FIG. 4.

Force on the human round window membrane (HRWM) during microneedle perforation as a function of time. The microneedle contacts the HRWM at (a) and the measured force begins to increase. The first peak at (b) and subsequent drop (c) correspond to the microneedle tip perforation and the second larger local peak (d) corresponds to the full microneedle diameter perforation. This larger peak (d) is considered to be the peak force during a perforation. Subsequently there is a sharp drop to baseline and the microneedle is withdrawn (e). As the microneedle withdraws, it encounters a period of negative force due to friction between the membrane and the microneedle until the trough force (f). Finally the force reaches baseline. The microneedle displacement during perforation—the distance that the microneedle travels during perforation—is the interval (g), defined by the difference in microneedle position at event (d) and event (a).