Vikram Mukundan Ph.D. defense (Tue, 17th Feb, CIS-X Aud)

Vikram Mukundan mvikram at stanford.edu
Thu Feb 12 16:29:04 PST 2009


Title: Electrostatic Actuators in Aqueous Ionic Media for Applications in 
Cell Mechanics.

University Oral Examination
Vikram Mukundan
Department of Mechanical Engineering
Stanford University
Advisor: Beth Pruitt

Date: Tuesday, February 17, 2009
Time: 2pm (Refreshments served at 1:45pm)
Location: CIS-X 101 (Auditorium)

Abstract:

Cells generate forces during physiological processes that are essential to 
cellular structure and function. Measurement of biological forces employ 
techniques that range from pN at the molecular level to microN at the single 
cell level. Microelectromechanical Systems (MEMS) have been increasingly 
used for biological measurements due to appropriate sensitivity, response 
times, size and force ranges. MEMS techniques that utilize passive sensors 
or employ external actuators may face constraints when used in ionic 
solutions. This thesis presents a water immersible MEMS electrostatic 
actuator for dynamic force sensing of single adherent cells.

Operating electrostatic actuators in conducting liquid media requires 
minimizing ionic shielding and electrochemical corrosion. These primary 
challenges were overcome by exploiting the finite time interval required for 
the formation of the electric double layer. A high frequency signal which 
changes polarity at a faster rate than the relaxation time of the ions was 
used for actuation. The rectifying nature of electrostatic force and 
mechanical damping of higher harmonics leads to quasi-static actuation in 
ionic media. Circuit models were used to evaluate device behavior in a 
variety of conducting liquids. Comb-drive actuators fabricated in single 
crystal silicon were demonstrated to operate successfully in highly 
conducting media such as 150 mM Potassium Chloride and cell culture media 
and to apply deformations of up to around 10 microns and measuring forces 
with a resolution of around 300 pN.

Single adherent cells were attached to a planar micro-tensile tester which 
was coupled to underwater actuators for direct on-chip force application. 
Stiffness and hysteresis properties were measured for Madin-Darby Canine 
Kidney (MDCK) cells cultured on the device. The dynamic response and 
relaxation of MDCK cells to applied step forces were also measured to 
examine their viscoelastic properties.




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