Could clothing monitor a person's health in real time, because the clothing itself is a self-powered sensor? A new material created through electrospinning, which is a process that draws out fibers using electricity, brings this possibility one step closer.
A team led by researchers at Penn State developed a new fabrication approach that optimizes the internal structure of electrospun fibers to improve their performance in electronic applications. They published their findings in the Journal of Applied Physics.
This novel electrospinning approach could open the door to more efficient, flexible and scalable electronics for wearable sensors, health monitoring and sustainable energy harvesting, according to Guanchun Rui, a visiting postdoctoral student in the Department of Electrical Engineering and the Materials Research Institute and co-lead author of the study.
The material is based on poly(vinylidene fluoride-trifluoroethylene), or PVDF-TrFE, a lightweight, flexible polymer known for its ability to generate an electric charge when pressed or bent. That quality, called piezoelectricity, makes it a strong candidate for use in electronics that convert motion into energy or signals.
"PVDF-TrFE has strong ferroelectric, piezoelectric and pyroelectric properties," Rui said, explaining that like piezoelectricity, pyroelectricity can generate electric charges when temperature change and thus influence the material. "It's thermally stable, lightweight and flexible, which makes it ideal for things like wearable electronics and energy harvesters."
Electrospinning is a technique that uses electric force to stretch a polymer solution into extremely thin fibers. As the fibers dry, the way the polymer chains pack together determines their performance. The researchers hypothesized that altering the concentration and molecular weight of the polymer solution could lead to more organized molecular structures.
"Crystallinity means the molecules are more ordered," Rui said, noting that the team also theorized the structure could have higher polar phase content. "And when we talk about polar phase content, we mean that the positive and negative charges in the molecules are aligned in specific directions. That alignment is what allows the material to generate electricity from motion."
The researchers explained that electrospinning plays a key role in enabling this alignment.
"The process stretches the fibers in a highly mobile state, which predisposes the polymer chains to crystallize into the form we want," said Patrick Mather, a co-author of the study and professor of chemical engineering and dean of the Schreyer Honors College. "You start with a liquid, and it dries over a split second as it travels to the collector. All the packing happens during that brief flight."
One surprising discovery, Mather said, came from experimenting with unusually high concentrations of polymer in the solution.
"These were very high concentrations, roughly around 30%, and much higher than we typically use," Mather said. "My initial thought was that this isn't going to work. But we were using a low molecular weight polymer, and that turned out to be essential. The chains were still mobile enough to pack well during crystallization. That was the biggest surprise. Sometimes, as scientists, we have doubts even when the theory says it should work. But Rui boldly proceeded, and it worked."
The implications are significant, according to Mather. By improving the internal structure of the fibers without requiring high-voltage treatment or complex post-processing, the team created a material that could be both low-cost and scalable.
Rui noted that the material's first intended application was actually for face masks, with funding from the National Institutes of Health (NIH).
"When electrospun into a mask, the material holds a charge that can attract and trap bacteria or viruses," Rui said. "But it also has broader uses in sensors and energy harvesters. If you press it, it can generate electricity."
Qiming Zhang, professor of electric engineering and Harvey F. Brush Chair in the College of Engineering and co-lead author of the study, added that the material's cloth-like texture could make it more comfortable than traditional plastic-based sensors - it could even be directly incorporated into clothing.
"If you wear it like clothing, it's much better," he said. "You could even incorporate sensors into bandages."
Mather pointed out that electrospinning is well suited to producing large sheets, which could be important for energy-harvesting systems. Currently, he notes, most sensors and actuators, material that will change or deform via external stimuli, are small films.
"Most sensors or actuators are small films," he said. "But this process could be scaled to wide-area sheets. The equipment exists, but we'd just need to pair it with an electrode manufacturing process."
Looking ahead, the researchers said they see opportunities to further improve the material through post-processing. Right now, the electrospun sheets are about 70% porous. Applying heat and pressure could densify them and increase sensitivity and output.
"We already have ideas about the next steps," Mather said. "One is densification. We could remove the air between fibers by compacting the sheets after electrospinning, which could boost their performance for certain applications."
For broader adoption, the team said industrial partners will be key.
"We need to find an industrial partner," Mather said. "Someone in the device space or energy harvesting who's interested in taking this to the next level. In my experience, if something works early, it will work commercially. If it's very delicate, it won't hold up. This is a very robust system."
Along with Rui, Mather and Zhang, other authors of the study are Wenyi Zhu, graduate research assistant in electric engineering at Penn State; and Yongsheng Chen, Bo Li and Shihai Zhang, PolyK Technologies.
The U.S. National Institutes of Health and U.S. National Science Foundation supported this research.
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