. The friction values ​​we obtained in the artificial system we created are similar to those obtained in natural joints." Part of the success of the research is attributed to the scientists' use of highly sensitive devices for measuring friction - which were designed and built for the first time by Prof. Klein's group at the Weizmann Institute of Science.

The advanced equipment made it possible to identify friction shielding mechanisms that were previously unknown, which helped in the design of the advanced lubrication system. Polymeric brushes, and in particular those coated with water molecules, work in a completely different way from the way traditional lubrication systems - based on the use of oils - work. This new approach may lead to the development of methods for the production of artificial joints for transplantation as well as the development of medical systems that are currently limited by friction and wear. This research may also offer new insights and reveal nature's professional secrets regarding the modes of action of the molecular mechanisms that underlie the natural lubricating ability of the cartilage in the joints.

On the same topic on the science site

• Surfing on the water