Hyung-Kwan Chang

Concentration gradient generation of multiple chemicals using spatially controlled self-assembly of particles in microchannels

We present a robust microfluidic platform for the stable generation of multiple chemical gradients simultaneously using in situ self-assembly of particles in microchannels. This proposed device enables us to generate stable and reproducible diffusion-based gradients rapidly without convection flow: gradients are stabilized within 5 min and are maintained steady for several hours. Using this device, we demonstrate the dynamic position control of bacteria by introducing the sequential directional change of chemical gradients. Green Fluorescent Protein (GFP)-expressing bacterial cells, allowing quantitative monitoring, show not only tracking motion according to the directional control of chemical gradients, but also the gradual loss of sensitivity when exposed to the sequential attractants because of receptor saturation. In addition, the proposed system can be used to study the preferential chemotaxis assay of bacteria toward multiple chemical sources, since it is possible to produce multiple chemical gradients in the main chamber; aspartate induces the most preferential chemotaxis over galactose and ribose. The microfluidic device can be easily fabricated with a simple and cost effective process based on capillary pressure and evaporation for particle assembly. The assembled particles create uniform porous membranes in microchannels and its porosity can be easily controlled with different size particles. Moreover, the membrane is biocompatible and more robust than hydrogel-based porous membranes. The proposed system is expected to be a useful tool for the characterization of bacterial responses to various chemical sources, screening of bacterial cells, synthetic biology and understanding many cellular activities.

Design and analysis of portable loadless wind power source for ubiquitous sensor network

Nowadays, green technology solution using natural power such as solar, wind, waves et al. has been interested as an alternative power source instead of battery. This paper presents a portable power source driven by natural wind power. One of very efficient energy conversion mechanisms to change mechanical energy of natural power with electrical energy is a direct piezoelectric effect. However, this scheme requires AC stress with relative high frequency, which cannot be obtained from wind in the natural environment, because mechanical energy induced by wind shows almost constant power level. We propose a novel and simple mechanism, which converts the constant wind power to AC-like wind power using a propeller. Then, the resultant AC-like flow induces the vibration of piezoelectric cantilever, whose material is polyvinylidene fluoride (PVDF), and thereby electrical energy can be generated. Under 3.5 m/s wind speed condition (mean wind speed in city of Seoul), constant wind was converted to AC-like wind with 42.724 Hz frequency successfully and maximum output voltage from of PVDF cantilever was 4.05 V. This system, combined with processing and storage of the electrical energy, is expected to be a practical solution of wireless energy supply to ubiquitous sensor network.

Microcontact printing of polydopamine on thermally expandable hydrogels for controlled cell adhesion and delivery of geometrically defined microtissues

Scaffold-free harvest of microtissue with a defined structure has received a great deal of interest in cell-based assay and regenerative medicine. In this study, we developed thermally expandable hydrogels with spatially controlled cell adhesive patterns for rapid harvest of geometrically controlled microtissue. We patterned polydopamine (PD) on to the hydrogel via microcontact printing (μCP), in linear shapes with widths of 50, 100 and 200μm. The hydrogels facilitated formation of spatially controlled strip-like microtissue of human dermal fibroblasts (HDFBs). It was possible to harvest and translocate microtissues with controlled widths of 61.4±14.7, 104.3±15.6, and 186.6±22.3μm from the hydrogel to glass substrates by conformal contact upon expansion of the hydrogel in response to a temperature change from 37 to 4°C, preserving high viability, extracellular matrix, and junction proteins. Microtissues were readily translocated in vivo to the subcutaneous tissue of mouse. The microtissues were further utilized as a simple assay model for monitoring of contraction in response to ROCK1 inhibitor. Collectively, micro-sized patterning of PD on the thermally expandable hydrogels via μCP holds promise for the development of microtissue harvesting systems that can be employed to ex vivo tissue assay and cell-based therapy. STATEMENT OF SIGNIFICANCE Harvest of artificial tissue with controlled cellular arrangement independently from external materials has been widely studied in cell-based assay and regenerative medicine. In this study, we developed scaffold-free harvest system of microtissues with anisotropic arrangement and controlled width by exploiting thermally expandable hydrogels with cell-adhesive patterns of polydopamine formed by simple microcontact printing. Cultured strips of human dermal fibroblasts on the hydrogels were rapidly delivered to various targets ranging from flat coverglass to mice subcutaneous tissue by thermal expansion of the hydrogel at 4°C for 10min. These were further utilized as a drug screening model responding to ROCK1 inhibitor, which imply its versatile applicability.

Ultra-fast responsive colloidal–polymer composite-based volatile organic compounds (VOC) sensor using nanoscale easy tear process

There is an immense need for developing a simple, rapid, and inexpensive detection assay for health-care applications or monitoring environments. To address this need, a photonic crystal (PC)-based sensor has been extensively studied due to its numerous advantages such as colorimetric measurement, high sensitivity, and low cost. However, the response time of a typical PC-based sensor is relatively slow due to the presence of the inevitable upper residual layer in colloidal structures. Hence, we propose an ultra-fast responsive PC-based volatile organic compound (VOC) sensor by using a “nanoscale easy tear (NET) process” inspired by commercially available “easy tear package”. A colloidal crystal-polydimethylsiloxane (PDMS) composite can be successfully realized through nanoscale tear propagation along the interface between the outer surface of crystallized nanoparticles and bulk PDMS. The response time for VOC detection exhibits a significant decrease by allowing the direct contact with VOCs, because of perfect removal of the residual on the colloidal crystals. Moreover, vapor-phase VOCs can be monitored, which had been previously impossible. High-throughput production of the patterned colloidal crystal–polymer composite through the NET process can be applied to other multiplexed selective sensing applications or may be used for nanomolding templates.

Use of Piezoelectric Effect in Portable Loadless Wind-Power Source for Ubiquitous Sensor Networks

This paper presents a wind-power-driven portable power source based on piezoelectric effect. Positive piezoelectric effect is one of efficient and widely used mechanisms for converting mechanical energy to electrical energy. However, for this mechanism, a periodic mechanical stress with a high frequency, as in the case of AC, has to be exerted; such stress cannot be exerted by the natural wind in the environment. The natural wind has a constant velocity with slow and irregular variations, as in the case of DC. In this paper, we propose a novel and simple mechanism to convert mechanical energy into electrical energy. The DC-like wind flow is passed through a propeller to convert it to an AC-like wind flow; the resultant AC-like periodic flow induces vibrations in a piezoelectric cantilever, thereby, generating electrical power. This system is expected to be one of practical solutions for wireless energy supply to ubiquitous sensor networks (USNs).

Nanoscale easy tear process for ultra-fast responsive colloidal crystal-PDMS composite VOCs sensors

In this study, we propose an ultra-fast responsive PC based volatile organic compounds (VOCs) sensor using a nanoscale easy tear process inspired by commercially available ‘easy tear package’. Colloidal crystal-polydimethylsiloxane (PDMS) composite can be realized through nanoscale tear propagation along the interface between the outer surface of crystallized nanoparticles and bulk PDMS. Not only cuboid but also dome shaped colloidal crystal-PDMS composite is successfully obtained using the assembled nanoparticles constructed by wettability contrast patterning or inkjet printing. The response time for VOCs detection is dramatically improved by allowing the direct contact with VOCs because of removing the residual on colloidal crystals perfectly.

Paper based reverse electrodialysis power generator

This study represents a paper based energy generator from mixing waters with different salinity. Since the proposed device consists of wax patterned papers that can control microflow and a flexible cation exchange membrane (polycarbonate track-etched (PCTE)), the efficient pumpless reverse electrodialysis (RED) micro platform can be realized successfully. The presented system is flexible, portable and cost-effective, and moreover, shows that the power generation is possible, even in very high concentrated salinity, so it is very promising for practical application.

Quantitative analysis of bacterial preference for cancer secreting proteins

We present a robust microfluidic platform for stable generation of multiple chemical gradients simultaneously using in situ self-assembly of microparticles in microchannels and show the potential of the proposed system for analysis of bacterial preference for cancer cell secreting proteins. We demonstrate the proof of concept as a preferential chemotaxis assay of bacteria toward multiple chemical sources. Aspartate induces the most preferential chemotaxis over galactose and ribose. The microfluidic device can be easily fabricated with simple and cost effective process based on capillary pressure and evaporation for particle assembly. The assembled particles create uniform porous membranes in microchannels and its porosity can be easily controlled with different size particles. Moreover, the membrane is biocompatible and more robust than hydrogel based porous membranes. The proposed system is expected to be a useful tool for characterization of bacterial responses to various chemical sources, screening of bacterial cells, synthetic biology, and understanding many cellular activities.

Control of Highly Organized Nanostructures in Microchannels Using Nanoliter Droplets

In this study, we introduce a novel method for control of self-organization of nanoparticles in microchannels using the control of nanoliter droplets and show its useful applications. By controlling capillary force and evaporation process, nanoparticles can be assembled at the desired area and they can be used from nanoporous membranes to biosensor itself. As the biosensor applications, biologically inspired humidity sensor and IgG antibody detector were developed. They can recognize the target materials by the change of visual color without using any fluorescent probe and external electrical power source. These highly organized nanoparticles also induce the unique nanoelectrokinetics, which open new application fields such as such as separation, filtering, accumulation, and analysis of biomolecules, energy generation, and optofluidic system. Among them, we introduce two techniques that are diffuse based chemical gradient generation and sea water desalination.Copyright

Formation of hydrogel membranes in microchannels and its applications

Formation of membranes in microfluidic channels have been paid attention for their great potentials in separation of components, cell culture supports for tissue engineering and chemical process involving molecular transports. In this study, we introduce a few techniques for formation of hydrogel membranes in microfluidic channels and their applications. First, using enzymatically crosslinkable hydrogels and microfluidic techniques, we realize the width-controllable hydrogel membrane, which was difficult to adjust so far. Second, we suggest a simple one step stamping method for the in-situ formation of hydrogel membranes with various patterns and closed loop shapes. Finally, as the applications of hydrogel membrane, we proposed a multiple directional chemical gradients method, which mimics the real cell environments. Using the proposed membrane formation method, the stable multiple chemical generation was established and the chemotactic response of bacteria was successfully monitored. These results suggest that our suggested systems can be used to understand many cellular activity including cell adhesion and migration directed by chemotaxis or complex diffusions from biological fluids in three dimensional microstructures.