Using Video Microscopy to Characterize Micromechanics of Biological and Man-Made Micromachines (invited)

Dennis M. Freeman and C. Quentin Davis

Presented at the Solid-State Sensor and Actuator Workshop

Hilton Head Island, SC, June 1996.

Part I: Biological Micromechanics: Inner Ear

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Sensory cells in the inner ear are called hair cells. This slide (from A.J. Hudspeth, Science, 230:745-752, 1985) shows a light microscopic image of a hair cell that has been isolated from the tissues that normally surround it. Hair cells convert mechanical stimulations of the microscopic sensory hairs that project from their apical surface into an electrical signal. This hair cell is about 30 micrometers long and the hair bundle is about 5 micrometers wide. To put these dimensions into perspective, let me compare the size of the hair cell to that of a structure that may be more familiar to this audience.
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This slide shows a 1 cm silicon die that contains a variety of MEMS test structures. To illustrate the size of a hair cell, I have electronically pasted the image of the hair cell to scale on this slide. You will have to zoom in on the hair cell to be able to see it.
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Hair cells are small. But hair cells are themselves complex micromechanical systems whose function relies on an array of even smaller mechanical parts. Displacements of hair bundles generate electrical responses in hair cells via mechanically sensitive ion channels in the cell membrane.
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Ion channels are proteins that are integral parts of the membranes of all cells. Most of the cell's membrane is lipid and acts as a chemical and electrical barrier between the inside and outside of the cell. Ion channels have pores that act as conduits for ionic currents to pass through the membrane.

The state of conduction of an ion channel can be modulated by external stimuli. For example, some ion channels have chemical binding sites, and their state of ionic conduction depends on whether the binding site is occupied. Such ion channels act as chemical sensors. They underlie sensory functions ranging from olfaction to the detection of neurotransmitters. Many ion channels are sensitive to other stimuli, such as voltage. The mechanically-sensitive ion-channels in hair cells are sensitive to mechanical stress.

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Mechanical stress is transmitted to ion channels in hair cells via tiny filaments that connect neighboring hairs in a hair bundle. The left panel shows a transmission electron micrograph (from A.J. Hudspeth and P.G. Gillespie, Neuron, 12:1-9, 1994) of two adjacent hairs in one hair cell and the tiny tip link that connects them. The hairs are about 500 nm in diameter and tip links are on the order of 2 nm in diameter.
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Displacements of the hair bundle modulate the mechanical stress in the tip links and that stress is transmitted to ion channels, as illustrated in the attached animation. These trap doors represent highly simplified models of ion channels, which are actually complex molecules built out of thousands of amino acids. Nevertheless, these complex molecules can exist in a number of stable states, and this cartoon accurately depicts the fact that mechanical stress modulates the state of conduction of ion channels.
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I began my introduction to inner ear micromechanics by talking about hair cells. Each hair cell is micromachine that is constructed from hundreds of much smaller components (e.g. ion channels and tip links). But hair cells do not operate in isolation. Hair cells are part of a larger system: the inner ear. Thus the inner ear represents VERY LARGE SCALE INTEGRATION of hundreds of thousands of biological microelectromechanical devices.

The inner ear performs some very remarkable signal processing. For example, the inner ear can detect motions of the eardrum on the order of a PICOMETER -- i.e., much smaller than the diameter of a hydrogen atom. Furthermore, the inner ear performs a frequency analysis of low level sounds, and that analysis is very frequency selective. In bats, the quality of tuning of inner ear structures can exceed 800! And the Q of 800 is not achieved in a vacuum; it is not even achieved in air; it is achieved UNDER WATER! We are interested to understand how hair cells and other structures in the inner ear are organized to obtain these remarkable signal processing properties.

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Hair cells are organized in a tissue that separates two fluid-filled compartments of the inner ear. The mechanically sensitive hair bundles project into a gelatinous structure called the tectorial membrane. Sound induces motions of the stapes, the last of the three middle ear bones, so that the stapes moves in a piston-like fashion that modulates the pressures in the inner ear fluids. The sound-induced pressures deform the inner ear tissues and thereby displace the hair bundles of hair cells, as illustrated in the the attached video.

This type of a model has been investigated for more than twenty years, and no one has yet been able to account for the remarkable signal processing properties of the ear using such a model. There has been a wide range of theories for why these models are inadequate. For example, for my doctoral thesis, I studied the effects of hydrodynamic forces on hair bundles and showed that hair bundles can exhibit a mechanical resonance. Another important theory is that the tectorial membrane could resonate. In the video, the displacements of all of the moving structures had similar magnitudes and identical phases. However, if the tectorial membrane were resonant, then its displacement and phase would depend on frequency, and at resonance, the displacements of the tectorial membrane would far exceed those of other structures.

Further progress in understanding cochlear mechanics depends not only on developing new theories but also on testing theories with direct observations. With that idea, let me move to the second part of my talk.

Computer Microvision: Video Microscopy + Computer Vision