May 30, 2012 by
John Sullivan The sensor, shown here on
a cow's tooth, detects bacteria in the body and passes a signal to a nearby receiver.
(Photo by Michael McAlpine) Using silk
strands pulled from cocoons and gold wires thinner than a spider's web,
researchers at Princeton
University Princeton 's Engineering
Quadrangle, a graduate student demonstrated the system's wireless capability,
breathing across a sensor attached to a cow's tooth. Instantaneously, the
sensor generated a response to the student's breath and transmitted a signal to
a nearby monitor. "This is a real-time, wireless response
from a sensor that can be directly interfaced with a variety of
biomaterials," said Michael McAlpine, the team's principal investigator.
He said the system not only has the ability to supply fast results, but is able
to detect very small amounts of bacteria — a feature that could prove critical
in treating certain diseases. The researchers created the tattoo by bundling
the silk and gold with graphene — an extremely thin sheet of carbon in which
atoms are arranged in a honeycomb lattice. The material's unique properties
allowed the researchers to construct a small, flexible device able to detect
bacteria at a much higher sensitivity level than traditional methods. In tests,
the researchers detected samples of bacteria that can cause surgical infections
and others that can lead to stomach ulcers. "In
principle, the graphene can be tailored to detect a range of different
things," said McAlpine, an assistant professor of mechanical and aerospace
engineering at Princeton . "It can be
configured to detect DNA or certain viruses. Here, we detect a single
bacterium." By combining the graphene array with a small
antenna, the detection can be picked up by a remote reader device that is small
enough to be held in a user's hand. The results were reported March 27 in the journal Nature Communications. In
addition to McAlpine, the paper's authors included graduate student Manu
Mannoor, undergraduate Jefferson Clayton, Assistant Professor of Electrical
Engineering Naveen Verma and associate research scholar Amartya Sengupta at
Princeton; Hu Tao, David Kaplan and Fiorenzo Omenetto of Tufts University; and
Rajesh Naik, of the Air Force Research Laboratory. Support for the research was
provided by the American Asthma Foundation and the Air Force Office of
Scientific Research. To build the devices, McAlpine's team first imprinted tiny
graphene sensors onto an extremely thin film of water-soluble silk. (The Tufts
researchers pulled silk strands from cocoons, dissolved them in a solution and
dried the mixture to create the silk base.) Next,
the researchers made an antenna by depositing a pattern of thin gold strands
onto the silk film, and connected it to the graphene sensors. When completed,
the device resembles a common removable tattoo. To attach the sensor, the
researchers place it against a tooth, or a person's skin, and wash it with
water. The silk base dissolves in the water, but the graphene sensor and the
antenna remain securely fastened to the spot. To
allow the device to detect certain types of bacteria, the researchers attached
peptides — fragments of proteins — to the graphene sensors. The peptides bind
to bacterial cells and allow the researchers to detect a signal change from the
graphene sensors. Graduate student Manu
Mannoor works with a tooth to which he has attached a biosensor capable of
detecting minute amounts of bacteria. The sensor, which can be applied like a
child's temporary tattoo, could be useful for diagnosing and monitoring a
variety of illnesses. (Photo by Frank Wojciechowski) McAlpine said one of the
goals was to create a device that was small, flexible and passive, capable of
providing detection from within the body or other remote location. So the
researchers designed the device without a power supply. Instead, an external
radio transmitter held nearby the device delivers a signal that causes the
device to resonate and transmit back its information. "The
antenna coil is what transmits the signal," he said. "You don't need
a battery." Designing the antenna was one of the project's
challenges. The gold coil needs to be big enough to transmit a readable signal,
but small enough to fit within the sensor's compact footprint. The team was
able to attach the current version of the system to a cow's tooth; reducing the
size of the sensor in order to fit onto a human's tooth would require further
work. "Typically, the quality with which you
can transmit depends on the size of the antenna," said Verma, an assistant
professor of electrical engineering. The researchers had to deal with the small
size of the antenna by choosing geometry for the coil that resulted in
sufficient coupling between an external reading unit and the device — this was
achieved through a series of concentric twists. The
current design allows for detection at a relatively short but practical
distance, roughly a centimeter. Verma said if longer range was needed for other
applications, modifications could be developed for the system. The
researchers said one of the key developments of the research was the use of
graphene with a biocompatible base, in this case silk. Current biosensors tend
to be relatively rigid and heavy, and they are often uncomfortable for
patients. In large part, that is a result of sensors' base material, called a
substrate. "When you make biosensors the traditional
way, silicon is the substrate," said Mannoor, a graduate student in
mechanical and aerospace engineering and the paper's lead author. "When
you think of interfacing that on the body, silicon is very brittle. Silk allows
for a dissolvable platform." In addition to its flexibility and
biocompatibility, the solubility of silk meant that it could wash away with
water or be dissolved by the body's enzymes. The
team plans to conduct further studies to better understand the adhesion between
the tooth enamel and the graphene sensor with the goal of achieving a
longer-lasting bond and enhancing the longevity of the system. One of the
challenges for a dental system is protecting the sensor from inadvertent damage
from things like brushing. "Ideally, you want something that would
be there for a while," McAlpine said. "We have a ways to go before we
could master that."
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