D.L. Howard

Removable tubing interconnects for glass-based micro-fluidic systems made using ECDM

Reversible tubing connections for glass micro-fluidic systems are realized using electro-chemical discharge machining of three-dimensional glass vias. The connections reversibly connect standard sized plastic tubing to holes in borosilicate microscope slides. Tubing connections are demonstrated on a sealed, micro-fluidic channel which is fabricated between two glass slides using SU-8. The connections are experimentally tested to withstand up to 30 psi (~206 kPa) of air pressure without leaking.

A comparison of noninvasive objective and subjective measurements of the optical density of human ocular media

Background. It is important for both researchers and clinicians to know the spectral composition of the light that reaches the retina. With this in mind, Johnson et al. developed a quick, objective instrument called the Lens Absorption Monitor (LAM) to measure the attenuation of light by the ocular media. Using a more refined method of measuring and correcting for scatter, we compared the results of the LAM against those with a subjective method utilizing a bipartite scotopic brightness-matching procedure (BIP). Methods. We tested 41 healthy subjects (mean age, 35.9 years) with the LAM and BIP. Results. Intereye correlation coefficients for the LAM were 0.70 at 410 nm and 0.83 at both 430 and 450 nm; for BIP, intereye correlation coefficients were 0.56 at 410 nm, 0.42 at 430 nm, and 0.38 at 450 nm. For optical density as a function of age, there was no statistically significant difference between the two instruments at 410 nm in either the slope (p > 0.43) or y intercept (p > 0.75). However, at both 430 and 450 nm, there was a significant difference in both slope (p < 0.001) and y intercept (p < 0.05) for the two instruments. Conclusions. With the latest refinements, LAM density measures correlated well with BIP estimates at 410 nm, but not at 430 or 450 nm. This underscores the fact that the LAM measures the spectral density of the anterior segment, whereas the BIP method is a measure of all the ocular media except for macular pigment. The difference between these two measures is consistent with the existence of a prereceptoral, presumably intraretinal, pigment (or pigments) located outside the anatomic fovea that absorbs light at short wavelengths.

Micromachined, silicon filament light source for spectrophotometric microsystems.

A miniature broadband light source is a critical element in a spectrophotometric microsystem. The design, fabrication, and characterization of a highly stable, miniature broadband light source that comprises filaments of single-crystal silicon are presented. Electrical current versus voltage and radiant emittance spectra under constant voltage bias are measured and related to filament dimensions. A maximum stable operating temperature for these filaments is estimated to be 1200 K. Resistance drift is demonstrated to be less than 0.5% over a 10-h period of continuous operation with visible incandescence. Emittance spectra of a multifilament array, measured at three different electrical biases, are presented and shown to compare well with theoretical blackbody radiation spectra. A continuous, total radiated power of 10.7 mW was achieved with a 1 mm x 1 mm filament array with peak emittance at lambda=2.7 micrometers.

A single-fringe etalon silicon pressure transducer

This paper describes the development of an interferometric pressure transducer using a single etalon fringe as an intensity mode signal for a deflectable silicon diagram. The last fringe before the mechanical contact of the two etalon surfaces is employed, thus, providing an integral overpressure limit. The etalon design permits the illumination source to be only approximately monochromatic thereby allowing a conventional LED to be used in place of the laser diode normally required in interferometric designs and greatly simplifies the pressure sensor and instrumentation package.

Thermally actuated, bistable, oxide/silicon/metal membranes

Thermally actuated, bistable, snapping membranes have been fabricated on a silicon wafer and tested. Nonlinear finite element analysis has been used to gain an understanding of the thermomechanical behavior of these devices. A unique feature of these membranes is that they buckle downward with an increase in temperature beyond a critical value, and remain in the downward buckled state as the temperature decreases back to its initial value. The demonstration devices consist of 2 mm2, 3–4 µm thick silicon membranes with a 1 µm thick layer of silicon dioxide grown on one side and a 3 µm thick layer of aluminum deposited onto the other side. The critical snapping temperature range is measured to be 55 °C ± 5 °C. The snapping temperature is varied by changing the membrane dimensions and/or the materials used to realize the composite membrane structure. Applications to thermal actuation devices and temperature indicators are envisioned.

Electrostatic actuators with long range translation

A general design and development of an electrostatic translational microsystem based on the Inchworm/sup /spl reg// principal of actuation is presented. The inchworm microsystem consists of an oscillating electrostatically actuated driver arm, and two synchronous electrostatic clamps to control direction. Actuation step size is delimited with an additional spring loaded electrostatic actuator, which adjusts the oscillation amplitude of the driver arm in the range of 0.05 /spl mu/m-2/spl mu/m. The translational microstage actuation requires no direct electrical wiring to power the actuations. Voltage to the microstage is delivered through coupled series capacitors allowing all external electrical connections to reside on the stationary substrate. Precision translation is provided by dovetail microjoints that constrain microstage motion. The inchworm microsystem is capable of arbitrarily large translation distances with nanometer resolution.

Thermally actuated, bi-stable, snapping silicon membranes

Thermally actuated, bistable, snapping silicon membranes have been fabricated on a silicon wafer and tested. Nonlinear finite element analysis has been used to model the snapping behavior of the membranes and to refine their design. A unique feature of these membranes is that after snapping down, they remain in the downward buckled state as the temperature decreases back to its starting point. The critical snapping temperature range is 50/spl deg/C-60/spl deg/C for 2 mm square, 6 /spl mu/m thick membranes with 1 /spl mu/m SiO/sub 2/ on top and 3 /spl mu/m aluminum deposited on the backside. The snapping temperature can be varied by changing the dimensions and materials used in the membrane fabrication.

Miniature linear synchronous motor

A miniature linear synchronous motor was designed, fabricated and tested. Actuation was achieved through interaction of traveling magnetic wave, generated by linear array of microcoils on a stator, and permanent magnets on a rotor. Two configurations of the motor were investigated. One with a single, hand assembled permanent magnet on rotor and corresponding array of multiturn microcoils, the other, a fully microfabricated rotor with embedded array of screenplated permanent magnets and serpentine microcoils on stator. Motion of the rotor is constrained by silicon dovetail microjoints. A numerical model was developed for modeling and control. The motor was tested under various operating conditions with both open and closed loop control.

Frequency doubling perimetry using a liquid crystal display

PURPOSE To compare frequency doubling contrast thresholds using a new liquid crystal window display with those obtained with the commercial video-based Frequency Doubling Technology perimeter. METHODS One eye of 49 glaucoma patients and one eye of 49 normal controls were tested with the liquid crystal window and Frequency Doubling Technology systems. Both displays employed identical stimulus conditions and test strategies, although the dynamic range of the liquid crystal window-based display was approximately 30% smaller than that of the Frequency Doubling Technology system. Measurements were repeated using the video-based Frequency Doubling Technology perimeter in a subset of 21 eyes. Relationships between and within displays were assessed using a chance-corrected agreement measure (quadratic weighted kappa) and paired measurement differences. Variability was quantified using standard deviation from the mean paired measurement difference. RESULTS Over the restricted operating range of the liquid crystal display system, between-display and within-video display variability was 2.3 dB and 3.2 dB, respectively, between-display agreement was 0.66, and within-display agreement (test-retest for Frequency Doubling Technology) was 0.65. CONCLUSIONS Levels of agreement and variability between the two frequency doubling displays were of similar magnitude to repeated (test-retest) Frequency Doubling Technology measures, suggesting that contrast threshold measurements made using the two displays may be used interchangeably. However, the operating range of the current liquid crystal window-based display is smaller.

Long-range translation actuator

A long range translation actuator designed for optic and robotic applications is presented. Specifically, the microstage is designed to operate as the moving mirror in a miniature version of a traditional Michelson Fourier transform spectrometer. The translational microstage utilizes an electromagnetic actuation mechanism to realize linear translation of centimeters of precision travel. Motion is constrained in the normal and lateral directions using silicon dovetail microjoints. The electromagnetic actuation is based on macro linear synchronous motor design using a linear array of microcoils. Microcoils are arranged in a 3-phase configuration to enable both velocity and direction control. The electromagnetic force is characterized by finite element computer simulations to develop the input signal for translational travel at constant velocity. Optical position detection was used to measure the translation with time. Operation was demonstrated at various drive frequencies.