Kyounghwan Na

Quantitative evaluation of cardiomyocyte contractility in a 3D microenvironment

Three-dimensional cultures in a microfabricated environment provide in vivo-like conditions for cells, and have been used in a variety of applications in basic and clinical studies. In this study, the contractility of cardiomyocytes in a 3D environment using complex 3D hybrid biopolymer microcantilevers was quantified and compared with that observed in a 2D environment. By measuring the deflections of the microcantilevers with different surfaces and carrying out finite element modeling (FEM) of the focal pressures of the microcantilevers, it was found that the contractile force of high-density cardiomyocytes on 3D grooved surfaces was 65-85% higher than that of cardiomyocytes on flat surfaces. These results were supported by immunostaining, which showed alignment of the cytoskeleton and elongation of the nuclei, as well as by quantitative RT-PCR, which revealed that cells on the grooved surface had experienced sustained stimuli and tighter cell-to-cell interactions.

Wetting behavior and nanotribological properties of silicon nanopatterns combined with diamond-like carbon and perfluoropolyether films

A large number of silicon (Si) patterns consisting of nanopillars of varying diameter and pitch have been fabricated and further coated with diamond-like carbon (DLC) and perfluoropolyether (Z-DOL) films. The wetting behavior and nano-adhesion/friction of the patterns are investigated experimentally in relation to the nanostructures and the hydrophobicity of the materials. Measurements of water contact angle illustrate that the patterning-enhanced wettability of the Si flat surface, along with two distinct wettings which are in good agreement with the Wenzel and hemi-wicking states, depended on the value of the pitch-over-diameter ratio. In the case of the coated patterns, three wetting states are observed: the Cassie-Baxter, the Wenzel, and a transition from the Cassie-Baxter into the Wenzel, which varies with regard to the hydrophobic properties of the DLC and Z-DOL. In terms of tribological properties, it is demonstrated that a combination of the nanopatterns and the films is effective in reducing adhesive and frictional forces. In addition, the pitch and diameter of the patterns are found to significantly influence their adhesion/friction behaviors.

Nanomechanical measurement of astrocyte stiffness correlated with cytoskeletal maturation

Astrocytes are known to serve as scaffolding cells that shape the brain. The physical properties of astrocytes, such as stiffness, are important for their scaffolding function. These properties may be altered in certain pathological conditions, such as in brain cancer. However, actual stiffness of astrocytes is not yet well understood. Here, we report that the astrocyte stiffness is positively correlated with the density of cytoskeletal proteins, such as actin filaments, microtubules, and intermediate filaments. The value of the stiffness of astrocytes as measured by atomic force microscopy (AFM) increases 38-fold in five-week-old rats compared to postnatal-day zero pups. Using multicolor confocal microscopy, we found that the complexity of cytoskeletal proteins, such as actin filaments, microtubules, and intermediate filaments, increase as the animal gets older. Our findings indicate that the change of stiffness positively correlates with the maturation of cytoskeletal proteins, and suggest that AFM can be useful as an analytical and diagnostic tool for neuroscience.

Hydrophobicity and micro-/nanotribological properties of polymeric nanolines

Abstract This paper presents an investigation of the effects of a topographical modification, namely, nanolines, on the hydrophobicity and micro-/nanotribological properties of poly(methyl methacrylate) surfaces. The polymeric line patterns were fabricated on the poly(methyl methacrylate) films by using the capillary force lithography technique. Examinations of the water contact angle revealed that the line patterns exhibited increased hydrophobicity compared to the flat film, along with an anisotropic wetting. It was observed that the presence of the nanolines greatly reduced the adhesion and micro-/nanofriction. Furthermore, the friction behaviour varied depending upon the sliding direction as the counter bodies slid over the line structure. It was also observed that the shape of the top of the nanolines noticeably influenced nanoscale adhesion and friction. Both the flat film and the nanolines were damaged in the microscale tests; however, the nanolines exhibited less damage than the film, presumably due to their enhanced adhesion and friction properties.

Flexible latching-type tactile display system actuated by combination of electromagnetic and pneumatic forces

In this paper, we have proposed and fabricated a new flexible latching-type tactile display system, which is actuated by combination of electromagnetic and pneumatic forces for large displacement and high tactile force. The proposed tactile display system is implemented on a flexible PDMS structure and can be easily attached on any curved body to transfer reliable tactile information to human. From the proto-type, the actuation displacement has been measured to be 700 μm and the generated force to be 100 mN. With an air pressure 19.5 kPa, the status of tactile actuator can be controlled with an applied current of 10 mA. Also, we have successfully implemented 2 × 2 tactile display array controlled with an applied current and it can be used to transfer different combination of tactile information to human.