Engineers at the McKelvey School of Engineering at Washington University in St. Louis say they have developed a microneedle patch that can be applied to the skin, capture a biomarker of interest, and allow clinicians to detect its presence. The technology is low cost, easy for a clinician or patients themselves to use, and could eliminate the need for a trip to the hospital just for a blood draw, according to the researchers.

The team’s work (“Microneedle patch for the ultrasensitive quantification of protein biomarkers in interstitial fluid”), from the lab of Srikanth Singamaneni, PhD, the Lilyan & E. Lisle Hughes professor in the department of mechanical engineering & material sciences, was published in Nature Biomedical Engineering.

“The detection and quantification of protein biomarkers in interstitial fluid is hampered by challenges in its sampling and analysis. Here we report the use of a microneedle patch for fast in vivo sampling and on-needle quantification of target protein biomarkers in interstitial fluid. We used plasmonic fluor—an ultrabright fluorescent label—to improve the limit of detection of various interstitial fluid protein biomarkers by nearly 800-fold compared with conventional fluorophores, and a magnetic backing layer to implement conventional immunoassay procedures on the patch and thus improve measurement consistency,” the investigators wrote.

“We used the microneedle patch in mice for minimally invasive evaluation of the efficiency of a cocaine vaccine, for longitudinal monitoring of the levels of inflammatory biomarkers, and for efficient sampling of the calvarial periosteum—a challenging site for biomarker detection—and the quantification of its levels of the matricellular protein periostin, which cannot be accurately inferred from blood or other systemic biofluids.

“Microneedle patches for the minimally invasive collection and analysis of biomarkers in interstitial fluid might facilitate point-of-care diagnostics and longitudinal monitoring.”

In addition to the low cost and ease of use, these microneedle patches have another advantage over blood draws, perhaps the most important feature for some: “They are entirely pain-free,” Singamaneni said.

Finding a biomarker using these microneedle patches is similar to blood testing. But instead of using a solution to find and quantify the biomarker in blood, the microneedles directly capture it from the liquid that surrounds our cells in skin, i.e., the dermal interstitial fluid (ISF). Once the biomarkers have been captured, they’re detected in the same way, using fluorescence to indicate their presence and quantity.

ISF is a rich source of biomolecules, densely packed with everything from neurotransmitters to cellular waste, continued Singamaneni. However, to analyze biomarkers in ISF, conventional methods generally require extraction of ISF from skin. This method is difficult and usually the amount of ISF that can be obtained is not sufficient for analysis. That has been a major hurdle for developing microneedle-based biosensing technology.

Another method involves direct capture of the biomarker in ISF without having to extract ISF. However, the biomarker has to maneuver through a crowded “soup” of ISF before reaching the microneedle in the skin tissue. Under such conditions, being able to capture enough of the biomarker to see using the traditional assay isn’t easy.

Instead, the scientists used plasmonic-fluors, an ultrabright fluorescence nanolabel. Compared with traditional fluorescent labels, when an assay was done on microneedle patch using plasmonic-fluor, the signal of target protein biomarkers shined about 1,400 times as bright and become detectable even when they are present at low concentrations.

“Previously, concentrations of a biomarker had to be on the order of a few micrograms per milliliter of fluid,” said Zheyu (Ryan) Wang, a graduate student in the Singamaneni lab and one of the lead authors of the paper. That’s far beyond the real-world physiological range. But using plasmonic-fluor, the research team was able to detect biomarkers on the order of picograms per milliliter.

“That’s orders of magnitude more sensitive,” Ryan noted, adding that these patches have a host of qualities that can make a real impact on medicine, patient care, and research. They would allow providers to monitor biomarkers over time, particularly important when it comes to understanding how immunity plays out in new diseases.

For example, researchers working on COVID-19 vaccines need to know if people are producing the right antibodies and for how long. “Let’s put a patch on,” Singamaneni suggested, “and let’s see whether the person has antibodies against COVID-19 and at what level.”

Or, in an emergency, “when someone complains of chest pain and they are being taken to the hospital in an ambulance, we’re hoping right then and there, the patch can be applied,” said Jingyi Luan, a student who recently graduated from the Singamaneni lab and one of the lead authors of the paper. Instead of having to get to the hospital and have blood drawn, EMTs could use a microneedle patch to test for troponin, the biomarker that indicates myocardial infarction.

For people with chronic conditions that require regular monitoring, microneedle patches could eliminate unnecessary trips to the hospital, saving money, time, and discomfort, said Singamaneni, who explained that the patches are almost pain-free and “go about 400 microns deep into the dermal tissue. They don’t even touch sensory nerves.”

In the lab, using this technology could limit the number of animals needed for research. Sometimes research necessitates a lot of measurements in succession to capture the ebb and flow of biomarkers, e.g., for example, to monitor the progression of sepsis. Sometimes, that means a lot of small animals.

“We could significantly lower the number of animals required for such studies,” according to Singamaneni, whose lab wants to make sure all the implications surrounding the technology are explored.

The team will have to determine clinical cutoffs, i.e., the range of biomarker in ISF that corresponds to a normal vs. abnormal level. “We’ll [also] have to determine what levels of biomarker are normal, what levels are pathological,” he said, adding that his research group is working on delivery methods for long distances and harsh conditions, providing options for improving rural healthcare.

“But we don’t have to do all of this ourselves,” Singamaneni said. Instead, the technology will be available to experts in different areas of medicine. “We have created a platform technology that anyone can use. And they can use it to find their own biomarker of interest.”

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