Eun Joong Lee

Differentiation of Neural Progenitor Cells in a Microfluidic Chip‐Generated Cytokine Gradient

In early embryonic development, spatial gradients of diffusible signaling molecules play important roles in controlling differentiation of cell types or arrays in diverse tissues. Thus, the concentration of exogenous cytokines or growth factors at any given time is crucial to the formation of an enriched population of a desired cell type from primitive stem cells in vitro. Microfluidic technology has proven very useful in the creation of cell‐friendly microenvironments. Such techniques are, however, currently limited to a few cell types. Improved versatility is required if these systems are to become practically applicable to stem cells showing various plasticity ranges. Here, we built a microfluidic platform in which cells can be exposed to stable concentration gradients of various signaling molecules for more than a week with only minimal handling and no external power source. To maintain stability of the gradient concentration, the osmotic pumping performance was optimized by balancing the capillary action and hydraulic pressure in the inlet reagent reservoirs. We cultured an enriched population of neural progenitors derived from human embryonic stem cells in our microfluidic chamber for 8 days under continuous cytokine gradients (sonic hedgehog, fibroblast growth factor 8, and bone morphogenetic protein 4). Neural progenitors successfully differentiated into neurons, generating a complex neural network. The average numbers of both neuronal cell body clusters and neurite bundles were directly proportional to sonic hedgehog concentrations in the gradient chip. The system was shown to be useful for both basic and translational research, with straightforward mechanisms and operational schemes. STEM CELLS 2009;27:2646–2654

Fabrication of three-dimensional microarray structures by controlling the thickness and elasticity of poly(dimethylsiloxane) membrane

In this paper, we propose a method to construct three-dimensional curved microstructures with easy control of the size, position and shape, by exploiting the elasticity of poly(dimethylsiloxane) (PDMS) membranes and basic physics. For this end, we developed the method to handle thin PDMS membrane safely, and to replicate PDMS microstructure from the PDMS mold. Using this method, we demonstrated two potential applications: (1) the use of concave well for the formation of embryoid body (EB) to differentiate into neuronal cells, and (2) the fabrication of SU-8 and hydrogel microparticles having diverse curved shapes. The curved structures were successfully fabricated with simple process, and EBs were formed in the concave well and differentiated into the neuronal cells. Microparticles with diverse shapes were fabricated from a range of materials for potential use as drug carrier and pH responsive micro-actuator elements.

Interconnection of Multichannel Polyimide Electrodes Using Anisotropic Conductive Films (ACFs) for Biomedical Applications

In this paper, we propose a method for interconnecting soft polyimide (PI) electrodes using anisotropic conductive films (ACFs). Reliable and automated bonding was achieved through development of a desktop thermocompressive bonding device that could simultaneously deliver appropriate temperatures and pressures to the interconnection area. The bonding conditions were optimized by changing the bonding temperature and bonding pressure. The electrical properties were characterized by measuring the contact resistance of the ACF bonding area, yielding a measure that was used to optimize the applied pressure and temperature. The optimal conditions consisted of applying a pressure of 4 kgf/cm2 and a temperature of 180 °C for 20 s. Although ACF base bonding is widely used in industry (e.g., liquid crystal display manufacturing), this study constitutes the first trial of a biomedical application. We performed a preliminary in vivo biocompatibility investigation of ACF bonded area. Using the optimized temperature and pressure conditions, we interconnected a 40-channel PI multielectrode device for measuring electroencephalography (EEG) signals from the skulls of mice. The electrical properties of electrode were characterized by measuring the impedance. Finally, EEG signals were measured from the mice skulls using the fabricated devices to investigate suitability for application to biomedical devices.

PDMS- and Silver-Ball-Based Flexible Multichannel Surface Electrode: Fabrication and Application in Nerve Conduction Study on Patients With Diabetic Polyneuropathy

In this paper, we describe a multichannel surface electrode for measuring nerve conduction in patients with neuromuscular disorders. The electrode was constructed using a nontoxic, nonflammable poly(dimethylsiloxane) substrate and a contacting silver-ball electrode using a simple, cost-effective fabrication process. The fabricated electrode is sufficiently flexible and embossed to maintain excellent skin contact, while preventing interference from neighboring electrodes under the wet environment. It is also biocompatible, as demonstrated by the absence of skin problems after a one-week test. The electrical and mechanical properties and durability of the electrode were tested. The multichannel surface electrode was strong and durable, enduring repeated bending through 120deg and resisting damage after four million repetitions in a bending test. The electrode surface was easily coated with conducting gel and recordings could be made under wet conditions without causing interference to neighboring electrodes. The enhanced impedance was comparable to that of a large commercial electrode and signals measured from the abductor pollicis brevis were noiseless. A quantitative investigation of the latencies of compound muscle action potentials in normal subjects and patients with diabetes mellitus was carried out to evaluate clinical applicability.

Polyimide-based multi-channel arrayed electrode for measuring EEG signal on the skull of mouse

In this paper, we have developed 40 channel multiple electrodes mounted on the surface of mouse’s skull using polyimide substrate and tested its performance by measuring EEG signals. The recording site of the electrode was electroplated by Pt to enhance both contact impedance and adhesive strength by applying proper current, cleaning surface and removing H2 gas bubbles. For in vivo test, the electrode was placed on the skull of F1 mouse and EEG signals were measured. We observed the suitability of electrode for measuring EEG signals from multiple areas on the skull. The spectrum of EEG signal to change was observed by urethane administration.

Fabrication of microfluidic system for the assessment of cell migration on 3D micropatterned substrates

Cell migration and proliferation are major process in wound healing, cancer metastasis and organogenesis during development. Many cells are related to recovery process of wound. Especially, fibroblasts act an important role in wound healing. Various cytokines such as platelet derived growth factor (PDGF) can induce fibroblast migration and widely studied to investigate the cell response under controlled cytokine microenvironments during wound healing. In real tissue healing process, cell microenvironments change with tissue types and anatomical characteristics of organs. With microfluidic system, we tried to mimic the natural microenvironment of wound healing, with gradient of PDGF, a fibroblast migration inducing cytokine, and patterned substrate with different orientation to PDGF gradient. Fibroblasts cultured in PDGF gradient micro fluidic chip showed cell migration under various micro environmental gradient conditions. Cells were cultured under PDGF gradient condition and different substrate pattern. Mouse fibroblast L929 cells were cultured in the microfluidic gradient. The results showed that most cells migrated along the substrate topological patterns under high concentration of PDGF. We developed long range sustaining micro fluidic channel and could analyze cell migration along the gradient of PDGF. Also, the cell migration on patterned extracellular environment shows that cells migrate along the extracellular 3D pattern rather than directly along the cytokine gradient when the pattern height is less than 1 μm. In this study, we could demonstrate that the extracellular pattern is more dominant to cell migration in combination with cytokine gradient in the wounded tissue when the environmental cues are 20 μm.

P24-22 Development of Polydimethylsiloxane (PDMS) and silver ball based dry type flexible EEG electrode for EEG recording

band, age-group under 30 has a smallest Z-score among three groups due to suppression by photic stimulus. The age-group from 30 to 60 showed the value intermediate between under 30 and over 60. Conclusions: We proposed the analysis method eliminating individual difference and then applied the method to photic driving response of normal subjects. Z-scores at frequencies such as the fundamental wave and higher harmonics is closely related to age. Significance: By using our statistical analysis method, the frequency characteristics of Z-score varied with advancing age drastically. Our method may be helpful to evaluate brain aging.

Multichannel surface electrode based on PDMS and silver ball, and its application to nerve conduction study

In this paper we have developed the multichannel surface electrode for the nerve conduction study in patients with neuromuscular disorder. As electrode material, we used PDMS (polydimethylsiloxane) which is flexible, biocompatible, non-toxic and non-flammable as substrate and silver ball as contacting electrode. Conducting gel was easily coatable on the electrode surface without causing interference of neighboring electrode. The signals measured from abductor pollicis brevis were noiseless and the clinical feasibility was evaluated by the quantitative investigation of the latencies of compound muscle action potentials in normal subject and patients with diabetes mellitus.