Eunsuk Choi

Flexible and transparent touch sensor using single-wall carbon nanotube thin-films

Here we report on the fabrication and demonstration of flexible and transparent touch sensors using carbon nanotube thin film(CNTF). We fabricated two types of touch sensors. The first type was a compressively strained CNTF pressure sensor, and the second type was a 5-wire resistive CNTF touch sensor. The change in compressively strained CNTF conductivity depended on the strength of the applied vertical pressure and with 0.2 MPa applied vertical pressure, the CNTF sheet conductance increased rapidly by 10 %. We applied cyclic pressures (0.2 MPa) at 15 second intervals and found that the tactile pressure dependence was reproducible for several hundred cycles. In the case of 5-wire resistive CNTF touch sensors, the touch activation pressure of is 23 Pa. And it showed good repeatability over 105 times.

Spatially digitized tactile pressure sensors with tunable sensitivity and sensing range

When developing an electronic skin with touch sensation, an array of tactile pressure sensors with various ranges of pressure detection need to be integrated. This requires low noise, highly reliable sensors with tunable sensing characteristics. We demonstrate the operation of tactile pressure sensors that utilize the spatial distribution of contact electrodes to detect various ranges of tactile pressures. The device consists of a suspended elastomer diaphragm, with a carbon nanotube thin-film on the bottom, which makes contact with the electrodes on the substrate with applied pressure. The electrodes separated by set distances become connected in sequence with tactile pressure, enabling consecutive electrodes to produce a signal. Thus, the pressure is detected not by how much of a signal is produced but by which of the electrodes is registering an output. By modulating the diaphragm diameter, and suspension height, it was possible to tune the pressure sensitivity and sensing range. Also, adding a fingerprint ridge structure enabled the sensor to detect the periodicity of sub-millimeter grating patterns on a silicon wafer.

Spray-coated carbon nanotube thin-film transistors with striped transport channels

We present results for the transfer characteristics of carbon nanotube thin-film transistors (CNT-TFTs) that utilize single-walled carbon nanotube thin-films prepared by direct spray-coating on the substrate. By varying the number of spray-coatings (N(sp)) and the concentration of nanotubes in solution (C(NT)), it was possible to control the conductivity of the spray-coated nanotube thin-film from 129 to 0.1 kΩ/□. Also, by introducing stripes into the channel of the CNT-TFT, and thereby reducing the number of metallic percolation paths between source and drain, it was possible to enhance the on/off current ratio 1000-fold, from 10 to 10(4), demonstrating that it may be possible to utilize spray-coating as a method to fabricate CNT-TFTs for large area switching array applications.

Simultaneous Detection of Displacement, Rotation Angle, and Contact Pressure Using Sandpaper Molded Elastomer Based Triple Electrode Sensor

In this article, we report on a flexible sensor based on a sandpaper molded elastomer that simultaneously detects planar displacement, rotation angle, and vertical contact pressure. When displacement, rotation, and contact pressure are applied, the contact area between the translating top elastomer electrode and the stationary three bottom electrodes change characteristically depending on the movement, making it possible to distinguish between them. The sandpaper molded undulating surface of the elastomer reduces friction at the contact allowing the sensor not to affect the movement during measurement. The sensor showed a 0.25 mm−1 displacement sensitivity with a ±33 μm accuracy, a 0.027 degree−1 of rotation sensitivity with ~0.95 degree accuracy, and a 4.96 kP−1 of pressure sensitivity. For possible application to joint movement detection, we demonstrated that our sensor effectively detected the up-and-down motion of a human forefinger and the bending and straightening motion of a human arm.

Separation and capture of circulating tumor cells from whole blood using a bypass integrated microfluidic trap array

We report on a microfluidic trap array that separates and captures circulating tumor cells (CTCs) from whole blood. The device is a series array of microfluidic branches that utilizes the difference in flow rates between the bypass channel and the trap channel to allow CTCs in whole blood to be separated and trapped. Once a trap has captured a cell with diameter larger than the narrow trap outlet, additional cells arriving at the branch would flow towards the bypass channel due to its lower flow resistance. Results demonstrated that it was possible to capture CTCs from the whole blood of a mouse with full-blown metastasis. With further developments, the bypass integrated microfluidic trap array could become a useful tool for the early prognosis of cancer metastasis.We report on a microfluidic trap array that separates and captures circulating tumor cells (CTCs) from whole blood. The device is a series array of microfluidic branches that utilizes the difference in flow rates between the bypass channel and the trap channel to allow CTCs in whole blood to be separated and trapped. Once a trap has captured a cell with diameter larger than the narrow trap outlet, additional cells arriving at the branch would flow towards the bypass channel due to its lower flow resistance. Results demonstrated that it was possible to capture CTCs from the whole blood of a mouse with full-blown metastasis. With further developments, the bypass integrated microfluidic trap array could become a useful tool for the early prognosis of cancer metastasis.

Blood pressure measurement using finger cuff

Many research groups have studied blood pressure measurement in finger artery because of its convenience. But, low accuracy prohibits many hypertension patients from using this device. So, we suggest measurement algorithm that measure systolic and diastolic blood pressure in finger artery. And we also develop calibration method that decreases the error from difference of finger circumference by subjects. We apply our methods for 90 subjects (age from 20 to 49, 55 male, 35 female) to test feasibility of our method by AAMI SP10 standard. The mean difference of our system is plusmn4.7 mmHg for systolic pressure, plusmn4.2 mmHg for systolic pressure. It proved that the feasibility of our method is clinically acceptable (under plusmn5 mmHg)

A Portable Stiffness Measurement System

A new stiffness measurement method is proposed that utilizes the lateral deformation profile of an object under indentation. The system consists of a force measurement module between a pair of equidistant touch sensing modules. Unique feature of the method is that by adjusting the touch module separation, indenter protrusion, and spring constant of the force sensing module, one can choose a desired sensing range for the force module. This feature helps to enhance the stiffness differentiation between objects of similar hardness and avoids measurement saturation. We devised a portable measurement system based on the method, and tested its performance with several materials including polymer foams and human skin.

Selective Dirac voltage engineering of individual graphene field-effect transistors for digital inverter and frequency multiplier integrations

The ambipolar band structure of graphene presents unique opportunities for novel electronic device applications. A cycle of gate voltage sweep in a conventional graphene transistor produces a frequency-doubled output current. To increase the frequency further, we used various graphene doping control techniques to produce Dirac voltage engineered graphene channels. The various surface treatments and substrate conditions produced differently doped graphene channels that were integrated on a single substrate and multiple Dirac voltages were observed by applying a single gate voltage sweep. We applied the Dirac voltage engineering techniques to graphene field-effect transistors on a single chip for the fabrication of a frequency multiplier and a logic inverter demonstrating analog and digital circuit application possibilities.

Contact Pressure Level Indication Using Stepped Output Tactile Sensors.

In this article, we report on a novel diaphragm-type tactile pressure sensor that produces stepwise output currents depending on varying low contact pressures. When contact pressures are applied to the stepped output tactile sensor (SOTS), the sensor’s suspended diaphragm makes contact with the substrate, which completes a circuit by connecting resistive current paths. Then the contact area, and therefore the number of current paths, would determine the stepped output current produced. This mechanism allows SOTS to have high signal-to-noise ratio (>20 dB) in the 3–500 Hz frequency range at contact pressures below 15 kPa. Moreover, since the sensor’s operation does not depend on a material’s pressure-dependent electrical properties, the SOTS is able to demonstrate high reproducibility and reliability. By forming a 4 × 4 array of SOTS with a surface bump structure, we demonstrated shear sensing as well as surface (1 × 1 cm2) pressure mapping capabilities.

Mapping the process dependent conductivity of carbon nanotube thin-films using a non-invasive contact probing system

We report on a non-invasive contact probing (NICP) system for measuring the distribution of local surface conductivity of macroscopic thin-films of carbon nanotubes. Using the NICP system, we were able to obtain the local sheet resistance of the conducting thin-films continuously at ∼10 μm resolution over few centimeters which would not have been possible using conventional contact probing methods. Measurements performed on carbon nanotube thin-films with various nanotube densities, physical, and chemical treatments revealed that the local variation in electrical characteristics was not reflected in global conductance measurements. This demonstrated the usefulness of the NICP system for evaluating the effect of processing on the electrical uniformity of conducting thin-films made using nanomaterials.