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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