May 23, 2012

Wearable Electronics

Berkeley Science Review - advances in materials science and electrical engineering have paved the way for a new type of electronic device: one that can bend and fold just like a piece of paper. From flexible displays to disposable RFID tags, these new materials have enabled electronics to end up in places they never have before.

Although we may be far from wearing electronics as a fashion fad, the applications of such technologies range from dynamic displays woven right into our shirts to embedded mechanical and ambient atmospheric sensors to obtain and store data about human movement and biological signals. These smart devices could change the way we use clothing. Professor Arias is already in the process of modifying her wearable electronics to fit a multitude of uses. Her previous work focused on defense applications, and she has branched into developing flexible magnetic resonance imaging (MRI) coils and wearable medical sensors. While there are still challenges involving performance, stability, and scalability, in Professor Arias’ words, “This is just the beginning; wearable sensors that measure environmental and biological signals can open up many applications for people who play sports, are in the hospital, or just want to monitor their daily health.” Flexible electronics can even be art; in collaboration with Professor Elad Alon and John Wawrzynek at the Berkeley Wireless Research Center, Professor Arias is also working on creating electronic wallpaper that will, she notes, cover walls with “electronics instead of flowers.”

Arias’s research has produced sensors that can detect ambient light (through photosensitive inks) and pressure. The basic structure of these sensors uses a slice of an organic polymer (PVDF) sandwiched between two electrodes. Exerting pressure on the polymer changes the polymer’s electrical characteristics, altering the voltage measured between the two electrodes. This design is modeled after similar sensors commonly used in car airbags, cell phones, and hard drives. Tailoring the thickness of the polymers and the materials used alters pressure sensitivity. One use for this type of sensor is as a blast dosimeter, a wearable device that detects shocks and stores data about them. Using existing technology for thin film batteries and a layer of circuitry including common electrical elements, a device as thin as a piece of tape becomes a functional, bendable pressure sensor.

With the ability to sense information, the next step was to find a way to store and display such information. Professor Arias’s group developed an organic, inkjet-printed method to create flexible small-area displays that make use of “e-inks”—positively and negatively charged particles suspended in a liquid. Depending on the voltage applied tothe device, either the negatively or positively charged particles will float upward. Because the differently charged particles are white and black, respectively, voltage controls the number of black or white particles at the surface, corresponding to the shade of black displayed for one pixel. A full black-andwhite image can be displayed on an array of these pixels, and by using the flexible substrate polyethylene naphthalate (PEN) as a base Professor Arias’s group created a flexible electrophoretic display.

While physical parts like displays and transistors can easily benefit from flexible materials, what about more intangible electrical properties like memory? Arias’s lab has created non-volatile memory produced with inkjet processing. In most computers, a constant trickle of electricity is needed to keep memory stable, but Arias’ organic material maintains previous states without power. This is accomplished using a transistor that relies on magnetism (not electricity) to store information. Although memory retention is only 8 hours, the technology could be used in applications that require continuous turnover.

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