Double duty: Magnetic nanotechnology holds promise in = fighting cancer, advancing computing

Detecting cancer and reinventing computing are two challenges = that=20 seemingly have little, if anything, to do with each other. That = is, unless=20 you are a nanotechnologist like Shan Wang, an associate professor = of=20 materials science and engineering and of electrical engineering at = Stanford. To him, the problems are two sides of the same coin, or = more=20 aptly, opposite poles of the same magnet.

"We have known for a long time that magnetism is a fundamental = property=20 of all materials and it has found wide applications in electronics = and=20 biology, like hard disk drives and magnetic resonance imaging, but = there=20 is also great potential to now apply magnetism at the nanoscale," = Wang=20 said in an interview in his office at the Geballe Laboratory for = Advanced=20 Materials.

There Wang is tuning the characteristics of tiny magnets=97on = the scale=20 of a billionth of a meter=97to help address both cancer and = computing. One=20 part of his research group is developing an ultrasensitive = detector of DNA=20 and proteins, including proteins associated with cancer. With some = of his=20 students, Wang also is making key advances in "spintronics," a new = computing technology that could augment or replace silicon=20 microelectronics when progress there is no longer possible because = of=20 physical limitations.

Wang's expertise and promising results have made him an = important=20 member of two research centers announced this year. On Feb. 27, = the=20 National Cancer Institute awarded Stanford $20 million over five = years to=20 establish a Center of Cancer Nanotechnology Excellence Wang = co-directs=20 with radiology Professor Sanjiv Gambhir. Then on March 9, the = university=20 joined with three University of California campuses to announce = the=20 Western Institute of Nanoelectronics, a center headquartered at = UCLA and=20 dedicated to spintronics research.

Spin for doctors=20

Wang's specialty in magnetism is particularly important in = medical=20 applications because a magnetic field stands out like a flare in = the night=20 sky in magnetically neutral biological settings. Magnetism stands = out more=20 than fluorescence, the current standard for signaling the = detection of a=20 cancer-related protein. That means if a cancer marker could be = made to=20 trigger a magnetic change, the result could be production of a = more=20 sensitive cancer detector. With better detectors, doctors could = diagnose=20 emerging cancers earlier and know sooner whether a particular = treatment is=20 working.

The trademarked MagArray biodetection chips Wang is building, = each=20 about half a square centimeter, are like little traps for target = proteins=20 or DNA strands. The chips are orderly arrays of "ferromagnetic = spin valve"=20 sensors, little magnetically sensitive platforms where magnetism = and=20 biology converge. Like other microarray chips, they work by = exploiting a=20 well-understood phenomenon called "biorecognition." Specific = targets, such=20 as proteins or DNA strands, will only link up with specific = proteins or=20 complementary DNA strands, respectively. In other words, one can = catch a=20 target in a blood or biopsy sample if one provides the right = "bait," or=20 probe.

Detection of a particular target with the MagArray chip first = involves=20 attaching the probes to sensors on the chip. The sensors, each = less than a=20 millionth of a meter wide, are specially designed so that their = electrical=20 resistance will change in a predictable way in the presence of a=20 particular magnetic field. The sample is then pumped onto the chip = via a=20 system of tiny "microfluidic" pipes. Probes capture the targets. = Then=20 magnetically sensitive nanoparticles coated in a chemical that = will bond=20 to the target are pumped in. In the presence of an applied = magnetic field,=20 the nanoparticles emit their own field=97the kind that would = predictably=20 change the resistance of the sensor.

When the nanoparticle links to the target, its proximity = changes the=20 sensor's resistance. The change is read electrically by a computer = as a=20 clear signal of the presence of the target. In a paper in the = journal=20 Sensors and Actuators A in January 2006, Wang and = collaborators=20 published the results of a simplified demonstration of MagArray = chips=20 without biological targets and probes. They showed that the change = in=20 resistance on a chip is directly proportional to the number of=20 nanoparticles on the chip's sensors. Collaborators on the study, = which was=20 funded by the Defense Advanced Research Projects Agency, include=20 electrical engineering Professor Emeritus Robert White, Wang's = former=20 doctoral student Guanxiong Li, research associates Robert Wilson = and Nader=20 Pourmand, and Brown University Professor Shouheng Sun.

Since doing those experiments, Wang and his current students = and=20 collaborators have done further work, as yet unpublished, = demonstrating=20 the efficacy of the chip with biodetection. Wang and his team now = plan to=20 test for proteins associated with breast and prostate cancers. The = researchers aim to produce a handheld device that could rapidly = test for a=20 number of diseases. "Our ultimate goal is that if you are sitting = in a=20 doctor's office or an emergency room, we'll be providing the = doctor with=20 firsthand diagnostics in a time well below one hour," Wang says. = "That=20 would be the holy grail."

Spintronic filters and switches=20

Meanwhile, Wang has made important progress in spintronics as = well.=20 While electronic circuits shuffle electrons around based on their = charge,=20 spintronic circuits would route electrons based on their magnetic = "spin,"=20 a quantum mechanical property that can be described as pointing = "up" or=20 "down." Spintronics holds great promise as an augmentation or even = a=20 replacement for electronics, because circuit operations such as = switching=20 (the mechanism that produces the zeroes and ones of binary code) = could be=20 performed more quickly using less energy.

To make spintronics work in practice, however, engineers must = build=20 working devices, such as filters that can let electrons with one = kind of=20 spin through but block the other kind. The most desirable filters = would=20 work at room temperature rather than require the extreme cooling = typical=20 of many quantum mechanics devices.

Wang's group has indeed done just that, although not yet = perfectly. In=20 a paper accepted by the journal Physical Review Letters B, = Wang and=20 materials science and engineering doctoral student Michael G. = Chapline=20 announce the first room-temperature electron spin filter, which = can block=20 electrons of one spin and let through electrons of the other more = than 75=20 percent of the time. Ideally, the filter would sort electrons of = opposite=20 spins with virtually 100 percent effectiveness. The research was = funded in=20 part by the National Science Foundation.

The whole device is a sandwich of four incredibly thin (just a = few=20 nanometers) layers of exotic materials selected for their magnetic = properties. On one end is a layer of iron oxide that emits = electrons of a=20 particular spin state. Then a layer of magnesium, aluminum and = oxygen=20 magnetically insulates this emitting layer from the most important = layer=97the one that actually does the filtering. That layer is = made of=20 cobalt, iron and oxygen. Finally, a gold layer conducts the = electrons that=20 have made it through the filter to an atomic force microscope for=20 detection.

In addition to finding materials that will increase the = filter's=20 effectiveness, Wang wants to find materials whose magnetic = properties can=20 be rapidly switched back and forth to block different spin = electrons at=20 different times. Such a switching capability would enable the = spintronic=20 equivalent of a transistor.

"In five to 10 years we will really have trouble maintaining = Moore's=20 Law," says Wang, referring to the doubling of transistors on a = chip=20 roughly every 18 months that has underpinned the information = technology=20 industry. "Spintronics is one of the answers to the challenge = posed by=20 Moore's Law as we get down to the nanoscale."

David Orenstein is the communications and public relations = manager=20 at the Stanford School of Engineering.=20