The sensor, shown here on a cow’s tooth, detects bacteria in the body and passes a signal to a nearby receiver. Image: Michael McAlpine
Using silk strands pulled from cocoons and gold wires thinner than a
spider’s web, researchers at Princeton
University have created a
removable tattoo that adheres to dental enamel and could eventually monitor a
patient’s health with unprecedented sensitivity.
In a laboratory in 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
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
The results were reported in 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.
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
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
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.”