Soft wireless sternal patch to detect systemic vasoconstriction using photoplethysmography

Introduction:: Vasoconstriction is a crucial physiological process that serves as the body’s primary blood pressure regulation mechanism and a key marker of numerous harmful health conditions. The ability to detect vasoconstriction in real time would be crucial for detecting blood pressure, identifying sympathetic arousals, characterizing patient wellbeing, detecting sickle cell anemia attacks early, and identifying complications caused by hypertension medications. However, vasoconstriction manifests weakly in traditional photoplethysmogram (PPG) measurement locations, like the finger, toe, and ear. Here, we report a wireless, fully integrated, soft sternal patch to capture PPG signals from the sternum, an anatomical region that exhibits a robust vasoconstrictive response. With healthy controls, the device is highly capable of detecting vasoconstriction induced endogenously and exogenously. Furthermore, in overnight trials with patients with sleep apnea, the device shows a high agreement (r2 = 0.74) in vasoconstriction detection with a commercial system, demonstrating its potential use in portable, continuous, long-term vasoconstriction monitoring.

Materials and Methods:: We developed a soft sternal patch comprised of a photolithographically patterned, ultrathin flexible circuit embedded in an elastomeric membrane and functionalized with integrated components. Finite element analysis (FEA) was conducted to simulate the conformal contact of the PPG unit to the skin and optimize the device's design for maximal conformality. Mechanical studies were then conducted to empirically determine the soft device’s bending stiffness and reliability. The first human-subjects trial was conducted to assess the hypothesis that the soft mechanics innovations can improve signal quality and reduce motion artifacts compared to a rigid device. After the optimal device mechanics were determined, the sensing performance was validated with human subjects, and the optimal anatomical region on the chest was identified. Additional controlled experiments in human subjects were then conducted to validate the system’s efficacy in endogenous and exogenous vasoconstriction detection. Finally, the soft device’s ability to detect naturally occurring vasoconstrictions in patients with sleep apnea resulting from sympathetic arousals during sleep was studied.

Results, Conclusions, and Discussions:: We report a soft wireless wearable patch with mechanics optimized to record vasoconstrictions in PPG signals from the chest. This work includes computational, empirical, and human pilot studies to design a skin-sensor interface based on a compressed elastomer and highly flexible microfabricated system. This system was used to identify the optimal measurement location on the chest, and then finalized based on the unique anatomical characteristics of the sternum. The mechanical insights derived from these studies were validated in controlled experiments consisting of endogenous and exogenous vasoconstrictions, where the device proved highly capable of identifying vasoconstrictions. Finally, the device was tested in symptomatic patients to determine if vasoconstrictions caused by obstructive sleep apnea could be detected. In a simultaneous study with the commercial gold-standard method using WatchPAT, an accuracy of 78% and r2 of 0.74 was reported, both of which are the highest reported in a wearable patch to the author's knowledge. Furthermore, the device demonstrated a high ability to detect vasoconstrictions across various magnitudes. As validated in the preceding studies, the mechanical insights presented here enable a new method for continuously detecting vasoconstrictions in real time, which has significant medical applications. For instance, this device could be used to identify the degree of vasoconstriction induced by blood pressure medication in a small trial dose, and the medicine’s physiological impact could be tracked over time. This would allow for precise medicine dosing, avoiding the risk of serious side effects. In addition, this device could be implemented to detect the warning signs of a sickle cell crisis, monitor shock victims, and assess sympathetic nervous system activation in patients with sleep disorders. Furthermore, the simple sternal patch could serve as an initial screening, augment, or even replace expensive and low-throughput MRI and CT scan to assess contracted vessels. Overall, the mechanical insights and integrated electronics presented here enable a new paradigm of continuous vasoconstriction monitoring with potentially significant medical applications. In addition, the fundamental knowledge gained from this work is broadly applicable to monitoring bio-signals from non-traditional anatomical regions, like the sternum.

Acknowledgements (Optional): : This work was supported by the Georgia Tech IEN Center for Human-Centric Interfaces and Engineering. Electronic devices in this work were fabricated at the Institute for Electronics and Nanotechnology, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the NSF (grant ECCS-2025462). The study involved healthy and symptomatic subjects. For healthy control subjects, the study was conducted by following the Georgia Tech approved IRB protocol (#H20211)

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