THE smooth cartilage that covers the ends of long bones provides a low-friction surface that artificial alternatives have so far been unable to rival 鈥� until now.
A team of researchers in Israel has made a surface covered with 鈥渕olecular brushes鈥� that matches the friction performance of real cartilage. The new material could help to make better, longer-lasting artificial joints.
Every step we take causes the bones in our knee and hip joints to rub against each other. They would quickly wear away if it were not for the protection afforded by the thick layers of smooth, slippery that coats their ends.
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For the same reason, artificial joints must also be covered with a cartilage-like layer. However, while it is possible to match cartilage鈥檚 slick properties at low pressure, the high pressures found in real joints dramatically increase the friction in synthetic alternatives, making them stiff. This extra friction also increases wear, which eventually destroys the joint.
鈥淟ike bone, artificial joints must be covered with a cartilage-like layer that reduces friction鈥�
Now at the Weizmann Institute of Science in Rehovot, Israel, and colleagues have discovered a possible solution in the form of molecular brushes that slide past each other with levels of friction matching those of cartilage, even under pressure.
The brushes are formed from filaments just 60 nanometres long, which consist of a polymer backbone with dozens of small molecular groups sticking out. These are positively charged at one end and negatively charged at the other (see diagram).
In water, each of these molecular groups attracts up to 25 water molecules, enveloping the filament in a slick watery sheath. The sheaths ensure that the brushes are lubricated as they move past each other, even at pressures above those experienced in real bone joints. Cartilage performs well in pressures up to 5 megapascals, a natural threshold that is rarely exceeded in real joints. Klein鈥檚 brushes reduce friction at pressures of up to 7.5 megapascals.
The new material is an 鈥渋mportant step forward鈥� for joint lubrication studies, says , a biomedical engineering specialist at Johns Hopkins University in Baltimore, Maryland.
, an orthopaedic surgeon at Duke University in Durham, North Carolina, agrees. 鈥淚t is very exciting to see that artificial polymer brushes can be designed to provide such low-frictional properties,鈥� he says.
Both commentators wonder how well the brushes will perform in real-world applications. Guilak points out that low-friction Teflon was used in early artificial joints, but that it wore out more rapidly than had been seen in laboratory experiments.
