Seok Yun

Mechanical force response of single living cells using a microrobotic system

In this paper, we investigate mechanical force response of single living cells at different conditions using a microrobotic system. Zebrafish eggs at different developmental stages were collected and an integrated biomanipulation system was employed to measure cellular force during penetrating the egg envelope, the chorion. First, the biomanipulation system integrated with cellular force sensing instrument is implemented to measure the penetration force of the chorion envelope and then to characterize mechanical properties of zebrafish embryos. Second, the cellular force sensing of penetrating the chorion envelope at each developmental stages was experimentally performed. The results demonstrated that the biomanipulation system with force sensing capability can measure cellular force in real-time while the injection operation is undergoing. The magnitude of the measured cellular force decrease as an embryo develops. This result quantitatively describes the chorion softening in zebrafish embryos. Experimental results also demonstrate that subtle modification of the chorion, the extracellular matrix of the egg, can be monitored physically using the developed real-time force sensing system.

Cellular force measurement for force reflected biomanipulation

In biological cell manipulations, manual thrust or a skilled operator, relying only on the visual feedback information only, usually performs penetration of injection pipette into single cells. Accurately measuring cellular force is a requirement for minimally invasive cell injection and it is essential in investigating biomechanical properties of cell membranes. We have fabricated an end effector with microforce sensing capability for biocell injections using PVDF piezoelectric polymer and measured several mN level injection force for zebrafish egg cell. Experimental results show that the cellular force level is changed as the injection pipette penetrates yolk and chorion sequentially. As well, the experimental setup with haptic interface suggests that a more reliable biomanipulation can be achieved by supplying real-time cellular force feedback to the operator.

Novel platform for minimizing cell loss on separation process: Droplet-based magnetically activated cell separator.

To reduce the problem of cell loss due to adhesion, one of the basic phenomena in microchannel, we proposed the droplet-based magnetically activated cell separator (DMACS). Based on the platform of the DMACS-which consists of permanent magnets, a coverslip with a circle-shaped boundary, and an injection tube-we could collect magnetically (CD45)-labeled (positive) cells with high purity and minimize cell loss due to adhesion. To compare separation efficiency between the MACS and the DMACS, the total number of cells before and after separation with both the separators was counted by flow cytometry. We could find that the number (3241/59 940) of cells lost in the DMACS is much less than that (22 360/59 940) in the MACS while the efficiency of cell separation in the DMACS (96.07%) is almost the same as that in the MACS (96.72%). Practically, with fluorescent images, it was visually confirmed that the statistical data are reliable. From the viability test by using Hoechst 33 342, it was also demonstrated that there was no cell damage on a gas-liquid interface. Conclusively, DMACS will be a powerful tool to separate rare cells and applicable as a separator, key component of lab-on-a-chip.

Droplet-based magnetically activated cell separation

In this study, we developed a method that target cells in suspension can be separated by combining magnetic force and gravitation force. Since the newly developed method involves a separating process of a droplet containing nontarget cells in suspension by applying magnetic force to separate target cells, we called it droplet-based magnetic activated cell sorting (dMACS). To demonstrate the efficiency of the dMACS system, Ter119 (+) cells from mouse bone marrow cells were separated by both conventional MACS and our dMACS systems. Effects of three parameters on separation efficiency were examined in the dMACS system. As a result, both volume of droplet of cell suspension, and magnetic force did not affect the efficiency of cell separation markedly. However, the time for cell settlement in the droplet showed a critical role in the efficiency of cell separation according to increasing time. Therefore, we tried to verify that the saturation time affected increase of its efficiency and that flow rate injected to get rid of the negative cell resulted in the decrease of its efficiency. Using this dMACS system, we were able to pinpoint that the flow rate of cell suspension injected into a magnetic platform results in disturbance in the droplet, leading to turbulence in the cell suspension.

Enhancement of sensitivity of DACS (dielectrophoretically activated cell sorting) using 3D-asymmetric microelectrodes

In this paper, a novel 3D-asymmetric microelectrode system has been designed and fabricated to enhance the sorting sensitivity about cells for high-throughput. Basic concept of the presented system, which utilizes relative strength between the negative dielectrophoretic force and the drag force, is the same with conventional 3D microelectrode system. However, varied dielectrophoretic force along the transverse direction of microchannel is realized by presented microelectrode system with changing width in half sphere shaped channel. This varied force increases the sorting sensitivity about cells by inducing the different forces to different kinds of cells distinctly. Based on numerical analysis for the 3D-asymmetric microelectrode, significantly improved sensitivity about cells was verified, and the feasibility of this device was shown by experimental study.

Design and fabrication of a large-deformed smart sensorized polymer actuator

The demands for actuators featuring biomimetic properties, such as high power density, large strain, and biocompatibility are growing in microrobotics and bioengineering. PPy is one candidate for biomimetic actuator since it is biocompatible, easy to be fabricated by MEMS technique, has large strain, and consumes low energy. In this paper, a novel sensorized polymer actuator which can measure its bending motion precisely with real-time is presented. It is fabricated by integrating polypyrrole(PPy) actuator with polyvinylidenefluoride(PVDF) sensor. Since PVDF is also biocompatible, stable to chemical, flexible, and polymer sensor, it could be well combined with PPy actuator. In experimental results, the proposed actuator shows the feasibility which can not only be actuated with large strain (a few mm) but also produce meaningful signals which made from bending motion and vice versa.