Micromachined Ultrasonic Silicon Horn Actuators For Biomedical Applications: Surgical Tools, Cardiac Electrophysiological Recordings, Testicular Tubule-Size Detection And Fluid Viscosity Measurement
This thesis presents the continuing research effort towards the use of ultrasonic silicon-based microelectromechanical systems (MEMS) systems for biomedical applications. Microfabricated silicon-horn based surgical microprobes are developed to reduce penetration force into biological tissues by actuating the surgical tool at its ultrasonic resonance. Silicon serves as an ideal platform for integration of a variety of microfabricated sensors on the surgical tools to monitor tissue activity. In this thesis, two sensors are integrated on the microprobes namely platinum electrodes and piezoresistive polysilicon strain gauge sensors. The use of these microprobes in biomedical applications is explored including ultrasonically actuated microprobes with platinum electrodes for cardiac signal recording and stimulation, and ultrasonically actuated microprobes with strain sensors for a testicular tubule-size assay and fluid viscosity measurement. The first part of the thesis presents silicon microprobes integrated with hornPZT actuator for reduction in penetration force in cardiac left ventricular tissue. Platinum electrodes integrated on the microprobes measure the action potentials along the ventricular wall. This device can potentially help provide a 3D map of the electrophysiological activity (wave propagation) in the heart, which may lead to the understanding of cardiac arrhythmias, as well as the prevention and cure of the disease. Also, by ultrasonically stimulating the tissue invasively using the horn with microprobes, the ability to stimulate cardiac tissue and initiate electrophysiological activity in the tissue is demonstrated. The second part of the thesis presents ultrasonic silicon microprobes integrated with strain gauges to monitor the reaction force when inserted into tissue, and its potential for in vitro microscale tissue characterization is demonstrated. A testicular tubule-size assay is demonstrated by monitoring the strain signal output recorded during insertion of the microprobe in rat testis tissue to estimate the average diameter of the seminiferous tubules. This information is important for the surgeon to distinguish between tubules with (larger diameter) and without (smaller diameter) fertile sperm during microdissection-TESE (testicular sperm extraction) surgery, thus enabling a microprobe-based assay for sperm viability. This technique is effectively a new biomedical imaging technique that can be used to image physical characteristics of tissue non-invasively. This can prove to be an invaluable tool during surgery for intelligent tissue biopsy by identifying specific regions of the tissue that exhibit detectable physical characteristics (stiffness, temperature, etc.). The final part of the thesis presents a silicon horn-based ultrasonic microprobes for fluid viscosity measurement with integrated capacitance-based microprobe immersion depth sensors. The longitudinal and flexural vibrations induced in the microprobes due to the PZT-based ultrasonic actuation of the silicon horn structure is precisely monitored by means of the strain gauge, and its damping when the microprobes are immersed in fluid is used to estimate the fluid viscosity. The high sensitivity demonstrated by the viscosity sensor allows for measurement in small sample volumes (approximate 5 mu-l).
dissertation or thesis