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PASSIVE WIRELESS DIRECT SHEAR STRESS MEASUREMENT
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This paper presents an implantable parylene-based wireless pressure sensor for biomedical pressure sensing applications specifically designed for continuous intraocular pressure (IOP) monitoring in glaucoma patients. It has an electrical LC tank resonant circuit formed by an integrated capacitor and an inductor coil to facilitate passive wireless sensing using an external interrogating coil connected to a readout unit. Two surface-micromachined sensor designs incorporating variable capacitor and variable capacitor/inductor resonant circuits have been implemented to realize the pressure-sensitive components. The sensor is monolithically microfabricated by exploiting parylene as a biocompatible structural material in a suitable form factor for minimally invasive intraocular implantation. Pressure responses of the microsensor have been characterized to demonstrate its high pressure sensitivity ( > 7000 ppm/mmHg) in both sensor designs, which confirms the feasibility of pressure sensing with smaller than 1 mmHg of resolution for practical biomedical applications. A six-month animal study verifies the in vivo bioefficacy and biostability of the implant in the intraocular environment with no surgical or postoperative complications. Preliminary ex vivo experimental results verify the IOP sensing feasibility of such device. This sensor will ultimately be implanted at the pars plana or on the iris of the eye to fulfill continuous, convenient, direct, and faithful IOP monitoring.[2008-0111].
This paper reports an integrated inductor-capacitor (LC) resonator structure fabricated using bulk micromachining and anodic bonding technologies. In this resonator structure, pressure change monitored by a capacitive pressure sensor results in the changeof resonance frequency. The resonance frequency shift is detected by inductive coupling from an external transmission coil; therefore, pressure can be monitored remotely using the passive LC resonator. The fabricated device size measures3 mm×3 mm×0.6 mm, and pressure responsivity has been estimated to be 2 MHz/mmHg. This micromachined, hermetically sealed structure is suitable for biomedical applications such as intraocular, cardiovascular and brain pressure monitoring.
This paper reviews three categories of skin-friction measurement techniques: MEMS-based sensors, oil-¯lm interferometry, and liquid crystals. For each of the techniques, the paper discusses the theory, development, use, limitation, and uncertainties of the techniques are presented. Current and future uses of the techniques are also dis-cussed. From this review, it is evident that the use of the MEMS-based and oil-¯lm interferometric skin-friction techniques is accelerating, which should ensure a future emphasis on the continued development of these techniques.
The paper presents a direct, capacitive shear stress sensor with performance sufficient for time-resolved turbulence measurements. The device employs a bulk-micromachined, metal-plated, differential capacitive floating-element design. A simple, two-mask fabrication process is used with DRIE on an SOI wafer to form a tethered floating element structure with comb fingers for transduction. Experimental results indicate a linear sensitivity of 7.66 mV/Pa up to the testing limit of 1.9 Pa at a bias voltage of 10 V , and a bandwidth of 6.2 kHz . The sensor possesses a dynamic range Gt 102 dB and a noise floor of 14.9 muPa/radic(Hz) at 1 kHz , outperforming previously reported sensors by nearly two orders of magnitude in MDS.
This paper presents a single-chip integrated humidity sensor (IHS) capable of wireless operation through inductive coupling with a remote antenna. The mm sensor chip consists of a planar electroplated copper coil (20 μm thick, 30 μm pitch, and 23 turns) and a silicon substrate separated by a 4100–5600 Å polyimide film. The resonant frequency of the IHS changes with humidity and the measured sensitivity ranges from 4 to 16 kHz/% RH. Measurements show a hysteresis of 4.5% RH over a range of 30–70% RH for a 5600 Å thick polyimide film IHS.
This paper reports the development of an absolute wireless pressure sensor that consists of a capacitive sensor and a gold-electroplated planar coil. Applied pressure deflects a 6 μm-thin silicon diaphragm, changing the capacitance formed between it and a metal electrode supported on a glass substrate. The resonant frequency of the LC circuit formed by the capacitor and the inductor changes as the capacitance changes; this change is sensed remotely through inductive coupling, eliminating the need for wire connection or implanted telemetry circuits. The sensor is fabricated using the dissolved-wafer process and utilizes a boron-doped silicon diaphragm supported on an insulating glass substrate. The complete sensor measures mm in size and incorporates a 24-turns gold-electroplated coil that has a measured inductance of 1.2 μH. The sensor is designed to provide a resonant frequency change in the range 95–103 MHz for a pressure change in the range 0–50 mmHg with respect to ambient pressure, providing a pressure responsivity and sensitivity of 160 kHz/mmHg and 1553 ppm/mmHg, respectively. The measured pressure responsivity and sensitivity of the fabricated device are 120 kHz/mmHg and 1579 ppm/mmHg, respectively.
A gas sensor, comprised of a gas-responsive multiwall carbon
nanotube (MWNT)-silicon dioxide (SiO<sub>2</sub>) composite layer
deposited on a planar inductor-capacitor resonant circuit is presented
here for the monitoring of carbon dioxide (CO<sub>2</sub>), oxygen (O
<sub>2</sub>), and ammonia (NH<sub>3</sub>). The absorption of different
gases in the MWNT-SiO<sub>2</sub> layer changes the permittivity and
conductivity of the material and consequently alters the resonant
frequency of the sensor. By tracking the frequency spectrum of the
sensor with a loop antenna, humidity, temperature, as well as CO<sub>2
</sub>, O<sub>2</sub> and NH<sub>3</sub> concentrations can be
determined, enabling applications such as remotely monitoring conditions
inside opaque, sealed containers. Experimental results show the sensor
response to CO<sub>2</sub> and O<sub>2</sub> is both linear and
reversible. Both irreversible and reversible responses are observed in
response to NH<sub>3</sub>, indicating both physisorption and
chemisorption of NH<sub>3</sub> by the carbon nanotubes. A sensor array,
comprised of an uncoated, SiO<sub>2</sub> coated, and MWNT-SiO<sub>2
</sub> coated sensor, enables CO<sub>2</sub> measurement to be
automatically calibrated for operation in a variable humidity and
temperature environment