A multi axial bioimplantable MEMS array bone stress sensor

This thesis presents the initial steps in the development of a sensor to fully extract multi axial stress components in situ. A minimally invasive MEMS sensor is designed in the Jazz 0.35 μm Bi-CMOS process, and embedded in a mock bone material. Using tensile and bending tests, mechanical loads transmitted into the sensor are measured and correlated with the stiffness of the mock bone material. The specific thesis aims are: 1) to provide the theory and methodology for analysis of the design space using piezoresistive bridges sensors in a textured chip for osteoconduction; 2) to design a textured topography on the chip’s surface to enhance cell growth and conduct in-vitro experiments to assess cell attachment; 3) to extract multi-axis stress components from a bone-like material and provide a feasible design for a mm-scale chip; and, 4) to experimentally verify the design theory and approach within mock bone material for a subset of stress components. The 3 mm x 3 mm multi axial bioimplantable MEMS bone stress sensor comprises an array of piezoresistive sensor “pixels” designed to detect stress across the tissue / sensor interface with resolution to 100Pa, in 1 sec averaging. The sensors are integrated within a textured surface to accommodate bone growth. From initial research, surface topography with 30-60 μm features was found to be conducive to guiding new cell growth. In-vitro studies were conducted to assess the viability of the proposed surface topographies. After completion, finite element analysis led to sensor design for multi-axis stress components extraction within a proposed integrated MEMS fabrication process. The micro-machined sensor was characterized in a material to simulate bone controlled axial and shear loads. Tensile tests and bending tests were performed in ASTM D 638 specimens with an embedded bone sensor array. Temperature, hysteresis, and repeatability tests are presented to demonstrate the functionality of the sensor.