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Thursday, August 28, 2008

A Helping Hand for Surgery

Get a grip: The fingers of this metal-and-polymer gripping gadget curl around a tiny bead when the device senses a certain triggering chemical. The gripper could someday help doctors perform minimally invasive surgeries.
Credit: Timothy Leong/JHU

A tiny handlike gripper that can grasp tissue or cell samples could make it easier for doctors to perform minimally invasive surgery, such as biopsies. The tiny device curls its "fingers" around an object when triggered chemically, and it can be moved around remotely with a magnet.

Minimally invasive, or "keyhole," surgery currently involves making several centimeter-size incisions and inserting surgical tools through hollow tubes placed in these incisions. Wires connect the tools to external controls that a surgeon uses to operate inside the body. This is less damaging than conventional surgery, but it limits a surgeon's ability to maneuver the instruments.

The new technology is a step toward surgical tools that move more freely inside the human body. "We want to make mobile surgical tools," says David Gracias, a biomolecular- and chemical-engineering professor at Johns Hopkins University, who led the development of the new gripper. "The ultimate goal is to have a machine that you can swallow, or [to] inject small structures that move and can do things [on their own]."

A gripper based on the current design could respond autonomously to chemical cues in the body. For example, it might react to the biochemicals released by infected tissue by closing around the tissue, so that pieces can be removed for analysis.

Gracias and his colleagues presented the microgripper at the American Chemical Society meeting earlier this month. To demonstrate the device, they used it to grasp and maneuver tiny beads and clumps of cells in a petri dish. They have also used the device in the laboratory to perform an in vitro biopsy on a cow's bladder. "This is the first micromachine that has been shown convincingly to do very useful things," Gracias says. "And it does not require electric power for operation."

he open gripper is 500 micrometers (0.05 centimeters) in diameter, and it is made of a film of copper and chromium covered with polymer. As long as the polymer stays rigid, the gripper remains open. But introducing a chemical trigger or lowering the temperature causes the polymer to soften, actuating the gripper's fingers so that they curl inward to form a ball that is 190 micrometers wide. Another chemical signal can be used to reopen the gripper. All of the chemicals used as triggers in experiments are harmless to the body.

Since the new technology does not need to be connected to controls outside the body, it could mean more dexterous microsurgery, says Chang-Jin Kim, a mechanical-engineering professor at the University of California, Los Angeles. "You don't have to have a physical connection, and that is pretty attractive," he says.

Microgrippers could also be important for lab-on-a-chip applications--for example, moving samples around a chip or cleaning away debris. But Kim says that using chemical triggers from the environment makes the Johns Hopkins device tricky to control. "If the environment changes, your performance changes," he adds.

Kim and his colleagues previously developed a four-fingered "microhand" that opens and closes when gas pressure is changed inside tiny polymer balloons at the finger joints. The microhand offers more precise control but must be tethered to a control unit. Nonetheless, Kim says that his device could have a wider range of uses--as a tool for remotely removing detonators from explosives, for example.

The new technology, meanwhile, is designed exclusively for surgery. Gracias hopes to shrink the gripper further--to about 10 micrometers wide--and to enable it to move in response to different chemical concentrations, like a bacteria moving toward higher concentrations of sugars.