Masking of Korotkoff sounds used in blood pressure measurement through auscultation.

Measurement of blood pressure (BP) through manual auscultation and the observation of Korotkoff sounds (KSs) remains the gold standard in BP methodology. Critical to determining BP levels via auscultation is the determination of KS audibility. While absolute sound level audibility is well researched, the problem has not been approached from the point of view of psychoacoustic masking of the sounds. Here, during manual auscultation of BP, a direct comparison is made between what an observer perceives as audible and the electronic analysis of audibility level determined from masking of sound signal levels. KSs are collected during auscultation with an electronic stethoscope, which allows simultaneously observing sound audibility and recording the sound electronically. By time-segmenting the recorded sound around Korotkoff peaks into a test segment and a masking segment, performing Fourier transforms on the segments, and comparing frequency-band sound energy levels, signal-to-noise ratios of a sound to its masking counterpart can be defined. Comparing these ratios to difference limen in the psychoacoustic masking literature, an approximate threshold for sound audibility is obtained. It is anticipated that this approach could have profound effects on future development of automated auscultation BP measurements.

Optimum waveform envelopes and amplitude ratios in oscillometric blood pressure estimation.

OBJECTIVE: To determine if, when using the oscillometric method, there is a specific range of amplitude ratios in the fixed-ratio algorithm that will result in blood pressure estimates that consistently fall within a mean error ≤5 mmHg and a SD of the error <8 mmHg. Additionally, to apply different representations of the oscillometric waveform envelope to verify if this will affect the accuracy of the results. METHODS: SBP and DBP were obtained using the fixed-ratios method applied to a dataset of 219 oscillometric measurements obtained from 73 healthy volunteers and compared to their corresponding auscultation values. Ratio and envelope analysis were done on Matlab (The MathWorks, Inc., Natick, Massachusetts, USA). RESULTS: Depending on the envelope representation, ratios between 0.44-0.74 for systolic pressure and 0.51-0.85 for diastolic pressure yield results within the limits mentioned above. When a set of optimum envelope representations and ratios are selected based on population mean, the highest percentage of subjects presenting blood pressure estimates within the limits were 72.6% for systolic and 69.9% for diastolic. CONCLUSION: The range of ratios presenting optimum results appears to be independent of the degree of arterial stiffness given the wide range of ages of the subjects in the study. Different representations of the oscillometric waveform envelope may improve the accuracy of the method. However, there remains a considerable percentage of the population with unreliable results. It is therefore important to only use devices that have been properly validated according to standard protocol.

Efficient actuation design for optomechanical sensors.

For any nanomechanical device intended for sensing applications, actuation is an important consideration. Many different actuation mechanisms have been used, including self-oscillation, piezoelectric shakers, capacitive excitation, and optically pumping via the optical gradient force. Despite the relatively frequent use of optical pumping, the limits of optical actuation with a pump laser have not been fully explored. We provide a practical framework for designing optical cavities and optomechanical systems to maximize the efficiency of the optical pumping technique. The effects of coherent backscattering on detection and actuation are included. We verify our results experimentally and show good agreement between the model and experiment. Our model for efficient actuation will be a useful resource for the future design of optomechanical cavities for sensor and other high-amplitude applications.

Porous Nanophotonic Optomechanical Beams for Enhanced Mass Adsorption.

We have developed a porous silicon nanocantilever for a nano-optomechanical system (NOMS) with a universal sensing surface for enhanced sensitivity. Using electron beam lithography, we selectively applied a V2O5/HF stain etch to the mechanical elements while protecting the silicon-on-insulator photonic ring resonators. This simple, rapid, and electrodeless approach generates tunable device porosity simultaneously with the mechanical release step. By controlling the porous etchant concentration and etch time, the porous etch depth, resonant frequency, and the adsorption surface area could be precisely manipulated. Using this control, cantilever sensors ranging from nonporous to fully porous were fabricated and tested as gas-phase mass sensors of volatile organic compounds coming from a gas chromatography stream. The fully porous cantilever produced a dramatic 10-fold increase in sensing signal and a 6-fold improvement in limit of detection (LOD) compared to an otherwise identical nonporous cantilever. This signal improvement could be separated into mass responsivity increase and adsorption increase components. Allan deviation measurements indicate that a further 4-fold improvement in LOD could be expected upon speeding up characteristic peak response time from 1 s to 50 ms. These results show promise for performance enhancement in nanomechanical sensors for applications in gas sensing, gas chromatography, and mass spectrometry.

Improving mechanical sensor performance through larger damping.

Mechanical resonances are used in a wide variety of devices, from smartphone accelerometers to computer clocks and from wireless filters to atomic force microscopes. Frequency stability, a critical performance metric, is generally assumed to be tantamount to resonance quality factor (the inverse of the linewidth and of the damping). We show that the frequency stability of resonant nanomechanical sensors can be improved by lowering the quality factor. At high bandwidths, quality-factor reduction is completely mitigated by increases in signal-to-noise ratio. At low bandwidths, notably, increased damping leads to better stability and sensor resolution, with improvement proportional to damping. We confirm the findings by demonstrating temperature resolution of 60 microkelvin at 300-hertz bandwidth. These results open the door to high-performance ultrasensitive resonators in gaseous or liquid environments, single-cell nanocalorimetry, nanoscale gas chromatography, atmospheric-pressure nanoscale mass spectrometry, and new approaches in crystal oscillator stability.

Nano-Optomechanical Systems for Gas Chromatography.

Microgas chromatography (GC) is promising for portable chemical analysis. We demonstrate a nano-optomechanical system (NOMS) as an ultrasensitive mass detector in gas chromatography. Bare, native oxide, silicon surfaces are sensitive enough to monitor volatile organic compounds at ppm levels, while simultaneously demonstrating chemical selectivity. The NOMS is able to sense GC peaks from derivatized metabolites at physiological concentrations. This is an important milestone for small-molecule quantitation assays in next generation metabolite analyses for applications such as disease diagnosis and personalized medicine. The optical microring, which plays an important role in the nanomechanical signal transduction mechanism, can also be used as an analyte concentration sensor. Different adsorption kinetics regimes are realized at different temperatures allowing temporary condensation of the analyte onto the sensor surfaces. This effect amplifies the signal, resulting in a 1 ppb level limit of detection, without partition enhancement from absorbing media. This sensitivity bodes well for NOMS as universal, ultrasensitive detectors in micro-GC, breath analysis, and other chemical-sensing applications.

Wavelength-division multiplexing of nano-optomechanical doubly clamped beam systems.

Wavelength-division multiplexing is demonstrated for a set of two doubly clamped beams. Using a single input/output waveguide in a nanophotonic detection system, the two mechanical beams are independently addressable using different wavelength channels as determined by their respective racetrack resonator detection cavities. The two cavities slightly overlap, which also enables the mechanical frequency of both beams to be detected simultaneously with a single wavelength. Finally, to physically map which wavelength channel corresponds to which specific device, a heating laser is targeted individually on each beam to create a reversible mechanical frequency shift. This multiplexing method would allow for the simpler detection of large arrays of nanomechanical devices in a sensor system.

Nanoelectromechanical devices in a fluidic environment.

We present a comprehensive study of nanoelectromechanical systems in pressurized fluids. Resonant responses and quality factors are monitored in five different gases and one liquid, in pressures ranging from vacuum to 20 MPa, in order to evaluate theoretical models of device-fluid interactions at the nanoscale. The traditional Newell picture of microresonator damping in different pressure regimes is found to be inadequate in describing nanoresonators in general. Damping at intermediate pressure ranges is better physically characterized by a Weissenberg number (which compares oscillation frequencies with fluid relaxation rates) than a Knudsen number (which compares mean free paths with device widths) and most adequately described by the Yakhot and Colosqui model. At high-pressure ranges, two models are found to give good agreement with data: the phenomenological model of vibrating spheres and the Sader and Bhiladvala model for the viscous regime. The latter is also successful in explicitly predicting pressure-dependent behavior of the viscous mass load and damping. We observe significant increases in damping due to the squeezed film (SF) of gas between the device and substrate as well as due to undercut (an unavoidable artifact of the standard fabrication technique); correcting the shape of the devices with a focused ion beam allows us to differentiate these two factors. Application of the SF model accounts well for additional damping at high pressures while only qualitatively agreeing at lower pressures. The extensive data collected allow additional insight into fundamental processes underlying fluid damping at the nanoscale, particularly in the intermediate- and high-pressure regimes.

Pressurized fluid damping of nanoelectromechanical systems.

Interactions of nanoscale structures with fluids are of current interest both in the elucidation of fluid dynamics at these small scales, and in determining the ultimate performance of nanoelectromechanical systems outside of vacuum. We present a comprehensive study of nanomechanical damping in three gases (He, N2, CO2), and liquid CO2. Resonant dynamics in multiple devices of varying size and frequency is measured over 10 decades of pressure (1 mPa-20 MPa) using time-domain stroboscopic optical interferometry. The wide pressure range allows full exploration of the regions of validity of Newtonian and non-Newtonian flow damping models. Observing free molecular flow behavior extending above 1 atm, we find a fluid relaxation time model to be valid throughout, but not beyond, the non-Newtonian regime, and a Newtonian flow vibrating spheres model to be valid in the viscous limit.

A simple cell for the analysis of nanoelectromechanical systems under gas pressure.

A simple yet versatile apparatus for optical microscopy investigations of solid-state devices under high gas pressures is presented. Interchangeable high-grade sapphire windows with different thicknesses allow variable choice of trade-off between the maximum operating pressure and maximum spatial resolution. The capabilities of this compact chamber were tested by performing stroboscopic optical interferometry on nanoelectromechanical systems (NEMSs) under capacitive excitation. With a 1.7 mm thick sapphire window, the cell is safe to operate at pressures ranging from vacuum to 5 MPa. Minimal optical wavefront distortion allows NEMSs with linear dimensions of 0.1x1.6 microm(2) to be explored. For a sapphire window with a maximum thickness of 6 mm, the safe operating pressure increases up to an estimated 60 MPa; however, the increasing distortions inhibit signal from NEMSs smaller than approximately 0.5x1 microm(2). The cell can be used for confocal microscopy, microphotoluminescence and electroluminescence, light scattering spectroscopy, and reflectivity. The light weight and compact design of the chamber allow mounting on a precision piezomotion control stage or inside a volume tight apparatus such as cryostats.