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Hearing Journal:
doi: 10.1097/01.HJ.0000445222.29735.31
Journal Club

Surface Refinements to Cochlear Implants Could Have Deeper Effects

Cullington, Helen PhD

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Dr. Cullington is associate professor, research coordinator, and clinical scientist with the University of Southampton Auditory Implant Service.

I have worked in the field of cochlear implants for 20 years. Not a day goes by that I don't feel thankful for the pioneers who have allowed me to play a small part in changing the lives of so many patients.

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The research underpinning rehabilitation for deafness with cochlear implants is impressive, from speech coding to therapy techniques. Implant devices themselves have reached high levels of reliability; in most cases, 99 percent of internal devices are still working after 10 years of use.

I must admit that I have never thought too much about the surface composition of the materials that make up the implanted devices.

Sure, I knew they had to be very stable in the body (who could forget secondary-school electrolysis experiments—inspired by my chemistry teacher, I coated some coral in copper simply by applying electricity to a chemical solution), and, of course, implant packages have to be strong enough to withstand the rough-and-tumble of active toddlers.

Figure. Helen Cullin...
Figure. Helen Cullin...
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I've also been following the stories of metal-on-metal hip implants degrading and silicone breast implants leaking, recognizing that safety is the number-one concern—especially when we consider that a baby being implanted now may live for another 100 years.

What I didn't really realize was that the materials forming the surfaces of implants could actually be more than passive carriers of electricity, containing special substances to improve the performance of the implant.

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Surface Biotechnology for Refining Cochlear Implants

Tan F,Walshe P,Viani L,Al-Rubeai M

Trends Biotechnol

2013;31(12):678-687 https://www.cell.com/trends/biotechnology/abstract/S0167-7799(13)00199-6

The paper from Trends in Biotechnologyprovides an exciting glimpse into current and future developments in implant material design. I am going to discuss a few of the topics covered in this article.

The surface of the cochlear implant is, of course, the part that actually communicates with the living tissue in the body and eventually influences the success or failure of the device.

The various surfaces of the implant have very different requirements. Image 1 shows the implanted part of the cochlear implant device.

The electronics are housed in a metal box that needs to be very strong, light, and resistant to corrosion when placed in body fluids. Since this part is placed in the bone of the skull, it needs to have properties that allow for bone growth around it.

The receiver coil also must be protected, but, unfortunately, it cannot be included in the metal case, as that would affect the signal transmission.

The other end of the internal implant is the electrode array—a series of electrode contacts connected to the electronics with tiny wires. These metal contacts and wires obviously need to be highly conductive and, again, very resistant to corrosion when permanently in fluid.

They also must have low chemical reactivity with electrical stimulation (No electrolysis!), and the tiny wires need to be very strong, even when flexed.

The whole package is coated in a soft casing for protection, like the silicon drop-proof case on my smartphone. The difference, though, is that I can replace the smartphone case every year or two when it wears out.

On an implant, the soft case needs to be nonreactive, flexible, and resistant to deterioration as it ages (perhaps over 100 years), and it can't be rejected by the human body.

The surface materials that meet these stringent requirements and are currently used in cochlear implants mainly are platinum for the electrode contacts, silicone for the coating, and titanium for the casing. Could enhancements of these surface materials improve cochlear implants?

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REPLACEMENT WITH BIOMATERIALS

One potential enhancement is replacing the surface materials of cochlear implants with biomaterials, which are biological or synthetic substances that interact with biological systems.

Nanofibers are one example of a biomaterial. These tiny fibers are less than one-thousandth the width of a human hair. Bundles of these fibers have been found to help guide neurite (projection from the cell body of a neuron) outgrowth and promote neural regrowth.

The success of a cochlear implant depends on electrical stimulation reaching spiral ganglion neurons: this is the group of nerve cells sending sound information from the ear to the brain.

If new materials could encourage the growth of the nerve cells toward the electrode array, hearing might improve.

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ENHANCING EXISTING MATERIALS

Another possibility is to make changes to the surfaces of the existing implant materials.

The most researched approach is topographical surface modification. One example of this technique is etching a microscopic pattern of grooves on the electrode contacts to enhance electrode–neuron contact while minimizing scar tissue formation.

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COATING THE SURFACE

A coating could be applied to cochlear implant electrodes. Unlike the surface modifications already described, this approach means that a totally different material would be added onto the surface. For example, a collagen coating can promote spiral ganglion neuron survival.

Two other coatings that have been investigated are carbon nanotubes and hydrogels. Carbon nanotubes are long, thin molecules of carbon that are shaped like tiny tubes. They are extremely strong and conduct electricity well.

Hydrogels, in contrast, are soft gels that have been shown to cause minimal tissue damage.

The disadvantage of applying coatings to cochlear implants, though, is that they can crack or peel.

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DRUG DELIVERY

The cochlear implant is ideally placed to deliver drugs topically to the inner ear and surrounding areas.

Implants could be designed to release neurotrophins—which help the survival, development, and function of neurons—or steroids. These drugs could also promote the growth of nerves toward the electrode.

Another possibility is to surface-load the receiver–stimulator package with a steroid drug that would reduce scar formation on the skin wound.

Drug delivery could also prevent the formation of biofilms, which can cause infections like meningitis, by coating the device with slow-release antibiotics.

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GLIMPSE OF THE FUTURE

While the article in Trends in Biotechnology did not give any indication as to how close some of these developments are to being implemented in clinically available devices, it is an exciting glimpse into the future.

One concern is that the biointegration of these new devices could make it more difficult to explant and reimplant devices, which may be necessary several times in the life of a child implanted today.

However, these surface enhancements may be just what we need to take the next step forward in cochlear implant performance. Can the focus on the interface between the implant and the tissue bring us closer to the restoration of normal hearing?

Read past Journal Club columns in a special collection on the HJ website: http://www.bit.ly/HJJournalClub

© 2014 by Lippincott Williams & Wilkins, Inc.

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