## 4 Discussion  
In the current study, it is shown that cochlear electrode insertion speed and pulling speed has a significant impact on its biomechanical behavior. For the first time, it is indicated (i) a significant dictation of pulling speed to the frictional behavior of the cochlear electrode array acting on endosteum lining mimicking tissue and (ii) revealed a statistical dependence between the electrode insertion force and the friction coefficient with a correlation coefficient of approximately 1 (0.9). During CI implantation, the endosteum lining of the scala typpani acts as a friction partner, and the perilymph usually acts as a lubricant for the electrode array. To simulate these clinical conditions a custom-made pulling device was established. Porcine endothoracic fascia was used for mimicking endosteum lining since both consist mainly of the same tissue type [24]. Moreover, artificial perilymph was used as a lubricant. Applied pulling speeds were identical to the insertion speed used in cochlear implant studies. Since most force-related reports have applied insertion speed of 0.16-3.3 mm/s [14; 22; 25; 26], a similar speed ranging from 0.1 to 2.0 mm/s was used in this study. The present experiments show that the friction coefficient increases when the speed is increased. Interestingly, this dependence is non-linear. Lowest friction coefficient was calculated at 0.1 mm/s (0.24 \(\pm\) 0.13), rises to a maximum at 1.5 mm/s (0.59 \(\pm\) 0.12) and decreased at 2 mm/s (0.45 \(\pm\) 0.11). The fluctuated curve plateau at 1.5 mm/s indicates a constant change of adherence and sliding between the electrode array and friction partner at a given speed. The second steep ascent in friction force, observed at a speed of 2.0 mm/s, could be attributed to the fact that the wetting rate was too low. Hence, insufficient lubrication of the entire electrode resulting in an increased friction force. Incidentally, all statistical analyses refer to the lowest calculated coefficient. [PERSON] et al. [18] reported multiple frictional coefficients when applying different lubricates or electrode array types in a slippage model. Compared to the frictional coefficients in our study (0.27-0.59) the authors have calculated lower coefficients (0.04-0.19). Nonetheless, our calculated coefficients are difficult to directly compare with the aforementioned study since the tested electrode arrays and used conditions were different. Uniquely, artificial perilymph was used as a lubricant to better simulate physiological conditions. In addition and in contrast to [PERSON] et al. [18], the electrode in our setup was moved mechanically along the friction partner, which is similar to the steady surgical movement during implantation of a CI electrode. In comparison, an impact of the electrode insertion speed on the insertion force was demonstrated in this work. Thereby, previous studies which reported the dependence of the electrode insertion speed to the insertion force [14; 15] were followed up. Noteworthy, [PERSON] and [PERSON] et al. [14] reported that high insertion speeds cause a significant increase of insertion force and investigated a quiet linear dependence between both parameters. In contrast, our data suggest a non-linear dependence. Lowest insertion force was recorded at 0.5 mm/s (0.014 N \(\pm\) 0.003 N), rises to a maximum at 1.5 mm/s (0.25 N \(\pm\) 0.007 N) and decreased at 2 mm/s (0.019 N \(\pm\) 0.003 N). However, the comparability with the antecedent study is limited, since the used scala typpani model and electrode arrays were different. In contrast to our friction data demonstrated in this study, insertion forces detected at a speed of 0.1 mm/s are significantly increased, if compared to insertion forces recorded at a speed of 0.5 mm/s. This could be related to the fact that the insertion force at 0.1 mm/s is subject to high statistical variance and more variable course of the force progression at a given speed.