Waveguides for neurostimulation in the cochlea

A potential new method of neural stimulation in cochlear implants (CIs) is light. It could improve the performance of cochlear implant users in speech, speech in noise, and music perception by increasing the spectral selectivity and number of independent channels. Inserting bundles of optical waveguides into the scala tympani of the cochlea is one possible method to deliver light to the spiral ganglion neurons. In this study, we tested waveguides made of OrmoComp®, a hybrid polymer. Waveguides were fabricated via injection molding and coated using dip-coating or thermal reflow. The resulting core diameters of the waveguides were 300 μm with outer diameters of 306 and 940 μm, respectively. For the 306 μm total diameter waveguides, the coupling losses were 10.4 ± 2.7, 4.3 ± 2.7, 6.0 ± 2.3, 6.3 ± 1.4, 6.6 ± 2.5, and 23. ± 3.6 dB, at l=405, l=534, l=680, l=1375, l=1460, and l=1550 nm, respectively. The propagation losses at the same wavelengths were 0.8 ± 1.4, 1.4 ± 1.4, 0 ± 1.2, 0 ± 0.7, 2.0 ± 1.5, and 3.1 ± 1.8 dB/cm, respectively. Bending losses for 360 degrees at l=1375 nm were 5.0, 2.4, and 0.46 for a bending radius of 2.5, 3, and 4 mm, respectively. Insertion forces for the 306 μm diameter waveguides into an acrylic human-size scala tympani model were about 100 mN. For the 940 μm total diameter waveguides, the coupling losses were 0.96 ± 1.4, 1.4 ± 1.0, 0.1 ± 0.56, 1.5 ± 0.45, 0.7 ± 1.3, and 0 ± 1.56 dB, at l=405, l=534, l=680, l=1375, l=1460, and l=1550 nm, respectively. The propagation losses at the same wavelengths were 2.2 ± 0.46, 0.6 ± 0.32, 0.87 ± 0.18, 1.46 ± 0.14, 3.7 ± 0.61, and 2.42 ± 0.54 dB/cm, respectively. Waveguides were also fabricated by injecting OrmoComp® into polyimide tubing, 132-μm outer and 100-μm inner diameter, followed by curing OrmoComp® with ultraviolet (UV) light at l=365 nm for 5 minutes. The coupling losses for the OrmoComp® filled 100- μm core diameter waveguides were less than 1 dB, and the propagation losses were 3.7 dB/cm. The coupling was not optimized in our measurements, and losses can be reduced. Propagation losses are more challenging to address because they depend on the waveguide's material properties and cladding. The bending stiffness of a 1000 μm segment of the 100- μm-diameter waveguides was 18.9 ± 2.2 N/m. Mechanical properties compare well with corresponding measures obtained from conventional cochlear implant electrodes.