Kian Meng Lim

NIR-to-visible upconversion nanoparticles for fluorescent labeling and targeted delivery of siRNA

Near-infrared (NIR)-to-visible upconversion fluorescent nanoparticles were synthesized and used for imaging and targeted delivery of small interfering RNA (siRNA) to cancer cells. Silica-coated NaYF(4) upconversion nanoparticles (UCNs) co-doped with lanthanide ions (Yb/Er) were synthesized. Folic acid and anti-Her2 antibody conjugated UCNs were used to fluorescently label the folate receptors of HT-29 cells and Her2 receptors of SK-BR-3 cells, respectively. The intracellular uptake of the folic acid and antibody conjugated UCNs was visualized using a confocal fluorescence microscope equipped with an NIR laser. siRNA was attached to anti-Her2 antibody conjugated UCNs and the delivery of these nanoparticles to SK-BR-3 cells was studied. Meanwhile, a luciferase assay was established to confirm the gene silencing effect of siRNA. Upconversion nanoparticles can serve as a fluorescent probe and delivery system for simultaneous imaging and delivery of biological molecules.

An extended level set method for shape and topology optimization

In this paper, the conventional level set methods are extended as an effective approach for shape and topology optimization by the introduction of the radial basis functions (RBFs). The RBF multiquadric splines are used to construct the implicit level set function with a high level of accuracy and smoothness and to discretize the original initial value problem into an interpolation problem. The motion of the dynamic interfaces is thus governed by a system of coupled ordinary differential equations (ODEs) and a relatively smooth evolution can be maintained without reinitialization. A practical implementation of this method is further developed for solving a class of energy-based optimization problems, in which approximate solution to the original Hamilton-Jacobi equation may be justified and nucleation of new holes inside the material domain is allowed for. Furthermore, the severe constraints on the temporal and spatial discretizations can be relaxed, leading to a rapid convergence to the final design insensitive to initial guesses. The normal velocities are chosen to perform steepest gradient-based optimization by using shape sensitivity analysis and a bi-sectioning algorithm. A physically meaningful and efficient extension velocity method is also presented. The proposed method is implemented in the framework of minimum compliance design and its efficiency over the existing methods is highlighted. Numerical examples show its accuracy, convergence speed and insensitivity to initial designs in shape and topology optimization of two-dimensional (2D) problems that have been extensively investigated in the literature.

A three-dimensional nonlinear active cochlear model analyzed by the WKB-numeric method

A physiologically based nonlinear active cochlear model is presented. The model includes the three-dimensional viscous fluid effects, an orthotropic cochlear partition with dimensional and material property variation along its length, and a nonlinear active feed-forward mechanism of the organ of Corti. A hybrid asymptotic and numerical method combined with Fourier series expansions is used to provide a fast and efficient iterative procedure for modeling and simulation of the nonlinear responses in the active cochlea. The simulation results for the chinchilla cochlea compare very well with experimental measurements, capturing several nonlinear features observed in basilar membrane responses. These include compression of response with stimulus level, two-tone suppressions, and generation of harmonic distortion and distortion products.

A mesh-free method for static and free vibration analysis of shear deformable laminated composite plates

A mesh-free method is presented to analyze the static deflection and natural frequencies of thin and thick laminated composite plates using high order shear deformation theory. In the present method, the problem domain is represented by a set of properly scattered nodes and no element conformability is required. Moving least-squares method is applied to construct the shape functions. Variational principle is used to derive the discrete system equations based on the third order shear deformation theory (TSDT) of Reddy. Essential boundary conditions are efficiently implemented by a penalty technique for both the static deflection and natural frequency analysis. Several examples are solved to demonstrate the convergence, accuracy and validity of the proposed method. The present solutions are verified with those available values by analytical as well as finite element method. The results from classical plate theory and first order shear deformation theory are also computed and compared with those of TSDT. The effects of the material coefficients, side-to-thickness ratio, nodal distribution and shear correction factor are discussed.

Drop impact test - mechanics & physics of failure

This paper deals with the mechanics and physics of board-level drop test with the intention of providing the fundamental understanding required to design and analyse the results of a drop test. Three finite element analyses were performed to understand the physics of failure in board-level drop impact: (i) velocity impact of a PCB - modeled as a beam; (ii) velocity impact of a PCB with centrally mounted package - modeled as a beam; (iii) velocity impact of a drop assembly - solid elements with submodeling. Parametric studies have been performed on the solid model for a number design variables: drop height, fall plate thickness, PCB length, PCB thickness, solder bump height, solder bump size, solder bump number, and impact cone diameter. Differential flexing as well as inertia has been identified as the key failure drivers. In both cases, the transverse stress, S33, is the most critical stress component. Geometrical stress concentration and intermetallics of the interconnection are critical in the impact strength of interconnection.

First-principles calculations of structural and mechanical properties of Cu6Sn5

The elastic constants of polycrystalline Cu6Sn5—an intermetallic in lead-free alternatives of several material systems—are presented. The results are obtained by applying: (i) Reported crystallographic structure of monoclinic single crystal Cu6Sn5, (ii) structure optimization and determination of single crystal elastic constants from first principle calculations, and (iii) limit analysis of polycrystal stiffness based on single crystal properties. The agreement between the calculated Young’s modulus (120 GPa) and those from nanoindentation experiments (112–125 GPa), and the tight bounds on the predicted polycrystal values give a measure of confidence in other calculated properties for which experimental data are unavailable.

A radial point interpolation method for simulation of two-dimensional piezoelectric structures

A meshfree, radial point interpolation method (RPIM) is presented for the analysis of piezoelectric structures, in which the fundamental electrostatic equations governing piezoelectric media are solved numerically without mesh generation. In the present method, the problem domain is represented by a set of scattered nodes and the field variable is interpolated using the values of nodes in its support domain based on the radial basis functions with polynomial reproduction. The shape functions so constructed possess a delta function property, and hence the essential boundary conditions can be implemented with ease as in the conventional finite element method (FEM). The method is successfully applied to determine deflections or electric potentials of a bimorph beam and mode shapes and natural frequencies of transducers. The present results agree well with those of experiments as well as the FEM by ABAQUS. Some shape parameters are also investigated thoroughly for the future convenience of applying the RPIM for smart materials and structures without the use of elements.

An implicit immersed boundary method for three-dimensional fluid-membrane interactions

We present an implicit immersed boundary method for the incompressible Navier-Stokes equations capable of handling three-dimensional membrane-fluid flow interactions. The goal of our approach is to greatly improve the time step by using the Jacobian-free Newton-Krylov method (JFNK) to advance the location of the elastic membrane implicitly. The most attractive feature of this Jacobian-free approach is Newton-like nonlinear convergence without the cost of forming and storing the true Jacobian. The Generalized Minimal Residual method (GMRES), which is a widely used Krylov-subspace iterative method, is used to update the search direction required for each Newton iteration. Each GMRES iteration only requires the action of the Jacobian in the form of matrix-vector products and therefore avoids the need of forming and storing the Jacobian matrix explicitly. Once the location of the boundary is obtained, the elastic forces acting at the discrete nodes of the membrane are computed using a finite element model. We then use the immersed boundary method to calculate the hydrodynamic effects and fluid-structure interaction effects such as membrane deformation. The present scheme has been validated by several examples including an oscillatory membrane initially placed in a still fluid, capsule membranes in shear flows and large deformation of red blood cells subjected to stretching force.

An immersed interface method for solving incompressible viscous flows with piecewise constant viscosity across a moving elastic membrane

This paper presents an implementation of the second-order accurate immersed interface method to simulate the motion of the flexible elastic membrane immersed in two viscous incompressible fluids with different viscosities, which further develops the work reported in Tan et al. [Z.-J. Tan, D.V. Le, K.M. Lim, B.C. Khoo, An Immersed Interface Method for the Incompressible Navier-Stokes Equations with Discontinuous Viscosity Across the Interface, submitted for publication] focussing mainly on the fixed interface problems. In this work, we introduce the velocity components at the membrane as two augmented unknown interface variables to decouple the originally coupled jump conditions for the velocity and pressure. Three forms of augmented equation are derived to determine the augmented variables to satisfy the continuous condition of the velocity. The velocity at the membrane, which determine the motion of the membrane, is then solved by the GMRES iterative method. The forces calculated from the configuration of the flexible elastic membrane and the augmented variables are interpolated using cubic splines and applied to the fluid through the jump conditions. The position of the flexible elastic membrane is updated implicitly using a quasi-Newton method (BFGS) within each time step. The Navier-Stokes equations are solved on a staggered Cartesian grid using a second order accurate projection method with the incorporation of spatial and temporal jump conditions. In addition, we also show that the inclusion of the temporal jump contributions has non-negligible effect on the simulation results when the grids are crossed by the membrane. Using the above method, we assess the effect of different viscosities on the flow solution and membrane motion.

Static and vibration control of composite laminates integrated with piezoelectric sensors and actuators using the radial point interpolation method

A meshfree model based on the first-order shear deformation theory is presented for the shape and vibration control of laminated composite plates with integrated piezoelectric sensors and actuators. A point interpolation method using radial basis functions (RPIM) is employed to construct shape functions for mechanical and electrical variables, which possess the delta function property and show linear reproduction behavior. The method shows a high convergence rate equivalent to that of the second-order finite elements approach. Comparing, one sees that a very simple nodal topology can be used for the field representation and no element continuity is required. A constant displacement and velocity feedback control algorithm is used for the active control of the static deflection as well as the dynamic response of plates through closed loop control. Numerical results for the static deformation, vibration modes and dynamic responses are in good agreement with those from the finite element method.