*2.6. Impedance Results*

The influence of different PLLA-coatings with varying thicknesses (2.5; 5; 10 μm; *N* = 5 each) on electrical contacts was investigated using flat silicone samples, each having included three Pt-contacts comparable to cochlear implant electrode arrays (Figure 5a–c).

**Figure 5.** Mean ( ± SEM) impedance values as measured for contacts coated with PLLA at a thickness of 2.5 μm (**a**), 5 μm (**b**) or 10 μm (**c**). All measurements from *t* = 0 to 24 h were performed with the samples being immersed in 0.9% NaCl. Between all later measurements, left and right contacts were electrically stimulated. *N* = 5 each; l—left contact of the samples, m—middle, r—right contacts.

Initial impedances measured at 1 kHz were above 10 M Ω for 44 of the 45 contacts. The last contact showed impedances between 1 and 10 M Ω. Impedances for all contacts were stable during 24 h incubation in 0.9% NaCl. During the following 24 h of electrical stimulation of two of the three contacts (left and right contacts) on each sample, impedances of stimulated contacts were much more variable which is indicated by larger standard errors of the mean (SEM) in Figure 5a–c. Impedance could remain stable or drop to less than 10 k Ω (examples provided in Figure 6).

Average values for uncoated contacts ( *N* = 6) were 1.65 k Ω and are indicated by a horizontal dashed line in Figure 6 for comparison. An overview on number of contacts and measured impedances after 24 h of electrical stimulation is provided in Table 1.

**Table 1.** Overview on the number of contacts with very low or very high (unchanged) impedances after 24 h of electrical stimulation ( *N* = 5 samples per condition with 2 stimulated contacts on each sample).


Similar results were found when 10% or 20% DCF were incorporated into a 10 μm PLLA coating (Figure 7). With 10% DCF, initial impedances were >10 M Ω for 11 out of 12 contacts whereas with 20% DCF, initial impedances were between 1 M Ω and 10 M Ω for 11 out of 12 contacts. For both concentrations, impedances could drop under electrical stimulation to below 10 k Ω (2/8 with 10% DCF and 4/8 with 20%) or remain at >1 M Ω (1/8 for both concentrations).

**Figure 7.** Mean (± SEM) impedance values as measured for contacts coated with PLLA and loaded with 10% or 20% DCF. All measurements from *t* = 0 to 24 h were performed with the samples being immersed in 0.9% NaCl. Between all later measurements, contacts were electrically stimulated (*N* = 8).

## *2.7. Effect of Electrical Stimulation on Coating*

All samples were also examined for morphological changes after impedance measurements. As shown in Figure 8, for each coating thickness of 2.5, 5 and 10 μm, undamaged coating around the platinum contacts (Figure 8a–c) and platinum contacts with cracks and erosion of the coating (Figure 8a`–c`) were found.

**Figure 8.** Representative SEM micrographs of platinum contacts after impedance measurements with intact (**<sup>a</sup>**–**<sup>c</sup>**) and damaged (**a**`–**c**`) PLLA coating of different thicknesses of 2.5 μm (**<sup>a</sup>**,**a**`); 5 μm (**b**,**b**`) and 10 μm (**<sup>c</sup>**,**c**`).

## *2.8. Prevention of Coating of Contacts*

In order to prevent the electrical contacts from being coated, masking was attached to the surface of the research electrode contacts prior to the PLLA coating process. After clearing the masked areas, impedances as measured at 1 kHz were between 5 and 24 k Ω (mean 14.0 ± 5.5 k Ω). When measuring the same contacts with the clinical system, impedances were between 2.71 and 4.13 k Ω (mean 3.31 ± 0.51 k Ω).
