Magnetic tweezers move DNA molecules - Johns Hopkins Univ researcher Denis Wirtz develops microscopic tweezers made of oxide beads coated with biotin - Brief Article

USA Today (Society for the Advancement of Education), June, 1997

Denis Wirtz switches on his magnetic tweezers, grabs hold of a single DNA molecule, and uses a joystick to steer it carefully through a solution. Wirtz. assistant professor of chemical engineering, Johns Hopkins University, Baltimore, Md., is not playing an electronic game. He is manipulating molecules in groundbreaking research that could lead to new surgical tools and drug delivery systems. Using the device. a physician soon may magnetically move a microscopic sac filled with cancer-killing medicine through a patient's veins, then empty it directly into diseased cells.

"You could use these magnetic tweezers to transport a vesicle or fluid-filled cell containing a drug," Wirtz explains. You could use the tweezers as sort of a surgical tool to take it to the targeted area and then penetrate the diseased cells. It could be done very easily. Right now, you can move things inside a vein mechanically, but it's very invasive. With magnetic force, it's non-invasive."

The forces that allow the device to work are familiar to any child who has played with a magnet. If you place a sheet of paper over a steel button, then hold the magnet on top of the paper.. you can move the button without touching it. Wirtz's magnets are three sets of coiled copper wire, each connected to a power source. The coils are positioned to produce electromagnetic fields in three dimensions: backward and forward, side to side, and up and down. With a joystick, Wirtz can shift the amount of electricity flowing to each set of coils to let him move a magnetic object in any direction.

A DNA molecule, by itself, is not magnetic. Wirtz overcomes this hurdle with iron oxide beads, each no larger than one-hundredth of a micron in diameter. (A micron is one-thousandth of a millimeter.) He coats these magnetic beads with a protein called biotin. Within a solution, biotin automatically attaches itself to an end of a DNA molecule, giving it a tiny magnetic partner. When he switches on his electromagnets, Wirtz can move the DNA molecules. By coating the beads and the DNA with fluorescent dye, he is able to observe their activity beneath a microscope.

This breakthrough has allowed Wirtz to make critical measurements of how a single DNA molecule moves and changes shape. At rest, DNA is a long, chain-like molecule that coils itself into the shape of a ball. When one end is pulled by a magnet, the molecule stretches into a narrow trumpet-like shape.