We present our efforts towards enabling a wearable sensor system that allows for the correlation of individual environmental exposures to physiologic and subsequent adverse health responses. This system will permit a better understanding of the impact of increased ozone levels and other pollutants on chronic asthma conditions. We discuss the inefficiency of existing commercial off-the-shelf components to achieve continuous monitoring and our system-level and nano-enabled efforts towards improving the wearability and power consumption. Our system consists of a wristband, a chest patch, and a handheld spirometer. We describe our preliminary efforts to achieve a sub-milliwatt system ultimately powered by the energy harvested from thermal radiation and motion of the body with the primary contributions being an ultra-low power ozone sensor, an volatile organic compounds sensor, spirometer, and the integration of these and other sensors in a multimodal sensing platform. The measured environmental parameters include ambient ozone concentration, temperature, and relative humidity. Our array of sensors also assesses heart rate via photoplethysmography and electrocardiography, respiratory rate via photoplethysmography, skin impedance, three-axis acceleration, wheezing via a microphone, and expiratory airflow. The sensors on the wristband, chest patch, and spirometer consume 0.83, 0.96, and 0.01 milliwatts respectively. The data from each sensor is continually streamed to a peripheral data aggregation device and is subsequently transferred to a dedicated server for cloud storage. Future work includes reducing the power consumption of the system-on-chip including radio to reduce the entirety of each described system in the sub-milliwatt range.
This work demonstrates ultra-low power ozone sensors for real time, continuous, and portable monitoring. Atomic Layer Deposition (ALD) of SnO 2 enables precise control of ultrathin film thickness on the order of the Debye length to enhance sensitivity at room temperature. Correlation between ozone concentration and the rate of resistance change is used to maintain fast response times and ultraviolet (UV) illumination hastens recovery. ALD SnO 2 ultrathin film sensors realize room temperature operation with highly selective detection of 50 ppb ozone with average power consumption of 150 μW making them well suited for real time, portable environmental monitoring systems. High levels of ozone (O 3 ) have been shown to contribute to respiratory symptoms such as chronic cough, wheeze, and shortness of breath and chest colds with phlegm.1 Individuals suffering with respiratory diseases such as asthma are particularly sensitive to O 3 which can trigger an asthma attack hours after exposure.2 According to the Environmental Protection Agency, O 3 concentrations are higher near urban areas, highways and in general outside on hot sunny days highlighting the need for continuous, portable, real time monitoring of an individual's exposure levels in order to correlate personal health with surrounding environments. Sensors used for these applications must have high sensitivity, appropriate selectivity against other gases, low total power consumption, stability, accuracy, reliability and low cost. Among several sensing materials, SnO 2 has been shown to be sensitive toward gases in a variety of papers however metal oxide sensors typically require high operating temperatures to achieve good sensitivity and fast recovery resulting in mW of power consumption making them unsuitable for portable monitoring.3-6 Several reports exist utilizing metal oxides at room temperature but they typically display response and recovery times greater than 10 minutes which also renders them unsuitable for real time monitoring. [7][8][9] Our work utilizes Atomic Layer Deposition of SnO 2 to tailor film thickness to twice the Debye length which has been shown to provide maximum sensitivity due to electron mobility modulation. [10][11][12] We correlate the derivative of the response to ozone concentration and utilize ultraviolet light to compensate for slow response and recovery times, respectively. These methods enable detection of 10s of ppb of ozone with room temperature operation for average power consumption of just 150 μW. Additionally, our sensors demonstrate selectivity over interfering gases NO 2 and CO at typical atmospheric levels making them good candidates for continuous, portable monitoring. ExperimentalThe fabrication of sensor device started with 500nm thermal oxidation of Si (100) substrate for electrical isolation. After oxidation, the SnO 2 sensing material was deposited in an ALD system (Cambridge Nanotech Savannah 100 model). Tetrakis(dimethylamino)tin precursor and O 3 reactants were used to deposit SnO 2 at 200• C. The films were...
Ultra-low power room temperature NO 2 sensors are demonstrated using AlGaN/GaN. The chemically stable semiconductor was sensitized to increase the sensitivity to enable ultra-low power, low ppb level detection without additional heaters. Sensors were sensitized by two methods, ultra-thin ALD SnO 2 and surface enhancement by ICP-RIE in BCl 3 gas. Both sensitization techniques demonstrate room temperature response, while the unsensitized sensors did not respond. At room temperature, surface enhanced sensors show a significant increase in sensitivity compared to SnO 2 sensitized sensors. The Environmental Protection Agency has identified Nitrogen Oxides (NO x ) as one of 6 major air pollutants for health concern.1 This means people with existing respiratory conditions may have severe reactions requiring hospitalization. Even more concerning is the variability in concentrations of these pollutants. NO x varies quite heavily outdoors due to combustion sources, such as near highways and high traffic cities.2,3 These effects create the need for wearable, low power, continuous environmental monitoring systems for correlating health effects. Current state of the art NO 2 sensors are not suited for wearable platforms due to high power, slow response, and maintenance constraints.4 Gallium Nitride (GaN) has been identified as a promising sensing material due to its chemically stable surface.5 AlGaN/GaN heterojunctions have been shown to have good sensitivity to H 2 gas due to the exploitation of surface state defects that allow for modulation of the high mobility 2-Dimensional Election Gas (2DEG), but generally require sensor heating to obtain high sensitivity. [5][6][7] In this paper, AlGaN/GaN heterostructures were studied to evaluate low power NO 2 sensors will be explored and optimized to obtain ultra-low power, low noise, room temperature (RT = 20• C) sensing response by surface sensitization. ExperimentalAlGaN/GaN substrates were first patterned for device to device isolation using conventional photolithography and dry ICP-RIE etching in BCl 3 gas. Following isolation, interdigitated electrodes were defined using photolithography and lift off with a metal stack Ti/Al/Ni/Ti/Au (20 nm/100 nm/20 nm/5 nm/100 nm). The sensors received a rapid thermal anneal at 850• C for 30s in Nitrogen environment to obtain Ohmic contact to the 2DEG. Finally, the surface was sensitized by two approaches. The first approach utilizes ultra-thin SnO 2 (7 nm) by Atomic Layer Deposition (ALD) at 200• C, whereas the second approach is enhancement of adsorption sites at the surface of the GaN layer by Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE) in BCl 3 . The optical image and cross-sectional schematic of interdigitated electrodes are shown in Fig. 1.Sensor testing was performed in a custom-built stainless steel chamber equipped with a borosilicate chuck for high temperature testing, as well as ultra-violet (UV) LEDs for sensor recovery. Using the NIST-certified Teledyne T700U gas calibrator, NO 2 concentrations of 50-500 ppb were ...
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