*4.5. Immunhistochemistry*

Following fixation, cells were incubated with the monoclonal mouse 200 kD neurofilament antibody (Novocastra, Leica, Wetzlar, Germany) for 1 h at 37 ◦C, 5% CO2. After rinsing with PBS, the secondary biotinylated anti-mouse antibody (Vector Laboratories Inc., Burlingame, CA, USA) was added for 30 min at room temperature before rinsing again with PBS. ABC complex solution (Vectastain® Elite® ABC-Kit, Vector Laboratories, Burlingame, CA, USA) was added to the cells using the protocol of the Vectastain® Elite® ABC Kit. Addition of diaminobenzidine (Peroxidase Substrate Kit DAB; Vector Laboratories, Burlingame, CA, USA) visualized the stained SGNs.

All surviving neurons of each well exhibiting a neurite length of at least three cell soma diameters were counted using a transmission light microscope (Olympus CKX41, Hamburg, Germany). For neurite length measurements, the five longest neurons in each field of view (one in the center and four around the perimeter of the well) were measured using the imaging software cellSens (Olympus, Hamburg, Germany) (compare Figure S6, Supplementary Materials). The survival rate was calculated by normalizing average cell numbers for each condition to average cell numbers in BDNF-treated control wells of the same 96-well plate before averaging across different plates. The same procedure was followed for evaluation of the neurite length. If not otherwise stated, values are presented as mean ± SD.

Statistical analysis of cell-culture results was performed by repeated measures ANOVA followed by Dunnett's posttest using GraphPad Prism version 5.02 (GraphPad, La Jolla, CA, USA). *p* values of less than 0.05 were considered to be statistically significant.

## *4.6. Coating of the Silicone Surface*

The cleaned silicone surfaces were activated via O2-plasma using 100 W power at 0.3 mbar for 1 min in a plasma chamber (Diener, Ebhausen, Germany). Then the samples were incubated in pure GOPS for 4 h at 90 ◦C. The activated samples were rinsed 3 times with ethanol and dried at 80 ◦C overnight under vacuum at 40 mbar.

The coating of the activated silicone samples was prepared via an established and characterized in-house manufactured spray-coating process. First, the activated silicone samples were spray coated with a thin polymer layer of PLLA-NH2 using a chloroform PLLA-NH2 (2 wt%) spray solution. Afterwards the samples were dried at 80 ◦C overnight and coated with pure polymer at thicknesses of 2.5, 5 or 10 μm (measured via microscopy (Olympus SZX16, Olympus, Hamburg, Germany)) or polymer/drug mixture, containing DCF to PLLA at ratios 10:90 wt% or 20:80 wt% in order to reach a layer thickness of about 10 μm. A chloroform PLLA (0.2 wt%) spray solution was used.

The surfaces were examined in a QUANTA FEG 250 (FEI Company, Germany) scanning electron microscope (SEM). A Contact Angle System (OCA 20, DataPhysics Instruments GmbH, Filderstadt, Germany) was used for analyzing surface modifications by contact angle measurements of ultra-pure water sessile drops. Presented mean values and standard deviations were calculated from *N* = 5 samples. Data were analyzed by Wilcoxon's test using SPSS 27.0.

Attenuated total reflection—Fourier-transform infrared spectroscopy (ATR-FTIR)— measurements of the investigated silicone samples were performed using a Bruker Vertex 70 IR-spectrometer (Bruker, Ettlingen, Germany) equipped with a DLaTGS-detector. Each spectrum has been recorded in the range of 4000–500 cm<sup>−</sup><sup>1</sup> at a spectral resolution of 4 cm<sup>−</sup><sup>1</sup> and with 32 scans on the average using a Graseby Golden Gate Diamond ATR-unit (Bruker, Ettlingen, Germany). All spectra were analyzed using OPUS software (Bruker, Ettlingen, Germany) and were subsequently atmosphere and baseline corrected.

## *4.7. In Vitro Drug Release*

The in vitro drug release studies were carried out under quasi-stationary and sink conditions. Between defined withdrawal time-periods each polymer sample (Ø = 6 mm) was left in the dark at 37 ◦C immersed in 1 mL artificial perilymph (145 mM NaCl; 2.7 mM KCl; 2 mM MgSO4; 1.2 mM CaCl2; 5 mM HEPES at a pH of 7.3). At the specific time, the elution medium was completely removed, replaced by 1 mL fresh artificial perilymph, and the drug concentration was quantified by HPLC. The chromatography was performed under isocratic conditions with a mobile phase consisting of acetonitrile/ultrapure water (50.5/49.5) (*v*/*v*), 0.15 M acetic acid and 4.7 mM trimethylamine at a pH 4.35. The flow rate was set to 1.0 mL/min. UV detection was conducted at a wavelength of 275 nm. The retention times for DMS and DCF are 3.8 and 8.3 min, respectively. MV and SD were calculated from *N* = 3 samples. In order to compare the release, the released amounts were normalized and referred to as the total amount of DMS (100%) and DCF (100%) in the samples. Data were analyzed by Kruskal-Wallis test using SPSS 27.0.

## *4.8. Impedance Measurements of Coated Samples*

For impedance measurements of electrical contacts, flat rectangular silicone samples (1 cm by 1 cm) were generated (Figure S1, Supplementary Materials) with three Pt-contacts (approximate size: 480 × 800 μm) being fixed to one side of the sample. Two of these samples were placed in a beaker filled with 0.9% NaCl such that the samples were positioned approximately parallel at a distance of 2 cm with the contacts facing each other. Measurements were performed between one contact of sample 1 and one contact of sample 2 at 1 kHz by using a 3522-50 LCR-HiTESTER (Hioki, Ueda, Japan). One of the samples in the setup remained uncoated and served as reference electrode whereas the other was the test subject being either uncoated or having received one of the different coatings. Impedances were measured at room temperature without additional electrical stimulation after placing the samples in the beaker (*t* = 0), then every 30 min during the first 7 h and again 24 h after start. Pulsatile electrical stimulation (biphasic, pulse width: 400 μs, interphase gap: 120 μs, repetition rate: 1 kHz; current: 0.44 mA) was applied for the next 24 h to two of the three contacts (left and right contacts) by using a pulse generator (TGP 110, AIM-TTI, Huntingdon, UK). Impedance measurements were continued on all three contacts of the test sample according to the measurement regime of the first day. After the impedance measurements, the investigated surfaces were examined by scanning electron microscopy (SEM).

In additional measurements with coated research electrodes (MED-EL), the clinical MAESTRO software together with an attached MAX-box (MED-EL) was also used.

Data were analyzed using repeated measures ANOVA (two-way) followed by Bonferroni posttest.
