Cochlear implants—the neural prosthetic cousins of standard hearing aids—can be a tremendous boon for people with profound hearing loss. But many would-be users are turned off by the device’s cumbersome external hardware, which must be worn to process signals passing through the implant. So researchers have been working to make a cochlear implant that sits entirely inside the ear, to restore speech and sound perception without the lifestyle restrictions imposed by current devices.

A new biocompatible microphone offers a bridge to such fully internal cochlear implants. About the size of a grain of rice, the microphone is made from a flexible piezoelectric material that directly measures the sound-induced motion of the eardrum. The tiny microphone’s sensitivity matches that of today’s best external hearing aids.

Cochlear implants create a novel pathway for sounds to reach the brain. An external microphone and processor, worn behind the ear or on the scalp, collect and translate incoming sounds into electrical signals, which get transmitted to an electrode that’s surgically implanted in the cochlea, deep within the inner ear. There, the electrical signals directly stimulate the auditory nerve, sending information to the brain to interpret as sound.

But, says Hideko Heidi Nakajima, an associate professor of otolaryngology at Harvard Medical School and Massachusetts Eye and Ear, “people don’t like the external hardware.” They can’t wear it while sleeping, or while swimming or doing many other forms of exercise, and so many potential candidates forgo the device altogether. What’s more, incoming sound goes directly into the microphone and bypasses the outer ear, which would otherwise perform the key functions of amplifying sound and filtering noise. “Now the big idea is instead to get everything—processor, battery, microphone—inside the ear,” says Nakajima. But even in clinical trials of fully internal designs, the microphone’s sensitivity—or lack thereof—has remained a roadblock.

Nakajima, along with colleagues from MIT, Harvard, and Columbia University, fabricated a cantilever microphone that senses the motion of a bone attached behind the eardrum called the umbo. Sound entering the ear canal causes the umbo to vibrate unidirectionally, with a displacement 10 times as great as other nearby bones. The tip of the “UmboMic” touches the umbo, and the umbo’s movements flex the material and produce an electrical charge through the piezoelectric effect. These electrical signals can then be processed and transmitted to the auditory nerve. “We’re using what nature gave us, which is the outer ear,” says Nakajima.

Why a cochlear implant needs low-noise, low-power electronics

Making a biocompatible microphone that can detect the eardrum’s minuscule movements isn’t easy, however. Jeff Lang, a professor of electrical engineering at MIT who jointly led the work, points out that only certain materials are tolerated by the human body. Another challenge is shielding the device from internal electronics to reduce noise. And then there’s long-term reliability. “We’d like an implant to last for decades,” says Lang.

In tests of the implantable microphone prototype, a laser beam measures the umbo’s motion, which gets transferred to the sensor tip. JEFF LANG & HEIDI NAKAJIMA

The researchers settled on a triangular design for the 3-by-3-millimeter sensor made from two layers of polyvinylidene fluoride (PVDF), a biocompatible piezoelectric polymer, sandwiched between layers of flexible, electrode-patterned polymer. When the cantilever tip bends, one PVDF layer produces a positive charge and the other produces a negative charge—taking the difference between the two cancels much of the noise. The triangular shape provides the most uniform stress distribution within the bending cantilever, maximizing the displacement it can undergo before it breaks. “The sensor can detect sounds below a quiet whisper,” says Lang.

Emma Wawrzynek, a graduate student at MIT, says that working with PVDF is tricky because it loses its piezoelectric properties at high temperatures, and most fabrication techniques involve heating the sample. “That’s a challenge especially for encapsulation,” which involves encasing the device in a protective layer so it can remain safely in the body, she says. The group had success by gradually depositing titanium and gold onto the PVDF while using a heat sink to cool it. That approach created a shielding layer that protects the charge-sensing electrodes from electromagnetic interference.

The other tool for improving a microphone’s performance is, of course, amplifying the signal. “On the electronics side, a low-noise amp is not necessarily a huge challenge to build if you’re willing to spend extra power,” says Lang. But, according to MIT graduate student John Zhang, cochlear implant manufacturers try to limit power for the entire device to 5 milliwatts, and just 1 mW for the microphone. “The trade-off between noise and power is hard to hit,” Zhang says. He and fellow student Aaron Yeiser developed a custom low-noise, low-power charge amplifier that outperformed commercially available options.

“Our goal was to perform better than or at least equal the performance of high-end capacitative external microphones,” says Nakajima. For leading external hearing-aid microphones, that means sensitivity down to a sound pressure level of 30 decibels—the equivalent of a whisper. In tests of the UmboMic on human cadavers, the researchers implanted the microphone and amplifier near the umbo, input sound through the ear canal, and measured what got sensed. Their device reached 30 decibels over the frequency range from 100 hertz to 6 kilohertz, which is the standard for cochlear implants and hearing aids and covers the frequencies of human speech. “But adding the outer ear’s filtering effects means we’re doing better [than traditional hearing aids], down to 10 dB, especially in speech frequencies,” says Nakajima.

Plenty of testing lies ahead, at the bench and on sheep before an eventual human trial. But if their UmboMic passes muster, the team hopes that it will help more than 1 million people worldwide go about their lives with a new sense of sound.

The work was published on 27 June in the Journal of Micromechanics and Microengineering.