Subproject T1 - Microstructured Cochlear Implant Electrodes

Cochlear implantation, in which an electrode array is inserted into the inner ear, is the standard present-day procedure in the treatment of deaf patients. Signal transmission from the implant to the auditory nerve is adversely affected by formation of new connective tissue around the electrode array (silicone and platinum contacts). The aim is, by specially structuring the surface using laser, to reduce connective tissue growth on the electrode arrays, so that stimulation of the auditory nerve - with improved selectivity and reduced energy consumption - will be possible.

Cochlear implants currently represent the gold standard in the treatment of deafened adults and congenitally deaf children. The successes of this means of clinical intervention are undisputed and have, in recent years, considerably brightened the general prospects for the severely hearing impaired. The majority of recipients are able to achieve open speech understanding without the aid of reinforcing measures such as lip reading. However, inter-individual variability with regard to the achieved outcomes of cochlear implantation is considerable. Among other causes, the local tissue reaction of the cochlea following insertion of the cochlear implant electrode array plays a particularly important role. Earlier studies have shown that the insertion of the array triggers the formation of connective tissue – sometimes in substantial amounts – around the array within the cochlea [T1-3]. This covering of connective tissue leads, among other things, to the stimulation thresholds being raised [T1-4]. In a clinical study, the applicants measured a marked postoperative increase in impedance, which turned out to be significantly lower in the long term when there was intracochlear administration of glucocorticoid [TI-5]. Reducing the formation of new connective tissue – indeed, preventing it altogether - on real electrode arrays is the chief aim of this subproject. The approach described in the submitted project application represents the first time that a systematic experimental approach has been chosen (in order) to systematically investigate the formation and reduction of connective tissue around the cochlear implant electrode array. A major focus of the work of subproject D2 in the initial funding period of German Collaborative Research Centre (SFB) 599 was on working out the basic principles of laser-based surface modification of cochlear implant materials with regard to the factors influencing connective tissue growth. Preliminary work shows that microstructuring of the surface using fs laser is capable of reducing growth of fibroblasts not only on silicon but also on platinum. These materials form the main components of the electrodes in clinical use today. In order to be able to test both their physiological effectiveness and the predicted biocompatibility, and to be able to transfer the initial findings of pure research work to application-oriented research, entire electrodes are to be microstructured by laser and used in in vivo models (animal experiments) in compliance with the established standard for electrode production. As a first step, the most promising structures from the in vitro experiments are to be transferred onto actual CI electrodes using direct laser ablation of these electrodes. It is at present unclear whether the alignment of the microstructure (i.e. longitudinal or transverse arrangement) on the electrode has any influence on the insertion properties of the electrode (such as trauma insertion) and the resulting growth of connective tissue. As this aspect may have a considerable impact on the biological behaviour of the electrodes in the cochlea, detailed investigations are required into the insertion forces in relation to the alignment of the structures on the electrode array. As the way in which this structural alignment influences connective tissue growth cannot be studied in cell culture experiments, the structures are to be inserted into the silicone both longitudinally and transversely, and characterised in an animal experiment. Parallel work will be done on integrating the structure into the production process. For this reason, a laser-based assembly technique for creating modified moulds for electrodes will be developed, as made possible by the nature of the silicone structure during the production process. In a further step, a method will be devised by which the ring-shaped Pt-contacts of the electrodes can also be structured. The electrode prototypes created will subsequently be characterised (as to biocompatibility and functionality) in an animal model (guinea pig and cat). The “best” structures in each case will then be combined. Additionally, the density of the cochleostomy must then be investigated in an in vivo model (cat). This is of great importance, especially in cases of incomplete electrode insertion where the microstructures are located near the cochleostomy site (with attendant reduction of connective tissue growth). The aim of the transfer project applied for is, therefore, to develop new techniques for creating cochlear implants with surfaces that have been microstructured and investigated as to their biological activity. A femtosecond laser-based machining technique will be used to microstructure injection moulds for the production of cochlear implants. This technique will thus allow the outcomes to be incorporated into the industrial production of implants. It is anticipated that the findings obtained from this example application – namely, the cochlear implant electrode – will be used in order to also allow, for example, enhanced in situ integration into the surrounding tissue for electrode systems currently undergoing clinical testing (inferior colliculus electrode; auditory midbrain implant, AMI). The findings of the transfer project will thus be incorporated back into this German Collaborative Research Centre (SFB 599) proper, thus allowing the immediate continuation of pure research.