A continuous pressure monitoring technology for biomedical applications
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The diverse physiological pressures within the human body are crucial in maintaining optimal health and homeostasis. Long-term and continuous monitoring of these physiological pressures can provide key information in the early diagnosis and treatment of several critical diseases, such as eye disease, bladder dysfunction, neurological disorders, and cardiovascular conditions. Among the vital physiological pressures in the human body, blood pressure (BP) is the most important clinical parameter to monitor. Approximately 1.13 billion people suffer from high blood pressure (hypertension), and nearly 10% of the global healthcare budget is spent on controlling BP. The gold standard BP monitors currently used in hypertension management are uncomfortable, lack precision, and, most importantly, unsuitable for continuous monitoring. With the recent development in wearable devices, a variety of new BP monitoring technologies have been introduced with the potential to monitor the BP continuously. However, the literature review suggested that the accuracy and suitability of these devices for clinical use are still questionable. To further validate the application of these devices and test the hypothesis that “the new wearable BP devices are suitable for clinical use", a clinical study was performed to validate the existing wearable home-based blood pressure monitors (HBPMs). The output of these HBPMs was compared against the gold standard brachial sphygmomanometer using the ISO81060-2 (2019/A1:2020) protocol, and none of the devices passed the ISO validation criteria. The results of this study strengthened the case for an unmet clinical need of a highly accurate, nonocclusive continuous BP monitoring solution for hypertension management, especially for high-risk patients. To address the unmet need, a miniaturized wireless pressure sensor was designed, developed, and tested for safety and functional performance in preclinical models. The sensor consists of a planar inductor and a capacitive sensing element to form an LC (inductor-capacitor) resonance circuit. The initial prototypes of the sensor were designed and optimized to monitor the pressure for wearable medical applications. The sensor prototypes were manufactured using a cost-effective, accessible, and scalable fabrication process. The performance of these prototypes was characterized by preclinical bench models, including a customized pressure chamber and physiological flow models. The fabrication process was optimized using an epoxy thiol click chemistry technique to improve the sensor integrity and reliability. This technique allowed achieving sensors with irreversible bonding between various material layers in the sensor assembly. The initial prototypes designed for wearable medical applications, such as pressure monitoring in compression bandages, were relatively larger in size to meet the requirements of the application. Following the proof of concept with the initial design, the sensor was optimized to meet the requirements of continuous BP monitoring with a novel thin-film miniaturized design. The modified sensor design was created for a subcutaneous BP monitoring system comprising a highly sensitive foldable sensor to be placed on the radial artery and a reader system embedded in a wrist bracelet. The design of the sensor was optimized to an elliptical shape that can be folded in compact dimensions along the major axis, making it suitable for subcutaneous delivery. Moreover, the elliptical sensors can cover more area over the artery. There was no existing analytical or numerical model for elliptical shape planar inductors designed for the sensor; therefore, a numerical model was also developed during this research project. This numerical model was validated thoroughly against experimental prototypes. The novel elliptical sensors were characterized for the wireless linkage between the implantable sensor and external reader coil in porcine and ovine tissue environments. During this ex vivo investigation study, the impact of the biological medium on the relative magnitude of the reflection coefficient and resonance frequency was analyzed. An external repeater coil was introduced between the implantable sensor and the external reader coil to improve the wireless linkage. It was observed that the repeater coil improved the wireless linkage 3 to 3.5 times when there was a 2.5 cm thick tissue (skin, fat, and muscle) layer between the sensor and reader coil. Subsequently, a preclinical in vivo study on an animal model (sheep) was conducted to validate the efficacy and safety of the novel elliptical thin-film pressure sensor. On the implantation day (day 0), 9 sensors were implanted on the left and right carotid arteries of sheep. After implantation on day 0, simultaneous BP data were recorded from a reference device (Millar Mikro-Tip SPR-524) and from the detectable implanted sensors. Another measurement session was performed on the follow-up day (day 14), and data was recorded from the detectable working sensors. The recorded data were post-processed and compared with Millar catheter data (Reference device), and a good correlation between the sensor and reference data was observed from day 0 and day 14. However, the quality of day 0 data was better than the data recorded on day 14. After the measurement session, the sensors and tissue samples from the implantation site were analyzed for biological evaluation according to ISO 109936:2016 (biological evaluation of implanted medical devices) protocol. No significant inflammations or reactions were seen, proving the biocompatibility and safety of the thin-film pressure sensors. This in vivo study successfully achieved its primary objectives, confirming the sensors' efficacy (continuous BP monitoring) in acute conditions and their safety during the follow-up session, as evidenced by their optimal performance on day 0 and the absence of infections or inflammation on day 14. The results suggested biocompatibility and the potential for a future minimally invasive, nonocclusive subcutaneous device for continuous long-term BP monitoring through further advancements.
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