Dynamics of a surface-modified miniaturized SiN mechanical resonator via a nanometer-scale pore array.

We have fabricated porous miniaturized SiN resonators with various dimensions and studied their mechanical dynamics at their resonant modes. The surface modification of the resonators has been achieved by etching through a thin porous anodic aluminum oxide (AAO) mask, prepared by two-step anodization. Even though these porous resonators show well-defined Lorentzian line-shapes at their resonant modes, the corresponding fundamental flexural resonance frequencies are lower than those from typical non-porous resonators. The change in the resonance frequency is due to the presence of the pores on the surface, which reduces the effective tensile stress across the beam structure, as shown from both experimental measurements and the computational model. In addition, the observed quality factor reveals the level of dissipation originating from the surface modification. The principal dissipation mechanism is found to be gas damping in the free molecular flow regime. Based on the dissipation measurement, one can see an increase in the surface-to-mass ratio, which is responsible for the increased dissipation in the porous beam structure. The work presented here demonstrates simple integration of mechanical elements with a nanopatterning technique based on an AAO as well as the tuning of mechanics via surface modification at a small scale. Such a scheme could provide an additional degree of freedom in developing a mechanical sensing element with enhanced effective surface area.

Pressure-dependent dissipation effect at multiple cantilever resonant modes.

Based on the optical deflection method, the resonant characteristics of a microcantilever under various pressure have been observed at room temperature to understand the pressure-dependent dissipation effect. Especially, the quality factor of the cantilever has been measured for up to fourth harmonic mode of cantilever resonance as a function of pressure between 0.1 and 1000 Torr. By considering the intrinsic dissipation present in the system at 0.1 Torr, the pressure-dependent fluidic quality factors were determined for the multiple cantilever resonant modes. The inverse of the fluidic quality factor appears to follow two different asymptotic behaviors at high and low pressure limits, which indicates that the dynamics of the fluid, due to the oscillating cantilever, changes from Newtonian to non-Newtonian with decreasing pressure. The experimentally observed transition of the fluidic dissipation effect agrees well with the recently proposed rapidly oscillating flow model based on the Boltzmann equation, regardless of the different mode shapes.

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 kg f/cm(2) 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.

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.

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 microm. 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 microm.

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 H(2) 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.

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.

Study of cellular behaviors on concave and convex microstructures fabricated from elastic PDMS membranes.

Cells respond to geometrical cues, as well as to biochemical and mechanical stimuli. Recent progress in micro- and nano-technology has allowed researchers to create microbeads, micro-circular islands, and microposts, that can be used to examine the effect of geometrical cues on cellular behavior. Knowledge of changes in cell mechanics and morphology in response to geometric cues is important for understanding the basic behavior of cells during development and pathological processes. Most previous research in this area has focused on cell responses to two-dimensional planar or rectilinear structures. Very few studies have examined cell responses to three-dimensional curved structures because of the difficulty of fabricating such microstructures. Here we describe a novel method for the fabrication of convex and concave microstructures by use of a thin poly(dimethylsiloxane) (PDMS) membrane, SU-8 shadow mask, and negative air pressure without using any complicated silicon processes. We successfully fabricated concave and convex microstructures, with base diameters of 200-300 microm and depth (or height) of 50-150 microm (aspect ratios up to 1 : 0.5), and used these microstructures to study the responses of cultured L929 mouse fibroblast cells and human mesenchymal stem cells. These cells clearly sensed the three-dimensional microscale curvature and actively "escaped" from concave patterns, but not from those which were convex. Thus, it appears that microscale concave structures suppress cell adhesion and proliferation. We hypothesized that this might relate to deformation of the plasma membrane and subsequent opening of membrane channels. We anticipate that our system will be useful for various bio-MEMS (micro electro mechanical system) applications, including formation of uniformly-sized embryoid bodies, embryonic stem cell differentiation, and the fabrication of cell docking devices, microbioreactors, and microlenses as well as cell mechanics study.