Wearable medical sensors that can monitor biosignals have the potential to transform healthcare - they encourage healthy living by providing individuals feedback on personal vital signs and enable facile implementation of both in-hospital and in-home health monitoring. To date, fabrication of wearable medical sensors heavily relies on conventional semiconductor vacuum-processing, which is expensive and has limited large-area scalability. Taking advantage of the unique manufacturing capabilities of printed electronics, we can now design wearables that are soft, lightweight, and skin-like. These soft and conformable sensors significantly improve the signal-to-noise ratio (SNR) by establishing high-fidelity sensor-skin interfaces. This thesis presents a review of existing literature on flexible and wearable health monitoring devices, discusses different printing techniques for fabricating wearable medical sensors, and highlights two sensing modalities: bioelectronic and biophotonic. For bioelectronic sensing, the design and fabrication of flexible and inkjet-printed gold electrode arrays are demonstrated, which are implemented in a smart bandage for early-detection of pressure ulcers. Also, the efficacy of the electrodes is demonstrated on conformal surfaces and on the skin to record electrocardiography (ECG) and electromyography (EMG) signals. For biophotonic sensing, an all-organic optoelectronic sensor is demonstrated for transmission-mode pulse oximetry, which accurately measures pulse rate and oxygenation. Since transmission-mode pulse oximetry can only be performed at the extremities of the body and requires a pulsatile arterial blood signal - printed, organic, reflection-mode oximeters are reported. The design, sensing methodology, and fabrication of a flexible and printed sensor array composed of organic light-emitting diodes (OLEDs) and organic photodiodes (OPDs) are shown, which senses reflected light from tissue to determine the oxygen saturation. The sensor is implemented to measure oxygen saturation on different parts of the body and to create 2D oxygenation maps of adult forearms under pressure-cuff-induced ischemia. Finally, a key enabling technology for wearables - flexible hybrid electronics (FHE) is presented. The implementation of FHE in an integrated multi-sensor platform is discussed, where soft sensors are interfaced with hard silicon-based integrated circuits for wearable health monitoring.




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