Yong-Gu Lee

Single-cell optoporation and transfection using femtosecond laser and optical tweezers

In this paper, we demonstrate a new single-cell optoporation and transfection technique using a femtosecond Gaussian laser beam and optical tweezers. Tightly focused near-infrared (NIR) femtosecond laser pulse was employed to transiently perforate the cellular membrane at a single point in MCF-7 cancer cells. A distinct technique was developed by trapping the microparticle using optical tweezers to focus the femtosecond laser precisely on the cell membrane to puncture it. Subsequently, an external gene was introduced in the cell by trapping and inserting the same plasmid-coated microparticle into the optoporated cell using optical tweezers. Various experimental parameters such as femtosecond laser exposure power, exposure time, puncture hole size, exact focusing of the femtosecond laser on the cell membrane, and cell healing time were closely analyzed to create the optimal conditions for cell viability. Following the insertion of plasmid-coated microparticles in the cell, the targeted cells exhibited green fluorescent protein (GFP) under the fluorescent microscope, hence confirming successful transfection into the cell. This new optoporation and transfection technique maximizes the level of selectivity and control over the targeted cell, and this may be a breakthrough method through which to induce controllable genetic changes in the cell.

Smoothing haptic interaction using molecular force calculations

This paper presents a new method for smoothing haptic interaction with molecular force calculations that uses Lennard-Jones forcefield. The gradient of the forcefield is used unaltered when the distance between two atoms is greater than the sum of their van der Waals radii. However, when they are smaller, a hard-surface wall implemented using a spring model is used to repel two atoms. This eliminates the instability when two atoms are in contact in the presence of forcefields that have strong gradients. This method is tested on rigid hydrocarbon molecules with no bond creation or breaking. Published by Elsevier Science Ltd.

Geometric detail suppression by the Fourier transform

As the Finite Element Method is widely used in strength analysis, automatic mesh generation draws more attention these days. For a given geometric tolerance value, the purpose of mesh generators is to discretize the continuous model within this error limit. Faithfulness to this condition produces many small elements at small features. Often, these regions are of little interest and computer resources are thus wasted. It is wished to suppress selectively, small features from the model before discretizing. This can be achieved by low-pass filtering a CAD model. A method to apply the techniques in signal processing to the manipulation of a three-dimensional model is proposed.

Trapping of a micro-bubble by non-paraxial Gaussian beam: computation using the FDTD method

Optical forces on a micro-bubble were computed using the Finite Difference Time Domain method. Non-paraxial Gaussian beam equation was used to represent the incident laser with high numerical aperture, common in optical tweezers. The electromagnetic field distribution around a micro-bubble was computed using FDTD method and the electromagnetic stress tensor on the surface of a micro-bubble was used to compute the optical forces. By the analysis of the computational results, interesting relations between the radius of the circular trapping ring and the corresponding stability of the trap were found.

Computing the medial surface of a 3-D boundary representation model

Abstract Medial surface, the skeletal abstraction of a solid shape, can be used in applications such as shell (solid) mesh generation, robot path planning, and feature recognition. Unfortunately, computing the medical surface is very difficult. Those few algorithms proposed invariably need to solve systems of non-linear polynomial equations, which requires complicated numerical techniques. Recently, an algorithm has been proposed by the authors that is applicable to a polyhedral model. It uses voxel-bisector intersection relationships to calculate the intersection points and, from these, constructs the boundaries of the medical surface patches. Unfortunately, it does not handle the degenerate cases. This paper extends the scope of the voxel-bisector intersection algorithm by proposing a supplementary algorithm to detect the existence of the degenerate intersection curve in a specified region (voxel). An enhancement to the voxel-bisector intersection algorithm by using the Voronoi territory, which needs no polynomial root-finding scheme, is also proposed.

Corrected field enhancement factor for the floating sphere model of carbon nanotube emitter

We have corrected the field enhancement factor for the “floating sphere at emitter-plane potential” model with the finite anode-cathode distance. If ρ is the radius of sphere, h is the distance from cathode to the center of sphere, and l is the distance from the center to the anode, then the field enhancement factor is given as the following expression βsph=(2+7η−η2)(λ2−2λ+2)/[2η(1−λ)(2−λ)], where η=ρ/h, λ=ρ/l. This expression demonstrates reasonable behavior for three limiting cases: if h→ρ, if l→∞, and if l→ρ. We have compared our factor βsph with the field enhancement factor βtube for the “hemisphere on a post” model and the factor βell for the “hemiellipsoid on plane” model. We have shown realization of the approximate evaluation βtube≈(βsph+βell)/2.

A micromachined friction meter for silicon sidewalls with consideration of contact surface shape

A friction meter with consideration of contact surface shape is proposed for the evaluation of the static and dynamic friction coefficients on the sidewalls of micromachined structures. In order to validate the proposed friction measurement method, a friction meter for sidewalls was designed employing simple beam springs with holding and driving comb actuators fabricated using a silicon deep reactive ion etching process. In experiments to assess the meter, a shuttle was placed at a certain position by the driving actuator, and a symmetric normal holding force was subsequently applied to the sidewalls of the shuttle. After increasing the driving voltage with a ramp slope, the sliding distance was measured so as to determine the static and dynamic friction coefficients with consideration of the spring nonlinearity. To characterize the suggested friction meter, experiments were performed to investigate the effects of the normal force and the contact surface shape on friction coefficients by varying the contact widths and the number of contact points. The results indicate that the friction coefficients increased with the normal holding force, whereas the contact surface shape did not show a noticeable effect on the friction coefficients.

Screened field enhancement factor for the floating sphere model of a carbon nanotube array

The screened field enhancement factor for a carbon nanotube (CNT) placed in a CNT array (which is reduced due to the screening effect) is derived based on the “floating sphere” model. We obtain an expression for the field enhancement factor for a CNT in the array as γ=3+2(1+η)/{(2+η)[2πα(2+η)δ2+η]}, where ρ is the radius of sphere, h is the distance from cathode to the center of sphere, and D is the distance between the nearest spheres, η=ρ/h, δ=ρ/D, and α=1 for square or 2/3 for hexagonal lattice made of CNTs. Explicit algebraic formulas for optimizing the distance between tubes, areal density of emitters, and the anode current are also obtained.

Trapping of a single DNA molecule using nanoplasmonic structures for biosensor applications

Conventional optical trapping using a tightly focused beam is not suitable for trapping particles that are smaller than the diffraction limit because of the increasing need of the incident laser power that could produce permanent thermal damages. One of the current solutions to this problem is to intensify the local field enhancement by using nanoplasmonic structures without increasing the laser power. Nanoplasmonic tweezers have been used for various small molecules but there is no known report of trapping a single DNA molecule. In this paper, we present the trapping of a single DNA molecule using a nanohole created on a gold substrate. Furthermore, we show that the DNA of different lengths can be differentiated through the measurement of scattering signals leading to possible new DNA sensor applications.

Simulation of an oil immersion objective lens: A simplified ray-optics model considering Abbe’s sine condition

In this paper, a simplified mathematical ray-optics model for an oil immersion objective lens, considering Abbes sine condition, is presented. Based on the given parameters of the objective lens, the proposed model utilizes an approach based on a paraxial thin lens formulation. This is done to simplify the complexity of the objective lens by avoiding the consideration of many lens elements inside a single objective lens. To demonstrate the performance of the proposed model, comparisons with exact ray tracing method, based on the specification of real objective lens, are presented in terms of several different criteria including the variation of shape of the light cone, the extent of vignetting and the focus displacement. From the exemplary simulations, it was demonstrated that the proposed model can describe the focusing of light through the objective lens precisely, even when the incident beam rotates.