Crystal fix

By Ian Sample LIQUID crystals are better known as the stuff of TV and laptop displays but surgeons could one day use them to rebuild shattered bones, say researchers in York. A common fix for severe bone fractures is to insert a hollow metal rod into the bone, skewering the marrow. Metal screws are then used to secure the rod while the fracture heals. But once the patient has recovered, they either face a second major operation to remove the rod, or it stays inside them for the rest of their lives. In some cases, this latter option has left patients with bones that rattle. Now scientists at the British healthcare company Smith & Nephew believe they’ve found an alternative. The company says the rods and screws could be made from biodegradable polymers absorbed by the body once the bone has healed. But while existing biodegradable polymers can be used to make sutures—stitches—they are not yet strong enough to hold a smashed leg together. “We knew we’d need to make much stiffer polymers than the ones around at the moment,” says team leader John Rose. One common trick to make polymers stiffer is to extrude them—force them through a narrow aperture—so that their molecular chains uncoil and are forced to run parallel to one another. This stiffens them in the direction of the chains. With this in mind, Rose started looking at the properties of liquid crystal polymers. When these materials melt, they retain a crystal structure, with strong, elongated rod-like molecules. If a liquid crystal is extruded, its polymer chains line up even more efficiently than those of a standard polymer. “With liquid crystals you can get a much higher degree of alignment and the stiffness goes up dramatically,” Rose says. The scientists developed a form of liquid crystal polyester that contains molecular links that biodegrade very slowly. Above 150 °C, the polymer can be extruded to align the polymer chains. Rose found that the material was nearly twice as stiff as the strongest degradable polymer currently being used in some orthopaedic screws. And from further tests, the scientists estimate that half the polymer’s bonds would break down in the body within six months. This allows ample time for most fractures to heal. Human bone has a Young’s modulus—a measure of its stiffness—of between 10 and 30 gigapascals, while the usual supporting steel rods offer very high stiffness of 220 gigapascals. But this can sometimes be too stiff—causing damage to the bone. However, the new liquid crystal material has a more acceptable stiffness of 20 gigapascals, letting the bone flex more naturally. The research team has now handed the project over to the company’s orthopaedic product development group. Something the researchers will have to confirm, says David Dandy, an orthopaedic surgeon at Addenbrookes Hospital in Cambridge, is that the polymer retains sufficient strength while the fracture heals. “Also, some biodegradable polymers cause inflammation as they degrade,” he adds. “It’s an attractive idea,
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