Make a Power Move

From laptops to tablets and phones to smart watches, the miniaturization of electronic devices has been progressing steadily through the years. As these devices get smaller and smaller, new challenges arise. In particular, keeping these devices powered up has proven to be difficult, as battery technology has not kept pace with other advances. Furthermore, the tiny sizes of today’s devices makes them suitable for use in wearable devices that can capture information that was inaccessible to hardware of the past, but their rigid components make them uncomfortable to wear for extended periods of time.

A group led by engineers at The Pennsylvania State University has created a new material that could one day deal with both of these problems in one fell swoop. The unique properties of this material that make it suitable for use in creating sensors also give it the ability to harvest energy from motion and temperature changes. And since it is also soft and flexible, it can be integrated directly into articles of clothing to create self-powered sensors that are comfortable to wear and have no need for batteries.

Spinning a yarn

The material is made with a special class of polymers known for their piezoelectric and ferroelectric properties. The polymers give it the ability to produce an electric charge when subjected to mechanical stress or temperature changes. Specifically, the material is based on a polymer called poly(vinylidene fluoride-trifluoroethylene), which is lightweight, thermally stable, and highly flexible. These characteristics make it an ideal candidate for wearable electronics, biomedical devices, and energy-harvesting applications.

To manufacture their new material, the researchers used a method called electrospinning. This technique uses a high-voltage electric field to draw out ultra-fine fibers from a liquid polymer solution. As the polymer jets stretch and dry in mid-air, their internal molecular structures lock into place, affecting the final properties of the fibers. The researchers discovered that by optimizing the molecular weight of the polymer and the concentration of the solution, they could create a fiber structure that exhibits high crystallinity and strong polar alignment, which are two characteristics that enhance piezoelectric performance.

An unexpected approach to an old problem

While earlier approaches typically relied on high molecular weight polymers, the team unexpectedly found that polymers with a much lower molecular weight actually performed better. As it turned out, the shorter chains in the low molecular weight polymer had greater mobility during the rapid electrospinning process, allowing for more organized crystal structures and a higher proportion of the most electroactive conformations. This resulted in fibers with up to 67% crystallinity and 79% polar phase content, boosting electroactive performance.

The researchers initially pursued this work for applications in medical face masks that could trap viruses and bacteria using electrostatic charges. However, the potential extends far beyond personal protective equipment. Pressing or flexing the material generates electricity, making it ideal for powering small sensors in wearable health monitors or environmental sensing devices, for example.

Looking ahead, the team is exploring post-processing steps such as densification (using heat and pressure to compress the porous fiber mats) to further boost performance. They are also seeking industrial partners to help transition the technology from the lab to real-world products.