Wireless, Self-Powered Sensors for Continuous and Long-term Monitoring of Spinal Fusion Process

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PROJECT SUMMARY Achieving better surgical outcomes and research studies involving lumbar spinal fusion requires reliable determination and degree of fusion. Current imaging technologies are not suitable to accurately and reliably determine different degrees of spinal fusion. These modalities are costly and expose the patient to significant radiation. In addition, nearly all of the previously developed implantable telemetry systems comprise on-board energy storage devices (batteries and super-capacitors) for sensing, computation, storage, and wireless communication. The use of batteries in biomedical implants is not suitable due to their limited life time, large size, and chemical risks. The rest of these spinal implants use radio-frequency identification (RFID) technology or other inductive methods to interrogate the sensor, which faces severe limitations inside the tissue. Similar to the imaging techniques, the current spinal implants evaluate the fusion condition at a given instant and present only a “snapshot at the time” where the measurements are taken. In this research study, we propose to investigate the feasibility of a wireless, self-powered piezo-floating-gate (PFG) sensor capable of monitoring the spinal fusion progress by continuously recording the mechanical usage of the spinal fixation device during the entire time course of fusion. The uniqueness of the proposed sensor is that the operation is completely self-powered by the micro-motion of the spine without the need for any implanted batteries or any external powering. Data collected by the sensor will be wirelessly retrieved using a portable ultrasound-scanner and the resulting output will be time-evolution curves, which will be correlated with the changes of functional spinal unit (FSU) stiffness. These evolution curves would enable clinicians to differentiate between conditions of osseous union, assess the effective fusion period, and schedule for more accurate implant removal in several types of spinal fusion procedures. Our first objective for this research will be to design a fully integrated spinal fusion implant with self-powered monitoring and wireless data retrieval capabilities. The research activity will involve designing and prototyping the ultrasonic energy harvesting and telemetry circuits in silicon and subsequently validating the functionality of the fabricated modules using a cadaver model. The challenge will be to achieve high energy efficiency of the telemetry circuit modules given the limited amount of energy that can be delivered by the ultrasound scanner to a millimeter-scale sensor. Our second objective will be bench-top testing to evaluate the performance of the PFG sensor and the ultrasonic telemetry interface for the monitoring of simulated posterior lumbar spinal fusion in human cadaver spines. Prior to testing on cadaver spines, the PFG spinal implants will be tested using a corpectomy model. Upon successful completion of this study, we will have demonstrated acute in-vitro monitoring of clinically relevant dynamics underlying the process of spinal fusion. Such a powerful tool would enable design of the next-generation, smart fixation-devices with self-monitoring capabilities.