Tiny ‘nanotrains’ could power big changes in the future

One use of nanotrains could be imaging surfaces. Above, time-lapse photography shows how glowing microtubules can bring out details of a surface. The circle in the center is a raised point.

For Viola Vogel, thinking big naturally comes coupled with the smallest objects imaginable.

According to Vogel, director of the University of Washington’s Center for Nanotechnology, understanding how nature does things at the molecular level and adapting those techniques into the synthetic world could drastically alter just about every aspect of our lives.

Imagine clothing that repairs itself when damaged. Or structures that grow and change to adapt to changes in the environment, in much the same way as living things.

A key area of research leading toward this vision of the future involves building nanoengines and tiny shuttle systems that could be woven into the fabric of objects to transport materials at the molecular level. Vogel and her colleagues have been working on such systems — a monorail at the nanoscale, as it were — with some success.

“One of our big areas is exploring what it takes to bring these attributes from nature into our synthetic world,”Vogel said. “And one of the first steps is learning how to work with nanoscale motors and how to control them.”

Vogel discussed her research today as a panelist in the nanotechnology seminar at the annual meeting of the American Association for the Advancement of Science in Denver.

In delving into the concept of nanoshuttles, Vogel and her team studied two motor proteins used for transport in the body: kinesin and myosin. Kinesin is found in all of the body’s cells and moves along a cell’s cytoskeleton — a network of rigid microtubules that help impart shape and structure to a cell — to carry neurotransmitters to where they are needed. Myosin is found in muscles, where it interacts with another protein, actin, to cause muscle contraction.

Part of the beauty of such motor proteins is their ability to convert chemical energy into mechanical energy with high efficiency, Vogel says. Macroscale motors are less efficient and the engineering concepts that govern them break down as scale diminishes.

“The motor proteins convert chemical energy into linear motion with an efficiency exceeding 50 percent,”she said.

Kinesin and myosin both use adenosine triphosphate, or ATP, as fuel. The proteins hydrolyze the ATP — a process that attaches a molecule of water and knocks off a phosphate, converting the ATP to adenosine diphosphate, or ADP. The energy from the chemical reaction causes a confirmation change in the protein. In other words, the protein changes shape, or bends. So chemical energy is converted to mechanical energy as the movement of the protein propels it forward.

Problems to be addressed in building a molecular shuttle include designing methods to control speed, guiding the movement of the shuttle, and loading and unloading cargo. The prototype shuttle Vogel built used kinesin, partly because it was easier to control the movement.

Kinesin walks along stiff microtubules, whereas myosin travels along relatively flexible actin filaments. “The actin filaments were too flexible,”Vogel explained. “They tended to take wrong turns.”

The shuttle was fabricated using a scenario that inverted the system found in nature. Instead of having kinesin walk along the microtubule, the protein was attached to a surface where its motion propelled a microtubule.

To govern where the shuttle went, Vogel used surface topography. “We have them run into a wall and bend into a wall, inching forward along it,”she said. Other potential methods could use electricity or magnetic fields to control direction.

To regulate speed, the team engineered a way to control the fuel supply.

“We used caged ATP,”Vogel said, or ATP that is bound together in a large group. The group is too big to fit into the binding pocket on kinesin. However, a burst of optical light allows the ATP to slip from the group, fit into the pocket and be used as fuel. So light regulated how much useable ATP was available.

To bind cargo, Vogel said, the team used biotin, a bacterial growth factor present in all living things, and its receptor, streptavidin. Biotin could be linked to the microtubules. It would then attach to any cargo coated with streptavidin.

Whether natural motor proteins will eventually be incorporated into synthetic systems is an open question, according to Viola.

“We’re just starting our exploratory journey into the nanoscale world,”Vogel said. “Our fabricated nanosystems are primitive compared to what nature has to offer. But as we decipher the engineering principles found in biology and use reverse engineering in designing synthetic systems, we can benefit from the billions of years nature has had to develop nanoscale systems. And I’m confident we can eventually go beyond what nature was able to evolve from biological building blocks.”

For more information, contact Vogel at vvogel@u.washington.edu or (206) 543-1776.

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