In Situ NANO-Indentation of Round Window Membrane

The round window membrane (RWM) is the terminal boundary between the fluid-filled inner ear and air-filled middle ear where the sound wave is released from the inner ear after exciting the neuronal sound transducers in the basilar membrane. For the treatment of inner ear diseases, the RWM is an attractive entrance to introduce therapeutic reagents by producing micro-scale perforations. Therefore, the mechanical properties of this collagen-fiber-reinforced membrane are critical in understanding the functional role in hearing and engineering a device for drug delivery. In this study, NANO-indentation was performed to determining the rupture criteria of freshly harvested RWMs of guinea pigs in situ. Tungsten probe tips with 20 and 5 curvature radius and 2 mm length were used to obtain load-displacement curves until rupture. As the tip size decrease, the rupture force decreased approximately from 5 to 0.3 mN. The Young's modulus of the membrane during the deformation and the stresses within the membrane at the rupture were analyzed inversely via finite element modeling. Biological variation of the mechanical properties was shown. These findings facilitate modeling of sound propagation within the inner ear in a normal condition and with a prosthetic device and developing microneedles for drug delivery.