Research Article: Polymer Coatings of Cochlear Implant Electrode Surface – An Option for Improving Electrode-Nerve-Interface by Blocking Fibroblast Overgrowth

Date Published: July 8, 2016

Publisher: Public Library of Science

Author(s): C. Hadler, P. Aliuos, G. Brandes, A. Warnecke, J. Bohlmann, W. Dempwolf, H. Menzel, T. Lenarz, G. Reuter, K. Wissel, Masaya Yamamoto.


Overgrowth of connective tissue and scar formation induced by the electrode array insertion increase the impedance and, thus, diminish the interactions between neural probes as like cochlear implants (CI) and the target tissue. Therefore, it is of great clinical interest to modify the carrier material of the electrodes to improve the electrode nerve interface for selective cell adhesion. On one side connective tissue growth needs to be reduced to avoid electrode array encapsulation, on the other side the carrier material should not compromise the interaction with neuronal cells. The present in vitro-study qualitatively and quantitatively characterises the interaction of fibroblasts, glial cells and spiral ganglion neurons (SGN) with ultrathin poly(N,N-dimethylacrylamide) (PDMAA), poly(2-ethyloxazoline) (PEtOx) and poly([2-methacryloyloxy)ethyl]trimethylammoniumchlorid) (PMTA) films immobilised onto glass surfaces using a photoreactive anchor layer. The layer thickness and hydrophilicity of the polymer films were characterised by ellipsometric and water contact angle measurement. Moreover the topography of the surfaces was investigated using atomic force microscopy (AFM). The neuronal and non-neuronal cells were dissociated from spiral ganglions of postnatal rats and cultivated for 48 h on top of the polymer coatings. Immunocytochemical staining of neuronal and intermediary filaments revealed that glial cells predominantly attached on PMTA films, but not on PDMAA and PEtOx monolayers. Hereby, strong survival rates and neurite outgrowth were only found on PMTA, whereas PDMAA and PEtOx coatings significantly reduced the SG neuron survival and neuritogenesis. As also shown by scanning electron microscopy (SEM) SGN strongly survived and retained their differentiated phenotype only on PMTA. In conclusion, survival and neuritogenesis of SGN may be associated with the extent of the glial cell growth. Since PMTA was the only of the polar polymers used in this study bearing a cationic charge, it can be assumed that this charge favours adhesion of both glial cells and SG neurons glial cells and SGN.

Partial Text

So far, the only therapeutic intervention for patients with profound sensory neural hearing loss is the chronic electrical stimulation of the residual auditory neurons via a cochlea implant (CI) [1–3]. However, insertion of the CI into the scala tympani evokes electrode insertion trauma resulting in mechanical damage of the lateral wall, basilar membrane and even the medial wall [4–5] as well as in inflammation and programmed cell death [6–7]. Moreover, fibrosis and new bone formation inside the scala tympani [8–11] and most adversely, growth of fibrous tissue on the implant surface [11–12] were found. In consequence, not only the impedance at the electrode–tissue interface increases [13–14] and higher power impact is needed to ensure CI performance, but also selective neuronal stimulation for discrimination between different sound frequencies is disturbed. Therefore, it is of great clinical interest to modify the surface of carrier material not only of auditory implants but also for other stimulating neural probes to inhibit connective tissue formation.

The growth of intracochlear tissue following the insertion of the electrode array is thought to be the result of an inflammatory foreign body reaction, the contamination of perilymph with blood or bone dust, or a local infection following surgery [54–56]. In consequence the impedance at electrode–tissue interface increases [13–14] and the selective neuronal stimulation for discrimination between different sound frequencies is disturbed. Thus, future strategies aim at preventing fibrotic tissue formation and encapsulation of the electrode contacts either by pharmacological impact, chemical or topographical modification of the carrier material. Implant-based local drug delivery without systemic toxic side-effects is an attractive therapeutic approach to control such cellular processes. Pharmacological substances as like mytomycin C, fluorouracil-5, paclitaxel [28] and dexamethasone [57–61] were described as anti-proliferative, anti-migrating and anti-inflammatory agents. Other studies investigated topographically modified biomedical material surfaces in the micro- and nanometer scale to study their impact on the cell behaviour in vitro [62–68]. Beside controlling the topography for minimizing cell adhesion chemical surface modifications may be another approach to gain cell selective implant surfaces. So far, the anti-adhesive and–proliferative impact of PDMAA and PEtOx coatings had been demonstrated on the murine fibroblast cell line NIH 3T3 as a cell model for connective tissue [32, 35]. Here, we present for the first time their effects on primary cells of the inner ear of postnatal rats comprising SGN, fibroblasts and glial cells.