It is the first time muscle tissue has been used to propel a micromachine. This distinctly futuristic development could lead to muscle-based nerve stimulators that would allow paralysed people to breathe without the help of a ventilator. And NASA- which is funding the research- hopes swarms of crawling "musclebots" could one day help maintain spacecraft by plugging holes made by micrometeorites.

Whatever the ultimate applications of the technology, no one was more surprised to see the tiny musclebots finally move than Carlos Montemagno, the microengineer whose team is developing them at the University of California, Los Angeles.

He has spent three disappointing years trying, and failing, to harness living muscle tissue to propel a micromachine. But when he and his team looked into their microscopes, they were amazed to see the latest version of their musclebot crawling around.

The device is an arch of silicon 5o micrometres wide. Attached to the underside of the arch, the team has grown a cord of heart muscle fibres (see Graphic). It is the contraction and relaxation of this cardiac tissue that makes the arch bend and stretch to produce the bot's crawling motion. And the muscle is fuelled by a simple glucose nutrient in a Petri dish.

The prospect of using living muscle to power microelectromechanical systems (MEMS) is an attractive alternative to micromotors. While motors need electricity, muscles can draw their energy from glucose- perhaps deposited on the surface where the robot will be working. The UCLA team's breakthrough is to have developed an automated way of anchoring muscle tissue to a substance like silicon.

The team carved an arch-shaped skeleton from a wafer of silicon using automated microchip manufacturing equipment, and coated it with an etchable polymer. They then etched away the coating on the underside of the arch and deposited a gold film there.

This acts as an adherent for the muscle cells. To grow the muscle, the skeleton was placed in a Petri dish containing rat cardiac muscle cells in a glucose culture medium. Over three days, the muscle cells grew into muscle fibres that attached themselves to the gold underside, forming a cable of cardiac muscle running the length of the arch.

During this process, the arch was held in place by a restraining beam. When this was removed the musclebot immediately started crawling at speeds up to 40 micrometres per second. The geometry of the musclebot ensures that its flexing pushes it in one direction, rather than simply contracting and relaxing on the spot.

Montemagno now wants to use the technology to help people who have damaged phrenic nerves. These stimulate the diaphragm to make us breathe and damage means patients often need ventilators instead.

Rather than moving the legs of a musclebot, the muscle fibres would flex a piece of piezoelectric material and generate a few millivolts to stimulate the phrenic nerve. Using cells from the patient's own heart would prevent rejection of the implant, and the muscle could be powered by blood glucose.

Montemagno's initial brief from NASA's Institute for Advanced Concepts was to design a muscle-powered micromachine that could seek and repair micrometeorite punctures on spacecraft.

However, he stresses that such applications are several decades away. "The issue of all of the microbots talking to one another hasn't even been addressed," he stresses. Or, indeed, how they would be fuelled. Watch out for the sugar-coated space station.

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