PhD Defense Announcement: Joey Doll

Joey Doll jcdoll at stanford.edu
Mon Feb 13 12:58:01 PST 2012


University PhD Dissertation Defense

*MEMS force probes for mechanobiology at the microsecond scale          *





Joey Doll
Department of Mechanical Engineering
Advisor: Prof. Beth L. Pruitt

Wednesday, March 7, 2012
9:00 am (refreshments at 8:45 am)
Paul G. Allen Auditorium (CIS-X 101)
http://cis.stanford.edu/directions/

Life is built upon mechanical forces, which play a central role in
everything from cell division to embryonic development. Rather than acting
as passive mechanical elements, cells and molecules sense and actively
respond to mechanical loads. One example of cellular force sensing is
mechanotransduction, the conversion of mechanical energy into an electrical
signal, which underlies our senses of hearing, touch and balance. For
example, the cochlear hair cells in your inner ear are exquisitely
sensitive and fast, capable of sensing piconewton-scale forces at the
microsecond-time scale. But in order to understand such fast
mechanotransduction processes we must first be able to apply and measure
small, fast forces. A variety of instruments have been developed for the
precise measurement of biological forces and displacements in the past 25
years. The most commonly used techniques are atomic force microscopy,
magnetic tweezers, and optical tweezers. Each provides a tradeoff in force,
displacement and time resolution, but none of them are capable of applying
and detecting forces fast enough for the study of cochlear hair cells.

In order to address this technological gap we have developed
microfabricated force probes for the application and measurement of forces
at the piconewton- and microsecond-scale. In order to simultaneously
achieve a high resonant frequency (10-200 kHz in water), low stiffness
(0.3-30 mN/m) and low minimum detectable force (10-100 pN), the probes are
roughly 300 nm thick, 1 micron wide and 30-200 microns long. Force applied
to the cantilever tip is transduced into a voltage by a piezoresistive
silicon strain gauge that is embedded in the beam. Actuation is
accomplished through a piezoelectric aluminum nitride film or a resistively
heated aluminum film to enable high-speed operation without spurious
resonant modes. The probes are mass produced on silicon wafers using
conventional batch fabrication techniques, and their dimensions are
individually adjusted lithographically to accommodate a wide range of
desired force and time resolutions. Optics are not required for sensing or
actuation so the probes can be integrated with any standard upright up
inverted microscope.

This talk will begin with a discussion of the mechanical, electrical and
thermal design of the force probes. Numerical design optimization is
utilized to satisfy the numerous design and performance constraints. Next,
I will present the seven- and nine-mask fabrication processes used to
manufacture the thermally and piezoelectrically actuated probes. The
sensing and actuation performance of the probes will be individually
addressed before discussing their integration, particularly crosstalk
compensation. I will conclude by presenting preliminary data on the
measurement of mammalian hair cell kinetics using the piezoelectrically
actuated probes.
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