Micro musclebot: Wee walker moves by heart cells' beats.

In a Los Angeles laboratory, researchers have let loose scores of what amount to living micromachines. Dwarfed by a comma, each tiny device consists of an arch of gold coated along its inner surface with a sheath of cardiac muscle grown from rat cells. With each of the muscle bundles' automatic cycles of contraction and relaxation, the device takes a step.

Viewed under a microscope, "they move very fast," says bioengineer Jianzhong Xi of the University of California, Los Angeles (UCLA). "The first time I saw that, it was kind of scary."

Xi and his UCLA colleagues Jacob J. Schmidt and Carlo D. Montemagno describe their musclebots in the February Nature Materials.

Microcontraptions of this sort may someday make pinpoint deliveries of drugs to cells or shuttle minuscule components during the manufacture of other itsy machines or structures, Xi says. Variations on the same design could lead to muscle-driven power supplies for microdevices or laboratory test beds for studying properties of muscle tissue.

Because the musclebot is both minuscule and designed to operate in body fluids, "this is the Fantastic Voyage kind of thing" that might someday roam the bloodstream and carry out on-the-spot surgery or disease treatments, comments physicist James Castracane of the State University of New York in Albany.

In the past, researchers have incorporated living muscle tissue into much larger machines. For instance, several years ago, a team at the Massachusetts Institute of Technology demonstrated a palm-size device, called the biomechatronic fish, that swam under the power of muscle tissue extracted from frogs' legs.

However, transferring fully developed muscles from an organism to a micromachine is impractical, notes Xi. Instead, the UCLA engineers grew a thin film of heart-muscle tissue directly on their device.

The researchers used chip-industry methods, which would be harmful to living cells, to construct temporary supporting beams on a silicon chip. Next, they deposited a biocompatible polymer, a layer of gold, and--with the chip immersed in a cell-culture medium--muscle cells, which grew into bundles. In the final step, the team dissolved the polymer and snapped away the beams that secured the devices to the chip. The result: many muscle-coated golden arches.

When immersed in a glucose-containing solution, the heart cells beat, causing the device to scoot along. With each muscle contraction, the arch tightens, dragging its back leg forward and planting it on the surface. Then, as the muscle relaxes, the springy arch loosens and the front leg takes a step forward.

At their current stage of development, the musclebots can trundle along in only one direction. More-versatile devices are on the way, Xi claims.

He says that the team is creating a new version using skeletal muscle instead of heart muscle. Whereas heart cells follow an intrinsic rhythm, skeletal muscle can be induced by electricity or chemicals to contract, he explains, and that should provide means for switching the motion on and off.