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
still picture (111K gif)
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.
still picture (130K gif)
This slide shows a 1 cm silicon die that contains a variety of MEMS
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.
still picture (9K gif)
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.
still picture (19K gif)
Ion channels are proteins that are integral parts of the membranes of all
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.
still picture (73K gif)
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.
still picture (18K gif)
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
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.
still picture (8K gif)
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
still picture (12K gif)
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
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
With that idea, let me move to the second part of my talk.
Computer Microvision: Video Microscopy +