Nanoscience and Nanotechnology
Double duty: Magnetic nanotechnology fights cancer, advances computing
Detecting cancer and reinventing computing are two challenges that seemingly have
little, if anything, to do with each other. That is, unless you are a nanotechnologist
like Shan Wang, an associate professor of materials science and engineering and
of electrical engineering at Stanford. To him, the problems are two sides of the
same coin, or more aptly, opposite poles of the same magnet.
“We have known for a long time that magnetism is a fundamental property
of all materials and it has found wide applications in electronics and biology,
like hard disk drives and magnetic resonance imaging [MRI], but there is also
great potential to now apply magnetism at the nanoscale,” Wang said in an
interview in his office at the Geballe Laboratory for Advanced Materials.
There Wang is tuning the characteristics of tiny magnets—on the scale of
a billionth of a meter—to help address both cancer and computing. One part
of his research group is developing an ultrasensitive detector of DNA and proteins,
including proteins associated with cancer. With another group of his students,
Wang is making key advances in “spintronics,” a new computing technology
that could augment or replace silicon microelectronics when improvement there is
no longer possible because of physical limitations producing heat and electrical
problems.
Wang’s expertise and promising results have made him an important member
of two research centers announced this year. On Feb. 27, the National Cancer Institute
awarded Stanford $20 million over 5 years to establish a Center of Cancer Nanotechnology
Excellence. Wang is co-director along with radiology and bioengineering Professor
Sanjiv Gambhir. Then on March 9, the university joined with three University of
California campuses to announce the Western Institute of Nanotechnology, a center
headquartered at the University of California-Los Angeles and dedicated to spintronics
research.
Spin for doctors
Wang’s specialty in magnetism is particularly important in medical applications
because a magnetic field stands out like a flare in the night sky in magnetically
neutral biological settings. Magnetism stands out more than fluorescence, the
current standard for signaling the detection of a cancer-related protein. That
means if a cancer protein could be made to trigger a magnetic change, the result
could be a more sensitive cancer detector. With better detectors,
doctors could diagnose emerging cancers earlier and know sooner whether a particular
treatment is working.
Wang's group aims to enable a handheld device that could rapidly test for a
number of diseases. “Our ultimate goal is that if you are sitting in a doctor’s
office or an emergency room, we’ll be providing the doctor with first-hand
diagnostics in a time well below one hour,” Wang says. “That would
be the holy grail.”
Such a device would be based on the trademarked MagArray biodetection chips Wang
is currently building. The device would pump a blood or tissue sample onto a chip that, at a half a square centimeter in area, is essentially a series of tiny traps for specific
proteins. Like other microarray chips, they work by exploiting
a well-understood phenomenon called “biorecognition.” Specific targets—the
cancer proteins doctors are looking for in a sample, for example —will only link
up with complementary proteins on the chip. In other words, the chips
can catch a target floating by in a blood or biopsy sample because they have the exact “bait,” or probes (biorecognition can work for DNA strands in other applications as well).
Detection of a particular target with the MagArray chip, therefore, involves attaching
the complementary protein probes to sensors, each less than a millionth
of a meter wide, arrayed on the chip. The sensors are ferromagnetic “spin
valve” sensors, meaning they are specially designed so that their electrical
resistance will change in a predictable way in the presence of a particular magnetic
field. When the sample is pumped on to the chip via a system of tiny “microfluidic”
pipes, the probes will capture the targets, if any exist in the sample. The next step is to pump in magnetically
sensitive nanoparticles coated in a chemical that will bond to the target. In
the presence of an applied magnetic field, the nanoparticles emit their own field—the
kind that would predictably change the resistance of the sensor.
When the nanoparticle links to the target, its proximity allows it to change the
sensor’s resistance with its field. The change is read electrically by a
computer as a clear signal of the presence of the target. In a paper in the journal
Sensors and Actuators A in January 2006, Wang and collaborators published
the results of a simplified demonstration of MagArray chips without biological
targets and probes. They showed that the change in resistance on a chip is directly
proportional to the number of nanoparticles on the chip’s sensors. In short, they showed that the chip can direclty measure the prevalence of targets in a sample.
The collaborators
on the study funded by the Defense Advanced Research Projects Agency include electrical
engineering Professor Emeritus Robert White, Wang’s former doctoral student
Guanxiong Li, research associates Robert Wilson and Nader Pourmand and Brown University
Professor Shouheng Sun.
Since doing those experiments, Wang and his current students and collaborators
have done further work, as yet unpublished, demonstrating the efficacy of the
chip with biodetection. Wang and his team now plan to test for proteins associated
with breast and prostate cancers.
Spintronic filters and switches
Meanwhile, Wang has made important progress in spintronics as well. While electronic
circuits shuffle electrons around based on their charge, spintronic circuits would
route electrons based on their magnetic “spin,” a quantum mechanical
property that can be described as pointing “up” or “down.” Spintronics holds great promise as an augmentation or even a replacement for electronics,
because circuit operations such as switching (the mechanism that produces the
zeroes and ones of binary code) could be performed more quickly and using less
energy.
To make spintronics work in practice, however, engineers must build working devices,
such as filters that can let electrons with one kind of spin through but block
the other kind. The most desirable filters would work at room temperature, rather
than require the extreme cooling typical of many quantum mechanics devices.
Wang’s group has indeed done just that, although not yet perfectly. In a
paper accepted by the journal Physical Review Letters B, Wang and materials
science and engineering doctoral student Michael G. Chapline announce the first
room-temperature electron spin filter, which can block electrons of one spin and
let through electrons of the other more than 75 percent of the time. Ideally,
the filter would sort electrons of opposite spins with virtually 100 percent effectiveness.
The research was partly funded by the National Science Foundation.
The whole device is a sandwich of four incredibly thin (just a few nanometers)
layers of exotic materials selected for their magnetic properties. On one end
is a layer of iron oxide that emits electrons of a particular spin state. Then
a layer of magnesium, aluminum and oxygen magnetically insulates this emitting
layer from the most important layer—the one that actually does the filtering.
That layer is made of cobalt, iron, and oxygen. Finally, a gold layer conducts
the electrons that have made it through the filter to an atomic force microscope
for detection.
In addition to finding materials that will increase the filter’s effectiveness,
Wang wants to find materials whose magnetic properties can be rapidly switched
back and forth, to block different spin electrons at different times. Such a switching
capability would enable the spintronic equivalent of a transistor.
“In five to ten years we will really have trouble maintaining Moore’s
Law,” says Wang, referring to the doubling of transistors on a chip roughly
every 18 months that has underpinned the information technology industry. “Spintronics
is one of the answers to the challenge posed by Moore’s Law as we get down
to the nanoscale.”
May 2006
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Strategic Priorities
