There are various ways people acquire hearing loss, from aging to damage from long-term noise exposure, infection, or an accident. Some are born with it. Also, many different parts of the ear may need healing to fix the condition. In some situations, this is possible, albeit difficult.
In many cases, the tiny hairs in the cochlea (part of the inner ear) – which allow the brain to recognize the sound as electrical pulses – are damaged. In other instances, the eardrum has suffered perforations. Two different teams of researchers have found solutions to both.
Focusing on fixing a damaged cochlea, Yunming Wang and colleagues at China’s Huazhong University of Science and Technology developed a prototype implantable device that translates soundwaves into matching electrical signals when placed inside a model ear. Meanwhile, Harvard scientists focused on repairing a damaged eardrum and developed the PhonoGraft device, a 3D-printed, biocompatible graft implant to facilitate the healing process of eardrum perforations.
A Step Toward An Artificial Cochlea
There’s no way to undo damage to the inner ear’s hair cells. As a result, treatment is currently limited to cochlear implants and hearing aids. Unfortunately, such devices entail external power sources and tend to have difficulty amplifying speech well or accurately enough to be understood by the user. A hearing aid that doesn’t require batteries would address these shortcomings, and Chinese scientists have designed such a device. It simulates healthy cochlear hairs, successfully converting noise into the brain’s electrical signals as recognizable sound without an external power source.
To accomplish this, the scientists fabricated a material that used compression and friction, which are piezoelectric and triboelectric qualities. Piezoelectric materials are self-powered, becoming charged when compressed by the pressure that accompanies sound waves. Triboelectric materials produce friction and static electricity when moved by these sound waves. They used the new piezo-triboelectric spongey material to make a prototype acoustic sensing device that delivers high efficiency and sensitivity across a wide range of audio frequencies.
How the team made the material:
- First, they coated barium titanate nanoparticles with silicon dioxide.
- Second, they mixed those coated particles into a liquid conductive polymer.
- Third, they dried the mixture into a thin, flexible membrane.
- Lastly, they used an alkaline solution to dissolve the silicon dioxide shells, leaving the nanoparticles sitting loosely within holes in the polymer matrix so they could jostle around when hit by sound waves. Also, this is how the material became sponge-like.
To test the membrane, the scientists sandwiched it between two thin metal grids and subjected it to sound waves. The vibrating membrane generated an electrical current by the piezoelectric effect, and a triboelectric charge was induced by the nanoparticles bouncing off the walls of their hollow chambers.
The effect of the bouncing particles boosted the material’s total electrical output by 55% (compared to piezoelectricity on its own). As a result, the membrane produced a frequency within the range of most adults’ voices (170 hertz).
Then, the team performed a second experiment. They implanted the device in a model ear and played music, recording the electrical output and converting it into a new audio file. The recording displayed a substantial similarity to the original song file. This proves that the self-powered device is sensitive to a wide acoustic range and should be capable of picking up most sounds and voices in the human range of hearing.
A 3D-Printed Eardrum Repair Patch
Perforations in the eardrum, known scientifically as the tympanic membrane, can be tricky to repair. Unfortunately, they can be painful and lead to impaired hearing. The thin, circular piece of tissue conducts sound by vibrating in response to sound waves and converting that movement into electrical signals interpretable by the brain. It also stands as a protective barrier against invading pathogens, thus making it vulnerable to chronic infections that can damage the delicate tissue. Other ways the membrane can become perforated include traumatic injuries, blasts, and loud sounds.
The eardrum has remarkable self-healing powers, but many perforations still require assistance in the healing process. Surgeons can repair a hole through reconstructive “tympanoplasty” procedures that involve tissue grafts harvested from the patient. However, failures commonly follow, making revision surgeries necessary. Furthermore, the structure of patient-derived tissue grafts doesn’t match that of the normal eardrum, resulting in imperfect sound-conducting abilities.
The Harvard team’s PhonoGraft technology is a regenerative medicine solution to this situation. It’s a 3D-printed implant that encourages natural cells to regrow, patching up the damage; It provides a scaffold for the body’s cells to regenerate and restore hearing. It worked so well in trials that it has entered commercial production.
Elliott Kozin, an ear surgeon who is an otologist and neurologist at MEE, and Assistant Professor at HMS, said:
“Ear surgery has seen a lot of advances over the last ten years. An important one has been the development of endoscopic procedures that allow us to treat patients directly through the ear canal, which in many cases avoids skin incisions and drilling behind the ear. This approach, together with innovations from the Lewis lab, enabled us to imagine, design, and ultimately manufacture a device that may one day improve the outcomes of eardrum surgeries.”
The PhonoGraft implant mimics the intricate shape of a natural eardrum, patterned with “spokes” like a bicycle wheel. It’s made of a synthetic polymer-based 3D-printing ink, specially developed for the product.
Jennifer Lewis, co-inventor of the PhonoGraft, said:
“When I learned that the native tympanic membrane is composed of a web-like radial and concentric architecture, I was really excited, as it builds upon earlier work from our lab, in which we first demonstrated the printing of 3D spiderweb-like structures.”
“To create a truly regenerative graft that mimics the circular and radial collagen pattern of the tympanic membrane, which guides the migration of cells as they deposit new collagen fibers in the same pattern, while simultaneously matching the acoustic and mechanical properties of native tympanic membranes, was a grand challenge. These grafts must be 3D printed to truly recapitulate these complex features and functionality.”
Nicole Black, Ph.D., also a co-inventor of the PhonoGraft, added:
“It soon became clear that we had to invent a new ‘ink’ for soft tissue repair. We were aiming for a biodegradable, programmable ink that could be used to control the structure of a graft across multiple length scales, ultimately enabling the production of a regenerative graft with the sound-conducting structure of the native eardrum. To achieve this goal, we developed a synthetic polymer-based ink system that could be aligned during 3D printing.”
The researchers implanted prototypes of the device into a chinchilla animal model’s eardrum since their ears are very similar to human ears regarding hearing range and size. What’s more, the surgery is less invasive than tympanoplasty since it can be inserted through the ear canal, whereas the latter has to be performed by an incision at the back of the ear.
MEE ear surgeon Aaron Remenschneider said:
“Three months after implanting our optimized graft into the chinchilla’s ear, we had a genuine eureka moment. The hearing tests indicated full restoration of sound conduction, which has been a big hurdle. Then we took our first peek down the ear canal with the endoscope. What we were seeing was merely the ghost of our graft that was being replaced with new tissue – a beautifully reconstructed eardrum with its radial-circular pattern. The different cell types of the tympanic membrane and a vascular system for blood support had aligned along the patterned polymer fibers and integrated into a functional tissue. Also, no signs of infection were visible in the reconstructed eardrum.”
Harvard’s Wyss Institute launched a startup called Beacon Bio to bring the invention to the market. Then, the company was acquired by Desktop Health. And now, its founders are working to obtain FDA clearance. Other similar implants, such as the ClearDrum in Australia, are also in the works.
