After developing a heat-powered graphene chip that could replace speakers in phones, scientists at Michigan State University have found a way to turn fabrics into speakers.

In late 2016, Nelson Sepulveda, MSU associate professor of electrical and computer engineering, and his team demonstrated a sheet-like device—known as a ferroelectret nanogenerator (FENG)—by using it to power a keyboard, LED lights and an LCD touchscreen. That process worked with a finger swipe or a light pressing motion to activate the devices, converting mechanical energy to electrical energy.

Now, the team has discovered that the material can also act as a microphone by capturing the vibrations from sound and converting it to electrical energy, as well as a loudspeaker by converting electrical energy to mechanical energy.

The audio breakthrough could eventually lead to such consumer products as a foldable loudspeaker, a voice-activated security patch for computers and even a talking newspaper, the researchers said.

"This is the first transducer that is ultrathin, flexible, scalable and bidirectional, meaning it can convert mechanical energy to electrical energy and electrical energy to mechanical energy," said Sepulveda.

MSU_FENG_01 (cr) Figure 1: Like a traditional loudspeaker, the sheet-like, flexible device can transmit sound. Created by MSU engineers, the device can be embedded into a flag or other fabric. (Source: G.L. Kohuth/MSU)

To demonstrate the microphone effect, the researchers developed a FENG security patch that uses voice recognition to access a computer. “The device is so sensitive to the vibrations that it catches the frequency components of your voice,” Sepulveda said.

To demonstrate the loudspeaker effect, the FENG fabric was embedded into an MSU Spartan flag. Music was piped from an iPad through an amplifier and into the flag, which then reproduced the sound.

“The flag itself became the loudspeaker,” Sepulveda said. “So we could use it in the future by taking traditional speakers, which are big, bulky and use a lot of power, and replacing them with this very flexible, thin, small device.”

Wei Li, an MSU engineering researcher and lead author of the paper in Nature Communications, said other potential applications of the FENG include noise-cancelling sheeting and a health-monitoring wristband that is voice-protected.

“Many people are focusing on the sight and touch aspects of flexible electronics,” Li said, “but we’re also focusing on the speaking and listening aspects of the technology.”

The process of creating the FENG starts with a silicone wafer, which is then fabricated with several thin sheets of environmentally friendly substances including silver, polyimide and polypropylene ferroelectret. Ions are added so that each layer in the device contains charged particles, according to researchers.