Take a silicon chip as small as a grain of rice and carve out barely visible
diving-board–like projections at one edge. Coat these cantilevers with gold. Attach
thiolated single-stranded DNA to the gold-coated cantilevers. Allow single-stranded
DNA of different sequences to come in contact with the DNA attached to the
cantilevers. If a DNA sequence complements the DNA on a cantilever, they will bind
together, or hybridize, to form double-stranded DNA. Thomas Thundat and Karolyn
Hansen, both of ORNL's Life Sciences Division, use this recipe to distinguish between
numbers of base pairs in DNA sequences on cantilevers.
"We immobilized single strands of DNA containing 20 bases on a series of cantilevers,"
Thundat says. "The DNA bases on these cantilevers paired with 20, 15, 10, and 9 bases
of single-stranded DNA introduced to the cantilevers. We found that the cantilevers all
bend because of the changes in surface tension as a result of DNA hybridization." The
more bases a cantilever holds, the more it bends, changing the angle of deflection of
laser light bounced off the cantilever, as recorded in a detector.
Thundat believes that this technology could be used for DNA
sequencing and that the approach would cost less and take less
time than conventional techniques because it would avoid the step
of adding fluorescent dyes to label the DNA bases. This
technology has been licensed to Graviton, Inc.
Thundat believes that cantilevers can be used to detect defective
genes that cause breast cancer, colorectal cancer, and cystic
fibrosis. These mutant genes have one incorrect DNA base. ORNL experiments have
shown that a DNA sequence in a liquid sample will hybridize with a complementary
DNA sequence bound to a cantilever, even if the sample sequence has one wrong base,
or a mismatch.
"We found that a mismatch causes the cantilever
to bend up instead of down," Thundat says. "This
change in bending direction could be used to
detect defective genes that cause disease."
The cantilever technology could also be used to
detect prostate cancer. ORNL researchers have
immobilized on a cantilever the antibody for
prostate-specific antigen (PSA), the chemical
signal for the disease. An ORNL collaboration
with the University of California's Professor Arun
Majumdar has shown that the cantilever bends
when its antibody matches PSA in serum samples
supplied by Majumdar. The sensitivity of this
technology is 10 times higher than that of
By detecting a mutant breast cancer gene, a doctor can predict that a patient will get
breast cancer. By detecting a certain protein, a doctor can determine that the patient has
adult-onset diabetes. Someday, physicians will be able to rapidly analyze both genes and
proteins from a single drop of a patient's blood, using a palm-size device. At least that's
the goal of Tuan Vo-Dinh and his co-workers David Stokes, Minoo Askari, and Guy
Griffin, all of the Life Sciences Division, and Alan Wintenberg of ORNL's
Instrumentation and Controls Division.
"We can do genomics and proteomics on a single
platform using our multifunctional biochip," says
Vo-Dinh. "The biochip is being designed to
process up to 100 samples in 30 minutes." The
multifunctional biochip is an advanced version of
the group's DNA biochip, which contains only
DNA probes. This technology has been licensed
to HealthSpex, Inc., in Oak Ridge.
To sample a patient's blood for a DNA sequence
that is a red flag for a genetic disease, the
multifunctional biochip has a complementary
DNA sequence to which this mutant sequence will
bind. To sample for a specific disease-related
protein, the biochip has a probe (e.g., an antibody
or protein receptor) that will bind with this
ORNL experiments have shown that the biochip
can detect the tuberculosis bacterium, the HIV
gene, a cancer suppressor gene, the anthrax
bacterium used in biological warfare, and
Escherichia coli found in contaminated food.
Thus, the biochip could be used for medical
diagnosis, defense, and food safety applications.
Small though they are, both the cantilever device and biochip could make a big
contribution to health care.