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Open Access Publications from the University of California

Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2018)

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.

    Cover page: Wearable Medical Sensors Enabled by Printed Bioelectronics and Biophotonics
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2016)
    This paper reports on the design and operation of a flexible power source integrating a lithium ion battery and amorphous silicon solar module, optimized to supply power to a wearable health monitoring device. The battery consists of printed anode and cathode layers based on graphite and lithium cobalt oxide, respectively, on thin flexible current collectors. It displays energy density of 6.98 mWh/cm(2) and demonstrates capacity retention of 90% at 3C discharge rate and ~99% under 100 charge/discharge cycles and 600 cycles of mechanical flexing. A solar module with appropriate voltage and dimensions is used to charge the battery under both full sun and indoor illumination conditions, and the addition of the solar module is shown to extend the battery lifetime between charging cycles while powering a load. Furthermore, we show that by selecting the appropriate load duty cycle, the average load current can be matched to the solar module current and the battery can be maintained at a constant state of charge. Finally, the battery is used to power a pulse oximeter, demonstrating its effectiveness as a power source for wearable medical devices.
      Cover page: High-performance flexible energy storage and harvesting system for wearable electronics
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2020)
      Human skin perceives external mechanical stimuli by sensing the variation in the membrane potential of skin sensory cells. Many scientists have attempted to recreate skin functions and develop electronic skins (e-skins) based on active and passive sensing mechanisms. Inspired by the skin sensory behavior, we investigated materials and electronic devices that allow us to encode mechanical stimuli into potential differences measured between two electrodes, resulting in a potentiometric mechanotransduction mechanism. We present here a potentiometric mechanotransducer that is fabricated through an all-solution processing approach. This mechanotransducer shows ultralow-power consumption, highly tunable sensing behavior, and capability to detect both static and low-frequency dynamic mechanical stimuli. Furthermore, we developed two novel classes of sensing devices, including strain-insensitive sensors and single-electrode-mode e-skins, which are challenging to achieve using the existing methods. This mechanotransduction mechanism has broad impact on robotics, prosthetics, and health care by providing a much improved human-machine interface.
        Cover page: A potentiometric mechanotransduction mechanism for novel electronic skins
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2017)
        A method to print two materials of different functionality during the same printing step is presented. In printed electronics, devices are built layer by layer and conventionally only one type of material is deposited in one pass. Here, the challenges involving printing of two emissive materials to form polymer light-emitting diodes (PLEDs) that emit light of different wavelengths without any significant changes in the device characteristics are described. The surface-energy-patterning technique is utilized to print materials in regions of interest. This technique proves beneficial in reducing the amount of ink used during blade coating and improving the reproducibility of printed films. A variety of colors (green, red, and near-infrared) are demonstrated and characterized. This is the first known attempt to print multiple materials by blade coating. These devices are further used in conjunction with a commercially available photodiode to perform blood oxygenation measurements on the wrist, where common accessories are worn. Prior to actual application, the threshold conditions for each color are discussed, in order to acquire a stable and reproducible photoplethysmogram (PPG) signal. Finally, based on the conditions, PPG and oxygenation measurements are successfully performed on the wrist with green and red PLEDs.
          Cover page: Flexible Blade‐Coated Multicolor Polymer Light‐Emitting Diodes for Optoelectronic Sensors
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2018)
          Transmission-mode pulse oximetry, the optical method for determining oxygen saturation in blood, is limited to only tissues that can be transilluminated, such as the earlobes and the fingers. The existing sensor configuration provides only single-point measurements, lacking 2D oxygenation mapping capability. Here, we demonstrate a flexible and printed sensor array composed of organic light-emitting diodes and organic photodiodes, which senses reflected light from tissue to determine the oxygen saturation. We use the reflectance oximeter array beyond the conventional sensing locations. The sensor is implemented to measure oxygen saturation on the forehead with 1.1% mean error and to create 2D oxygenation maps of adult forearms under pressure-cuff-induced ischemia. In addition, we present mathematical models to determine oxygenation in the presence and absence of a pulsatile arterial blood signal. The mechanical flexibility, 2D oxygenation mapping capability, and the ability to place the sensor in various locations make the reflectance oximeter array promising for medical sensing applications such as monitoring of real-time chronic medical conditions as well as postsurgery recovery management of tissues, organs, and wounds.
            Cover page: A flexible organic reflectance oximeter array
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