Materials and microfabrication approaches for completely biodegradable wireless micromachined sensors
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Implantable sensors have been extensively investigated to facilitate diagnosis or to provide a means to generated closed loop control of therapy by yielding in vivo measurements of physical and chemical signals. Biodegradable sensors which degrade gradually after they are no longer functionally needed exhibit great potential in acute or shorter-term medical diagnostic and sensing applications due to the advantages of (a) exclusion of the need to a secondary surgery for sensor removal, and (b) reduction of the risk of long-term infection. The objective of this research is to design and characterize microfabricated RF wireless pressure sensors that are made of completely biodegradable materials and degrade at time-controlled manner (in the order of years and months). This was achieved by means of investigation of appropriate biodegradable materials and development of appropriate fabrication processes for these non-standard (Microelectromechanical systems) MEMS materials. Four subareas of research are performed: (1) Design of sensors that operate wirelessly and are made of biodegradable materials. The structure of the wireless sensor consists a very compact and relatively simple design of passive LC resonant circuits embedded in a polymer dielectric package. To design the sensor with a particular resonant frequency range, an electromagnetic model of the sensor and a mechanical model for circular plate are developed. The geometry of the sensor is established based on the analytical and finite element simulations results. (2) Investigation of the biodegradable materials in the application of implantable biodegradable wireless sensors to achieve controllable degradation lifetimes. Commercially available and FDA approved biodegradable polymers poly(L-lactic acid) (PLLA) and a "shell-core" structure of poly(lactic-co-glycolic acid) (PLGA) and polyvinyl alcohol (PVA) are utilized as the dielectric package for slow and rapid degradation sensors, respectively. Biodegradable metallic zinc and zinc/iron couples are chosen as conductor materials. The degradation behavior of Zn and Zn/Fe-couple are investigated in vitro. (3) Development of novel fabrication processes. The process exploit the advantages of MEMS technology in fabricating miniaturized devices, while protecting vulnerable biodegradable materials from the strong and/or hazardous chemicals that are commonly used in conventional MEMS fabrication process. These new processes enable the fabrication of biocompatible and biodegradable 3-D devices with embedded, near-hermetic cavities. (4) Testing the pressure response functionality and studying the degradation behavior of the wireless biodegradable pressure sensors. Both PLLA-based and PLGA/PVA-based sensors are characterized in vitro by being immersed in 0.9% saline for prolonged time. All the sensors exhibit three stages of behavior in vitro: equilibration, functional lifetime, and performance degradation. During the functional lifetime, most sensors exhibit fully stable functionality. The PLLA-based sensors show no significant weight loss within 8 month and are expected to fully degrade after 2 years, while the PLGA/PVA-based sensors can degrade completely within 26 days.