The cochlear implant (CI) represents, for almost 25 years now, the gold standard in the treatment of children born deaf and for post-lingually deafened adults. These devices thus constitute the greatest success story in the field of ‘neuro bionic’ prostheses. Their fitting in adults, and especially in young children and even babies, places exacting demands on these implants, particularly with regard to the biocompatibility of a CI’s surface components. Furthermore, certain parts of the implant face considerable mechanical challenges, such as the need for the electrode array to be flexible and resistant to breakage, and for the implant casing to be able to withstand external forces.
As these implants are in the immediate vicinity of the middle-ear mucosa and of the junction to the perilymph of the cochlea, the risk exists at least in principle that bacteria may spread along the electrode array into the cochlea. The wide-ranging requirements made of the CI in terms of biocompatibility and the electrode mechanism mean that there is still further scope despite the fact that CIs are already technically highly sophisticated for ongoing improvements to the properties of these implants and their constituent materials, thus enhancing the effectiveness of these devices.
The chief clinical requirements that have to be fulfilled by a cochlear implant derive from the main functional principle of the implant: the differential, site-specific transfer of charge to the auditory nerve and the tonotopic hearing sensation that this generates.
The intention is to give implants a functional life of several decades. The choice of implant materials is thus of crucial significance, the aim being to prevent the need for revision surgery or for operations to replace the implant, as this delays auditory rehabilitation and places a strain on the recipient.
The clinical demands made of a cochlear implant were described as follows in 1992 by E. Lehnhardt
- The materials used must be biocompatible (i.e. physically tolerated)
- The insertion of the electrode array should not cause any additional damage
- The surgical technique used should be as non-invasive as possible
- There needs to be efficient, non-damaging electrical stimulation of the auditory nerve on a sustained basis
- There must be no increased risk of infection caused by the implant and by the access route to the fluid-filled spaces of the cochlea.
The mechanical requirements for a cochlear implant vary greatly with regard to the individual components. The implant’s outer casing needs to be a stable and fluid-tight enclosure that securely houses the electronics. This is a not insignificant aspect in terms of ensuring the device’s long-term functioning. Important materials that have found application over the years include titanium and ceramics, each of which combines a number of beneficial properties.
The second part of the implant, the electrode array, needs to exhibit not only great flexibility but also long-term mechanical stability if it is not to give rise to cable breaks in the bend of the array, or to a lack of leak-tightness.
The chief function of the cochlear implant involves facilitating charge transfer from the electrode array to the auditory nerve and the hearing sensation that this generates. At present, all CI manufacturers use platinum contacts in electrode production. Moreover, iridium oxide coatings have been investigated in certain studies, which showed beneficial effects in terms of what happens to impedance following implantation.
CLINICAL APPLICATION OF BIOMATERIALS USED
1. Allergic reaction/material incompatibility
There are cases of demonstrable allergic reaction or intolerance to silicone. These cases, however, have prompted the provision of test kits in which material samples are made available by the manufacturers in laminar or rod form for implantation at a site that is well away from the ear. This procedure allows individualized compatibility testing involving an observation period of around six weeks intended to rule out allergic responses to the CI material which is to be implanted.
2. Formation of connective tissue around the implant
Although silicone possesses a number of beneficial properties that make it possible to use it for producing both flexible electrodes and biocompatible implant sheaths, it does also have certain disadvantageous characteristics. These include a relatively strong ability to induce connective-tissue growth, which can be observed around both the electrode array and the implant casing. A long-term study by a research team headed by De Ceulaer, in which the effect of one-time intracochlear administration of steroids immediately prior to electrode insertion was investigated, revealed a significant reduction in impedance even two months after implantation.
Working on this project was an amazing experience as not only did I learn more about Biomaterials but also fueled my interest in Biology as a subject. As a student who took up Biology in Class 12th it was really nostalgic to do all the research and feel like a Biology student again. I would like to thank our faculty Sir Hari Murthy and Christ University(Kengeri) for giving us this opportunity to prove our keen interest in the field of Biology.
Have a great day ahead!