Vascular voyage

Nano platforms deliver drugs, genes and more to individual cells

In the 1960s blockbuster film Fantastic Voyage, miniaturized rescuers, injected along with their submarine into the bloodstream of an intelligence agent, rush to destroy a blood clot in his brain. Surprisingly, that's the sort of problem-solving adventure Tim McKnight and his research team launch every day—without the secret agent and the submarine.

McKnight, a scientist in ORNL's Microelectronic Systems Research Group, heads a team of researchers who are developing tools designed to deliver therapeutic agents to localized regions of tissue. These agents include genes that are introduced into the tissue to elicit certain responses, as well as other genetic material designed to "silence," or suppress, the actions of existing genes. One of the most promising applications of this technology is its ability to treat tissue inside blood vessels before and after various surgical procedures.

Surgeons' efforts to widen narrowed vessels, remove clots, and insert stents to keep vessels open often result in post-surgical inflammation and a buildup of vascular smooth muscle cells—the kind that make up the walls of blood vessels—around the affected area. This buildup can restrict blood flow and may require follow-up surgery.

McKnight notes that stents coated with drugs to minimize the inflammatory response of blood vessels produce mixed results. Problems sometimes arise from the fact that blood vessels are composed of two kinds of cells: a thin lining of endothelial cells and a thick, underlying layer of smooth muscle cells. The latter often reacts to surgery by rapidly reproducing in an effort to isolate the foreign body (in this case, the stent) from the rest of the body. Unfortunately, drugs that prevent this overgrowth of muscle cells can also inhibit the healing of the endothelial layer of cells, which is necessary to fully repair the blood vessel.

Genetic back door
To reduce unwanted cell growth, McKnight and his colleagues are developing genetic techniques that can be applied during surgery. The key to their approach is a very small device that is inserted into a blood vessel the way an angioplasty balloon is inserted into a narrowed artery. One side of the device is covered in vertically aligned carbon nanofibers (VACNF). An array of these closely spaced fibers extends from the device like bristles on a hairbrush. When the array is pressed into the interior surface of the blood vessel, the individual nanofibers deliver genetic material or drugs to a large number of cells in the vessel wall.

The length of the nanofibers determines whether they deliver their payload to the vessel's lining or into the smooth muscle cells below. Genetic-level strategies, including either introducing new genes or silencing the expression of existing genes, can then be used to influence the response of the surrounding tissue. "The purely mechanical nature of this approach provides flexibility with respect to what can be delivered to the cells, and may circumvent some of the existing limitations of viralor chemical-mediated gene delivery methods," McKnight explains. "This strategy can be used to address many medical conditions where highly localized effects are desired, such as dampening the inflammatory response after coronary artery surgery or the removal of blood clots in cases of ischemic stroke. It might also be used to limit the reactive response that tends to occur in tissue surrounding some biomedical implants."

In addition to introducing genetic material into tissues, McKnight and his team have tested methods of "immobilizing" DNA on nanofibers in the array. Under this scenario, the investigators hope to provide a measure of control over genetic-level manipulation. For example, suppose a nanofiber carries a small segment of DNA containing the genetic instructions for producing a particular protein. When the nanofiber is inserted into the nucleus of a cell, the cell is able to produce that protein. However, when the fiber and its attached DNA are removed, the cell loses the ability because it no longer has access to the necessary genetic information.

Neural interface
In addition to blood-vessel-related applications, nanofiber arrays have shown promise for use in implants in the central nervous system, such as deep brain stimulators used to treat Parkinson's disease and other neurological conditions. Working through ORNL's Laboratory Directed Research and Development program, microelectronics researcher Nance Ericson and his team have taken the first steps in this direction with their research into electrically-addressable nanofiber electrode arrays.

"These arrays may have critical advantages over traditional neural implants," McKnight says. "The performance of these central nervous system implants is often compromised by formation of scar tissue. Nanofiber arrays may address this problem by providing genetic-level strategies to reduce the inflammation of the surrounding tissue. This should allow implants to remain effective longer."

It may also be possible to develop implants that vary the amount of stimulus the electrode arrays deliver in response to the reaction of the surrounding tissue. McKnight notes that his team has developed techniques to fabricate nanofiber electrode systems on flexible films. These electrodes can be used either to inject current through the nanofibers to stimulate the neural tissue, or to record the bioelectrical activity of the nerve cells. "We have shown we can use these electrodes to measure the levels of neurotransmitters— the chemicals that enable nerve cells to communicate with one another—in the surrounding tissue," McKnight says. "This may allow us to vary the stimulus required to ensure that the tissue responds at optimal levels."

Rapid response
McKnight and his team foresee a wide range of applications for this suite of nanofiber-based tools. "We can stimulate excitable tissues, such as nerve cells," he says. "We can monitor their response to stimulation, and we have a genetic-level interface which enables us to reprogram localized cells to do things we want them to do—or stop them from doing things we don't want them to do. The VACNF platform is extremely versatile.— Jim Pearce