Carbon Nanotube – Silicone rubber compound investigated as interface material on electrode contacts for neural interfacing

K. Tegtmeier, F. Borrmann, T. Doll
Hannover Medical School,
Germany

Keywords: CNTs, neural interfacing, nanostructuring

Summary:

Carbon Nanotubes (CNTs) have been investigated as filler material in a large number of polymers including silicone rubber. Immersing CNTs in silicone rubber provides a flexible electrode material due to the CNTs good conductivity and silicone rubbers flexibility. Areas of application for such a material may be in development of neuronal implants, such as cochlea implants (CI) or electrocortical grid arrays (ECoG). In CIs a replacement for currently used platinum wires and electrodes may thus be found, to be able to increase the number of electrode contacts whilst leaving the implants stiffness unchanged. A promising application is the use as electrode-nerve-interface on electrode contacts in CIs or ECoGs. Neuronal cells thrive on CNTs because their size is similar to the neurites extensions. However, immersion leads to completely insulated CNTs and reduces the nanostructuring of the interface significantly. To regain the nanostructure and also reduce insulation to a minimum, surface etching is implemented. Wet etching techniques including Tetra-n-butylammonium fluoride solution (75 % (w/w) in H2O, Sigma Aldrich; TBAF) combined with three different solvents, dimethyl sulfoxide (DMSO), N-Methyl-2-pyrrolidone (NMP) or Dimethylformamide (DMF) in a 3:1 ratio as well as dry etching using Tetrafluoromethane (CF4) in a 3:1 ratio with Oxygen (O2) have been established. The investigated composite materials consists of the CNTs NC7000 (Nanocyl, Belgium) and the PDMS Sylgard 184 (Dow Corning, USA). To cure the PDMS, the compound was mixed with the curing agent in a 1:10 ratio. The investigation of successful etching was done using scanning electron microscopy, electrochemical impedance spectroscopy and contact angle measurements. Biocompatibility was establishes using both cell lines and primary cells. All tests were done with the compound spread on copper foil as substrate material. Following the positive results of these mentioned test, the methods were transferred to ECoGs, to investigate the usability of spreading the material on the electrode contacts and the feasibility of the established etching methods and durations without damaging the electrodes. Electrode contacts were successfully covered with the CNT-silicone rubber compound. The etching methods could also be used to structure the surface of these small contacts without further damage to the electrodes. Impedance measurements showed low impedances for the surfaces and first cell recordings also showed a high signal-to-noise ratio.