A Multi Axial BioImplantable MEMS Array Bone Stress Sensor

Abstract

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 um 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 um 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 embed- ded bone sensor array. Temperature, hysteresis, and repeatability tests are presented to demonstrate the functionality of the sensor. The long term goal is to use this sensor to monitor the stiffness of regenerating bone (open fractures or grafts) or the interfaces between bone and prosthetic implants in order to help guide clinical management of skeletal trauma and disease. Current technology relies on radiographic imaging to infer bone quality (measuring bone mineral density). However, bone stiffness does not necessarily correlate well with image intensity. A practical means to directly measure and quantify biomechanical properties of healing or diseased bone in situ could provide improved and timely information for treatment management options, including drugs, fixation adjustments, rehabilitation regiments, or pre-emptive surgical intervention.