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Wireless Sensing of Tissue Deformations Featuring Polymeric Magnets
Measurement of physiological deformations in specific tissues can provide significant information for the diagnosis, monitoring, and treatment of medical conditions. Yet these deformation measurements can be hard to obtain, especially when the targeted tissue is inside the body where optical access is denied. Current medical imaging technologies, including ultrasound, magnetic resonance imaging (MRI) and X-ray, can image soft tissues and bones with decent spatial resolution. However, they are not feasible for chronic tissue monitoring or cases in which rapid tissue deformation/vibration measurements are required. Wireless magnetic sensing is a favorable option for implantable pressure, strain, or deformation sensing systems due to its compact size, passiveness, high sampling rate and minimal interference from biological materials. Polymeric magnets, made from polymer carrier and embedded magnetic micro/nano-particles, possess the traits of flexibility, stretchability and biocompatibility that are preferred for biomedical applications. Nonetheless, their magnetic field is much weaker comparing to that of traditional ferrous/rare earth magnets. Emergence of highly sensitive magnetic sensors based on various principles (Hall effect, anisotropic magneto-resistance (AMR), giant magneto-resistance (GMR), giant magneto-impedance (GMI), tunneling magneto-resistance (TMR)) has enabled precise magnetic sensing of such polymeric magnets. To this end, we developed wireless magnetic sensing systems capable of measuring tissue deformations through implantable polymeric magnets for biomedical applications. This thesis work details the end-to-end development (magnetic sensor selection, magnetic transducer design & fabrication, measurement algorithm development) and the collaborative, interdisciplinary experiment result of a wireless brain deformation sensing system for blast induced traumatic brain injury (bTBI) featuring a polymeric magnetic disk, and a wireless strain sensing system for bladder dysfunction or heart failure (HF) featuring a stretchable polymeric magnetic band. Both systems comprise of one or more polymeric magnetic transducers, an external magnetic sensor / sensor array, and a signal processing unit. Upon tissue deformation, the magnetic transducers attached to the tissue deform jointly, inducing a change in the magnetic field that can be measured wirelessly by the external magnetic sensor / sensor array. Tissue deformation is then recovered from the measured magnetic field signal via the signal processing unit.