M. Bulut Coskun

Ultrafast Dynamic Piezoresistive Response of Graphene‐Based Cellular Elastomers

Ultralight graphene-based cellular elastomers are found to exhibit nearly frequency-independent piezoresistive behaviors. Surpassing the mechanoreceptors in the human skin, these graphene elastomers can provide an instantaneous and high-fidelity electrical response to dynamic pressures ranging from quasi-static up to 2000 Hz, and are capable of detecting ultralow pressures as small as 0.082 Pa.

Nanoscale displacement sensing using microfabricated variable-inductance planar coils

Microfabricated spiral inductors were employed for nanoscale displacement detection, suitable for use in implantable pressure sensor applications. We developed a variable inductor sensor consisting of two coaxially positioned planar coils connected in series to a measurement circuit. The devices were characterized by varying the air gap between the coils hence changing the inductance, while a Colpitts oscillator readout was used to obtain corresponding frequencies. Our approach shows significant advantages over existing methodologies combining a displacement resolution of 17 nm and low hysteresis (0.15%) in a 1 × 1 mm2 device. We show that resolution could be further improved by shrinking the devices lateral dimensions.

Ultrasensitive Strain Sensor Produced by Direct Patterning of Liquid Crystals of Graphene Oxide on a Flexible Substrate

Ultrasensitive flexible strain sensors were developed through the combination of shear alignment of a high concentration graphene oxide (GO) dispersion with fast and precise patterning of multiple rectangular features on a flexible substrate. Resistive changes in the reduced GO films were investigated under various uniaxial strain cycles ranging from 0.025 to 2%, controlled with a motorized nanopositioning stage. The devices uniquely combine a very small detection limit (0.025%) and a high gauge factor with a rapid fabrication process conducive to batch production.

A microfabricated fringing field capacitive pH sensor with an integrated readout circuit

This work presents a microfabricated fringe-field capacitive pH sensor using interdigitated electrodes and an integrated modulation-based readout circuit. The changes in capacitance of the sensor result from the permittivity changes due to pH variations and are converted to frequency shifts using a crossed-coupled voltage controlled oscillator readout circuit. The shift in resonant frequency of the readout circuit is 30.96 MHz for a change in pH of 1.0–5.0. The sensor can be used for the measurement of low pH levels, such as gastric acid, and can be integrated with electronic pills. The measurement results show high repeatability, low noise, and a stable output.

Zero displacement microelectromechanical force sensor using feedback control

Conventional microscale force sensors use moving parts to infer applied forces. Whenever physical deformations are involved, the sensor characteristics become a function of mechanical parameters, and there is an inevitable trade-off between the sensitivity and measurement range. We developed a microfabricated force sensor that uses feedback control to nullify any displacements within the device, directly transducing forces as high as 1.5 mN with a 7.8 nN resolution. The range and sensitivity of the device no longer depend on mechanical parameters, which allow the same device to be used to test samples with a wide range of stiffnesses without loss of accuracy.

Feedback-Controlled MEMS Force Sensor for Characterization of Microcantilevers

This paper outlines the design and characterization of a setup used to measure the stiffness of microcantilevers and other small mechanical devices. Due to the simplicity of fabrication, microcantilevers are used as the basis for a variety of mechanical sensor designs. In a range of applications, knowledge of the stiffness of microcantilevers is essential for the accurate calibration of the sensors in which they are used. Stiffness is most commonly identified through measurement of the microcantilevers resonance frequency, which is applied to an empirically derived model. This paper uses a microelectromechanical system (MEMS)-based force sensor to measure the forces produced by a microcantilever when deformed and a piezoelectric tube-based nanopositioner to displace the microcantilever. A method of calibrating the force sensor is presented that takes advantage of the lumped nature of the mechanical system and the nonlinearity of MEMS electrostatic drives.

A MEMS capacitive pH sensor for high acidic and basic solutions

We have developed a micro-scale chip-based pH sensing system, which can effectively measure the changes in pH ranging from 1 to 4 and 10 to 12. This method relies on fringing field capacitive measurements. A change in pH of the medium results in a permittivity change for the fringing electric field, which in turn affects the capacitance values. Capacitance changes are then converted to resonant frequency shifts via a readout circuit. This technique provides high sensitivity, low hysteresis, and low noise as well as low fabrication cost. Very importantly, the device and the corresponding measurement circuitry have the potential to be integrated in an electronic pill for continuous non-invasive measurement within the body.

Detecting Subtle Vibrations Using Graphene-Based Cellular Elastomers

Ultralight graphene elastomer-based flexible sensors are developed to detect subtle vibrations within a broad frequency range. The same device can be employed as an accelerometer, tested within the experimental bandwidth of 20-300 Hz as well as a microphone, monitoring sound pressures from 300 to 20 000 Hz. The sensing element does not contain any metal parts, making them undetectable by external sources and can provide an acceleration sensitivity of 2.6 mV/g, which is higher than or comparable to those of rigid Si-based piezoresistive microelectromechanical systems (MEMS).

On-Chip Feedthrough Cancellation Methods for Microfabricated AFM Cantilevers With Integrated Piezoelectric Transducers

Active microcantilevers with on-chip sensing and actuation capabilities provide significant advantages in tapping-mode atomic force microscopy. The collocated transduction in active cantilevers enables effective control of their dynamics, allowing for the modification of the quality (

Q control of a microfabricated piezoelectric cantilever with on-chip feedthrough cancellation

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