Steven Mills

Low-Power Wearable Systems for Continuous Monitoring of Environment and Health for Chronic Respiratory Disease.

We present our efforts toward enabling a wearable sensor system that allows for the correlation of individual environmental exposures with 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 toward 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 submilliwatt system ultimately powered by the energy harvested from thermal radiation and motion of the body with the primary contributions being an ultralow-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 mW, respectively. The data from each sensor are continually streamed to a peripheral data aggregation device and are 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 submilliwatt range.

Ultra-low power sensing platform for personal health and personal environmental monitoring

The vision of the NSF Center on Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST) is to develop nano-enabled technologies to achieve a paradigm shift towards long-term health and wellness management. To achieve this, the center is building self-powered, wearable and multimodal sensing systems for correlation of environmental exposures to physiological parameters. This paper presents the latest advances in environmental and personal health sensors that have ultra-low power consumption and are highly selective and sensitive to enable real time, continuous, and wearable platforms.

On Using the Volatile Mem-Capacitive Effect of TiO 2 Resistive Random Access Memory to Mimic the Synaptic Forgetting Process

In this work, we report on mimicking the synaptic forgetting process using the volatile mem-capacitive effect of a resistive random access memory (RRAM). TiO2 dielectric, which is known to show volatile memory operations due to migration of inherent oxygen vacancies, was used to achieve the volatile mem-capacitive effect. By placing the volatile RRAM candidate along with SiO2 at the gate of a MOS capacitor, a volatile capacitance change resembling the forgetting nature of a human brain is demonstrated. Furthermore, the memory operation in the MOS capacitor does not require a current flow through the gate dielectric indicating the feasibility of obtaining low power memory operations. Thus, the mem-capacitive effect of volatile RRAM candidates can be attractive to the future neuromorphic systems for implementing the forgetting process of a human brain.

Room temperature sensing of VOCs by atomic layer deposition of metal oxide

This work demonstrates room temperature sensing of volatile organic compound (VOC) — acetone via an ultrathin film metal oxide sensing layer. Atomic layer deposition (ALD) enables a high quality ultrathin film with precise thickness control. The 14nm ultrathin SnO2 thin film was deposited by ALD resulting in VOCs sensing at room temperature. The ultra-low power consumption (less than 50nW) and the room temperature operation of these devices make them compatible with wearable devices for real-time health and environment monitoring.

Metal oxide gas sensing characterization by low frequency noise spectroscopy

This work demonstrates a new method for selective identification of low ppb concentrations of O3. Atomic layer deposited thin film SnO2 was used as a sensing layer. SnO2 sensitized quartz crystal microbalances (QCM) demonstrate expected mass loading behavior as well as unique frequency domain response towards synthetic air, O3, and NO2 at room temperature. Power spectral densities (PSD) of the response of each gas were calculated and contain peaks at different normalized frequencies. These PSD peaks are found to have significant differences in magnitude for each analyte and provide evidence of selective room temperature adsorption of gases on SnO2.

Atomic Layer Deposited TiO 2 thin films for environmental gas sensing

Functional nanostructured metal oxide thin films produced by Atomic Layer Deposition (ALD) are promising for environmental gas sensing. NO2 sensing is particularly interesting as short term exposure has been shown to cause respiratory problems including asthma attacks. The film thickness control and conformal coating offered by ALD enable tailoring of film thickness to the Debye length for enhancing depletion region effects and high surface to volume ratios for enhanced sensitivity. Exploration of annealing conditions to control surface morphology, crystallinity and oxygen vacancy density will enable highly optimized sensing films for fast and reliable sensors. Here we have characterized our ALD TiO2 process and for the first time, demonstrated the sensing ability of ALD TiO2 films with thicknesses on the order of the Debye length toward NO2 in the low ppm range.