Hear, Hear For The Bionic Ear

One goal of tissue engineering is to create devices that give humans abilities that they don’t naturally possess. For example, imagine being able to detect sounds outside the normal range of human hearing. Human ears can typically pick up sounds within the frequency range of 20 Hz to 20 kHz. At the low end are the rumblings of engines; at the high end are the shrill screeches of high-pitched whistles.

A new bionic ear, developed by a team of engineering researchers at Princeton University, can detect not just those frequencies but others in the megahertz to gigahertz range—the range of radio waves. The bionic ear “hears” by detecting electromagnetic waves instead of sound waves, although the signals it detects can be converted into sounds audible to humans.

To develop the device, researchers had to integrate sophisticated electronics—capable of transmitting signals to the auditory nerve—into engineered tissue that looks and functions like an ear. The team, led by Michael C. McAlpine, an assistant professor of mechanical and aerospace engineering, used three-dimensional printing to pattern the tissue (Nano Lett. 2013, DOI:10.1021/nl4007744). Other researchers have previously made molds for ears using 3-D printing, or they have placed flexible electronics on top of bioengineered tissue. But McAlpine’s team is the first to have inserted electronics directly into growing tissue layers during the printing process.

“This research team is the first to combine these individually demonstrated components into an integrated bionic construct,” says Jennifer A. Lewis, an engineering professor at Harvard University who was not involved in the work.

“The ear is one of the simpler organs to make, in the sense that the cartilage has no vasculature,” McAlpine says. But at the same time, the outer ear’s complex geometry is difficult to mimic with conventional tissue engineering methods. So he and his coworkers adopt a new approach.

In typical tissue engineering methods, cells are seeded on a scaffold. The cells excrete their own scaffold as the tissue grows, and the original scaffold dissolves. McAlpine’s team instead uses computer-aided design to make a 3-D model of an ear, which they print using a combination of biological, electronic, and structural “inks.” The biological ink is a hydrogel matrix containing cartilage-forming cells, the electronic ink is silver nanoparticles, and the structural ink is silicone.

The 3-D printer builds the structure layer by layer in a few hours. “If it took longer, the cells would die,” McAlpine says. Then they put the ear in a cell culture medium to grow the tissue. “The tissue we have at the end completely surrounds the electronics, such that the electronics are interwoven with the biology,” he says.

The engineered ear consists of cartilaginous tissue with an electronic receiver coil near the surface. The coil penetrates the ear and connects to a cochlea-shaped helical structure with nanoparticle electrodes. The cochlea is a spiral-shaped cavity containing hair cells that convert acoustic vibrations to nerve impulses that are then interpreted as sounds by the brain.

McAlpine and coworkers exposed the 3-D printed ears to left and right channels of stereo music. They connected the cochlear electrodes to a digital oscilloscope, which allowed them to visualize the sounds. They also attached the cochlear electrodes to speakers, which allowed them to play back the output. After going through the entire system, the music was recognizable as Beethoven’s “Für Elise.”

The ears detect radio waves and other electromagnetic radiation, but McAlpine envisions 3-D-printing other structures that detect acoustic signals directly.

The ears are a long way from being used in a person, but “this work represents a first step,” Harvard’s Lewis says. “An important next step is to extend this work by printing more complex 3-D tissue with embedded vascular and conductive networks.”