The next frontier for continuous health monitoring may be skin deep.
Biomedical engineers at the University of Cincinnati say that interstitial fluid, the watery fluid found between and around cells, tissues or organs in the body, can provide an excellent medium for early diagnosis of disease or long-term health monitoring.
In a paper published in the journal The nature of biomedical engineeringthey identified the potential advantages and technological challenges of using interstitial fluid.
“Why we consider it a valuable diagnostic fluid is continuous access. With blood, you can’t easily take continuous readings,” said Mark Friedel, a UCSD doctoral graduate student and co-lead author of the study.
“Can you imagine spending your day with a needle stuck in your vein all day? So we need other tools.”
Researchers are looking for alternatives to monitoring a person’s health and wellness. Sweat is a good way to measure things like stress or anxiety because it contains hormones like cortisol. But Friedel said the body is stingy with other chemicals that aren’t easily released into sweat.
“The sweat glands are big filters that don’t let everything through,” he said. “So more than half of the things we want to monitor don’t sweat at all.”
Blood is the gold standard for health monitoring. But people also have liters of interstitial fluid that makes up to 15% of their body weight.
“The main feature of blood that makes it so useful is that we understand blood really well,” Friedel said. He said, “If you have something in your blood, we know what will happen to your heart or liver.”
The researchers said the interstitial fluid contains many of the same chemicals in the same proportions as blood, providing a potential alternative to costly and time-consuming laboratory work.
The study outlined the different ways doctors can take samples of interstitial fluid, from applying suction to the skin to deploying dialysis.
“As biomedical engineers, one of our greatest goals is to help people better manage their health by making diagnosis more accessible,” said co-lead author Ian Thompson of Stanford University.
“A significant barrier to this accessibility is that most current diagnoses rely on blood sampling, which can be painful and require trained personnel to perform. Thus, in recent years, there has been increasing interest in using interstitial fluid just under the skin as a diagnostic sample that facilitates accessible and less painful to extract.”
In the UCLA College of Engineering and Applied Sciences Laboratory for New Devices, students are developing sensors to measure hormones and other chemicals in the interstitial fluid. They use tiny needles less than 1 millimeter long that pierce the skin through a small patch.
“If you have a splinter, it’s probably deeper in your skin than the microneedles,” Friedel said. “They’re generally painless. Most of the time I don’t feel it. The most uncomfortable part is removing the tape that secures the device.”
But even if you don’t know it’s there, your body knows, Friedel said. This subtle reaction can affect test results.
“There is a Schrödinger’s observer effect with the interstitial fluid. Any time you try to collect and measure it, you are inherently changing the fluid itself,” Friedel said. “If you insert a needle into your skin, then your body becomes inflamed [sample] Levels change. For continuous vital monitoring, we want to know these concentrations as they are when you are not being pricked with a small needle.
“That’s why it’s a challenging liquid that hasn’t been used outside of diabetes control.”
However, the researchers say that interstitial fluid holds enormous promise for health monitoring through wearable technology. This can help doctors track the effectiveness of medications to ensure proper dosage or provide early diagnosis of disease by monitoring the immune system.
But Friedel said there is still a lot to learn.
“We try to open the box and read the instructions inside to understand what is in the interstitial fluid and what are the possibilities to exploit it,” he said.
Friedel and Thompson worked with co-author Hickenfeld, UCLA’s James L. Winkle School of Pharmacy, Sandia National Laboratories in New Mexico, and Southeast Missouri State University.
The study was funded by grants from the National Science Foundation, the US Air Force Office of Scientific Research and the US Office of Naval Research.