Although it's now possible to create lab-grown cartilage, there's still at least one big challenge in doing so – cartilage grown in a flat Petri dish may not be optimally-shaped for replacing the body's own natural cartilage parts. Scientists from a consortium of UK universities, however, are developing a possible solution. They're using "ultrasonic tweezers" to grow cartilage in mid-air.

The so-called sonotweezers incorporate multiple tiny beams of ultrasonic waves that focus onto a central chamber from all directions. Their converging, opposing force is sufficient to levitate small objects within the chamber for weeks at a time.

Those objects can include pieces of lab-grown human tissue, such as cartilage. As that tissue is being grown, it's possible to move its cells around by selectively adjusting the intensity of the individual waves. Using a microscope for guidance, the researchers have been able to position cells where required, rotate them, and hold them in place. By doing so, the shape of the tissue can be custom-formed in three dimensions.

Additionally, because the cells aren't resting on a surface, nutrients in a carrier fluid are able to reach them from all sides. This, combined with the stimulating effect produced by the ultrasound, resulted in the sonotweezer-levitated cells growing into better implant-ready tissue than cells cultured in a Petri dish.

Polystyrene spheres are levitated by the sonotweezers (Photo: Bruce Drinkwater, Bristol University)

It is hoped that the technology could be in practical use within five years, and not just in the field of medicine. "Ultrasonic tweezers have all kinds of possible uses in bioscience, nanotechnology and more widely across industry," said Bristol University's Prof. Bruce Drinkwater, the program coordinator. "They offer big advantages over optical tweezers that rely on light waves and also over electromagnetic methods of cell manipulation; for example, they have a complete absence of moving parts and can manipulate not just one or two cells (or other objects) at a time but clusters of several centimeters across."

The research program is being funded by the the Engineering and Physical Sciences Research Council, and includes researchers from the Universities of Bristol, Dundee, Glasgow and Southampton, plus several industrial partners. Along with the ultrasonic tweezers, it has also resulted in the creation of an acoustic tractor beam.

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