PhD Defense Announcement: Joey Doll

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.