News & Events

Ph.D. Thesis Defense Seminar: “Investigating the Mechanics of the Mammalian Cochlear Partition Using a Novel Microfluidic Device”

Daniel Marnell Advised by: Prof. Jong-Hoon Nam

Hopeman Bldg, Room 224

Abstract

The cochlea is the mammalian hearing organ that encodes sound information into neural impulses carried to the central nervous system. According to prevailing theory, the cochlea achieves frequency tuning through mechanical resonance of a structure known as the organ of Corti complex (OCC). Unfortunately, there are few approaches to observe the mechanics of the OCC, which has left the theory weakly grounded.

The first goal of this thesis work was to develop and validate a new experimental approach that combines the advantages of both in vivo and in vitro approaches, while minimizing their drawbacks. A microfluidic chamber device, fabricated by stereolithography, replaces the mechanical function of the cochlea: delivery of fluid pressure to the OCC across a range of audible frequencies. A membrane or tissue sample placed over a small slit opening separates fluid spaces in the top and bottom compartments of the microchamber. Hydrostatic and hydrodynamic pressures are applied to slit samples, and their mechanical responses are measured using laser interferometry, a paired dual photodiode, and image correlation analysis. Artificial membranes of known mechanical properties were used to verify the feasibility of accurately measuring the mechanical properties of slit samples.

The second goal of the thesis work was to measure the mechanical properties of isolated segments of the OCC from Mongolian gerbils with the microchamber. More specifically, I measured the OCC stiffness in response to fluid pressure, overcoming several challenges from previous stiffness measurements that have exclusively used Hertzian contact mechanics with compliant microprobes.  I measured a compliance of 115 nm/Pa at the location 9 mm from the gerbil cochlear base using both hydrodynamic and hydrostatic stimulation. In order to facilitate comparison of my results with those from previous studies, a computational model of the OCC was used. The model showed that the experimentally measured compliance compared favorably with the lower bound of past stiffness measurements at the similar cochlear location.