Detecting Ultra-Rare Proteins with a Cellphone Camera

The ability to detect extremely rare proteins in blood could make a life-saving difference for many conditions, such as the early detection of certain cancers or the diagnosis of traumatic brain injury, where the relevant biomarkers only appear in vanishingly small quantities.

Commercial approaches to ultrasensitive protein detection are based on expensive optics and fluid handlers, which make them relatively bulky and expensive and constrain their use to laboratory settings. Engineers at Penn have developed a test that uses off-the-shelf components and can detect single proteins with results in a matter of minutes, compared to the traditional workflow, which can take days.

The researchers, led by David Issadore, assistant professor in Penn Engineering’s department of bioengineering, and graduate student Venkata R. Yelleswarapu, demonstrated their system in a study published in the Proceedings of the National Academy of Sciences.

Dr. Issadore’s approach works by measuring one protein at a time by breaking apart the sample into microdroplets, each of which contain either a single protein or none at all. His lab has produced microchips etched with hundreds of microdroplet generators, all working in parallel. Microdroplets that contain the protein in question are tagged with a fluorescent marker.

“Normally, you’d have to measure very precisely how much a sample changes color or fluoresces, but here we’re turning it into tens of millions of yes-or-no questions,” Dr. Issadore said. “Digitizing that question brings down the cost of the camera and the surrounding fluid handling equipment, but shifts the problem into how to process tens of millions of those questions, in a way that is reproducible, accurate, inexpensive and portable.”

Existing digital droplet detectors line the droplets up so they can be measured one at a time. To speed up the process, the researchers flow droplets into hundreds of channels that pass by the camera at the same time.

A typical cell-phone camera is too slow, Mr. Yelleswarapu said, “but you can use that camera if the light source you’re using to illuminate the droplet strobes a thousand times faster than the frame-rate of the camera.”

Dr. Issadore’s team encodes a strobing light with a signal that allows them to tease apart one microdroplet from its neighbors. “We’re strobing the light in a very specific pattern that never repeats itself, which is a technique we borrowed from radar,” Dr. Issadore said. “As the signals are going across the screen they get imprinted with this barcode. So even though they overlap with one another, we can tell them apart by which strobe pulse illuminated each droplet.”