FAST’s sensor is composed of a flexible and stretchable skin-like polymer that includes an embedded layer of gold circuitry. [Alex Abramson, Bao Group, Stanford University]

Engineers at Stanford University have created a wearable sensor device that can be adhered to the skin to measure the changing size of tumors below. Known as FAST for “Flexible Autonomous Sensor measuring Tumors” the technology is a battery-operated device that can capture changes to a one-hundredth of a millimeter (10 micrometers) in real-time and provide those measurements via a smartphone app. The sensor might provide an efficient means for cancer drug screening. A paper describing the device is published in Science Advances.

Historically, preclinical studies in mice often involve subcutaneously implanted tumors underneath the skin of mice. “That allows you to test a ton of different tumor types and drugs in a very easily accessible tumor,” explained Alex Abramson, first author of the study. “With FAST, we’ve developed a better way to measure how the tumors respond, overcoming the limitations of current electronics that are too large to be implanted or for tumors that need to be measured by hand with calipers or by imaging with CT scans or bioluminescence.”

Those technologies can be labor-intensive and limited to a set point in time. “With this new technology, a sensor backpack is attached to the top of the mouse allowing it to read out measurements continuously in real time, giving us a dataset that has much more information than what you would get using an imaging system,” said Abramson.

FAST’s sensor is composed of a flexible and stretchable skin-like polymer that includes an embedded layer of gold circuitry. When that sensor is stretched, it develops small cracks that change the electrical conductivity of the material. Stretch the material and number of cracks increases, causing the electronic resistance in the sensor to increase as well. When the material contracts, the cracks come back into contact and conductivity improves.

“People have used gold patterning to connect flexible and stretchable electronic interconnects for quite a while now,” said Abramson. “But what we did is we pre-stretched stretched it quite a bit, about 30% to 50% more, allowing it to sense ultra-sensitive changes.” If a tumor grows, more cracks appear forcing electrons to travel a convoluted pathway increasing resistance. Conversely, if the tumor shrinks, those cracks begin to reconnect, changing the conductivity of the sensor, allowing a clearer path for the electrons to pass through.

The sensor is connected to a small electronic backpack that measures the strain on the membrane—how much it stretches or shrinks—and transmits that data to a smartphone. “The sensor gives us real time data set, which hasn’t been shown before,” Abramson added. “Because the sensor is ultra-sensitive down to 10 micrometers, taking measurements every five minutes, it allows us to read out really, really small changes in tumor progression or regression almost as it happens.”  In experiments in mice, the researchers could detect discernable differences in tumor volume dynamics within five hours following drug initiation.

Other studies showed the sensor can also enshroud a tumor increasing the ability to measure shape changes. The authors claim the FAST packs are reusable with a cost of about $60 or so to assemble.

“We see the main application of the FAST system is in the drug screening process,” said Abramson. “It would allow for a fully automated version of drug screening which is not possible in animals right now, allowing one to test many more drugs more efficiently with much less human effort.”

Abramson believes FAST can also help researchers perform different types of experiments on various tumor types. “For example, we would be able to know how the tumor is impacted in real time by very small doses continuously compared to a large dose all at once,” he adds.

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