Development of Piezoresistive Microcantilever Based Force Feedback System for Analysis of Mechanosensation in Caenorhabditis Elegans Nematodes

Cellular mechanotransduction, or the conversion of a force into an electrochemical signal, is a fundamental process underlying our senses of hearing, touch and balance. While hearing and balance have been studied in detail, our sense of touch is only poorly understood. Caenorhabditis elegans, a nematode only about 1 mm in length, is a powerful model organism in which to analyze the mechanism of touch sensation and widely used in biology and medicine. However, few techniques exist to provide the minute forces and displacements appropriate for studies of touch sensation in C. elegans. To address this technological gap, we developed a metrology using piezoresistive cantilevers as force-displacement sensors coupled to a feedback system in order to apply and maintain defined load profiles in the nN-mN range. This thesis presents (1) the design and optimization of piezoresistive cantilevers, (2) integration and development of the force clamp system, and (3) direct application of the system to biological studies of C. elegans mechanotransduction. Analytical modeling and optimization techniques are very useful in the design of piezoresistive devices with complex design constraints. We developed and validated a new analytical model for piezoresistive cantilevers. The model accurately predicted the force sensitivity and force resolution of a wide variety of piezoresistive cantilevers, as measured using laser doppler velocimetry. Using the analytical model, which also utilized doping profile results from TSUPREM simulations, we systemically analyzed the effect of process parameters on device sensitivity and force resolution. With this analytical model, we also developed an optimization approach for piezoresistive cantilever design. We evaluated the approach by fabricating and testing cantilevers designed using the optimization technique. The optimization technique was utilized to produce an optimal cantilever with a minimum force resolution of 69 pN over a 1-1000 Hz bandwidth. We also conducted biological studies of C. elegans mechanotransduction by integrating the developed force probe with a force and displacement feedback control system. We measured body stiffness of wild type and mutant animals with altered body shape and cuticle proteins exposed to solutions of varying osmolarity. The results suggest that shell mechanics dominates C. elegans stiffness rather than hydrostatic pressure, which is contrary to common belief. We also studied the behavioral response of C. elegans to touch stimuli by utilizing the system in force-clamp mode. We applied a 100 nN to 10 mN step force to freely-moving wild type and mec-4 mutant animals, which lack touch receptor neurons. The behavioral result agrees with prior in-vivo work, which suggests that the physiological responses of wild type animals to touch saturate near a force threshold between 100 nN and 1 mN. These initial analyses provide new insights into the mechanism of touch sensation in C. elegans, and open the door to a wide variety of future experiments with C. elegans nematodes, from mechanics to electrophysiology.