Hybrid GMR Sensor Detecting 950 pT/sqrt(Hz) at 1 Hz and Room Temperature.

Advances in the magnetic sensing technology have been driven by the increasing demand for the capability of measuring ultrasensitive magnetic fields. Among other emerging applications, the detection of magnetic fields in the picotesla range is crucial for biomedical applications. In this work Picosense reports a millimeter-scale, low-power hybrid magnetoresistive-piezoelectric magnetometer with subnanotesla sensitivity at low frequency. Through an innovative noise-cancelation mechanism, the 1/f noise in the MR sensors is surpassed by the mechanical modulation of the external magnetic fields in the high frequency regime. A modulation efficiency of 13% was obtained enabling a final device's sensitivity of ~950 pT/Hz1/2 at 1 Hz. This hybrid device proved to be capable of measuring biomagnetic signals generated in the heart in an unshielded environment. This result paves the way for the development of a portable, contactless, low-cost and low-power magnetocardiography device.

Faster-than-anticipated Na(+)/Cl(-) diffusion across lipid bilayers in vesicles.

Maintenance of electrochemical potential gradients across lipid membranes is critical for signal transduction and energy generation in biological systems. However, because ions with widely varying membrane permeabilities all contribute to the electrostatic potential, it can be difficult to measure the influence of diffusion of a single ion type across the bilayer. To understand the electrodiffusion of H(+) across lipid bilayers, we used a pH-sensitive fluorophore to monitor the lumenal pH in vesicles after a stepwise change in the bulk pH. In vesicles containing the ion channel gramicidin, the lumenal pH rapidly approached the external pH. In contrast, the lumen of intact vesicles showed a two stage pH response: an initial rapid change occurred over ~1min, followed by a much slower change over ~24h. We provide a quantitative interpretation of these results based on the Goldman-Hodgkin-Katz ion fluxes discharging the electrical capacitance of the bilayer membrane. This interpretation provides an estimate of the permeability of the membranes to Na(+) and Cl(-) ions of ~10(-8)cm/s, which is ~3 orders of magnitude faster than previous reports. We discuss possible mechanisms to account for this considerably higher permeability in vesicle membranes.

A 32 × 32 optical phased array using polysilicon sub-wavelength high-contrast-grating mirrors.

We report on microelectromechanical systems (MEMS)-actuated 32 × 32 optical phased arrays (OPAs) with high fill-factors and microsecond response time. To reduce the mirror weight and temperature-dependent curvature, we use high-contrast-grating (HCG) mirrors comprising a single layer of sub-wavelength polysilicon gratings with 400 nm thickness, 1250 nm pitch, and 570 nm grating bar width. The mirror has a broad reflection band and a peak reflectivity of 99.9% at 1550 nm wavelength. With 20 × 20 µm2 pixels and 2 µm, the OPA has a total aperture of 702 × 702 µm2 and a fill factor of 85%. The OPA is electrostatically controlled by voltage and has a total field of view of ± 2°, an instantaneous field of view (beam width) of 0.14°, and a response time of 3.8 µs. The latter agrees well with the mechanical resonance frequency of the HCG mirror (0.42 MHz).

High speed optical phased array using high contrast grating all-pass filters.

We report a high speed 8x8 optical phased array using tunable 1550 nm all-pass filters with ultrathin high contrast gratings (HCGs) as the microelectromechanical-actuated top reflectors. The all-pass filter design enables a highly efficient phase tuning (1.7 π) with a small actuation voltage (10 V) and actuation displacement of the HCG (50 nm). The microelectromechanical HCG structure facilitates a high phase tuning speed >0.5 MHz. Beam steering is experimentally demonstrated with the optical phased array.

Optical beamsteering using an 8 × 8 MEMS phased array with closed-loop interferometric phase control.

We present a high-speed optical beamsteering system based on an 8x8 MEMS phased array. The system incorporates an in situ interferometer that provides a real-time, dynamic measure of the phase of each mirror in the array during beamsteering. A closed-loop phase-control algorithm results in <π/100 mirror phase accuracy and far field beam steering is shown. Stroboscopic measurement capabilities are demonstrated which allow us to show feedforward control to eliminate micromirror ringing.

Optical phased array using high contrast gratings for two dimensional beamforming and beamsteering.

We have developed a microelectromechanical system (MEMS) optical phased array incorporating a high-index-contrast subwavelength grating (HCG) for beamforming and beamsteering in a range of ± 1.26° × 1.26°. Our approach needs only a thin single-layer HCG made of silicon, considerably improving its speed thanks to the low mass, and is suitable for high optical power applications. The measured resonant frequency of HCG is 0.32 MHz.

Nanopore-spanning lipid bilayers on silicon nitride membranes that seal and selectively transport ions.

We report the formation of POPC lipid bilayers that span 130 nm pores in a freestanding silicon nitride film supported on a silicon substrate. These solvent-free lipid membranes self-assemble on organosilane-treated Si3N4 via the fusion of 200 nm unilamellar vesicles. Membrane fluidity is verified by fluorescence recovery after photobleaching (FRAP), and membrane resistance in excess of 1 GΩ is demonstrated using electrical impedance spectroscopy (EIS). An array of 40,000 membranes maintained high impedance over 72 h, followed by rupture of most of the membranes by 82 h. Membrane incorporation of gramicidin, a model ion channel, resulted in increased membrane conductance. This membrane conductance was diminished when the gramicidin channels were blocked with CaCl2, indicating that the change in membrane conductance results from gramicidin-mediated ion transport. These very stable, biologically functional pore-spanning membranes open many possibilities for silicon-based ion-channel devices for applications such as biosensors and high-throughput drug screening.

High-SPL Air-Coupled Piezoelectric Micromachined Ultrasonic Transducers Based on 36% ScAlN Thin-Film.

In this paper, air-coupled piezoelectric micromachined ultrasonic transducers (PMUTs) using 36% scandium-doped aluminum nitride (ScAlN) thin-film are presented. ScAlN is known to exhibit higher piezoelectric properties compared to pure AlN leading to significant performance improvements in various piezoelectric micro-electromechanical systems (MEMS) applications including PMUTs. Here, the concentration of Sc in the actual sputtered 1- [Formula: see text]-thick ScAlN film was 36%, which is slightly below the maximum at the phase boundary. The ScAlN PMUTs were fabricated from an SOI wafer, where the dry etching of ScAlN film was optimized. The frequency response and displacement sensitivity of the PMUTs were characterized in the air using laser Doppler vibrometry confirming 2× higher transverse piezoelectric coefficient than AlN. The acoustic transmission and reception of the PMUTs were evaluated from a high-sensitivity microphone and pulse-echo measurements. The PMUTs were designed to operate below 100 kHz in order to mitigate the absorption loss, which resulted in a high transmit pressure of 105-dB sound pressure level (SPL) at 10 cm and only 30-dB attenuation at the 2-m range. Through implementing 36% ScAlN film, the presented PMUTs exhibited a large displacement and consequently, a high SPL compared to the state-of-the-art PMUTs and the bulk transducer considering the size and the excitation voltage.

Microfluidic mixers for the investigation of rapid protein folding kinetics using synchrotron radiation circular dichroism spectroscopy.

We have developed a microfluidic mixer optimized for rapid measurements of protein folding kinetics using synchrotron radiation circular dichroism (SRCD) spectroscopy. The combination of fabrication in fused silica and synchrotron radiation allows measurements at wavelengths below 220 nm, the typical limit of commercial instrumentation. At these wavelengths, the discrimination between the different types of protein secondary structure increases sharply. The device was optimized for rapid mixing at moderate sample consumption by employing a serpentine channel design, resulting in a dead time of less than 200 micros. Here, we discuss the design and fabrication of the mixer and quantify the mixing efficiency using wide-field and confocal epi-fluorescence microscopy. We demonstrate the performance of the device in SRCD measurements of the folding kinetics of cytochrome c, a small, fast-folding protein. Our results show that the combination of SRCD with microfluidic mixing opens new possibilities for investigating rapid conformational changes in biological macromolecules that have previously been inaccessible.

A MEMS light modulator based on diffractive nanohole gratings.

We present the design, fabrication, and testing of a microelectromechanical systems (MEMS) light modulator based on pixels patterned with periodic nanohole arrays. Flexure-suspended silicon pixels are patterned with a two dimensional array of 150 nm diameter nanoholes using nanoimprint lithography. A top glass plate assembled above the pixel array is used to provide a counter electrode for electrostatic actuation. The nanohole pattern is designed so that normally-incident light is coupled into an in-plane grating resonance, resulting in an optical stop-band at a desired wavelength. When the pixel is switched into contact with the top plate, the pixel becomes highly reflective. A 3:1 contrast ratio at the resonant wavelength is demonstrated for gratings patterned on bulk Si substrates. The switching time is 0.08 ms and the switching voltage is less than 15V.

Frequency-domain birefringence measurement of biological binding to magnetic nanoparticles.

Optical detection of the frequency-dependent magnetic relaxation signal is used to monitor the binding of biological molecules to magnetic nanoparticles in a ferrofluid. Biological binding reactions cause changes in the magnetic relaxation signal due to an increase in the average hydrodynamic diameter of the nanoparticles. To allow the relaxation signal to be detected in dilute ferrofluids, measurements are made using a balanced photodetector, resulting in a 25 µV/√Hz noise floor, within 50% of the theoretical limit imposed by photon shot noise. Measurements of a ferrofluid composed of magnetite nanoparticles coated with anti-IgG antibodies show that the average hydrodynamic diameter increases from 115.2 to 125.4 nm after reaction with IgG.

Characterization of a bimorph deformable mirror using stroboscopic phase-shifting interferometry.

The static and dynamic characteristics of a bimorph deformable mirror (DM) for use in an adaptive optics system are described. The DM is a 35-actuator device composed of two disks of lead magnesium niobate (PMN), an electrostrictive ceramic that produces a mechanical strain in response to an imposed electric field. A custom stroboscopic phase-shifting interferometer was developed to measure the deformation of the mirror in response to applied voltage. The ability of the mirror to replicate optical aberrations described by the Zernike polynomials was tested as a measure of the mirror's static performance. The natural frequencies of the DM were measured up to 20 kHz using both stroboscopic interferometry as well as a commercial laser Doppler vibrometer (LDV). Interferometric measurements of the DM surface profile were analyzed by fitting the surface with mode-shapes predicted using classical plate theory for an elastically supported disk. The measured natural frequencies were found to be in good agreement with the predictions of the theoretical model.

Ultrasonic Fingerprint Sensor With Transmit Beamforming Based on a PMUT Array Bonded to CMOS Circuitry.

In this paper, we present a single-chip 65 ×42 element ultrasonic pulse-echo fingerprint sensor with transmit (TX) beamforming based on piezoelectric micromachined ultrasonic transducers directly bonded to a CMOS readout application-specific integrated circuit (ASIC). The readout ASIC was realized in a standard 180-nm CMOS process with a 24-V high-voltage transistor option. Pulse-echo measurements are performed column-by-column in sequence using either one column or five columns to TX the ultrasonic pulse at 20 MHz. TX beamforming is used to focus the ultrasonic beam at the imaging plane where the finger is located, increasing the ultrasonic pressure and narrowing the 3-dB beamwidth to [Formula: see text], a factor of 6.4 narrower than nonbeamformed measurements. The surface of the sensor is coated with a poly-dimethylsiloxane (PDMS) layer to provide good acoustic impedance matching to skin. Scanning laser Doppler vibrometry of the PDMS surface was used to map the ultrasonic pressure field at the imaging surface, demonstrating the expected increase in pressure, and reduction in beamwidth. Imaging experiments were conducted using both PDMS phantoms and real fingerprints. The average image contrast is increased by a factor of 1.5 when beamforming is used.

Monolithic ultrasound fingerprint sensor.

This paper presents a 591×438-DPI ultrasonic fingerprint sensor. The sensor is based on a piezoelectric micromachined ultrasonic transducer (PMUT) array that is bonded at wafer-level to complementary metal oxide semiconductor (CMOS) signal processing electronics to produce a pulse-echo ultrasonic imager on a chip. To meet the 500-DPI standard for consumer fingerprint sensors, the PMUT pitch was reduced by approximately a factor of two relative to an earlier design. We conducted a systematic design study of the individual PMUT and array to achieve this scaling while maintaining a high fill-factor. The resulting 110×56-PMUT array, composed of 30×43-µm2 rectangular PMUTs, achieved a 51.7% fill-factor, three times greater than that of the previous design. Together with the custom CMOS ASIC, the sensor achieves 2 mV kPa-1 sensitivity, 15 kPa pressure output, 75 µm lateral resolution, and 150 µm axial resolution in a 4.6 mm×3.2 mm image. To the best of our knowledge, we have demonstrated the first MEMS ultrasonic fingerprint sensor capable of imaging epidermis and sub-surface layer fingerprints.

Self-induced parametric amplification arising from nonlinear elastic coupling in a micromechanical resonating disk gyroscope.

Parametric amplification, resulting from intentionally varying a parameter in a resonator at twice its resonant frequency, has been successfully employed to increase the sensitivity of many micro- and nano-scale sensors. Here, we introduce the concept of self-induced parametric amplification, which arises naturally from nonlinear elastic coupling between the degenerate vibration modes in a micromechanical disk-resonator, and is not externally applied. The device functions as a gyroscope wherein angular rotation is detected from Coriolis coupling of elastic vibration energy from a driven vibration mode into a second degenerate sensing mode. While nonlinear elasticity in silicon resonators is extremely weak, in this high quality-factor device, ppm-level nonlinear elastic effects result in an order-of-magnitude increase in the observed sensitivity to Coriolis force relative to linear theory. Perfect degeneracy of the primary and secondary vibration modes is achieved through electrostatic frequency tuning, which also enables the phase and frequency of the parametric coupling to be varied, and we show that the resulting phase and frequency dependence of the amplification follow the theory of parametric resonance. We expect that this phenomenon will be useful for both fundamental studies of dynamic systems with low dissipation and for increasing signal-to-noise ratio in practical applications such as gyroscopes.

Magnetic scanometric DNA microarray detection of methyl tertiary butyl ether degrading bacteria for environmental monitoring.

A magnetoresistive biosensing platform based on a single magnetic tunnel junction (MTJ) scanning probe and DNA microarrays labeled with magnetic particles has been developed to provide an inexpensive, sensitive and reliable detection of DNA. The biosensing platform was demonstrated on a DNA microarray assay for quantifying bacteria capable of degrading methyl tertiary butyl ether (MTBE), where concentrations as low as 10 pM were detectable. Synthetic probe bacterial DNA was immobilized on a microarray glass slide surface, hybridized with the 48 base pair long biotinylated target DNA and subsequently incubated with streptavidin-coated 2.8 µm diameter magnetic particles. The biosensing platform then makes use of a micron-sized MTJ sensor that was raster scanned across a 3 mm by 5 mm glass slide area to capture the stray magnetic field from the tagged DNA and extract two dimensional magnetic field images of the microarray. The magnetic field output is then averaged over each 100 µm diameter DNA array spot to extract the magnetic spot intensity, analogous to the fluorescence spot intensity used in conventional optical scanners. The magnetic scanning result is compared with results from a commercial laser scanner and particle coverage optical counting to demonstrate the dynamic range and linear sensitivity of the biosensing platform as a potentially inexpensive, sensitive and portable alternative for DNA microarray detection for field applications.

Mapping protein collapse with single-molecule fluorescence and kinetic synchrotron radiation circular dichroism spectroscopy.

We have used the combination of single-molecule Förster resonance energy transfer and kinetic synchrotron radiation circular dichroism experiments to probe the conformational ensemble of the collapsed unfolded state of the small cold shock protein CspTm under near-native conditions. This regime is physiologically most relevant but difficult to access experimentally, because the equilibrium signal in ensemble experiments is dominated by folded molecules. Here, we avoid this problem in two ways. One is the use of single-molecule Förster resonance energy transfer, which allows the separation of folded and unfolded subpopulations at equilibrium and provides information on long-range intramolecular distance distributions. From experiments with donor and acceptor chromophores placed at different positions within the chain, we find that the distance distributions in unfolded CspTm agree surprisingly well with a Gaussian chain not only at high concentrations of denaturant, where the polypeptide chain is expanded, but also at low denaturant concentrations, where the chain is collapsed. The second, complementary approach is synchrotron radiation circular dichroism spectroscopy of collapsed unfolded molecules transiently populated with a microfluidic device that enables rapid mixing. The results indicate a beta-structure content of the collapsed unfolded state of approximately 20% compared with the folded protein. This suggests that collapse can induce secondary structure in an unfolded state without interfering with long-range distance distributions characteristic of a random coil, which were previously found only for highly expanded unfolded proteins.