In vivo intracochlear imaging and guided insertion of cochlear implant with biosensing capability

Cochlear Implants (CI) are a proven surgical solution to treat perceptive auditory problems. CIs are composed of an external microphone and a sound processor that sends electrical signals to a platinum electrode array in the cochlea, the organ where the auditory hair cells and nerves are located. These electric signals directly trigger local neurons, bypassing the conventional hearing pathway, and restoring hearing by stimulation.

The surgical insertion of the electrode array follows the cochlear spaces. Strikingly, trauma related to the insertion is difficult to assess, rendering the cochlea a kind of black box to the surgeon. The cochlea lies within the temporal bone and is enclosed by a dense otic capsule, limiting direct visualization of its microstructure. Current radiographic clinical imaging (X-ray and CT scan) lacks resolution to properly distinguish the intracochlear structure. Insufficient information on the patient-specific internal structure can lead to insertion-related trauma during implantation. Visual feedback during the surgical procedure would greatly diminish the risk of damage to (residual) hearing. Unsurprisingly, the preservation of hearing structures is a priority and leads to a higher sound quality from the CI (reflected as better temporal responses and tonotopicity).

Optical Coherence Tomography (OCT) is proposed as a visual means to verify correct CI placement. It is a cross-sectional imaging modality, based on optical interferometry, that employs non-ionizing IR light. OCT allows 3D tomographic imaging of microstructure with a resolution ranging from 1 to 15 µm. The equipment is able to penetrate thin bone, which spans from 3-4 mm thick in a human temporal bone, and soft tissue and can resolve middle4 and inner ear structures. As a result, OCT may assist the surgeon to locate the scala tympani and to insert the CI electrode array.

After the insertion of the electrode array an acute inflammatory response may take place. Mast cells and monocytes release compounds such as histamine and serotonin, inducing vasodilatation and capillary permeability. The acute response may be superseded by a chronic inflammation phase if the device is recognized as a foreign body. An accumulation of macrophages and lymphocytes (along with their derivatives) takes place as well as tissue granulation (identified by the presence of macrophages and fibroblasts). The CI efficiency has a negative correlation to connective tissue growth after operation. The tissue growth is thought to be the reason behind an electrical impedance increase. The result of this phenomena is the soft failure (slowly aggravating function) of the cochlear device, reflected as a steep efficiency decrease.

To gain insight into the acute inflammatory response, a histamine sensor can be incorporated into the CI electrode array. The sensor is a thin layer of polymer imprinted with recognition sites for the marker. The sensor is attached to the CI by electropolymerizing a monomer in the presence of a template molecule. The resulting polymer will contain cavities complementary to the target molecule, interacting by (non)-covalent bonding. The functionalized electrode array will be able to rebind the inflammation marker with high affinity and specificity (similar to an antibody-antigen or receptor-ligand), which can be detected by electrical impedance spectroscopy.

This project attempts to improve the cochlear implantation surgical procedure and gain insight of the inflammation process in the cochlea by facing 2 important limitations of the current treatment: (1) lacking intra-operatively imaging during a CI insertion procedure to avoid unnecessary trauma and (2) provide insight of the acute inflammatory reaction post-operatively by extending the CI function with a biosensing scheme for inflammatory marker detection. As a result, the knowledge of the precise position of a CI after insertion in vivo can be assessed, as well as an insight to the inflammatory pathway significance of inflammatory-mediated response molecules post-operatively.