Yeon Hwa Kwak

Flexible glucose sensor using CVD-grown graphene-based field effect transistor

A flexible glucose sensor using a CVD-grown graphene-based field-effect-transistor (FET) is demonstrated. The CVD-grown graphene was functionalized with linker molecules in order to immobilize the enzymes that induce the catalytic response of glucose. Polyethylene terephthalate (PET) was employed as the substrate material to realize a flexible sensor. The fabricated graphene-based FET sensor showed ambipolar transfer characteristics. Through measurements of the Dirac point shift and differential drain-source current, the fabricated FET sensor could detect glucose levels in the range of 3.3-10.9 mM, which mostly covers the reference range of medical examination or screen test for diabetes diagnostic. This CVD-grown graphene-based FET sensor, which provides excellent fitting to a model curve even when deformed, high resolution, and continuous real-time monitoring, holds great promise, especially for portable, wearable, and implantable glucose level monitoring applications.

Flexible heartbeat sensor for wearable device

We demonstrate a flexible strain-gauge sensor and its use in a wearable application for heart rate detection. This polymer-based strain-gauge sensor was fabricated using a double-sided fabrication method with polymer and metal, i.e., polyimide and nickel-chrome. The fabrication process for this strain-gauge sensor is compatible with the conventional flexible printed circuit board (FPCB) processes facilitating its commercialization. The fabricated sensor showed a linear relation for an applied normal force of more than 930 kPa, with a minimum detectable force of 6.25Pa. This sensor can also linearly detect a bending radius from 5mm to 100mm. It is a thin, flexible, compact, and inexpensive (for mass production) heart rate detection sensor that is highly sensitive compared to the established optical photoplethysmography (PPG) sensors. It can detect not only the timing of heart pulsation, but also the amplitude or shape of the pulse signal. The proposed strain-gauge sensor can be applicable to various applications for smart devices requiring heartbeat detection.

A single cell tracking system in real-time

We describe here a novel real-time cell tracking system which can measure cell migration routes under cell culture condition. This system includes a mini incubator which controls temperature and CO(2) gas flow and a PDMS (polydimethylsiloxane) chip for chemotaxis measurement. The main differences from previous ones are real-timely long-term (24h) tracking for single cell quantitatively, simple and inexpensive constitution of optical parts for illumination and imaging, and compatible to commercial well plate. The tracking principle is to trace cell images for each 0.2s by converting the live cell images to binary images of black and white. Migration results of HUVEC and NCI-H23 cells are obtained respectively using this system. The results are single cell path (x, y) during migration, cell size, migration distance, migration speed, real-time pictures and so on. This system is applicable to all kinds of researches related to cell migration such as cell angiogenesis, chemotaxis, and moreover cancer metastasis.

A new 2D structure for chemotaxis measurement of adhesive cells

This paper presents a new 2D structure to measure a chemotactic migration of adhesive cells for analysis of cell kinetics. This structure was designed to overcome Boyden chambers disadvantages and made real-time measurement possible in 2D. The 2D chip was made of PDMS and glass for transparency. To create more than 2% of concentration gradient for chemotaxis measurement, the PDMS chip has a gradually narrowing width towards the chemotactic material. The concentration gradient was checked by measuring fluorescent gradient of Cy3 tagged to EGF as a chemotactic material. More than 2% of concentration gradient was maintained for more than 24 hours, and which was confirmed by fluorescent microscope. HUVEC chemotaxis using VEGF as a chemoattractant was checked in the 2D chip. It was verified that HUVEC migrated towards attractant position and the migration direction and migration distance were almost in proportion to the concentration of VEGF. This chip will be applicable to all kinds of migration related researches including cancer metastasis.

A flexible strain-gauge sensor for flexible input devices

This study demonstrates a novel polymer-based flexible strain-gauge sensor for flexible input devices in flexible displays. Proposed novel strain-gauge sensor for user interfaces (UIs) in flexible display measures the deformation of an object by using its electrical resistance change. This strain-gauge sensor was fabricated by the double-sided fabrication method with polymer and metal, i.e., polyimide and Nickel-Chrome. Experimental results show that this sensor linearly detects the forces more than 500gf, i.e., 5N, with the force resolution of less than 10gf, i.e., 0.1N. Also it can linearly detect the bending radius from 5mm to 100mm. This flexible strain-gauge sensor array may be positioned on the bezel area of a flexible display device enabling various input methodology as well as UIs.

Comparison of two types of tactile sensing layer in touch screen panel for force sensitive detection

Here we present two types of Touch Screen Panels (TSPs) consisted of silicone gel and glycerin as the transparent Tactile Sensing Layer (TSL) measuring touch force(z axis) and touch position(x-y axis). The principle of the TSP is based on capacitive methods in which the distance between top and bottom substrates is varied by touch or interaction force leading the capacitance change between two substrates. Silicone gel as the TSL showed the force detection resolution of about 50gf and the dynamic range of 0-500gf. For the same test, Glycerin showed the detection resolution of 10gf and the dynamic range of 200gf. These relatively excellent results are caused from the permittivity and hardness of the new two TSL materials.

High-throughput and real-time microalgae monitoring platform using lens-free shadow imaging system (LSIS)

Floc size analysis is one of the major processes in bio-flocculant efficiency determination, which is critical for microalgae harvesting. Till now the flocculation analysis has been performed by using photospectrometry. In this conventional method, optical density of a sample is measured in a time interval by collecting the sample from a fixed point of the container, up to ~5 hrs of observation period. This time consuming process may not guarantee the viability of the algae, which limits the efficient harvesting. To address this issue, we introduce a real time flocculation monitoring system based on the lens-free shadow imaging technique (LSIT). This simple, fast, and cost-effective system automatically analyzes the thousands of single microalgae all in parallel and monitors their flocculation by using a custom developed algorithm. We evaluate the performance of this approach by comparing the results with the standard method, showing a good agreement between two modalities.

Lens-free automated cell detection system for telemedicine application

Cell and micro particle analysis is one of the major tasks in many clinical labs culturing cells. The functional profiles of cells such as concentration, size, morphology and viability are the key parameters for early diagnosis of many diseases. However in most labs, especially in the resource limited settings, these processes are done fully manually using conventional optical microscopes, which are slow and prone to subjective errors. Recently the lens-free shadow imaging system has been introduced as an alternative for the conventional optical microscopy. In this paper we demonstrate a dedicated algorithm for lens-free shadow imaging system, which was implemented as a smartphone application. This android application can wirelessly acquire the images from the lens-free system and process them automatically. The results can be obtained locally or sent to a remote expert for further analysis. The feasibility of this system was evaluated by comparing the results with the standard optical microscope.

Live cells detection in breast cell-line by FTIR micro-spectrometer

Infrared absorption for living cells in aqueous media has been measured for normal and cancer cells using Fourier transform infrared (FTIR) point micro-spectrometer. IR characteristics of live cells were measured under cell culture conditions, while most of previous works were performed with dehydrated or fixed cells. In this paper, the infrared spectra of adhered cells were measured in transmission mode by point microscopy with spot sizes of 200 mum times 200 mum. Some remarkable differences are observed among normal (MCF10A), cancer(MCF7) and invasive cancer (MDAMB231) cells in breast cell line. Peaks at 3621 and 2873 cm-1 appeared except for MCF10A, which shows the distinction between cancer and normal cell. At 3621 cm-1 peak also, appeared in MCF10A after subculture over fifteen times, which seemed to be an evidence that normal cell was being transformed to cancer cell. These results show a potential of spectroscopic methodology instead of image analysis with fluorescence material in cell.