Mechanism of an arched basilar membrane in mammalian cochlea
Chan, Wei Xuan
Date of Issue2016-06-24
School of Mechanical and Aerospace Engineering
Energy Research Group
Energy Research Institute @NTU
The mammalian cochlea is a highly sensitive transducer which converts acoustic vibration into electrical signal. The acoustic vibration enters via the stapes and travels in the scala fluid along the cochlea from its basal to apical end. Prior to the acoustic-electrical conversion, the acoustic vibration is mapped onto the basilar membrane based on decreasing frequencies from base to apex. The variation of dynamic structural properties of the basilar membrane contributes to the frequency-mapping and sensitivity of the cochlea. The basilar membrane in most species of mammals, including humans, varies in width and thickness. However, in few species of mammals, such as gerbil, their basilar membranes are arched and have a resting radial tension. These mammals retain their cochlear sensitivity despite the lack of varying width and thickness in their basilar membrane. The present research analyses the mechanism of an arched basilar membrane in contributing to the sharp frequency tuning in a gerbil cochlea. The findings provide understanding of an arched membrane in mammalian cochlea and enables development for application of the cochlear mechanics in areas such as microfluidics and artificial cochlear development where limitations on the channel width are critical. Among the more commonly researched species, the basilar membrane in human cochlea varies significantly in width (300% increase) and thickness (75% decrease) from its basal to apical end. The bandwidth of human cochlear auditory nerve fiber tuning curve is estimated with Yoon et. al.'s 3-Dimensional, push-pull mechanism, two-box model and compared to the bandwidth of gerbil cochlea. The results show comparative sensitivity between gerbil and human cochlea. In order to understand the difference between the two types of basilar membranes, effects of the bending stiffness and radial tension on the acoustic traveling wave in the passive gerbil cochlea is analyzed. The traveling wave number obtained from experimental measurements is compared to that calculated from Steele et. al.'s 3-Dimensional, two-box model which assumed a flat basilar membrane. Significant variation in bending stiffness along the cochlea (1-2 orders in section of 2.2 mm to 3 mm from base) is required in the Steele et. al.'s model in order to match the wave number obtained from experimental measurements. With knowledge of contributing factors in the mechanism of an arched membrane, the dynamic equation is formulated with experimental measurements of gerbil basilar membrane and substituted to the eikonal equation of the two-box model. The wave number coefficients in the eikonal equation of the present arched basilar membrane model matched Yoon et at.'s verified gerbil cochlear model which used estimated effective basilar membrane properties. For integration of cochlea mechanics design into microfluidic applications and the development of artificial cochlea, a method of fabricating and bonding the thin, flexible, anisotropic, 3-dimensional basilar membrane is required as the boundary conditions and properties of the basilar membrane are critical to the frequency tuning of the cochlea. Current methodologies of bonding thermo-set elastomer (basilar membrane) and thermoplastic (cochlear walls) is complex. The present ultrasonic bonding method using a thin film of thermoplastic-elastomeric composite is a simple process which provides better flow control by restricting melted thermoplastic with elastomer. The theoretical analysis of the energy absorption and distribution in the thin film composite provides a guideline for selection of thermoplastic and elastomer as well as estimation of the power required for ultrasonic bonding of the film composite. The experiment shows sufficient bonding strength in the bonded samples and verified the feasibility of this methodology. In a mammalian cochlea, the middle ear acts as a force transducer (acoustic--acoustic) which converts high amplitude--low power into low amplitude--high power vibration in order to match the impedance between air and fluid. This research presents an electromagnetic-acoustic transducer which transfer acoustic energy into the fluid with a lower operating voltage. The transducer consists of a voice coil attached to a membrane and a magnet base. The transducer is shown to operate at a similar efficiency (as a micromixer) while requiring a lower voltage input.