Mapping the Young's modulus distribution of the human tympanic membrane by microindentation.

The human tympanic membrane (TM, or eardrum) is composed primarily of layers of collagen fibers oriented in the radial and circumferential directions, as well as epidermal and mucosal layers at the lateral and medial surfaces. The mechanical properties of the TM depend on the microstructures of the collagen fibers, which vary with location, resulting in a spatial variation of Young's modulus. In this study, the Young's modulus of the human TM is measured using microindentation. A 10 µm diameter spherical nanoindenter tip is used to indent the TM at different locations in the lateral and medial surfaces. Through a viscoelastic contact analysis, the steady state out-of-plane (through thickness) Young's modulus at a constant strain rate for the TM is determined from the uniaxial relaxation modulus. The measured spatial distribution of Young's modulus is reported for the entire TM pars tensa on both lateral and medial surfaces. The Young's modulus, for the four TM quadrants, is analyzed statistically using a normal quantile-quantile (Q-Q) plot. The obtained S-shaped curve indicates a bi-modal Gaussian distribution in the Q-Q plot. The spatial distribution of the Young's modulus is modeled by a bivariate Gaussian function in the polar coordinates over the entire TM on both the lateral and medial surfaces. It is shown that the anterior-superior quadrant has the smallest value of Young's modulus. Differences are observed in the spatial distribution of the Young's modulus for both the lateral and medial surfaces. For the medial surface, Young's modulus varies mainly along the radial direction following a small-large-small trend, emanating from the umbo. For the lateral surface, the modulus at the anterior-superior quadrant shows the smallest modulus; the modulus decreases gradually along the radial directions. The quantitative results presented in this paper will help improve future simulation models of the middle ear by using spatial dependence of Young's modulus over the entire TM.

Characterization of the nonlinear elastic behavior of chinchilla tympanic membrane using micro-fringe projection.

The mechanical properties of an intact, full tympanic membrane (TM) inside the bulla of a fresh chinchilla were measured under quasi-static pressure from -1.0 kPa to 1.0 kPa applied on the TM lateral side. Images of the fringes projected onto the TM were acquired by a digital camera connected to a surgical microscope and analyzed using a phase-shift method to reconstruct the surface topography. The relationship between the applied pressure and the resulting volume displacement was determined and analyzed using a finite element model implementing a hyperelastic 2(nd)-order Ogden model. Through an inverse method, the best-fit model parameters for the TM were determined to allow the simulation results to agree with the experimental data. The nonlinear stress-strain relationship for the TM of a chinchilla was determined up to an equibiaxial tensile strain of 31% experienced by the TM in the experiments. The average Young's modulus of the chinchilla TM from ten bullas was determined as approximately 19 MPa.