phd defense

Mechanical Properties of the Tympanic Membrane: Measurement and ModelingJef AernoutsLaboratory of Biomedical Physics (BIMEF)University of Antwerp

PhD defenseSeptember 24th, 2012

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A PhD study

• PhD: Doctor of Philosophy- “philosophy” = “love of wisdom”

• PhD in Physics- “to understand the behavior of natural phenomena…”

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A PhD study

• PhD: Doctor of Philosophy- “philosophy” = “love of wisdom”

• PhD in Physics- “to understand the behavior of natural phenomena…

…and to use this knowledge for new technologies”

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Mechanical Properties of the Tympanic Membrane: Measurement and Modeling

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The human ear

tympanic

membrane

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Middle ear & tympanic membrane

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Function of the ear

Convert sound (20-20000 Hz) > nerve activity in our brain

What is role middle ear?

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air fluid

Role of the middle ear

?

Impedance matching

between air & fluid

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Middle ear impedance matching

1. Area ratio

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1. Area ratio2. Lever action

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1. Area ratio2. Lever action3. ‘Buckling effect’

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Mechanics

• “Behavior of solids when subjected to forces”

rubbersteelstrain

Dl/l

F/A

stre

ss

linear

non-linear

Elasticity

biological

tissue

visco-elastic

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Mechanics

• “Behavior of solids when subjected to forces”

Vibrational mechanics

100 Hz

(deformation x1e4)

5000 Hz

(deformation x4e6)

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Why study TM mechanics?

• Middle ear finite element modeling

normal

diseasedreconstructed

tympanic membrane!

(Aerts J, Aernouts J. 2012)

(Gan et al., 2009)

(Kelly et al., 2003)

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1. Validation measurements and modeling

2. Gerbil tympanic membrane pars tensa elasticity

3. Gerbil tympanic membrane pars flaccida elasticity

4. Human tympanic membrane elasticity

5. Human tympanic membrane vibrations

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Human tympanic membrane

- Base diameter: 9 mm- Apex height: 1.7 mm

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TM elasticity

• Literature: experiments on cut-out strips- Erroneous analyses (non-uniform thickness)- Difficult specimen clamping

• In my work: experiments on intact samples

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Needle indentation

• Approach- Apply indentations- Measure forces

(1) TM, (2): force transducer,

(3): piston, (4): LVDT , (5): signal

generator, (6): feedback control unit

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Needle indentation


• Sample preparation

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Needle indentation



• Results- Quasi-static

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Needle indentation



• Results- Quasi-static- Step

sample

piston that

drives needle

mounter

attached

to load

cellLVDT

camera

monitor

vaporizer

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• Moiré profilometry

Shape measurement

Buytaert JAN, Dirckx JJJ. Phase-shifting Moiré topography using optical demodulation on liquid crystal matrices. Optics and Lasers in Engineering.

2010;48(2):172–181.

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• Moiré profilometry

Shape measurement

Buytaert JAN, Dirckx JJJ. Phase-shifting Moiré topography using optical demodulation on liquid crystal matrices. Optics and Lasers in Engineering.

2010;48(2):172–181.

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Finite element model

1. Import geometry

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1. Import geometry

2. Mesh geometry

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1. Import geometry

2. Mesh geometry

3. Apply loadings &boundary conditions

In rest

Indented

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1. Import geometry

2. Mesh geometry

3. Apply loadings &boundary conditions

4. Quasi-static stiffness:E = (2.9±1.3) MPa

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Visco-elastic properties

• Relaxation function in time domain

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Visco-elastic properties

• Relaxation function in time domaintime domain

frequency domain

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Work at Boston

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TM mechanics at acoustic freqs

• Sample

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• Sample

• Laser Doppler vibrometry- Sounds: 100 Hz –

18 kHz, 80-120 dB- Umbo velocity

front view

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• Sample

• Laser Doppler vibrometry- Sounds: 100 Hz –

18 kHz, 80-120 dB- Umbo velocity

• Stroboscopic holography- Sounds: 0.5 kHz –

19 kHz, 80-120 dB- Full-field displacement

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• Principle

• Digital holography- CCD- Virtual reconstruction:

hologram before and after > deformation

Holography

CCD

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Stroboscopic holography

• Shutter laser beam/pulsed laser on specific phases

• Both magnitude and phaseof vibration pattern

sample

holography setup

speaker

probe microphone

camera

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FE model

• Geometry (from micro-CT)

(Aerts Johan, 2012)

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FE model

• Geometry (from micro-CT)• Boundary conditions & Loadings

sound wave

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TM transfer function

- Measured with laser Doppler vibrometry:

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TM transfer function

- Measured with laser Doppler vibrometry: - Finite element model outcome

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TM full-field displacement

- Measured with stroboscopic holography:

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TM full-field displacement

- Measured with stroboscopic holography:- Finite element outcome

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TM wave motion

1000 Hz

(deformation x6e3)

7000 Hz

(deformation x1e5)

16000 Hz

(deformation x8e5)

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TM curvature

• Cochlear load at umbo (tip malleus)• Natural curved versus artificially flat

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TM curvature

• Umbo velocity response800 Hz – 4 kHz:

17.5 dB difference

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General conclusion

• Indentation approach- Quasi-static regime (0.001 Hz – 3 Hz)- Elastic characterization pars tensa

• Static inflation experiments- Elastic characterization pars flaccida

• Stroboscopic holography- Acoustic regime (20 Hz – 20 kHz)- Vibrational properties tympanic membrane

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Thanks for your attention!

• Questions? I’m all ears…

phd defense

Education

shifting moir topography

laser doppler vibrometry

fe model geometry

needle indentation approach

finite element model1

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apply loadings

tm transfer function